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diff --git a/42018.txt b/42018.txt deleted file mode 100644 index 7e47325..0000000 --- a/42018.txt +++ /dev/null @@ -1,1195 +0,0 @@ -The Project Gutenberg EBook of Natural Stability and the Parachute -Principle in Aeroplanes, by W. LeMaitre - -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: Natural Stability and the Parachute Principle in Aeroplanes - -Author: W. LeMaitre - -Release Date: February 4, 2013 [EBook #42018] - -Language: English - -Character set encoding: ASCII - -*** START OF THIS PROJECT GUTENBERG EBOOK NATURAL STABILITY IN AEROPLANES *** - - - - -Produced by Chris Curnow, Matthew Wheaton and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - - - - - - NATURAL STABILITY - AND - THE PARACHUTE PRINCIPLE - IN AEROPLANES - - BY - - W. LEMAITRE - - - _Hon. Sec., Aeroplane Building and Flying Society_ - - WITH 34 ILLUSTRATIONS - - - - LONDON - E. & F. N. SPON, LTD., 57 HAYMARKET - - New York - SPON & CHAMBERLAIN, 123 LIBERTY STREET - - 1911 - - - - - CONTENTS - - - PAGE - - FRONTISPIECE iv - - PREFACE ix - - CHAPTER - - I. THE IMPORTANCE OF STABILITY 11 - - II. SPEED AS A MEANS OF STABILITY 14 - - III. THE LOW CENTRE OF GRAVITY 17 - - IV. SHORT SPAN AND AREA 28 - - V. VARIABLE SPEED AND THE PARACHUTE PRINCIPLE 36 - - VI. THE DESIGN WHICH FULFILS THE CONDITIONS 39 - - - - -PREFACE - - -Since there is nothing new under the sun, it is useless to pretend -that there is anything new in the design here advocated or the -theories advanced. Both are rather the result of a commonsense -consideration of the different points of all flying machines, natural -and artificial, and an endeavour to select from the great number of -good points those which seem most likely to blend together into a -practical machine. The conclusions reached are the result of a quite -independent investigation, carried on over three years by means of -numberless experiments, and the writer has endeavoured to make no -single statement which he cannot by some experiment amply prove. - - - - -NATURAL STABILITY IN AEROPLANES - - - - -CHAPTER I. - -THE IMPORTANCE OF STABILITY. - - -In considering the whole question of aviation, it becomes evident that -the one point to strive for at the present juncture is stability. If -we are ever to have a practical flying machine, that is, a machine -which we can use as we do a yacht, a motor car, or a bicycle, it must -be one that we can trust to keep its balance by reason of the natural -forces embodied in it, and without any effort of control on the part -of the pilot. It may be objected that a bicycle does not do this, and -this is true, but, on the other hand, the upsetting of a bicycle is a -very small matter, whereas the tilting of an aeroplane mostly means -sudden death to its occupant, and it is probable that if the same -consequences followed the tilting of a bicycle, bicycles would soon -have been made with four wheels. - -At present aeroplanes are the most unstable of all things. The least -gust, the least shifting of weight, the slightest difference in the -density of the strata of the supporting air, and the machine sways, -and if not instantly corrected by the pilot the sway becomes a tilt, -the tilt a dive, and the rest is silence. The first aeroplanes, the -Wrights' for instance, were so unstable that twenty minutes in one of -them was as much as the most iron-nerved man could stand, the -continual strain being too exhausting to keep up for any length of -time. By throwing out extensions and outriggers in all directions we -have altered that to a certain extent, but only to an extent--we have -not yet got rid of it. The monoplane is probably the most unstable, as -might be expected from its smaller surface, but the bi-plane runs it -pretty closely. - -And the difficulty seems to arise chiefly from the fact that the -machines are built round the propeller. In the case of a yacht or a -car, the machine is built first and the propelling means is fitted on -as an auxiliary. The consequence is that an aeroplane which is safe -enough while the propeller is exerting a tractive force of some 250 -lbs., becomes, the moment this power is for any reason stopped, merely -a shapeless construction at the mercy of the wind and the force of -gravitation. It is true that most machines may be made to glide if the -pilot is clever enough and quick enough to steer them into the proper -gliding angle, but the machine that will naturally and by reason of -its design assume its proper gliding angle when the propelling force -is withdrawn, has not yet been built. - -Such a machine would have "Natural Stability." - -It will be recognized that this natural stability, which