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+The Project Gutenberg EBook of The Wright Brothers' Engines and Their
+Design, by Leonard S. Hobbs
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: The Wright Brothers' Engines and Their Design
+
+Author: Leonard S. Hobbs
+
+Release Date: February 2, 2012 [EBook #38739]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE WRIGHT BROTHERS' ENGINES ***
+
+
+
+
+Produced by Chris Curnow, Joe Cooper, Christine P. Travers
+and the Online Distributed Proofreading Team at
+http://www.pgdp.net
+
+
+
+
+
+
+
+
+
+SERIAL PUBLICATIONS OF THE SMITHSONIAN INSTITUTION
+
+
+The emphasis upon publications as a means of diffusing knowledge was
+expressed by the first Secretary of the Smithsonian Institution. In
+his formal plan for the Institution, Joseph Henry articulated a
+program that included the following statement: "It is proposed to
+publish a series of reports, giving an account of the new discoveries
+in science, and of the changes made from year to year in all branches
+of knowledge not strictly professional." This keynote of basic
+research has been adhered to over the years in the issuance of
+thousands of titles in serial publications under the Smithsonian
+imprint, commencing with _Smithsonian Contributions to Knowledge_ in
+1848 and continuing with the following active series:
+
+  _Smithsonian Annals of Flight_
+
+  _Smithsonian Contributions to Anthropology_
+
+  _Smithsonian Contributions to Astrophysics_
+
+  _Smithsonian Contributions to Botany_
+
+  _Smithsonian Contributions to the Earth Sciences_
+
+  _Smithsonian Contributions to Paleobiology_
+
+  _Smithsonian Contributions to Zoology_
+
+  _Smithsonian Studies in History and Technology_
+
+In these series, the Institution publishes original articles and
+monographs dealing with the research and collections of its several
+museums and offices and of professional colleagues at other
+institutions of learning. These papers report newly acquired facts,
+synoptic interpretations of data, or original theory in specialized
+fields. Each publication is distributed by mailing lists to libraries,
+laboratories, institutes, and interested specialists throughout the
+world. Individual copies may be obtained from the Smithsonian
+Institution Press as long as stocks are available.
+
+                                                 S. DILLON RIPLEY
+                                                    _Secretary_
+                                               Smithsonian Institution
+
+
+
+
+  The Wright Brothers' Engines
+  And Their Design
+
+
+
+
+[Illustration: Kitty Hawk Flyer with original Wright engine poised on
+launching rail at Kill Devil Hill, near Kitty Hawk, North Carolina, 24
+November 1903, the month before the Wrights achieved man's first
+powered and controlled flight in a heavier-than-air craft.]
+
+[Illustration: Reproduction of the first engine, built by Pratt &
+Whitney, as displayed in Wright Brothers National Memorial at Kitty
+Hawk. Engine is mounted in a reproduction of the Wrights' Flyer built
+by the National Capital Section of the Institute of the Aeronautical
+Sciences (now the American Institute of Aeronautics and Astronautics).
+Engine and plane were donated in 1963 to the National Park Service
+Cape Hatteras National Seashore.]
+
+
+
+
+  SMITHSONIAN ANNALS OF FLIGHT * NUMBER 5
+
+  SMITHSONIAN INSTITUTION * NATIONAL AIR AND SPACE MUSEUM
+
+  The Wright Brothers' Engines
+  And Their Design
+
+  _Leonard S. Hobbs_
+
+
+  SMITHSONIAN INSTITUTION PRESS
+  CITY OF WASHINGTON
+  1971
+
+
+
+
+_Smithsonian Annals of Flight_
+
+
+Numbers 1-4 constitute volume one of _Smithsonian Annals of Flight_.
+Subsequent numbers will bear no volume designation, which has been
+dropped. The following earlier numbers of _Smithsonian Annals of
+Flight_ are available from the Superintendent of Documents as
+indicated below:
+
+  1. The First Nonstop Coast-to-Coast Flight and the Historic T-2
+  Airplane, by Louis S. Casey. 1964. 90 pages, 43 figures, appendix,
+  bibliography. Out of print.
+
+  2. The First Airplane Diesel Engine: Packard Model DR-980 of
+  1928, by Robert B. Meyer. 1964. 48 pages, 37 figures, appendix,
+  bibliography. Price 60˘.
+
+  3. The Liberty Engine 1918-1942, by Philip S. Dickey. 1968.
+  110 pages, 20 figures, appendix, bibliography. Price 75˘.
+
+  4. Aircraft Propulsion: A Review of the Evolution of Aircraft Piston
+  Engines, by C. Fayette Taylor. 1971 viii + 134 pages,
+  72 figures, appendix, bibliography of 601 items. Price $1.75.
+
+
+For sale by Superintendent of Documents, Government Printing Office
+Washington, D.C. 20402--Price 60 cents
+
+
+
+
+Foreword
+
+
+In this fifth number of _Smithsonian Annals of Flight_ Leonard S.
+Hobbs analyzes the original Wright _Kitty Hawk Flyer_ engine from the
+point of view of an aeronautical engineer whose long experience in the
+development of aircraft engines gives him unique insight into the
+problems confronting these remarkable brothers and the ingenious
+solutions they achieved. His review of these achievements also
+includes their later vertical 4-and 6-cylinder models designed and
+produced between 1903 and 1915.
+
+The career of Leonard S. (Luke) Hobbs spans the years that saw the
+maturing of the aircraft piston engine and then the transition from
+reciprocating power to the gas turbine engine. In 1920 he became a
+test engineer in the Power Plant Laboratory of the Army Air Service at
+McCook Field in Dayton, Ohio. There, and later as an engineer with the
+Stromberg Motor Devices Corporation, he specialized in aircraft engine
+carburetors and developed the basic float-type to the stage of utility
+where for the first time it provided normal operation during airplane
+evolutions, including inverted flight.
+
+Joining Pratt & Whitney Aircraft in 1927 as Research Engineer, Hobbs
+advanced to engineering manager in 1935 and in 1939 took over complete
+direction of its engineering. He was named vice president for
+engineering for all of United Aircraft in 1944, and was elected vice
+chairman of United Aircraft in 1956, serving in that capacity until
+his retirement in 1958. He remained a member of the board of directors
+until 1968. Those years saw the final development of Pratt & Whitney's
+extensive line of aircraft piston engines which were utilized by the
+United States and foreign air forces in large quantities and were
+prominent in the establishment of worldwide air transportation.
+
+In 1963 Hobbs was awarded the Collier Trophy for having directed the
+design and development of the J57 turbojet, the country's first such
+engine widely used in both military service and air transportation.
+
+He was an early fellow of the Institute of Aeronautical Sciences
+(later the American Institute of Aeronautics and Astronautics), served
+for many years on the Powerplant Committee of the National Advisory
+Committee for Aeronautics, and was the recipient of the Presidential
+Certificate of Merit.
+
+                                    FRANK A. TAYLOR, _Acting Director_
+                                       _National Air and Space Museum_
+
+_March 1970_
+
+
+
+
+Contents
+
+
+  Foreword                                                           v
+  Acknowledgments                                                   ix
+  The Beginnings                                                     1
+  The Engine of the First Flight, 1903                               9
+  The Engines With Which They Mastered the Art of Flying            29
+  The Four-Cylinder Vertical Demonstration Engine and the First
+      Production Engine                                             34
+  The Eight-Cylinder Racing Engine                                  47
+  The Six-Cylinder Vertical Engine                                  49
+
+  Minor Design Details and Performance of the Wright Engines        57
+  Appendix                                                          62
+      Characteristics of the Wright Flight Engines                  62
+      The Wright Shop Engine                                        64
+  Bibliography                                                      69
+  Index                                                             71
+
+
+
+
+Acknowledgments
+
+
+As is probably usual with most notes such as this, however short,
+before completion the author becomes indebted to so many people that
+it is not practical to record all the acknowledgments that should be
+made. This I regret extremely, for I am most appreciative of the
+assistance of the many who responded to my every request. The mere
+mention of the Wright name automatically opened almost every door and
+brought forth complete cooperation. I do not believe that in the
+history of the country there has been another scientist or engineer as
+admired and revered as they are.
+
+I must, however, name a few who gave substantially of their time and
+effort and without whose help this work would not be as complete as it
+is. Gilmoure N. Cole, A. L. Rockwell, and the late L. Morgan Porter
+were major contributors, the latter having made the calculations of
+the shaking forces, the volumetric efficiency, and the connecting rod
+characteristics of the 1903 engine. Louis P. Christman, who was
+responsible for the Smithsonian drawings of this engine and also
+supervised the reconstruction of the 1905 Wright airplane, supplied
+much information, including a great deal of the history of the early
+engines. Opie Chenoweth, one of the early students of the subject, was
+of much assistance; and I am indebted to R. V. Kerley for the major
+part of the data on the Wrights' shop engine.
+
+Also, I must express my great appreciation to the many organizations
+that cooperated so fully, and to all the people of these organizations
+and institutions who gave their assistance so freely. These include
+the following:
+
+  Air Force Museum, Wright-Patterson Air Force Base, Ohio
+  Carillon Park Museum, Dayton, Ohio
+  Connecticut Aeronautical Historical Association, Hebron, Connecticut
+  Fredrick C. Crawford Museum, Cleveland, Ohio
+  Historical Department, Daimler Benz A. G., Stuttgart-Untertürkheim,
+    West Germany
+  Engineers Club, Dayton, Ohio
+  Deutsches Museum, Munich, West Germany
+  Educational and Musical Arts, Inc., Dayton, Ohio
+  Henry Ford Museum, Dearborn, Michigan
+  Franklin Institute, Philadelphia, Pennsylvania
+  Howell Cheney Technical School, Manchester, Connecticut
+  Library of Congress, Washington, D.C.
+  Naval Air Systems Command, U.S. Navy, Washington, D.C.
+  Science Museum, London, England
+  Victoria and Albert Museum, London, England
+
+In particular, very extensive contributions were made by the
+Smithsonian Institution and by the United Aircraft Corporation through
+its Library, through the Pratt & Whitney Aircraft Division's entire
+Engineering Department and its Marketing and Product Support
+Departments, and through United Aircraft International.
+
+
+
+
+The Beginnings
+
+
+The general history of the flight engines used by the Wright Brothers
+is quite fascinating and fortunately rather well recorded.[1] The
+individual interested in obtaining a reasonably complete general story
+quickly is referred to three of the items listed in the short
+bibliography on page 69. The first, _The Papers of Wilbur and Orville
+Wright_, is a primary source edited by the authority on the Wright
+brothers, Marvin W. McFarland of the Library of Congress; a compact
+appendix to volume 2 of the _Papers_ contains most of the essential
+facts. This source is supplemented by the paper of Baker[2] and the
+accompanying comments by Chenoweth, presented at the National
+Aeronautics Meeting of the Society of Automotive Engineers on 17 April
+1950. Aside from their excellence as history, these publications are
+outstanding for the manner in which those responsible demonstrate
+their competence and complete mastery of the sometimes complex
+technical part of the Wright story.
+
+[Footnote 1: An extensive bibliography, essentially as complete at
+this time as when it was compiled in the early 1950s, is given on
+pages 1240-1242 of volume 2 of _The Papers of Wilbur and Orville
+Wright_, 1953.]
+
+[Footnote 2: Max P. Baker was a technical adviser to the Wright estate
+and as such had complete access to all of the material it contained.]
+
+The consuming interest of the Wrights, of course, was in flight as
+such, and in their thinking the required power unit was of only
+secondary importance. However, regardless of their feeling about it,
+the unit was an integral part of their objective and, due to the
+prevailing circumstances, they very early found themselves in the
+aircraft engine business despite their inexperience. This business was
+carried on very successfully, against increasingly severe competition,
+until Orville Wright withdrew from commercial activity and dissolved
+the Wright Company. The time span covered approximately the twelve
+years from 1903 to 1915, during the first five years of which they
+designed and built for their own use several engines of three
+different experimental and demonstration designs. In the latter part
+of the period, they manufactured and sold engines commercially, and
+during this time they marketed three models, one of which was
+basically their last demonstration design. A special racing engine was
+also built and flown during this period. Accurate records are not
+available but altogether, they produced a total of something probably
+close to 200 engines of which they themselves took a small number for
+their various activities, including their school and flying exhibition
+work which at one time accounted for a very substantial part of their
+business. A similar lack of information concerning their competition,
+which expanded rapidly after the Wright's demonstrations, makes any
+comparisons a difficult task. The Wrights were meticulous about
+checking the actual performance of their engines but at that time
+ratings generally were seldom authenticated and even when different
+engines were tried in the same airplane the results usually were not
+measured with any accuracy or recorded with any permanency. There is
+evidence that the competition became effective enough to compel the
+complete redesign of their engine so that it was essentially a new
+model.
+
+For their initial experimentation the Wrights regarded gravity as not
+only their most reliable power source but also the one most economical
+and readily available, hence their concentration on gliding. They had
+correctly diagnosed the basic problem of flight to be that of control,
+the matter of the best wing shapes being inherently a simpler one
+which they would master by experiment, utilizing at first gravity and
+later a wind tunnel. Consequently, the acquisition of a powerplant
+intended for actual flight was considerably deferred.
+
+Nevertheless, they were continuously considering the power requirement
+and its problems. In his September 1901 lecture to the Western Society
+of Engineers, Wilbur Wright made two statements: "Men also know how to
+build engines and screws of sufficient lightness and power to drive
+these planes at sustaining speed"; and in conjunction with some
+figures he quoted of the required power and weight: "Such an engine is
+entirely practicable. Indeed, working motors of one-half this weight
+per horsepower [9 pounds per horsepower] have been constructed by
+several different builders." It is quite obvious that with their
+general knowledge and the experience they had acquired in designing
+and building a successful shop engine for their own use, they had no
+cause to doubt their ability to supply a suitable powerplant when the
+need arose. After the characteristics of the airframe had been
+settled, and the engine requirements delineated in rather detailed
+form, they had reached the point of decision on what they termed the
+motor problem. Only one major element had changed greatly since their
+previous consideration of the matter; they had arrived at the point
+where they not only needed a flight engine, they wanted it quickly.
+
+Nothing has been found that would indicate how much consideration they
+had given to forms of power for propulsion other than the choice they
+had apparently made quite early--the internal-combustion,
+four-stroke-cycle piston engine. Undoubtedly, steam was dismissed
+without being given much, if any, thought. On the face of it, the
+system was quite impractical for the size and kind of machine they
+planned; but it had been chosen by Maxim for his experiments,[3] and
+some thirty-five or forty years later a serious effort to produce an
+aviation engine utilizing steam was initiated by Lockheed. On the
+other hand internal-combustion two-stroke-cycle piston engines had
+been built and used successfully in a limited way. And since, at that
+time, it was probably not recognized that the maximum quantity of heat
+it is possible to dissipate imposed an inherent limitation on the
+power output of the internal-combustion engine, the two-stroke-cycle
+may have appeared to offer a higher output from a given engine size
+than the four-stroke-cycle could produce. Certainly, it would have
+seemed to promise much less torque variation for the same output,
+something that was of great importance to the Wrights. Against this,
+the poor scavenging efficiency of the two-stroke operation, and most
+probably its concurrent poor fuel economy, were always evident; and,
+moreover, at that time the majority of operating engines were
+four-stroke-cycle. Whatever their reasoning, they selected for their
+first powered flight the exact form of prime mover that continued to
+power the airplane until the advent of the aircraft gas turbine more
+than forty years later.
+
+[Footnote 3: In the 1890s the wealthy inventor Sir Hiram Stevens Maxim
+conducted an experiment of considerable magnitude with a flying
+machine that utilized a twin-cylinder compound steam powerplant. It
+was developed to the flight-test stage.]
+
+The indicated solution to their problem of obtaining the engine--and
+the engine that would seem by all odds most reliable--would have been
+to have a unit produced to their specifications by one of the best of
+the experienced engine builders, and to accomplish this, the most
+effective method would be to use the equivalent of a bid procedure.
+This they attempted, and sent out a letter of inquiry to a fairly
+large number of manufacturers. Although no copy of the letter is
+available, it is rather well established that it requested the price
+of an engine of certain limited specifications which would satisfy
+their flight requirements, but beyond this there is little in the
+record.
+
+A more thorough examination of the underlying fundamentals, however,
+discloses many weaknesses in the simple assumptions that made the
+choice of an experienced builder seem automatic. A maximum requirement
+limited to only one or two units offered little incentive to a
+manufacturer already successfully producing in his field, and the
+disadvantage of the limited quantity was only accentuated by the basic
+requirement for a technical performance in excess of any standard of
+the time. Certainly there was no promise of any future quantity
+business or any other substantial reward. Orville Wright many times
+stated that they had no desire to produce their own engine, but it is
+doubtful that they had any real faith in the buying procedure, for
+they made no attempt to follow up their first inquiries or to expand
+the original list.
+
+Whatever the reasoning, their judgment of the situation is obvious;
+they spent no time awaiting results from the letter but almost
+immediately started on the task of designing and building the engine
+themselves. Perhaps the generalities were not as governing as the two
+specific factors whose immediate importance were determining: cost and
+time. The Wrights no doubt realized that a specially designed,
+relatively high performance engine in very limited hand-built
+quantities would not only be an expensive purchased article but would
+also take considerable time to build, even under the most favorable
+circumstances. So the lack of response to their first approach did not
+have too much to do with their ultimate decision to undertake this
+task themselves.
+
+The question of the cost of the Wrights' powerplants is most
+intriguing, as is that of their entire accomplishment. No detailed
+figures of actual engine costs are in the record, and it is somewhat
+difficult to imagine just how they managed to conduct an operation
+requiring so much effort and such material resources, given the income
+available from their fairly small bicycle business. The only evidence
+bearing on this is a statement that the maximum income from this
+business averaged $3,000 a year,[4] which of course had to cover not
+only the airplane and engine but all personal and other expenses. Yet
+they always had spare engines and spare parts available; they
+seemingly had no trouble acquiring needed materials and supplies, both
+simple and complex; and they apparently never were hindered at any
+time by lack of cash or credit. The only mention of any concern about
+money is a statement by Wilbur Wright in a letter of 20 May 1908 when,
+about to sail for France for the first public demonstrations, he
+wrote: "This plan would put it to the touch quickly and also help ward
+off an approaching financial stringency which has worried me very much
+for several months." It is a remarkable record in the economical use
+of money, considering all they had done up to that time. The myth that
+they had been aided by the earnings of their sister Katherine as a
+school teacher was demolished long ago.
+
+[Footnote 4: Fred C. Kelly, _Miracle at Kitty Hawk_, 1951.]
+
+The decision to build the engine themselves added one more
+requirement, and possibly to some extent a restriction, to the design.
+They undoubtedly desired to machine as much of the engine as possible
+in their own shop, and the very limited equipment they had would
+affect the variety of features and constructions that could be
+utilized, although experienced machine shops with sophisticated
+equipment were available in Dayton and it is obvious that the Wrights
+intended to, and did, utilize these when necessary. The use of their
+own equipment, of course, guaranteed that the parts they could handle
+themselves would be more expeditiously produced. They commenced work
+on the design and construction shortly before Christmas in 1902.
+
+The subject of drawings of the engine is interesting, not only as
+history but also because it presents several mysteries. Taylor[5]
+stated, "We didn't make any drawings. One of us would sketch out the
+part we were talking about on a piece of scrap paper ..." Obviously
+somewhere in the operation some dimensions were added, for the design
+in many places required quite accurate machining. Orville Wright's
+diary of 1904 has the entry, "Took old engine apart to get
+measurements for making new engine." Finally, no Wright drawings of
+the original engine have been seen by anyone connected with the
+history or with the Wright estate. In the estate were two drawings
+(now at the Franklin Institute), on heavy brown wrapping paper,
+relating to one of the two very similar later engines built in 1904;
+one is of a cylinder and connecting rod, the other is an end view of
+the engine. Thus even if the very ingenious drafting board now in the
+Wright Museum at Carillon Park was available at the time there is no
+indication that it was used to produce what could properly be called
+drawings of the first engine.
+
+[Footnote 5: Charles E. Taylor (Charley Taylor to the many who knew
+him) was in effect the superintendent of and also the only employee to
+work in the original small machine shop. A most versatile and
+efficient mechanic and machine operator, he made many parts for all of
+the early engines, and in the manner of the experimental machinist,
+worked mainly from sketches. He also had charge of the bicycle shop
+and its business in the absence of the Wrights.]
+
+There are in existence, however, two complete sets of drawings, both
+of which purport to represent the 1903 flight engine. One set was made
+in England for the Science Museum in the two years 1928 and 1939. The
+1928 drawings were made on receipt of the engine, which was not
+disassembled, but in 1939 the engine was removed from the airplane,
+disassembled, the original 1928 drawings were corrected and added to,
+and the whole was made into one very complete and usable set. The
+other set was prepared in Dayton, Ohio, for Educational and Musical
+Arts, Inc.,[6] and was donated to the Smithsonian Institution. This
+latter set was started under the direction of Orville Wright, who died
+shortly after the work had been commenced.
+
+[Footnote 6: This is a charitable agency set up by the late Colonel
+and Mrs. E. A. Deeds primarily for the purpose of building and
+supporting the Deeds Carillon and the Carillon Park Museum in Dayton,
+Ohio.]
+
+The two sets of drawings, that is, the one of the Science Museum and
+that made in Dayton for the Smithsonian Institution, cannot be
+reconciled in the matter of details. Hardly any single dimension is
+exactly the same and essentially every part differs in some respect.
+Many of the forms of construction differ and even the firing order of
+the two engines is not the same, so that in effect the drawings show
+two different engines.
+
+[Illustration: Figure 1.--First flight engine, 1903, valve side.
+(Photo courtesy Science Museum, London.)]
+
+The primary trouble is, of course, that the exact engine which flew in
+1903 is no longer in existence, and since no original drawings of it
+exist, there is considerable doubt about its details. The engine had
+its crankcase broken in an accident to the airframe (this was caused
+by a strong wind gust immediately following the last of the first
+series of flights at Kitty Hawk), and when it was brought back to
+Dayton it was for some inexplicable reason completely laid aside, even
+though it presumably contained many usable parts. When the engine was
+disassembled to obtain measurements for constructing the 1904 engines,
+again apparently no drawings were made. In February 1906 Orville
+Wright wrote that all the parts of the engine were still in existence
+except the crankcase; but shortly after this the crankshaft and
+flywheel were loaned for exhibition purposes and were never recovered.
+In 1926 the engine was reassembled for an exhibition and in 1928 it
+was again reassembled for shipment to England. The only parts of this
+particular engine whose complete history is definitely known are the
+crankshaft and flywheel, which were taken from the 1904-1905 flight
+engine. This latter engine, now in the restored 1905 airplane in the
+Carillon Park Museum in Dayton, does not contain a crankshaft, and in
+its place incorporates a length of round bar stock.
+
+[Illustration: Figure 2.--First flight engine, 1903, underside and
+flywheel end. (Photo courtesy Science Museum, London.)]
+
+In late 1947 work on the Educational and Musical Arts drawings was
+initiated under the direction of Louis P. Christman and carried
+through to completion by him. Christman has stated that Orville Wright
+was critical of the Science Museum drawings but just what he thought
+incorrect is not known. Whatever his reasons, he did encourage
+Christman to undertake the major task of duplication. Christman worked
+directly with Orville Wright for a period of six weeks and had access
+to all the records and parts the Wrights had preserved. The resultant
+drawings are also very complete and, regardless of the differences
+between these two primary sets, both give a sufficiently accurate
+picture of the first engine for all purposes except that of exact
+reproduction in every detail.
+
+There exists a still unsolved puzzle in connection with what seems to
+be yet another set of drawings of the first engine. In December 1943,
+in writing to the Science Museum telling of his decision to have the
+airplane and engine brought back to the United States, Orville Wright
+stated, "I have complete and accurate drawings of the engine. I shall
+be glad to furnish them if you decide to make a replica."[7] No trace
+of these particular drawings can be found in any of the museums,
+institutions, or other repositories that normally should have acquired
+them and the executors of Orville Wright's estate have no record or
+knowledge of them. The date of his letter is four years before the
+Dayton drawings were commenced; and when Christman was working on
+these with Orville Wright they had copies of the Science Museum
+drawings, with complete knowledge of their origin, yet Christman has
+no knowledge of the drawings referred to in Orville's letter to the
+Museum. Finally, the evidence is quite conclusive that there were no
+reproducible or permanent drawings made at the time the first engine
+was constructed, and, of course, the reconstructed engine itself was
+sent to England in 1928 and not returned to this country until
+1948.[8]
+
+[Footnote 7: The Science Museum expressed a desire to have these but
+never received them. There is a reference to them in a letter to the
+Museum from the executors of his estate dated 20 February 1948, but is
+seems rather obvious from the text that by this time the drawings
+mentioned by Orville Wright in his 1943 letter had become confused
+with those being prepared by Christman for the Smithsonian
+Institution. The Science Museum did have constructed from its own
+drawings a very fine replica which is completely operable at this
+time.]
+
+[Footnote 8: There is a third set of drawings prepared by the Ford
+Motor Company also marked as being of the 1903 engine and these are
+rather well distributed in various museums and institutions. What this
+set is based on has been impossible to determine but it is indicated
+from the existence of actual engines and parts and the probable date
+of their preparation (no date is given on the drawings themselves)
+that they were copied from drawings previously made, and therefore add
+nothing to them. The Orville Wright-Henry Ford friendship originated
+rather late, considering Ford's avid interest in history and
+mechanical things. This tardiness could possibly have been the result
+of Wright coolness--a coolness caused by a report, at the time the
+validity of the Wright patents was being so strongly contested, that
+Ford had advised some of those opposing the Wrights to persevere and
+to obtain the services of his patent counsel who had been successful
+in overturning the Selden automobile patent. If this barrier ever
+existed it was surmounted, and Ford spent much effort and went to
+considerable expense to collect the Wright home and machine shop for
+his Dearborn museum. The shop equipment apparently had been widely
+scattered and its retrieval was a major task. It is most likely that
+the drawings resulted from someone's effort to follow out an order to
+produce a set of Ford drawings of the original engine. A small scale
+model of the 1903 flight engine, constructed under the supervision of
+Charles Taylor, is contained in the Dearborn Museum.]
+
+
+
+
+The Engine of the First Flight, 1903
+
+
+In commencing the design of the first engine, the first important
+decision arrived at was that of the number and size of the cylinders
+to be employed and the form in which they would be combined, although
+it is unlikely that this presented any serious problem. In a similar
+situation Manly, when he was working on the engine for the Langley
+Aerodrome,[9] was somewhat perturbed because he did not have access to
+the most advanced technical knowledge, since the automobile people who
+were at that time the leaders in the development of the internal
+combustion engine, tended for competitive reasons to be rather
+secretive about their latest advancements and designs. But although
+the standard textbooks may not have been very helpful to him, there
+were available such volumes as W. Worby Beaumont's _Motor Vehicles and
+Motors_ which contained in considerable detail descriptions and
+illustrations of the best of the current automobile engines. The
+situations of Manly and the Wrights differed, however, in that whereas
+the Wrights' objective was certainly a technical performance
+considerably above the existing average, Manly's goal was that of
+something so far beyond this average as to have been considered by
+many impossible. Importantly, the Wrights had their own experience
+with their shop engine and a good basic general knowledge of the size
+of engine that would be necessary to meet their requirements.
+
+[Footnote 9: Charles L. Manly was engaged in the development of the
+engine for the Langley Aerodrome. See also footnote to Table on page
+62.]
+
+Engine roughness was of primary concern to them. In the 1902
+description of the engine they sent to various manufacturers, they had
+stated: "... and the engine would be free from vibration." Even though
+their requirement for a smooth engine was much more urgent than merely
+to avoid the effect of roughness on the airplane frame, they were
+faced, before they made their first powered flight, with the basic
+problem with which the airplane has had to contend for over
+three-quarters of its present life span: that is, it was necessary to
+utilize an explosion engine in a structure which, because of weight
+limitations, had to be made the lightest and hence frailest that could
+possibly be devised and yet serve its primary purpose. However great
+the difficulty may have appeared, in the long view, the fault was
+certainly a relatively minor one in the overall development of the
+internal combustion engine--that wonderful invention without which
+their life work would probably never have been so completely
+successful while they lived, and which, even aside from its
+partnership with the airplane, has so profoundly affected the nature
+of the world in which we live.
+
+It seems quite obvious that to the Wrights vibration, or roughness,
+was predominantly if not entirely caused by the explosion forces, and
+they were either not completely aware of the effects of the other
+vibratory forces or they chose to neglect them. Although crankshaft
+counterweights had been in use as far back as the middle 1800s, the
+Wrights never incorporated them in any of their engines; and despite
+the inherent shaking force in the 4-inline arrangement, they continued
+to use it for many years.
+
+The choice of four cylinders was obviously made in order to get, for
+smoothness, what in that day was "a lot of small cylinders"; and this
+was sound judgment. Furthermore, although the majority of automobiles
+at that time had engines with fewer than four cylinders, for those
+that did the inline form was standard and well proven, and, in fact,
+Daimler was then operating engines of this general design at powers
+several times the minimum the Wrights had determined necessary for
+their purpose.
+
+What fixed the exact cylinder size, that is, the "square" 4×4-in.
+form, is not recorded, nor is it obvious by supposition. Baker says it
+was for high displacement and low weight, but these qualities are also
+greatly affected by many other factors. The total displacement of just
+over 200 cu in. was on the generous side, given the horsepower they
+had determined was necessary, but here again the Wrights were
+undoubtedly making the conservative allowances afterwards proven
+habitual, to be justified later by greatly increased power
+requirements and corresponding outputs. The Mean Effective Pressure
+(MEP), based on their indicated goal of 8 hp, would be a very modest
+36 psi at the speed of 870 rpm at which they first tested the engine,
+and only 31 psi at the reasonably conservative speed of 1000 rpm. The
+4×4-in. dimension would provide a cylinder large enough so that the
+engine was not penalized in the matter of weight and yet small enough
+to essentially guarantee its successful operation, as cylinders of
+considerably larger bore were being utilized in automobiles. That
+their original choice was an excellent one is rather well supported by
+the fact that in all the different models and sizes of engines they
+eventually designed and built, they never found it necessary to go to
+cylinders very much larger than this.
+
+[Illustration: _Figure 3._--First flight engine, 1903, installed in
+the Kitty Hawk airplane, as exhibited in the Science Museum. (Photo
+courtesy the Science Museum, London.)]
+
+A second basic determination which was made either concurrently or
+even possibly in advance of that of the general form and size was in
+the matter of the type of cylinder cooling to adopt. Based on current
+practice that had proven practical, there were three possibilities,
+all of which were in use in automobiles: air, water, or a combination
+of the two. It is an interesting commentary that Fernand Forest's[10]
+proposed 32-cylinder aircraft engine of 1888 was to be air-cooled,
+that Santos-Dumont utilized an air-cooled Clement engine in his
+dirigible flights of 1903, and that the Wrights had chosen air cooling
+for their shop engine. With the promise of simplicity and elimination
+of the radiator, water and piping, it would seem, offhand, that this
+would be the Wrights' choice for their airplane; but they were
+probably governed by the fact that not only was the water-cooled type
+predominant in automobile practice, but that the units giving the best
+and highest performance in general service were all water cooled. In
+their subsequent practice they never departed from this original
+decision, although Wilbur Wright's notebook of 1904-1907 contains an
+undated weight estimate by detailed parts for an 8-cylinder air-cooled
+engine. Unfortunately, the proposed power output is not recorded, so
+their conception of the relative weight of the air-cooled form is not
+disclosed.
+
+[Footnote 10: Fernand Forest, _Les Bateaux Automobiles_, 1906.]
+
+One of the most important decisions relating to the powerplant--one
+which was probably made long before they became committed to the
+design itself--was a determination of the method of transmission of
+power to the propeller, or propellers. A lingering impression exists
+that the utilization of a chain drive for this purpose was a natural
+inheritance from their bicycle background. No doubt this experience
+greatly simplified the task of adaptation but a merely cursory
+examination shows that even if they had never had any connection with
+bicycles, the chain drive was a logical solution, considering every
+important element of the problem. The vast majority of automobiles of
+the time were chain driven, and chains and sprockets capable of
+handling a wide range of power were completely developed and
+available. Further, at that time they had no accurate knowledge of
+desirable or limiting propeller and engine speeds. The chain drive
+offered a very simple and inexpensive method of providing for a
+completely flexible range of speed ratios. The other two possibilities
+were both undesirable: the first, a simple direct-driven single
+propeller connected to the crankshaft, provided essentially no
+flexibility whatsoever in experimentally varying engine or propeller
+speed ratios, it added an out-of-balance engine torque force to the
+problem of airplane control, and, finally, it dictated that the pilot
+would be in the propeller slipstream or the airflow to it; the second,
+drive shafts and gearing for dual propellers, would have been very
+heavy and expensive, and most probably would have required a long-time
+development, with every experimental change in speed ratios requiring
+a complete change in gears. Again, their original choice was so
+correct that it lasted them through essentially all their active
+flying years.
+
+The very substantial advantages of the chain drive were not, however,
+obtained at no cost. Torque variations in the engine would tend to
+cause a whipping action in the chain, so that it was vulnerable to
+rough running caused by misfiring cylinders and, with the right timing
+and magnitude of normal regular variations, the action could result in
+destructive forces in the transmission system. This was the basic
+reason for the Wrights' great fear of "engine vibration," which
+confined them to the use of small cylinders and made a fairly heavy
+flywheel necessary on all their engines. When they were requested to
+install an Austro-Daimler engine in one of their airplanes, they
+designed a flexible coupling which was interposed between the engine
+and the propeller drive and this was considered so successful that it
+was applied to the flywheel of some engines of their last model, the
+6-70, "which had been giving trouble in this regard."[11]
+
+[Footnote 11: Grover Loening, letter of 10 April 1963, to the
+Smithsonian Institution.]
+
+Although flat, angled, and vertical engines had all been operated
+successfully, the best and most modern automotive engines of the time
+were vertical, so their choice of a horizontal position was probably
+dictated either by considerations of drag or their desire to provide a
+sizable mounting base for the engine, or both. There is no record of
+their ever having investigated the matter of the drag of the engine,
+either alone or in combination with the wing. The merit of a vertical
+versus a horizontal position of the engine was not analogous to that
+of the pilot, which they had studied, and where the prone position
+undoubtedly reduced the resistance.
+
+Having decided on the general makeup of their engine, the next major
+decision was that of just what form the principal parts should take,
+the most important of these being the cylinders and crankcase. Even at
+this fairly early date in the history of the internal combustion
+engine various successful arrangements and combinations were in
+existence. Individual cylinder construction was by far the most used,
+quite probably due to its case of manufacture and adaptability to
+change. Since 4-cylinder engines were just coming into general use (a
+few production engines of this type had been utilized as early as
+1898), there were few examples of en-bloc or one-piece construction.
+The original German Daimler Company undoubtedly was at this time the
+leader in the development of high-output internal-combustion engines,
+and in 1902, as an example of what was possible, had placed in service
+one that possibly approximated 40 hp, which was an MEP of 70 psi.
+(Almost without exception, quoted power figures of this period were
+not demonstrated quantities but were based on a formula, of which the
+only two factors were displacement and rpm.) The cylinders of this
+Daimler engine were cast iron, the cylinder barrel, head, and water
+jacket being cast in one piece. The upper part of the barrel and the
+cylinder head were jacketed, but, surprisingly, the bottom 60 percent
+of the barrel had no cooling. The cylinders were cast in pairs and
+bolted to a two-piece aluminum case split at the line of the
+crankshaft. Ignition was make-and-break and the inlet valves were
+mechanically actuated. Displacement was 413 cu in. and the rpm was
+1050.
+
+Although a few examples of integral crankcase and water jacket
+combinations were in use, the Wrights were being somewhat radical when
+they decided to incorporate all four cylinders in the one-piece
+construction, particularly since they also proposed to include the
+entire crankcase and not just one part of it. It was undoubtedly the
+most important decision that they were required to make on all the
+various construction details, and probably the one given the most
+study and investigation. Many factors were involved, but fundamentally
+everything went back to their three basic requirements: suitability,
+time, and cost. There was no obvious reason why the construction would
+not work, and it eliminated a very large number of individual parts
+and the required time for procuring, machining, and joining them.
+Probably one very strong argument was the advanced state of the
+casting art, one of the oldest of the mechanical arts in existence and
+one the Wrights used in many places, even though other processes were
+available. What no doubt weighed heavily was that Dayton had some
+first-class foundries. The casting, though intricate and not
+machinable in their own shop, could be easily handled in one that was
+well outfitted. The pattern was fairly complex but apparently not
+enough to delay the project or cause excessive cost.
+
+[Illustration: _Figure 4._--First flight engine, 1903, left side and
+rear views, with dimensions. (Drawing courtesy Howell Cheney Technical
+School.)
+
+LEFT SIDE VIEW.]
+
+[Illustration: REAR VIEW]
+
+The selection of aluminum for the material was an integral part of
+the basic design decision. Despite the excellence and accuracy of the
+castings that could be obtained, there was nevertheless a minimum
+dimension beyond which wall thickness could not be reduced; and the
+use of either one of the two other proven materials, cast iron or
+bronze, would have made the body, as they called it, prohibitively
+heavy. The use of aluminum was not entirely novel at this time, as it
+had been utilized in many automobile engine parts, particularly
+crankcases; but its incorporation in this rather uncommon combination
+represented a bold step. There was no choice in the matter of the
+alloy to be used, the only proven one available was an 8 percent
+copper 92 percent aluminum combination.
+
+By means of the proper webs, brackets and bosses, the crankcase would
+also carry the crankshaft, the rocker arms and bearings, and the
+intake manifold. The open section of the case at the top was covered
+with a screw-fastened thin sheet of cold-rolled steel. The main
+bearing bosses were split at a 45° angle for ease of assembly. The
+engine support and fastening were provided by four feet, or lugs, cast
+integral on the bottom corners of the case, and by accompanying bolts
+(Figure 2). Although the crankcase continued to be pretty much the
+"body" of the internal combustion aircraft engine throughout its life,
+the Wrights managed to incorporate in this original part a major
+portion of the overall engine, and certainly far more than had ever
+previously been included.
+
+The design of the cylinder barrel presented fairly simple problems
+involving not much more than those of keeping the sections as thin as
+possible and devising means of fastening it and of keeping the water
+jacket tight. They saved considerable weight by making the barrel
+quite short, so that in operation a large part of the piston extended
+below the bottom of it; but this could be accepted, as there were no
+rings below the piston pin (Figure 6). The barrel material, a good
+grade of cast iron, was an almost automatic choice. In connection with
+these seemingly predetermined decisions, however, it should be
+remembered that their goal was an engine which would work without
+long-time development, and that, with no previous experience in
+lightweight construction to guide them they were nevertheless
+compelled to meet a weight limit, so that the thickness of every wall
+and flange and the length of every thread was important.
+
+With the separate cylinder barrel they were now almost committed to a
+three-piece cylinder. It would have been possible to combine the
+barrel and head in a one-piece casting and then devise a method of
+attachment, but this would have been more complex and certainly
+heavier. For housing the valves, what was in effect a separate
+cylindrical, or tubular, box was decided upon. This would lie across
+the top of the cylinder proper at right angles to the cylinder axis,
+and the two valves would be carried in the two ends of this box. The
+cylinder barrel would be brought in at its head end to form a portion
+of the cylinder head and then extended along its axis in the form of a
+fairly large boss, a mating boss being provided on one side of the
+valve box. The cylinder barrel would then be threaded into the valve
+box and the whole tightened or fastened to the crankcase by means of
+two sets of threads, one at each end of the barrel proper. This meant
+that three joints had to be made tight with only two sets of threads.
+This was accomplished by accurate machining and possibly even hand
+fitting in combination with a rather thick gasket at the head end, one
+flat of which bore against two different surfaces. This can be seen in
+Figure 6, where the circular flange on the valve box contacts both the
+crankcase and the cylinder barrel. Altogether it was a simple, light,
+and ingenious solution to a rather complex problem.
+
+At this point the question arises: Why was the engine layout such that
+the exhaust took place close to the operator's ears? It would have
+been possible, starting with the original design, to turn the engine
+around so that the exhaust was on the other side. This would have
+little effect on the location of the center of gravity, and the two
+main drive chains would then have been of more equal length. However,
+of the many factors involved, probably one of the principal
+considerations in arriving at their final decision was the location of
+the spark-advance control, which was in effect the only control they
+had of engine output, except for complete shutoff. In their design
+this was immediately adjacent to the operator; with a turned-around
+engine, an extension control mechanism of some sort would have been
+required. The noise of the exhaust apparently became of some concern
+to them, as Orville's diary in early 1904 contains an entry with a
+sketch labeled "Design for Muffler for Engine," but there is no
+further comment.
+
+The problem of keeping joints tight, and for that matter the entire
+construction itself, were both greatly simplified by their decision to
+water-jacket only a part of the cylinder head proper, and the valve
+box not at all. This was undoubtedly the correct decision for their
+immediate purpose, as again they were effecting savings in time, cost,
+complexity, and weight. There is nothing in the record, however, to
+show why they continued this practice long after they had advanced to
+much greater power outputs and longer flight times. Their own
+statements show that they were well aware of the effect of the very
+hot cylinder head on power output and they must also have realized its
+influence on exhaust-valve temperature.
+
+The cylinder assembly was made somewhat more complicated by their
+desire to oil the piston and cylinder by means of holes near the
+crankshaft end in what was, with the engine in the horizontal
+position, the upper side of the cylinder barrel. This complication was
+no doubt taken care of by not drilling the holes until a tight
+assembly had been made by screwing the barrel into place, and by
+marking the desired location on the barrel. Since this position was
+determined by a metal-to-metal jam fit of the crankcase and cylinder
+barrel flange, the barrel would reassemble with the holes in very
+nearly the same relative position after disassembly.
+
+With the valve box, or housing, cylindrical, the task of locking and
+fastening the intake and exhaust valve guides and seats in place was
+easy. The guide was made integral with and in the center of one end of
+a circular cage, the other end of which contained the valve seat (see
+Figure 5). Four sections were cut out of the circular wall of the cage
+so that in effect the seat and guide were joined by four narrow legs,
+the spaces between which provided passages for the flow of the
+cylinder gases. These cages were then dropped into the ends of the
+valve boxes until they came up against machined shoulders and were
+held in place by internal ring nuts screwed into the valve box. The
+intake manifold or passage was placed over the intake valves so that
+the intake charge flowed directly into and through the valve cage
+around the open valve and into the cylinder. The exhaust gas, after
+flowing through the passages in the valve cage, was discharged
+directly to the atmosphere through a series of holes machined in one
+side of the valve box.
+
+[Illustration: _Figure 5._--First flight engine, 1903, assembly.
+(Phantom cutaway by J. H. Clark, with key, courtesy _Aeroplane_.)
+
+KEY
+
+1 and 2. Bearing caps in one piece with plate 3.
+
+3. Plated screwed over hole 4 in crankcase end.
+
+4. Key-shaped hole as hole 5 in intermediate ribs.
+
+6. Inter-bearings cap (white-metal lined) and screwed to inter-rib
+halves 7.
+
+8. Splash-drip feed to bearings.
+
+9. Return to pump from each compartment of crankcase base ("sump") via
+gallery 10 and pipe to pump 11 underneath jacket.
+
+12. Oil feed from pump via rubber tube 13.
+
+13. Drip feeds to cylinders and pistons.
+
+14. Gear drive to pump.
+
+15. Big-end nuts, lock-strip, and shims.
+
+16. Gudgeon-pin lock.
+
+17. Piston-ring retainer pegs.
+
+18. Cylinder liner screwing into jacket.
+
+19. Open-ended "can" admits air.
+
+20. Fuel supply.
+
+21. (Hot) side of water jacket makes surface carburetter.
+
+22. Sparking plug (comprising positive electrode 23 and
+spark-producing make-and-break 24).
+
+25. Lever attached to lever 26 via bearing 27 screwed into chamber
+neck 28.
+
+26. Levers with mainspring 29 and inter-spring 30, and rocked by "cam"
+31.
+
+31. Cam with another alongside (for adjacent cylinder).
+
+32. Positive busbar feed to all four cylinders.
+
+33. Assembly retaining-rings.
+
+34. Sealing disc.
+
+35. Exhaust outlet ports.
+
+36. Camshaft right along on underside of jacket and also driving oil
+pump 11 via 14.
+
+37. Spring-loaded sliding pinion drives make-and-break shaft 38
+through peg in inclined slot 39.
+
+40. Cam to push pinion 37 along and so alter its angular relation with
+shaft 38 (to vary timing).
+
+41. Exhaust-valve cams bear on rollers 42 mounted in end of
+rocker-arms 43.
+
+44. Generator floating coils.
+
+45. Friction-drive off flywheel.
+
+46. Sight-feed lubricator (on stationary sleeve).
+
+47. Hardwood chain tensioner.]
+
+The intake and exhaust valves were identical and of two-piece
+construction, with the stems screwed tightly into and through the
+heads and the protruding ends then peened over. This construction was
+not novel, having had much usage behind it, and it continued for a
+long time in both automobile and aircraft practice. One-piece cast
+and forged valves were available but here again it was a choice of the
+quick, cheap, and proven answer.
+
+The entire valve system, including guides and seats, was of cast iron,
+a favorite material of the Wrights, except for the valve stems, which
+were, at different times, of various carbon steels. Ordinary
+cold-rolled apparently was used in those of the original engine, but
+in later engines this was changed to a high-carbon steel.
+
+The piston design presented no difficulty. In some measure this was
+due to the remarkable similarity that seems to have existed among all
+the different engines of the time in the construction of this
+particular part, for, although there were some major variations, it
+was, in fact, almost as if some standard had been adopted. Pistons all
+were of cast iron and comparatively quite long (it was a number of
+years before they evolved into the short ones of modern practice);
+they were almost invariably equipped with three wide piston rings
+between the piston pin and the head; and, although there were in
+existence a few pistons with four rings, no oil wiper or other ring
+seems to have been placed below the piston pin. The Wrights' piston
+was typical of the time, with the rings pinned in the grooves to
+prevent turning and the piston pin locked in the piston with a
+setscrew. In designing this first engine they were, however,
+apparently somewhat unsure about this latter feature, as they provided
+the rod with a split little end and a clamping bolt (see Figure 6), so
+that the pin could be held in the rod if desired; but no examples of
+this use have been encountered.
+
+The Wrights' selection of an "automatic" or suction-operated inlet
+valve was entirely logical. Mechanically operated inlet valves were in
+use and their history went back many years, but the great majority of
+the engines of that time still had the automatic type, and with this
+construction one complete set of valve-operating mechanisms was
+eliminated. They were well aware of the loss of volumetric efficiency
+inherent in this valve, and apparently went to some pains to obtain
+from it the best performance possible. Speaking of the first engine,
+Orville Wright wrote, "Since putting in heavier springs to actuate the
+valves on our engine we have increased its power to nearly 16 hp and
+at the same time reduced the amount of gasoline consumed per hour to
+about one-half of what it was."[12]
+
+[Footnote 12: Assuming a rich mixture, consumption of all the air, and
+an airbrake thermal efficiency of 24.50% for the original engine, the
+approximate volumetric efficiency of the cylinder is calculated to
+have been just under 40%.]
+
+Why they continued with this form on their later engines is a question
+a little more difficult to answer, as they were then seeking more and
+more power and were building larger engines. The advantages of
+simplicity and a reduced number of parts still existed, but there also
+was a sizable power increase to be had which possibly would have more
+than balanced off the increased cost and weight. They did not utilize
+mechanical operation until after a major redesign of their last engine
+model. Very possibly the answer lies in the phenomenon of fuel
+detonation. This was only beginning to be understood in the late
+1920s, and it is quite evident from their writings that they had
+little knowledge of what made a good fuel in this respect. It is
+fairly certain, however, that they did know of the existence of
+cylinder "knock," or detonation, and particularly that the compression
+ratio had a major effect on it. The ratios they utilized on their
+different engines varied considerably, ranging from what, for that
+time, was medium to what was relatively high. The original flight
+engine had a compression ratio of 4.4:1. The last of their service
+engines had a compression ratio about twenty percent under that of the
+previous series--a clear indication that they considered that they had
+previously gone too high. Quite possibly they concluded that
+increasing the amount of the cylinder charge seemed to bring on
+detonation, and that the complication of the mechanical inlet valve
+was therefore not warranted.
+
+[Illustration: _Figure 6._--First flight engine, 1903, cross section.
+(Drawing courtesy Science Museum, London.)]
+
+The camshaft for the exhaust valves (101, Figure 6), was chain driven
+from the crankshaft and was carried along the bottom of the crankcase
+in three babbit-lined bearings in bearing boxes or lugs cast integral
+with the case. Both the driving chain and the sprockets were standard
+bicycle parts, and a number of bicycle thread standards and other
+items of bicycle practice were incorporated in several places in the
+engine, easing their construction task. The shaft itself, of mild
+carbon steel, was hollow and on each side of an end bearing sweated-on
+washers provided shoulders to locate it longitudinally. Its location
+adjacent to the valves, with the cam operating directly on the rocker
+arm, eliminated push rods and attendant parts, a major economy. The
+cams were machined as separate parts and then sweated onto the shaft.
+Their shape shows the principal concern in the design to have been
+obtaining maximum valve capacity--that is, a quite rapid opening with
+a long dwell. This apparent desire to get rid of the exhaust gas
+quickly is manifested again in the alacrity with which they adopted a
+piston-controlled exhaust port immediately they had really mastered
+flight and were contemplating more powerful and more durable engines.
+This maximum-capacity theory of valve operation, with its neglect of
+acceleration forces and seating velocities, may well have been at
+least partially if not largely the cause of their exhaust-valve
+troubles and the seemingly disproportionate amount of development they
+devoted to this part, as reported by Chenoweth, although it is also
+true that the exhaust valve continued to present a problem in the
+aircraft piston engine for a great many years after, even with the
+most scientific of cam designs.
+
+The rocker arm (102, Figure 6) is probably the best example of a small
+part which met all of their many specific requirements with an extreme
+of simplicity. It consisted of two identical side pieces, or walls, of
+sheet steel shaped to the desired side contour of the assembly, in
+which were drilled three holes, one in each end, to carry the roller
+axles, and the third in the approximate middle for the rocker axle
+shaft proper. This consisted of a piece of solid rod positioned by
+cotter pins in each end outside the side walls (see Figure 5). The
+assembly was made by riveting over the ends of the roller axles so
+that the walls were held tightly against the shoulders on the axles,
+thus providing the correct clearance for the rollers. The construction
+was so light and serviceable that it was essentially carried over to
+the last engine the Wrights ever built.
+
+The basic intake manifold (see Figure 5) consisted of a very low flat
+box of sheet steel which ran across the tops of the valve boxes and
+was directly connected to the top of each of them so that the cages,
+and thus the valves, were open to the interior of the manifold.
+Through an opening in the side toward the engine the manifold was
+connected to a flat induction chamber (21, Figure 5) which served to
+vaporize the fuel and mix it with the incoming air. This chamber was
+formed by screw-fastening a piece of sheet steel to vertical ribs cast
+integral with the crankcase, the crankcase wall itself thus forming
+the bottom of the chamber. A beaded sheet-steel cylinder resembling a
+can (73, Figure 6) but open at both ends was fastened upright to the
+top of this chamber. In the absence of anything else, this can could
+be called the carburetor, as a fuel supply line entered the cylinder
+near the top and discharged the fuel into the incoming air stream,
+both the fuel and air then going directly into the mixing chamber. The
+can was attached near one corner of the chamber, and vertical baffles,
+also cast integral with the case, were so located that the incoming
+mixture was forced to circulate over the entire area of exposed
+crankcase inside the chamber before it reached the outlet to the
+manifold proper, the hot surface vaporizing that part of the fuel
+still liquid.
+
+[Illustration: _Figure 7._--First flight engine, 1903: cylinder, valve
+box, and gear mechanism; below, miscellaneous parts. (Photos courtesy
+Science Museum, London, and Louis P. Christman.)]
+
+Fuel was gravity fed to the can through copper and rubber tubing from
+a tank fastened to a strut, several feet above the engine. Of the two
+valves placed in the fuel line, one was a simple on-off shutoff cock
+and the other a type whose opening could be regulated. The latter was
+adjusted to supply the correct amount of fuel under the desired flight
+operating condition; the shutoff cock was used for starting and
+stopping. The rate of fuel supply to the engine would decrease as the
+level in the fuel tank dropped, but as the head being utilized was a
+matter of several feet and the height of the supply tank a matter of
+inches, the fuel-air ratio was still maintained well within the range
+that would ignite and burn properly in the contemplated one-power
+condition of their flight operation.
+
+This arrangement is one of the best of the many illustrations of how
+by the use of foresight and ingenuity the Wrights met the challenge of
+a complex requirement with a simple device, for while carburetors were
+not in the perfected stage later attained, quite good ones that would
+both control power output and supply a fairly constant fuel-air
+mixture over a range of operating conditions were available, but they
+were complex, heavy, and expensive. The arrangement, moreover, secured
+at no cost a good vaporizer, or modern "hot spot." In their subsequent
+engines they took the control of the fuel metering away from the
+regulating valve and gravity tank combination and substituted an
+engine-driven fuel pump which provided a fuel supply bearing a fairly
+close relationship to engine speed.
+
+The reasons behind selection of the type of ignition used, and the
+considerations entering into the decision, are open to speculation, as
+are those concerning many other elements that eventually made up the
+engine. Both the high-tension spark plug and low-tension
+make-and-break systems had been in wide use for many years, with the
+latter constituting the majority in 1902. Both were serviceable and
+therefore acceptable, and both required a "magneto". The art of the
+spark plug was in a sense esoteric (to a certain extent it so remains
+to this day), but the spark-plug system did involve a much simpler
+combination of parts: in addition to the plug and magneto there would
+be needed only a timer, or distributor, together with coils and
+points, or some substitute arrangement. The make-and-break system, on
+the other hand, required for each cylinder what was physically the
+equivalent of a spark plug, that is, a moving arm and contact point
+inside the cylinder, a spring-loaded snap mechanism to break the
+contact outside the cylinder, and a camshaft and cams to actuate the
+breaker mechanism at the proper time. Furthermore, as the Wrights
+applied it, the system required dry cells and a coil for starting,
+although these did not accompany the engine in flight. And finally
+there was the problem of keeping tight the joint where the
+oscillating shaft required to operate the moving point in the spark
+plug entered the cylinder.
+
+This is one of the few occasions, if not the only one, when the
+Wrights chose the more complex solution in connection with a major
+part--in this particular case, one with far more bits and pieces.
+However, it did carry with it some quite major advantages. The common
+spark plug, always subject to fouling or failure to function because
+of a decreased gap, was not very reliable over a lengthy period, and
+was undoubtedly much more so in those days when control of the amount
+of oil inside the cylinder was not at all exact. Make-and-break
+points, on the other hand, were unaffected by excess oil in the
+cylinder. Because of this resistance to fouling, the system was
+particularly suitable for use with the compression-release method of
+power control which they later utilized, although they probably could
+not have been looking that far ahead at the time they chose it.
+High-tension current has always, and rightfully so, been thought of as
+a troublemaker in service; in Beaumont's 1900 edition of _Motor
+Vehicles and Motors_, which seems to have been technically the best
+volume of its time, the editor predicted that low-tension
+make-and-break ignition would ultimately supersede all other methods.
+And finally, the large number of small parts required for the
+make-and-break system could all be made in the Wright Brothers' shop
+or easily procured, and in the end this was probably the factor, plus
+reliability, that determined the decision which, all things
+considered, was the correct one.
+
+There was nothing exceptional about the exact form the Wrights
+devised. It displayed the usual refined simplicity (the cams were made
+of a single small piece of strip steel bent to shape and clamped to
+the ignition camshaft with a simple self-locking screw), and
+lightness. The ignition camshaft (38, Figure 5), a piece of
+small-diameter bar stock, was located on the same side as the exhaust
+valve camshaft, approximately midway between it and the valve boxes,
+and was operated by the exhaust camshaft through spur gearing. That
+the Wrights were thinking of something beyond mere hops or short
+flights is shown by the fact that the ignition points were
+platinum-faced, whereas even soft iron would have been satisfactory
+for the duration of all their flying for many years.
+
+The control of the spark timing was effected by advancing or retarding
+the ignition camshaft in relation to the exhaust valve camshaft. The
+spur gear (37, Figure 5) driving the ignition camshaft had its hub on
+one side extended out to provide what was in effect a sleeve around
+the camshaft integral with the gear. The gear and integral sleeve were
+slidable on the shaft and the sleeve at one place (39, Figure 5) was
+completely slotted through to the shaft at an angle of 45° to the
+longitudinal axis of the shaft. The shaft was driven by a pin tightly
+fitted in it and extending into the slot. The fore-and-aft position of
+the sleeve on the shaft was determined by a lever-operated cam (40,
+Figure 5) on one side and a spring on the other. The movement of the
+sleeve along the shaft would cause the shaft to rotate in relation to
+it because of the angle of the slot, thus providing the desired
+variation in timing of the spark. The "magneto" was a purchased item
+driven by means of a friction wheel contacting the flywheel, and
+several different makes were used later, but the original is indicated
+to have been a Miller-Knoblock (see Figure 5).
+
+The connecting rod is another example of how, seemingly without
+trouble, they were able to meet the basic requirements they had set
+for themselves. It consisted of a piece of seamless steel tubing with
+each end fastened into a phosphor-bronze casting, these castings
+comprising the big and little ends, drilled through to make the
+bearings (See Figures 5 and 6). It was strong, stiff and light.[13]
+Forged rods were in rather wide use at the time and at least one
+existing engine even had a forged I-beam section design that was
+tapered down from big to little end. The Wrights' rod was obtained in
+little more time than it took to make the simple patterns for the two
+ends. The weight was easily controlled, no bearing liners were
+necessary, and a very minimum of machining was required. Concerning
+the big-end material, there exists a contradiction in the records:
+Baker, whose data are generally most accurate, states that these were
+babbited, but this must be in error, as the existing engine has
+straight bronze castings without babbiting, and there is no record, or
+drawing, or other indication of the bearings having been otherwise.
+
+[Footnote 13: A rather thorough stress analysis of the rod shows it to
+compare very favorably with modern practice. In the absence of an
+indicator card for the 1903 engine, if a maximum gas pressure of five
+times the MEP is assumed, the yield-tension factor of safety is
+measurably higher than that of two designs of piston engines still in
+wide service, and the column factor of safety only slightly less. The
+shear stresses in the brazed and threaded joints are so low as to be
+negligible.]
+
+Different methods of assembling the rod were used. At one time the
+tube ends were screwed into the bronze castings and pinned, and at
+another the ends were pinned and soldered. There is an indication that
+at one time soldering and threads were used in combination. One of the
+many conflicts between the two primary sets of drawings exists at this
+point. The Smithsonian drawings show the use at each end of adapters
+between the rod and end castings, the adapters being first screwed
+into the castings and pinned and then brazed to the inside of the
+tube. The Science Museum drawings show the tube section threaded and
+screwed into the castings. The direct screw assembly method called for
+accurate machining and hand fitting in order to make the ends of the
+tubing jam against the bottom of the threaded holes in the castings,
+and at the same time have the end bearings properly lined up. The
+weakness of the basic design patently lies in the joints. It is an
+attempt to utilize what was probably in the beginning a combination
+five-piece assembly and later three, in a very highly stressed part
+where the load was reversing. It gave them considerable trouble from
+time to time, particularly in the 4-cylinder vertical engines, and was
+abandoned for a forged I-beam section type in their last engine model;
+but it was nevertheless the ideal solution for their first engine.
+
+The crankshaft was made from a solid block of relatively high carbon
+steel which, aside from its bulk and the major amount of machining
+required, presented no special problems. It was heat-treated to a
+machinable hardness before being worked on, but was not further
+tempered. The design was an orthodox straight pin and cheek
+combination and, as previously noted, there were no counterweights to
+complicate the machining or assembly. A sizable bearing was provided
+on each side of each crank of the shaft, which helped reduce the
+stiffness requirement.
+
+Their only serious design consideration was to maintain the desired
+strength and still keep within weight limitations. A fundamental that
+every professional designer knows is that it is with this particular
+sort of part that weight gets out of control; even an additional 1/16
+in., if added in a few places, can balloon the weight. With their
+usual foresight and planning, the Wrights carefully checked and
+recorded the weight of each part as it was finished, but even this
+does not quite explain how these two individuals, inexperienced in
+multicylinder engines--much less in extra-light construction--could,
+in two months, bring through an engine which was both operable and
+somewhat lighter than their specification.
+
+In one matter it would seem that they were quite fortunate. The
+records are not complete, but with one exception there is no
+indication of any chronic or even occasional crankshaft failure. This
+would seem to show that it apparently never happened that any of their
+designs came out such that the frequency of a vibrating force of any
+magnitude occurred at the natural frequency of the shaft. Much later,
+when this type of vibration became understood, it was found virtually
+impossible, with power outputs of any magnitude, to design an
+undampened shaft, within the space and weight limitations existing in
+an ordinary engine, strong enough to withstand the stress generated
+when the frequency of the imposed vibration approximated the natural
+frequency of the shaft. The vibratory forces were mostly relatively
+small in their engines, so that forced vibration probably was not
+encountered, and the operating speed range of the engines was so
+limited that the natural frequency always fell outside this range.
+
+The flywheel was about the least complex of any of their engine parts
+and required little studied consideration, although they did have to
+balance its weight against the magnitude of the explosion forces which
+would reach the power transmission chains, with their complete lack of
+rigidity, a problem about which they were particularly concerned. The
+flywheel was made of cast iron and was both keyed to and shrunk on the
+shaft.
+
+Some doubt still exists about the exact method of lubricating the
+first engine. The unit presently in the airplane has a gear-type oil
+pump driven by the crankshaft through a worm gear and cross shaft, and
+the Appendix to the _Papers_ states that it was lubricated by a small
+pump; nevertheless Baker says, after careful research, that despite
+this evidence, it was not. Also, the drawings prepared by Christman
+(they were commenced under the supervision of Orville Wright) do not
+show the oil pump. In March 1905 Wilbur Wright wrote to Chanute,
+"However we have added oiling and feeding devices to the engine ...";
+but this could possibly have referred to something other than an oil
+pump. But even if a pump was not included originally, its presence in
+the present engine is easily explained. Breakage of the crankcase
+casting caused the retirement of this engine, which was not rebuilt
+until much later, and the pattern for this part had no doubt long
+since been altered to incorporate a pump. It was therefore easier in
+rebuilding to include than to omit the pump, even though this required
+the addition of a cross shaft and worm gear combination. On later
+engines, when the pump was used, oil was carried to a small pipe,
+running along the inside of the case, which had four small drill holes
+so located as to throw the oil in a jet on the higher, thrust-loaded
+side of each cylinder. The rods had a sharp scupper on the outside of
+the big end so placed as also to throw the oil on this same thrust
+face. Some scuppers were drilled through to carry oil to the rod
+bearing and some were not.
+
+The first engine was finished and assembled in February 1903 and given
+its first operating test on 22 February. The Wrights were quite
+pleased with its operation, and particularly with its smoothness.
+Their father, Bishop Wright, was the recorder of their satisfaction
+over its initial performance, but what he noted was probably the
+afterglow of the ineffable feeling of deep satisfaction that is the
+reward that comes to every maker of a new engine when it first comes
+to life and then throbs. They obtained 13 hp originally: later figures
+went as high as almost 16, but as different engine speeds were
+utilized it is rather difficult to settle on any single power figure.
+The most realistic is probably that given in the _Papers_ as having
+been attained later, after an accurate check had been made of the
+power required to turn a set of propellers at a given rpm. This came
+out at approximately 12 hp, the design goal having been 8. Following
+exactly the procedure that exists to this day, the engine went through
+an extended development period, and it was the end of September 1903
+before it was taken, with the airplane, to Kitty Hawk where the
+historic flights, which have had such a profound effect on the lives
+of all men, were made on 17 December 1903.
+
+
+
+
+The Engines With Which They Mastered The Art of Flying
+
+
+Two more engines of this first general design were built but they
+differed somewhat from each other as well as from the original.
+Together with a third 8-cylinder engine these were begun right after
+the first of the year in 1904, shortly after the Wrights' return from
+Kitty Hawk. In planning the 8-cylinder engine they were again only
+being forehanded, but considerably so, in providing more power for
+increased airplane performance beyond that which might possibly be
+obtained from the 4-cylinder units. Progress with the 4-cylinder
+engines was such that they fairly quickly concluded that the
+8-cylinder size would not be necessary, and it was abandoned before
+completion. Exactly how far it was carried is not known. The record
+contains only a single note covering the final scrapping of the parts
+that had been completed; and apparently there were no drawings, so
+that even its intended appearance is not known with any exactness. It
+was probably a 90° V-type using their original basic cylinder
+construction.
+
+The changes carried through in the two 4-cylinder engines were not
+major. The water-cooled area of the cylinder barrel was increased by
+nearly ten percent but the head remained only partially cooled. In
+hindsight, this consistent avoidance of complete cylinder-head cooling
+presents the one most inexplicable of the more important design
+decisions they made, as it does not appear logical. In the original
+engine, where the factors of time and simplicity were of paramount
+importance, this made sense, but now they were contemplating
+considerably increased power requirements, knowing the effect of
+temperature on both the cylinder and the weight of cylinder charge,
+and knowing that valve failure was one of their most troublesome
+service problems. Nor does it seem that they could have been avoiding
+complete cylinder cooling through fear of the slightly increased
+complexity or the difficulty of keeping the water connections and
+joints tight, for they had faced a much more severe problem in their
+first engine, where their basic design required that three joints be
+kept tight with only two sets of threads, and had rather easily
+mastered it; so there must have been some much more major but not
+easily discernible factor which governed, for they still continued to
+use the poorly cooled head, even carrying it over to their next engine
+series. Very probably they did not know the effect on detonation of a
+high-temperature fuel-charge.
+
+One of the new engines was intended for use in their future
+experimental flying and has become known as _No. 2._ It had a bore of
+4-1/8 in., incorporated an oil pump, and at some time shortly after
+its construction a fuel pump was added. The fuel pump was undoubtedly
+intended to provide a metering system responsive to engine speed and
+possibly also to eliminate the small inherent variation in flow of the
+original gravity system.
+
+This engine incorporated a cylinder compression release device not on
+the original. The exact reason or reasons for the application of the
+compression release have not been determined, although the record
+shows it to have been utilized for several different purposes under
+different operating conditions. Whatever the motivation for its
+initial application, it was apparently useful, as it was retained in
+one form or another in subsequent engine models up to the last
+6-cylinder design. Essentially it was a manually controlled mechanism
+whereby all the exhaust valves could be held open as long as desired,
+thus preventing any normal charge intake or compression in the
+cylinder. Its one certain and common use was to facilitate starting,
+the open exhaust valves easing the task of turning the engine over by
+hand and making priming easy. In flight, its operation had the effect
+of completely shutting off the power. The propellers would then
+"windmill" and keep the engine revolving. One advantage stated for
+this method of operation was that when power was required and the
+control released, the engine would be at fairly high speed, so that
+full power was delivered immediately fuel reached the engine. It is
+also reported to have been used both in making normal landings and in
+emergencies, when an instant power shutdown was desired. Although it
+is not clear whether the fuel shutoff cock was intended to be
+manipulated when the compression release was used for any of these
+reasons, over the many years of its availability, undoubtedly at one
+time or another every conceivable combination of operating conditions
+of the various elements was tried. Because of the pumping power
+required with at least one valve open during every stroke, the
+windmilling speed of the engine was probably less than with any other
+method of completely stopping power output, but whether this
+difference was large enough to be noticeable, or was even considered,
+is doubtful.
+
+Since a simple ignition switch was all that was required to stop the
+power output, regardless of whether a fuel-control valve or a
+spark-advance control was used, it must be concluded that the primary
+function of the compression release was to facilitate starting, and
+any other useful result was something obtained at no cost. The
+compression release was later generally abandoned, and until the
+advent of the mechanical starter during the 1920s, starting an engine
+by "pulling the propeller through" could be a difficult task. With the
+Wrights' demonstrated belief that frugality was a first principle of
+design, it is hardly conceivable that they would have accepted for any
+other reason the complication of the compression-release mechanism if
+a simple ignition switch would have sufficed.
+
+The compression-release mechanism was kept relatively simple,
+considering what it was required to accomplish. A small non-revolving
+shaft was located directly under the rocker arm rollers that actuated
+the exhaust valves. Four slidable stops were placed on this shaft,
+each in the proper location, so that at one extreme of their travel
+they would be directly underneath the rocker roller and at the other
+extreme completely in the clear. They were positioned along the shaft
+by a spring forcing them in one direction against a shoulder integral
+with the shaft, and the shaft was slidable in its bearings, its
+position being determined by a manually controlled lever. When the
+lever was moved in one direction the spring pressure then imposed on
+the stops would cause each of them to move under the corresponding
+rocker roller as the exhaust valve opened, thus holding the exhaust
+valve in the open position. When the shaft was moved in the other
+direction the collar on the shaft would mechanically move the stop
+from underneath the roller, allowing the valve to return to normal
+operation.
+
+[Illustration: _Figure 8._--Development engine No. 3, 1904-1906,
+showing auxiliary exhaust port, separate one-piece water-jacket block.
+(Photo by author.)]
+
+If the 1903 engine is the most significant of all that the Wrights
+built and flew, then certainly the _No. 2_ unit was the most useful,
+for it was their sole power source during all their flying of 1904 and
+1905 and, as they affirmed, it was during this period that they
+perfected the art, progressing from a short straightaway flight of 59
+seconds to a flight controllable in all directions with the duration
+limited only by the fuel supply. It is to be greatly regretted that no
+complete log or record was kept of this engine.
+
+The Wrights again exhibited their engineering mastery of a novel basic
+situation when, starting out to make flight a practical thing, they
+provided engine _No. 3_ to be used for experimental purposes. In so
+doing they initiated a system which continues to be fundamental in the
+art of providing serviceable aircraft engines to this day--one that is
+expensive and time consuming, but for which no substitute has yet been
+found. Their two objectives were: improvement in performance and
+improvement in reliability, and the engine was operated rather
+continuously from early 1904 until well into 1906. Unfortunately,
+again, no complete record exists of the many changes made and the
+ideas tested, although occasional notes are scattered through the
+diaries and notebooks.
+
+In its present form--it is on exhibition at the Engineers Club in
+Dayton, Ohio--the _No. 3_ engine embodies one feature which became
+standard construction on all the Wright 4-cylinder models. This was
+the addition of a number of holes in a line part way around the
+circumference of the cylinder barrel so that they were uncovered by
+the piston at the end of its stroke toward the shaft, thus becoming
+exhaust ports (see Figure 9). This arrangement, although not entirely
+novel, was just beginning to come into use, and in its original form
+the ports exhausted into a separate chamber, which in turn was
+evacuated by means of a mechanically operated valve, so that two
+exhaust valves were needed per cylinder. Elimination of this chamber
+and the valve arrangement is typical of the Wrights' simplifying
+procedure, and it would seem that they were among the very first to
+use this form.[14]
+
+[Footnote 14: Rankin Kennedy, _Flying Machines--Practice and Design_,
+1909.]
+
+The primary purpose of the scheme was to reduce, by this early release
+and consequent pressure and temperature drop, the temperature of the
+exhaust gases passing the exhaust valve, this valve being one of their
+main sources of mechanical trouble. It is probable that with the
+automatic intake valves being used there was also a slight effect in
+the direction of increasing the inlet charge, although with the small
+area of the ports and the short time of opening, the amount of this
+was certainly minor. With the original one-piece crankcase and
+cylinder jacket construction, the incorporation of this auxiliary
+porting was not easy, but this difficulty was overcome in the
+development engine by making different castings for the crankcase
+itself and for the cylinder jacket and separating them by several
+inches, so that room was provided between the two for the ports.
+
+This engine demonstrated the most power of any of the flat 4s,
+eventually reaching an output of approximately 25 hp, which was even
+somewhat more than that developed by the slightly larger
+4-1/8-in.-bore flight engine, with which 21 hp was not exceeded.
+Indicative of the development that had taken place, the performance of
+the _No. 3_ engine was twice the utilized output of the original
+engine of the same size, an increase that was accomplished in a period
+of less than three years.
+
+The Wrights were only twice charged with having plagiarized others'
+work, a somewhat unusual record in view of their successes, and both
+times apparently entirely without foundation. A statement was
+published that the 1903 flight engine was a reworked Pope Toledo
+automobile unit, and it was repeated in an English lecture on the
+Wright brothers. This was adequately refuted by McFarland but
+additionally, it must be noted, there was no Pope Toledo company or
+car when the Wright engine was built. This company, an outgrowth of
+another which had previously manufactured one-and two-cylinder
+automobiles, was formed, or reformed, and a Pope license arrangement
+entered into during the year 1903.
+
+The other incident was connected with Whitehead's activities and
+designs. Whitehead was an early experimenter in flying, about the time
+of the Wrights, whose rather extraordinary claims of successful flight
+were published in the 1901-1903 period but received little attention
+until very much later. His first engines were designed by a clever
+engineer, Anton Pruckner, who left at the end of 1901, after which
+Whitehead himself became solely responsible for them. It was stated
+that the Wrights visited the Whitehead plant in Bridgeport,
+Connecticut, and that Wilbur remained for several days, spending his
+time in their machine shop. This was not only categorically denied by
+Orville Wright when he heard of it but it is quite obvious that the
+1903 or any other of the Wright engine designs bears little
+resemblance to Pruckner's work. In fact, its principal design features
+are just the opposite of Pruckner's, who utilized vertical cylinders,
+the 2-stroke cycle, and air-cooling, which Whitehead at some point
+changed to water-cooling.[15]
+
+[Footnote 15: Considerable doubt surrounds Whitehead's actual flight
+accomplishments, but Pruckner's engines were certainly used, as
+several were sold to early pioneers, including Charles Wittemann. It
+is probable that the specific power output was not very great, for the
+air-cooled art of this time was not very advanced and Pruckner had a
+rather poor fin design. But the change to water cooling eliminated
+this trouble, and the engines were most simple, should have been
+relatively quite light, and with enough development could probably
+have been made into sufficiently satisfactory flying units for that
+period.]
+
+
+
+
+The Four-Cylinder Vertical Demonstration Engine and the First
+Production Engine
+
+
+In 1906, while still doing general development work on the flat
+experimental engine, the Wrights started two new engines, and for the
+first time the brothers engaged in separate efforts. One was "a
+modification of the old ones" by Wilbur and the other, "an entirely
+new pattern" by Orville. There is no record of any of the features of
+Wilbur's project or what was done in connection with it. Two months
+after the experimental operation of the two designs began, an entry in
+Wilbur's diary gives some weight and performance figures for the "4" x
+4" rebuilt horizontal," and since Orville's design was vertical the
+data clearly refer to Wilbur's; but since the output is given only in
+test-fan rpm it does not serve to indicate what had been accomplished
+and there is no further mention of it.
+
+Orville's design became the most used of any model they produced. It
+saw them through the years from 1906 to 1911 or 1912, which included
+the crucial European and United States Army demonstrations, and more
+engines of this model were manufactured than any of their others
+including their later 6-cylinder. Although its ancestry is traceable
+to the original 1903 engine, the design form, particularly the
+external configuration, was considerably altered. Along with many
+individual parts it retained the basic conception of four medium-size
+cylinders positioned in line and driving the propellers through two
+sprocket wheels. From the general tenor of the record it would seem,
+despite there being no specific indication, that from this time on
+Orville served as the leader in engine design, although this occurred
+with no effect whatsoever on their finely balanced, exactly equal
+partnership which endured until Wilbur's death in 1912.
+
+The first major change from the 1903 design, putting the engine in an
+upright instead of flat position, was probably done primarily to
+provide for a minimum variation in the location of the center of
+gravity with and without a passenger. Whether or not it had any
+influence, the vertical cylinder arrangement was becoming predominant
+in automobile powerplants by this time, and the Wright engines now
+began to resemble this prevailing form of the internal combustion
+engine--a basic form that, in a wide variety of uses, was to endure
+for a long time.
+
+Over the years, the Wrights seem to have made many changes in the
+engine: the bore was varied at different times, rod assembly methods
+were altered, and rod ends were changed from bronze to steel.
+Chenoweth states that on later engines an oil-control ring was added
+on the bottom of the piston, necessitating a considerable increase in
+the length of the cylinder barrel. This arrangement could not have
+been considered successful, as it apparently was applied to only a
+limited number of units and was not carried over to the later
+6-cylinder engine model. There was much experimentation with cam
+shapes and most probably variations of these got into production.
+
+With the crankcase, they did not go all the way to the modern
+two-piece form but instead retained the one-piece construction.
+Assembly was effected through the ends and a detachable plate was
+provided on one side for access to the interior. It is clear that they
+regarded this ability to get at the interior of the case without major
+disassembly as a valuable characteristic, and later featured it in
+their sales literature. They were apparently willing to accept the
+resultant weakening of the case and continued the construction through
+their last engine model. The integrally cast cylinder water jackets
+were abandoned and the top of the crankcase was machined flat to
+provide a mounting deck for individual cylinders. The use of aluminum
+alloy was continued, and the interior of the case was provided with
+strengthening webs of considerable thickness, together with supporting
+ribs. The cam shaft was supported directly in the case.
+
+The individual cylinder design was of extreme simplicity, a single
+iron casting embodying everything except the water jacket. The valves
+seated directly on the cast-iron cylinder head and the guides and
+ports were all contained in an integral boss on top of the head. The
+exhaust valve location on the side of the engine opposite the pilot
+was a decided advantage over that of the 1903 design, where the
+exhaust was toward the pilot. A four-cornered flange near the bottom
+of the cylinder provided for fastening it to the crankcase, and a
+threaded hole in the top of the head received a vertical eyebolt which
+served as the rocker-arm support. The cylinder was machined all over;
+two flanges, one at the bottom and the other about two-thirds of the
+way down provided the surfaces against which the water jacket was
+shrunk. The jacket was an aluminum casting incorporating the necessary
+bosses and double shrunk on the barrel; that is, the jacket itself was
+shrunk on the cylinder-barrel flanges and then steel rings were shrunk
+on the ends of the jacket over the flanges. The jacket thickness was
+reduced by machining at the ends, making a semigroove into which the
+steel shrink rings fitted. These rings insured the maintenance of a
+tight joint despite the tendency of the aluminum jacket to expand away
+from the cast-iron barrel.
+
+[Illustration: _Figure 9._--4-Cylinder vertical engine: a, magneto
+side; b, valve port side with intake manifold removed; c, flywheel end
+of engine at Carillon Park Museum, Dayton, Ohio; d, magneto side with
+crankcase cover removed. (Photos: a, Smithsonian A-3773; b, d, Pratt &
+Whitney D-15003, 15007; c, by A. L. Rockwell.)]
+
+Why the one-piece crankcase and cylinder jacket combination of the
+1903 engine was abandoned for the individual cylinder construction can
+only be surmised. The difference in weight was probably slight, as the
+inherent weight advantage of the original crankcase casting was
+largely offset by the relatively heavy valve boxes, and the difference
+in the total amount of machining required, because of the separate
+valve boxes, cages, and attaching parts, also was probably slight.
+Although the crankcase had shown itself to be structurally weak, this
+could have been cared for by proper strengthening. The 1903 design did
+have some fundamental disadvantages: it required a fairly complex
+pattern and expensive casting, plus some difficult machining, part of
+which had to be very accurate in order to maintain both gas and water
+joints tight; and the failure of any one cylinder that affected the
+jacket meant a complete crankcase replacement.
+
+It seems probable that a change was initially made mandatory by their
+intention to utilize the ported exhaust feature, the value of which
+they had proved in the experimental engine. The separate one-piece
+water jacket construction they had arrived at in this engine was
+available, but once the decision to change was made, the individual
+cylinder with its shrunk-on jacket had much to commend it--simplicity,
+cost, ease of manufacture and assembly and attachment, and
+serviceability. The advantages of the auxiliary, or ported, exhaust
+were not obtained without cost, however, as the water jacket around
+the barrel could not very easily be extended below the ports. Thus,
+even though the water was carried as high as possible on the upper
+end, a large portion of the barrel was left uncooled, and the lack of
+cooling at the lower end, in conjunction with the uncooled portion of
+the head, meant that only approximately half the entire cylinder
+surface was cooled directly.
+
+The piston was generally the same as in the 1903 engine, except that
+six radial ribs were added on the under side of the head, tapering
+from maximum thickness at the center to nothing near the wall. They
+were probably incorporated as an added path for heat to flow from the
+center of the piston toward the outside, as their shape was not the
+best use of material for strength. The piston pin was locked in the
+piston by the usual set screw, but here no provision was made for the
+alternate practice of clamping the rod on the pin. This piston-pin
+setscrew construction had become a standard arrangement in automobile
+practice. The piston rings were the normal wide design of that time,
+with what would now be considered a low unit pressure.
+
+Quite early in the life of this engine model the practice was
+initiated of incorporating shallow grooves in the surface of the more
+highly loaded thrust face of the piston below the piston pin to
+provide additional lubrication. This development apparently proceeded
+haphazardly. Figure 10c shows three of the pistons from an engine of
+low serial number--the first of this model to be delivered to the U.S.
+Navy--and it will be noted that one has no grooves, another has one,
+and the other has three. The eventual standardized arrangement
+provided three of these grooves, approximately 1/16 in. wide,
+extending halfway around the piston, and, although the depth was only
+a few thousandths of an inch, the amount of oil carried in them was
+apparently sufficient to assist in the lubrication of the face, as
+they were used in both the 4-and 6-cylinder engines.
+
+Each cylinder was fastened to the crankcase by four nuts on studs
+driven into the aluminum case. Valves and rocker arms were similar to
+those of the early engines, the automatic inlet valve being retained.
+The continued use of the two-piece valve is not notable, even though
+one-piece forgings were available and in use at this time; the
+automobile continued for many years to use this construction. The
+camshaft was placed at the bottom of the engine, inside the crankcase,
+and the rocker arms were actuated by pushrods which were operated by
+hinged cam followers. The pushrod was fastened in the rocker by a pin,
+about which it operated, through its upper end and was positioned near
+the bottom by a guide in the crankcase deck. The lower end of the rod
+bore directly on the flat upper surface of the cam follower, and valve
+clearance adjustment was obtained by grinding this end. The camshaft
+and magneto were driven by the crankshaft through a three-member train
+of spur gears (see Figures 9, 10 and 11).
+
+The built-up construction of the connecting rod was carried over from
+the first engine, and in the beginning apparently the same materials
+were used, except that the big end was babbited. Later the rod ends
+were changed from bronze to steel. The big end incorporated a small
+pointed scupper on one side for lubrication, as with the original, and
+this was sometimes drilled to feed a groove which carried oil to the
+rod bearing, but where the drilling was omitted, the only function the
+scupper then could perform was, as in the original engine, to throw a
+small amount of oil on the cylinder wall.
+
+The crankshaft and flywheel were similar in design to those on the
+1903 engine, except that the sharp corners at the top and bottom of
+the crank cheeks were machined off to save weight (see Figure 10f). An
+oil pump and a fuel pump were mounted side by side in bosses cast on
+the valve side of the crankcase; they were driven from the camshaft by
+worm gears and small shafts crossing the case.
+
+[Illustration: _Figure 10._--4-Cylinder vertical engine: a, cylinder
+assembly with valve mechanism parts; b, cylinder disassembled, and
+parts; c, pistons and connecting rods; d, bottom side of piston; e,
+crankshaft, flywheel and crankcase end closure; f, crankcase, with
+compression release parts. (Pratt & Whitney photos D-14996, 15001,
+14998, 14994, 14999, 14989, respectively.)]
+
+The camshaft construction was considerably altered from the 1903
+design. Although the reason is not entirely clear, one indication
+suggests that breakage or distortion of the shaft may have been
+encountered: whereas in the 1903 engine there had been no relationship
+between the location of the cams and the camshaft bearings, in this
+engine the exhaust valves were carefully positioned so that all cams
+were located very close to the supporting bearings in the crankcase.
+Also, the camshaft was solid, although it would seem that the original
+hollow shaft construction could have provided equal stiffness with
+less weight. The final decision was possibly determined by the
+practicality that there existed no standard tubing even approximating
+the size and wall thickness desired.
+
+There still was no carburetor, a gear pump metering the fuel in the
+same manner as on the 1904-1905 engine. Basically, the intake charge
+was fed to the cylinders by a round gallery manifold running alongside
+the engine. This was split internally by a baffle extending almost
+from end to end, so that the fuel mixture entering the manifold on one
+side of the baffle was compelled to travel to the two ends before it
+could return to the inside cylinder, this feature being a copy of
+their 1903 general intake arrangement. Apparently various shapes and
+positions of entrance pipes with which to spray the fuel into the
+manifold were used; and the injection arrangement seems also to have
+been varied at different times. The fuel pump was not necessarily
+always used, as the engine in some of the illustrations did not
+incorporate one, the fuel apparently being fed by gravity, as on the
+original engine. Chenoweth describes an arrangement in which exhaust
+heat was applied to the inlet manifold to assist the fuel vaporization
+process, but it is believed that this was one of the many changes made
+in the engine during its lifetime and not necessarily a standard
+feature.
+
+A water circulation pump was provided, driven directly by the
+crankshaft through a two-arm universal joint intended to care for any
+misalignment between the shaft and the pump. The water was piped to a
+horizontal manifold running along the cylinders just below the intake
+manifold, and a similar manifold on the other side of the engine
+collected it for delivery to the radiator. It is a little difficult to
+understand why it was not introduced at the bottom of the water
+jackets.
+
+The crankcase was a relatively strong and well proportioned structure
+with three heavy strengthening ribs running from side to side, its
+only weakness being the one open side. A sheet-iron sump was fastened
+to the bottom by screws and it would appear from its design, method of
+attachment, and location of the engine mounting pads that this was
+added some time after the crankcase had been designed; but if so it
+was apparently retrofitted, as engines with quite low serial numbers
+have this part.
+
+The ignition was by high-tension magneto and spark plug and this
+decision to change from the make-and-break system was undoubtedly the
+correct one, just as adoption of the other form originally was logical
+under the circumstances that existed then. The high-tension system was
+simpler and had now collected more service experience. The magneto
+was driven through the camshaft gear, and a shelf, or bracket, cast as
+an integral part of the case, was provided for mounting it. The spark
+advance control was in the magneto and, since spark timing was the
+only means of regulating the engine power and speed, a wide range of
+adjustment was provided.
+
+The engine had the controllable compression release which had been
+added to the _No. 2_ and _No. 3_ flat engines, although mechanically
+it was considerably altered from the original design. Instead of the
+movable stop operating directly on the rocker roller to hold the
+exhaust valve open, it was located underneath a collar on the pushrod.
+This stop was hinged to the crankcase and actuated by a small rod
+running along and supported by the crankcase deck. Longitudinal
+movement of this rod in one direction would, by spring pressure on
+each stop, push them underneath the collars as the exhaust valves were
+successively opened. A reverse movement of the rod would release them
+(see Figure 10f). Why they retained the method of manually operating
+the compression release, which was the same as had been used in the
+1904-1905 engine, is not quite clear. That is, the mechanism was put
+into operation by pulling a wire running from the pilot to a lever
+actuating the cam which moved the control rod. When normal valve
+operation was subsequently desired, the pilot was compelled to reach
+with his hand and operate the lever manually, whereas a second wire or
+push-pull mechanism would have obviated the necessity for both the
+awkward manual operation of the lever and the gear guard which was
+added to protect the pilot's hand, the lever being located close to
+the camshaft gear.
+
+The 4-cylinder vertical engine was a considerable improvement over the
+previous designs. They had obtained a power increase of about 40
+percent, with a weight decrease of 10 percent, and now had an engine
+whose design was almost standard form for good internal combustion
+engines for years to come. In fact, had they split the crankcase at
+the crankshaft center line and operated the inlet valves mechanically,
+they would have had what could be termed a truly modern design. They
+needed more cylinder cooling, both barrel and head, particularly the
+latter, and an opened-up induction system for maximum power output,
+but this was not what they were yet striving for. They had directly
+stated that they were much more interested in reliability than light
+weight.
+
+This engine model was the only one of the Wright designs to be
+licensed and produced abroad, being manufactured in Germany by the
+Neue Automobil-Gesellschaft and by Bariquand et Marré in France. The
+latter was much more prominent and their engines were used in several
+early European airplanes.
+
+[Illustration: _Figure 11._--4-Cylinder vertical engine assembly,
+Bariquand et Marré version. (Drawing courtesy Bristol Siddeley
+Engines, Ltd.)]
+
+[Illustration: THE WRIGHT BROTHERS AERO ENGINE]
+
+The French manufacturer, without altering the basic design, made a
+number of changes of detail which seem to have greatly annoyed
+Wilbur Wright, although some of them could probably be listed as
+improvements, based on several features of later standard design. One
+consisted of an alteration in the position of the fuel and oil pumps,
+the latter being lowered to the level of the sump. The crankcase was
+drilled to provide forced-feed lubrication to the connecting rod big
+end and crankshaft main bearings. Strengthening ribs were added to the
+pistons running from the upper side of the pin bosses to the piston
+wall, and the crankcase studs holding down the cylinders were replaced
+with bolts having their heads inside the case. The hinged cam follower
+was omitted and the pushrod bore directly on the cam through a roller
+in its end. The magneto was moved toward the rear of the engine a
+considerable distance and an ignition timing control device was
+introduced between it and its driving gear. Instead of the magneto
+being mounted directly on the special bracket integral with the
+crankcase, a wooden board running from front to rear of the engine was
+used and this was fastened to the two engine support pads, the magneto
+bracket being omitted entirely.
+
+Despite his criticism of the French motor and the quality of its
+manufacture, Wilbur was compelled to install one in his own exhibition
+airplane during his early French demonstrations at Le Mans after rod
+failure had broken his spare crankcase, and much of his subsequent
+demonstration flying was made with the French product.
+
+
+
+
+The Eight-Cylinder Racing Engine
+
+
+By 1909 regular and special air meets and races were being held and
+various competitions for trophies conducted. Among these the Gordon
+Bennett Cup Race for many years was considered a major event. For the
+1910 competition it was decided to enter a Wright machine and, since
+this was a race with speed the sole objective, the available
+4-cylinder engine, even in a version pushed to its maximum output, was
+deemed too small. They built for it a special 8-cylinder unit in a
+90°V form. They were thus resorting to one of their 1904
+concepts--modifying and enlarging a version known and proved in
+use--as the proper method of most quickly increasing output.
+Unfortunately again, there are essentially no detailed drawings
+available, so that the design cannot be studied.[16]
+
+[Footnote 16: A drawing of the camshaft is held by The Franklin
+Institute.]
+
+Only one engine is historically recorded as having been built,
+although in view of the Wrights' record of foresight and preparation
+it is almost certain that at least one spare unit, assembled or in
+parts, was provided. In any case, the airplane--it was called the
+_Baby Grand Racer_--and engine were wrecked just before the race, and
+no physical parts were retained, so that the sole descriptions come
+from external photographs, memory, and hearsay. McFarland thinks that
+possibly Orville Wright, particularly, was somewhat discomfited over
+the accident that eliminated the machine, as he had previously flown
+it quite successfully at a speed substantially higher than that of the
+ultimate winner, and he wanted to get it out of sight and mind as
+quickly as possible. The Air Force Museum at Wright Field, Dayton,
+Ohio, has an incomplete set of drawings of a 90°V, 8-cylinder Wright
+engine, but it is quite obvious from the basic design and individual
+features, as well as from at least one date on the drawings, that this
+conception is of a considerably later vintage than that of the _Baby
+Grand Racer_.
+
+The racing engine was in essence a combination of two of the standard
+4s on a redesigned crankcase utilizing as many of the 4-cylinder
+engine parts as possible. The rods were reported to have been placed
+side by side, and the regular 4-cylinder crankshaft, with alterations
+to accommodate the rods, was utilized. A single cam operated all the
+exhaust valves. It was compact and light, its only fundamental
+disadvantage being the inherent unbalance of the 90°V-8. The
+arrangement provided a much higher powered unit in the cheapest and
+quickest manner, and one that could be expected to operate
+satisfactorily with the least development.
+
+
+
+
+The Six-Cylinder Vertical Engines
+
+
+Shortly after the construction of the 8-cylinder engine the Wrights
+were again faced with the ever-recurrent problem of providing a higher
+powered standard production engine for their airplanes, which were now
+being produced in some numbers. By this time, 1911, there had been a
+relatively tremendous growth in both flying and automotive use of the
+internal combustion engine and as a result many kinds and sizes had
+been produced and utilized, so that numerous choices were presented to
+them. But if they were both to make use of their past experience and
+retain the simplicity they had always striven for, the more practical
+possibilities narrowed down to three: they could increase the cylinder
+size in the 4-cylinder combination, or they could go either to 6 or 8
+cylinders in the approximate size they had previously used.
+
+[Illustration: _Figure 12._--Original 6-cylinder engine: a, push-rod
+side; b, valve-port side; c, crankcase with sump removed. (Photos:
+Smithsonian A-3773A, 45598; Pratt & Whitney D-15015, respectively.)]
+
+The 4-in. cylinder in combination with a 5-in. stroke would provide in
+four cylinders about the displacement they wanted. Strokes of 6 in.
+were not uncommon and cylinders of 6-in. bore had been very
+successfully utilized in high-output automobile racing engines many
+years before this, so there was seemingly no reason to doubt that the
+5-in. cylinder could be made to operate satisfactorily, but it is not
+difficult to imagine the Wrights' thoughts concerning the roughness of
+an engine with cylinders of this diameter. The question of the grade
+of available fuel may possibly have entered into their decision to
+some extent, but it seems far more likely that roughness, their
+perennial concern, was the predominant reason for not staying with the
+more simple 4-cylinder form (as we have seen, roughness to them meant
+the effect of the cylinder explosion forces). Actually, of course,
+they never went larger than a 4-3/8-in. cylinder bore, and later
+aircraft engine experience would seem generally to confirm their
+judgment, for with the piston engine it has always been much more
+difficult to make the larger bores operate satisfactorily at any given
+specific output.
+
+While the 90°V, 8-cylinder arrangement would have enabled them to
+utilize a great number of the 4-cylinder-engine parts, it would have
+given them a somewhat larger engine than was their apparent desire,
+unless they reduced the cylinder size. And while they had had some
+limited experience in building and operating this kind of engine, and
+twice had chosen it when seeking more power, both of these choices
+were greatly influenced by the desire to obtain quickly an engine of
+higher power. It is also possible that something in their experience
+with the V-8 moved them away from it; the unbalanced shaking force
+inherent in the arrangement may well have become evident to them. What
+probably also helped them to their final conclusion was the
+fundamental consideration that the V-8 provided two extra cylinders
+which were not really needed.
+
+The eventual selection of the 6-cylinder was a slight compromise. In
+order to get the desired output the cylinder displacement was
+increased, but this was done by lengthening the stroke--the first time
+this had been altered since the original design. The increase (from 4
+to 4-1/2 in.) was only 1/2 in., and the bore, the more important
+influence on fuel performance, was kept the same. Overall, the choice
+was quite logical. They were utilizing the in-line construction upon
+which almost all of their now considerable experience had been based,
+and the sizes of and requirements for parts also conformed to this
+experience. They could, in fact, use many of the same parts. The
+natural balance of the 6-cylinder arrangement gave them a very smooth
+engine, and had they stiffened the shaft and counter-weighted the
+cranks, they would have produced the smoothest engine that could have
+been built at that time.
+
+In the literature are two references to a Wright 6-cylinder engine
+constructed around the cylinders of the vertical 4. One of these is in
+Angle's _Airplane Engine Encyclopedia_, published in 1921, and the
+other is in _Aerosphere 1939_, published in 1940. The wording of the
+latter is essentially identical with that of the former; it seems a
+reasonable conclusion that it is a copy. Although it is possible that
+such an engine was built at some time, just as the 8-cylinder racing
+engine was cobbled up out of parts from the 4-cylinder vertical, no
+other record, no engines, and no illustrations have been found. It is
+thus quite certain that no significant quantity was ever manufactured
+or utilized.
+
+The crankcase was considerably changed from that of the vertical 4,
+and was now in two pieces, with the split on the crankshaft center
+line. However, the shaft was not supported by the lower half of the
+case, as eventually became standard practice, but by bearing caps
+bolted to the ends of the upper case and, in between, to heavy ribs
+running across the upper case between the cylinders. The lower half of
+the case thus received none of the dynamic or explosion loads, and,
+serving only to support the engine and to provide for its mounting,
+was lightly ribbed. In it were incorporated integral-boss standpipe
+oil drains which discharged into a bolted-on sump. The upper half of
+the case was again left open on one side, giving the desired access to
+the interior, and, additionally, the design was altered to provide a
+method of camshaft assembly that was much simpler than that of the
+vertical 4 (see p. 42).
+
+The cylinder was also greatly altered from that of the vertical 4. It
+was made in three parts, a piece of seamless steel tubing being shrunk
+on a cast-iron barrel to form the water jacket, with a cast-iron
+cylinder head shrunk on the upper end of the barrel. This construction
+compelled the use of long studs running from the cylinder head to the
+case for fastening down the cylinder (see Figures 12a-c). For the
+first time the cylinder heads were water-cooled, cored passages being
+provided, and more barrel surface was jacketed than previously,
+although a considerable area at the bottom was still left uncooled,
+obviously by direct intent, as the ported exhaust arrangement was no
+longer employed.
+
+Also for the first time one-piece forged valves were used, but just
+when these were incorporated is not certain and, surprisingly, they
+were applied to the inlet only, the exhaust valve being continued in
+the previous two-piece screwed and riveted construction. The reasoning
+behind this is not evident. If a satisfactory two-piece exhaust valve
+had finally been developed it would be logical to carry it over to
+the new design; but exhaust valves normally being much more
+troublesome, it would seem that a good exhaust valve would make an
+even better inlet valve and, in the quantities utilized, the two-piece
+design should have been much cheaper. In the original 6-cylinder
+engine the inlet valves operated automatically as in all previous
+models, but at the time of a later extensive redesign (1913) this was
+changed to mechanical actuation, and the succeeding engines
+incorporated this feature. All the valve-actuating mechanism was
+similar to that of the vertical 4, and the engine had the usual
+compression-release mechanism, the detail design being carried over
+directly from the 4-cylinder.
+
+Design of the piston followed their previous practice, with wide rings
+above the pin and shallow grooves below the pin on the thrust face,
+and with the pin fastened in the piston by a set screw. The piston had
+four ribs underneath the head (see Figure 13b) radiating from the
+center and with the two over the pin bosses incorporating
+strengthening webs running down and joining the bosses. The piston
+length was reduced by 1 in., thus giving a much less clumsy appearance
+and, with other minor alterations, a weight saving of 40 percent (see
+Figures 13b and c). The rods were for the first time made of I-section
+forgings, a major departure, machined on the sides and hand
+finished at the ends, with a babbit lining in the big end, the piston
+pin bearing remaining steel on steel.
+
+[Illustration: _Figure 13._--Original 6-cylinder engine: a, cylinder
+assembly and valve parts; b, bottom side of piston; c, piston, piston
+pin and connecting rod; d, valve mechanism; e, crankshaft and
+flywheel. (Pratt & Whitney photos D-15012, 15017, 15013, 15018,
+respectively.)]
+
+At least two different general carburetion and induction systems were
+utilized, possibly three. One, and most probably the original,
+consisted of a duplicate of the injection pump of the 4-cylinder
+fitted to a manifold which ran the length of the engine, with three
+takeoffs, each of which then split into two, one for each cylinder. Of
+this arrangement they tried at least two variations involving changes
+in the location and method of injecting the fuel into the manifold;
+and there seems to have been an intermediate manifold arrangement,
+using fuel-pump injection at the middle of the straight side, or
+gallery, manifold, which was fed additional air at both ends through
+short auxiliary inlet pipes. This would indicate that with the
+original arrangement, the end cylinders were receiving too rich a
+mixture, when the fuel in the manifold was not properly vaporized.
+Although the exhaust was on the same side of the engine as the inlet
+system, no attempt was made to heat the incoming charge at any point
+in its travel. An entirely different system adopted at the time of the
+complete redesign in 1913 consisted of two float-feed Zenith
+carburetors each feeding a conventional three-outlet manifold. This
+carburetor was one of the first of the plain-tube type, that is, with
+the airflow through a straight venturi without any spring-loaded or
+auxiliary air valves, and was the simplest that could be devised. When
+properly fitted to the engine, it gave a quite good approximation of
+the correct fuel and air mixture ratio over the speed-load running
+range, although it is considerably more than doubtful that this was
+maintained at altitude, as is stated in one of the best descriptions
+of the engine published at the time the carburetors were applied.
+
+The compression ratio of this engine was lowered by almost 20 percent
+from that of the vertical 4. This, in combination with the low
+bore-to-stroke ratio, the unheated charge, and the later mechanically
+operated inlet valve, indicates that the Wrights were now attempting
+for the first time to secure from an engine something approaching the
+maximum output of which it was capable.
+
+As the engine originally came out, it continued to utilize only one
+spark plug in each cylinder. The high-tension magneto had a wide range
+of spark advance adjustment, which again provided the only control of
+the engine when equipped with the original fuel pump injection.
+
+The location of the valves and pushrods was similar to that in the 4,
+so that the cams were immediately adjacent to the camshaft bearings,
+which were carried in the crankcase ends and in the heavy webs. The
+camshaft was gear-driven and the cam shape was similar to that of the
+last 4s, with a quite rapid opening and closing and a long dwell,
+leaving the valve opening accelerations and seating velocities still
+quite high.
+
+The crankshaft was a continuation of their basic design of rather
+light construction, particularly in the webs. The cheeks were even
+thinner (by 1/4 in.) than those of the 4 although the width was
+increased by 1/8 in. (see Figure 13e). For the first time they went to
+a forging, the rough contour type of the time, and utilized a
+chrome-nickel alloy steel.
+
+Lubrication was by means of the usual gear pump, and the piston and
+rod bearings continued to be splash-fed. The rod big-end bearing
+carried a small sharp undrilled boss at the point where, on the other
+engines, had been located scuppers whose purpose was apparently still
+to throw lubricating oil on the cylinder wall carrying the more highly
+loaded side of the piston. The rod big-end bearing was lubricated by a
+hole on the top of the big-end boss catching some of the crankcase
+splash, which was then carried to the bearing by a groove.
+
+When the 6-cylinder engine was completely redesigned in 1913 this led
+to the introduction in late fall of that year of a new model called
+the 6-60, the 60 designating the rating in horsepower. There is little
+in the Wright records to show why such a radical revision was thought
+necessary, but the general history of the period gives a rather clear
+indication. The competition had caught up to the Wrights in
+powerplants. Other engines were being installed in Wright airplanes,
+and Navy log books show these other engines being used interchangeably
+with those of the Wrights.
+
+Most of the descriptions of the new model published at the time it was
+introduced concentrate on the addition of the two carburetors and the
+mechanical operation of the inlet valves, but these were only two of
+many major changes. The cylinder was completely revised, the intake
+being moved to the camshaft side of the engine from its position
+adjacent to the exhaust, so that the two ports were now on opposite
+sides of the cylinder. By proper positioning of the rocker-arm
+supports and choice of their length and angles, all valves were made
+operable from a single camshaft. The shrunk-on steel water jacket
+cylinder was retained, but the water connections were repositioned so
+that the water entered at the bottom and came out at the top of the
+cylinder. Over the life of the 6-cylinder engine several different
+valve types were used but the published specifications for the model
+6-60 called for "cast iron heads"--the old two-piece construction. The
+piston pins were case hardened and ground and the crankshaft pins and
+journals were heat treated and ground.
+
+The fuel and oil pumps were removed from the side of the crankcase and
+a different ignition system was applied, although still of the
+high-tension spark-plug type which by this time had become general
+practice on all so-called high-speed internal-combustion engines. A
+second threaded spark-plug hole was provided in the cylinder head and
+despite its more common use for other purposes, it is evident that the
+intention was to provide two-plug ignition. It is doubtful that at the
+specific output of this engine any power difference would be found
+between one-and two-plug operation, so that the objective was clearly
+to provide a reserve unit in case of plug failure. However, it was
+also used for the installation of a priming cock for starting and
+because of the prevalence of single-wire ignition systems on existing
+and illustrated engines, it seems to have been used mostly in this
+manner, even though dual-ignition systems later became an unvarying
+standard for aircraft engines.
+
+Viewed externally, the only part of the engine that appears the same
+as the original 6 is the small lower portion of the crankcase; but
+what is more visually striking is the beauty of the new lines and
+extreme cleanness of the exterior design (see Figures 14 and 15). Many
+of their individual parts had shown the beauty of the sparse design of
+pure utility but it was now in evidence in the whole. Despite the
+proven practical value of their other models, this is the only one
+that can be called a good-looking engine, instantly appealing to the
+aesthetic sense, even though the vertical 4 is not an ugly engine. The
+appearance of their final effort, in a field they were originally
+reluctant to enter and concerning which they always deprecated the
+results of their own work, was a thing of which a technically trained
+professional engine designer could be proud.
+
+The 6-60 was continued in production and development until it became
+the 6-70, and indications are that it eventually approached an output
+of 80 horsepower.
+
+[Illustration: _Figure 14._--6-Cylinder 6-60 and 6-70 engine, right
+rear intake side. (Pratt & Whitney photo.)]
+
+[Illustration: _Figure 15._--6-Cylinder 6-70 engine, incorporating
+flexible flywheel drive, exhaust side. (Smithsonian photo A-54381.)]
+
+
+
+
+Minor Design Details and Performance of the Wright Engines
+
+
+In the Wright brothers' various models were many minor design items
+which altogether required a great deal of consideration, but which did
+not materially affect overall engine performance. The results
+generally could all be classed as good practice; however, one of these
+utilized in the 4-cylinder vertical engine was rather unorthodox and
+consisted of offsetting the cylinders with relation to the crankshaft.
+This arrangement, which can be seen in the drawing (Figure 11) was
+apparently an attempt to reduce the maximum side load on the piston
+during the power stroke, but since the peak gas loading usually occurs
+at about 10 to 15 percent of the power stroke, this probably did not
+have much effect, and it was not carried over to the 6-cylinder
+design.
+
+All engine bearings were of the plain sleeve type and, except for the
+bronze and steel bearings in the connecting rod, were of babbit. The
+advantages of babbit for bearings were discovered very early in the
+development of the mechanical arts, and apparently the Wrights never
+encountered a bearing loading sufficiently high to cause a structural
+breakdown in this relatively weak material.
+
+Valve openings show no variation through the successive production
+engines, although the Wrights most probably experimented with
+different amounts. The 1903 engine and the vertical 4-and 6-cylinder
+all had lifts of 5/16 in., but the valve-seat angles varied somewhat;
+the records show included angles of 110° to 90°--not a large
+difference.
+
+The valve-operating mechanism was the same from the first vertical 4
+onward. The high side thrust caused by the cam shape required for the
+very rapid valve opening they chose was, no doubt, the reason for the
+use of the hinged cam follower, and since the same general cam design
+was used in their last engine, the 6-cylinder, the same method of
+operation which had apparently proved very serviceable was continued.
+How satisfactory was the considerably simpler substitute used in the
+Bariquand et Marré version of the 4-cylinder engine is not known.
+Possibly it was one of the alterations in the Wrights' design that
+Wilbur Wright objected to, although in principle it more closely
+conforms to the later fairly standard combination valve tappet and
+roller construction: The available drawings do indicate, however, that
+the cam of the Bariquand et Marré engine was also altered to give a
+considerably less abrupt valve opening than the Wright design, so that
+there was less side thrust. For the Wright 6-cylinder engine their
+4-cylinder cam was slightly altered to provide a rounding off near the
+top of the lobe, thus providing some reduction in the velocity before
+maximum opening was reached. All their cam designs indicate a somewhat
+greater fear of the effect of seating velocities than of opening
+accelerations.
+
+Since the range of cylinder diameters utilized did not vary greatly,
+the valve sizes were correspondingly fairly uniform. The diameter of
+the valves for the original 4-in.-bore cylinder was 2 in., while that
+for the 4-3/8-in. bore used in the 6-cylinder engine was actually
+slightly smaller, 1-7/8 in. Possibly the Wrights clung too long to the
+automatic inlet valve, although it did serve them well; but possibly,
+as has been previously noted, there were valid reasons for continuing
+its use despite the inherently low volumetric efficiency this
+entailed.
+
+The inherent weakness in the joints of the three-piece connecting rod
+has been pointed out, but aside from this, the design was excellent,
+for all the materials and manufacturing methods required were readily
+available, and structurally it was very sound. Tubular rods were still
+in use in aircraft engines in the 1920s.
+
+The Wrights had a surprisingly thorough grasp of the metallurgy of the
+time, and their choice of materials could hardly have been improved
+upon. Generally they relied upon the more simple and commonly used
+metals even though more sophisticated and technically better alloys
+and combinations were available.[17] Case hardening was in widespread
+use in this period but their only utilization of it was in some parts
+of the drive chains purchased completely assembled and in the piston
+pins of their last engine. The treatment of the crankshafts of all
+their engines except the final 6-cylinder was typical of their
+uncomplicated procedure: the particular material was chosen on the
+basis of many years of experience with it, hardening was a very simple
+process, and the expedient of carrying this to a point just below the
+non-machinable range gave them bearing surfaces that were sufficiently
+hard, yet at the same time it eliminated the possibility--present in a
+heat-treating operation--of warping the finished piece.
+
+[Footnote 17: Baker states that the first crankshaft was made from a
+slab of armor plate and if this is correct the alloy was a rather
+complex one of approximately .30-.35 carbon, .30-.80 manganese, .10
+silicon, .04 phosphorus, .02 sulphur, 3.25-3.50 nickel, 0.00-1.90
+chromium; however, all the rest of the evidence, including Orville
+Wright's statement to Dr. Gough, would seem to show that it was made
+of what was called tool steel (approximately 1.0 carbon).]
+
+In the entire 1903 engine only five basic materials--excepting those
+in the purchased "magneto" and the platinum facing on the
+ignition-system firing points--were used: steel, cast iron, aluminum,
+phosphor bronze, and babbit. The steels were all plain carbon types
+with the exception of the sheet manifold, which contained manganese,
+and no doubt this was used because the sheet available came in a
+standard alloy of the time.
+
+Overall, the Wright engines performed well, and in every case met or
+exceeded the existing requirements. Even though aircraft engines then
+were simpler than they became later and the design-development time
+much shorter, their performance stands as remarkable. As a result, the
+Wrights never lacked for a suitable powerplant despite the rapid
+growth in airplane size and performance, and the continual demand for
+increased power and endurance.
+
+Few service records dating from before 1911, when the military
+services started keeping log books, have been found. Some of those for
+the period toward the end of their active era have been preserved, but
+for that momentous period spanning the first few years when the
+Wrights had the only engines in actual continuous flight operation,
+there seems to be essentially nothing--perhaps because there were no
+standard development methods or routines to follow, no requirements to
+be met with respect to pre-flight demonstrations or the keeping of
+service records. Beginning in 1904, however, and continuing as long as
+they were actively in business, they apparently had in progress work
+on one or more developmental or experimental engines. This policy, in
+combination with the basic simplicity of design of these engines,
+accounted in large measure for their ability to conduct both
+demonstrations and routine flying essentially whenever they chose.
+
+Time between engine overhauls obviously varied. In mid 1906 an engine
+was "rebuilt after running about 12 hours." This is comparatively
+quite a good performance, particularly when it is remembered that
+essentially all the "running" was at full power output. It was
+considerably after 1920 before the Liberty engine was redesigned and
+developed to the stage where it was capable of operating 100 hours
+between overhauls, even though it was being used at cruising, or less
+than full, power for most of this time.
+
+The Wrights of course met with troubles and failures, but it is
+difficult, from the limited information available, to evaluate these
+and judge their relative severity. Lubrication seems to have been a
+rather constant problem, particularly in the early years. Although
+some bearing lubrication troubles were encountered from time to time,
+this was not of major proportions, and they never had to resort to
+force-feed lubrication of the main or rod big-end bearings. The piston
+and cylinder-barrel bearing surfaces seem to have given them the most
+trouble by far, and examination of almost any used early Wright engine
+will usually show one or more pistons with evidence of scuffing in
+varying degrees, and this is also apparent in the photographs in the
+record. This is a little difficult to understand inasmuch as most of
+the time they had the very favorable operating condition of cast iron
+on cast iron. Many references to piston seizure or incipient seizure,
+indicated by a loss of power, occur, and this trouble may have been
+aggravated by the very small piston clearances utilized. Why these
+small clearances were continued is also not readily explainable,
+except that with no combination of true oil-scraper rings, which was
+the basic reason why the final form of aviation piston engine was able
+to reach its unbelievably low oil consumptions, their large and rather
+weak compression rings were probably not doing an adequate job of oil
+control, and they were attempting to overcome this with a quite tight
+piston fit.[18] In any event, they did encounter scuffing or seizing
+pistons and cylinder over-oiling at the same time. As late as 4 May
+1908 in the Wright _Papers_ there appears the notation: "The only
+important change has been in the oiling. The engine now feeds entirely
+by splash...."
+
+[Footnote 18: Their intended piston ring tension is not known.
+Measurements of samples from the 4-and 6-cylinder vertical engines
+vary greatly, ranging from less than 1/2 lb per sq in. to almost 1-1/4
+lb. The validity of these data is very questionable as they apply to
+parts with unknown length of service and amount of wear. It seems
+quite certain, however, that even when new the unit tension figure
+with their wide rings was only a small fraction of that of the modern
+aircraft piston engine.]
+
+Their troubles tended to concentrate in the cylinder-piston
+combination, as has been true of almost all piston engines. References
+to broken cylinders are frequent. These were quite obviously cylinder
+barrels, as replacement was common, and this again is not readily
+explainable. The material itself, according to Orville Wright, had a
+very high tensile strength, and in the 1903 engine more than ample
+material was provided, as the barrel all the way down to well below
+the attachment to the case was 7/32 in. thick. The exact location of
+the point of failure was never recorded, but in its design are many
+square corners serving as points of stress concentration. Also, of
+course, no method was then available for determining a faulty casting,
+except by visual observation of imperfections on the surface, and this
+was probably the more common cause. It is interesting, however, that
+the engine finally assembled in 1928 for installation in the 1903
+airplane sent to England has a cracked cylinder barrel, the crack
+originating at a sharp corner in the slot provided at the bottom of
+the barrel for screwing it in place.
+
+Valve failures were also a continuing problem, and Chenoweth reports
+that a large proportion of the operating time of the 1904-1906
+development engine was concentrated on attempts to remedy this
+trouble. None of their cams, including those of the 6-cylinder engine,
+evidence any attempt to effect a major reduction in seating
+velocities. United States Navy log books of 1912 and 1913 record many
+instances of inlet valves "broken at the weld," indicating that some
+of the earlier 6-cylinder engines were fitted with valves of welded
+construction.
+
+For the engineer particularly, the fascination of the Wrights' engine
+story lies in its delineation of the essentially perfect engineering
+achievement by the classic definition of engineering--to utilize the
+available art and science to accomplish the desired end with a minimum
+expenditure of time, energy, and material. Light weight and
+operability were the guiding considerations; these could be obtained
+only through constant striving for the utmost simplicity. Always
+modest, the Wrights seem to have been even more so in connection with
+their engine accomplishments. Although the analogy is somewhat
+inexact, the situation is reminiscent of the truism often heard in the
+aircraft propulsion business--few people know the name of Paul
+Revere's horse. Yet, as McFarland has pointed out, "The engine was in
+fact far from their meanest achievement." With hardly any experience
+in this field and only a meagerly equipped machine shop, they designed
+and assembled an internal combustion engine that exceeded the
+specifications they had laid down as necessary for flight and had it
+operating in a period of about two months elapsed time. The basic form
+they evolved during this unequalled performance carried them through
+two years of such successful evolutionary flight development that
+their flying progressed from a hop to mastery of the art. And the
+overall record of their powerplants shows them to have been remarkably
+reliable in view of the state of the internal combustion engine at
+that time.
+
+
+
+
+Appendix
+
+
+Characteristics of the Wright Flight Engines
+
+  -------------------------------------------------------------------------
+                          _1903       _1904-1905    _1908-1911   _1911-1915
+                        First flight Experimental  Demonstrations  service_
+                        engine[a]_     flights_        and
+                                                      service_
+  -------------------------------------------------------------------------
+  Cyl./Form              4/flat       4/flat       4/vertical   6/vertical
+  Bore and stroke (in.)  4×4          4-1/8×4      4-3/8×4      4-3/8×4-1/2
+  Displacement (cu. in.) 201          214          240          406
+  Horsepower             8.25-16      15-21        28-42        50-75
+  RPM                    670-1200     1070-1360    1325-1500    1400-1560
+  MEP                    49-53        52-57        70-87        70-94
+  Weight (lb)            140-180      160-170      160-180      265-300
+  -------------------------------------------------------------------------
+
+[Footnote a: Concurrently with the Wrights' first engine work, Manly
+was developing the engine for the Langley Aerodrome, and a comparison
+of the Wrights' engine development with that of Manly is immediately
+suggested, but no meaningful comparison of the two efforts can be
+drawn. Beyond the objective of producing a power unit to accomplish
+human flight and the fact that all three individuals were superb
+mechanics, the two efforts had nothing in common. The Wrights' goal
+was an operable and reasonably lightweight unit to be obtained quickly
+and cheaply. Manly's task was to obtain what was for the time an
+inordinately light engine and, although the originally specified power
+was considerably greater than that of the Wrights, it was still
+reasonable even though Manly himself apparently increased it on the
+assumption that Langley would need more power than he thought. The
+cost and time required were very much greater than the Wrights
+expended. He ended up with an engine of extraordinary performance for
+its time, containing many features utilized in much later important
+service engines. His weight per horsepower was not improved upon for
+many years. The Wrights' engine proved its practicability in actual
+service. The Manly engine never had this opportunity but its
+successful ground tests indicated an equal potential in this respect.
+A description of the Langley-Manly engine and the history of its
+development is contained in _Smithsonian Annals of Flight_ number 6,
+"Langley's Aero Engine of 1903," by Robert B. Meyer (xi+193 pages, 44
+figures; Smithsonian Institution Press, 1971)]
+
+It is not possible to state the exact quantities of each engine that
+the Wrights produced up to the time that their factory ceased
+operation in 1915. Chenoweth gives an estimate, based on the
+recollection of their test foreman, of 100 vertical 4s and 50 6s. My
+estimate (see page 2) places the total of all engines at close to 200.
+Original Wright-built engines of all four of these basic designs are
+in existence, although they are rather widely scattered. The
+Smithsonian's National Air and Space Museum has examples of them all,
+including, of course, the unique first-flight engine. Their condition
+varies, but many are operable, or could easily be made so. Among the
+best are the first-flight engine and the last vertical 6, at the
+Smithsonian, the first vertical 6, at the United States Air Force
+Museum, and the vertical 4, at the Carillon Park Museum.
+
+The Wrights were constantly experimenting and altering, and this in
+connection with the lack of complete records makes it almost
+impossible to state with any certainty specific performances of
+individual engines at given times. Weights sometimes included
+accessories and at others did not. Often they were of the complete
+powerplant unit, including radiator and water and fuel, with no
+clarification. In the table, performance is given in ranges which are
+thought to be the most representative of those actually utilized.
+Occasionally performances were attained even beyond the ranges given.
+For example, the 4×4-in. flat development engine eventually
+demonstrated 25 hp at an MEP of approximately 65 psi.
+
+One important figure--the horsepower actually utilized during the
+first flight--is quite accurately known. In 1904 the 1904-1905 flight
+engine, after having been calibrated by their prony-brake test-fan
+method, was used to turn the 1903 flight propellers, and Orville
+Wright calculated this power to be 12.05 bhp by comparing the
+calibrated engine results with those obtained with the flight engine
+at Kitty Hawk when tested under similar conditions. However, since the
+tests were conducted in still air with the engine stationary, this did
+not exactly represent the flight condition. No doubt the rotational
+speed of the engine and propellers increased somewhat with the forward
+velocity of the airplane so that unless the power-rpm curve of the
+engine was flat, the actual horsepower utilized was probably a small
+amount greater than Orville's figures. The lowest power figure shown
+for this engine is that of its first operation.
+
+No fuel consumption figures are given, primarily because no
+comprehensive data have been found. This is most probably because in
+the early flight years, when the Wrights were so meticulously
+measuring and recording technical information on the important factors
+affecting their work, the flights were of such short duration that
+fuel economy was of very minor importance. After success had been
+achieved, they ceased to keep detailed records on very much except
+their first interest--the flying machine itself--and when the time of
+longer flights arrived, the fuel consumption that resulted from their
+best engine design efforts was simply accepted. The range obtained
+became mostly a matter of aerodynamic design and weight carried.
+Orville Wright quotes an early figure of brake thermal efficiency for
+the 1903 engine that gives a specific fuel consumption of .580 lb of
+fuel per bhp/hr based on an estimate of the heating value of the fuel
+they had. This seems low, considering the compression ratio and
+probable leakage past their rather weak piston rings, but it is
+possible. In an undated entry, presumably in 1905, Orville Wright's
+notebook covered fuel consumption in terms of miles of flight; one of
+the stated assumptions in the entry is, "One horsepower consumes .60
+pounds per horsepower hour"--still quite good for the existing
+conditions. Published figures for the 6-60 engine centered around .67
+lb/hp hr for combined fuel and oil consumption.
+
+
+The Wright Shop Engine
+
+Despite the fact that the Wright shop engine was not a flight unit, it
+is interesting both because it was a well designed stationary
+powerplant with several exceedingly ingenious features, and because
+its complete success was doubtless a major factor in the Wrights'
+decision to design and build their own first flight engine. Put in
+service in their small shop in the fall of 1901, it was utilized in
+the construction of engine and airframe parts during the vital years
+from 1902 through 1908 and, in addition, it provided the sole means of
+determining the power output of all of their early flight engines. By
+means of a prony brake, its power output was carefully measured and
+from this the amount of power required for it to turn certain fans or
+test clubs was determined. These were then fitted to the flight
+engines and the power developed calculated from the speed at which the
+engines under test would turn the calibrated clubs. Although a
+somewhat complex method of using power per explosion of the shop
+engine was made necessary by the basic governor control of the engine,
+the final figures calculated by means of the propeller cube law seem
+to have been surprisingly accurate.[19] Restored under the personal
+direction of Charles Taylor, it is in the Henry Ford Museum in
+Dearborn, Michigan, together with the shop machinery it operated.
+
+[Footnote 19: _The Papers of Wilbur and Orville Wright_, volume 2,
+Appendix.]
+
+The engine was a single cylinder, 4-stroke-cycle "hot-tube" ignition
+type. The cylinder, of cast iron quite finely and completely finned
+for its day, was air-cooled, or rather, air-radiated, as there was no
+forced circulation of air over it, the atmosphere surrounding the
+engine simply soaking up the dissipated heat. Although this was
+possibly a desirable adjunct in winter, inside the small shop in
+Dayton, the temperature there in summer must have been quite high at
+times. The operating fuel was city illuminating gas, which was also
+utilized to heat, by means of a burner, the ignition tube. This part
+was of copper, with one completely closed end positioned directly in
+the burner flame; the other end was open and connected the interior of
+the tube to the combustion chamber. The inlet valve was of the usual
+automatic type while the exhaust valve was mechanically operated. The
+fuel gas flow was controlled by a separate valve mechanically
+connected to the inlet valve so that the opening of the inlet valve
+also opened the gas valve, and gas and air were carried into the
+cylinder together.
+
+[Illustration: _Figure 16._--Shop engine, 1901, showing governor and
+exhaust valve cam. (Photo courtesy R. V. Kerley.)]
+
+The engine was of normal stationary powerplant design, having a heavy
+base and two heavy flywheels, one on each side of the crank. These
+were necessary to ensure reasonably uniform rotational speed, as, in
+addition to having only one cylinder, the governing was of the
+hit-and-miss type. It had a 6×7-in. bore and stroke and would develop
+slightly over 3 hp at what was apparently its normal operating speed
+of 447 rpm, which gives an MEP of 27 psi.
+
+The engine is noteworthy not only for its very successful operation
+but also because it incorporated two quite ingenious features. One was
+the speed-governing mechanism. As in the usual hit-and-miss operation,
+the engine speed was maintained at a constant value, the output then
+being determined by the number of power strokes necessary to
+accomplish this. The governor proper was a cylindrical weight free to
+slide along its axis on a shaft fastened longitudinally to a spoke of
+one of the flywheels. A spring forced it toward the center of the
+wheel, while centrifugal force pulled it toward the rim against the
+spring pressure. After each opening of the valve the exhaust-valve
+actuating lever was automatically locked in the valve-open position by
+a spring-loaded pawl, or catch. The lever had attached to it a small
+side extension, or bar, which, when properly forced, would release the
+catch and free the actuating lever. This bar was so positioned as to
+be contacted by the governor weight when the engine speed was of the
+desired value or lower, thus maintaining regular valve operation; but
+an excessive speed would move the governor weight toward the rim and
+the exhaust valve would then be held in the open position during the
+inlet stroke, so no cylinder charge would be ingested. Since the
+ignition was not mechanically timed, the firing of the charge was
+dependent only on the compression of the inlet charge in the cylinder,
+so it made no difference whether the governor caused the engine to
+cease firing for an odd or even number of revolutions, even though the
+engine was operating on a 4-stroke cycle at all times.
+
+[Illustration: _Figure 17._--Shop engine, 1901, showing operation of
+exhaust valve cam. (Pratt & Whitney drawing.)]
+
+The exhaust valve operating cam was even more ingenious. To obtain
+operation on a 4-stroke cycle and still avoid the addition of a
+half-speed camshaft, a cam traveling at crankshaft speed was made to
+operate the exhaust valve every other revolution (see Figure 17). It
+consisted of a very slim quarter-moon outline fastened to a disc on
+the crankshaft by a single bearing bolt through its middle which
+served as the pivot about which it moved. Just enough clearance was
+provided between the inside of the quarter-moon and the crankshaft to
+allow the passage of the cam-follower roller. The quarter-moon,
+statically balanced and free to move about its pivot, basically had
+two positions. In one the leading edge was touching the shaft (Figure
+17b), so that when the cam came to the cam follower, the follower was
+forced to go over the top of the cam, thus opening the exhaust valve.
+When the cam pivot point had passed the roller, the pressure of the
+exhaust valve spring forced the following edge of the cam into
+contact with the shaft and this movement, which separated the leading
+edge of the cam from the shaft, provided sufficient space between it
+and the shaft for the roller to enter (Figure 17c). Thus, when the
+leading edge of the cam next reached the roller, the roller, being
+held against the crankshaft by the valve spring pressure (Figure 17d),
+entered the space between the cam and the shaft and there was no
+actuation of the valve. In exiting from the space, it raised the
+trailing edge of the cam, forcing the leading edge against the shaft
+(Figure 17a) so that at the next meeting a normal valve opening would
+take place. The cam was maintained by friction alone in the position
+in which it was set by the roller, but since the amount of this could
+be adjusted to any value, it could be easily maintained sufficient to
+offset the small centrifugal force tending to put the cam in a neutral
+position.[20]
+
+[Footnote 20: The Wrights apparently never applied for an engine
+patent of any kind. This no doubt grew out of their attitude of
+regarding the engine as an accessory and deprecating their work in
+this field. A reasonably complete patent search indicates that this
+particular cam device has never been patented, although a much more
+complex arrangement accomplishing the same purpose was patented in
+1900, and a patent application on a cam-actuating mechanism
+substantially identical to that of the Wrights and intended for use in
+a golf practice apparatus is pending at the present time.]
+
+
+
+
+Bibliography
+
+
+ANGLE, GLENN D. Wright. Pages 521-523 in _Airplane Engine
+Encyclopedia, an Alphabetically Arranged Compilation of All Available
+Data on the World's Airplane Engines_. Dayton, Ohio: The Otterbein
+Press, 1921.
+
+BAKER, MAX P. The Wright Brothers as Aeronautical Engineers. _Annual
+Report of ... the Smithsonian Institution ... for the Year Ended June
+30, 1950_, pages 209-223, 4 figures, 9 plates.
+
+BEAUMOUNT, WILLIAM WORBY. _Motor Vehicles and Motors: Their Design,
+Construction, and Working by Steam, Oil, and Electricity._ 2 volumes.
+Philadelphia: J. B. Lippincott, 1901-1902.
+
+CHENOWETH, OPIE. Power Plants Built by the Wright Brothers. _S.A.E.
+Quarterly Transactions_ (January 1951), 5:14-17.
+
+FOREST, FERNAND. _Les Bateaux automobiles._ Paris: H. Dunod et E.
+Pinat, Éditeurs, 1906.
+
+GOUGH, DR. H. J. Materials of Aircraft Construction. _Journal of the
+Royal Aeronautical Society_ (November 1938), 42:922-1032. Illustrated.
+
+KELLY, FRED C. _Miracle at Kitty Hawk; the Letters of Wilbur and
+Orville Wright._ New York: Farrar, Straus and Young, 1951.
+
+---------- _The Wright Brothers, a Biography Authorized by Orville
+Wright._ New York: Harcourt, Brace & Co., 1943.
+
+KENNEDY, RANKIN. _Flying Machines: Practice and Design. Their
+Principles, Construction and Working._ 158 pages. London: Technical
+Publishing Co., Ltd., 1909.
+
+LAWRANCE, CHARLES L. _The Development of the Aeroplane Engine in the
+United States._ Pages 409-429 in International Civil Aeronautics
+Conference, Washington, D.C., 12-14 December 1928, Papers Submitted by
+the Delegates for Consideration by the Conference. Washington:
+Government Printing Office, 1928.
+
+MCFARLAND, MARVIN W. _The Papers of Wilbur and Orville Wright._ 2
+volumes. New York: McGraw Hill Book Co., 1953.
+
+RENSTROM, ARTHUR G. Wilbur and Orville Wright: A Bibliography
+Commemorating the Hundredth Anniversary of the Birth of Wilbur Wright,
+April 16, 1867. Washington, D.C.: The Library of Congress [Government
+Printing Office], 1968. Contains 2055 entries.
+
+The 6-Cylinder 60-Horsepower Wright Motor. _Aeronautics_ (November
+1913), 13(5):177-179.
+
+Wright Brothers. Pages 829-830 in _Aerosphere 1939, Including World's
+Aircraft Engines, with Aircraft Directory_, Glenn D. Angle, editor.
+New York: Aircraft Publishers, 1940.
+
+
+
+
+Index
+
+
+  Angle, Glenn D., 51
+
+
+  _Baby Grand Racer_, 47
+
+  Baker, Max P. 1, 10, 26, 28
+
+  Bariquand et Marré, 43, 44-45, 57-58
+
+  Beaumount, William Worby, 9, 25
+
+  Bristol Siddeley Engines, Ltd., 44-45
+
+
+  Carillon Park Museum, Dayton, Ohio, ix, 5n, 7, 37
+
+  Chanute, Octave, 28
+
+  Chenoweth, Opie, ix, 22, 35, 42, 63
+
+  Christman, Louis P., ix, 7, 8, 28
+
+  Cole, Gilmoure N., ix
+
+  Clarke, J. H., 18
+
+
+  Daimler-Benz A. G., ix, 10, 13
+
+
+  Engineers Club, Dayton, Ohio, ix, 32
+
+
+  Ford, Henry, 8
+
+  Ford, Henry, Museum, Dearborn, Michigan, 8, 64
+
+  Forest, Fernand, 11
+
+  Franklin Institute, Philadelphia, Pennsylvania, ix, 47
+
+
+  Gough, Dr. H. J., 58n
+
+
+  Howell Cheney Technical School, Manchester, Connecticut, x, 14, 15
+
+
+  Kelly, Fred C, 4n
+
+  Kerley, R. V., ix, 65
+
+  _Kitty Hawk Flyer_, ii, 3
+
+
+  Langley [Samuel P.] Aerodrome, 9, 62
+
+  Loening, Grover C, 13n
+
+
+  Manly, Charles L., 9, 62
+
+  Maxim, Sir Hiram Stevens, 3
+
+  McFarland, Marvin W., 1, 33, 47, 61
+
+  Miller-Knoblock Manufacturing Co., South Bend, Indiana, 26
+
+
+  National Park Service, Cape Hatteras National Seashore, ii, ix
+
+  Neue Automobil-Gesellschaft, 43
+
+
+  Porter, L. Morgan, ix
+
+  Pratt & Whitney Aircraft Corp., v, x, 37, 40-41, 49, 52, 53, 67
+
+  Pruckner, Anton, 33
+
+
+  Rockwell, A. L., ix, 37
+
+
+  Santos-Dumont, Alberto, 11
+
+  Science Museum, London, x, 5, 6, 7, 8, 11, 21, 23, 26
+
+
+  Taylor, Charles E., 5, 64
+
+
+  United Aircraft Corp., v, x
+
+
+  Western Society of Engineers, 2
+
+  Whitehead, Gustave, 33
+
+  Wittemann, Charles, 33n
+
+  Wright, Bishop Milton (father), 28
+
+  Wright, Katherine (sister), 4
+
+
+  Zenith carburetor, 52
+
+
+*U.S. GOVERNMENT PRINTING OFFICE: 1971--397-764
+
+
+
+
+Publication in Smithsonian Annals of Flight
+
+
+_Manuscript_ for serial publications are accepted by the Smithsonian
+Institution Press, subject to substantive review, only through
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+<pre>
+
+The Project Gutenberg EBook of The Wright Brothers' Engines and Their
+Design, by Leonard S. Hobbs
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: The Wright Brothers' Engines and Their Design
+
+Author: Leonard S. Hobbs
+
+Release Date: February 2, 2012 [EBook #38739]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE WRIGHT BROTHERS' ENGINES ***
+
+
+
+
+Produced by Chris Curnow, Joe Cooper, Christine P. Travers
+and the Online Distributed Proofreading Team at
+http://www.pgdp.net
+
+
+
+
+
+
+</pre>
+
+
+<a id="img000" name="img000"></a>
+<div class="figcenter">
+<img src="images/img000.jpg" width="400" height="423" alt="Cover" title="">
+</div>
+
+<p class="p4 center">SERIAL PUBLICATIONS OF THE SMITHSONIAN INSTITUTION</p>
+
+<p>The emphasis upon publications as a means of diffusing knowledge was
+expressed by the first Secretary of the Smithsonian Institution. In
+his formal plan for the Institution, Joseph Henry articulated a
+program that included the following statement: "It is proposed to
+publish a series of reports, giving an account of the new discoveries
+in science, and of the changes made from year to year in all branches
+of knowledge not strictly professional." This keynote of basic
+research has been adhered to over the years in the issuance of
+thousands of titles in serial publications under the Smithsonian
+imprint, commencing with <i>Smithsonian Contributions to Knowledge</i> in
+1848 and continuing with the following active series:</p>
+
+<ul class="none center">
+<li><i>Smithsonian Annals of Flight</i></li>
+
+<li><i>Smithsonian Contributions to Anthropology</i></li>
+
+<li><i>Smithsonian Contributions to Astrophysics</i></li>
+
+<li><i>Smithsonian Contributions to Botany</i></li>
+
+<li><i>Smithsonian Contributions to the Earth Sciences</i></li>
+
+<li><i>Smithsonian Contributions to Paleobiology</i></li>
+
+<li><i>Smithsonian Contributions to Zoology</i></li>
+
+<li><i>Smithsonian Studies in History and Technology</i></li>
+</ul>
+
+<p>In these series, the Institution publishes original articles and
+monographs dealing with the research and collections of its several
+museums and offices and of professional colleagues at other
+institutions of learning. These papers report newly acquired facts,
+synoptic interpretations of data, or original theory in specialized
+fields. Each publication is distributed by mailing lists to libraries,
+laboratories, institutes, and interested specialists throughout the
+world. Individual copies may be obtained from the Smithsonian
+Institution Press as long as stocks are available.</p>
+
+<p class="right10"><span class="smcap">S. Dillon Ripley</span><br>
+  <i>Secretary</i><br>
+  Smithsonian Institution</p>
+
+<h1>The Wright Brothers' Engines<br>
+  And Their Design</h1>
+
+<a id="img001" name="img001"></a>
+<div class="figcenter4">
+<img src="images/img001.jpg" width="500" height="327" alt="" title="">
+<p>Kitty Hawk Flyer with original Wright engine poised on
+launching rail at Kill Devil Hill, near Kitty Hawk, North Carolina, 24
+November 1903, the month before the Wrights achieved man's first
+powered and controlled flight in a heavier-than-air craft.</p>
+</div>
+
+<a id="img002" name="img002"></a>
+<div class="figcenter">
+<img src="images/img002.jpg" width="400" height="436" alt="" title="">
+<p>Reproduction of the first engine, built by Pratt &amp;
+Whitney, as displayed in Wright Brothers National Memorial at Kitty
+Hawk. Engine is mounted in a reproduction of the Wrights' Flyer built
+by the National Capital Section of the Institute of the Aeronautical
+Sciences (now the American Institute of Aeronautics and Astronautics).
+Engine and plane were donated in 1963 to the National Park Service
+Cape Hatteras National Seashore.</p>
+</div>
+
+<p class="p4 center">SMITHSONIAN ANNALS OF FLIGHT * NUMBER 5</p>
+
+<p class="center">SMITHSONIAN INSTITUTION * NATIONAL AIR AND SPACE MUSEUM</p>
+
+<h1>The Wright Brothers' Engines<br>
+  And Their Design</h1>
+
+<p class="p2 center"><i>Leonard S. Hobbs</i></p>
+
+<a id="img003" name="img003"></a>
+<div class="figcenter4">
+<img src="images/img003.jpg" width="100" height="94" alt="" title="">
+</div>
+
+<p class="p4 center">SMITHSONIAN INSTITUTION PRESS<br>
+  CITY OF WASHINGTON<br>
+  1971</p>
+
+<p class="p4 center"><i>Smithsonian Annals of Flight</i></p>
+
+<p>Numbers 1-4 constitute volume one of <i>Smithsonian Annals of Flight</i>.
+Subsequent numbers will bear no volume designation, which has been
+dropped. The following earlier numbers of <i>Smithsonian Annals of
+Flight</i> are available from the Superintendent of Documents as
+indicated below:</p>
+
+<ul class="decimal">
+<li>The First Nonstop Coast-to-Coast Flight and the Historic T-2
+  Airplane, by Louis S. Casey. 1964. 90 pages, 43 figures, appendix,
+  bibliography. Out of print.</li>
+
+<li>The First Airplane Diesel Engine: Packard Model DR-980 of
+  1928, by Robert B. Meyer. 1964. 48 pages, 37 figures, appendix,
+  bibliography. Price 60˘.</li>
+
+<li>The Liberty Engine 1918-1942, by Philip S. Dickey. 1968.
+  110 pages, 20 figures, appendix, bibliography. Price 75˘.</li>
+
+<li>Aircraft Propulsion: A Review of the Evolution of Aircraft Piston
+  Engines, by C. Fayette Taylor. 1971 viii + 134 pages,
+  72 figures, appendix, bibliography of 601 items. Price $1.75.</li>
+</ul>
+
+<p class="p2 small center">For sale by Superintendent of Documents, Government Printing Office
+Washington, D.C. 20402&mdash;Price 60 cents</p>
+
+<h2><span class="pagenum"><a id="pagev" name="pagev"></a>(p. v)</span> Foreword</h2>
+
+<p>In this fifth number of <i>Smithsonian Annals of Flight</i> Leonard S.
+Hobbs analyzes the original Wright <i>Kitty Hawk Flyer</i> engine from the
+point of view of an aeronautical engineer whose long experience in the
+development of aircraft engines gives him unique insight into the
+problems confronting these remarkable brothers and the ingenious
+solutions they achieved. His review of these achievements also
+includes their later vertical 4-and 6-cylinder models designed and
+produced between 1903 and 1915.</p>
+
+<p>The career of Leonard S. (Luke) Hobbs spans the years that saw the
+maturing of the aircraft piston engine and then the transition from
+reciprocating power to the gas turbine engine. In 1920 he became a
+test engineer in the Power Plant Laboratory of the Army Air Service at
+McCook Field in Dayton, Ohio. There, and later as an engineer with the
+Stromberg Motor Devices Corporation, he specialized in aircraft engine
+carburetors and developed the basic float-type to the stage of utility
+where for the first time it provided normal operation during airplane
+evolutions, including inverted flight.</p>
+
+<p>Joining Pratt &amp; Whitney Aircraft in 1927 as Research Engineer, Hobbs
+advanced to engineering manager in 1935 and in 1939 took over complete
+direction of its engineering. He was named vice president for
+engineering for all of United Aircraft in 1944, and was elected vice
+chairman of United Aircraft in 1956, serving in that capacity until
+his retirement in 1958. He remained a member of the board of directors
+until 1968. Those years saw the final development of Pratt &amp; Whitney's
+extensive line of aircraft piston engines which were utilized by the
+United States and foreign air forces in large quantities and were
+prominent in the establishment of worldwide air transportation.</p>
+
+<p>In 1963 Hobbs was awarded the Collier Trophy for having directed the
+design and development of the J57 turbojet, the country's first such
+engine widely used in both military service and air transportation.</p>
+
+<p>He was an early fellow of the Institute of Aeronautical Sciences
+(later the American Institute of Aeronautics and Astronautics), served
+for many years on the Powerplant Committee of the National Advisory
+Committee for Aeronautics, and was the recipient of the Presidential
+Certificate of Merit.</p>
+
+<p class="right10"><span class="smcap">Frank A. Taylor</span>, <i>Acting Director</i><br>
+  <i>National Air and Space Museum</i></p>
+
+<p><i>March 1970</i></p>
+
+<h2><span class="pagenum"><a id="pagevii" name="pagevii"></a>(p. vii)</span> Contents</h2>
+
+<div class="toc">
+<ul class="none">
+<li>Foreword
+<span class="ralign10"><a href="#pagev">v</a></span></li>
+<li>Acknowledgments
+<span class="ralign10"><a href="#pageix">ix</a></span></li>
+<li>The Beginnings
+<span class="ralign10"><a href="#page1">1</a></span></li>
+<li>The Engine of the First Flight, 1903
+<span class="ralign10"><a href="#page9">9</a></span></li>
+<li>The Engines With Which They Mastered the Art of Flying
+<span class="ralign10"><a href="#page29">29</a></span></li>
+<li>The Four-Cylinder Vertical Demonstration Engine and the First
+      Production Engine
+<span class="ralign10"><a href="#page34">34</a></span></li>
+<li>The Eight-Cylinder Racing Engine
+<span class="ralign10"><a href="#page47">47</a></span></li>
+<li>The Six-Cylinder Vertical Engine
+<span class="ralign10"><a href="#page49">49</a></span></li>
+<li>Minor Design Details and Performance of the Wright Engines
+<span class="ralign10"><a href="#page57">57</a></span></li>
+<li>Appendix
+<span class="ralign10"><a href="#page62">62</a></span></li>
+<li class="add2em">Characteristics of the Wright Flight Engines
+<span class="ralign10"><a href="#page62">62</a></span></li>
+<li class="add2em">The Wright Shop Engine
+<span class="ralign10"><a href="#page64">64</a></span></li>
+<li>Bibliography
+<span class="ralign10"><a href="#page69">69</a></span></li>
+<li>Index
+<span class="ralign10"><a href="#page71">71</a></span></li>
+</ul>
+</div>
+
+<h2><span class="pagenum"><a id="pageix" name="pageix"></a>(p. ix)</span> Acknowledgments</h2>
+
+<p>As is probably usual with most notes such as this, however short,
+before completion the author becomes indebted to so many people that
+it is not practical to record all the acknowledgments that should be
+made. This I regret extremely, for I am most appreciative of the
+assistance of the many who responded to my every request. The mere
+mention of the Wright name automatically opened almost every door and
+brought forth complete cooperation. I do not believe that in the
+history of the country there has been another scientist or engineer as
+admired and revered as they are.</p>
+
+<p>I must, however, name a few who gave substantially of their time and
+effort and without whose help this work would not be as complete as it
+is. Gilmoure N. Cole, A. L. Rockwell, and the late L. Morgan Porter
+were major contributors, the latter having made the calculations of
+the shaking forces, the volumetric efficiency, and the connecting rod
+characteristics of the 1903 engine. Louis P. Christman, who was
+responsible for the Smithsonian drawings of this engine and also
+supervised the reconstruction of the 1905 Wright airplane, supplied
+much information, including a great deal of the history of the early
+engines. Opie Chenoweth, one of the early students of the subject, was
+of much assistance; and I am indebted to R. V. Kerley for the major
+part of the data on the Wrights' shop engine.</p>
+
+<p>Also, I must express my great appreciation to the many organizations
+that cooperated so fully, and to all the people of these organizations
+and institutions who gave their assistance so freely. These include
+the following:</p>
+
+<ul class="none">
+<li>Air Force Museum, Wright-Patterson Air Force Base, Ohio</li>
+<li>Carillon Park Museum, Dayton, Ohio</li>
+<li>Connecticut Aeronautical Historical Association, Hebron, Connecticut</li>
+<li>Fredrick C. Crawford Museum, Cleveland, Ohio</li>
+<li>Historical Department, Daimler Benz A. G., Stuttgart-Untertürkheim, West Germany</li>
+<li>Engineers Club, Dayton, Ohio</li>
+<li>Deutsches Museum, Munich, West Germany</li>
+<li>Educational and Musical Arts, Inc., Dayton, Ohio</li>
+<li>Henry Ford Museum, Dearborn, Michigan</li>
+<li>Franklin Institute, Philadelphia, Pennsylvania</li>
+<li><span class="pagenum"><a id="pagex" name="pagex"></a>(p. x)</span> Howell Cheney Technical School, Manchester, Connecticut</li>
+<li>Library of Congress, Washington, D.C.</li>
+<li>Naval Air Systems Command, U.S. Navy, Washington, D.C.</li>
+<li>Science Museum, London, England</li>
+<li>Victoria and Albert Museum, London, England</li>
+</ul>
+
+<p>In particular, very extensive contributions were made by the
+Smithsonian Institution and by the United Aircraft Corporation through
+its Library, through the Pratt &amp; Whitney Aircraft Division's entire
+Engineering Department and its Marketing and Product Support
+Departments, and through United Aircraft International.</p>
+
+<h2><span class="pagenum"><a id="page1" name="page1"></a>(p. 1)</span> The Beginnings</h2>
+
+
+<p>The general history of the flight engines used by the Wright Brothers
+is quite fascinating and fortunately rather well recorded.<a id="footnotetag1" name="footnotetag1"></a><a href="#footnote1" title="Go to footnote 1"><span class="smaller">[1]</span></a> The
+individual interested in obtaining a reasonably complete general story
+quickly is referred to three of the items listed in the short
+bibliography on page <a href="#page69">69</a>. The first, <i>The Papers of Wilbur and Orville
+Wright</i>, is a primary source edited by the authority on the Wright
+brothers, Marvin W. McFarland of the Library of Congress; a compact
+appendix to volume 2 of the <i>Papers</i> contains most of the essential
+facts. This source is supplemented by the paper of Baker<a id="footnotetag2" name="footnotetag2"></a><a href="#footnote2" title="Go to footnote 2"><span class="smaller">[2]</span></a> and the
+accompanying comments by Chenoweth, presented at the National
+Aeronautics Meeting of the Society of Automotive Engineers on 17 April
+1950. Aside from their excellence as history, these publications are
+outstanding for the manner in which those responsible demonstrate
+their competence and complete mastery of the sometimes complex
+technical part of the Wright story.</p>
+
+<p>The consuming interest of the Wrights, of course, was in flight as
+such, and in their thinking the required power unit was of only
+secondary importance. However, regardless of their feeling about it,
+the unit was an integral part of their objective and, due to the
+prevailing circumstances, they very early found themselves in the
+aircraft engine business despite their inexperience. This business was
+carried on very successfully, against increasingly severe competition,
+until Orville Wright withdrew from commercial activity and dissolved
+the Wright Company. The time span covered approximately the twelve
+years from 1903 to 1915, during the first five years of which they
+designed and built for their own use several engines of three
+different experimental and demonstration designs. In the latter part
+of the period, they manufactured and sold engines commercially, and
+during this time they marketed three models, one of which was
+basically their last demonstration design. A special racing engine was
+also built and flown <span class="pagenum"><a id="page2" name="page2"></a>(p. 2)</span> during this period. Accurate records are
+not available but altogether, they produced a total of something
+probably close to 200 engines of which they themselves took a small
+number for their various activities, including their school and flying
+exhibition work which at one time accounted for a very substantial
+part of their business. A similar lack of information concerning their
+competition, which expanded rapidly after the Wright's demonstrations,
+makes any comparisons a difficult task. The Wrights were meticulous
+about checking the actual performance of their engines but at that
+time ratings generally were seldom authenticated and even when
+different engines were tried in the same airplane the results usually
+were not measured with any accuracy or recorded with any permanency.
+There is evidence that the competition became effective enough to
+compel the complete redesign of their engine so that it was
+essentially a new model.</p>
+
+<p>For their initial experimentation the Wrights regarded gravity as not
+only their most reliable power source but also the one most economical
+and readily available, hence their concentration on gliding. They had
+correctly diagnosed the basic problem of flight to be that of control,
+the matter of the best wing shapes being inherently a simpler one
+which they would master by experiment, utilizing at first gravity and
+later a wind tunnel. Consequently, the acquisition of a powerplant
+intended for actual flight was considerably deferred.</p>
+
+<p>Nevertheless, they were continuously considering the power requirement
+and its problems. In his September 1901 lecture to the Western Society
+of Engineers, Wilbur Wright made two statements: "Men also know how to
+build engines and screws of sufficient lightness and power to drive
+these planes at sustaining speed"; and in conjunction with some
+figures he quoted of the required power and weight: "Such an engine is
+entirely practicable. Indeed, working motors of one-half this weight
+per horsepower [9 pounds per horsepower] have been constructed by
+several different builders." It is quite obvious that with their
+general knowledge and the experience they had acquired in designing
+and building a successful shop engine for their own use, they had no
+cause to doubt their ability to supply a suitable powerplant when the
+need arose. After the characteristics of the airframe had been
+settled, and the engine requirements delineated in rather detailed
+form, they had reached the point of decision on what they termed the
+motor problem. Only one major element had changed greatly since their
+previous consideration of the matter; they had arrived at the point
+where they not only needed a flight engine, they wanted it quickly.</p>
+
+<p>Nothing has been found that would indicate how much consideration they
+had given to forms of power for propulsion other than the choice they
+had apparently made quite early&mdash;the internal-combustion,
+four-stroke-cycle <span class="pagenum"><a id="page3" name="page3"></a>(p. 3)</span> piston engine. Undoubtedly, steam was
+dismissed without being given much, if any, thought. On the face of
+it, the system was quite impractical for the size and kind of machine
+they planned; but it had been chosen by Maxim for his experiments,<a id="footnotetag3" name="footnotetag3"></a><a href="#footnote3" title="Go to footnote 3"><span class="smaller">[3]</span></a>
+and some thirty-five or forty years later a serious effort to produce
+an aviation engine utilizing steam was initiated by Lockheed. On the
+other hand internal-combustion two-stroke-cycle piston engines had
+been built and used successfully in a limited way. And since, at that
+time, it was probably not recognized that the maximum quantity of heat
+it is possible to dissipate imposed an inherent limitation on the
+power output of the internal-combustion engine, the two-stroke-cycle
+may have appeared to offer a higher output from a given engine size
+than the four-stroke-cycle could produce. Certainly, it would have
+seemed to promise much less torque variation for the same output,
+something that was of great importance to the Wrights. Against this,
+the poor scavenging efficiency of the two-stroke operation, and most
+probably its concurrent poor fuel economy, were always evident; and,
+moreover, at that time the majority of operating engines were
+four-stroke-cycle. Whatever their reasoning, they selected for their
+first powered flight the exact form of prime mover that continued to
+power the airplane until the advent of the aircraft gas turbine more
+than forty years later.</p>
+
+<p>The indicated solution to their problem of obtaining the engine&mdash;and
+the engine that would seem by all odds most reliable&mdash;would have been
+to have a unit produced to their specifications by one of the best of
+the experienced engine builders, and to accomplish this, the most
+effective method would be to use the equivalent of a bid procedure.
+This they attempted, and sent out a letter of inquiry to a fairly
+large number of manufacturers. Although no copy of the letter is
+available, it is rather well established that it requested the price
+of an engine of certain limited specifications which would satisfy
+their flight requirements, but beyond this there is little in the
+record.</p>
+
+<p>A more thorough examination of the underlying fundamentals, however,
+discloses many weaknesses in the simple assumptions that made the
+choice of an experienced builder seem automatic. A maximum requirement
+limited to only one or two units offered little incentive to a
+manufacturer already successfully producing in his field, and the
+disadvantage of the limited quantity was only accentuated by the basic
+requirement for a technical performance in excess of any standard of
+the time. Certainly there was no promise of any future quantity
+business or any other substantial reward. Orville Wright many times
+stated that they had no desire to produce their own <span class="pagenum"><a id="page4" name="page4"></a>(p. 4)</span> engine,
+but it is doubtful that they had any real faith in the buying
+procedure, for they made no attempt to follow up their first inquiries
+or to expand the original list.</p>
+
+<p>Whatever the reasoning, their judgment of the situation is obvious;
+they spent no time awaiting results from the letter but almost
+immediately started on the task of designing and building the engine
+themselves. Perhaps the generalities were not as governing as the two
+specific factors whose immediate importance were determining: cost and
+time. The Wrights no doubt realized that a specially designed,
+relatively high performance engine in very limited hand-built
+quantities would not only be an expensive purchased article but would
+also take considerable time to build, even under the most favorable
+circumstances. So the lack of response to their first approach did not
+have too much to do with their ultimate decision to undertake this
+task themselves.</p>
+
+<p>The question of the cost of the Wrights' powerplants is most
+intriguing, as is that of their entire accomplishment. No detailed
+figures of actual engine costs are in the record, and it is somewhat
+difficult to imagine just how they managed to conduct an operation
+requiring so much effort and such material resources, given the income
+available from their fairly small bicycle business. The only evidence
+bearing on this is a statement that the maximum income from this
+business averaged $3,000 a year,<a id="footnotetag4" name="footnotetag4"></a><a href="#footnote4" title="Go to footnote 4"><span class="smaller">[4]</span></a> which of course had to cover not
+only the airplane and engine but all personal and other expenses. Yet
+they always had spare engines and spare parts available; they
+seemingly had no trouble acquiring needed materials and supplies, both
+simple and complex; and they apparently never were hindered at any
+time by lack of cash or credit. The only mention of any concern about
+money is a statement by Wilbur Wright in a letter of 20 May 1908 when,
+about to sail for France for the first public demonstrations, he
+wrote: "This plan would put it to the touch quickly and also help ward
+off an approaching financial stringency which has worried me very much
+for several months." It is a remarkable record in the economical use
+of money, considering all they had done up to that time. The myth that
+they had been aided by the earnings of their sister Katherine as a
+school teacher was demolished long ago.</p>
+
+<p>The decision to build the engine themselves added one more
+requirement, and possibly to some extent a restriction, to the design.
+They undoubtedly desired to machine as much of the engine as possible
+in their own shop, and the very limited equipment they had would
+affect the variety of features and constructions that could be
+utilized, although experienced <span class="pagenum"><a id="page5" name="page5"></a>(p. 5)</span> machine shops with
+sophisticated equipment were available in Dayton and it is obvious
+that the Wrights intended to, and did, utilize these when necessary.
+The use of their own equipment, of course, guaranteed that the parts
+they could handle themselves would be more expeditiously produced.
+They commenced work on the design and construction shortly before
+Christmas in 1902.</p>
+
+<p>The subject of drawings of the engine is interesting, not only as
+history but also because it presents several mysteries. Taylor<a id="footnotetag5" name="footnotetag5"></a><a href="#footnote5" title="Go to footnote 5"><span class="smaller">[5]</span></a>
+stated, "We didn't make any drawings. One of us would sketch out the
+part we were talking about on a piece of scrap paper ..." Obviously
+somewhere in the operation some dimensions were added, for the design
+in many places required quite accurate machining. Orville Wright's
+diary of 1904 has the entry, "Took old engine apart to get
+measurements for making new engine." Finally, no Wright drawings of
+the original engine have been seen by anyone connected with the
+history or with the Wright estate. In the estate were two drawings
+(now at the Franklin Institute), on heavy brown wrapping paper,
+relating to one of the two very similar later engines built in 1904;
+one is of a cylinder and connecting rod, the other is an end view of
+the engine. Thus even if the very ingenious drafting board now in the
+Wright Museum at Carillon Park was available at the time there is no
+indication that it was used to produce what could properly be called
+drawings of the first engine.</p>
+
+<p>There are in existence, however, two complete sets of drawings, both
+of which purport to represent the 1903 flight engine. One set was made
+in England for the Science Museum in the two years 1928 and 1939. The
+1928 drawings were made on receipt of the engine, which was not
+disassembled, but in 1939 the engine was removed from the airplane,
+disassembled, the original 1928 drawings were corrected and added to,
+and the whole was made into one very complete and usable set. The
+other set was prepared in Dayton, Ohio, for Educational and Musical
+Arts, Inc.,<a id="footnotetag6" name="footnotetag6"></a><a href="#footnote6" title="Go to footnote 6"><span class="smaller">[6]</span></a> and was donated to the Smithsonian Institution. This
+latter set was started under the direction of Orville Wright, who died
+shortly after the work had been commenced.</p>
+
+<p><span class="pagenum"><a id="page6" name="page6"></a>(p. 6)</span> The two sets of drawings, that is, the one of the Science
+Museum and that made in Dayton for the Smithsonian Institution, cannot
+be reconciled in the matter of details. Hardly any single dimension is
+exactly the same and essentially every part differs in some respect.
+Many of the forms of construction differ and even the firing order of
+the two engines is not the same, so that in effect the drawings show
+two different engines.</p>
+
+<a id="img004" name="img004"></a>
+<div class="figcenter">
+<img src="images/img004.jpg" width="500" height="388" alt="" title="">
+<p><i>Figure 1</i>.&mdash;First flight engine, 1903, valve side.
+(Photo courtesy Science Museum, London.)</p>
+</div>
+
+<p>The primary trouble is, of course, that the exact engine which flew in
+1903 is no longer in existence, and since no original drawings of it
+exist, there is considerable doubt about its details. The engine had
+its crankcase broken in an accident to the airframe (this was caused
+by a strong wind gust immediately following the last of the first
+series of flights at Kitty Hawk), and when it was brought back to
+Dayton it was for some inexplicable reason completely laid aside, even
+though it presumably contained many usable parts. When the engine was
+disassembled to obtain measurements for constructing the 1904 engines,
+again apparently no drawings were made. In February 1906 Orville
+Wright wrote that all the parts of the engine were still in existence
+except the crankcase; but shortly after <span class="pagenum"><a id="page7" name="page7"></a>(p. 7)</span> this the crankshaft
+and flywheel were loaned for exhibition purposes and were never
+recovered. In 1926 the engine was reassembled for an exhibition and in
+1928 it was again reassembled for shipment to England. The only parts
+of this particular engine whose complete history is definitely known
+are the crankshaft and flywheel, which were taken from the 1904-1905
+flight engine. This latter engine, now in the restored 1905 airplane
+in the Carillon Park Museum in Dayton, does not contain a crankshaft,
+and in its place incorporates a length of round bar stock.</p>
+
+<a id="img005" name="img005"></a>
+<div class="figcenter">
+<img src="images/img005.jpg" width="500" height="343" alt="" title="">
+<p><i>Figure 2</i>.&mdash;First flight engine, 1903, underside and
+flywheel end. (Photo courtesy Science Museum, London.)</p>
+</div>
+
+<p>In late 1947 work on the Educational and Musical Arts drawings was
+initiated under the direction of Louis P. Christman and carried
+through to completion by him. Christman has stated that Orville Wright
+was critical of the Science Museum drawings but just what he thought
+incorrect is not known. Whatever his reasons, he did encourage
+Christman to undertake the major task of duplication. Christman worked
+directly with Orville Wright for a period of six weeks and had access
+to all the records and parts the Wrights had preserved. The resultant
+drawings are also very complete and, regardless of the differences
+between these two primary sets, <span class="pagenum"><a id="page8" name="page8"></a>(p. 8)</span> both give a sufficiently
+accurate picture of the first engine for all purposes except that of
+exact reproduction in every detail.</p>
+
+<p>There exists a still unsolved puzzle in connection with what seems to
+be yet another set of drawings of the first engine. In December 1943,
+in writing to the Science Museum telling of his decision to have the
+airplane and engine brought back to the United States, Orville Wright
+stated, "I have complete and accurate drawings of the engine. I shall
+be glad to furnish them if you decide to make a replica."<a id="footnotetag7" name="footnotetag7"></a><a href="#footnote7" title="Go to footnote 7"><span class="smaller">[7]</span></a> No trace
+of these particular drawings can be found in any of the museums,
+institutions, or other repositories that normally should have acquired
+them and the executors of Orville Wright's estate have no record or
+knowledge of them. The date of his letter is four years before the
+Dayton drawings were commenced; and when Christman was working on
+these with Orville Wright they had copies of the Science Museum
+drawings, with complete knowledge of their origin, yet Christman has
+no knowledge of the drawings referred to in Orville's letter to the
+Museum. Finally, the evidence is quite conclusive that there were no
+reproducible or permanent drawings made at the time the first engine
+was constructed, and, of course, the reconstructed engine itself was
+sent to England in 1928 and not returned to this country until
+1948.<a id="footnotetag8" name="footnotetag8"></a><a href="#footnote8" title="Go to footnote 8"><span class="smaller">[8]</span></a></p>
+
+<h2><span class="pagenum"><a id="page9" name="page9"></a>(p. 9)</span> The Engine of the First Flight, 1903</h2>
+
+<p>In commencing the design of the first engine, the first important
+decision arrived at was that of the number and size of the cylinders
+to be employed and the form in which they would be combined, although
+it is unlikely that this presented any serious problem. In a similar
+situation Manly, when he was working on the engine for the Langley
+Aerodrome,<a id="footnotetag9" name="footnotetag9"></a><a href="#footnote9" title="Go to footnote 9"><span class="smaller">[9]</span></a> was somewhat perturbed because he did not have access to
+the most advanced technical knowledge, since the automobile people who
+were at that time the leaders in the development of the internal
+combustion engine, tended for competitive reasons to be rather
+secretive about their latest advancements and designs. But although
+the standard textbooks may not have been very helpful to him, there
+were available such volumes as W. Worby Beaumont's <i>Motor Vehicles and
+Motors</i> which contained in considerable detail descriptions and
+illustrations of the best of the current automobile engines. The
+situations of Manly and the Wrights differed, however, in that whereas
+the Wrights' objective was certainly a technical performance
+considerably above the existing average, Manly's goal was that of
+something so far beyond this average as to have been considered by
+many impossible. Importantly, the Wrights had their own experience
+with their shop engine and a good basic general knowledge of the size
+of engine that would be necessary to meet their requirements.</p>
+
+<p>Engine roughness was of primary concern to them. In the 1902
+description of the engine they sent to various manufacturers, they had
+stated: "... and the engine would be free from vibration." Even
+though their requirement for a smooth engine was much more urgent than
+merely to avoid the effect of roughness on the airplane frame, they
+were faced, before they made their first powered flight, with the
+basic problem with which the airplane has had to contend for over
+three-quarters of its present life span: that is, it was necessary to
+utilize an explosion engine in a structure which, because of weight
+limitations, had to be made the lightest and hence frailest that could
+possibly be devised and yet serve its primary purpose. However
+<span class="pagenum"><a id="page10" name="page10"></a>(p. 10)</span> great the difficulty may have appeared, in the long view, the
+fault was certainly a relatively minor one in the overall development
+of the internal combustion engine&mdash;that wonderful invention without
+which their life work would probably never have been so completely
+successful while they lived, and which, even aside from its
+partnership with the airplane, has so profoundly affected the nature
+of the world in which we live.</p>
+
+<p>It seems quite obvious that to the Wrights vibration, or roughness,
+was predominantly if not entirely caused by the explosion forces, and
+they were either not completely aware of the effects of the other
+vibratory forces or they chose to neglect them. Although crankshaft
+counterweights had been in use as far back as the middle 1800s, the
+Wrights never incorporated them in any of their engines; and despite
+the inherent shaking force in the 4-inline arrangement, they continued
+to use it for many years.</p>
+
+<p>The choice of four cylinders was obviously made in order to get, for
+smoothness, what in that day was "a lot of small cylinders"; and this
+was sound judgment. Furthermore, although the majority of automobiles
+at that time had engines with fewer than four cylinders, for those
+that did the inline form was standard and well proven, and, in fact,
+Daimler was then operating engines of this general design at powers
+several times the minimum the Wrights had determined necessary for
+their purpose.</p>
+
+<p>What fixed the exact cylinder size, that is, the "square" 4×4-in.
+form, is not recorded, nor is it obvious by supposition. Baker says it
+was for high displacement and low weight, but these qualities are also
+greatly affected by many other factors. The total displacement of just
+over 200 cu in. was on the generous side, given the horsepower they
+had determined was necessary, but here again the Wrights were
+undoubtedly making the conservative allowances afterwards proven
+habitual, to be justified later by greatly increased power
+requirements and corresponding outputs. The Mean Effective Pressure
+(MEP), based on their indicated goal of 8 hp, would be a very modest
+36 psi at the speed of 870 rpm at which they first tested the engine,
+and only 31 psi at the reasonably conservative speed of 1000 rpm. The
+4×4-in. dimension would provide a cylinder large enough so that the
+engine was not penalized in the matter of weight and yet small enough
+to essentially guarantee its successful operation, as cylinders of
+considerably larger bore were being utilized in automobiles. That
+their original choice was an excellent one is rather well supported by
+the fact that in all the different models and sizes of engines they
+eventually designed and built, they never found it necessary to go to
+cylinders very much larger than this.</p>
+
+<a id="img006" name="img006"></a>
+<div class="figcenter">
+<img src="images/img006.jpg" width="500" height="394" alt="" title="">
+<p><i>Figure 3.</i>&mdash;First flight engine, 1903, installed in
+the Kitty Hawk airplane, as exhibited in the Science Museum. (Photo
+courtesy the Science Museum, London.)</p>
+</div>
+
+<p>A second basic determination which was made either concurrently or
+even possibly in advance of that of the general form and size was in
+the matter of the type of cylinder cooling to adopt. Based on current
+practice <span class="pagenum"><a id="page11" name="page11"></a>(p. 11)</span> that had proven practical, there were three
+possibilities, all of which were in use in automobiles: air, water, or
+a combination of the two. It is an interesting commentary that Fernand
+Forest's<a id="footnotetag10" name="footnotetag10"></a><a href="#footnote10" title="Go to footnote 10"><span class="smaller">[10]</span></a> proposed 32-cylinder aircraft engine of 1888 was to be
+air-cooled, that Santos-Dumont utilized an air-cooled Clement engine
+in his dirigible flights of 1903, and that the Wrights had chosen air
+cooling for their shop engine. With the promise of simplicity and
+elimination of the radiator, water and piping, it would seem, offhand,
+that this would be the Wrights' choice for their airplane; but they
+were probably governed by the fact that not only was the water-cooled
+type predominant in automobile practice, but that the units giving the
+best and highest performance in general service were all water cooled.
+In their subsequent practice they never departed from this original
+decision, although <span class="pagenum"><a id="page12" name="page12"></a>(p. 12)</span> Wilbur Wright's notebook of 1904-1907
+contains an undated weight estimate by detailed parts for an
+8-cylinder air-cooled engine. Unfortunately, the proposed power output
+is not recorded, so their conception of the relative weight of the
+air-cooled form is not disclosed.</p>
+
+<p>One of the most important decisions relating to the powerplant&mdash;one
+which was probably made long before they became committed to the
+design itself&mdash;was a determination of the method of transmission of
+power to the propeller, or propellers. A lingering impression exists
+that the utilization of a chain drive for this purpose was a natural
+inheritance from their bicycle background. No doubt this experience
+greatly simplified the task of adaptation but a merely cursory
+examination shows that even if they had never had any connection with
+bicycles, the chain drive was a logical solution, considering every
+important element of the problem. The vast majority of automobiles of
+the time were chain driven, and chains and sprockets capable of
+handling a wide range of power were completely developed and
+available. Further, at that time they had no accurate knowledge of
+desirable or limiting propeller and engine speeds. The chain drive
+offered a very simple and inexpensive method of providing for a
+completely flexible range of speed ratios. The other two possibilities
+were both undesirable: the first, a simple direct-driven single
+propeller connected to the crankshaft, provided essentially no
+flexibility whatsoever in experimentally varying engine or propeller
+speed ratios, it added an out-of-balance engine torque force to the
+problem of airplane control, and, finally, it dictated that the pilot
+would be in the propeller slipstream or the airflow to it; the second,
+drive shafts and gearing for dual propellers, would have been very
+heavy and expensive, and most probably would have required a long-time
+development, with every experimental change in speed ratios requiring
+a complete change in gears. Again, their original choice was so
+correct that it lasted them through essentially all their active
+flying years.</p>
+
+<p>The very substantial advantages of the chain drive were not, however,
+obtained at no cost. Torque variations in the engine would tend to
+cause a whipping action in the chain, so that it was vulnerable to
+rough running caused by misfiring cylinders and, with the right timing
+and magnitude of normal regular variations, the action could result in
+destructive forces in the transmission system. This was the basic
+reason for the Wrights' great fear of "engine vibration," which
+confined them to the use of small cylinders and made a fairly heavy
+flywheel necessary on all their engines. When they were requested to
+install an Austro-Daimler engine in one of their airplanes, they
+designed a flexible coupling which was interposed between the engine
+and the propeller drive and this was considered so successful that it
+was applied to the flywheel of some engines of their last <span class="pagenum"><a id="page13" name="page13"></a>(p. 13)</span>
+model, the 6-70, "which had been giving trouble in this regard."<a id="footnotetag11" name="footnotetag11"></a><a href="#footnote11" title="Go to footnote 11"><span class="smaller">[11]</span></a></p>
+
+<p>Although flat, angled, and vertical engines had all been operated
+successfully, the best and most modern automotive engines of the time
+were vertical, so their choice of a horizontal position was probably
+dictated either by considerations of drag or their desire to provide a
+sizable mounting base for the engine, or both. There is no record of
+their ever having investigated the matter of the drag of the engine,
+either alone or in combination with the wing. The merit of a vertical
+versus a horizontal position of the engine was not analogous to that
+of the pilot, which they had studied, and where the prone position
+undoubtedly reduced the resistance.</p>
+
+<p>Having decided on the general makeup of their engine, the next major
+decision was that of just what form the principal parts should take,
+the most important of these being the cylinders and crankcase. Even at
+this fairly early date in the history of the internal combustion
+engine various successful arrangements and combinations were in
+existence. Individual cylinder construction was by far the most used,
+quite probably due to its case of manufacture and adaptability to
+change. Since 4-cylinder engines were just coming into general use (a
+few production engines of this type had been utilized as early as
+1898), there were few examples of en-bloc or one-piece construction.
+The original German Daimler Company undoubtedly was at this time the
+leader in the development of high-output internal-combustion engines,
+and in 1902, as an example of what was possible, had placed in service
+one that possibly approximated 40 hp, which was an MEP of 70 psi.
+(Almost without exception, quoted power figures of this period were
+not demonstrated quantities but were based on a formula, of which the
+only two factors were displacement and rpm.) The cylinders of this
+Daimler engine were cast iron, the cylinder barrel, head, and water
+jacket being cast in one piece. The upper part of the barrel and the
+cylinder head were jacketed, but, surprisingly, the bottom 60 percent
+of the barrel had no cooling. The cylinders were cast in pairs and
+bolted to a two-piece aluminum case split at the line of the
+crankshaft. Ignition was make-and-break and the inlet valves were
+mechanically actuated. Displacement was 413 cu in. and the rpm was
+1050.</p>
+
+<p>Although a few examples of integral crankcase and water jacket
+combinations were in use, the Wrights were being somewhat radical when
+they decided to incorporate all four cylinders in the one-piece
+construction, particularly since they also proposed to include the
+entire crankcase and not just one part of it. It was undoubtedly the
+most important decision that they were required to make on all the
+various construction details, and <span class="pagenum"><a id="page14" name="page14"></a>(p. 14)</span> probably the one given the
+most study and investigation. Many factors were involved, but
+fundamentally everything went back to their three basic requirements:
+suitability, time, and cost. There was no obvious reason why the
+construction would not work, and it eliminated a very large number of
+individual parts and the required time for procuring, machining, and
+joining them. Probably one very strong argument was the advanced state
+of the casting art, one of the oldest of the mechanical arts in
+existence and one the Wrights used in many places, even though other
+processes were available. What no doubt weighed heavily was that
+Dayton had some first-class foundries. The casting, though intricate
+and not machinable in their own shop, could be easily handled in one
+that was well outfitted. The pattern was fairly complex but apparently
+not enough to delay the project or cause excessive cost.</p>
+
+<a id="img007" name="img007"></a>
+<div class="figcenter">
+<img src="images/img007.jpg" width="500" height="356" alt="" title="">
+<p><i>Figure 4.</i>&mdash;First flight engine, 1903, left side and
+rear views, with dimensions. (Drawing courtesy Howell Cheney Technical
+School.)</p>
+<p>LEFT SIDE VIEW.</p>
+</div>
+
+<a id="img008" name="img008"></a>
+<div class="figcenter">
+<img src="images/img008.jpg" width="500" height="496" alt="" title="">
+<p>REAR VIEW</p>
+</div>
+
+<p><span class="pagenum"><a id="page15" name="page15"></a>(p. 15)</span> The selection of aluminum for the material was an integral
+part of the basic design decision. Despite the excellence and accuracy
+of the castings that could be obtained, there was nevertheless a
+minimum dimension beyond which wall thickness could not be reduced;
+and the use of either one of the two other proven materials, cast iron
+or bronze, would have made the body, as they called it, prohibitively
+heavy. The use of aluminum was not entirely novel at this time, as it
+had been utilized in many automobile engine parts, particularly
+crankcases; but its incorporation in this rather uncommon combination
+represented a bold step. There was no choice in the matter of the
+alloy to be used, the only proven one available was an 8 percent
+copper 92 percent aluminum combination.</p>
+
+<p>By means of the proper webs, brackets and bosses, the crankcase would
+also carry the crankshaft, the rocker arms and bearings, and the
+intake <span class="pagenum"><a id="page16" name="page16"></a>(p. 16)</span> manifold. The open section of the case at the top was
+covered with a screw-fastened thin sheet of cold-rolled steel. The
+main bearing bosses were split at a 45° angle for ease of assembly.
+The engine support and fastening were provided by four feet, or lugs,
+cast integral on the bottom corners of the case, and by accompanying
+bolts (Figure <a href="#img005">2</a>). Although the crankcase continued to be pretty much
+the "body" of the internal combustion aircraft engine throughout its
+life, the Wrights managed to incorporate in this original part a major
+portion of the overall engine, and certainly far more than had ever
+previously been included.</p>
+
+<p>The design of the cylinder barrel presented fairly simple problems
+involving not much more than those of keeping the sections as thin as
+possible and devising means of fastening it and of keeping the water
+jacket tight. They saved considerable weight by making the barrel
+quite short, so that in operation a large part of the piston extended
+below the bottom of it; but this could be accepted, as there were no
+rings below the piston pin (Figure <a href="#img010">6</a>). The barrel material, a good
+grade of cast iron, was an almost automatic choice. In connection with
+these seemingly predetermined decisions, however, it should be
+remembered that their goal was an engine which would work without
+long-time development, and that, with no previous experience in
+lightweight construction to guide them they were nevertheless
+compelled to meet a weight limit, so that the thickness of every wall
+and flange and the length of every thread was important.</p>
+
+<p>With the separate cylinder barrel they were now almost committed to a
+three-piece cylinder. It would have been possible to combine the
+barrel and head in a one-piece casting and then devise a method of
+attachment, but this would have been more complex and certainly
+heavier. For housing the valves, what was in effect a separate
+cylindrical, or tubular, box was decided upon. This would lie across
+the top of the cylinder proper at right angles to the cylinder axis,
+and the two valves would be carried in the two ends of this box. The
+cylinder barrel would be brought in at its head end to form a portion
+of the cylinder head and then extended along its axis in the form of a
+fairly large boss, a mating boss being provided on one side of the
+valve box. The cylinder barrel would then be threaded into the valve
+box and the whole tightened or fastened to the crankcase by means of
+two sets of threads, one at each end of the barrel proper. This meant
+that three joints had to be made tight with only two sets of threads.
+This was accomplished by accurate machining and possibly even hand
+fitting in combination with a rather thick gasket at the head end, one
+flat of which bore against two different surfaces. This can be seen in
+Figure <a href="#img010">6</a>, where the circular flange on the valve box contacts both the
+crankcase and the cylinder barrel. Altogether it was a simple, light,
+and ingenious solution to a rather complex problem.</p>
+
+<p><span class="pagenum"><a id="page17" name="page17"></a>(p. 17)</span> At this point the question arises: Why was the engine layout
+such that the exhaust took place close to the operator's ears? It
+would have been possible, starting with the original design, to turn
+the engine around so that the exhaust was on the other side. This
+would have little effect on the location of the center of gravity, and
+the two main drive chains would then have been of more equal length.
+However, of the many factors involved, probably one of the principal
+considerations in arriving at their final decision was the location of
+the spark-advance control, which was in effect the only control they
+had of engine output, except for complete shutoff. In their design
+this was immediately adjacent to the operator; with a turned-around
+engine, an extension control mechanism of some sort would have been
+required. The noise of the exhaust apparently became of some concern
+to them, as Orville's diary in early 1904 contains an entry with a
+sketch labeled "Design for Muffler for Engine," but there is no
+further comment.</p>
+
+<p>The problem of keeping joints tight, and for that matter the entire
+construction itself, were both greatly simplified by their decision to
+water-jacket only a part of the cylinder head proper, and the valve
+box not at all. This was undoubtedly the correct decision for their
+immediate purpose, as again they were effecting savings in time, cost,
+complexity, and weight. There is nothing in the record, however, to
+show why they continued this practice long after they had advanced to
+much greater power outputs and longer flight times. Their own
+statements show that they were well aware of the effect of the very
+hot cylinder head on power output and they must also have realized its
+influence on exhaust-valve temperature.</p>
+
+<p>The cylinder assembly was made somewhat more complicated by their
+desire to oil the piston and cylinder by means of holes near the
+crankshaft end in what was, with the engine in the horizontal
+position, the upper side of the cylinder barrel. This complication was
+no doubt taken care of by not drilling the holes until a tight
+assembly had been made by screwing the barrel into place, and by
+marking the desired location on the barrel. Since this position was
+determined by a metal-to-metal jam fit of the crankcase and cylinder
+barrel flange, the barrel would reassemble with the holes in very
+nearly the same relative position after disassembly.</p>
+
+<p>With the valve box, or housing, cylindrical, the task of locking and
+fastening the intake and exhaust valve guides and seats in place was
+easy. The guide was made integral with and in the center of one end of
+a circular cage, the other end of which contained the valve seat (see
+Figure <a href="#img009">5</a>). Four sections were cut out of the circular wall of the cage
+so that in effect the seat and guide were joined by four narrow legs,
+the spaces between which provided passages for the flow of the
+cylinder gases. These cages were then dropped into the ends of the
+valve boxes until they came up <span class="pagenum"><a id="page18" name="page18"></a>(p. 18)</span> against machined shoulders and
+were held in place by internal ring nuts screwed into the valve box.
+The intake manifold or passage was placed over the intake valves so
+that the intake charge flowed directly into and through the valve cage
+around the open valve and into the cylinder. The exhaust gas, after
+flowing through the passages in the valve cage, was discharged
+directly to the atmosphere through a series of holes machined in one
+side of the valve box.</p>
+
+<a id="img009" name="img009"></a>
+<div class="figcenter">
+<a href="images/img009.jpg">
+<img src="images/img009tb.jpg" width="500" height="446" alt="" title=""></a>
+<p><i>Figure 5.</i>&mdash;First flight engine, 1903, assembly.
+(Phantom cutaway by J. H. Clark, with key, courtesy <i>Aeroplane</i>.)</p>
+</div>
+
+<div class="key">
+<p class="center">KEY</p>
+
+<ul class="none">
+<li>1 and 2. Bearing caps in one piece with plate 3.</li>
+
+<li>3. Plated screwed over hole 4 in crankcase end.</li>
+
+<li>4. Key-shaped hole as hole 5 in intermediate ribs.</li>
+
+<li>6. Inter-bearings cap (white-metal lined) and screwed to
+     inter-rib halves 7.</li>
+
+<li>8. Splash-drip feed to bearings.</li>
+
+<li>9. Return to pump from each compartment of crankcase base
+     ("sump") via gallery 10 and pipe to pump 11 underneath jacket.</li>
+
+<li>12. Oil feed from pump via rubber tube 13.</li>
+
+<li>13. Drip feeds to cylinders and pistons.</li>
+
+<li>14. Gear drive to pump.</li>
+
+<li>15. Big-end nuts, lock-strip, and shims.</li>
+
+<li>16. Gudgeon-pin lock.</li>
+
+<li>17. Piston-ring retainer pegs.</li>
+
+<li>18. Cylinder liner screwing into jacket.</li>
+
+<li>19. Open-ended "can" admits air.</li>
+
+<li>20. Fuel supply.</li>
+
+<li>21. (Hot) side of water jacket makes surface carburetter.</li>
+
+<li>22. Sparking plug (comprising positive electrode 23 and
+     spark-producing make-and-break 24).</li>
+
+<li>25. Lever attached to lever 26 via bearing 27 screwed into
+     chamber neck 28.</li>
+
+<li>26. Levers with mainspring 29 and inter-spring 30, and rocked by
+     "cam" 31.</li>
+
+<li>31. Cam with another alongside (for adjacent cylinder).</li>
+
+<li>32. Positive busbar feed to all four cylinders.</li>
+
+<li>33. Assembly retaining-rings.</li>
+
+<li>34. Sealing disc.</li>
+
+<li>35. Exhaust outlet ports.</li>
+
+<li>36. Camshaft right along on underside of jacket and also driving
+     oil pump 11 via 14.</li>
+
+<li>37. Spring-loaded sliding pinion drives make-and-break shaft 38
+     through peg in inclined slot 39.</li>
+
+<li>40. Cam to push pinion 37 along and so alter its angular relation
+     with shaft 38 (to vary timing).</li>
+
+<li>41. Exhaust-valve cams bear on rollers 42 mounted in end of
+     rocker-arms 43.</li>
+
+<li>44. Generator floating coils.</li>
+
+<li>45. Friction-drive off flywheel.</li>
+
+<li>46. Sight-feed lubricator (on stationary sleeve).</li>
+
+<li>47. Hardwood chain tensioner.</li>
+</ul>
+</div>
+
+<p>The intake and exhaust valves were identical and of two-piece
+construction, with the stems screwed tightly into and through the
+heads and the protruding ends then peened over. This construction was
+not novel, having had much usage behind it, and it continued for a
+long time in both automobile and aircraft practice. One-piece cast
+and forged valves were available <span class="pagenum"><a id="page19" name="page19"></a>(p. 19)</span> but here again it was a
+choice of the quick, cheap, and proven answer.</p>
+
+<p>The entire valve system, including guides and seats, was of cast iron,
+a favorite material of the Wrights, except for the valve stems, which
+were, at different times, of various carbon steels. Ordinary
+cold-rolled apparently was used in those of the original engine, but
+in later engines this was changed to a high-carbon steel.</p>
+
+<p>The piston design presented no difficulty. In some measure this was
+due to the remarkable similarity that seems to have existed among all
+the different engines of the time in the construction of this
+particular part, for, although there were some major variations, it
+was, in fact, almost as if some standard had been adopted. Pistons all
+were of cast iron and comparatively quite long (it was a number of
+years before they evolved into the short ones of modern practice);
+they were almost invariably equipped with three wide piston rings
+between the piston pin and the head; and, although <span class="pagenum"><a id="page20" name="page20"></a>(p. 20)</span> there were
+in existence a few pistons with four rings, no oil wiper or other ring
+seems to have been placed below the piston pin. The Wrights' piston
+was typical of the time, with the rings pinned in the grooves to
+prevent turning and the piston pin locked in the piston with a
+setscrew. In designing this first engine they were, however,
+apparently somewhat unsure about this latter feature, as they provided
+the rod with a split little end and a clamping bolt (see Figure <a href="#img010">6</a>), so
+that the pin could be held in the rod if desired; but no examples of
+this use have been encountered.</p>
+
+<p>The Wrights' selection of an "automatic" or suction-operated inlet
+valve was entirely logical. Mechanically operated inlet valves were in
+use and their history went back many years, but the great majority of
+the engines of that time still had the automatic type, and with this
+construction one complete set of valve-operating mechanisms was
+eliminated. They were well aware of the loss of volumetric efficiency
+inherent in this valve, and apparently went to some pains to obtain
+from it the best performance possible. Speaking of the first engine,
+Orville Wright wrote, "Since putting in heavier springs to actuate the
+valves on our engine we have increased its power to nearly 16 hp and
+at the same time reduced the amount of gasoline consumed per hour to
+about one-half of what it was."<a id="footnotetag12" name="footnotetag12"></a><a href="#footnote12" title="Go to footnote 12"><span class="smaller">[12]</span></a></p>
+
+<p>Why they continued with this form on their later engines is a question
+a little more difficult to answer, as they were then seeking more and
+more power and were building larger engines. The advantages of
+simplicity and a reduced number of parts still existed, but there also
+was a sizable power increase to be had which possibly would have more
+than balanced off the increased cost and weight. They did not utilize
+mechanical operation until after a major redesign of their last engine
+model. Very possibly the answer lies in the phenomenon of fuel
+detonation. This was only beginning to be understood in the late
+1920s, and it is quite evident from their writings that they had
+little knowledge of what made a good fuel in this respect. It is
+fairly certain, however, that they did know of the existence of
+cylinder "knock," or detonation, and particularly that the compression
+ratio had a major effect on it. The ratios they utilized on their
+different engines varied considerably, ranging from what, for that
+time, was medium to what was relatively high. The original flight
+engine had a compression ratio of 4.4:1. The last of their service
+engines had a compression ratio about twenty percent under that of the
+previous series&mdash;a clear indication that they considered that they had
+previously gone too high. Quite possibly they concluded <span class="pagenum"><a id="page21" name="page21"></a>(p. 21)</span> that
+increasing the amount of the cylinder charge seemed to bring on
+detonation, and that the complication of the mechanical inlet valve
+was therefore not warranted.</p>
+
+<a id="img010" name="img010"></a>
+<div class="figcenter">
+<a href="images/img010.jpg">
+<img src="images/img010tb.jpg" width="500" height="358" alt="" title=""></a>
+<p><i>Figure 6.</i>&mdash;First flight engine, 1903, cross section.
+(Drawing courtesy Science Museum, London.)</p>
+</div>
+
+<p>The camshaft for the exhaust valves (101, Figure <a href="#img010">6</a>), was chain driven
+from the crankshaft and was carried along the bottom of the crankcase
+in three babbit-lined bearings in bearing boxes or lugs cast integral
+with the case. Both the driving chain and the sprockets were standard
+bicycle parts, and a number of bicycle thread standards and other
+items of bicycle practice were incorporated in several places in the
+engine, easing their construction task. The shaft itself, of mild
+carbon steel, was hollow and on each side of an end bearing sweated-on
+washers provided shoulders to locate it longitudinally. Its location
+adjacent to the valves, with the cam operating directly on the rocker
+arm, eliminated push rods and attendant parts, a <span class="pagenum"><a id="page22" name="page22"></a>(p. 22)</span> major
+economy. The cams were machined as separate parts and then sweated
+onto the shaft. Their shape shows the principal concern in the design
+to have been obtaining maximum valve capacity&mdash;that is, a quite rapid
+opening with a long dwell. This apparent desire to get rid of the
+exhaust gas quickly is manifested again in the alacrity with which
+they adopted a piston-controlled exhaust port immediately they had
+really mastered flight and were contemplating more powerful and more
+durable engines. This maximum-capacity theory of valve operation, with
+its neglect of acceleration forces and seating velocities, may well
+have been at least partially if not largely the cause of their
+exhaust-valve troubles and the seemingly disproportionate amount of
+development they devoted to this part, as reported by Chenoweth,
+although it is also true that the exhaust valve continued to present a
+problem in the aircraft piston engine for a great many years after,
+even with the most scientific of cam designs.</p>
+
+<p>The rocker arm (102, Figure <a href="#img010">6</a>) is probably the best example of a small
+part which met all of their many specific requirements with an extreme
+of simplicity. It consisted of two identical side pieces, or walls, of
+sheet steel shaped to the desired side contour of the assembly, in
+which were drilled three holes, one in each end, to carry the roller
+axles, and the third in the approximate middle for the rocker axle
+shaft proper. This consisted of a piece of solid rod positioned by
+cotter pins in each end outside the side walls (see Figure <a href="#img009">5</a>). The
+assembly was made by riveting over the ends of the roller axles so
+that the walls were held tightly against the shoulders on the axles,
+thus providing the correct clearance for the rollers. The construction
+was so light and serviceable that it was essentially carried over to
+the last engine the Wrights ever built.</p>
+
+<p>The basic intake manifold (see Figure <a href="#img009">5</a>) consisted of a very low flat
+box of sheet steel which ran across the tops of the valve boxes and
+was directly connected to the top of each of them so that the cages,
+and thus the valves, were open to the interior of the manifold.
+Through an opening in the side toward the engine the manifold was
+connected to a flat induction chamber (21, Figure <a href="#img009">5</a>) which served to
+vaporize the fuel and mix it with the incoming air. This chamber was
+formed by screw-fastening a piece of sheet steel to vertical ribs cast
+integral with the crankcase, the crankcase wall itself thus forming
+the bottom of the chamber. A beaded sheet-steel cylinder resembling a
+can (73, Figure <a href="#img010">6</a>) but open at both ends was fastened upright to the
+top of this chamber. In the absence of anything else, this can could
+be called the carburetor, as a fuel supply line entered the cylinder
+near the top and discharged the fuel into the incoming air stream,
+both the fuel and air then going directly into the mixing chamber. The
+can was attached near one corner of the chamber, and vertical baffles,
+also cast integral with the case, were so located that the incoming
+mixture was <span class="pagenum"><a id="page24" name="page24"></a>(p. 24)</span> forced to circulate over the entire area of
+exposed crankcase inside the chamber before it reached the outlet to
+the manifold proper, the hot surface vaporizing that part of the fuel
+still liquid.</p>
+
+<a id="img011" name="img011"></a>
+<div class="figcenter">
+<img src="images/img011.jpg" width="300" height="236" alt="" title="">
+<p><i>Figure 7.</i>&mdash;First flight engine, 1903: cylinder, valve
+box, and gear mechanism; below, miscellaneous parts. (Photos courtesy
+Science Museum, London, and Louis P. Christman.)</p>
+</div>
+
+<a id="img012" name="img012"></a>
+<div class="figcenter">
+<img src="images/img012.jpg" width="400" height="251" alt="" title="">
+</div>
+
+<p>Fuel was gravity fed to the can through copper and rubber tubing from
+a tank fastened to a strut, several feet above the engine. Of the two
+valves placed in the fuel line, one was a simple on-off shutoff cock
+and the other a type whose opening could be regulated. The latter was
+adjusted to supply the correct amount of fuel under the desired flight
+operating condition; the shutoff cock was used for starting and
+stopping. The rate of fuel supply to the engine would decrease as the
+level in the fuel tank dropped, but as the head being utilized was a
+matter of several feet and the height of the supply tank a matter of
+inches, the fuel-air ratio was still maintained well within the range
+that would ignite and burn properly in the contemplated one-power
+condition of their flight operation.</p>
+
+<p>This arrangement is one of the best of the many illustrations of how
+by the use of foresight and ingenuity the Wrights met the challenge of
+a complex requirement with a simple device, for while carburetors were
+not in the perfected stage later attained, quite good ones that would
+both control power output and supply a fairly constant fuel-air
+mixture over a range of operating conditions were available, but they
+were complex, heavy, and expensive. The arrangement, moreover, secured
+at no cost a good vaporizer, or modern "hot spot." In their subsequent
+engines they took the control of the fuel metering away from the
+regulating valve and gravity tank combination and substituted an
+engine-driven fuel pump which provided a fuel supply bearing a fairly
+close relationship to engine speed.</p>
+
+<p>The reasons behind selection of the type of ignition used, and the
+considerations entering into the decision, are open to speculation, as
+are those concerning many other elements that eventually made up the
+engine. Both the high-tension spark plug and low-tension
+make-and-break systems had been in wide use for many years, with the
+latter constituting the majority in 1902. Both were serviceable and
+therefore acceptable, and both required a "magneto". The art of the
+spark plug was in a sense esoteric (to a certain extent it so remains
+to this day), but the spark-plug system did involve a much simpler
+combination of parts: in addition to the plug and magneto there would
+be needed only a timer, or distributor, together with coils and
+points, or some substitute arrangement. The make-and-break system, on
+the other hand, required for each cylinder what was physically the
+equivalent of a spark plug, that is, a moving arm and contact point
+inside the cylinder, a spring-loaded snap mechanism to break the
+contact outside the cylinder, and a camshaft and cams to actuate the
+breaker mechanism at the proper time. Furthermore, as the Wrights
+applied it, the system required dry cells and a coil for starting,
+although these did not accompany the engine in flight. And finally
+there was the problem of keeping tight the joint where <span class="pagenum"><a id="page25" name="page25"></a>(p. 25)</span> the
+oscillating shaft required to operate the moving point in the spark
+plug entered the cylinder.</p>
+
+<p>This is one of the few occasions, if not the only one, when the
+Wrights chose the more complex solution in connection with a major
+part&mdash;in this particular case, one with far more bits and pieces.
+However, it did carry with it some quite major advantages. The common
+spark plug, always subject to fouling or failure to function because
+of a decreased gap, was not very reliable over a lengthy period, and
+was undoubtedly much more so in those days when control of the amount
+of oil inside the cylinder was not at all exact. Make-and-break
+points, on the other hand, were unaffected by excess oil in the
+cylinder. Because of this resistance to fouling, the system was
+particularly suitable for use with the compression-release method of
+power control which they later utilized, although they probably could
+not have been looking that far ahead at the time they chose it.
+High-tension current has always, and rightfully so, been thought of as
+a troublemaker in service; in Beaumont's 1900 edition of <i>Motor
+Vehicles and Motors</i>, which seems to have been technically the best
+volume of its time, the editor predicted that low-tension
+make-and-break ignition would ultimately supersede all other methods.
+And finally, the large number of small parts required for the
+make-and-break system could all be made in the Wright Brothers' shop
+or easily procured, and in the end this was probably the factor, plus
+reliability, that determined the decision which, all things
+considered, was the correct one.</p>
+
+<p>There was nothing exceptional about the exact form the Wrights
+devised. It displayed the usual refined simplicity (the cams were made
+of a single small piece of strip steel bent to shape and clamped to
+the ignition camshaft with a simple self-locking screw), and
+lightness. The ignition camshaft (38, Figure <a href="#img009">5</a>), a piece of
+small-diameter bar stock, was located on the same side as the exhaust
+valve camshaft, approximately midway between it and the valve boxes,
+and was operated by the exhaust camshaft through spur gearing. That
+the Wrights were thinking of something beyond mere hops or short
+flights is shown by the fact that the ignition points were
+platinum-faced, whereas even soft iron would have been satisfactory
+for the duration of all their flying for many years.</p>
+
+<p>The control of the spark timing was effected by advancing or retarding
+the ignition camshaft in relation to the exhaust valve camshaft. The
+spur gear (37, Figure <a href="#img009">5</a>) driving the ignition camshaft had its hub on
+one side extended out to provide what was in effect a sleeve around
+the camshaft integral with the gear. The gear and integral sleeve were
+slidable on the shaft and the sleeve at one place (39, Figure <a href="#img009">5</a>) was
+completely slotted through to the shaft at an angle of 45° to the
+longitudinal axis of the shaft. The shaft was driven by a pin tightly
+fitted in it and extending into the slot. The fore-and-aft position of
+the sleeve on the shaft was determined <span class="pagenum"><a id="page26" name="page26"></a>(p. 26)</span> by a lever-operated
+cam (40, Figure <a href="#img009">5</a>) on one side and a spring on the other. The movement
+of the sleeve along the shaft would cause the shaft to rotate in
+relation to it because of the angle of the slot, thus providing the
+desired variation in timing of the spark. The "magneto" was a
+purchased item driven by means of a friction wheel contacting the
+flywheel, and several different makes were used later, but the
+original is indicated to have been a Miller-Knoblock (see Figure <a href="#img009">5</a>).</p>
+
+<p>The connecting rod is another example of how, seemingly without
+trouble, they were able to meet the basic requirements they had set
+for themselves. It consisted of a piece of seamless steel tubing with
+each end fastened into a phosphor-bronze casting, these castings
+comprising the big and little ends, drilled through to make the
+bearings (See Figures <a href="#img009">5</a> and <a href="#img010">6</a>). It was strong, stiff and light.<a id="footnotetag13" name="footnotetag13"></a><a href="#footnote13" title="Go to footnote 13"><span class="smaller">[13]</span></a>
+Forged rods were in rather wide use at the time and at least one
+existing engine even had a forged I-beam section design that was
+tapered down from big to little end. The Wrights' rod was obtained in
+little more time than it took to make the simple patterns for the two
+ends. The weight was easily controlled, no bearing liners were
+necessary, and a very minimum of machining was required. Concerning
+the big-end material, there exists a contradiction in the records:
+Baker, whose data are generally most accurate, states that these were
+babbited, but this must be in error, as the existing engine has
+straight bronze castings without babbiting, and there is no record, or
+drawing, or other indication of the bearings having been otherwise.</p>
+
+<p>Different methods of assembling the rod were used. At one time the
+tube ends were screwed into the bronze castings and pinned, and at
+another the ends were pinned and soldered. There is an indication that
+at one time soldering and threads were used in combination. One of the
+many conflicts between the two primary sets of drawings exists at this
+point. The Smithsonian drawings show the use at each end of adapters
+between the rod and end castings, the adapters being first screwed
+into the castings and pinned and then brazed to the inside of the
+tube. The Science Museum drawings show the tube section threaded and
+screwed into the castings. The direct screw assembly method called for
+accurate machining and hand fitting in order to make the ends of the
+tubing jam against the bottom of the threaded holes in the castings,
+and at the same time have the end bearings properly lined up. The
+weakness of the basic design patently lies <span class="pagenum"><a id="page27" name="page27"></a>(p. 27)</span> in the joints. It
+is an attempt to utilize what was probably in the beginning a
+combination five-piece assembly and later three, in a very highly
+stressed part where the load was reversing. It gave them considerable
+trouble from time to time, particularly in the 4-cylinder vertical
+engines, and was abandoned for a forged I-beam section type in their
+last engine model; but it was nevertheless the ideal solution for
+their first engine.</p>
+
+<p>The crankshaft was made from a solid block of relatively high carbon
+steel which, aside from its bulk and the major amount of machining
+required, presented no special problems. It was heat-treated to a
+machinable hardness before being worked on, but was not further
+tempered. The design was an orthodox straight pin and cheek
+combination and, as previously noted, there were no counterweights to
+complicate the machining or assembly. A sizable bearing was provided
+on each side of each crank of the shaft, which helped reduce the
+stiffness requirement.</p>
+
+<p>Their only serious design consideration was to maintain the desired
+strength and still keep within weight limitations. A fundamental that
+every professional designer knows is that it is with this particular
+sort of part that weight gets out of control; even an additional 1/16
+in., if added in a few places, can balloon the weight. With their
+usual foresight and planning, the Wrights carefully checked and
+recorded the weight of each part as it was finished, but even this
+does not quite explain how these two individuals, inexperienced in
+multicylinder engines&mdash;much less in extra-light construction&mdash;could,
+in two months, bring through an engine which was both operable and
+somewhat lighter than their specification.</p>
+
+<p>In one matter it would seem that they were quite fortunate. The
+records are not complete, but with one exception there is no
+indication of any chronic or even occasional crankshaft failure. This
+would seem to show that it apparently never happened that any of their
+designs came out such that the frequency of a vibrating force of any
+magnitude occurred at the natural frequency of the shaft. Much later,
+when this type of vibration became understood, it was found virtually
+impossible, with power outputs of any magnitude, to design an
+undampened shaft, within the space and weight limitations existing in
+an ordinary engine, strong enough to withstand the stress generated
+when the frequency of the imposed vibration approximated the natural
+frequency of the shaft. The vibratory forces were mostly relatively
+small in their engines, so that forced vibration probably was not
+encountered, and the operating speed range of the engines was so
+limited that the natural frequency always fell outside this range.</p>
+
+<p>The flywheel was about the least complex of any of their engine parts
+and required little studied consideration, although they did have to
+balance its weight against the magnitude of the explosion forces which
+would reach the power transmission chains, with their complete lack of
+rigidity, <span class="pagenum"><a id="page28" name="page28"></a>(p. 28)</span> a problem about which they were particularly
+concerned. The flywheel was made of cast iron and was both keyed to
+and shrunk on the shaft.</p>
+
+<p>Some doubt still exists about the exact method of lubricating the
+first engine. The unit presently in the airplane has a gear-type oil
+pump driven by the crankshaft through a worm gear and cross shaft, and
+the Appendix to the <i>Papers</i> states that it was lubricated by a small
+pump; nevertheless Baker says, after careful research, that despite
+this evidence, it was not. Also, the drawings prepared by Christman
+(they were commenced under the supervision of Orville Wright) do not
+show the oil pump. In March 1905 Wilbur Wright wrote to Chanute,
+"However we have added oiling and feeding devices to the engine ...";
+but this could possibly have referred to something other than an oil
+pump. But even if a pump was not included originally, its presence in
+the present engine is easily explained. Breakage of the crankcase
+casting caused the retirement of this engine, which was not rebuilt
+until much later, and the pattern for this part had no doubt long
+since been altered to incorporate a pump. It was therefore easier in
+rebuilding to include than to omit the pump, even though this required
+the addition of a cross shaft and worm gear combination. On later
+engines, when the pump was used, oil was carried to a small pipe,
+running along the inside of the case, which had four small drill holes
+so located as to throw the oil in a jet on the higher, thrust-loaded
+side of each cylinder. The rods had a sharp scupper on the outside of
+the big end so placed as also to throw the oil on this same thrust
+face. Some scuppers were drilled through to carry oil to the rod
+bearing and some were not.</p>
+
+<p>The first engine was finished and assembled in February 1903 and given
+its first operating test on 22 February. The Wrights were quite
+pleased with its operation, and particularly with its smoothness.
+Their father, Bishop Wright, was the recorder of their satisfaction
+over its initial performance, but what he noted was probably the
+afterglow of the ineffable feeling of deep satisfaction that is the
+reward that comes to every maker of a new engine when it first comes
+to life and then throbs. They obtained 13 hp originally: later figures
+went as high as almost 16, but as different engine speeds were
+utilized it is rather difficult to settle on any single power figure.
+The most realistic is probably that given in the <i>Papers</i> as having
+been attained later, after an accurate check had been made of the
+power required to turn a set of propellers at a given rpm. This came
+out at approximately 12 hp, the design goal having been 8. Following
+exactly the procedure that exists to this day, the engine went through
+an extended development period, and it was the end of September 1903
+before it was taken, with the airplane, to Kitty Hawk where the
+historic flights, which have had such a profound effect on the lives
+of all men, were made on 17 December 1903.</p>
+
+<h2><span class="pagenum"><a id="page29" name="page29"></a>(p. 29)</span> The Engines With Which They Mastered The Art of Flying</h2>
+
+<p>Two more engines of this first general design were built but they
+differed somewhat from each other as well as from the original.
+Together with a third 8-cylinder engine these were begun right after
+the first of the year in 1904, shortly after the Wrights' return from
+Kitty Hawk. In planning the 8-cylinder engine they were again only
+being forehanded, but considerably so, in providing more power for
+increased airplane performance beyond that which might possibly be
+obtained from the 4-cylinder units. Progress with the 4-cylinder
+engines was such that they fairly quickly concluded that the
+8-cylinder size would not be necessary, and it was abandoned before
+completion. Exactly how far it was carried is not known. The record
+contains only a single note covering the final scrapping of the parts
+that had been completed; and apparently there were no drawings, so
+that even its intended appearance is not known with any exactness. It
+was probably a 90° V-type using their original basic cylinder
+construction.</p>
+
+<p>The changes carried through in the two 4-cylinder engines were not
+major. The water-cooled area of the cylinder barrel was increased by
+nearly ten percent but the head remained only partially cooled. In
+hindsight, this consistent avoidance of complete cylinder-head cooling
+presents the one most inexplicable of the more important design
+decisions they made, as it does not appear logical. In the original
+engine, where the factors of time and simplicity were of paramount
+importance, this made sense, but now they were contemplating
+considerably increased power requirements, knowing the effect of
+temperature on both the cylinder and the weight of cylinder charge,
+and knowing that valve failure was one of their most troublesome
+service problems. Nor does it seem that they could have been avoiding
+complete cylinder cooling through fear of the slightly increased
+complexity or the difficulty of keeping the water connections and
+joints tight, for they had faced a much more severe problem in their
+first engine, where their basic design required that three joints be
+kept tight with only two sets of threads, and had rather easily
+mastered it; so there must have been some much more major but not
+easily discernible factor <span class="pagenum"><a id="page30" name="page30"></a>(p. 30)</span> which governed, for they still
+continued to use the poorly cooled head, even carrying it over to
+their next engine series. Very probably they did not know the effect
+on detonation of a high-temperature fuel-charge.</p>
+
+<p>One of the new engines was intended for use in their future
+experimental flying and has become known as <i>No. 2.</i> It had a bore of
+4-1/8 in., incorporated an oil pump, and at some time shortly after
+its construction a fuel pump was added. The fuel pump was undoubtedly
+intended to provide a metering system responsive to engine speed and
+possibly also to eliminate the small inherent variation in flow of the
+original gravity system.</p>
+
+<p>This engine incorporated a cylinder compression release device not on
+the original. The exact reason or reasons for the application of the
+compression release have not been determined, although the record
+shows it to have been utilized for several different purposes under
+different operating conditions. Whatever the motivation for its
+initial application, it was apparently useful, as it was retained in
+one form or another in subsequent engine models up to the last
+6-cylinder design. Essentially it was a manually controlled mechanism
+whereby all the exhaust valves could be held open as long as desired,
+thus preventing any normal charge intake or compression in the
+cylinder. Its one certain and common use was to facilitate starting,
+the open exhaust valves easing the task of turning the engine over by
+hand and making priming easy. In flight, its operation had the effect
+of completely shutting off the power. The propellers would then
+"windmill" and keep the engine revolving. One advantage stated for
+this method of operation was that when power was required and the
+control released, the engine would be at fairly high speed, so that
+full power was delivered immediately fuel reached the engine. It is
+also reported to have been used both in making normal landings and in
+emergencies, when an instant power shutdown was desired. Although it
+is not clear whether the fuel shutoff cock was intended to be
+manipulated when the compression release was used for any of these
+reasons, over the many years of its availability, undoubtedly at one
+time or another every conceivable combination of operating conditions
+of the various elements was tried. Because of the pumping power
+required with at least one valve open during every stroke, the
+windmilling speed of the engine was probably less than with any other
+method of completely stopping power output, but whether this
+difference was large enough to be noticeable, or was even considered,
+is doubtful.</p>
+
+<p>Since a simple ignition switch was all that was required to stop the
+power output, regardless of whether a fuel-control valve or a
+spark-advance control was used, it must be concluded that the primary
+function of the compression release was to facilitate starting, and
+any other useful result was something obtained at no cost. The
+compression release was later generally abandoned, and until the
+advent of the mechanical starter during the <span class="pagenum"><a id="page31" name="page31"></a>(p. 31)</span> 1920s, starting
+an engine by "pulling the propeller through" could be a difficult
+task. With the Wrights' demonstrated belief that frugality was a first
+principle of design, it is hardly conceivable that they would have
+accepted for any other reason the complication of the
+compression-release mechanism if a simple ignition switch would have
+sufficed.</p>
+
+<p>The compression-release mechanism was kept relatively simple,
+considering what it was required to accomplish. A small non-revolving
+shaft was located directly under the rocker arm rollers that actuated
+the exhaust valves. Four slidable stops were placed on this shaft,
+each in the proper location, so that at one extreme of their travel
+they would be directly underneath the rocker roller and at the other
+extreme completely in the clear. They were positioned along the shaft
+by a spring forcing them in one direction against a shoulder integral
+with the shaft, and the shaft was slidable in its bearings, its
+position being determined by a manually controlled lever. When the
+lever was moved in one direction the spring pressure then imposed on
+the stops would cause each of them to move under the corresponding
+rocker roller as the exhaust valve opened, thus holding the exhaust
+valve in the open position. When the shaft was moved in the other
+direction the collar on the shaft would mechanically move the stop
+from underneath the roller, allowing the valve to return to normal
+operation.</p>
+
+<a id="img013" name="img013"></a>
+<div class="figcenter">
+<img src="images/img013.jpg" width="400" height="267" alt="" title="">
+<p><i>Figure 8.</i>&mdash;Development engine No. 3, 1904-1906,
+showing auxiliary exhaust port, separate one-piece water-jacket block.
+(Photo by author.)</p>
+</div>
+
+<p>If the 1903 engine is the most significant of all that the Wrights
+built and <span class="pagenum"><a id="page32" name="page32"></a>(p. 32)</span> flew, then certainly the <i>No. 2</i> unit was the most
+useful, for it was their sole power source during all their flying of
+1904 and 1905 and, as they affirmed, it was during this period that
+they perfected the art, progressing from a short straightaway flight
+of 59 seconds to a flight controllable in all directions with the
+duration limited only by the fuel supply. It is to be greatly
+regretted that no complete log or record was kept of this engine.</p>
+
+<p>The Wrights again exhibited their engineering mastery of a novel basic
+situation when, starting out to make flight a practical thing, they
+provided engine <i>No. 3</i> to be used for experimental purposes. In so
+doing they initiated a system which continues to be fundamental in the
+art of providing serviceable aircraft engines to this day&mdash;one that is
+expensive and time consuming, but for which no substitute has yet been
+found. Their two objectives were: improvement in performance and
+improvement in reliability, and the engine was operated rather
+continuously from early 1904 until well into 1906. Unfortunately,
+again, no complete record exists of the many changes made and the
+ideas tested, although occasional notes are scattered through the
+diaries and notebooks.</p>
+
+<p>In its present form&mdash;it is on exhibition at the Engineers Club in
+Dayton, Ohio&mdash;the <i>No. 3</i> engine embodies one feature which became
+standard construction on all the Wright 4-cylinder models. This was
+the addition of a number of holes in a line part way around the
+circumference of the cylinder barrel so that they were uncovered by
+the piston at the end of its stroke toward the shaft, thus becoming
+exhaust ports (see Figure <a href="#img014">9</a>). This arrangement, although not entirely
+novel, was just beginning to come into use, and in its original form
+the ports exhausted into a separate chamber, which in turn was
+evacuated by means of a mechanically operated valve, so that two
+exhaust valves were needed per cylinder. Elimination of this chamber
+and the valve arrangement is typical of the Wrights' simplifying
+procedure, and it would seem that they were among the very first to
+use this form.<a id="footnotetag14" name="footnotetag14"></a><a href="#footnote14" title="Go to footnote 14"><span class="smaller">[14]</span></a></p>
+
+<p>The primary purpose of the scheme was to reduce, by this early release
+and consequent pressure and temperature drop, the temperature of the
+exhaust gases passing the exhaust valve, this valve being one of their
+main sources of mechanical trouble. It is probable that with the
+automatic intake valves being used there was also a slight effect in
+the direction of increasing the inlet charge, although with the small
+area of the ports and the short time of opening, the amount of this
+was certainly minor. With the original one-piece crankcase and
+cylinder jacket construction, the incorporation of this auxiliary
+porting was not easy, but this difficulty was overcome in the
+development engine by making different castings for the <span class="pagenum"><a id="page33" name="page33"></a>(p. 33)</span>
+crankcase itself and for the cylinder jacket and separating them by
+several inches, so that room was provided between the two for the
+ports.</p>
+
+<p>This engine demonstrated the most power of any of the flat 4s,
+eventually reaching an output of approximately 25 hp, which was even
+somewhat more than that developed by the slightly larger
+4-1/8-in.-bore flight engine, with which 21 hp was not exceeded.
+Indicative of the development that had taken place, the performance of
+the <i>No. 3</i> engine was twice the utilized output of the original
+engine of the same size, an increase that was accomplished in a period
+of less than three years.</p>
+
+<p>The Wrights were only twice charged with having plagiarized others'
+work, a somewhat unusual record in view of their successes, and both
+times apparently entirely without foundation. A statement was
+published that the 1903 flight engine was a reworked Pope Toledo
+automobile unit, and it was repeated in an English lecture on the
+Wright brothers. This was adequately refuted by McFarland but
+additionally, it must be noted, there was no Pope Toledo company or
+car when the Wright engine was built. This company, an outgrowth of
+another which had previously manufactured one-and two-cylinder
+automobiles, was formed, or reformed, and a Pope license arrangement
+entered into during the year 1903.</p>
+
+<p>The other incident was connected with Whitehead's activities and
+designs. Whitehead was an early experimenter in flying, about the time
+of the Wrights, whose rather extraordinary claims of successful flight
+were published in the 1901-1903 period but received little attention
+until very much later. His first engines were designed by a clever
+engineer, Anton Pruckner, who left at the end of 1901, after which
+Whitehead himself became solely responsible for them. It was stated
+that the Wrights visited the Whitehead plant in Bridgeport,
+Connecticut, and that Wilbur remained for several days, spending his
+time in their machine shop. This was not only categorically denied by
+Orville Wright when he heard of it but it is quite obvious that the
+1903 or any other of the Wright engine designs bears little
+resemblance to Pruckner's work. In fact, its principal design features
+are just the opposite of Pruckner's, who utilized vertical cylinders,
+the 2-stroke cycle, and air-cooling, which Whitehead at some point
+changed to water-cooling.<a id="footnotetag15" name="footnotetag15"></a><a href="#footnote15" title="Go to footnote 15"><span class="smaller">[15]</span></a></p>
+
+<h2><span class="pagenum"><a id="page34" name="page34"></a>(p. 34)</span> The Four-Cylinder Vertical Demonstration Engine and the First
+Production Engine</h2>
+
+<p>In 1906, while still doing general development work on the flat
+experimental engine, the Wrights started two new engines, and for the
+first time the brothers engaged in separate efforts. One was "a
+modification of the old ones" by Wilbur and the other, "an entirely
+new pattern" by Orville. There is no record of any of the features of
+Wilbur's project or what was done in connection with it. Two months
+after the experimental operation of the two designs began, an entry in
+Wilbur's diary gives some weight and performance figures for the "4" x
+4" rebuilt horizontal," and since Orville's design was vertical the
+data clearly refer to Wilbur's; but since the output is given only in
+test-fan rpm it does not serve to indicate what had been accomplished
+and there is no further mention of it.</p>
+
+<p>Orville's design became the most used of any model they produced. It
+saw them through the years from 1906 to 1911 or 1912, which included
+the crucial European and United States Army demonstrations, and more
+engines of this model were manufactured than any of their others
+including their later 6-cylinder. Although its ancestry is traceable
+to the original 1903 engine, the design form, particularly the
+external configuration, was considerably altered. Along with many
+individual parts it retained the basic conception of four medium-size
+cylinders positioned in line and driving the propellers through two
+sprocket wheels. From the general tenor of the record it would seem,
+despite there being no specific indication, that from this time on
+Orville served as the leader in engine design, although this occurred
+with no effect whatsoever on their finely balanced, exactly equal
+partnership which endured until Wilbur's death in 1912.</p>
+
+<p>The first major change from the 1903 design, putting the engine in an
+upright instead of flat position, was probably done primarily to
+provide for a minimum variation in the location of the center of
+gravity with and without a passenger. Whether or not it had any
+influence, the vertical cylinder arrangement was becoming predominant
+in automobile powerplants by this time, and the Wright engines now
+began to resemble this prevailing form of the internal combustion
+engine&mdash;a basic form that, in a wide variety of uses, was to endure
+for a long time.</p>
+
+<p><span class="pagenum"><a id="page35" name="page35"></a>(p. 35)</span> Over the years, the Wrights seem to have made many changes in
+the engine: the bore was varied at different times, rod assembly
+methods were altered, and rod ends were changed from bronze to steel.
+Chenoweth states that on later engines an oil-control ring was added
+on the bottom of the piston, necessitating a considerable increase in
+the length of the cylinder barrel. This arrangement could not have
+been considered successful, as it apparently was applied to only a
+limited number of units and was not carried over to the later
+6-cylinder engine model. There was much experimentation with cam
+shapes and most probably variations of these got into production.</p>
+
+<p>With the crankcase, they did not go all the way to the modern
+two-piece form but instead retained the one-piece construction.
+Assembly was effected through the ends and a detachable plate was
+provided on one side for access to the interior. It is clear that they
+regarded this ability to get at the interior of the case without major
+disassembly as a valuable characteristic, and later featured it in
+their sales literature. They were apparently willing to accept the
+resultant weakening of the case and continued the construction through
+their last engine model. The integrally cast cylinder water jackets
+were abandoned and the top of the crankcase was machined flat to
+provide a mounting deck for individual cylinders. The use of aluminum
+alloy was continued, and the interior of the case was provided with
+strengthening webs of considerable thickness, together with supporting
+ribs. The cam shaft was supported directly in the case.</p>
+
+<p>The individual cylinder design was of extreme simplicity, a single
+iron casting embodying everything except the water jacket. The valves
+seated directly on the cast-iron cylinder head and the guides and
+ports were all contained in an integral boss on top of the head. The
+exhaust valve location on the side of the engine opposite the pilot
+was a decided advantage over that of the 1903 design, where the
+exhaust was toward the pilot. A four-cornered flange near the bottom
+of the cylinder provided for fastening it to the crankcase, and a
+threaded hole in the top of the head received a vertical eyebolt which
+served as the rocker-arm support. The cylinder was machined all over;
+two flanges, one at the bottom and the other about two-thirds of the
+way down provided the surfaces against which the water jacket was
+shrunk. The jacket was an aluminum casting incorporating the necessary
+bosses and double shrunk on the barrel; that is, the jacket itself was
+shrunk on the cylinder-barrel flanges and then steel rings were shrunk
+on the ends of the jacket over the flanges. The jacket thickness was
+reduced by machining at the ends, making a semigroove into which the
+steel shrink rings fitted. These rings insured the maintenance of a
+tight joint despite the tendency of the aluminum jacket to expand away
+from the cast-iron barrel.</p>
+
+<a id="img014" name="img014"></a>
+<div class="figcenter">
+<img src="images/img014.jpg" width="400" height="360" alt="" title="">
+</div>
+<a id="img015" name="img015"></a>
+<div class="figcenter">
+<img src="images/img015.jpg" width="400" height="313" alt="" title="">
+</div>
+
+<a id="img016" name="img016"></a>
+<div class="figcenter">
+<img src="images/img016.jpg" width="250" height="343" alt="" title="">
+</div>
+
+<a id="img017" name="img017"></a>
+<div class="figcenter">
+<img src="images/img017.jpg" width="400" height="323" alt="" title="">
+<p><i>Figure 9.</i>&mdash;4-Cylinder vertical engine: a, magneto
+side; b, valve port side with intake manifold removed; c, flywheel end
+of engine at Carillon Park Museum, Dayton, Ohio; d, magneto side with
+crankcase cover removed. (Photos: a, Smithsonian A-3773; b, d, Pratt &amp;
+Whitney D-15003, 15007; c, by A. L. Rockwell.)</p>
+</div>
+
+<p><span class="pagenum"><a id="page38" name="page38"></a>(p. 38)</span> Why the one-piece crankcase and cylinder jacket combination of
+the 1903 engine was abandoned for the individual cylinder construction
+can only be surmised. The difference in weight was probably slight, as
+the inherent weight advantage of the original crankcase casting was
+largely offset by the relatively heavy valve boxes, and the difference
+in the total amount of machining required, because of the separate
+valve boxes, cages, and attaching parts, also was probably slight.
+Although the crankcase had shown itself to be structurally weak, this
+could have been cared for by proper strengthening. The 1903 design did
+have some fundamental disadvantages: it required a fairly complex
+pattern and expensive casting, plus some difficult machining, part of
+which had to be very accurate in order to maintain both gas and water
+joints tight; and the failure of any one cylinder that affected the
+jacket meant a complete crankcase replacement.</p>
+
+<p>It seems probable that a change was initially made mandatory by their
+intention to utilize the ported exhaust feature, the value of which
+they had proved in the experimental engine. The separate one-piece
+water jacket construction they had arrived at in this engine was
+available, but once the decision to change was made, the individual
+cylinder with its shrunk-on jacket had much to commend it&mdash;simplicity,
+cost, ease of manufacture and assembly and attachment, and
+serviceability. The advantages of the auxiliary, or ported, exhaust
+were not obtained without cost, however, as the water jacket around
+the barrel could not very easily be extended below the ports. Thus,
+even though the water was carried as high as possible on the upper
+end, a large portion of the barrel was left uncooled, and the lack of
+cooling at the lower end, in conjunction with the uncooled portion of
+the head, meant that only approximately half the entire cylinder
+surface was cooled directly.</p>
+
+<p>The piston was generally the same as in the 1903 engine, except that
+six radial ribs were added on the under side of the head, tapering
+from maximum thickness at the center to nothing near the wall. They
+were probably incorporated as an added path for heat to flow from the
+center of the piston toward the outside, as their shape was not the
+best use of material for strength. The piston pin was locked in the
+piston by the usual set screw, but here no provision was made for the
+alternate practice of clamping the rod on the pin. This piston-pin
+setscrew construction had become a standard arrangement in automobile
+practice. The piston rings were the normal wide design of that time,
+with what would now be considered a low unit pressure.</p>
+
+<p>Quite early in the life of this engine model the practice was
+initiated of incorporating shallow grooves in the surface of the more
+highly loaded thrust face of the piston below the piston pin to
+provide additional lubrication. <span class="pagenum"><a id="page39" name="page39"></a>(p. 39)</span> This development apparently
+proceeded haphazardly. Figure <a href="#img018">10c</a> shows three of the pistons from an
+engine of low serial number&mdash;the first of this model to be delivered
+to the U.S. Navy&mdash;and it will be noted that one has no grooves,
+another has one, and the other has three. The eventual standardized
+arrangement provided three of these grooves, approximately 1/16 in.
+wide, extending halfway around the piston, and, although the depth was
+only a few thousandths of an inch, the amount of oil carried in them
+was apparently sufficient to assist in the lubrication of the face, as
+they were used in both the 4-and 6-cylinder engines.</p>
+
+<p>Each cylinder was fastened to the crankcase by four nuts on studs
+driven into the aluminum case. Valves and rocker arms were similar to
+those of the early engines, the automatic inlet valve being retained.
+The continued use of the two-piece valve is not notable, even though
+one-piece forgings were available and in use at this time; the
+automobile continued for many years to use this construction. The
+camshaft was placed at the bottom of the engine, inside the crankcase,
+and the rocker arms were actuated by pushrods which were operated by
+hinged cam followers. The pushrod was fastened in the rocker by a pin,
+about which it operated, through its upper end and was positioned near
+the bottom by a guide in the crankcase deck. The lower end of the rod
+bore directly on the flat upper surface of the cam follower, and valve
+clearance adjustment was obtained by grinding this end. The camshaft
+and magneto were driven by the crankshaft through a three-member train
+of spur gears (see Figures <a href="#img014">9</a>, <a href="#img018">10</a> and <a href="#img021">11</a>).</p>
+
+<p>The built-up construction of the connecting rod was carried over from
+the first engine, and in the beginning apparently the same materials
+were used, except that the big end was babbited. Later the rod ends
+were changed from bronze to steel. The big end incorporated a small
+pointed scupper on one side for lubrication, as with the original, and
+this was sometimes drilled to feed a groove which carried oil to the
+rod bearing, but where the drilling was omitted, the only function the
+scupper then could perform was, as in the original engine, to throw a
+small amount of oil on the cylinder wall.</p>
+
+<p>The crankshaft and flywheel were similar in design to those on the
+1903 engine, except that the sharp corners at the top and bottom of
+the crank cheeks were machined off to save weight (see Figure <a href="#img020">10f</a>). An
+oil pump and a fuel pump were mounted side by side in bosses cast on
+the valve side of the crankcase; they were driven from the camshaft by
+worm gears and small shafts crossing the case.</p>
+
+<a id="img018" name="img018"></a>
+<div class="figcenter">
+<img src="images/img018.jpg" width="300" height="544" alt="" title="">
+</div>
+
+<a id="img019" name="img019"></a>
+<div class="figcenter">
+<img src="images/img019.jpg" width="250" height="246" alt="" title="">
+</div>
+
+<a id="img020" name="img020"></a>
+<div class="figcenter">
+<img src="images/img020.jpg" width="400" height="551" alt="" title="">
+<p><i>Figure 10.</i>&mdash;4-Cylinder vertical engine: a, cylinder
+assembly with valve mechanism parts; b, cylinder disassembled, and
+parts; c, pistons and connecting rods; d, bottom side of piston; e,
+crankshaft, flywheel and crankcase end closure; f, crankcase, with
+compression release parts. (Pratt &amp; Whitney photos D-14996, 15001,
+14998, 14994, 14999, 14989, respectively.)</p>
+</div>
+
+<p>The camshaft construction was considerably altered from the 1903
+design. Although the reason is not entirely clear, one indication
+suggests that breakage or distortion of the shaft may have been
+encountered: whereas in the 1903 engine there had been no relationship
+between the location of <span class="pagenum"><a id="page42" name="page42"></a>(p. 42)</span> the cams and the camshaft bearings,
+in this engine the exhaust valves were carefully positioned so that
+all cams were located very close to the supporting bearings in the
+crankcase. Also, the camshaft was solid, although it would seem that
+the original hollow shaft construction could have provided equal
+stiffness with less weight. The final decision was possibly determined
+by the practicality that there existed no standard tubing even
+approximating the size and wall thickness desired.</p>
+
+<p>There still was no carburetor, a gear pump metering the fuel in the
+same manner as on the 1904-1905 engine. Basically, the intake charge
+was fed to the cylinders by a round gallery manifold running alongside
+the engine. This was split internally by a baffle extending almost
+from end to end, so that the fuel mixture entering the manifold on one
+side of the baffle was compelled to travel to the two ends before it
+could return to the inside cylinder, this feature being a copy of
+their 1903 general intake arrangement. Apparently various shapes and
+positions of entrance pipes with which to spray the fuel into the
+manifold were used; and the injection arrangement seems also to have
+been varied at different times. The fuel pump was not necessarily
+always used, as the engine in some of the illustrations did not
+incorporate one, the fuel apparently being fed by gravity, as on the
+original engine. Chenoweth describes an arrangement in which exhaust
+heat was applied to the inlet manifold to assist the fuel vaporization
+process, but it is believed that this was one of the many changes made
+in the engine during its lifetime and not necessarily a standard
+feature.</p>
+
+<p>A water circulation pump was provided, driven directly by the
+crankshaft through a two-arm universal joint intended to care for any
+misalignment between the shaft and the pump. The water was piped to a
+horizontal manifold running along the cylinders just below the intake
+manifold, and a similar manifold on the other side of the engine
+collected it for delivery to the radiator. It is a little difficult to
+understand why it was not introduced at the bottom of the water
+jackets.</p>
+
+<p>The crankcase was a relatively strong and well proportioned structure
+with three heavy strengthening ribs running from side to side, its
+only weakness being the one open side. A sheet-iron sump was fastened
+to the bottom by screws and it would appear from its design, method of
+attachment, and location of the engine mounting pads that this was
+added some time after the crankcase had been designed; but if so it
+was apparently retrofitted, as engines with quite low serial numbers
+have this part.</p>
+
+<p>The ignition was by high-tension magneto and spark plug and this
+decision to change from the make-and-break system was undoubtedly the
+correct one, just as adoption of the other form originally was logical
+under the circumstances that existed then. The high-tension system was
+simpler <span class="pagenum"><a id="page43" name="page43"></a>(p. 43)</span> and had now collected more service experience. The
+magneto was driven through the camshaft gear, and a shelf, or bracket,
+cast as an integral part of the case, was provided for mounting it.
+The spark advance control was in the magneto and, since spark timing
+was the only means of regulating the engine power and speed, a wide
+range of adjustment was provided.</p>
+
+<p>The engine had the controllable compression release which had been
+added to the <i>No. 2</i> and <i>No. 3</i> flat engines, although mechanically
+it was considerably altered from the original design. Instead of the
+movable stop operating directly on the rocker roller to hold the
+exhaust valve open, it was located underneath a collar on the pushrod.
+This stop was hinged to the crankcase and actuated by a small rod
+running along and supported by the crankcase deck. Longitudinal
+movement of this rod in one direction would, by spring pressure on
+each stop, push them underneath the collars as the exhaust valves were
+successively opened. A reverse movement of the rod would release them
+(see Figure <a href="#img020">10f</a>). Why they retained the method of manually operating
+the compression release, which was the same as had been used in the
+1904-1905 engine, is not quite clear. That is, the mechanism was put
+into operation by pulling a wire running from the pilot to a lever
+actuating the cam which moved the control rod. When normal valve
+operation was subsequently desired, the pilot was compelled to reach
+with his hand and operate the lever manually, whereas a second wire or
+push-pull mechanism would have obviated the necessity for both the
+awkward manual operation of the lever and the gear guard which was
+added to protect the pilot's hand, the lever being located close to
+the camshaft gear.</p>
+
+<p>The 4-cylinder vertical engine was a considerable improvement over the
+previous designs. They had obtained a power increase of about 40
+percent, with a weight decrease of 10 percent, and now had an engine
+whose design was almost standard form for good internal combustion
+engines for years to come. In fact, had they split the crankcase at
+the crankshaft center line and operated the inlet valves mechanically,
+they would have had what could be termed a truly modern design. They
+needed more cylinder cooling, both barrel and head, particularly the
+latter, and an opened-up induction system for maximum power output,
+but this was not what they were yet striving for. They had directly
+stated that they were much more interested in reliability than light
+weight.</p>
+
+<p>This engine model was the only one of the Wright designs to be
+licensed and produced abroad, being manufactured in Germany by the
+Neue Automobil-Gesellschaft and by Bariquand et Marré in France. The
+latter was much more prominent and their engines were used in several
+early European airplanes.</p>
+
+<a id="img021" name="img021"></a>
+<div class="figcenter">
+<a href="images/img021.jpg">
+<img src="images/img021tb.jpg" width="400" height="768" alt="" title=""></a>
+</div>
+
+<a id="img022" name="img022"></a>
+<div class="figcenter">
+<a href="images/img022.jpg">
+<img src="images/img022tb.jpg" width="400" height="893" alt="" title=""></a>
+<p><i>Figure 11.</i>&mdash;4-Cylinder vertical engine assembly,
+Bariquand et Marré version. (Drawing courtesy Bristol Siddeley
+Engines, Ltd.)</p>
+</div>
+
+<a id="img023" name="img023"></a>
+<div class="figcenter">
+<a href="images/img023.jpg">
+<img src="images/img023tb.jpg" width="400" height="565" alt="" title=""></a>
+<p>THE WRIGHT BROTHERS AERO ENGINE</p>
+</div>
+
+<p>The French manufacturer, without altering the basic design, made a
+<span class="pagenum"><a id="page46" name="page46"></a>(p. 46)</span> number of changes of detail which seem to have greatly
+annoyed Wilbur Wright, although some of them could probably be listed
+as improvements, based on several features of later standard design.
+One consisted of an alteration in the position of the fuel and oil
+pumps, the latter being lowered to the level of the sump. The
+crankcase was drilled to provide forced-feed lubrication to the
+connecting rod big end and crankshaft main bearings. Strengthening
+ribs were added to the pistons running from the upper side of the pin
+bosses to the piston wall, and the crankcase studs holding down the
+cylinders were replaced with bolts having their heads inside the case.
+The hinged cam follower was omitted and the pushrod bore directly on
+the cam through a roller in its end. The magneto was moved toward the
+rear of the engine a considerable distance and an ignition timing
+control device was introduced between it and its driving gear. Instead
+of the magneto being mounted directly on the special bracket integral
+with the crankcase, a wooden board running from front to rear of the
+engine was used and this was fastened to the two engine support pads,
+the magneto bracket being omitted entirely.</p>
+
+<p>Despite his criticism of the French motor and the quality of its
+manufacture, Wilbur was compelled to install one in his own exhibition
+airplane during his early French demonstrations at Le Mans after rod
+failure had broken his spare crankcase, and much of his subsequent
+demonstration flying was made with the French product.</p>
+
+<h2><span class="pagenum"><a id="page47" name="page47"></a>(p. 47)</span> The Eight-Cylinder Racing Engine</h2>
+
+<p>By 1909 regular and special air meets and races were being held and
+various competitions for trophies conducted. Among these the Gordon
+Bennett Cup Race for many years was considered a major event. For the
+1910 competition it was decided to enter a Wright machine and, since
+this was a race with speed the sole objective, the available
+4-cylinder engine, even in a version pushed to its maximum output, was
+deemed too small. They built for it a special 8-cylinder unit in a
+90°V form. They were thus resorting to one of their 1904
+concepts&mdash;modifying and enlarging a version known and proved in
+use&mdash;as the proper method of most quickly increasing output.
+Unfortunately again, there are essentially no detailed drawings
+available, so that the design cannot be studied.<a id="footnotetag16" name="footnotetag16"></a><a href="#footnote16" title="Go to footnote 16"><span class="smaller">[16]</span></a></p>
+
+<p>Only one engine is historically recorded as having been built,
+although in view of the Wrights' record of foresight and preparation
+it is almost certain that at least one spare unit, assembled or in
+parts, was provided. In any case, the airplane&mdash;it was called the
+<i>Baby Grand Racer</i>&mdash;and engine were wrecked just before the race, and
+no physical parts were retained, so that the sole descriptions come
+from external photographs, memory, and hearsay. McFarland thinks that
+possibly Orville Wright, particularly, was somewhat discomfited over
+the accident that eliminated the machine, as he had previously flown
+it quite successfully at a speed substantially higher than that of the
+ultimate winner, and he wanted to get it out of sight and mind as
+quickly as possible. The Air Force Museum at Wright Field, Dayton,
+Ohio, has an incomplete set of drawings of a 90°V, 8-cylinder Wright
+engine, but it is quite obvious from the basic design and individual
+features, as well as from at least one date on the drawings, that this
+conception is of a considerably later vintage than that of the <i>Baby
+Grand Racer</i>.</p>
+
+<p>The racing engine was in essence a combination of two of the standard
+4s on a redesigned crankcase utilizing as many of the 4-cylinder
+engine parts as possible. The rods were reported to have been placed
+side by side, and the regular 4-cylinder crankshaft, with alterations
+to accommodate <span class="pagenum"><a id="page48" name="page48"></a>(p. 48)</span> the rods, was utilized. A single cam operated
+all the exhaust valves. It was compact and light, its only fundamental
+disadvantage being the inherent unbalance of the 90°V-8. The
+arrangement provided a much higher powered unit in the cheapest and
+quickest manner, and one that could be expected to operate
+satisfactorily with the least development.</p>
+
+<h2><span class="pagenum"><a id="page49" name="page49"></a>(p. 49)</span> The Six-Cylinder Vertical Engines</h2>
+
+<a id="img024" name="img024"></a>
+<div class="figcenter">
+<img src="images/img024.jpg" width="400" height="273" alt="" title="">
+</div>
+
+<a id="img025" name="img025"></a>
+<div class="figcenter">
+<img src="images/img025.jpg" width="400" height="304" alt="" title="">
+</div>
+
+<a id="img026" name="img026"></a>
+<div class="figcenter">
+<img src="images/img026.jpg" width="400" height="357" alt="" title="">
+<p><i>Figure 12.</i>&mdash;Original 6-cylinder engine: a, push-rod
+side; b, valve-port side; c, crankcase with sump removed. (Photos:
+Smithsonian A-3773A, 45598; Pratt &amp; Whitney D-15015, respectively.)</p>
+</div>
+
+<p>Shortly after the construction of the 8-cylinder engine the Wrights
+were again faced with the ever-recurrent problem of providing a higher
+powered standard production engine for their airplanes, which were now
+being produced in some numbers. By this time, 1911, there had been a
+relatively tremendous growth in both flying and automotive use of the
+internal combustion engine and as a result many kinds and sizes had
+been produced and utilized, so that numerous choices were presented to
+them. But if they <span class="pagenum"><a id="page50" name="page50"></a>(p. 50)</span> were both to make use of their past
+experience and retain the simplicity they had always striven for, the
+more practical possibilities narrowed down to three: they could
+increase the cylinder size in the 4-cylinder combination, or they
+could go either to 6 or 8 cylinders in the approximate size they had
+previously used.</p>
+
+<p>The 4-in. cylinder in combination with a 5-in. stroke would provide in
+four cylinders about the displacement they wanted. Strokes of 6 in.
+were not uncommon and cylinders of 6-in. bore had been very
+successfully utilized in high-output automobile racing engines many
+years before this, so there was seemingly no reason to doubt that the
+5-in. cylinder could be made to operate satisfactorily, but it is not
+difficult to imagine the Wrights' thoughts concerning the roughness of
+an engine with cylinders of this diameter. The question of the grade
+of available fuel may possibly have entered into their decision to
+some extent, but it seems far more likely that roughness, their
+perennial concern, was the predominant reason for not staying with the
+more simple 4-cylinder form (as we have seen, roughness to them meant
+the effect of the cylinder explosion forces). Actually, of course,
+they never went larger than a 4-3/8-in. cylinder bore, and later
+aircraft engine experience would seem generally to confirm their
+judgment, for with the piston engine it has always been much more
+difficult to make the larger bores operate satisfactorily at any given
+specific output.</p>
+
+<p>While the 90°V, 8-cylinder arrangement would have enabled them to
+utilize a great number of the 4-cylinder-engine parts, it would have
+given them a somewhat larger engine than was their apparent desire,
+unless they reduced the cylinder size. And while they had had some
+limited experience in building and operating this kind of engine, and
+twice had chosen it when seeking more power, both of these choices
+were greatly influenced by the desire to obtain quickly an engine of
+higher power. It is also possible that something in their experience
+with the V-8 moved them away from it; the unbalanced shaking force
+inherent in the arrangement may well have become evident to them. What
+probably also helped them to their final conclusion was the
+fundamental consideration that the V-8 provided two extra cylinders
+which were not really needed.</p>
+
+<p>The eventual selection of the 6-cylinder was a slight compromise. In
+order to get the desired output the cylinder displacement was
+increased, but this was done by lengthening the stroke&mdash;the first time
+this had been altered since the original design. The increase (from 4
+to 4-1/2 in.) was only 1/2 in., and the bore, the more important
+influence on fuel performance, was kept the same. Overall, the choice
+was quite logical. They were utilizing the in-line construction upon
+which almost all of their now considerable experience had been based,
+and the sizes of and requirements for parts also conformed to this
+experience. They could, in fact, use many <span class="pagenum"><a id="page51" name="page51"></a>(p. 51)</span> of the same parts.
+The natural balance of the 6-cylinder arrangement gave them a very
+smooth engine, and had they stiffened the shaft and counter-weighted
+the cranks, they would have produced the smoothest engine that could
+have been built at that time.</p>
+
+<p>In the literature are two references to a Wright 6-cylinder engine
+constructed around the cylinders of the vertical 4. One of these is in
+Angle's <i>Airplane Engine Encyclopedia</i>, published in 1921, and the
+other is in <i>Aerosphere 1939</i>, published in 1940. The wording of the
+latter is essentially identical with that of the former; it seems a
+reasonable conclusion that it is a copy. Although it is possible that
+such an engine was built at some time, just as the 8-cylinder racing
+engine was cobbled up out of parts from the 4-cylinder vertical, no
+other record, no engines, and no illustrations have been found. It is
+thus quite certain that no significant quantity was ever manufactured
+or utilized.</p>
+
+<p>The crankcase was considerably changed from that of the vertical 4,
+and was now in two pieces, with the split on the crankshaft center
+line. However, the shaft was not supported by the lower half of the
+case, as eventually became standard practice, but by bearing caps
+bolted to the ends of the upper case and, in between, to heavy ribs
+running across the upper case between the cylinders. The lower half of
+the case thus received none of the dynamic or explosion loads, and,
+serving only to support the engine and to provide for its mounting,
+was lightly ribbed. In it were incorporated integral-boss standpipe
+oil drains which discharged into a bolted-on sump. The upper half of
+the case was again left open on one side, giving the desired access to
+the interior, and, additionally, the design was altered to provide a
+method of camshaft assembly that was much simpler than that of the
+vertical 4 (see p. <a href="#page42">42</a>).</p>
+
+<p>The cylinder was also greatly altered from that of the vertical 4. It
+was made in three parts, a piece of seamless steel tubing being shrunk
+on a cast-iron barrel to form the water jacket, with a cast-iron
+cylinder head shrunk on the upper end of the barrel. This construction
+compelled the use of long studs running from the cylinder head to the
+case for fastening down the cylinder (see Figures <a href="#img024">12a</a>-c). For the
+first time the cylinder heads were water-cooled, cored passages being
+provided, and more barrel surface was jacketed than previously,
+although a considerable area at the bottom was still left uncooled,
+obviously by direct intent, as the ported exhaust arrangement was no
+longer employed.</p>
+
+<p>Also for the first time one-piece forged valves were used, but just
+when these were incorporated is not certain and, surprisingly, they
+were applied to the inlet only, the exhaust valve being continued in
+the previous two-piece screwed and riveted construction. The reasoning
+behind this is not evident. If a satisfactory two-piece exhaust valve
+had finally been developed it <span class="pagenum"><a id="page52" name="page52"></a>(p. 52)</span> would be logical to carry it
+over to the new design; but exhaust valves normally being much more
+troublesome, it would seem that a good exhaust valve would make an
+even better inlet valve and, in the quantities utilized, the two-piece
+design should have been much cheaper. In the original 6-cylinder
+engine the inlet valves operated automatically as in all previous
+models, but at the time of a later extensive redesign (1913) this was
+changed to mechanical actuation, and the succeeding engines
+incorporated this feature. All the valve-actuating mechanism was
+similar to that of the vertical 4, and the engine had the usual
+compression-release mechanism, the detail design being carried over
+directly from the 4-cylinder.</p>
+
+<p>Design of the piston followed their previous practice, with wide rings
+above the pin and shallow grooves below the pin on the thrust face,
+and with the pin fastened in the piston by a set screw. The piston had
+four ribs underneath the head (see Figure <a href="#img027">13b</a>) radiating from the
+center and with the two over the pin bosses incorporating
+strengthening webs running down and joining the bosses. The piston
+length was reduced by 1 in., thus giving a much less clumsy appearance
+and, with other minor alterations, a weight saving of 40 percent (see
+Figures <a href="#img027">13b</a> and <a href="#img027">c</a>). The rods were for the first time made of I-section
+forgings, a major departure, machined on the sides and hand
+finished at the ends, with a babbit lining in the big end, the piston
+pin bearing remaining steel on steel.</p>
+
+<a id="img027" name="img027"></a>
+<div class="figcenter">
+<img src="images/img027.jpg" width="300" height="479" alt="" title="">
+<p><i>Figure 13.</i>&mdash;Original 6-cylinder engine: a, cylinder
+assembly and valve parts; b, bottom side of piston; c, piston, piston
+pin and connecting rod; d, valve mechanism; e, crankshaft and
+flywheel. (Pratt &amp; Whitney photos D-15012, 15017, 15013, 15018,
+respectively.)</p>
+</div>
+
+<p>At least two different general carburetion and induction systems were
+utilized, possibly three. One, and most probably the original,
+consisted of a duplicate of the injection pump of the 4-cylinder
+fitted to a manifold which ran the length of the engine, with three
+takeoffs, each of which then split into two, one for each cylinder. Of
+this arrangement they tried at least two variations involving changes
+in the location and method of injecting the fuel into the manifold;
+and there seems to have been an intermediate manifold arrangement,
+using fuel-pump injection at the middle of the straight side, or
+gallery, manifold, which was fed additional air at both ends through
+short auxiliary inlet pipes. This would indicate that with the
+original arrangement, the end cylinders were receiving too rich a
+mixture, when the fuel in the manifold was not properly vaporized.
+Although the exhaust was on the same side of the engine as the inlet
+system, no attempt was made to heat the incoming charge at any point
+in its travel. An entirely different system adopted at the time of the
+complete redesign in 1913 consisted of two float-feed Zenith
+carburetors each feeding a conventional <span class="pagenum"><a id="page54" name="page54"></a>(p. 54)</span> three-outlet
+manifold. This carburetor was one of the first of the plain-tube type,
+that is, with the airflow through a straight venturi without any
+spring-loaded or auxiliary air valves, and was the simplest that could
+be devised. When properly fitted to the engine, it gave a quite good
+approximation of the correct fuel and air mixture ratio over the
+speed-load running range, although it is considerably more than
+doubtful that this was maintained at altitude, as is stated in one of
+the best descriptions of the engine published at the time the
+carburetors were applied.</p>
+
+<p>The compression ratio of this engine was lowered by almost 20 percent
+from that of the vertical 4. This, in combination with the low
+bore-to-stroke ratio, the unheated charge, and the later mechanically
+operated inlet valve, indicates that the Wrights were now attempting
+for the first time to secure from an engine something approaching the
+maximum output of which it was capable.</p>
+
+<p>As the engine originally came out, it continued to utilize only one
+spark plug in each cylinder. The high-tension magneto had a wide range
+of spark advance adjustment, which again provided the only control of
+the engine when equipped with the original fuel pump injection.</p>
+
+<p>The location of the valves and pushrods was similar to that in the 4,
+so that the cams were immediately adjacent to the camshaft bearings,
+which were carried in the crankcase ends and in the heavy webs. The
+camshaft was gear-driven and the cam shape was similar to that of the
+last 4s, with a quite rapid opening and closing and a long dwell,
+leaving the valve opening accelerations and seating velocities still
+quite high.</p>
+
+<p>The crankshaft was a continuation of their basic design of rather
+light construction, particularly in the webs. The cheeks were even
+thinner (by 1/4 in.) than those of the 4 although the width was
+increased by 1/8 in. (see Figure <a href="#img027">13e</a>). For the first time they went to
+a forging, the rough contour type of the time, and utilized a
+chrome-nickel alloy steel.</p>
+
+<p>Lubrication was by means of the usual gear pump, and the piston and
+rod bearings continued to be splash-fed. The rod big-end bearing
+carried a small sharp undrilled boss at the point where, on the other
+engines, had been located scuppers whose purpose was apparently still
+to throw lubricating oil on the cylinder wall carrying the more highly
+loaded side of the piston. The rod big-end bearing was lubricated by a
+hole on the top of the big-end boss catching some of the crankcase
+splash, which was then carried to the bearing by a groove.</p>
+
+<p>When the 6-cylinder engine was completely redesigned in 1913 this led
+to the introduction in late fall of that year of a new model called
+the 6-60, the 60 designating the rating in horsepower. There is little
+in the Wright records to show why such a radical revision was thought
+necessary, but the general history of the period gives a rather clear
+indication. The competition <span class="pagenum"><a id="page55" name="page55"></a>(p. 55)</span> had caught up to the Wrights in
+powerplants. Other engines were being installed in Wright airplanes,
+and Navy log books show these other engines being used interchangeably
+with those of the Wrights.</p>
+
+<p>Most of the descriptions of the new model published at the time it was
+introduced concentrate on the addition of the two carburetors and the
+mechanical operation of the inlet valves, but these were only two of
+many major changes. The cylinder was completely revised, the intake
+being moved to the camshaft side of the engine from its position
+adjacent to the exhaust, so that the two ports were now on opposite
+sides of the cylinder. By proper positioning of the rocker-arm
+supports and choice of their length and angles, all valves were made
+operable from a single camshaft. The shrunk-on steel water jacket
+cylinder was retained, but the water connections were repositioned so
+that the water entered at the bottom and came out at the top of the
+cylinder. Over the life of the 6-cylinder engine several different
+valve types were used but the published specifications for the model
+6-60 called for "cast iron heads"&mdash;the old two-piece construction. The
+piston pins were case hardened and ground and the crankshaft pins and
+journals were heat treated and ground.</p>
+
+<p>The fuel and oil pumps were removed from the side of the crankcase and
+a different ignition system was applied, although still of the
+high-tension spark-plug type which by this time had become general
+practice on all so-called high-speed internal-combustion engines. A
+second threaded spark-plug hole was provided in the cylinder head and
+despite its more common use for other purposes, it is evident that the
+intention was to provide two-plug ignition. It is doubtful that at the
+specific output of this engine any power difference would be found
+between one-and two-plug operation, so that the objective was clearly
+to provide a reserve unit in case of plug failure. However, it was
+also used for the installation of a priming cock for starting and
+because of the prevalence of single-wire ignition systems on existing
+and illustrated engines, it seems to have been used mostly in this
+manner, even though dual-ignition systems later became an unvarying
+standard for aircraft engines.</p>
+
+<p>Viewed externally, the only part of the engine that appears the same
+as the original 6 is the small lower portion of the crankcase; but
+what is more visually striking is the beauty of the new lines and
+extreme cleanness of the exterior design (see Figures <a href="#img028">14</a> and <a href="#img029">15</a>). Many
+of their individual parts had shown the beauty of the sparse design of
+pure utility but it was now in evidence in the whole. Despite the
+proven practical value of their other models, this is the only one
+that can be called a good-looking engine, instantly appealing to the
+aesthetic sense, even though the vertical 4 is not an ugly engine. The
+appearance of their final effort, in a field they were originally
+reluctant to enter and concerning which they always deprecated
+<span class="pagenum"><a id="page56" name="page56"></a>(p. 56)</span> the results of their own work, was a thing of which a
+technically trained professional engine designer could be proud.</p>
+
+<p>The 6-60 was continued in production and development until it became
+the 6-70, and indications are that it eventually approached an output
+of 80 horsepower.</p>
+
+<a id="img028" name="img028"></a>
+<div class="figcenter">
+<img src="images/img028.jpg" width="400" height="392" alt="" title="">
+<p><i>Figure 14.</i>&mdash;6-Cylinder 6-60 and 6-70 engine, right
+rear intake side. (Pratt &amp; Whitney photo.)</p>
+</div>
+
+<a id="img029" name="img029"></a>
+<div class="figcenter">
+<img src="images/img029.jpg" width="400" height="308" alt="" title="">
+<p><i>Figure 15.</i>&mdash;6-Cylinder 6-70 engine, incorporating
+flexible flywheel drive, exhaust side. (Smithsonian photo A-54381.)</p>
+</div>
+
+<h2><span class="pagenum"><a id="page57" name="page57"></a>(p. 57)</span> Minor Design Details and Performance of the Wright Engines</h2>
+
+<p>In the Wright brothers' various models were many minor design items
+which altogether required a great deal of consideration, but which did
+not materially affect overall engine performance. The results
+generally could all be classed as good practice; however, one of these
+utilized in the 4-cylinder vertical engine was rather unorthodox and
+consisted of offsetting the cylinders with relation to the crankshaft.
+This arrangement, which can be seen in the drawing (Figure <a href="#img021">11</a>) was
+apparently an attempt to reduce the maximum side load on the piston
+during the power stroke, but since the peak gas loading usually occurs
+at about 10 to 15 percent of the power stroke, this probably did not
+have much effect, and it was not carried over to the 6-cylinder
+design.</p>
+
+<p>All engine bearings were of the plain sleeve type and, except for the
+bronze and steel bearings in the connecting rod, were of babbit. The
+advantages of babbit for bearings were discovered very early in the
+development of the mechanical arts, and apparently the Wrights never
+encountered a bearing loading sufficiently high to cause a structural
+breakdown in this relatively weak material.</p>
+
+<p>Valve openings show no variation through the successive production
+engines, although the Wrights most probably experimented with
+different amounts. The 1903 engine and the vertical 4-and 6-cylinder
+all had lifts of 5/16 in., but the valve-seat angles varied somewhat;
+the records show included angles of 110° to 90°&mdash;not a large
+difference.</p>
+
+<p>The valve-operating mechanism was the same from the first vertical 4
+onward. The high side thrust caused by the cam shape required for the
+very rapid valve opening they chose was, no doubt, the reason for the
+use of the hinged cam follower, and since the same general cam design
+was used in their last engine, the 6-cylinder, the same method of
+operation which had apparently proved very serviceable was continued.
+How satisfactory was the considerably simpler substitute used in the
+Bariquand et Marré version of the 4-cylinder engine is not known.
+Possibly it was one of the alterations in the Wrights' design that
+Wilbur Wright objected to, <span class="pagenum"><a id="page58" name="page58"></a>(p. 58)</span> although in principle it more
+closely conforms to the later fairly standard combination valve tappet
+and roller construction: The available drawings do indicate, however,
+that the cam of the Bariquand et Marré engine was also altered to give
+a considerably less abrupt valve opening than the Wright design, so
+that there was less side thrust. For the Wright 6-cylinder engine
+their 4-cylinder cam was slightly altered to provide a rounding off
+near the top of the lobe, thus providing some reduction in the
+velocity before maximum opening was reached. All their cam designs
+indicate a somewhat greater fear of the effect of seating velocities
+than of opening accelerations.</p>
+
+<p>Since the range of cylinder diameters utilized did not vary greatly,
+the valve sizes were correspondingly fairly uniform. The diameter of
+the valves for the original 4-in.-bore cylinder was 2 in., while that
+for the 4-3/8-in. bore used in the 6-cylinder engine was actually
+slightly smaller, 1-7/8 in. Possibly the Wrights clung too long to the
+automatic inlet valve, although it did serve them well; but possibly,
+as has been previously noted, there were valid reasons for continuing
+its use despite the inherently low volumetric efficiency this
+entailed.</p>
+
+<p>The inherent weakness in the joints of the three-piece connecting rod
+has been pointed out, but aside from this, the design was excellent,
+for all the materials and manufacturing methods required were readily
+available, and structurally it was very sound. Tubular rods were still
+in use in aircraft engines in the 1920s.</p>
+
+<p>The Wrights had a surprisingly thorough grasp of the metallurgy of the
+time, and their choice of materials could hardly have been improved
+upon. Generally they relied upon the more simple and commonly used
+metals even though more sophisticated and technically better alloys
+and combinations were available.<a id="footnotetag17" name="footnotetag17"></a><a href="#footnote17" title="Go to footnote 17"><span class="smaller">[17]</span></a> Case hardening was in widespread
+use in this period but their only utilization of it was in some parts
+of the drive chains purchased completely assembled and in the piston
+pins of their last engine. The treatment of the crankshafts of all
+their engines except the final 6-cylinder was typical of their
+uncomplicated procedure: the particular material was chosen on the
+basis of many years of experience with it, hardening was a very simple
+process, and the expedient of carrying this to a <span class="pagenum"><a id="page59" name="page59"></a>(p. 59)</span> point just
+below the non-machinable range gave them bearing surfaces that were
+sufficiently hard, yet at the same time it eliminated the
+possibility&mdash;present in a heat-treating operation&mdash;of warping the
+finished piece.</p>
+
+<p>In the entire 1903 engine only five basic materials&mdash;excepting those
+in the purchased "magneto" and the platinum facing on the
+ignition-system firing points&mdash;were used: steel, cast iron, aluminum,
+phosphor bronze, and babbit. The steels were all plain carbon types
+with the exception of the sheet manifold, which contained manganese,
+and no doubt this was used because the sheet available came in a
+standard alloy of the time.</p>
+
+<p>Overall, the Wright engines performed well, and in every case met or
+exceeded the existing requirements. Even though aircraft engines then
+were simpler than they became later and the design-development time
+much shorter, their performance stands as remarkable. As a result, the
+Wrights never lacked for a suitable powerplant despite the rapid
+growth in airplane size and performance, and the continual demand for
+increased power and endurance.</p>
+
+<p>Few service records dating from before 1911, when the military
+services started keeping log books, have been found. Some of those for
+the period toward the end of their active era have been preserved, but
+for that momentous period spanning the first few years when the
+Wrights had the only engines in actual continuous flight operation,
+there seems to be essentially nothing&mdash;perhaps because there were no
+standard development methods or routines to follow, no requirements to
+be met with respect to pre-flight demonstrations or the keeping of
+service records. Beginning in 1904, however, and continuing as long as
+they were actively in business, they apparently had in progress work
+on one or more developmental or experimental engines. This policy, in
+combination with the basic simplicity of design of these engines,
+accounted in large measure for their ability to conduct both
+demonstrations and routine flying essentially whenever they chose.</p>
+
+<p>Time between engine overhauls obviously varied. In mid 1906 an engine
+was "rebuilt after running about 12 hours." This is comparatively
+quite a good performance, particularly when it is remembered that
+essentially all the "running" was at full power output. It was
+considerably after 1920 before the Liberty engine was redesigned and
+developed to the stage where it was capable of operating 100 hours
+between overhauls, even though it was being used at cruising, or less
+than full, power for most of this time.</p>
+
+<p>The Wrights of course met with troubles and failures, but it is
+difficult, from the limited information available, to evaluate these
+and judge their relative severity. Lubrication seems to have been a
+rather constant problem, particularly in the early years. Although
+some bearing lubrication <span class="pagenum"><a id="page60" name="page60"></a>(p. 60)</span> troubles were encountered from time
+to time, this was not of major proportions, and they never had to
+resort to force-feed lubrication of the main or rod big-end bearings.
+The piston and cylinder-barrel bearing surfaces seem to have given
+them the most trouble by far, and examination of almost any used early
+Wright engine will usually show one or more pistons with evidence of
+scuffing in varying degrees, and this is also apparent in the
+photographs in the record. This is a little difficult to understand
+inasmuch as most of the time they had the very favorable operating
+condition of cast iron on cast iron. Many references to piston seizure
+or incipient seizure, indicated by a loss of power, occur, and this
+trouble may have been aggravated by the very small piston clearances
+utilized. Why these small clearances were continued is also not
+readily explainable, except that with no combination of true
+oil-scraper rings, which was the basic reason why the final form of
+aviation piston engine was able to reach its unbelievably low oil
+consumptions, their large and rather weak compression rings were
+probably not doing an adequate job of oil control, and they were
+attempting to overcome this with a quite tight piston fit.<a id="footnotetag18" name="footnotetag18"></a><a href="#footnote18" title="Go to footnote 18"><span class="smaller">[18]</span></a> In any
+event, they did encounter scuffing or seizing pistons and cylinder
+over-oiling at the same time. As late as 4 May 1908 in the Wright
+<i>Papers</i> there appears the notation: "The only important change has
+been in the oiling. The engine now feeds entirely by splash...."</p>
+
+<p>Their troubles tended to concentrate in the cylinder-piston
+combination, as has been true of almost all piston engines. References
+to broken cylinders are frequent. These were quite obviously cylinder
+barrels, as replacement was common, and this again is not readily
+explainable. The material itself, according to Orville Wright, had a
+very high tensile strength, and in the 1903 engine more than ample
+material was provided, as the barrel all the way down to well below
+the attachment to the case was 7/32 in. thick. The exact location of
+the point of failure was never recorded, but in its design are many
+square corners serving as points of stress concentration. Also, of
+course, no method was then available for determining a faulty casting,
+except by visual observation of imperfections on the surface, and this
+was probably the more common cause. It is interesting, however, that
+the engine finally assembled in 1928 for installation in the 1903
+airplane sent to England has a cracked cylinder barrel, the crack
+originating at a <span class="pagenum"><a id="page61" name="page61"></a>(p. 61)</span> sharp corner in the slot provided at the
+bottom of the barrel for screwing it in place.</p>
+
+<p>Valve failures were also a continuing problem, and Chenoweth reports
+that a large proportion of the operating time of the 1904-1906
+development engine was concentrated on attempts to remedy this
+trouble. None of their cams, including those of the 6-cylinder engine,
+evidence any attempt to effect a major reduction in seating
+velocities. United States Navy log books of 1912 and 1913 record many
+instances of inlet valves "broken at the weld," indicating that some
+of the earlier 6-cylinder engines were fitted with valves of welded
+construction.</p>
+
+<p>For the engineer particularly, the fascination of the Wrights' engine
+story lies in its delineation of the essentially perfect engineering
+achievement by the classic definition of engineering&mdash;to utilize the
+available art and science to accomplish the desired end with a minimum
+expenditure of time, energy, and material. Light weight and
+operability were the guiding considerations; these could be obtained
+only through constant striving for the utmost simplicity. Always
+modest, the Wrights seem to have been even more so in connection with
+their engine accomplishments. Although the analogy is somewhat
+inexact, the situation is reminiscent of the truism often heard in the
+aircraft propulsion business&mdash;few people know the name of Paul
+Revere's horse. Yet, as McFarland has pointed out, "The engine was in
+fact far from their meanest achievement." With hardly any experience
+in this field and only a meagerly equipped machine shop, they designed
+and assembled an internal combustion engine that exceeded the
+specifications they had laid down as necessary for flight and had it
+operating in a period of about two months elapsed time. The basic form
+they evolved during this unequalled performance carried them through
+two years of such successful evolutionary flight development that
+their flying progressed from a hop to mastery of the art. And the
+overall record of their powerplants shows them to have been remarkably
+reliable in view of the state of the internal combustion engine at
+that time.</p>
+
+<h2><span class="pagenum"><a id="page62" name="page62"></a>(p. 62)</span> Appendix</h2>
+
+<h3>Characteristics of the Wright Flight Engines</h3>
+
+<table  border="0" cellpadding="2" summary="Characteristics.">
+<tr>
+<td class="bordtop">&nbsp;</td>
+<td class="center bordtop"><i>1903<br> First flight<br> engine<a id="footnotetaga" name="footnotetaga"></a><a href="#footnotea" title="Go to footnote a"><span class="smaller">[a]</span></a></i></td>
+<td class="center bordtop"><i>1904-1905<br> Experimental<br> flights</i></td>
+<td class="center bordtop"><i>1908-1911<br> Demonstrations<br> and<br> service</i></td>
+<td class="center bordtop"><i>1911-1915<br> service</i></td>
+</tr>
+<tr>
+<td colspan="5" class="bordbot">&nbsp;</td>
+</tr>
+<tr>
+<td>Cyl./Form</td>
+<td class="center">4/flat</td>
+<td class="center">4/flat</td>
+<td class="center">4/vertical</td>
+<td class="center">6/vertical</td>
+</tr>
+<tr>
+<td>Bore and stroke (in.)</td>
+<td class="center">4×4</td>
+<td class="center">4-1/8×4</td>
+<td class="center">4-3/8×4</td>
+<td class="center">4-3/8×4-1/2</td>
+</tr>
+<tr>
+<td>Displacement (cu. in.)</td>
+<td class="center">201</td>
+<td class="center">214</td>
+<td class="center">240</td>
+<td class="center">406</td>
+</tr>
+<tr>
+<td>Horsepower</td>
+<td class="center">8.25-16</td>
+<td class="center">15-21</td>
+<td class="center">28-42</td>
+<td class="center">50-75</td>
+</tr>
+<tr>
+<td>RPM</td>
+<td class="center">670-1200</td>
+<td class="center">1070-1360</td>
+<td class="center">1325-1500</td>
+<td class="center">1400-1560</td>
+</tr>
+<tr>
+<td>MEP</td>
+<td class="center">49-53</td>
+<td class="center">52-57</td>
+<td class="center">70-87</td>
+<td class="center">70-94</td>
+</tr>
+<tr>
+<td class="bordbot">Weight (lb)</td>
+<td class="center bordbot">140-180</td>
+<td class="center bordbot">160-170</td>
+<td class="center bordbot">160-180</td>
+<td class="center bordbot">265-300</td>
+</tr>
+</table>
+
+<p><span class="pagenum"><a id="page63" name="page63"></a>(p. 63)</span> It is not possible to state the exact quantities of each
+engine that the Wrights produced up to the time that their factory
+ceased operation in 1915. Chenoweth gives an estimate, based on the
+recollection of their test foreman, of 100 vertical 4s and 50 6s. My
+estimate (see page <a href="#page2">2</a>) places the total of all engines at close to 200.
+Original Wright-built engines of all four of these basic designs are
+in existence, although they are rather widely scattered. The
+Smithsonian's National Air and Space Museum has examples of them all,
+including, of course, the unique first-flight engine. Their condition
+varies, but many are operable, or could easily be made so. Among the
+best are the first-flight engine and the last vertical 6, at the
+Smithsonian, the first vertical 6, at the United States Air Force
+Museum, and the vertical 4, at the Carillon Park Museum.</p>
+
+<p>The Wrights were constantly experimenting and altering, and this in
+connection with the lack of complete records makes it almost
+impossible to state with any certainty specific performances of
+individual engines at given times. Weights sometimes included
+accessories and at others did not. Often they were of the complete
+powerplant unit, including radiator and water and fuel, with no
+clarification. In the table, performance is given in ranges which are
+thought to be the most representative of those actually utilized.
+Occasionally performances were attained even beyond the ranges given.
+For example, the 4×4-in. flat development engine eventually
+demonstrated 25 hp at an MEP of approximately 65 psi.</p>
+
+<p>One important figure&mdash;the horsepower actually utilized during the
+first flight&mdash;is quite accurately known. In 1904 the 1904-1905 flight
+engine, after having been calibrated by their prony-brake test-fan
+method, was used to turn the 1903 flight propellers, and Orville
+Wright calculated this power to be 12.05 bhp by comparing the
+calibrated engine results with those obtained with the flight engine
+at Kitty Hawk when tested under similar conditions. However, since the
+tests were conducted in still air with the engine stationary, this did
+not exactly represent the flight condition. No doubt the rotational
+speed of the engine and propellers increased somewhat with the forward
+velocity of the airplane so that unless the power-rpm curve of the
+engine was flat, the actual horsepower utilized was probably a small
+amount greater than Orville's figures. The lowest power figure shown
+for this engine is that of its first operation.</p>
+
+<p>No fuel consumption figures are given, primarily because no
+comprehensive data have been found. This is most probably because in
+the early flight years, when the Wrights were so meticulously
+measuring and recording technical information on the important factors
+affecting their work, the flights were of such short duration that
+fuel economy was of very minor importance. After success had been
+achieved, they ceased to keep detailed records on very much except
+their first interest&mdash;the flying <span class="pagenum"><a id="page64" name="page64"></a>(p. 64)</span> machine itself&mdash;and when the
+time of longer flights arrived, the fuel consumption that resulted
+from their best engine design efforts was simply accepted. The range
+obtained became mostly a matter of aerodynamic design and weight
+carried. Orville Wright quotes an early figure of brake thermal
+efficiency for the 1903 engine that gives a specific fuel consumption
+of .580 lb of fuel per bhp/hr based on an estimate of the heating
+value of the fuel they had. This seems low, considering the
+compression ratio and probable leakage past their rather weak piston
+rings, but it is possible. In an undated entry, presumably in 1905,
+Orville Wright's notebook covered fuel consumption in terms of miles
+of flight; one of the stated assumptions in the entry is, "One
+horsepower consumes .60 pounds per horsepower hour"&mdash;still quite good
+for the existing conditions. Published figures for the 6-60 engine
+centered around .67 lb/hp hr for combined fuel and oil consumption.</p>
+
+<h3>The Wright Shop Engine</h3>
+
+<p>Despite the fact that the Wright shop engine was not a flight unit, it
+is interesting both because it was a well designed stationary
+powerplant with several exceedingly ingenious features, and because
+its complete success was doubtless a major factor in the Wrights'
+decision to design and build their own first flight engine. Put in
+service in their small shop in the fall of 1901, it was utilized in
+the construction of engine and airframe parts during the vital years
+from 1902 through 1908 and, in addition, it provided the sole means of
+determining the power output of all of their early flight engines. By
+means of a prony brake, its power output was carefully measured and
+from this the amount of power required for it to turn certain fans or
+test clubs was determined. These were then fitted to the flight
+engines and the power developed calculated from the speed at which the
+engines under test would turn the calibrated clubs. Although a
+somewhat complex method of using power per explosion of the shop
+engine was made necessary by the basic governor control of the engine,
+the final figures calculated by means of the propeller cube law seem
+to have been surprisingly accurate.<a id="footnotetag19" name="footnotetag19"></a><a href="#footnote19" title="Go to footnote 19"><span class="smaller">[19]</span></a> Restored under the personal
+direction of Charles Taylor, it is in the Henry Ford Museum in
+Dearborn, Michigan, together with the shop machinery it operated.</p>
+
+<p>The engine was a single cylinder, 4-stroke-cycle "hot-tube" ignition
+type. The cylinder, of cast iron quite finely and completely finned
+for its <span class="pagenum"><a id="page65" name="page65"></a>(p. 65)</span> day, was air-cooled, or rather, air-radiated, as
+there was no forced circulation of air over it, the atmosphere
+surrounding the engine simply soaking up the dissipated heat. Although
+this was possibly a desirable adjunct in winter, inside the small shop
+in Dayton, the temperature there in summer must have been quite high
+at times. The operating fuel was city illuminating gas, which was also
+utilized to heat, by means of a burner, the ignition tube. This part
+was of copper, with one completely closed end positioned directly in
+the burner flame; the other end was open and connected the interior of
+the tube to the combustion chamber. The inlet valve was of the usual
+automatic type while the exhaust valve was mechanically operated. The
+fuel gas flow was controlled by a separate valve mechanically
+connected to the inlet valve so that the opening of the inlet valve
+also opened the gas valve, and gas and air were carried into the
+cylinder together.</p>
+
+<a id="img030" name="img030"></a>
+<div class="figcenter">
+<img src="images/img030.jpg" width="400" height="321" alt="" title="">
+<p><i>Figure 16.</i>&mdash;Shop engine, 1901, showing governor and
+exhaust valve cam. (Photo courtesy R. V. Kerley.)</p>
+</div>
+
+<p>The engine was of normal stationary powerplant design, having a heavy
+base and two heavy flywheels, one on each side of the crank. These
+were <span class="pagenum"><a id="page66" name="page66"></a>(p. 66)</span> necessary to ensure reasonably uniform rotational speed,
+as, in addition to having only one cylinder, the governing was of the
+hit-and-miss type. It had a 6×7-in. bore and stroke and would develop
+slightly over 3 hp at what was apparently its normal operating speed
+of 447 rpm, which gives an MEP of 27 psi.</p>
+
+<p>The engine is noteworthy not only for its very successful operation
+but also because it incorporated two quite ingenious features. One was
+the speed-governing mechanism. As in the usual hit-and-miss operation,
+the engine speed was maintained at a constant value, the output then
+being determined by the number of power strokes necessary to
+accomplish this. The governor proper was a cylindrical weight free to
+slide along its axis on a shaft fastened longitudinally to a spoke of
+one of the flywheels. A spring forced it toward the center of the
+wheel, while centrifugal force pulled it toward the rim against the
+spring pressure. After each opening of the valve the exhaust-valve
+actuating lever was automatically locked in the valve-open position by
+a spring-loaded pawl, or catch. The lever had attached to it a small
+side extension, or bar, which, when properly forced, would release the
+catch and free the actuating lever. This bar was so positioned as to
+be contacted by the governor weight when the engine speed was of the
+desired value or lower, thus maintaining regular valve operation; but
+an excessive speed would move the governor weight toward the rim and
+the exhaust valve would then be held in the open position during the
+inlet stroke, so no cylinder charge would be ingested. Since the
+ignition was not mechanically timed, the firing of the charge was
+dependent only on the compression of the inlet charge in the cylinder,
+so it made no difference whether the governor caused the engine to
+cease firing for an odd or even number of revolutions, even though the
+engine was operating on a 4-stroke cycle at all times.</p>
+
+<a id="img031" name="img031"></a>
+<div class="figcenter">
+<a href="images/img031.jpg">
+<img src="images/img031tb.jpg" width="400" height="353" alt="" title=""></a>
+<p><i>Figure 17.</i>&mdash;Shop engine, 1901, showing operation of
+exhaust valve cam. (Pratt &amp; Whitney drawing.)</p>
+</div>
+
+<p>The exhaust valve operating cam was even more ingenious. To obtain
+operation on a 4-stroke cycle and still avoid the addition of a
+half-speed camshaft, a cam traveling at crankshaft speed was made to
+operate the exhaust valve every other revolution (see Figure <a href="#page17">17</a>). It
+consisted of a very slim quarter-moon outline fastened to a disc on
+the crankshaft by a single bearing bolt through its middle which
+served as the pivot about which it moved. Just enough clearance was
+provided between the inside of the quarter-moon and the crankshaft to
+allow the passage of the cam-follower roller. The quarter-moon,
+statically balanced and free to move about its pivot, basically had
+two positions. In one the leading edge was touching the shaft (Figure
+<a href="#img031">17b</a>), so that when the cam came to the cam follower, the follower was
+forced to go over the top of the cam, thus opening the exhaust valve.
+When the cam pivot point had passed the roller, the pressure of the
+exhaust valve spring forced the following edge of the cam <span class="pagenum"><a id="page67" name="page67"></a>(p. 67)</span>
+into contact with the shaft and this movement, which separated the
+leading edge of the cam from the shaft, provided sufficient space
+between it and the shaft for the roller to enter (Figure <a href="#img031">17c</a>). Thus,
+when the leading edge of the cam next reached the roller, the roller,
+being held against the crankshaft by the valve spring pressure (Figure
+<a href="#img031">17d</a>), entered the space between the cam and the shaft and there was no
+actuation of the valve. In exiting from the space, it raised the
+trailing edge of the cam, forcing the leading edge against the shaft
+(Figure <a href="#img031">17a</a>) so that at the next meeting a normal valve opening would
+take place. The cam was maintained by friction <span class="pagenum"><a id="page68" name="page68"></a>(p. 68)</span> alone in the
+position in which it was set by the roller, but since the amount of
+this could be adjusted to any value, it could be easily maintained
+sufficient to offset the small centrifugal force tending to put the
+cam in a neutral position.<a id="footnotetag20" name="footnotetag20"></a><a href="#footnote20" title="Go to footnote 20"><span class="smaller">[20]</span></a></p>
+
+<h2><span class="pagenum"><a id="page69" name="page69"></a>(p. 69)</span> Bibliography</h2>
+
+<ul class="none biblio">
+<li><span class="smcap">Angle, Glenn D.</span> Wright. Pages 521-523 in <i>Airplane Engine
+     Encyclopedia, an Alphabetically Arranged Compilation of All
+     Available Data on the World's Airplane Engines</i>. Dayton, Ohio:
+     The Otterbein Press, 1921.</li>
+
+<li><span class="smcap">Baker, Max P.</span> The Wright Brothers as Aeronautical Engineers.
+     <i>Annual Report of ... the Smithsonian Institution ... for the
+     Year Ended June 30, 1950</i>, pages 209-223, 4 figures, 9 plates.</li>
+
+<li><span class="smcap">Beaumount, William Worby.</span> <i>Motor Vehicles and Motors: Their
+     Design, Construction, and Working by Steam, Oil, and
+     Electricity.</i> 2 volumes. Philadelphia: J. B. Lippincott,
+     1901-1902.</li>
+
+<li><span class="smcap">Chenoweth, Opie.</span> Power Plants Built by the Wright Brothers.
+     <i>S.A.E. Quarterly Transactions</i> (January 1951), 5:14-17.</li>
+
+<li><span class="smcap">Forest, Fernand.</span> <i>Les Bateaux automobiles.</i> Paris: H. Dunod et E.
+     Pinat, Éditeurs, 1906.</li>
+
+<li><span class="smcap">Gough, Dr. H. J.</span> Materials of Aircraft Construction. <i>Journal of
+     the Royal Aeronautical Society</i> (November 1938), 42:922-1032.
+     Illustrated.</li>
+
+<li><span class="smcap">Kelly, Fred C.</span> <i>Miracle at Kitty Hawk; the Letters of Wilbur and
+     Orville Wright.</i> New York: Farrar, Straus and Young, 1951.</li>
+
+<li>&mdash;&mdash;&mdash;&mdash;&mdash; <i>The Wright Brothers, a Biography Authorized by
+     Orville Wright.</i> New York: Harcourt, Brace &amp; Co., 1943.</li>
+
+<li><span class="smcap">Kennedy, Rankin.</span> <i>Flying Machines: Practice and Design. Their
+     Principles, Construction and Working.</i> 158 pages. London:
+     Technical Publishing Co., Ltd., 1909.</li>
+
+<li><span class="smcap">Lawrance, Charles L.</span> <i>The Development of the Aeroplane Engine in
+     the United States.</i> Pages 409-429 in International Civil
+     Aeronautics Conference, Washington, D.C., 12-14 December 1928,
+     Papers Submitted by the Delegates for Consideration by the
+     Conference. Washington: Government Printing Office, 1928.</li>
+
+<li><span class="smcap">McFarland, Marvin W.</span> <i>The Papers of Wilbur and Orville Wright.</i> 2
+     volumes. New York: McGraw Hill Book Co., 1953.</li>
+
+<li><span class="smcap">Renstrom, Arthur G.</span> Wilbur and Orville Wright: A Bibliography
+     Commemorating the Hundredth Anniversary of the Birth of Wilbur
+     Wright, April 16, 1867. Washington, D.C.: The Library of Congress
+     [Government Printing Office], 1968. Contains 2055 entries.</li>
+
+<li>The 6-Cylinder 60-Horsepower Wright Motor. <i>Aeronautics</i>
+     (November 1913), 13(5):177-179.</li>
+
+<li>Wright Brothers. Pages 829-830 in <i>Aerosphere 1939, Including
+     World's Aircraft Engines, with Aircraft Directory</i>, Glenn D.
+     Angle, editor. New York: Aircraft Publishers, 1940.</li>
+</ul>
+
+<h2><span class="pagenum"><a id="page71" name="page71"></a>(p. 71)</span> Index</h2>
+
+<div class="index">
+<p>Angle, Glenn D.,
+<a href="#page51">51</a></p>
+
+
+<p><i>Baby Grand Racer</i>,
+<a href="#page47">47</a><br>
+
+  Baker, Max P.
+<a href="#page1">1</a>,
+<a href="#page10">10</a>,
+<a href="#page26">26</a>,
+<a href="#page28">28</a><br>
+
+  Bariquand et Marré,
+<a href="#page43">43</a>,
+<a href="#page43">44</a>-45,
+<a href="#page57">57</a>-58<br>
+
+  Beaumount, William Worby,
+<a href="#page9">9</a>,
+<a href="#page25">25</a><br>
+
+  Bristol Siddeley Engines, Ltd.,
+<a href="#page43">44</a>-45</p>
+
+
+<p>Carillon Park Museum, Dayton, Ohio,
+<a href="#pageix">ix</a>,
+<a href="#footnote5">5n</a>,
+<a href="#page7">7</a>,
+<a href="#page35">37</a><br>
+
+  Chanute, Octave,
+<a href="#page28">28</a><br>
+
+  Chenoweth, Opie,
+<a href="#pageix">ix</a>,
+<a href="#page22">22</a>,
+<a href="#page35">35</a>,
+<a href="#page42">42</a>,
+<a href="#page63">63</a><br>
+
+  Christman, Louis P.,
+<a href="#pageix">ix</a>,
+<a href="#page7">7</a>,
+<a href="#page8">8</a>,
+<a href="#page28">28</a><br>
+
+  Cole, Gilmoure N.,
+<a href="#pageix">ix</a><br>
+
+  Clarke, J. H.,
+<a href="#page18">18</a></p>
+
+
+<p>Daimler-Benz A. G.,
+<a href="#pageix">ix</a>,
+<a href="#page10">10</a>,
+<a href="#page13">13</a></p>
+
+
+<p>Engineers Club, Dayton, Ohio,
+<a href="#pageix">ix</a>,
+<a href="#page32">32</a></p>
+
+
+<p>Ford, Henry,
+<a href="#page8">8</a><br>
+
+  Ford, Henry, Museum, Dearborn, Michigan,
+<a href="#page8">8</a>,
+<a href="#page64">64</a><br>
+
+  Forest, Fernand,
+<a href="#page11">11</a><br>
+
+  Franklin Institute, Philadelphia, Pennsylvania,
+<a href="#pageix">ix</a>,
+<a href="#page47">47</a></p>
+
+
+<p>Gough, Dr. H. J.,
+<a href="#footnote17">58n</a></p>
+
+
+<p>Howell Cheney Technical School, Manchester, Connecticut,
+<a href="#pagex">x</a>,
+<a href="#page14">14</a>,
+<a href="#page15">15</a></p>
+
+
+<p>Kelly, Fred C,
+<a href="#footnote4">4n</a><br>
+
+  Kerley, R. V.,
+<a href="#pageix">ix</a>,
+<a href="#page65">65</a><br>
+
+  <i>Kitty Hawk Flyer</i>,
+<a href="#img001">ii</a>,
+<a href="#page3">3</a></p>
+
+
+<p>Langley [Samuel P.] Aerodrome,
+<a href="#page9">9</a>,
+<a href="#page62">62</a><br>
+
+  Loening, Grover C,
+<a href="#footnote11">13n</a></p>
+
+
+<p>Manly, Charles L.,
+<a href="#page9">9</a>,
+<a href="#page62">62</a><br>
+
+  Maxim, Sir Hiram Stevens,
+<a href="#page3">3</a><br>
+
+  McFarland, Marvin W.,
+<a href="#page1">1</a>,
+<a href="#page33">33</a>,
+<a href="#page47">47</a>,
+<a href="#page61">61</a><br>
+
+  Miller-Knoblock Manufacturing Co., South Bend, Indiana,
+<a href="#page26">26</a></p>
+
+
+<p>National Park Service, Cape Hatteras National Seashore,
+<a href="#img001">ii</a>,
+<a href="#pageix">ix</a><br>
+
+  Neue Automobil-Gesellschaft,
+<a href="#page43">43</a></p>
+
+
+<p>Porter, L. Morgan,
+<a href="#pageix">ix</a><br>
+
+  Pratt &amp; Whitney Aircraft Corp.,
+<a href="#pagev">v</a>,
+<a href="#pagex">x</a>,
+<a href="#page35">37</a>,
+<a href="#page39">40</a>-41,
+<a href="#page49">49</a>,
+<a href="#page52">52</a>,
+<a href="#page52">53</a>,
+<a href="#page67">67</a><br>
+
+  Pruckner, Anton,
+<a href="#page33">33</a></p>
+
+
+<p>Rockwell, A. L.,
+<a href="#pageix">ix</a>,
+<a href="#page35">37</a></p>
+
+
+<p>Santos-Dumont, Alberto,
+<a href="#page11">11</a><br>
+
+  Science Museum, London,
+<a href="#pagex">x</a>,
+<a href="#page5">5</a>,
+<a href="#page6">6</a>,
+<a href="#page7">7</a>,
+<a href="#page8">8</a>,
+<a href="#page11">11</a>,
+<a href="#page21">21</a>,
+<a href="#page22">23</a>,
+<a href="#page26">26</a></p>
+
+
+<p>Taylor, Charles E.,
+<a href="#page5">5</a>,
+<a href="#page64">64</a></p>
+
+
+<p>United Aircraft Corp.,
+<a href="#page5">v</a>,
+<a href="#pagex">x</a></p>
+
+
+<p>Western Society of Engineers,
+<a href="#page2">2</a><br>
+
+  Whitehead, Gustave,
+<a href="#page33">33</a><br>
+
+  Wittemann, Charles,
+<a href="#footnote15">33n</a><br>
+
+  Wright, Bishop Milton (father),
+<a href="#page28">28</a><br>
+
+  Wright, Katherine (sister),
+<a href="#page4">4</a></p>
+
+<p>Zenith carburetor,
+<a href="#page52">52</a></p>
+</div>
+
+
+<p class="p2 center smaller">*U.S. GOVERNMENT PRINTING OFFICE: 1971&mdash;397-764</p>
+
+<h3>Publication in Smithsonian Annals of Flight</h3>
+
+<p><i>Manuscript</i> for serial publications are accepted by the Smithsonian
+Institution Press, subject to substantive review, only through
+departments of the various Smithsonian museums. Non-Smithsonian
+authors should address inquiries to the appropriate department. If
+submission is invited, the following format requirements of the Press
+will govern the preparation of copy.</p>
+
+<p><i>Copy</i> must be typewritten, double-spaced, on one side of standard
+white bond paper, with 1-1/2" top and left margins, submitted in
+ribbon copy with a carbon or duplicate, and accompanied by the
+original artwork. Duplicate copies of all material, including
+illustrations, should be retained by the author. There may be several
+paragraphs to a page, but each page should begin with a new paragraph.
+Number all pages consecutively, including title page, abstract, text,
+literature cited, legends, and tables. A manuscript should consist of
+at least thirty pages, including typescript and illustrations.</p>
+
+<p>The <i>title</i> should be complete and clear for easy indexing by
+abstracting services. Include an <i>abstract</i> as an introductory part of
+the text, followed by an identification of the <i>author</i> that includes
+his professional mailing address. A <i>table of contents</i> is optional.
+An <i>index</i>, if required, may be supplied by the author when he returns
+page proof.</p>
+
+<p><i>Headings</i> are to be used discriminately and must be typed with extra
+space above and below.</p>
+
+<p>For matters of general style (including bibliography and footnotes or
+notes) follow <i>A Manual of Style</i>, 12th edition, University of Chicago
+Press, 1969. For more detailed treatment on footnotes and bibliography
+see Citation Style Manual prepared for MHT publications (October
+1963). Use the modern order of citing dates: 29 February 1972.</p>
+
+<p>Simple <i>tabulations</i> in the text (e.g., columns of data) may carry
+headings or not, but they should not contain rules. Formal tables must
+be submitted as pages separate from the text, and each table, no
+matter how large, should be pasted up as a single sheet of copy.</p>
+
+<p>Use the <i>metric system</i> instead of (or in addition to) the English
+system.</p>
+
+<p><i>Illustrations</i> (line drawings, maps, photographs, shaded drawings)
+can be intermixed throughout the printed text. They must be clearly
+numbered in sequence, as they are to appear in the text. They will be
+termed <i>Figures</i> and should be numbered consecutively; if a group of
+figures is treated as a single figure, however, the individual
+components should be indicated by lowercase italic letters on the
+illustration, in the legend, and in text references: "Figure 9<i>b</i>."
+Type (double spaced) all legends on a page or pages separate from the
+text and not attached to the artwork.</p>
+
+<p>In the <i>bibliography</i> spell out book, journal, and article titles,
+using initial caps with all words except minor terms such as "and, of,
+the." (For capitalization of foreign language titles, follow the
+national practice of the language.) Underscore (for italics) book and
+journal titles. Use the parentheses-colon system: 10(2):5-9 for
+volume, number, and page citations.</p>
+
+<p><i>Notes</i> and <i>footnotes</i>, accompanying a manuscript, are to be typed
+consecutively, double-spaced, and on sheets separate from the text. In
+typing the notes the number should be typed on the line and followed
+by a period. The footnote number should be typed slightly above the
+line and should follow any punctuation mark except a dash.</p>
+
+<p>For <i>free copies</i> of his own paper, a Smithsonian author should
+indicate his requirements on "Form 36" (submitted to the Press with
+the manuscript). A non-Smithsonian author will receive fifty free
+copies; order forms for quantities above this amount with instructions
+for payment will be supplied when page proof is forwarded.</p>
+
+<h2>Notes</h2>
+<div class="footnote">
+<p><a id="footnote1" name="footnote1"></a>
+<b><a href="#footnotetag1">1</a></b>: An extensive bibliography, essentially as complete at
+this time as when it was compiled in the early 1950s, is given on
+pages 1240-1242 of volume 2 of <i>The Papers of Wilbur and Orville
+Wright</i>, 1953.</p>
+
+<p><a id="footnote2" name="footnote2"></a>
+<b><a href="#footnotetag2">2</a></b>: Max P. Baker was a technical adviser to the Wright estate
+and as such had complete access to all of the material it contained.</p>
+
+<p><a id="footnote3" name="footnote3"></a>
+<b><a href="#footnotetag3">3</a></b>: In the 1890s the wealthy inventor Sir Hiram Stevens Maxim
+conducted an experiment of considerable magnitude with a flying
+machine that utilized a twin-cylinder compound steam powerplant. It
+was developed to the flight-test stage.</p>
+
+<p><a id="footnote4" name="footnote4"></a>
+<b><a href="#footnotetag4">4</a></b>: Fred C. Kelly, <i>Miracle at Kitty Hawk</i>, 1951.</p>
+
+<p><a id="footnote5" name="footnote5"></a>
+<b><a href="#footnotetag5">5</a></b>: Charles E. Taylor (Charley Taylor to the many who knew
+him) was in effect the superintendent of and also the only employee to
+work in the original small machine shop. A most versatile and
+efficient mechanic and machine operator, he made many parts for all of
+the early engines, and in the manner of the experimental machinist,
+worked mainly from sketches. He also had charge of the bicycle shop
+and its business in the absence of the Wrights.</p>
+
+<p><a id="footnote6" name="footnote6"></a>
+<b><a href="#footnotetag6">6</a></b>: This is a charitable agency set up by the late Colonel
+and Mrs. E. A. Deeds primarily for the purpose of building and
+supporting the Deeds Carillon and the Carillon Park Museum in Dayton,
+Ohio.</p>
+
+<p><a id="footnote7" name="footnote7"></a>
+<b><a href="#footnotetag7">7</a></b>: The Science Museum expressed a desire to have these but
+never received them. There is a reference to them in a letter to the
+Museum from the executors of his estate dated 20 February 1948, but is
+seems rather obvious from the text that by this time the drawings
+mentioned by Orville Wright in his 1943 letter had become confused
+with those being prepared by Christman for the Smithsonian
+Institution. The Science Museum did have constructed from its own
+drawings a very fine replica which is completely operable at this
+time.</p>
+
+<p><a id="footnote8" name="footnote8"></a>
+<b><a href="#footnotetag8">8</a></b>: There is a third set of drawings prepared by the Ford
+Motor Company also marked as being of the 1903 engine and these are
+rather well distributed in various museums and institutions. What this
+set is based on has been impossible to determine but it is indicated
+from the existence of actual engines and parts and the probable date
+of their preparation (no date is given on the drawings themselves)
+that they were copied from drawings previously made, and therefore add
+nothing to them. The Orville Wright-Henry Ford friendship originated
+rather late, considering Ford's avid interest in history and
+mechanical things. This tardiness could possibly have been the result
+of Wright coolness&mdash;a coolness caused by a report, at the time the
+validity of the Wright patents was being so strongly contested, that
+Ford had advised some of those opposing the Wrights to persevere and
+to obtain the services of his patent counsel who had been successful
+in overturning the Selden automobile patent. If this barrier ever
+existed it was surmounted, and Ford spent much effort and went to
+considerable expense to collect the Wright home and machine shop for
+his Dearborn museum. The shop equipment apparently had been widely
+scattered and its retrieval was a major task. It is most likely that
+the drawings resulted from someone's effort to follow out an order to
+produce a set of Ford drawings of the original engine. A small scale
+model of the 1903 flight engine, constructed under the supervision of
+Charles Taylor, is contained in the Dearborn Museum.</p>
+
+<p><a id="footnote9" name="footnote9"></a>
+<b><a href="#footnotetag9">9</a></b>: Charles L. Manly was engaged in the development of the
+engine for the Langley Aerodrome. See also footnote to Table on page
+<a href="#footnotea">62</a>.</p>
+
+<p><a id="footnote10" name="footnote10"></a>
+<b><a href="#footnotetag10">10</a></b>: Fernand Forest, <i>Les Bateaux Automobiles</i>, 1906.</p>
+
+<p><a id="footnote11" name="footnote11"></a>
+<b><a href="#footnotetag11">11</a></b>: Grover Loening, letter of 10 April 1963, to the
+Smithsonian Institution.</p>
+
+<p><a id="footnote12" name="footnote12"></a>
+<b><a href="#footnotetag12">12</a></b>: Assuming a rich mixture, consumption of all the air, and
+an airbrake thermal efficiency of 24.50% for the original engine, the
+approximate volumetric efficiency of the cylinder is calculated to
+have been just under 40%.</p>
+
+<p><a id="footnote13" name="footnote13"></a>
+<b><a href="#footnotetag13">13</a></b>: A rather thorough stress analysis of the rod shows it to
+compare very favorably with modern practice. In the absence of an
+indicator card for the 1903 engine, if a maximum gas pressure of five
+times the MEP is assumed, the yield-tension factor of safety is
+measurably higher than that of two designs of piston engines still in
+wide service, and the column factor of safety only slightly less. The
+shear stresses in the brazed and threaded joints are so low as to be
+negligible.</p>
+
+<p><a id="footnote14" name="footnote14"></a>
+<b><a href="#footnotetag14">14</a></b>: Rankin Kennedy, <i>Flying Machines&mdash;Practice and Design</i>,
+1909.</p>
+
+<p><a id="footnote15" name="footnote15"></a>
+<b><a href="#footnotetag15">15</a></b>: Considerable doubt surrounds Whitehead's actual flight
+accomplishments, but Pruckner's engines were certainly used, as
+several were sold to early pioneers, including Charles Wittemann. It
+is probable that the specific power output was not very great, for the
+air-cooled art of this time was not very advanced and Pruckner had a
+rather poor fin design. But the change to water cooling eliminated
+this trouble, and the engines were most simple, should have been
+relatively quite light, and with enough development could probably
+have been made into sufficiently satisfactory flying units for that
+period.</p>
+
+<p><a id="footnote16" name="footnote16"></a>
+<b><a href="#footnotetag16">16</a></b>: A drawing of the camshaft is held by The Franklin
+Institute.</p>
+
+<p><a id="footnote17" name="footnote17"></a>
+<b><a href="#footnotetag17">17</a></b>: Baker states that the first crankshaft was made from a
+slab of armor plate and if this is correct the alloy was a rather
+complex one of approximately .30-.35 carbon, .30-.80 manganese, .10
+silicon, .04 phosphorus, .02 sulphur, 3.25-3.50 nickel, 0.00-1.90
+chromium; however, all the rest of the evidence, including Orville
+Wright's statement to Dr. Gough, would seem to show that it was made
+of what was called tool steel (approximately 1.0 carbon).</p>
+
+<p><a id="footnote18" name="footnote18"></a>
+<b><a href="#footnotetag18">18</a></b>: Their intended piston ring tension is not known.
+Measurements of samples from the 4-and 6-cylinder vertical engines
+vary greatly, ranging from less than 1/2 lb per sq in. to almost 1-1/4
+lb. The validity of these data is very questionable as they apply to
+parts with unknown length of service and amount of wear. It seems
+quite certain, however, that even when new the unit tension figure
+with their wide rings was only a small fraction of that of the modern
+aircraft piston engine.</p>
+
+<p><a id="footnote19" name="footnote19"></a>
+<b><a href="#footnotetag19">19</a></b>: <i>The Papers of Wilbur and Orville Wright</i>, volume 2,
+Appendix.</p>
+
+<p><a id="footnote20" name="footnote20"></a>
+<b><a href="#footnotetag20">20</a></b>: The Wrights apparently never applied for an engine
+patent of any kind. This no doubt grew out of their attitude of
+regarding the engine as an accessory and deprecating their work in
+this field. A reasonably complete patent search indicates that this
+particular cam device has never been patented, although a much more
+complex arrangement accomplishing the same purpose was patented in
+1900, and a patent application on a cam-actuating mechanism
+substantially identical to that of the Wrights and intended for use in
+a golf practice apparatus is pending at the present time.</p>
+
+<p><a id="footnotea" name="footnotea"></a>
+<b><a href="#footnotetaga">a</a></b>: Concurrently with the Wrights' first engine work, Manly
+was developing the engine for the Langley Aerodrome, and a comparison
+of the Wrights' engine development with that of Manly is immediately
+suggested, but no meaningful comparison of the two efforts can be
+drawn. Beyond the objective of producing a power unit to accomplish
+human flight and the fact that all three individuals were superb
+mechanics, the two efforts had nothing in common. The Wrights' goal
+was an operable and reasonably lightweight unit to be obtained quickly
+and cheaply. Manly's task was to obtain what was for the time an
+inordinately light engine and, although the originally specified power
+was considerably greater than that of the Wrights, it was still
+reasonable even though Manly himself apparently increased it on the
+assumption that Langley would need more power than he thought. The
+cost and time required were very much greater than the Wrights
+expended. He ended up with an engine of extraordinary performance for
+its time, containing many features utilized in much later important
+service engines. His weight per horsepower was not improved upon for
+many years. The Wrights' engine proved its practicability in actual
+service. The Manly engine never had this opportunity but its
+successful ground tests indicated an equal potential in this respect.
+A description of the Langley-Manly engine and the history of its
+development is contained in <i>Smithsonian Annals of Flight</i> number 6,
+"Langley's Aero Engine of 1903," by Robert B. Meyer (xi+193 pages, 44
+figures; Smithsonian Institution Press, 1971)</p>
+
+</div>
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
+End of the Project Gutenberg EBook of The Wright Brothers' Engines and Their
+Design, by Leonard S. Hobbs
+
+*** END OF THIS PROJECT GUTENBERG EBOOK THE WRIGHT BROTHERS' ENGINES ***
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+</body>
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+The Project Gutenberg EBook of The Wright Brothers' Engines and Their
+Design, by Leonard S. Hobbs
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: The Wright Brothers' Engines and Their Design
+
+Author: Leonard S. Hobbs
+
+Release Date: February 2, 2012 [EBook #38739]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE WRIGHT BROTHERS' ENGINES ***
+
+
+
+
+Produced by Chris Curnow, Joe Cooper, Christine P. Travers
+and the Online Distributed Proofreading Team at
+http://www.pgdp.net
+
+
+
+
+
+
+
+
+
+SERIAL PUBLICATIONS OF THE SMITHSONIAN INSTITUTION
+
+
+The emphasis upon publications as a means of diffusing knowledge was
+expressed by the first Secretary of the Smithsonian Institution. In
+his formal plan for the Institution, Joseph Henry articulated a
+program that included the following statement: "It is proposed to
+publish a series of reports, giving an account of the new discoveries
+in science, and of the changes made from year to year in all branches
+of knowledge not strictly professional." This keynote of basic
+research has been adhered to over the years in the issuance of
+thousands of titles in serial publications under the Smithsonian
+imprint, commencing with _Smithsonian Contributions to Knowledge_ in
+1848 and continuing with the following active series:
+
+  _Smithsonian Annals of Flight_
+
+  _Smithsonian Contributions to Anthropology_
+
+  _Smithsonian Contributions to Astrophysics_
+
+  _Smithsonian Contributions to Botany_
+
+  _Smithsonian Contributions to the Earth Sciences_
+
+  _Smithsonian Contributions to Paleobiology_
+
+  _Smithsonian Contributions to Zoology_
+
+  _Smithsonian Studies in History and Technology_
+
+In these series, the Institution publishes original articles and
+monographs dealing with the research and collections of its several
+museums and offices and of professional colleagues at other
+institutions of learning. These papers report newly acquired facts,
+synoptic interpretations of data, or original theory in specialized
+fields. Each publication is distributed by mailing lists to libraries,
+laboratories, institutes, and interested specialists throughout the
+world. Individual copies may be obtained from the Smithsonian
+Institution Press as long as stocks are available.
+
+                                                 S. DILLON RIPLEY
+                                                    _Secretary_
+                                               Smithsonian Institution
+
+
+
+
+  The Wright Brothers' Engines
+  And Their Design
+
+
+
+
+[Illustration: Kitty Hawk Flyer with original Wright engine poised on
+launching rail at Kill Devil Hill, near Kitty Hawk, North Carolina, 24
+November 1903, the month before the Wrights achieved man's first
+powered and controlled flight in a heavier-than-air craft.]
+
+[Illustration: Reproduction of the first engine, built by Pratt &
+Whitney, as displayed in Wright Brothers National Memorial at Kitty
+Hawk. Engine is mounted in a reproduction of the Wrights' Flyer built
+by the National Capital Section of the Institute of the Aeronautical
+Sciences (now the American Institute of Aeronautics and Astronautics).
+Engine and plane were donated in 1963 to the National Park Service
+Cape Hatteras National Seashore.]
+
+
+
+
+  SMITHSONIAN ANNALS OF FLIGHT * NUMBER 5
+
+  SMITHSONIAN INSTITUTION * NATIONAL AIR AND SPACE MUSEUM
+
+  The Wright Brothers' Engines
+  And Their Design
+
+  _Leonard S. Hobbs_
+
+
+  SMITHSONIAN INSTITUTION PRESS
+  CITY OF WASHINGTON
+  1971
+
+
+
+
+_Smithsonian Annals of Flight_
+
+
+Numbers 1-4 constitute volume one of _Smithsonian Annals of Flight_.
+Subsequent numbers will bear no volume designation, which has been
+dropped. The following earlier numbers of _Smithsonian Annals of
+Flight_ are available from the Superintendent of Documents as
+indicated below:
+
+  1. The First Nonstop Coast-to-Coast Flight and the Historic T-2
+  Airplane, by Louis S. Casey. 1964. 90 pages, 43 figures, appendix,
+  bibliography. Out of print.
+
+  2. The First Airplane Diesel Engine: Packard Model DR-980 of
+  1928, by Robert B. Meyer. 1964. 48 pages, 37 figures, appendix,
+  bibliography. Price 60c.
+
+  3. The Liberty Engine 1918-1942, by Philip S. Dickey. 1968.
+  110 pages, 20 figures, appendix, bibliography. Price 75c.
+
+  4. Aircraft Propulsion: A Review of the Evolution of Aircraft Piston
+  Engines, by C. Fayette Taylor. 1971 viii + 134 pages,
+  72 figures, appendix, bibliography of 601 items. Price $1.75.
+
+
+For sale by Superintendent of Documents, Government Printing Office
+Washington, D.C. 20402--Price 60 cents
+
+
+
+
+Foreword
+
+
+In this fifth number of _Smithsonian Annals of Flight_ Leonard S.
+Hobbs analyzes the original Wright _Kitty Hawk Flyer_ engine from the
+point of view of an aeronautical engineer whose long experience in the
+development of aircraft engines gives him unique insight into the
+problems confronting these remarkable brothers and the ingenious
+solutions they achieved. His review of these achievements also
+includes their later vertical 4-and 6-cylinder models designed and
+produced between 1903 and 1915.
+
+The career of Leonard S. (Luke) Hobbs spans the years that saw the
+maturing of the aircraft piston engine and then the transition from
+reciprocating power to the gas turbine engine. In 1920 he became a
+test engineer in the Power Plant Laboratory of the Army Air Service at
+McCook Field in Dayton, Ohio. There, and later as an engineer with the
+Stromberg Motor Devices Corporation, he specialized in aircraft engine
+carburetors and developed the basic float-type to the stage of utility
+where for the first time it provided normal operation during airplane
+evolutions, including inverted flight.
+
+Joining Pratt & Whitney Aircraft in 1927 as Research Engineer, Hobbs
+advanced to engineering manager in 1935 and in 1939 took over complete
+direction of its engineering. He was named vice president for
+engineering for all of United Aircraft in 1944, and was elected vice
+chairman of United Aircraft in 1956, serving in that capacity until
+his retirement in 1958. He remained a member of the board of directors
+until 1968. Those years saw the final development of Pratt & Whitney's
+extensive line of aircraft piston engines which were utilized by the
+United States and foreign air forces in large quantities and were
+prominent in the establishment of worldwide air transportation.
+
+In 1963 Hobbs was awarded the Collier Trophy for having directed the
+design and development of the J57 turbojet, the country's first such
+engine widely used in both military service and air transportation.
+
+He was an early fellow of the Institute of Aeronautical Sciences
+(later the American Institute of Aeronautics and Astronautics), served
+for many years on the Powerplant Committee of the National Advisory
+Committee for Aeronautics, and was the recipient of the Presidential
+Certificate of Merit.
+
+                                    FRANK A. TAYLOR, _Acting Director_
+                                       _National Air and Space Museum_
+
+_March 1970_
+
+
+
+
+Contents
+
+
+  Foreword                                                           v
+  Acknowledgments                                                   ix
+  The Beginnings                                                     1
+  The Engine of the First Flight, 1903                               9
+  The Engines With Which They Mastered the Art of Flying            29
+  The Four-Cylinder Vertical Demonstration Engine and the First
+      Production Engine                                             34
+  The Eight-Cylinder Racing Engine                                  47
+  The Six-Cylinder Vertical Engine                                  49
+
+  Minor Design Details and Performance of the Wright Engines        57
+  Appendix                                                          62
+      Characteristics of the Wright Flight Engines                  62
+      The Wright Shop Engine                                        64
+  Bibliography                                                      69
+  Index                                                             71
+
+
+
+
+Acknowledgments
+
+
+As is probably usual with most notes such as this, however short,
+before completion the author becomes indebted to so many people that
+it is not practical to record all the acknowledgments that should be
+made. This I regret extremely, for I am most appreciative of the
+assistance of the many who responded to my every request. The mere
+mention of the Wright name automatically opened almost every door and
+brought forth complete cooperation. I do not believe that in the
+history of the country there has been another scientist or engineer as
+admired and revered as they are.
+
+I must, however, name a few who gave substantially of their time and
+effort and without whose help this work would not be as complete as it
+is. Gilmoure N. Cole, A. L. Rockwell, and the late L. Morgan Porter
+were major contributors, the latter having made the calculations of
+the shaking forces, the volumetric efficiency, and the connecting rod
+characteristics of the 1903 engine. Louis P. Christman, who was
+responsible for the Smithsonian drawings of this engine and also
+supervised the reconstruction of the 1905 Wright airplane, supplied
+much information, including a great deal of the history of the early
+engines. Opie Chenoweth, one of the early students of the subject, was
+of much assistance; and I am indebted to R. V. Kerley for the major
+part of the data on the Wrights' shop engine.
+
+Also, I must express my great appreciation to the many organizations
+that cooperated so fully, and to all the people of these organizations
+and institutions who gave their assistance so freely. These include
+the following:
+
+  Air Force Museum, Wright-Patterson Air Force Base, Ohio
+  Carillon Park Museum, Dayton, Ohio
+  Connecticut Aeronautical Historical Association, Hebron, Connecticut
+  Fredrick C. Crawford Museum, Cleveland, Ohio
+  Historical Department, Daimler Benz A. G., Stuttgart-Untertuerkheim,
+    West Germany
+  Engineers Club, Dayton, Ohio
+  Deutsches Museum, Munich, West Germany
+  Educational and Musical Arts, Inc., Dayton, Ohio
+  Henry Ford Museum, Dearborn, Michigan
+  Franklin Institute, Philadelphia, Pennsylvania
+  Howell Cheney Technical School, Manchester, Connecticut
+  Library of Congress, Washington, D.C.
+  Naval Air Systems Command, U.S. Navy, Washington, D.C.
+  Science Museum, London, England
+  Victoria and Albert Museum, London, England
+
+In particular, very extensive contributions were made by the
+Smithsonian Institution and by the United Aircraft Corporation through
+its Library, through the Pratt & Whitney Aircraft Division's entire
+Engineering Department and its Marketing and Product Support
+Departments, and through United Aircraft International.
+
+
+
+
+The Beginnings
+
+
+The general history of the flight engines used by the Wright Brothers
+is quite fascinating and fortunately rather well recorded.[1] The
+individual interested in obtaining a reasonably complete general story
+quickly is referred to three of the items listed in the short
+bibliography on page 69. The first, _The Papers of Wilbur and Orville
+Wright_, is a primary source edited by the authority on the Wright
+brothers, Marvin W. McFarland of the Library of Congress; a compact
+appendix to volume 2 of the _Papers_ contains most of the essential
+facts. This source is supplemented by the paper of Baker[2] and the
+accompanying comments by Chenoweth, presented at the National
+Aeronautics Meeting of the Society of Automotive Engineers on 17 April
+1950. Aside from their excellence as history, these publications are
+outstanding for the manner in which those responsible demonstrate
+their competence and complete mastery of the sometimes complex
+technical part of the Wright story.
+
+[Footnote 1: An extensive bibliography, essentially as complete at
+this time as when it was compiled in the early 1950s, is given on
+pages 1240-1242 of volume 2 of _The Papers of Wilbur and Orville
+Wright_, 1953.]
+
+[Footnote 2: Max P. Baker was a technical adviser to the Wright estate
+and as such had complete access to all of the material it contained.]
+
+The consuming interest of the Wrights, of course, was in flight as
+such, and in their thinking the required power unit was of only
+secondary importance. However, regardless of their feeling about it,
+the unit was an integral part of their objective and, due to the
+prevailing circumstances, they very early found themselves in the
+aircraft engine business despite their inexperience. This business was
+carried on very successfully, against increasingly severe competition,
+until Orville Wright withdrew from commercial activity and dissolved
+the Wright Company. The time span covered approximately the twelve
+years from 1903 to 1915, during the first five years of which they
+designed and built for their own use several engines of three
+different experimental and demonstration designs. In the latter part
+of the period, they manufactured and sold engines commercially, and
+during this time they marketed three models, one of which was
+basically their last demonstration design. A special racing engine was
+also built and flown during this period. Accurate records are not
+available but altogether, they produced a total of something probably
+close to 200 engines of which they themselves took a small number for
+their various activities, including their school and flying exhibition
+work which at one time accounted for a very substantial part of their
+business. A similar lack of information concerning their competition,
+which expanded rapidly after the Wright's demonstrations, makes any
+comparisons a difficult task. The Wrights were meticulous about
+checking the actual performance of their engines but at that time
+ratings generally were seldom authenticated and even when different
+engines were tried in the same airplane the results usually were not
+measured with any accuracy or recorded with any permanency. There is
+evidence that the competition became effective enough to compel the
+complete redesign of their engine so that it was essentially a new
+model.
+
+For their initial experimentation the Wrights regarded gravity as not
+only their most reliable power source but also the one most economical
+and readily available, hence their concentration on gliding. They had
+correctly diagnosed the basic problem of flight to be that of control,
+the matter of the best wing shapes being inherently a simpler one
+which they would master by experiment, utilizing at first gravity and
+later a wind tunnel. Consequently, the acquisition of a powerplant
+intended for actual flight was considerably deferred.
+
+Nevertheless, they were continuously considering the power requirement
+and its problems. In his September 1901 lecture to the Western Society
+of Engineers, Wilbur Wright made two statements: "Men also know how to
+build engines and screws of sufficient lightness and power to drive
+these planes at sustaining speed"; and in conjunction with some
+figures he quoted of the required power and weight: "Such an engine is
+entirely practicable. Indeed, working motors of one-half this weight
+per horsepower [9 pounds per horsepower] have been constructed by
+several different builders." It is quite obvious that with their
+general knowledge and the experience they had acquired in designing
+and building a successful shop engine for their own use, they had no
+cause to doubt their ability to supply a suitable powerplant when the
+need arose. After the characteristics of the airframe had been
+settled, and the engine requirements delineated in rather detailed
+form, they had reached the point of decision on what they termed the
+motor problem. Only one major element had changed greatly since their
+previous consideration of the matter; they had arrived at the point
+where they not only needed a flight engine, they wanted it quickly.
+
+Nothing has been found that would indicate how much consideration they
+had given to forms of power for propulsion other than the choice they
+had apparently made quite early--the internal-combustion,
+four-stroke-cycle piston engine. Undoubtedly, steam was dismissed
+without being given much, if any, thought. On the face of it, the
+system was quite impractical for the size and kind of machine they
+planned; but it had been chosen by Maxim for his experiments,[3] and
+some thirty-five or forty years later a serious effort to produce an
+aviation engine utilizing steam was initiated by Lockheed. On the
+other hand internal-combustion two-stroke-cycle piston engines had
+been built and used successfully in a limited way. And since, at that
+time, it was probably not recognized that the maximum quantity of heat
+it is possible to dissipate imposed an inherent limitation on the
+power output of the internal-combustion engine, the two-stroke-cycle
+may have appeared to offer a higher output from a given engine size
+than the four-stroke-cycle could produce. Certainly, it would have
+seemed to promise much less torque variation for the same output,
+something that was of great importance to the Wrights. Against this,
+the poor scavenging efficiency of the two-stroke operation, and most
+probably its concurrent poor fuel economy, were always evident; and,
+moreover, at that time the majority of operating engines were
+four-stroke-cycle. Whatever their reasoning, they selected for their
+first powered flight the exact form of prime mover that continued to
+power the airplane until the advent of the aircraft gas turbine more
+than forty years later.
+
+[Footnote 3: In the 1890s the wealthy inventor Sir Hiram Stevens Maxim
+conducted an experiment of considerable magnitude with a flying
+machine that utilized a twin-cylinder compound steam powerplant. It
+was developed to the flight-test stage.]
+
+The indicated solution to their problem of obtaining the engine--and
+the engine that would seem by all odds most reliable--would have been
+to have a unit produced to their specifications by one of the best of
+the experienced engine builders, and to accomplish this, the most
+effective method would be to use the equivalent of a bid procedure.
+This they attempted, and sent out a letter of inquiry to a fairly
+large number of manufacturers. Although no copy of the letter is
+available, it is rather well established that it requested the price
+of an engine of certain limited specifications which would satisfy
+their flight requirements, but beyond this there is little in the
+record.
+
+A more thorough examination of the underlying fundamentals, however,
+discloses many weaknesses in the simple assumptions that made the
+choice of an experienced builder seem automatic. A maximum requirement
+limited to only one or two units offered little incentive to a
+manufacturer already successfully producing in his field, and the
+disadvantage of the limited quantity was only accentuated by the basic
+requirement for a technical performance in excess of any standard of
+the time. Certainly there was no promise of any future quantity
+business or any other substantial reward. Orville Wright many times
+stated that they had no desire to produce their own engine, but it is
+doubtful that they had any real faith in the buying procedure, for
+they made no attempt to follow up their first inquiries or to expand
+the original list.
+
+Whatever the reasoning, their judgment of the situation is obvious;
+they spent no time awaiting results from the letter but almost
+immediately started on the task of designing and building the engine
+themselves. Perhaps the generalities were not as governing as the two
+specific factors whose immediate importance were determining: cost and
+time. The Wrights no doubt realized that a specially designed,
+relatively high performance engine in very limited hand-built
+quantities would not only be an expensive purchased article but would
+also take considerable time to build, even under the most favorable
+circumstances. So the lack of response to their first approach did not
+have too much to do with their ultimate decision to undertake this
+task themselves.
+
+The question of the cost of the Wrights' powerplants is most
+intriguing, as is that of their entire accomplishment. No detailed
+figures of actual engine costs are in the record, and it is somewhat
+difficult to imagine just how they managed to conduct an operation
+requiring so much effort and such material resources, given the income
+available from their fairly small bicycle business. The only evidence
+bearing on this is a statement that the maximum income from this
+business averaged $3,000 a year,[4] which of course had to cover not
+only the airplane and engine but all personal and other expenses. Yet
+they always had spare engines and spare parts available; they
+seemingly had no trouble acquiring needed materials and supplies, both
+simple and complex; and they apparently never were hindered at any
+time by lack of cash or credit. The only mention of any concern about
+money is a statement by Wilbur Wright in a letter of 20 May 1908 when,
+about to sail for France for the first public demonstrations, he
+wrote: "This plan would put it to the touch quickly and also help ward
+off an approaching financial stringency which has worried me very much
+for several months." It is a remarkable record in the economical use
+of money, considering all they had done up to that time. The myth that
+they had been aided by the earnings of their sister Katherine as a
+school teacher was demolished long ago.
+
+[Footnote 4: Fred C. Kelly, _Miracle at Kitty Hawk_, 1951.]
+
+The decision to build the engine themselves added one more
+requirement, and possibly to some extent a restriction, to the design.
+They undoubtedly desired to machine as much of the engine as possible
+in their own shop, and the very limited equipment they had would
+affect the variety of features and constructions that could be
+utilized, although experienced machine shops with sophisticated
+equipment were available in Dayton and it is obvious that the Wrights
+intended to, and did, utilize these when necessary. The use of their
+own equipment, of course, guaranteed that the parts they could handle
+themselves would be more expeditiously produced. They commenced work
+on the design and construction shortly before Christmas in 1902.
+
+The subject of drawings of the engine is interesting, not only as
+history but also because it presents several mysteries. Taylor[5]
+stated, "We didn't make any drawings. One of us would sketch out the
+part we were talking about on a piece of scrap paper ..." Obviously
+somewhere in the operation some dimensions were added, for the design
+in many places required quite accurate machining. Orville Wright's
+diary of 1904 has the entry, "Took old engine apart to get
+measurements for making new engine." Finally, no Wright drawings of
+the original engine have been seen by anyone connected with the
+history or with the Wright estate. In the estate were two drawings
+(now at the Franklin Institute), on heavy brown wrapping paper,
+relating to one of the two very similar later engines built in 1904;
+one is of a cylinder and connecting rod, the other is an end view of
+the engine. Thus even if the very ingenious drafting board now in the
+Wright Museum at Carillon Park was available at the time there is no
+indication that it was used to produce what could properly be called
+drawings of the first engine.
+
+[Footnote 5: Charles E. Taylor (Charley Taylor to the many who knew
+him) was in effect the superintendent of and also the only employee to
+work in the original small machine shop. A most versatile and
+efficient mechanic and machine operator, he made many parts for all of
+the early engines, and in the manner of the experimental machinist,
+worked mainly from sketches. He also had charge of the bicycle shop
+and its business in the absence of the Wrights.]
+
+There are in existence, however, two complete sets of drawings, both
+of which purport to represent the 1903 flight engine. One set was made
+in England for the Science Museum in the two years 1928 and 1939. The
+1928 drawings were made on receipt of the engine, which was not
+disassembled, but in 1939 the engine was removed from the airplane,
+disassembled, the original 1928 drawings were corrected and added to,
+and the whole was made into one very complete and usable set. The
+other set was prepared in Dayton, Ohio, for Educational and Musical
+Arts, Inc.,[6] and was donated to the Smithsonian Institution. This
+latter set was started under the direction of Orville Wright, who died
+shortly after the work had been commenced.
+
+[Footnote 6: This is a charitable agency set up by the late Colonel
+and Mrs. E. A. Deeds primarily for the purpose of building and
+supporting the Deeds Carillon and the Carillon Park Museum in Dayton,
+Ohio.]
+
+The two sets of drawings, that is, the one of the Science Museum and
+that made in Dayton for the Smithsonian Institution, cannot be
+reconciled in the matter of details. Hardly any single dimension is
+exactly the same and essentially every part differs in some respect.
+Many of the forms of construction differ and even the firing order of
+the two engines is not the same, so that in effect the drawings show
+two different engines.
+
+[Illustration: Figure 1.--First flight engine, 1903, valve side.
+(Photo courtesy Science Museum, London.)]
+
+The primary trouble is, of course, that the exact engine which flew in
+1903 is no longer in existence, and since no original drawings of it
+exist, there is considerable doubt about its details. The engine had
+its crankcase broken in an accident to the airframe (this was caused
+by a strong wind gust immediately following the last of the first
+series of flights at Kitty Hawk), and when it was brought back to
+Dayton it was for some inexplicable reason completely laid aside, even
+though it presumably contained many usable parts. When the engine was
+disassembled to obtain measurements for constructing the 1904 engines,
+again apparently no drawings were made. In February 1906 Orville
+Wright wrote that all the parts of the engine were still in existence
+except the crankcase; but shortly after this the crankshaft and
+flywheel were loaned for exhibition purposes and were never recovered.
+In 1926 the engine was reassembled for an exhibition and in 1928 it
+was again reassembled for shipment to England. The only parts of this
+particular engine whose complete history is definitely known are the
+crankshaft and flywheel, which were taken from the 1904-1905 flight
+engine. This latter engine, now in the restored 1905 airplane in the
+Carillon Park Museum in Dayton, does not contain a crankshaft, and in
+its place incorporates a length of round bar stock.
+
+[Illustration: Figure 2.--First flight engine, 1903, underside and
+flywheel end. (Photo courtesy Science Museum, London.)]
+
+In late 1947 work on the Educational and Musical Arts drawings was
+initiated under the direction of Louis P. Christman and carried
+through to completion by him. Christman has stated that Orville Wright
+was critical of the Science Museum drawings but just what he thought
+incorrect is not known. Whatever his reasons, he did encourage
+Christman to undertake the major task of duplication. Christman worked
+directly with Orville Wright for a period of six weeks and had access
+to all the records and parts the Wrights had preserved. The resultant
+drawings are also very complete and, regardless of the differences
+between these two primary sets, both give a sufficiently accurate
+picture of the first engine for all purposes except that of exact
+reproduction in every detail.
+
+There exists a still unsolved puzzle in connection with what seems to
+be yet another set of drawings of the first engine. In December 1943,
+in writing to the Science Museum telling of his decision to have the
+airplane and engine brought back to the United States, Orville Wright
+stated, "I have complete and accurate drawings of the engine. I shall
+be glad to furnish them if you decide to make a replica."[7] No trace
+of these particular drawings can be found in any of the museums,
+institutions, or other repositories that normally should have acquired
+them and the executors of Orville Wright's estate have no record or
+knowledge of them. The date of his letter is four years before the
+Dayton drawings were commenced; and when Christman was working on
+these with Orville Wright they had copies of the Science Museum
+drawings, with complete knowledge of their origin, yet Christman has
+no knowledge of the drawings referred to in Orville's letter to the
+Museum. Finally, the evidence is quite conclusive that there were no
+reproducible or permanent drawings made at the time the first engine
+was constructed, and, of course, the reconstructed engine itself was
+sent to England in 1928 and not returned to this country until
+1948.[8]
+
+[Footnote 7: The Science Museum expressed a desire to have these but
+never received them. There is a reference to them in a letter to the
+Museum from the executors of his estate dated 20 February 1948, but is
+seems rather obvious from the text that by this time the drawings
+mentioned by Orville Wright in his 1943 letter had become confused
+with those being prepared by Christman for the Smithsonian
+Institution. The Science Museum did have constructed from its own
+drawings a very fine replica which is completely operable at this
+time.]
+
+[Footnote 8: There is a third set of drawings prepared by the Ford
+Motor Company also marked as being of the 1903 engine and these are
+rather well distributed in various museums and institutions. What this
+set is based on has been impossible to determine but it is indicated
+from the existence of actual engines and parts and the probable date
+of their preparation (no date is given on the drawings themselves)
+that they were copied from drawings previously made, and therefore add
+nothing to them. The Orville Wright-Henry Ford friendship originated
+rather late, considering Ford's avid interest in history and
+mechanical things. This tardiness could possibly have been the result
+of Wright coolness--a coolness caused by a report, at the time the
+validity of the Wright patents was being so strongly contested, that
+Ford had advised some of those opposing the Wrights to persevere and
+to obtain the services of his patent counsel who had been successful
+in overturning the Selden automobile patent. If this barrier ever
+existed it was surmounted, and Ford spent much effort and went to
+considerable expense to collect the Wright home and machine shop for
+his Dearborn museum. The shop equipment apparently had been widely
+scattered and its retrieval was a major task. It is most likely that
+the drawings resulted from someone's effort to follow out an order to
+produce a set of Ford drawings of the original engine. A small scale
+model of the 1903 flight engine, constructed under the supervision of
+Charles Taylor, is contained in the Dearborn Museum.]
+
+
+
+
+The Engine of the First Flight, 1903
+
+
+In commencing the design of the first engine, the first important
+decision arrived at was that of the number and size of the cylinders
+to be employed and the form in which they would be combined, although
+it is unlikely that this presented any serious problem. In a similar
+situation Manly, when he was working on the engine for the Langley
+Aerodrome,[9] was somewhat perturbed because he did not have access to
+the most advanced technical knowledge, since the automobile people who
+were at that time the leaders in the development of the internal
+combustion engine, tended for competitive reasons to be rather
+secretive about their latest advancements and designs. But although
+the standard textbooks may not have been very helpful to him, there
+were available such volumes as W. Worby Beaumont's _Motor Vehicles and
+Motors_ which contained in considerable detail descriptions and
+illustrations of the best of the current automobile engines. The
+situations of Manly and the Wrights differed, however, in that whereas
+the Wrights' objective was certainly a technical performance
+considerably above the existing average, Manly's goal was that of
+something so far beyond this average as to have been considered by
+many impossible. Importantly, the Wrights had their own experience
+with their shop engine and a good basic general knowledge of the size
+of engine that would be necessary to meet their requirements.
+
+[Footnote 9: Charles L. Manly was engaged in the development of the
+engine for the Langley Aerodrome. See also footnote to Table on page
+62.]
+
+Engine roughness was of primary concern to them. In the 1902
+description of the engine they sent to various manufacturers, they had
+stated: "... and the engine would be free from vibration." Even though
+their requirement for a smooth engine was much more urgent than merely
+to avoid the effect of roughness on the airplane frame, they were
+faced, before they made their first powered flight, with the basic
+problem with which the airplane has had to contend for over
+three-quarters of its present life span: that is, it was necessary to
+utilize an explosion engine in a structure which, because of weight
+limitations, had to be made the lightest and hence frailest that could
+possibly be devised and yet serve its primary purpose. However great
+the difficulty may have appeared, in the long view, the fault was
+certainly a relatively minor one in the overall development of the
+internal combustion engine--that wonderful invention without which
+their life work would probably never have been so completely
+successful while they lived, and which, even aside from its
+partnership with the airplane, has so profoundly affected the nature
+of the world in which we live.
+
+It seems quite obvious that to the Wrights vibration, or roughness,
+was predominantly if not entirely caused by the explosion forces, and
+they were either not completely aware of the effects of the other
+vibratory forces or they chose to neglect them. Although crankshaft
+counterweights had been in use as far back as the middle 1800s, the
+Wrights never incorporated them in any of their engines; and despite
+the inherent shaking force in the 4-inline arrangement, they continued
+to use it for many years.
+
+The choice of four cylinders was obviously made in order to get, for
+smoothness, what in that day was "a lot of small cylinders"; and this
+was sound judgment. Furthermore, although the majority of automobiles
+at that time had engines with fewer than four cylinders, for those
+that did the inline form was standard and well proven, and, in fact,
+Daimler was then operating engines of this general design at powers
+several times the minimum the Wrights had determined necessary for
+their purpose.
+
+What fixed the exact cylinder size, that is, the "square" 4x4-in.
+form, is not recorded, nor is it obvious by supposition. Baker says it
+was for high displacement and low weight, but these qualities are also
+greatly affected by many other factors. The total displacement of just
+over 200 cu in. was on the generous side, given the horsepower they
+had determined was necessary, but here again the Wrights were
+undoubtedly making the conservative allowances afterwards proven
+habitual, to be justified later by greatly increased power
+requirements and corresponding outputs. The Mean Effective Pressure
+(MEP), based on their indicated goal of 8 hp, would be a very modest
+36 psi at the speed of 870 rpm at which they first tested the engine,
+and only 31 psi at the reasonably conservative speed of 1000 rpm. The
+4x4-in. dimension would provide a cylinder large enough so that the
+engine was not penalized in the matter of weight and yet small enough
+to essentially guarantee its successful operation, as cylinders of
+considerably larger bore were being utilized in automobiles. That
+their original choice was an excellent one is rather well supported by
+the fact that in all the different models and sizes of engines they
+eventually designed and built, they never found it necessary to go to
+cylinders very much larger than this.
+
+[Illustration: _Figure 3._--First flight engine, 1903, installed in
+the Kitty Hawk airplane, as exhibited in the Science Museum. (Photo
+courtesy the Science Museum, London.)]
+
+A second basic determination which was made either concurrently or
+even possibly in advance of that of the general form and size was in
+the matter of the type of cylinder cooling to adopt. Based on current
+practice that had proven practical, there were three possibilities,
+all of which were in use in automobiles: air, water, or a combination
+of the two. It is an interesting commentary that Fernand Forest's[10]
+proposed 32-cylinder aircraft engine of 1888 was to be air-cooled,
+that Santos-Dumont utilized an air-cooled Clement engine in his
+dirigible flights of 1903, and that the Wrights had chosen air cooling
+for their shop engine. With the promise of simplicity and elimination
+of the radiator, water and piping, it would seem, offhand, that this
+would be the Wrights' choice for their airplane; but they were
+probably governed by the fact that not only was the water-cooled type
+predominant in automobile practice, but that the units giving the best
+and highest performance in general service were all water cooled. In
+their subsequent practice they never departed from this original
+decision, although Wilbur Wright's notebook of 1904-1907 contains an
+undated weight estimate by detailed parts for an 8-cylinder air-cooled
+engine. Unfortunately, the proposed power output is not recorded, so
+their conception of the relative weight of the air-cooled form is not
+disclosed.
+
+[Footnote 10: Fernand Forest, _Les Bateaux Automobiles_, 1906.]
+
+One of the most important decisions relating to the powerplant--one
+which was probably made long before they became committed to the
+design itself--was a determination of the method of transmission of
+power to the propeller, or propellers. A lingering impression exists
+that the utilization of a chain drive for this purpose was a natural
+inheritance from their bicycle background. No doubt this experience
+greatly simplified the task of adaptation but a merely cursory
+examination shows that even if they had never had any connection with
+bicycles, the chain drive was a logical solution, considering every
+important element of the problem. The vast majority of automobiles of
+the time were chain driven, and chains and sprockets capable of
+handling a wide range of power were completely developed and
+available. Further, at that time they had no accurate knowledge of
+desirable or limiting propeller and engine speeds. The chain drive
+offered a very simple and inexpensive method of providing for a
+completely flexible range of speed ratios. The other two possibilities
+were both undesirable: the first, a simple direct-driven single
+propeller connected to the crankshaft, provided essentially no
+flexibility whatsoever in experimentally varying engine or propeller
+speed ratios, it added an out-of-balance engine torque force to the
+problem of airplane control, and, finally, it dictated that the pilot
+would be in the propeller slipstream or the airflow to it; the second,
+drive shafts and gearing for dual propellers, would have been very
+heavy and expensive, and most probably would have required a long-time
+development, with every experimental change in speed ratios requiring
+a complete change in gears. Again, their original choice was so
+correct that it lasted them through essentially all their active
+flying years.
+
+The very substantial advantages of the chain drive were not, however,
+obtained at no cost. Torque variations in the engine would tend to
+cause a whipping action in the chain, so that it was vulnerable to
+rough running caused by misfiring cylinders and, with the right timing
+and magnitude of normal regular variations, the action could result in
+destructive forces in the transmission system. This was the basic
+reason for the Wrights' great fear of "engine vibration," which
+confined them to the use of small cylinders and made a fairly heavy
+flywheel necessary on all their engines. When they were requested to
+install an Austro-Daimler engine in one of their airplanes, they
+designed a flexible coupling which was interposed between the engine
+and the propeller drive and this was considered so successful that it
+was applied to the flywheel of some engines of their last model, the
+6-70, "which had been giving trouble in this regard."[11]
+
+[Footnote 11: Grover Loening, letter of 10 April 1963, to the
+Smithsonian Institution.]
+
+Although flat, angled, and vertical engines had all been operated
+successfully, the best and most modern automotive engines of the time
+were vertical, so their choice of a horizontal position was probably
+dictated either by considerations of drag or their desire to provide a
+sizable mounting base for the engine, or both. There is no record of
+their ever having investigated the matter of the drag of the engine,
+either alone or in combination with the wing. The merit of a vertical
+versus a horizontal position of the engine was not analogous to that
+of the pilot, which they had studied, and where the prone position
+undoubtedly reduced the resistance.
+
+Having decided on the general makeup of their engine, the next major
+decision was that of just what form the principal parts should take,
+the most important of these being the cylinders and crankcase. Even at
+this fairly early date in the history of the internal combustion
+engine various successful arrangements and combinations were in
+existence. Individual cylinder construction was by far the most used,
+quite probably due to its case of manufacture and adaptability to
+change. Since 4-cylinder engines were just coming into general use (a
+few production engines of this type had been utilized as early as
+1898), there were few examples of en-bloc or one-piece construction.
+The original German Daimler Company undoubtedly was at this time the
+leader in the development of high-output internal-combustion engines,
+and in 1902, as an example of what was possible, had placed in service
+one that possibly approximated 40 hp, which was an MEP of 70 psi.
+(Almost without exception, quoted power figures of this period were
+not demonstrated quantities but were based on a formula, of which the
+only two factors were displacement and rpm.) The cylinders of this
+Daimler engine were cast iron, the cylinder barrel, head, and water
+jacket being cast in one piece. The upper part of the barrel and the
+cylinder head were jacketed, but, surprisingly, the bottom 60 percent
+of the barrel had no cooling. The cylinders were cast in pairs and
+bolted to a two-piece aluminum case split at the line of the
+crankshaft. Ignition was make-and-break and the inlet valves were
+mechanically actuated. Displacement was 413 cu in. and the rpm was
+1050.
+
+Although a few examples of integral crankcase and water jacket
+combinations were in use, the Wrights were being somewhat radical when
+they decided to incorporate all four cylinders in the one-piece
+construction, particularly since they also proposed to include the
+entire crankcase and not just one part of it. It was undoubtedly the
+most important decision that they were required to make on all the
+various construction details, and probably the one given the most
+study and investigation. Many factors were involved, but fundamentally
+everything went back to their three basic requirements: suitability,
+time, and cost. There was no obvious reason why the construction would
+not work, and it eliminated a very large number of individual parts
+and the required time for procuring, machining, and joining them.
+Probably one very strong argument was the advanced state of the
+casting art, one of the oldest of the mechanical arts in existence and
+one the Wrights used in many places, even though other processes were
+available. What no doubt weighed heavily was that Dayton had some
+first-class foundries. The casting, though intricate and not
+machinable in their own shop, could be easily handled in one that was
+well outfitted. The pattern was fairly complex but apparently not
+enough to delay the project or cause excessive cost.
+
+[Illustration: _Figure 4._--First flight engine, 1903, left side and
+rear views, with dimensions. (Drawing courtesy Howell Cheney Technical
+School.)
+
+LEFT SIDE VIEW.]
+
+[Illustration: REAR VIEW]
+
+The selection of aluminum for the material was an integral part of
+the basic design decision. Despite the excellence and accuracy of the
+castings that could be obtained, there was nevertheless a minimum
+dimension beyond which wall thickness could not be reduced; and the
+use of either one of the two other proven materials, cast iron or
+bronze, would have made the body, as they called it, prohibitively
+heavy. The use of aluminum was not entirely novel at this time, as it
+had been utilized in many automobile engine parts, particularly
+crankcases; but its incorporation in this rather uncommon combination
+represented a bold step. There was no choice in the matter of the
+alloy to be used, the only proven one available was an 8 percent
+copper 92 percent aluminum combination.
+
+By means of the proper webs, brackets and bosses, the crankcase would
+also carry the crankshaft, the rocker arms and bearings, and the
+intake manifold. The open section of the case at the top was covered
+with a screw-fastened thin sheet of cold-rolled steel. The main
+bearing bosses were split at a 45 deg. angle for ease of assembly. The
+engine support and fastening were provided by four feet, or lugs, cast
+integral on the bottom corners of the case, and by accompanying bolts
+(Figure 2). Although the crankcase continued to be pretty much the
+"body" of the internal combustion aircraft engine throughout its life,
+the Wrights managed to incorporate in this original part a major
+portion of the overall engine, and certainly far more than had ever
+previously been included.
+
+The design of the cylinder barrel presented fairly simple problems
+involving not much more than those of keeping the sections as thin as
+possible and devising means of fastening it and of keeping the water
+jacket tight. They saved considerable weight by making the barrel
+quite short, so that in operation a large part of the piston extended
+below the bottom of it; but this could be accepted, as there were no
+rings below the piston pin (Figure 6). The barrel material, a good
+grade of cast iron, was an almost automatic choice. In connection with
+these seemingly predetermined decisions, however, it should be
+remembered that their goal was an engine which would work without
+long-time development, and that, with no previous experience in
+lightweight construction to guide them they were nevertheless
+compelled to meet a weight limit, so that the thickness of every wall
+and flange and the length of every thread was important.
+
+With the separate cylinder barrel they were now almost committed to a
+three-piece cylinder. It would have been possible to combine the
+barrel and head in a one-piece casting and then devise a method of
+attachment, but this would have been more complex and certainly
+heavier. For housing the valves, what was in effect a separate
+cylindrical, or tubular, box was decided upon. This would lie across
+the top of the cylinder proper at right angles to the cylinder axis,
+and the two valves would be carried in the two ends of this box. The
+cylinder barrel would be brought in at its head end to form a portion
+of the cylinder head and then extended along its axis in the form of a
+fairly large boss, a mating boss being provided on one side of the
+valve box. The cylinder barrel would then be threaded into the valve
+box and the whole tightened or fastened to the crankcase by means of
+two sets of threads, one at each end of the barrel proper. This meant
+that three joints had to be made tight with only two sets of threads.
+This was accomplished by accurate machining and possibly even hand
+fitting in combination with a rather thick gasket at the head end, one
+flat of which bore against two different surfaces. This can be seen in
+Figure 6, where the circular flange on the valve box contacts both the
+crankcase and the cylinder barrel. Altogether it was a simple, light,
+and ingenious solution to a rather complex problem.
+
+At this point the question arises: Why was the engine layout such that
+the exhaust took place close to the operator's ears? It would have
+been possible, starting with the original design, to turn the engine
+around so that the exhaust was on the other side. This would have
+little effect on the location of the center of gravity, and the two
+main drive chains would then have been of more equal length. However,
+of the many factors involved, probably one of the principal
+considerations in arriving at their final decision was the location of
+the spark-advance control, which was in effect the only control they
+had of engine output, except for complete shutoff. In their design
+this was immediately adjacent to the operator; with a turned-around
+engine, an extension control mechanism of some sort would have been
+required. The noise of the exhaust apparently became of some concern
+to them, as Orville's diary in early 1904 contains an entry with a
+sketch labeled "Design for Muffler for Engine," but there is no
+further comment.
+
+The problem of keeping joints tight, and for that matter the entire
+construction itself, were both greatly simplified by their decision to
+water-jacket only a part of the cylinder head proper, and the valve
+box not at all. This was undoubtedly the correct decision for their
+immediate purpose, as again they were effecting savings in time, cost,
+complexity, and weight. There is nothing in the record, however, to
+show why they continued this practice long after they had advanced to
+much greater power outputs and longer flight times. Their own
+statements show that they were well aware of the effect of the very
+hot cylinder head on power output and they must also have realized its
+influence on exhaust-valve temperature.
+
+The cylinder assembly was made somewhat more complicated by their
+desire to oil the piston and cylinder by means of holes near the
+crankshaft end in what was, with the engine in the horizontal
+position, the upper side of the cylinder barrel. This complication was
+no doubt taken care of by not drilling the holes until a tight
+assembly had been made by screwing the barrel into place, and by
+marking the desired location on the barrel. Since this position was
+determined by a metal-to-metal jam fit of the crankcase and cylinder
+barrel flange, the barrel would reassemble with the holes in very
+nearly the same relative position after disassembly.
+
+With the valve box, or housing, cylindrical, the task of locking and
+fastening the intake and exhaust valve guides and seats in place was
+easy. The guide was made integral with and in the center of one end of
+a circular cage, the other end of which contained the valve seat (see
+Figure 5). Four sections were cut out of the circular wall of the cage
+so that in effect the seat and guide were joined by four narrow legs,
+the spaces between which provided passages for the flow of the
+cylinder gases. These cages were then dropped into the ends of the
+valve boxes until they came up against machined shoulders and were
+held in place by internal ring nuts screwed into the valve box. The
+intake manifold or passage was placed over the intake valves so that
+the intake charge flowed directly into and through the valve cage
+around the open valve and into the cylinder. The exhaust gas, after
+flowing through the passages in the valve cage, was discharged
+directly to the atmosphere through a series of holes machined in one
+side of the valve box.
+
+[Illustration: _Figure 5._--First flight engine, 1903, assembly.
+(Phantom cutaway by J. H. Clark, with key, courtesy _Aeroplane_.)
+
+KEY
+
+1 and 2. Bearing caps in one piece with plate 3.
+
+3. Plated screwed over hole 4 in crankcase end.
+
+4. Key-shaped hole as hole 5 in intermediate ribs.
+
+6. Inter-bearings cap (white-metal lined) and screwed to inter-rib
+halves 7.
+
+8. Splash-drip feed to bearings.
+
+9. Return to pump from each compartment of crankcase base ("sump") via
+gallery 10 and pipe to pump 11 underneath jacket.
+
+12. Oil feed from pump via rubber tube 13.
+
+13. Drip feeds to cylinders and pistons.
+
+14. Gear drive to pump.
+
+15. Big-end nuts, lock-strip, and shims.
+
+16. Gudgeon-pin lock.
+
+17. Piston-ring retainer pegs.
+
+18. Cylinder liner screwing into jacket.
+
+19. Open-ended "can" admits air.
+
+20. Fuel supply.
+
+21. (Hot) side of water jacket makes surface carburetter.
+
+22. Sparking plug (comprising positive electrode 23 and
+spark-producing make-and-break 24).
+
+25. Lever attached to lever 26 via bearing 27 screwed into chamber
+neck 28.
+
+26. Levers with mainspring 29 and inter-spring 30, and rocked by "cam"
+31.
+
+31. Cam with another alongside (for adjacent cylinder).
+
+32. Positive busbar feed to all four cylinders.
+
+33. Assembly retaining-rings.
+
+34. Sealing disc.
+
+35. Exhaust outlet ports.
+
+36. Camshaft right along on underside of jacket and also driving oil
+pump 11 via 14.
+
+37. Spring-loaded sliding pinion drives make-and-break shaft 38
+through peg in inclined slot 39.
+
+40. Cam to push pinion 37 along and so alter its angular relation with
+shaft 38 (to vary timing).
+
+41. Exhaust-valve cams bear on rollers 42 mounted in end of
+rocker-arms 43.
+
+44. Generator floating coils.
+
+45. Friction-drive off flywheel.
+
+46. Sight-feed lubricator (on stationary sleeve).
+
+47. Hardwood chain tensioner.]
+
+The intake and exhaust valves were identical and of two-piece
+construction, with the stems screwed tightly into and through the
+heads and the protruding ends then peened over. This construction was
+not novel, having had much usage behind it, and it continued for a
+long time in both automobile and aircraft practice. One-piece cast
+and forged valves were available but here again it was a choice of the
+quick, cheap, and proven answer.
+
+The entire valve system, including guides and seats, was of cast iron,
+a favorite material of the Wrights, except for the valve stems, which
+were, at different times, of various carbon steels. Ordinary
+cold-rolled apparently was used in those of the original engine, but
+in later engines this was changed to a high-carbon steel.
+
+The piston design presented no difficulty. In some measure this was
+due to the remarkable similarity that seems to have existed among all
+the different engines of the time in the construction of this
+particular part, for, although there were some major variations, it
+was, in fact, almost as if some standard had been adopted. Pistons all
+were of cast iron and comparatively quite long (it was a number of
+years before they evolved into the short ones of modern practice);
+they were almost invariably equipped with three wide piston rings
+between the piston pin and the head; and, although there were in
+existence a few pistons with four rings, no oil wiper or other ring
+seems to have been placed below the piston pin. The Wrights' piston
+was typical of the time, with the rings pinned in the grooves to
+prevent turning and the piston pin locked in the piston with a
+setscrew. In designing this first engine they were, however,
+apparently somewhat unsure about this latter feature, as they provided
+the rod with a split little end and a clamping bolt (see Figure 6), so
+that the pin could be held in the rod if desired; but no examples of
+this use have been encountered.
+
+The Wrights' selection of an "automatic" or suction-operated inlet
+valve was entirely logical. Mechanically operated inlet valves were in
+use and their history went back many years, but the great majority of
+the engines of that time still had the automatic type, and with this
+construction one complete set of valve-operating mechanisms was
+eliminated. They were well aware of the loss of volumetric efficiency
+inherent in this valve, and apparently went to some pains to obtain
+from it the best performance possible. Speaking of the first engine,
+Orville Wright wrote, "Since putting in heavier springs to actuate the
+valves on our engine we have increased its power to nearly 16 hp and
+at the same time reduced the amount of gasoline consumed per hour to
+about one-half of what it was."[12]
+
+[Footnote 12: Assuming a rich mixture, consumption of all the air, and
+an airbrake thermal efficiency of 24.50% for the original engine, the
+approximate volumetric efficiency of the cylinder is calculated to
+have been just under 40%.]
+
+Why they continued with this form on their later engines is a question
+a little more difficult to answer, as they were then seeking more and
+more power and were building larger engines. The advantages of
+simplicity and a reduced number of parts still existed, but there also
+was a sizable power increase to be had which possibly would have more
+than balanced off the increased cost and weight. They did not utilize
+mechanical operation until after a major redesign of their last engine
+model. Very possibly the answer lies in the phenomenon of fuel
+detonation. This was only beginning to be understood in the late
+1920s, and it is quite evident from their writings that they had
+little knowledge of what made a good fuel in this respect. It is
+fairly certain, however, that they did know of the existence of
+cylinder "knock," or detonation, and particularly that the compression
+ratio had a major effect on it. The ratios they utilized on their
+different engines varied considerably, ranging from what, for that
+time, was medium to what was relatively high. The original flight
+engine had a compression ratio of 4.4:1. The last of their service
+engines had a compression ratio about twenty percent under that of the
+previous series--a clear indication that they considered that they had
+previously gone too high. Quite possibly they concluded that
+increasing the amount of the cylinder charge seemed to bring on
+detonation, and that the complication of the mechanical inlet valve
+was therefore not warranted.
+
+[Illustration: _Figure 6._--First flight engine, 1903, cross section.
+(Drawing courtesy Science Museum, London.)]
+
+The camshaft for the exhaust valves (101, Figure 6), was chain driven
+from the crankshaft and was carried along the bottom of the crankcase
+in three babbit-lined bearings in bearing boxes or lugs cast integral
+with the case. Both the driving chain and the sprockets were standard
+bicycle parts, and a number of bicycle thread standards and other
+items of bicycle practice were incorporated in several places in the
+engine, easing their construction task. The shaft itself, of mild
+carbon steel, was hollow and on each side of an end bearing sweated-on
+washers provided shoulders to locate it longitudinally. Its location
+adjacent to the valves, with the cam operating directly on the rocker
+arm, eliminated push rods and attendant parts, a major economy. The
+cams were machined as separate parts and then sweated onto the shaft.
+Their shape shows the principal concern in the design to have been
+obtaining maximum valve capacity--that is, a quite rapid opening with
+a long dwell. This apparent desire to get rid of the exhaust gas
+quickly is manifested again in the alacrity with which they adopted a
+piston-controlled exhaust port immediately they had really mastered
+flight and were contemplating more powerful and more durable engines.
+This maximum-capacity theory of valve operation, with its neglect of
+acceleration forces and seating velocities, may well have been at
+least partially if not largely the cause of their exhaust-valve
+troubles and the seemingly disproportionate amount of development they
+devoted to this part, as reported by Chenoweth, although it is also
+true that the exhaust valve continued to present a problem in the
+aircraft piston engine for a great many years after, even with the
+most scientific of cam designs.
+
+The rocker arm (102, Figure 6) is probably the best example of a small
+part which met all of their many specific requirements with an extreme
+of simplicity. It consisted of two identical side pieces, or walls, of
+sheet steel shaped to the desired side contour of the assembly, in
+which were drilled three holes, one in each end, to carry the roller
+axles, and the third in the approximate middle for the rocker axle
+shaft proper. This consisted of a piece of solid rod positioned by
+cotter pins in each end outside the side walls (see Figure 5). The
+assembly was made by riveting over the ends of the roller axles so
+that the walls were held tightly against the shoulders on the axles,
+thus providing the correct clearance for the rollers. The construction
+was so light and serviceable that it was essentially carried over to
+the last engine the Wrights ever built.
+
+The basic intake manifold (see Figure 5) consisted of a very low flat
+box of sheet steel which ran across the tops of the valve boxes and
+was directly connected to the top of each of them so that the cages,
+and thus the valves, were open to the interior of the manifold.
+Through an opening in the side toward the engine the manifold was
+connected to a flat induction chamber (21, Figure 5) which served to
+vaporize the fuel and mix it with the incoming air. This chamber was
+formed by screw-fastening a piece of sheet steel to vertical ribs cast
+integral with the crankcase, the crankcase wall itself thus forming
+the bottom of the chamber. A beaded sheet-steel cylinder resembling a
+can (73, Figure 6) but open at both ends was fastened upright to the
+top of this chamber. In the absence of anything else, this can could
+be called the carburetor, as a fuel supply line entered the cylinder
+near the top and discharged the fuel into the incoming air stream,
+both the fuel and air then going directly into the mixing chamber. The
+can was attached near one corner of the chamber, and vertical baffles,
+also cast integral with the case, were so located that the incoming
+mixture was forced to circulate over the entire area of exposed
+crankcase inside the chamber before it reached the outlet to the
+manifold proper, the hot surface vaporizing that part of the fuel
+still liquid.
+
+[Illustration: _Figure 7._--First flight engine, 1903: cylinder, valve
+box, and gear mechanism; below, miscellaneous parts. (Photos courtesy
+Science Museum, London, and Louis P. Christman.)]
+
+Fuel was gravity fed to the can through copper and rubber tubing from
+a tank fastened to a strut, several feet above the engine. Of the two
+valves placed in the fuel line, one was a simple on-off shutoff cock
+and the other a type whose opening could be regulated. The latter was
+adjusted to supply the correct amount of fuel under the desired flight
+operating condition; the shutoff cock was used for starting and
+stopping. The rate of fuel supply to the engine would decrease as the
+level in the fuel tank dropped, but as the head being utilized was a
+matter of several feet and the height of the supply tank a matter of
+inches, the fuel-air ratio was still maintained well within the range
+that would ignite and burn properly in the contemplated one-power
+condition of their flight operation.
+
+This arrangement is one of the best of the many illustrations of how
+by the use of foresight and ingenuity the Wrights met the challenge of
+a complex requirement with a simple device, for while carburetors were
+not in the perfected stage later attained, quite good ones that would
+both control power output and supply a fairly constant fuel-air
+mixture over a range of operating conditions were available, but they
+were complex, heavy, and expensive. The arrangement, moreover, secured
+at no cost a good vaporizer, or modern "hot spot." In their subsequent
+engines they took the control of the fuel metering away from the
+regulating valve and gravity tank combination and substituted an
+engine-driven fuel pump which provided a fuel supply bearing a fairly
+close relationship to engine speed.
+
+The reasons behind selection of the type of ignition used, and the
+considerations entering into the decision, are open to speculation, as
+are those concerning many other elements that eventually made up the
+engine. Both the high-tension spark plug and low-tension
+make-and-break systems had been in wide use for many years, with the
+latter constituting the majority in 1902. Both were serviceable and
+therefore acceptable, and both required a "magneto". The art of the
+spark plug was in a sense esoteric (to a certain extent it so remains
+to this day), but the spark-plug system did involve a much simpler
+combination of parts: in addition to the plug and magneto there would
+be needed only a timer, or distributor, together with coils and
+points, or some substitute arrangement. The make-and-break system, on
+the other hand, required for each cylinder what was physically the
+equivalent of a spark plug, that is, a moving arm and contact point
+inside the cylinder, a spring-loaded snap mechanism to break the
+contact outside the cylinder, and a camshaft and cams to actuate the
+breaker mechanism at the proper time. Furthermore, as the Wrights
+applied it, the system required dry cells and a coil for starting,
+although these did not accompany the engine in flight. And finally
+there was the problem of keeping tight the joint where the
+oscillating shaft required to operate the moving point in the spark
+plug entered the cylinder.
+
+This is one of the few occasions, if not the only one, when the
+Wrights chose the more complex solution in connection with a major
+part--in this particular case, one with far more bits and pieces.
+However, it did carry with it some quite major advantages. The common
+spark plug, always subject to fouling or failure to function because
+of a decreased gap, was not very reliable over a lengthy period, and
+was undoubtedly much more so in those days when control of the amount
+of oil inside the cylinder was not at all exact. Make-and-break
+points, on the other hand, were unaffected by excess oil in the
+cylinder. Because of this resistance to fouling, the system was
+particularly suitable for use with the compression-release method of
+power control which they later utilized, although they probably could
+not have been looking that far ahead at the time they chose it.
+High-tension current has always, and rightfully so, been thought of as
+a troublemaker in service; in Beaumont's 1900 edition of _Motor
+Vehicles and Motors_, which seems to have been technically the best
+volume of its time, the editor predicted that low-tension
+make-and-break ignition would ultimately supersede all other methods.
+And finally, the large number of small parts required for the
+make-and-break system could all be made in the Wright Brothers' shop
+or easily procured, and in the end this was probably the factor, plus
+reliability, that determined the decision which, all things
+considered, was the correct one.
+
+There was nothing exceptional about the exact form the Wrights
+devised. It displayed the usual refined simplicity (the cams were made
+of a single small piece of strip steel bent to shape and clamped to
+the ignition camshaft with a simple self-locking screw), and
+lightness. The ignition camshaft (38, Figure 5), a piece of
+small-diameter bar stock, was located on the same side as the exhaust
+valve camshaft, approximately midway between it and the valve boxes,
+and was operated by the exhaust camshaft through spur gearing. That
+the Wrights were thinking of something beyond mere hops or short
+flights is shown by the fact that the ignition points were
+platinum-faced, whereas even soft iron would have been satisfactory
+for the duration of all their flying for many years.
+
+The control of the spark timing was effected by advancing or retarding
+the ignition camshaft in relation to the exhaust valve camshaft. The
+spur gear (37, Figure 5) driving the ignition camshaft had its hub on
+one side extended out to provide what was in effect a sleeve around
+the camshaft integral with the gear. The gear and integral sleeve were
+slidable on the shaft and the sleeve at one place (39, Figure 5) was
+completely slotted through to the shaft at an angle of 45 deg. to the
+longitudinal axis of the shaft. The shaft was driven by a pin tightly
+fitted in it and extending into the slot. The fore-and-aft position of
+the sleeve on the shaft was determined by a lever-operated cam (40,
+Figure 5) on one side and a spring on the other. The movement of the
+sleeve along the shaft would cause the shaft to rotate in relation to
+it because of the angle of the slot, thus providing the desired
+variation in timing of the spark. The "magneto" was a purchased item
+driven by means of a friction wheel contacting the flywheel, and
+several different makes were used later, but the original is indicated
+to have been a Miller-Knoblock (see Figure 5).
+
+The connecting rod is another example of how, seemingly without
+trouble, they were able to meet the basic requirements they had set
+for themselves. It consisted of a piece of seamless steel tubing with
+each end fastened into a phosphor-bronze casting, these castings
+comprising the big and little ends, drilled through to make the
+bearings (See Figures 5 and 6). It was strong, stiff and light.[13]
+Forged rods were in rather wide use at the time and at least one
+existing engine even had a forged I-beam section design that was
+tapered down from big to little end. The Wrights' rod was obtained in
+little more time than it took to make the simple patterns for the two
+ends. The weight was easily controlled, no bearing liners were
+necessary, and a very minimum of machining was required. Concerning
+the big-end material, there exists a contradiction in the records:
+Baker, whose data are generally most accurate, states that these were
+babbited, but this must be in error, as the existing engine has
+straight bronze castings without babbiting, and there is no record, or
+drawing, or other indication of the bearings having been otherwise.
+
+[Footnote 13: A rather thorough stress analysis of the rod shows it to
+compare very favorably with modern practice. In the absence of an
+indicator card for the 1903 engine, if a maximum gas pressure of five
+times the MEP is assumed, the yield-tension factor of safety is
+measurably higher than that of two designs of piston engines still in
+wide service, and the column factor of safety only slightly less. The
+shear stresses in the brazed and threaded joints are so low as to be
+negligible.]
+
+Different methods of assembling the rod were used. At one time the
+tube ends were screwed into the bronze castings and pinned, and at
+another the ends were pinned and soldered. There is an indication that
+at one time soldering and threads were used in combination. One of the
+many conflicts between the two primary sets of drawings exists at this
+point. The Smithsonian drawings show the use at each end of adapters
+between the rod and end castings, the adapters being first screwed
+into the castings and pinned and then brazed to the inside of the
+tube. The Science Museum drawings show the tube section threaded and
+screwed into the castings. The direct screw assembly method called for
+accurate machining and hand fitting in order to make the ends of the
+tubing jam against the bottom of the threaded holes in the castings,
+and at the same time have the end bearings properly lined up. The
+weakness of the basic design patently lies in the joints. It is an
+attempt to utilize what was probably in the beginning a combination
+five-piece assembly and later three, in a very highly stressed part
+where the load was reversing. It gave them considerable trouble from
+time to time, particularly in the 4-cylinder vertical engines, and was
+abandoned for a forged I-beam section type in their last engine model;
+but it was nevertheless the ideal solution for their first engine.
+
+The crankshaft was made from a solid block of relatively high carbon
+steel which, aside from its bulk and the major amount of machining
+required, presented no special problems. It was heat-treated to a
+machinable hardness before being worked on, but was not further
+tempered. The design was an orthodox straight pin and cheek
+combination and, as previously noted, there were no counterweights to
+complicate the machining or assembly. A sizable bearing was provided
+on each side of each crank of the shaft, which helped reduce the
+stiffness requirement.
+
+Their only serious design consideration was to maintain the desired
+strength and still keep within weight limitations. A fundamental that
+every professional designer knows is that it is with this particular
+sort of part that weight gets out of control; even an additional 1/16
+in., if added in a few places, can balloon the weight. With their
+usual foresight and planning, the Wrights carefully checked and
+recorded the weight of each part as it was finished, but even this
+does not quite explain how these two individuals, inexperienced in
+multicylinder engines--much less in extra-light construction--could,
+in two months, bring through an engine which was both operable and
+somewhat lighter than their specification.
+
+In one matter it would seem that they were quite fortunate. The
+records are not complete, but with one exception there is no
+indication of any chronic or even occasional crankshaft failure. This
+would seem to show that it apparently never happened that any of their
+designs came out such that the frequency of a vibrating force of any
+magnitude occurred at the natural frequency of the shaft. Much later,
+when this type of vibration became understood, it was found virtually
+impossible, with power outputs of any magnitude, to design an
+undampened shaft, within the space and weight limitations existing in
+an ordinary engine, strong enough to withstand the stress generated
+when the frequency of the imposed vibration approximated the natural
+frequency of the shaft. The vibratory forces were mostly relatively
+small in their engines, so that forced vibration probably was not
+encountered, and the operating speed range of the engines was so
+limited that the natural frequency always fell outside this range.
+
+The flywheel was about the least complex of any of their engine parts
+and required little studied consideration, although they did have to
+balance its weight against the magnitude of the explosion forces which
+would reach the power transmission chains, with their complete lack of
+rigidity, a problem about which they were particularly concerned. The
+flywheel was made of cast iron and was both keyed to and shrunk on the
+shaft.
+
+Some doubt still exists about the exact method of lubricating the
+first engine. The unit presently in the airplane has a gear-type oil
+pump driven by the crankshaft through a worm gear and cross shaft, and
+the Appendix to the _Papers_ states that it was lubricated by a small
+pump; nevertheless Baker says, after careful research, that despite
+this evidence, it was not. Also, the drawings prepared by Christman
+(they were commenced under the supervision of Orville Wright) do not
+show the oil pump. In March 1905 Wilbur Wright wrote to Chanute,
+"However we have added oiling and feeding devices to the engine ...";
+but this could possibly have referred to something other than an oil
+pump. But even if a pump was not included originally, its presence in
+the present engine is easily explained. Breakage of the crankcase
+casting caused the retirement of this engine, which was not rebuilt
+until much later, and the pattern for this part had no doubt long
+since been altered to incorporate a pump. It was therefore easier in
+rebuilding to include than to omit the pump, even though this required
+the addition of a cross shaft and worm gear combination. On later
+engines, when the pump was used, oil was carried to a small pipe,
+running along the inside of the case, which had four small drill holes
+so located as to throw the oil in a jet on the higher, thrust-loaded
+side of each cylinder. The rods had a sharp scupper on the outside of
+the big end so placed as also to throw the oil on this same thrust
+face. Some scuppers were drilled through to carry oil to the rod
+bearing and some were not.
+
+The first engine was finished and assembled in February 1903 and given
+its first operating test on 22 February. The Wrights were quite
+pleased with its operation, and particularly with its smoothness.
+Their father, Bishop Wright, was the recorder of their satisfaction
+over its initial performance, but what he noted was probably the
+afterglow of the ineffable feeling of deep satisfaction that is the
+reward that comes to every maker of a new engine when it first comes
+to life and then throbs. They obtained 13 hp originally: later figures
+went as high as almost 16, but as different engine speeds were
+utilized it is rather difficult to settle on any single power figure.
+The most realistic is probably that given in the _Papers_ as having
+been attained later, after an accurate check had been made of the
+power required to turn a set of propellers at a given rpm. This came
+out at approximately 12 hp, the design goal having been 8. Following
+exactly the procedure that exists to this day, the engine went through
+an extended development period, and it was the end of September 1903
+before it was taken, with the airplane, to Kitty Hawk where the
+historic flights, which have had such a profound effect on the lives
+of all men, were made on 17 December 1903.
+
+
+
+
+The Engines With Which They Mastered The Art of Flying
+
+
+Two more engines of this first general design were built but they
+differed somewhat from each other as well as from the original.
+Together with a third 8-cylinder engine these were begun right after
+the first of the year in 1904, shortly after the Wrights' return from
+Kitty Hawk. In planning the 8-cylinder engine they were again only
+being forehanded, but considerably so, in providing more power for
+increased airplane performance beyond that which might possibly be
+obtained from the 4-cylinder units. Progress with the 4-cylinder
+engines was such that they fairly quickly concluded that the
+8-cylinder size would not be necessary, and it was abandoned before
+completion. Exactly how far it was carried is not known. The record
+contains only a single note covering the final scrapping of the parts
+that had been completed; and apparently there were no drawings, so
+that even its intended appearance is not known with any exactness. It
+was probably a 90 deg. V-type using their original basic cylinder
+construction.
+
+The changes carried through in the two 4-cylinder engines were not
+major. The water-cooled area of the cylinder barrel was increased by
+nearly ten percent but the head remained only partially cooled. In
+hindsight, this consistent avoidance of complete cylinder-head cooling
+presents the one most inexplicable of the more important design
+decisions they made, as it does not appear logical. In the original
+engine, where the factors of time and simplicity were of paramount
+importance, this made sense, but now they were contemplating
+considerably increased power requirements, knowing the effect of
+temperature on both the cylinder and the weight of cylinder charge,
+and knowing that valve failure was one of their most troublesome
+service problems. Nor does it seem that they could have been avoiding
+complete cylinder cooling through fear of the slightly increased
+complexity or the difficulty of keeping the water connections and
+joints tight, for they had faced a much more severe problem in their
+first engine, where their basic design required that three joints be
+kept tight with only two sets of threads, and had rather easily
+mastered it; so there must have been some much more major but not
+easily discernible factor which governed, for they still continued to
+use the poorly cooled head, even carrying it over to their next engine
+series. Very probably they did not know the effect on detonation of a
+high-temperature fuel-charge.
+
+One of the new engines was intended for use in their future
+experimental flying and has become known as _No. 2._ It had a bore of
+4-1/8 in., incorporated an oil pump, and at some time shortly after
+its construction a fuel pump was added. The fuel pump was undoubtedly
+intended to provide a metering system responsive to engine speed and
+possibly also to eliminate the small inherent variation in flow of the
+original gravity system.
+
+This engine incorporated a cylinder compression release device not on
+the original. The exact reason or reasons for the application of the
+compression release have not been determined, although the record
+shows it to have been utilized for several different purposes under
+different operating conditions. Whatever the motivation for its
+initial application, it was apparently useful, as it was retained in
+one form or another in subsequent engine models up to the last
+6-cylinder design. Essentially it was a manually controlled mechanism
+whereby all the exhaust valves could be held open as long as desired,
+thus preventing any normal charge intake or compression in the
+cylinder. Its one certain and common use was to facilitate starting,
+the open exhaust valves easing the task of turning the engine over by
+hand and making priming easy. In flight, its operation had the effect
+of completely shutting off the power. The propellers would then
+"windmill" and keep the engine revolving. One advantage stated for
+this method of operation was that when power was required and the
+control released, the engine would be at fairly high speed, so that
+full power was delivered immediately fuel reached the engine. It is
+also reported to have been used both in making normal landings and in
+emergencies, when an instant power shutdown was desired. Although it
+is not clear whether the fuel shutoff cock was intended to be
+manipulated when the compression release was used for any of these
+reasons, over the many years of its availability, undoubtedly at one
+time or another every conceivable combination of operating conditions
+of the various elements was tried. Because of the pumping power
+required with at least one valve open during every stroke, the
+windmilling speed of the engine was probably less than with any other
+method of completely stopping power output, but whether this
+difference was large enough to be noticeable, or was even considered,
+is doubtful.
+
+Since a simple ignition switch was all that was required to stop the
+power output, regardless of whether a fuel-control valve or a
+spark-advance control was used, it must be concluded that the primary
+function of the compression release was to facilitate starting, and
+any other useful result was something obtained at no cost. The
+compression release was later generally abandoned, and until the
+advent of the mechanical starter during the 1920s, starting an engine
+by "pulling the propeller through" could be a difficult task. With the
+Wrights' demonstrated belief that frugality was a first principle of
+design, it is hardly conceivable that they would have accepted for any
+other reason the complication of the compression-release mechanism if
+a simple ignition switch would have sufficed.
+
+The compression-release mechanism was kept relatively simple,
+considering what it was required to accomplish. A small non-revolving
+shaft was located directly under the rocker arm rollers that actuated
+the exhaust valves. Four slidable stops were placed on this shaft,
+each in the proper location, so that at one extreme of their travel
+they would be directly underneath the rocker roller and at the other
+extreme completely in the clear. They were positioned along the shaft
+by a spring forcing them in one direction against a shoulder integral
+with the shaft, and the shaft was slidable in its bearings, its
+position being determined by a manually controlled lever. When the
+lever was moved in one direction the spring pressure then imposed on
+the stops would cause each of them to move under the corresponding
+rocker roller as the exhaust valve opened, thus holding the exhaust
+valve in the open position. When the shaft was moved in the other
+direction the collar on the shaft would mechanically move the stop
+from underneath the roller, allowing the valve to return to normal
+operation.
+
+[Illustration: _Figure 8._--Development engine No. 3, 1904-1906,
+showing auxiliary exhaust port, separate one-piece water-jacket block.
+(Photo by author.)]
+
+If the 1903 engine is the most significant of all that the Wrights
+built and flew, then certainly the _No. 2_ unit was the most useful,
+for it was their sole power source during all their flying of 1904 and
+1905 and, as they affirmed, it was during this period that they
+perfected the art, progressing from a short straightaway flight of 59
+seconds to a flight controllable in all directions with the duration
+limited only by the fuel supply. It is to be greatly regretted that no
+complete log or record was kept of this engine.
+
+The Wrights again exhibited their engineering mastery of a novel basic
+situation when, starting out to make flight a practical thing, they
+provided engine _No. 3_ to be used for experimental purposes. In so
+doing they initiated a system which continues to be fundamental in the
+art of providing serviceable aircraft engines to this day--one that is
+expensive and time consuming, but for which no substitute has yet been
+found. Their two objectives were: improvement in performance and
+improvement in reliability, and the engine was operated rather
+continuously from early 1904 until well into 1906. Unfortunately,
+again, no complete record exists of the many changes made and the
+ideas tested, although occasional notes are scattered through the
+diaries and notebooks.
+
+In its present form--it is on exhibition at the Engineers Club in
+Dayton, Ohio--the _No. 3_ engine embodies one feature which became
+standard construction on all the Wright 4-cylinder models. This was
+the addition of a number of holes in a line part way around the
+circumference of the cylinder barrel so that they were uncovered by
+the piston at the end of its stroke toward the shaft, thus becoming
+exhaust ports (see Figure 9). This arrangement, although not entirely
+novel, was just beginning to come into use, and in its original form
+the ports exhausted into a separate chamber, which in turn was
+evacuated by means of a mechanically operated valve, so that two
+exhaust valves were needed per cylinder. Elimination of this chamber
+and the valve arrangement is typical of the Wrights' simplifying
+procedure, and it would seem that they were among the very first to
+use this form.[14]
+
+[Footnote 14: Rankin Kennedy, _Flying Machines--Practice and Design_,
+1909.]
+
+The primary purpose of the scheme was to reduce, by this early release
+and consequent pressure and temperature drop, the temperature of the
+exhaust gases passing the exhaust valve, this valve being one of their
+main sources of mechanical trouble. It is probable that with the
+automatic intake valves being used there was also a slight effect in
+the direction of increasing the inlet charge, although with the small
+area of the ports and the short time of opening, the amount of this
+was certainly minor. With the original one-piece crankcase and
+cylinder jacket construction, the incorporation of this auxiliary
+porting was not easy, but this difficulty was overcome in the
+development engine by making different castings for the crankcase
+itself and for the cylinder jacket and separating them by several
+inches, so that room was provided between the two for the ports.
+
+This engine demonstrated the most power of any of the flat 4s,
+eventually reaching an output of approximately 25 hp, which was even
+somewhat more than that developed by the slightly larger
+4-1/8-in.-bore flight engine, with which 21 hp was not exceeded.
+Indicative of the development that had taken place, the performance of
+the _No. 3_ engine was twice the utilized output of the original
+engine of the same size, an increase that was accomplished in a period
+of less than three years.
+
+The Wrights were only twice charged with having plagiarized others'
+work, a somewhat unusual record in view of their successes, and both
+times apparently entirely without foundation. A statement was
+published that the 1903 flight engine was a reworked Pope Toledo
+automobile unit, and it was repeated in an English lecture on the
+Wright brothers. This was adequately refuted by McFarland but
+additionally, it must be noted, there was no Pope Toledo company or
+car when the Wright engine was built. This company, an outgrowth of
+another which had previously manufactured one-and two-cylinder
+automobiles, was formed, or reformed, and a Pope license arrangement
+entered into during the year 1903.
+
+The other incident was connected with Whitehead's activities and
+designs. Whitehead was an early experimenter in flying, about the time
+of the Wrights, whose rather extraordinary claims of successful flight
+were published in the 1901-1903 period but received little attention
+until very much later. His first engines were designed by a clever
+engineer, Anton Pruckner, who left at the end of 1901, after which
+Whitehead himself became solely responsible for them. It was stated
+that the Wrights visited the Whitehead plant in Bridgeport,
+Connecticut, and that Wilbur remained for several days, spending his
+time in their machine shop. This was not only categorically denied by
+Orville Wright when he heard of it but it is quite obvious that the
+1903 or any other of the Wright engine designs bears little
+resemblance to Pruckner's work. In fact, its principal design features
+are just the opposite of Pruckner's, who utilized vertical cylinders,
+the 2-stroke cycle, and air-cooling, which Whitehead at some point
+changed to water-cooling.[15]
+
+[Footnote 15: Considerable doubt surrounds Whitehead's actual flight
+accomplishments, but Pruckner's engines were certainly used, as
+several were sold to early pioneers, including Charles Wittemann. It
+is probable that the specific power output was not very great, for the
+air-cooled art of this time was not very advanced and Pruckner had a
+rather poor fin design. But the change to water cooling eliminated
+this trouble, and the engines were most simple, should have been
+relatively quite light, and with enough development could probably
+have been made into sufficiently satisfactory flying units for that
+period.]
+
+
+
+
+The Four-Cylinder Vertical Demonstration Engine and the First
+Production Engine
+
+
+In 1906, while still doing general development work on the flat
+experimental engine, the Wrights started two new engines, and for the
+first time the brothers engaged in separate efforts. One was "a
+modification of the old ones" by Wilbur and the other, "an entirely
+new pattern" by Orville. There is no record of any of the features of
+Wilbur's project or what was done in connection with it. Two months
+after the experimental operation of the two designs began, an entry in
+Wilbur's diary gives some weight and performance figures for the "4" x
+4" rebuilt horizontal," and since Orville's design was vertical the
+data clearly refer to Wilbur's; but since the output is given only in
+test-fan rpm it does not serve to indicate what had been accomplished
+and there is no further mention of it.
+
+Orville's design became the most used of any model they produced. It
+saw them through the years from 1906 to 1911 or 1912, which included
+the crucial European and United States Army demonstrations, and more
+engines of this model were manufactured than any of their others
+including their later 6-cylinder. Although its ancestry is traceable
+to the original 1903 engine, the design form, particularly the
+external configuration, was considerably altered. Along with many
+individual parts it retained the basic conception of four medium-size
+cylinders positioned in line and driving the propellers through two
+sprocket wheels. From the general tenor of the record it would seem,
+despite there being no specific indication, that from this time on
+Orville served as the leader in engine design, although this occurred
+with no effect whatsoever on their finely balanced, exactly equal
+partnership which endured until Wilbur's death in 1912.
+
+The first major change from the 1903 design, putting the engine in an
+upright instead of flat position, was probably done primarily to
+provide for a minimum variation in the location of the center of
+gravity with and without a passenger. Whether or not it had any
+influence, the vertical cylinder arrangement was becoming predominant
+in automobile powerplants by this time, and the Wright engines now
+began to resemble this prevailing form of the internal combustion
+engine--a basic form that, in a wide variety of uses, was to endure
+for a long time.
+
+Over the years, the Wrights seem to have made many changes in the
+engine: the bore was varied at different times, rod assembly methods
+were altered, and rod ends were changed from bronze to steel.
+Chenoweth states that on later engines an oil-control ring was added
+on the bottom of the piston, necessitating a considerable increase in
+the length of the cylinder barrel. This arrangement could not have
+been considered successful, as it apparently was applied to only a
+limited number of units and was not carried over to the later
+6-cylinder engine model. There was much experimentation with cam
+shapes and most probably variations of these got into production.
+
+With the crankcase, they did not go all the way to the modern
+two-piece form but instead retained the one-piece construction.
+Assembly was effected through the ends and a detachable plate was
+provided on one side for access to the interior. It is clear that they
+regarded this ability to get at the interior of the case without major
+disassembly as a valuable characteristic, and later featured it in
+their sales literature. They were apparently willing to accept the
+resultant weakening of the case and continued the construction through
+their last engine model. The integrally cast cylinder water jackets
+were abandoned and the top of the crankcase was machined flat to
+provide a mounting deck for individual cylinders. The use of aluminum
+alloy was continued, and the interior of the case was provided with
+strengthening webs of considerable thickness, together with supporting
+ribs. The cam shaft was supported directly in the case.
+
+The individual cylinder design was of extreme simplicity, a single
+iron casting embodying everything except the water jacket. The valves
+seated directly on the cast-iron cylinder head and the guides and
+ports were all contained in an integral boss on top of the head. The
+exhaust valve location on the side of the engine opposite the pilot
+was a decided advantage over that of the 1903 design, where the
+exhaust was toward the pilot. A four-cornered flange near the bottom
+of the cylinder provided for fastening it to the crankcase, and a
+threaded hole in the top of the head received a vertical eyebolt which
+served as the rocker-arm support. The cylinder was machined all over;
+two flanges, one at the bottom and the other about two-thirds of the
+way down provided the surfaces against which the water jacket was
+shrunk. The jacket was an aluminum casting incorporating the necessary
+bosses and double shrunk on the barrel; that is, the jacket itself was
+shrunk on the cylinder-barrel flanges and then steel rings were shrunk
+on the ends of the jacket over the flanges. The jacket thickness was
+reduced by machining at the ends, making a semigroove into which the
+steel shrink rings fitted. These rings insured the maintenance of a
+tight joint despite the tendency of the aluminum jacket to expand away
+from the cast-iron barrel.
+
+[Illustration: _Figure 9._--4-Cylinder vertical engine: a, magneto
+side; b, valve port side with intake manifold removed; c, flywheel end
+of engine at Carillon Park Museum, Dayton, Ohio; d, magneto side with
+crankcase cover removed. (Photos: a, Smithsonian A-3773; b, d, Pratt &
+Whitney D-15003, 15007; c, by A. L. Rockwell.)]
+
+Why the one-piece crankcase and cylinder jacket combination of the
+1903 engine was abandoned for the individual cylinder construction can
+only be surmised. The difference in weight was probably slight, as the
+inherent weight advantage of the original crankcase casting was
+largely offset by the relatively heavy valve boxes, and the difference
+in the total amount of machining required, because of the separate
+valve boxes, cages, and attaching parts, also was probably slight.
+Although the crankcase had shown itself to be structurally weak, this
+could have been cared for by proper strengthening. The 1903 design did
+have some fundamental disadvantages: it required a fairly complex
+pattern and expensive casting, plus some difficult machining, part of
+which had to be very accurate in order to maintain both gas and water
+joints tight; and the failure of any one cylinder that affected the
+jacket meant a complete crankcase replacement.
+
+It seems probable that a change was initially made mandatory by their
+intention to utilize the ported exhaust feature, the value of which
+they had proved in the experimental engine. The separate one-piece
+water jacket construction they had arrived at in this engine was
+available, but once the decision to change was made, the individual
+cylinder with its shrunk-on jacket had much to commend it--simplicity,
+cost, ease of manufacture and assembly and attachment, and
+serviceability. The advantages of the auxiliary, or ported, exhaust
+were not obtained without cost, however, as the water jacket around
+the barrel could not very easily be extended below the ports. Thus,
+even though the water was carried as high as possible on the upper
+end, a large portion of the barrel was left uncooled, and the lack of
+cooling at the lower end, in conjunction with the uncooled portion of
+the head, meant that only approximately half the entire cylinder
+surface was cooled directly.
+
+The piston was generally the same as in the 1903 engine, except that
+six radial ribs were added on the under side of the head, tapering
+from maximum thickness at the center to nothing near the wall. They
+were probably incorporated as an added path for heat to flow from the
+center of the piston toward the outside, as their shape was not the
+best use of material for strength. The piston pin was locked in the
+piston by the usual set screw, but here no provision was made for the
+alternate practice of clamping the rod on the pin. This piston-pin
+setscrew construction had become a standard arrangement in automobile
+practice. The piston rings were the normal wide design of that time,
+with what would now be considered a low unit pressure.
+
+Quite early in the life of this engine model the practice was
+initiated of incorporating shallow grooves in the surface of the more
+highly loaded thrust face of the piston below the piston pin to
+provide additional lubrication. This development apparently proceeded
+haphazardly. Figure 10c shows three of the pistons from an engine of
+low serial number--the first of this model to be delivered to the U.S.
+Navy--and it will be noted that one has no grooves, another has one,
+and the other has three. The eventual standardized arrangement
+provided three of these grooves, approximately 1/16 in. wide,
+extending halfway around the piston, and, although the depth was only
+a few thousandths of an inch, the amount of oil carried in them was
+apparently sufficient to assist in the lubrication of the face, as
+they were used in both the 4-and 6-cylinder engines.
+
+Each cylinder was fastened to the crankcase by four nuts on studs
+driven into the aluminum case. Valves and rocker arms were similar to
+those of the early engines, the automatic inlet valve being retained.
+The continued use of the two-piece valve is not notable, even though
+one-piece forgings were available and in use at this time; the
+automobile continued for many years to use this construction. The
+camshaft was placed at the bottom of the engine, inside the crankcase,
+and the rocker arms were actuated by pushrods which were operated by
+hinged cam followers. The pushrod was fastened in the rocker by a pin,
+about which it operated, through its upper end and was positioned near
+the bottom by a guide in the crankcase deck. The lower end of the rod
+bore directly on the flat upper surface of the cam follower, and valve
+clearance adjustment was obtained by grinding this end. The camshaft
+and magneto were driven by the crankshaft through a three-member train
+of spur gears (see Figures 9, 10 and 11).
+
+The built-up construction of the connecting rod was carried over from
+the first engine, and in the beginning apparently the same materials
+were used, except that the big end was babbited. Later the rod ends
+were changed from bronze to steel. The big end incorporated a small
+pointed scupper on one side for lubrication, as with the original, and
+this was sometimes drilled to feed a groove which carried oil to the
+rod bearing, but where the drilling was omitted, the only function the
+scupper then could perform was, as in the original engine, to throw a
+small amount of oil on the cylinder wall.
+
+The crankshaft and flywheel were similar in design to those on the
+1903 engine, except that the sharp corners at the top and bottom of
+the crank cheeks were machined off to save weight (see Figure 10f). An
+oil pump and a fuel pump were mounted side by side in bosses cast on
+the valve side of the crankcase; they were driven from the camshaft by
+worm gears and small shafts crossing the case.
+
+[Illustration: _Figure 10._--4-Cylinder vertical engine: a, cylinder
+assembly with valve mechanism parts; b, cylinder disassembled, and
+parts; c, pistons and connecting rods; d, bottom side of piston; e,
+crankshaft, flywheel and crankcase end closure; f, crankcase, with
+compression release parts. (Pratt & Whitney photos D-14996, 15001,
+14998, 14994, 14999, 14989, respectively.)]
+
+The camshaft construction was considerably altered from the 1903
+design. Although the reason is not entirely clear, one indication
+suggests that breakage or distortion of the shaft may have been
+encountered: whereas in the 1903 engine there had been no relationship
+between the location of the cams and the camshaft bearings, in this
+engine the exhaust valves were carefully positioned so that all cams
+were located very close to the supporting bearings in the crankcase.
+Also, the camshaft was solid, although it would seem that the original
+hollow shaft construction could have provided equal stiffness with
+less weight. The final decision was possibly determined by the
+practicality that there existed no standard tubing even approximating
+the size and wall thickness desired.
+
+There still was no carburetor, a gear pump metering the fuel in the
+same manner as on the 1904-1905 engine. Basically, the intake charge
+was fed to the cylinders by a round gallery manifold running alongside
+the engine. This was split internally by a baffle extending almost
+from end to end, so that the fuel mixture entering the manifold on one
+side of the baffle was compelled to travel to the two ends before it
+could return to the inside cylinder, this feature being a copy of
+their 1903 general intake arrangement. Apparently various shapes and
+positions of entrance pipes with which to spray the fuel into the
+manifold were used; and the injection arrangement seems also to have
+been varied at different times. The fuel pump was not necessarily
+always used, as the engine in some of the illustrations did not
+incorporate one, the fuel apparently being fed by gravity, as on the
+original engine. Chenoweth describes an arrangement in which exhaust
+heat was applied to the inlet manifold to assist the fuel vaporization
+process, but it is believed that this was one of the many changes made
+in the engine during its lifetime and not necessarily a standard
+feature.
+
+A water circulation pump was provided, driven directly by the
+crankshaft through a two-arm universal joint intended to care for any
+misalignment between the shaft and the pump. The water was piped to a
+horizontal manifold running along the cylinders just below the intake
+manifold, and a similar manifold on the other side of the engine
+collected it for delivery to the radiator. It is a little difficult to
+understand why it was not introduced at the bottom of the water
+jackets.
+
+The crankcase was a relatively strong and well proportioned structure
+with three heavy strengthening ribs running from side to side, its
+only weakness being the one open side. A sheet-iron sump was fastened
+to the bottom by screws and it would appear from its design, method of
+attachment, and location of the engine mounting pads that this was
+added some time after the crankcase had been designed; but if so it
+was apparently retrofitted, as engines with quite low serial numbers
+have this part.
+
+The ignition was by high-tension magneto and spark plug and this
+decision to change from the make-and-break system was undoubtedly the
+correct one, just as adoption of the other form originally was logical
+under the circumstances that existed then. The high-tension system was
+simpler and had now collected more service experience. The magneto
+was driven through the camshaft gear, and a shelf, or bracket, cast as
+an integral part of the case, was provided for mounting it. The spark
+advance control was in the magneto and, since spark timing was the
+only means of regulating the engine power and speed, a wide range of
+adjustment was provided.
+
+The engine had the controllable compression release which had been
+added to the _No. 2_ and _No. 3_ flat engines, although mechanically
+it was considerably altered from the original design. Instead of the
+movable stop operating directly on the rocker roller to hold the
+exhaust valve open, it was located underneath a collar on the pushrod.
+This stop was hinged to the crankcase and actuated by a small rod
+running along and supported by the crankcase deck. Longitudinal
+movement of this rod in one direction would, by spring pressure on
+each stop, push them underneath the collars as the exhaust valves were
+successively opened. A reverse movement of the rod would release them
+(see Figure 10f). Why they retained the method of manually operating
+the compression release, which was the same as had been used in the
+1904-1905 engine, is not quite clear. That is, the mechanism was put
+into operation by pulling a wire running from the pilot to a lever
+actuating the cam which moved the control rod. When normal valve
+operation was subsequently desired, the pilot was compelled to reach
+with his hand and operate the lever manually, whereas a second wire or
+push-pull mechanism would have obviated the necessity for both the
+awkward manual operation of the lever and the gear guard which was
+added to protect the pilot's hand, the lever being located close to
+the camshaft gear.
+
+The 4-cylinder vertical engine was a considerable improvement over the
+previous designs. They had obtained a power increase of about 40
+percent, with a weight decrease of 10 percent, and now had an engine
+whose design was almost standard form for good internal combustion
+engines for years to come. In fact, had they split the crankcase at
+the crankshaft center line and operated the inlet valves mechanically,
+they would have had what could be termed a truly modern design. They
+needed more cylinder cooling, both barrel and head, particularly the
+latter, and an opened-up induction system for maximum power output,
+but this was not what they were yet striving for. They had directly
+stated that they were much more interested in reliability than light
+weight.
+
+This engine model was the only one of the Wright designs to be
+licensed and produced abroad, being manufactured in Germany by the
+Neue Automobil-Gesellschaft and by Bariquand et Marre in France. The
+latter was much more prominent and their engines were used in several
+early European airplanes.
+
+[Illustration: _Figure 11._--4-Cylinder vertical engine assembly,
+Bariquand et Marre version. (Drawing courtesy Bristol Siddeley
+Engines, Ltd.)]
+
+[Illustration: THE WRIGHT BROTHERS AERO ENGINE]
+
+The French manufacturer, without altering the basic design, made a
+number of changes of detail which seem to have greatly annoyed
+Wilbur Wright, although some of them could probably be listed as
+improvements, based on several features of later standard design. One
+consisted of an alteration in the position of the fuel and oil pumps,
+the latter being lowered to the level of the sump. The crankcase was
+drilled to provide forced-feed lubrication to the connecting rod big
+end and crankshaft main bearings. Strengthening ribs were added to the
+pistons running from the upper side of the pin bosses to the piston
+wall, and the crankcase studs holding down the cylinders were replaced
+with bolts having their heads inside the case. The hinged cam follower
+was omitted and the pushrod bore directly on the cam through a roller
+in its end. The magneto was moved toward the rear of the engine a
+considerable distance and an ignition timing control device was
+introduced between it and its driving gear. Instead of the magneto
+being mounted directly on the special bracket integral with the
+crankcase, a wooden board running from front to rear of the engine was
+used and this was fastened to the two engine support pads, the magneto
+bracket being omitted entirely.
+
+Despite his criticism of the French motor and the quality of its
+manufacture, Wilbur was compelled to install one in his own exhibition
+airplane during his early French demonstrations at Le Mans after rod
+failure had broken his spare crankcase, and much of his subsequent
+demonstration flying was made with the French product.
+
+
+
+
+The Eight-Cylinder Racing Engine
+
+
+By 1909 regular and special air meets and races were being held and
+various competitions for trophies conducted. Among these the Gordon
+Bennett Cup Race for many years was considered a major event. For the
+1910 competition it was decided to enter a Wright machine and, since
+this was a race with speed the sole objective, the available
+4-cylinder engine, even in a version pushed to its maximum output, was
+deemed too small. They built for it a special 8-cylinder unit in a
+90 deg.V form. They were thus resorting to one of their 1904
+concepts--modifying and enlarging a version known and proved in
+use--as the proper method of most quickly increasing output.
+Unfortunately again, there are essentially no detailed drawings
+available, so that the design cannot be studied.[16]
+
+[Footnote 16: A drawing of the camshaft is held by The Franklin
+Institute.]
+
+Only one engine is historically recorded as having been built,
+although in view of the Wrights' record of foresight and preparation
+it is almost certain that at least one spare unit, assembled or in
+parts, was provided. In any case, the airplane--it was called the
+_Baby Grand Racer_--and engine were wrecked just before the race, and
+no physical parts were retained, so that the sole descriptions come
+from external photographs, memory, and hearsay. McFarland thinks that
+possibly Orville Wright, particularly, was somewhat discomfited over
+the accident that eliminated the machine, as he had previously flown
+it quite successfully at a speed substantially higher than that of the
+ultimate winner, and he wanted to get it out of sight and mind as
+quickly as possible. The Air Force Museum at Wright Field, Dayton,
+Ohio, has an incomplete set of drawings of a 90 deg.V, 8-cylinder Wright
+engine, but it is quite obvious from the basic design and individual
+features, as well as from at least one date on the drawings, that this
+conception is of a considerably later vintage than that of the _Baby
+Grand Racer_.
+
+The racing engine was in essence a combination of two of the standard
+4s on a redesigned crankcase utilizing as many of the 4-cylinder
+engine parts as possible. The rods were reported to have been placed
+side by side, and the regular 4-cylinder crankshaft, with alterations
+to accommodate the rods, was utilized. A single cam operated all the
+exhaust valves. It was compact and light, its only fundamental
+disadvantage being the inherent unbalance of the 90 deg.V-8. The
+arrangement provided a much higher powered unit in the cheapest and
+quickest manner, and one that could be expected to operate
+satisfactorily with the least development.
+
+
+
+
+The Six-Cylinder Vertical Engines
+
+
+Shortly after the construction of the 8-cylinder engine the Wrights
+were again faced with the ever-recurrent problem of providing a higher
+powered standard production engine for their airplanes, which were now
+being produced in some numbers. By this time, 1911, there had been a
+relatively tremendous growth in both flying and automotive use of the
+internal combustion engine and as a result many kinds and sizes had
+been produced and utilized, so that numerous choices were presented to
+them. But if they were both to make use of their past experience and
+retain the simplicity they had always striven for, the more practical
+possibilities narrowed down to three: they could increase the cylinder
+size in the 4-cylinder combination, or they could go either to 6 or 8
+cylinders in the approximate size they had previously used.
+
+[Illustration: _Figure 12._--Original 6-cylinder engine: a, push-rod
+side; b, valve-port side; c, crankcase with sump removed. (Photos:
+Smithsonian A-3773A, 45598; Pratt & Whitney D-15015, respectively.)]
+
+The 4-in. cylinder in combination with a 5-in. stroke would provide in
+four cylinders about the displacement they wanted. Strokes of 6 in.
+were not uncommon and cylinders of 6-in. bore had been very
+successfully utilized in high-output automobile racing engines many
+years before this, so there was seemingly no reason to doubt that the
+5-in. cylinder could be made to operate satisfactorily, but it is not
+difficult to imagine the Wrights' thoughts concerning the roughness of
+an engine with cylinders of this diameter. The question of the grade
+of available fuel may possibly have entered into their decision to
+some extent, but it seems far more likely that roughness, their
+perennial concern, was the predominant reason for not staying with the
+more simple 4-cylinder form (as we have seen, roughness to them meant
+the effect of the cylinder explosion forces). Actually, of course,
+they never went larger than a 4-3/8-in. cylinder bore, and later
+aircraft engine experience would seem generally to confirm their
+judgment, for with the piston engine it has always been much more
+difficult to make the larger bores operate satisfactorily at any given
+specific output.
+
+While the 90 deg.V, 8-cylinder arrangement would have enabled them to
+utilize a great number of the 4-cylinder-engine parts, it would have
+given them a somewhat larger engine than was their apparent desire,
+unless they reduced the cylinder size. And while they had had some
+limited experience in building and operating this kind of engine, and
+twice had chosen it when seeking more power, both of these choices
+were greatly influenced by the desire to obtain quickly an engine of
+higher power. It is also possible that something in their experience
+with the V-8 moved them away from it; the unbalanced shaking force
+inherent in the arrangement may well have become evident to them. What
+probably also helped them to their final conclusion was the
+fundamental consideration that the V-8 provided two extra cylinders
+which were not really needed.
+
+The eventual selection of the 6-cylinder was a slight compromise. In
+order to get the desired output the cylinder displacement was
+increased, but this was done by lengthening the stroke--the first time
+this had been altered since the original design. The increase (from 4
+to 4-1/2 in.) was only 1/2 in., and the bore, the more important
+influence on fuel performance, was kept the same. Overall, the choice
+was quite logical. They were utilizing the in-line construction upon
+which almost all of their now considerable experience had been based,
+and the sizes of and requirements for parts also conformed to this
+experience. They could, in fact, use many of the same parts. The
+natural balance of the 6-cylinder arrangement gave them a very smooth
+engine, and had they stiffened the shaft and counter-weighted the
+cranks, they would have produced the smoothest engine that could have
+been built at that time.
+
+In the literature are two references to a Wright 6-cylinder engine
+constructed around the cylinders of the vertical 4. One of these is in
+Angle's _Airplane Engine Encyclopedia_, published in 1921, and the
+other is in _Aerosphere 1939_, published in 1940. The wording of the
+latter is essentially identical with that of the former; it seems a
+reasonable conclusion that it is a copy. Although it is possible that
+such an engine was built at some time, just as the 8-cylinder racing
+engine was cobbled up out of parts from the 4-cylinder vertical, no
+other record, no engines, and no illustrations have been found. It is
+thus quite certain that no significant quantity was ever manufactured
+or utilized.
+
+The crankcase was considerably changed from that of the vertical 4,
+and was now in two pieces, with the split on the crankshaft center
+line. However, the shaft was not supported by the lower half of the
+case, as eventually became standard practice, but by bearing caps
+bolted to the ends of the upper case and, in between, to heavy ribs
+running across the upper case between the cylinders. The lower half of
+the case thus received none of the dynamic or explosion loads, and,
+serving only to support the engine and to provide for its mounting,
+was lightly ribbed. In it were incorporated integral-boss standpipe
+oil drains which discharged into a bolted-on sump. The upper half of
+the case was again left open on one side, giving the desired access to
+the interior, and, additionally, the design was altered to provide a
+method of camshaft assembly that was much simpler than that of the
+vertical 4 (see p. 42).
+
+The cylinder was also greatly altered from that of the vertical 4. It
+was made in three parts, a piece of seamless steel tubing being shrunk
+on a cast-iron barrel to form the water jacket, with a cast-iron
+cylinder head shrunk on the upper end of the barrel. This construction
+compelled the use of long studs running from the cylinder head to the
+case for fastening down the cylinder (see Figures 12a-c). For the
+first time the cylinder heads were water-cooled, cored passages being
+provided, and more barrel surface was jacketed than previously,
+although a considerable area at the bottom was still left uncooled,
+obviously by direct intent, as the ported exhaust arrangement was no
+longer employed.
+
+Also for the first time one-piece forged valves were used, but just
+when these were incorporated is not certain and, surprisingly, they
+were applied to the inlet only, the exhaust valve being continued in
+the previous two-piece screwed and riveted construction. The reasoning
+behind this is not evident. If a satisfactory two-piece exhaust valve
+had finally been developed it would be logical to carry it over to
+the new design; but exhaust valves normally being much more
+troublesome, it would seem that a good exhaust valve would make an
+even better inlet valve and, in the quantities utilized, the two-piece
+design should have been much cheaper. In the original 6-cylinder
+engine the inlet valves operated automatically as in all previous
+models, but at the time of a later extensive redesign (1913) this was
+changed to mechanical actuation, and the succeeding engines
+incorporated this feature. All the valve-actuating mechanism was
+similar to that of the vertical 4, and the engine had the usual
+compression-release mechanism, the detail design being carried over
+directly from the 4-cylinder.
+
+Design of the piston followed their previous practice, with wide rings
+above the pin and shallow grooves below the pin on the thrust face,
+and with the pin fastened in the piston by a set screw. The piston had
+four ribs underneath the head (see Figure 13b) radiating from the
+center and with the two over the pin bosses incorporating
+strengthening webs running down and joining the bosses. The piston
+length was reduced by 1 in., thus giving a much less clumsy appearance
+and, with other minor alterations, a weight saving of 40 percent (see
+Figures 13b and c). The rods were for the first time made of I-section
+forgings, a major departure, machined on the sides and hand
+finished at the ends, with a babbit lining in the big end, the piston
+pin bearing remaining steel on steel.
+
+[Illustration: _Figure 13._--Original 6-cylinder engine: a, cylinder
+assembly and valve parts; b, bottom side of piston; c, piston, piston
+pin and connecting rod; d, valve mechanism; e, crankshaft and
+flywheel. (Pratt & Whitney photos D-15012, 15017, 15013, 15018,
+respectively.)]
+
+At least two different general carburetion and induction systems were
+utilized, possibly three. One, and most probably the original,
+consisted of a duplicate of the injection pump of the 4-cylinder
+fitted to a manifold which ran the length of the engine, with three
+takeoffs, each of which then split into two, one for each cylinder. Of
+this arrangement they tried at least two variations involving changes
+in the location and method of injecting the fuel into the manifold;
+and there seems to have been an intermediate manifold arrangement,
+using fuel-pump injection at the middle of the straight side, or
+gallery, manifold, which was fed additional air at both ends through
+short auxiliary inlet pipes. This would indicate that with the
+original arrangement, the end cylinders were receiving too rich a
+mixture, when the fuel in the manifold was not properly vaporized.
+Although the exhaust was on the same side of the engine as the inlet
+system, no attempt was made to heat the incoming charge at any point
+in its travel. An entirely different system adopted at the time of the
+complete redesign in 1913 consisted of two float-feed Zenith
+carburetors each feeding a conventional three-outlet manifold. This
+carburetor was one of the first of the plain-tube type, that is, with
+the airflow through a straight venturi without any spring-loaded or
+auxiliary air valves, and was the simplest that could be devised. When
+properly fitted to the engine, it gave a quite good approximation of
+the correct fuel and air mixture ratio over the speed-load running
+range, although it is considerably more than doubtful that this was
+maintained at altitude, as is stated in one of the best descriptions
+of the engine published at the time the carburetors were applied.
+
+The compression ratio of this engine was lowered by almost 20 percent
+from that of the vertical 4. This, in combination with the low
+bore-to-stroke ratio, the unheated charge, and the later mechanically
+operated inlet valve, indicates that the Wrights were now attempting
+for the first time to secure from an engine something approaching the
+maximum output of which it was capable.
+
+As the engine originally came out, it continued to utilize only one
+spark plug in each cylinder. The high-tension magneto had a wide range
+of spark advance adjustment, which again provided the only control of
+the engine when equipped with the original fuel pump injection.
+
+The location of the valves and pushrods was similar to that in the 4,
+so that the cams were immediately adjacent to the camshaft bearings,
+which were carried in the crankcase ends and in the heavy webs. The
+camshaft was gear-driven and the cam shape was similar to that of the
+last 4s, with a quite rapid opening and closing and a long dwell,
+leaving the valve opening accelerations and seating velocities still
+quite high.
+
+The crankshaft was a continuation of their basic design of rather
+light construction, particularly in the webs. The cheeks were even
+thinner (by 1/4 in.) than those of the 4 although the width was
+increased by 1/8 in. (see Figure 13e). For the first time they went to
+a forging, the rough contour type of the time, and utilized a
+chrome-nickel alloy steel.
+
+Lubrication was by means of the usual gear pump, and the piston and
+rod bearings continued to be splash-fed. The rod big-end bearing
+carried a small sharp undrilled boss at the point where, on the other
+engines, had been located scuppers whose purpose was apparently still
+to throw lubricating oil on the cylinder wall carrying the more highly
+loaded side of the piston. The rod big-end bearing was lubricated by a
+hole on the top of the big-end boss catching some of the crankcase
+splash, which was then carried to the bearing by a groove.
+
+When the 6-cylinder engine was completely redesigned in 1913 this led
+to the introduction in late fall of that year of a new model called
+the 6-60, the 60 designating the rating in horsepower. There is little
+in the Wright records to show why such a radical revision was thought
+necessary, but the general history of the period gives a rather clear
+indication. The competition had caught up to the Wrights in
+powerplants. Other engines were being installed in Wright airplanes,
+and Navy log books show these other engines being used interchangeably
+with those of the Wrights.
+
+Most of the descriptions of the new model published at the time it was
+introduced concentrate on the addition of the two carburetors and the
+mechanical operation of the inlet valves, but these were only two of
+many major changes. The cylinder was completely revised, the intake
+being moved to the camshaft side of the engine from its position
+adjacent to the exhaust, so that the two ports were now on opposite
+sides of the cylinder. By proper positioning of the rocker-arm
+supports and choice of their length and angles, all valves were made
+operable from a single camshaft. The shrunk-on steel water jacket
+cylinder was retained, but the water connections were repositioned so
+that the water entered at the bottom and came out at the top of the
+cylinder. Over the life of the 6-cylinder engine several different
+valve types were used but the published specifications for the model
+6-60 called for "cast iron heads"--the old two-piece construction. The
+piston pins were case hardened and ground and the crankshaft pins and
+journals were heat treated and ground.
+
+The fuel and oil pumps were removed from the side of the crankcase and
+a different ignition system was applied, although still of the
+high-tension spark-plug type which by this time had become general
+practice on all so-called high-speed internal-combustion engines. A
+second threaded spark-plug hole was provided in the cylinder head and
+despite its more common use for other purposes, it is evident that the
+intention was to provide two-plug ignition. It is doubtful that at the
+specific output of this engine any power difference would be found
+between one-and two-plug operation, so that the objective was clearly
+to provide a reserve unit in case of plug failure. However, it was
+also used for the installation of a priming cock for starting and
+because of the prevalence of single-wire ignition systems on existing
+and illustrated engines, it seems to have been used mostly in this
+manner, even though dual-ignition systems later became an unvarying
+standard for aircraft engines.
+
+Viewed externally, the only part of the engine that appears the same
+as the original 6 is the small lower portion of the crankcase; but
+what is more visually striking is the beauty of the new lines and
+extreme cleanness of the exterior design (see Figures 14 and 15). Many
+of their individual parts had shown the beauty of the sparse design of
+pure utility but it was now in evidence in the whole. Despite the
+proven practical value of their other models, this is the only one
+that can be called a good-looking engine, instantly appealing to the
+aesthetic sense, even though the vertical 4 is not an ugly engine. The
+appearance of their final effort, in a field they were originally
+reluctant to enter and concerning which they always deprecated the
+results of their own work, was a thing of which a technically trained
+professional engine designer could be proud.
+
+The 6-60 was continued in production and development until it became
+the 6-70, and indications are that it eventually approached an output
+of 80 horsepower.
+
+[Illustration: _Figure 14._--6-Cylinder 6-60 and 6-70 engine, right
+rear intake side. (Pratt & Whitney photo.)]
+
+[Illustration: _Figure 15._--6-Cylinder 6-70 engine, incorporating
+flexible flywheel drive, exhaust side. (Smithsonian photo A-54381.)]
+
+
+
+
+Minor Design Details and Performance of the Wright Engines
+
+
+In the Wright brothers' various models were many minor design items
+which altogether required a great deal of consideration, but which did
+not materially affect overall engine performance. The results
+generally could all be classed as good practice; however, one of these
+utilized in the 4-cylinder vertical engine was rather unorthodox and
+consisted of offsetting the cylinders with relation to the crankshaft.
+This arrangement, which can be seen in the drawing (Figure 11) was
+apparently an attempt to reduce the maximum side load on the piston
+during the power stroke, but since the peak gas loading usually occurs
+at about 10 to 15 percent of the power stroke, this probably did not
+have much effect, and it was not carried over to the 6-cylinder
+design.
+
+All engine bearings were of the plain sleeve type and, except for the
+bronze and steel bearings in the connecting rod, were of babbit. The
+advantages of babbit for bearings were discovered very early in the
+development of the mechanical arts, and apparently the Wrights never
+encountered a bearing loading sufficiently high to cause a structural
+breakdown in this relatively weak material.
+
+Valve openings show no variation through the successive production
+engines, although the Wrights most probably experimented with
+different amounts. The 1903 engine and the vertical 4-and 6-cylinder
+all had lifts of 5/16 in., but the valve-seat angles varied somewhat;
+the records show included angles of 110 deg. to 90 deg.--not a large
+difference.
+
+The valve-operating mechanism was the same from the first vertical 4
+onward. The high side thrust caused by the cam shape required for the
+very rapid valve opening they chose was, no doubt, the reason for the
+use of the hinged cam follower, and since the same general cam design
+was used in their last engine, the 6-cylinder, the same method of
+operation which had apparently proved very serviceable was continued.
+How satisfactory was the considerably simpler substitute used in the
+Bariquand et Marre version of the 4-cylinder engine is not known.
+Possibly it was one of the alterations in the Wrights' design that
+Wilbur Wright objected to, although in principle it more closely
+conforms to the later fairly standard combination valve tappet and
+roller construction: The available drawings do indicate, however, that
+the cam of the Bariquand et Marre engine was also altered to give a
+considerably less abrupt valve opening than the Wright design, so that
+there was less side thrust. For the Wright 6-cylinder engine their
+4-cylinder cam was slightly altered to provide a rounding off near the
+top of the lobe, thus providing some reduction in the velocity before
+maximum opening was reached. All their cam designs indicate a somewhat
+greater fear of the effect of seating velocities than of opening
+accelerations.
+
+Since the range of cylinder diameters utilized did not vary greatly,
+the valve sizes were correspondingly fairly uniform. The diameter of
+the valves for the original 4-in.-bore cylinder was 2 in., while that
+for the 4-3/8-in. bore used in the 6-cylinder engine was actually
+slightly smaller, 1-7/8 in. Possibly the Wrights clung too long to the
+automatic inlet valve, although it did serve them well; but possibly,
+as has been previously noted, there were valid reasons for continuing
+its use despite the inherently low volumetric efficiency this
+entailed.
+
+The inherent weakness in the joints of the three-piece connecting rod
+has been pointed out, but aside from this, the design was excellent,
+for all the materials and manufacturing methods required were readily
+available, and structurally it was very sound. Tubular rods were still
+in use in aircraft engines in the 1920s.
+
+The Wrights had a surprisingly thorough grasp of the metallurgy of the
+time, and their choice of materials could hardly have been improved
+upon. Generally they relied upon the more simple and commonly used
+metals even though more sophisticated and technically better alloys
+and combinations were available.[17] Case hardening was in widespread
+use in this period but their only utilization of it was in some parts
+of the drive chains purchased completely assembled and in the piston
+pins of their last engine. The treatment of the crankshafts of all
+their engines except the final 6-cylinder was typical of their
+uncomplicated procedure: the particular material was chosen on the
+basis of many years of experience with it, hardening was a very simple
+process, and the expedient of carrying this to a point just below the
+non-machinable range gave them bearing surfaces that were sufficiently
+hard, yet at the same time it eliminated the possibility--present in a
+heat-treating operation--of warping the finished piece.
+
+[Footnote 17: Baker states that the first crankshaft was made from a
+slab of armor plate and if this is correct the alloy was a rather
+complex one of approximately .30-.35 carbon, .30-.80 manganese, .10
+silicon, .04 phosphorus, .02 sulphur, 3.25-3.50 nickel, 0.00-1.90
+chromium; however, all the rest of the evidence, including Orville
+Wright's statement to Dr. Gough, would seem to show that it was made
+of what was called tool steel (approximately 1.0 carbon).]
+
+In the entire 1903 engine only five basic materials--excepting those
+in the purchased "magneto" and the platinum facing on the
+ignition-system firing points--were used: steel, cast iron, aluminum,
+phosphor bronze, and babbit. The steels were all plain carbon types
+with the exception of the sheet manifold, which contained manganese,
+and no doubt this was used because the sheet available came in a
+standard alloy of the time.
+
+Overall, the Wright engines performed well, and in every case met or
+exceeded the existing requirements. Even though aircraft engines then
+were simpler than they became later and the design-development time
+much shorter, their performance stands as remarkable. As a result, the
+Wrights never lacked for a suitable powerplant despite the rapid
+growth in airplane size and performance, and the continual demand for
+increased power and endurance.
+
+Few service records dating from before 1911, when the military
+services started keeping log books, have been found. Some of those for
+the period toward the end of their active era have been preserved, but
+for that momentous period spanning the first few years when the
+Wrights had the only engines in actual continuous flight operation,
+there seems to be essentially nothing--perhaps because there were no
+standard development methods or routines to follow, no requirements to
+be met with respect to pre-flight demonstrations or the keeping of
+service records. Beginning in 1904, however, and continuing as long as
+they were actively in business, they apparently had in progress work
+on one or more developmental or experimental engines. This policy, in
+combination with the basic simplicity of design of these engines,
+accounted in large measure for their ability to conduct both
+demonstrations and routine flying essentially whenever they chose.
+
+Time between engine overhauls obviously varied. In mid 1906 an engine
+was "rebuilt after running about 12 hours." This is comparatively
+quite a good performance, particularly when it is remembered that
+essentially all the "running" was at full power output. It was
+considerably after 1920 before the Liberty engine was redesigned and
+developed to the stage where it was capable of operating 100 hours
+between overhauls, even though it was being used at cruising, or less
+than full, power for most of this time.
+
+The Wrights of course met with troubles and failures, but it is
+difficult, from the limited information available, to evaluate these
+and judge their relative severity. Lubrication seems to have been a
+rather constant problem, particularly in the early years. Although
+some bearing lubrication troubles were encountered from time to time,
+this was not of major proportions, and they never had to resort to
+force-feed lubrication of the main or rod big-end bearings. The piston
+and cylinder-barrel bearing surfaces seem to have given them the most
+trouble by far, and examination of almost any used early Wright engine
+will usually show one or more pistons with evidence of scuffing in
+varying degrees, and this is also apparent in the photographs in the
+record. This is a little difficult to understand inasmuch as most of
+the time they had the very favorable operating condition of cast iron
+on cast iron. Many references to piston seizure or incipient seizure,
+indicated by a loss of power, occur, and this trouble may have been
+aggravated by the very small piston clearances utilized. Why these
+small clearances were continued is also not readily explainable,
+except that with no combination of true oil-scraper rings, which was
+the basic reason why the final form of aviation piston engine was able
+to reach its unbelievably low oil consumptions, their large and rather
+weak compression rings were probably not doing an adequate job of oil
+control, and they were attempting to overcome this with a quite tight
+piston fit.[18] In any event, they did encounter scuffing or seizing
+pistons and cylinder over-oiling at the same time. As late as 4 May
+1908 in the Wright _Papers_ there appears the notation: "The only
+important change has been in the oiling. The engine now feeds entirely
+by splash...."
+
+[Footnote 18: Their intended piston ring tension is not known.
+Measurements of samples from the 4-and 6-cylinder vertical engines
+vary greatly, ranging from less than 1/2 lb per sq in. to almost 1-1/4
+lb. The validity of these data is very questionable as they apply to
+parts with unknown length of service and amount of wear. It seems
+quite certain, however, that even when new the unit tension figure
+with their wide rings was only a small fraction of that of the modern
+aircraft piston engine.]
+
+Their troubles tended to concentrate in the cylinder-piston
+combination, as has been true of almost all piston engines. References
+to broken cylinders are frequent. These were quite obviously cylinder
+barrels, as replacement was common, and this again is not readily
+explainable. The material itself, according to Orville Wright, had a
+very high tensile strength, and in the 1903 engine more than ample
+material was provided, as the barrel all the way down to well below
+the attachment to the case was 7/32 in. thick. The exact location of
+the point of failure was never recorded, but in its design are many
+square corners serving as points of stress concentration. Also, of
+course, no method was then available for determining a faulty casting,
+except by visual observation of imperfections on the surface, and this
+was probably the more common cause. It is interesting, however, that
+the engine finally assembled in 1928 for installation in the 1903
+airplane sent to England has a cracked cylinder barrel, the crack
+originating at a sharp corner in the slot provided at the bottom of
+the barrel for screwing it in place.
+
+Valve failures were also a continuing problem, and Chenoweth reports
+that a large proportion of the operating time of the 1904-1906
+development engine was concentrated on attempts to remedy this
+trouble. None of their cams, including those of the 6-cylinder engine,
+evidence any attempt to effect a major reduction in seating
+velocities. United States Navy log books of 1912 and 1913 record many
+instances of inlet valves "broken at the weld," indicating that some
+of the earlier 6-cylinder engines were fitted with valves of welded
+construction.
+
+For the engineer particularly, the fascination of the Wrights' engine
+story lies in its delineation of the essentially perfect engineering
+achievement by the classic definition of engineering--to utilize the
+available art and science to accomplish the desired end with a minimum
+expenditure of time, energy, and material. Light weight and
+operability were the guiding considerations; these could be obtained
+only through constant striving for the utmost simplicity. Always
+modest, the Wrights seem to have been even more so in connection with
+their engine accomplishments. Although the analogy is somewhat
+inexact, the situation is reminiscent of the truism often heard in the
+aircraft propulsion business--few people know the name of Paul
+Revere's horse. Yet, as McFarland has pointed out, "The engine was in
+fact far from their meanest achievement." With hardly any experience
+in this field and only a meagerly equipped machine shop, they designed
+and assembled an internal combustion engine that exceeded the
+specifications they had laid down as necessary for flight and had it
+operating in a period of about two months elapsed time. The basic form
+they evolved during this unequalled performance carried them through
+two years of such successful evolutionary flight development that
+their flying progressed from a hop to mastery of the art. And the
+overall record of their powerplants shows them to have been remarkably
+reliable in view of the state of the internal combustion engine at
+that time.
+
+
+
+
+Appendix
+
+
+Characteristics of the Wright Flight Engines
+
+  -------------------------------------------------------------------------
+                          _1903       _1904-1905    _1908-1911   _1911-1915
+                        First flight Experimental  Demonstrations  service_
+                        engine[a]_     flights_        and
+                                                      service_
+  -------------------------------------------------------------------------
+  Cyl./Form              4/flat       4/flat       4/vertical   6/vertical
+  Bore and stroke (in.)  4x4          4-1/8x4      4-3/8x4      4-3/8x4-1/2
+  Displacement (cu. in.) 201          214          240          406
+  Horsepower             8.25-16      15-21        28-42        50-75
+  RPM                    670-1200     1070-1360    1325-1500    1400-1560
+  MEP                    49-53        52-57        70-87        70-94
+  Weight (lb)            140-180      160-170      160-180      265-300
+  -------------------------------------------------------------------------
+
+[Footnote a: Concurrently with the Wrights' first engine work, Manly
+was developing the engine for the Langley Aerodrome, and a comparison
+of the Wrights' engine development with that of Manly is immediately
+suggested, but no meaningful comparison of the two efforts can be
+drawn. Beyond the objective of producing a power unit to accomplish
+human flight and the fact that all three individuals were superb
+mechanics, the two efforts had nothing in common. The Wrights' goal
+was an operable and reasonably lightweight unit to be obtained quickly
+and cheaply. Manly's task was to obtain what was for the time an
+inordinately light engine and, although the originally specified power
+was considerably greater than that of the Wrights, it was still
+reasonable even though Manly himself apparently increased it on the
+assumption that Langley would need more power than he thought. The
+cost and time required were very much greater than the Wrights
+expended. He ended up with an engine of extraordinary performance for
+its time, containing many features utilized in much later important
+service engines. His weight per horsepower was not improved upon for
+many years. The Wrights' engine proved its practicability in actual
+service. The Manly engine never had this opportunity but its
+successful ground tests indicated an equal potential in this respect.
+A description of the Langley-Manly engine and the history of its
+development is contained in _Smithsonian Annals of Flight_ number 6,
+"Langley's Aero Engine of 1903," by Robert B. Meyer (xi+193 pages, 44
+figures; Smithsonian Institution Press, 1971)]
+
+It is not possible to state the exact quantities of each engine that
+the Wrights produced up to the time that their factory ceased
+operation in 1915. Chenoweth gives an estimate, based on the
+recollection of their test foreman, of 100 vertical 4s and 50 6s. My
+estimate (see page 2) places the total of all engines at close to 200.
+Original Wright-built engines of all four of these basic designs are
+in existence, although they are rather widely scattered. The
+Smithsonian's National Air and Space Museum has examples of them all,
+including, of course, the unique first-flight engine. Their condition
+varies, but many are operable, or could easily be made so. Among the
+best are the first-flight engine and the last vertical 6, at the
+Smithsonian, the first vertical 6, at the United States Air Force
+Museum, and the vertical 4, at the Carillon Park Museum.
+
+The Wrights were constantly experimenting and altering, and this in
+connection with the lack of complete records makes it almost
+impossible to state with any certainty specific performances of
+individual engines at given times. Weights sometimes included
+accessories and at others did not. Often they were of the complete
+powerplant unit, including radiator and water and fuel, with no
+clarification. In the table, performance is given in ranges which are
+thought to be the most representative of those actually utilized.
+Occasionally performances were attained even beyond the ranges given.
+For example, the 4x4-in. flat development engine eventually
+demonstrated 25 hp at an MEP of approximately 65 psi.
+
+One important figure--the horsepower actually utilized during the
+first flight--is quite accurately known. In 1904 the 1904-1905 flight
+engine, after having been calibrated by their prony-brake test-fan
+method, was used to turn the 1903 flight propellers, and Orville
+Wright calculated this power to be 12.05 bhp by comparing the
+calibrated engine results with those obtained with the flight engine
+at Kitty Hawk when tested under similar conditions. However, since the
+tests were conducted in still air with the engine stationary, this did
+not exactly represent the flight condition. No doubt the rotational
+speed of the engine and propellers increased somewhat with the forward
+velocity of the airplane so that unless the power-rpm curve of the
+engine was flat, the actual horsepower utilized was probably a small
+amount greater than Orville's figures. The lowest power figure shown
+for this engine is that of its first operation.
+
+No fuel consumption figures are given, primarily because no
+comprehensive data have been found. This is most probably because in
+the early flight years, when the Wrights were so meticulously
+measuring and recording technical information on the important factors
+affecting their work, the flights were of such short duration that
+fuel economy was of very minor importance. After success had been
+achieved, they ceased to keep detailed records on very much except
+their first interest--the flying machine itself--and when the time of
+longer flights arrived, the fuel consumption that resulted from their
+best engine design efforts was simply accepted. The range obtained
+became mostly a matter of aerodynamic design and weight carried.
+Orville Wright quotes an early figure of brake thermal efficiency for
+the 1903 engine that gives a specific fuel consumption of .580 lb of
+fuel per bhp/hr based on an estimate of the heating value of the fuel
+they had. This seems low, considering the compression ratio and
+probable leakage past their rather weak piston rings, but it is
+possible. In an undated entry, presumably in 1905, Orville Wright's
+notebook covered fuel consumption in terms of miles of flight; one of
+the stated assumptions in the entry is, "One horsepower consumes .60
+pounds per horsepower hour"--still quite good for the existing
+conditions. Published figures for the 6-60 engine centered around .67
+lb/hp hr for combined fuel and oil consumption.
+
+
+The Wright Shop Engine
+
+Despite the fact that the Wright shop engine was not a flight unit, it
+is interesting both because it was a well designed stationary
+powerplant with several exceedingly ingenious features, and because
+its complete success was doubtless a major factor in the Wrights'
+decision to design and build their own first flight engine. Put in
+service in their small shop in the fall of 1901, it was utilized in
+the construction of engine and airframe parts during the vital years
+from 1902 through 1908 and, in addition, it provided the sole means of
+determining the power output of all of their early flight engines. By
+means of a prony brake, its power output was carefully measured and
+from this the amount of power required for it to turn certain fans or
+test clubs was determined. These were then fitted to the flight
+engines and the power developed calculated from the speed at which the
+engines under test would turn the calibrated clubs. Although a
+somewhat complex method of using power per explosion of the shop
+engine was made necessary by the basic governor control of the engine,
+the final figures calculated by means of the propeller cube law seem
+to have been surprisingly accurate.[19] Restored under the personal
+direction of Charles Taylor, it is in the Henry Ford Museum in
+Dearborn, Michigan, together with the shop machinery it operated.
+
+[Footnote 19: _The Papers of Wilbur and Orville Wright_, volume 2,
+Appendix.]
+
+The engine was a single cylinder, 4-stroke-cycle "hot-tube" ignition
+type. The cylinder, of cast iron quite finely and completely finned
+for its day, was air-cooled, or rather, air-radiated, as there was no
+forced circulation of air over it, the atmosphere surrounding the
+engine simply soaking up the dissipated heat. Although this was
+possibly a desirable adjunct in winter, inside the small shop in
+Dayton, the temperature there in summer must have been quite high at
+times. The operating fuel was city illuminating gas, which was also
+utilized to heat, by means of a burner, the ignition tube. This part
+was of copper, with one completely closed end positioned directly in
+the burner flame; the other end was open and connected the interior of
+the tube to the combustion chamber. The inlet valve was of the usual
+automatic type while the exhaust valve was mechanically operated. The
+fuel gas flow was controlled by a separate valve mechanically
+connected to the inlet valve so that the opening of the inlet valve
+also opened the gas valve, and gas and air were carried into the
+cylinder together.
+
+[Illustration: _Figure 16._--Shop engine, 1901, showing governor and
+exhaust valve cam. (Photo courtesy R. V. Kerley.)]
+
+The engine was of normal stationary powerplant design, having a heavy
+base and two heavy flywheels, one on each side of the crank. These
+were necessary to ensure reasonably uniform rotational speed, as, in
+addition to having only one cylinder, the governing was of the
+hit-and-miss type. It had a 6x7-in. bore and stroke and would develop
+slightly over 3 hp at what was apparently its normal operating speed
+of 447 rpm, which gives an MEP of 27 psi.
+
+The engine is noteworthy not only for its very successful operation
+but also because it incorporated two quite ingenious features. One was
+the speed-governing mechanism. As in the usual hit-and-miss operation,
+the engine speed was maintained at a constant value, the output then
+being determined by the number of power strokes necessary to
+accomplish this. The governor proper was a cylindrical weight free to
+slide along its axis on a shaft fastened longitudinally to a spoke of
+one of the flywheels. A spring forced it toward the center of the
+wheel, while centrifugal force pulled it toward the rim against the
+spring pressure. After each opening of the valve the exhaust-valve
+actuating lever was automatically locked in the valve-open position by
+a spring-loaded pawl, or catch. The lever had attached to it a small
+side extension, or bar, which, when properly forced, would release the
+catch and free the actuating lever. This bar was so positioned as to
+be contacted by the governor weight when the engine speed was of the
+desired value or lower, thus maintaining regular valve operation; but
+an excessive speed would move the governor weight toward the rim and
+the exhaust valve would then be held in the open position during the
+inlet stroke, so no cylinder charge would be ingested. Since the
+ignition was not mechanically timed, the firing of the charge was
+dependent only on the compression of the inlet charge in the cylinder,
+so it made no difference whether the governor caused the engine to
+cease firing for an odd or even number of revolutions, even though the
+engine was operating on a 4-stroke cycle at all times.
+
+[Illustration: _Figure 17._--Shop engine, 1901, showing operation of
+exhaust valve cam. (Pratt & Whitney drawing.)]
+
+The exhaust valve operating cam was even more ingenious. To obtain
+operation on a 4-stroke cycle and still avoid the addition of a
+half-speed camshaft, a cam traveling at crankshaft speed was made to
+operate the exhaust valve every other revolution (see Figure 17). It
+consisted of a very slim quarter-moon outline fastened to a disc on
+the crankshaft by a single bearing bolt through its middle which
+served as the pivot about which it moved. Just enough clearance was
+provided between the inside of the quarter-moon and the crankshaft to
+allow the passage of the cam-follower roller. The quarter-moon,
+statically balanced and free to move about its pivot, basically had
+two positions. In one the leading edge was touching the shaft (Figure
+17b), so that when the cam came to the cam follower, the follower was
+forced to go over the top of the cam, thus opening the exhaust valve.
+When the cam pivot point had passed the roller, the pressure of the
+exhaust valve spring forced the following edge of the cam into
+contact with the shaft and this movement, which separated the leading
+edge of the cam from the shaft, provided sufficient space between it
+and the shaft for the roller to enter (Figure 17c). Thus, when the
+leading edge of the cam next reached the roller, the roller, being
+held against the crankshaft by the valve spring pressure (Figure 17d),
+entered the space between the cam and the shaft and there was no
+actuation of the valve. In exiting from the space, it raised the
+trailing edge of the cam, forcing the leading edge against the shaft
+(Figure 17a) so that at the next meeting a normal valve opening would
+take place. The cam was maintained by friction alone in the position
+in which it was set by the roller, but since the amount of this could
+be adjusted to any value, it could be easily maintained sufficient to
+offset the small centrifugal force tending to put the cam in a neutral
+position.[20]
+
+[Footnote 20: The Wrights apparently never applied for an engine
+patent of any kind. This no doubt grew out of their attitude of
+regarding the engine as an accessory and deprecating their work in
+this field. A reasonably complete patent search indicates that this
+particular cam device has never been patented, although a much more
+complex arrangement accomplishing the same purpose was patented in
+1900, and a patent application on a cam-actuating mechanism
+substantially identical to that of the Wrights and intended for use in
+a golf practice apparatus is pending at the present time.]
+
+
+
+
+Bibliography
+
+
+ANGLE, GLENN D. Wright. Pages 521-523 in _Airplane Engine
+Encyclopedia, an Alphabetically Arranged Compilation of All Available
+Data on the World's Airplane Engines_. Dayton, Ohio: The Otterbein
+Press, 1921.
+
+BAKER, MAX P. The Wright Brothers as Aeronautical Engineers. _Annual
+Report of ... the Smithsonian Institution ... for the Year Ended June
+30, 1950_, pages 209-223, 4 figures, 9 plates.
+
+BEAUMOUNT, WILLIAM WORBY. _Motor Vehicles and Motors: Their Design,
+Construction, and Working by Steam, Oil, and Electricity._ 2 volumes.
+Philadelphia: J. B. Lippincott, 1901-1902.
+
+CHENOWETH, OPIE. Power Plants Built by the Wright Brothers. _S.A.E.
+Quarterly Transactions_ (January 1951), 5:14-17.
+
+FOREST, FERNAND. _Les Bateaux automobiles._ Paris: H. Dunod et E.
+Pinat, Editeurs, 1906.
+
+GOUGH, DR. H. J. Materials of Aircraft Construction. _Journal of the
+Royal Aeronautical Society_ (November 1938), 42:922-1032. Illustrated.
+
+KELLY, FRED C. _Miracle at Kitty Hawk; the Letters of Wilbur and
+Orville Wright._ New York: Farrar, Straus and Young, 1951.
+
+---------- _The Wright Brothers, a Biography Authorized by Orville
+Wright._ New York: Harcourt, Brace & Co., 1943.
+
+KENNEDY, RANKIN. _Flying Machines: Practice and Design. Their
+Principles, Construction and Working._ 158 pages. London: Technical
+Publishing Co., Ltd., 1909.
+
+LAWRANCE, CHARLES L. _The Development of the Aeroplane Engine in the
+United States._ Pages 409-429 in International Civil Aeronautics
+Conference, Washington, D.C., 12-14 December 1928, Papers Submitted by
+the Delegates for Consideration by the Conference. Washington:
+Government Printing Office, 1928.
+
+MCFARLAND, MARVIN W. _The Papers of Wilbur and Orville Wright._ 2
+volumes. New York: McGraw Hill Book Co., 1953.
+
+RENSTROM, ARTHUR G. Wilbur and Orville Wright: A Bibliography
+Commemorating the Hundredth Anniversary of the Birth of Wilbur Wright,
+April 16, 1867. Washington, D.C.: The Library of Congress [Government
+Printing Office], 1968. Contains 2055 entries.
+
+The 6-Cylinder 60-Horsepower Wright Motor. _Aeronautics_ (November
+1913), 13(5):177-179.
+
+Wright Brothers. Pages 829-830 in _Aerosphere 1939, Including World's
+Aircraft Engines, with Aircraft Directory_, Glenn D. Angle, editor.
+New York: Aircraft Publishers, 1940.
+
+
+
+
+Index
+
+
+  Angle, Glenn D., 51
+
+
+  _Baby Grand Racer_, 47
+
+  Baker, Max P. 1, 10, 26, 28
+
+  Bariquand et Marre, 43, 44-45, 57-58
+
+  Beaumount, William Worby, 9, 25
+
+  Bristol Siddeley Engines, Ltd., 44-45
+
+
+  Carillon Park Museum, Dayton, Ohio, ix, 5n, 7, 37
+
+  Chanute, Octave, 28
+
+  Chenoweth, Opie, ix, 22, 35, 42, 63
+
+  Christman, Louis P., ix, 7, 8, 28
+
+  Cole, Gilmoure N., ix
+
+  Clarke, J. H., 18
+
+
+  Daimler-Benz A. G., ix, 10, 13
+
+
+  Engineers Club, Dayton, Ohio, ix, 32
+
+
+  Ford, Henry, 8
+
+  Ford, Henry, Museum, Dearborn, Michigan, 8, 64
+
+  Forest, Fernand, 11
+
+  Franklin Institute, Philadelphia, Pennsylvania, ix, 47
+
+
+  Gough, Dr. H. J., 58n
+
+
+  Howell Cheney Technical School, Manchester, Connecticut, x, 14, 15
+
+
+  Kelly, Fred C, 4n
+
+  Kerley, R. V., ix, 65
+
+  _Kitty Hawk Flyer_, ii, 3
+
+
+  Langley [Samuel P.] Aerodrome, 9, 62
+
+  Loening, Grover C, 13n
+
+
+  Manly, Charles L., 9, 62
+
+  Maxim, Sir Hiram Stevens, 3
+
+  McFarland, Marvin W., 1, 33, 47, 61
+
+  Miller-Knoblock Manufacturing Co., South Bend, Indiana, 26
+
+
+  National Park Service, Cape Hatteras National Seashore, ii, ix
+
+  Neue Automobil-Gesellschaft, 43
+
+
+  Porter, L. Morgan, ix
+
+  Pratt & Whitney Aircraft Corp., v, x, 37, 40-41, 49, 52, 53, 67
+
+  Pruckner, Anton, 33
+
+
+  Rockwell, A. L., ix, 37
+
+
+  Santos-Dumont, Alberto, 11
+
+  Science Museum, London, x, 5, 6, 7, 8, 11, 21, 23, 26
+
+
+  Taylor, Charles E., 5, 64
+
+
+  United Aircraft Corp., v, x
+
+
+  Western Society of Engineers, 2
+
+  Whitehead, Gustave, 33
+
+  Wittemann, Charles, 33n
+
+  Wright, Bishop Milton (father), 28
+
+  Wright, Katherine (sister), 4
+
+
+  Zenith carburetor, 52
+
+
+*U.S. GOVERNMENT PRINTING OFFICE: 1971--397-764
+
+
+
+
+Publication in Smithsonian Annals of Flight
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