depends on -the design of the machine, is something entirely different from -"automatic stability" of which there are many systems, all having this -one defect; that, depending upon working devices, movable planes, -gyroscopes, compensating balancers, pendulums, etc., they are all -liable to go wrong and refuse to act the moment a sudden strain makes -their perfect action most important. - -Considering that the propeller is the only means the aeroplane has of -keeping in the air at all, the question arises: Is it possible to -design a machine that will be stable to the extent of descending -safely when the propeller stops, and that will yet be a good and -speedy flyer? - -That is the problem we have to solve. - - - - -CHAPTER II. - -SPEED AS A MEANS OF STABILITY. - - -It is recognized on all hands that speed is a great factor in the -problem of stability. To begin with, a machine going at high speed -would be practically untouched by gusts of wind, different densities -of air strata, holes in the air, etc. Also its greater momentum would -tend to keep it in a straight line, not only relative to its course -but also relative to itself. That is to say, its wings being started -in a horizontal plane, would tend to keep in the same plane and would -not easily tilt or sway out of it. Both these effects of natural law -show that a high speed machine must be more stable than a low speed -machine. How then are we to design a high speed machine? - -Leaving aside the question of higher power, the first point that -suggests itself is to lessen the head resistance. All fast things, -boats, birds, arrows, even motor-cars, are made long and narrow. It -will be objected that a bird with its wings outspread is not long and -narrow, but in the sense in which this illustration is meant, the -bird's wings, being merely its propelling apparatus, do not count, and -when the bird is at its fastest, as in the swoop of a hawk or an -eagle, the wings are shut tightly to the body so as to offer no -resistance to its lightning passage through the air. If we are to -follow previous experience in Nature's laws, our aeroplanes must be -considerably reduced in span. To drive through the air at a high speed -with a machine of 40 foot span is a practical impossibility, both -because of the tremendous power it would require and also by reason of -the great strength the plane must have to withstand the resistance of -the air. - -In reducing the span, however, we reduce the lifting surface of the -machine. But on the other hand it must be remembered that the lifting -efficiency is increased by increasing the speed. Lift is the product -of supporting surface and speed. A small plane driven at a high speed -will give as great a lift as a large plane driven at a low speed. -Speed, again, is the difference between the propelling power and the -head resistance, and we can increase the speed by decreasing the -resistance. It follows, then, that we need not necessarily give up -lifting power by reducing the span of the wings, since the shorter -span gives greater speed, and the increase of efficiency by reason of -the greater speed would go to make up for the loss of span. - -It is, then, quite possible to design a short span machine which shall -be as efficient for lift as a long span machine, and which will have -the advantage of possessing, by reason of its speed, much greater -stability. - -But the span is not the only factor in the speed problem. In the low -speed machines at present in use we have found it necessary to curve -the planes to get greater efficiency. This efficiency is also gained -at the expense of head resistance, and it is already recognized that -the higher the speed the less is the need of camber. This is the same -problem over again. A high speed flat plane will give as much lift as -a low speed cambered plane, and we gain in stability with every -additional mile per hour. - -The third point to be considered in the problem of speed is the -resistance caused by the multitude of struts and wires, the body of -the pilot, the tanks, engine, and all the other impedimenta projecting -in all directions from the body of the aeroplane. It has occurred to -our builders that if the whole of these things could be collected -together and enclosed in a light covered-in car of a proper shape, the -skin friction of such a car would be much less than the total head -resistance offered by the different obstructions so covered. And there -is another advantage to be gained here, for if, at 40 miles per hour, -the force of the wind is very seriously uncomfortable for the pilot, -the position at such speeds as 70 or 100 miles per hour would be quite -impossible. - - - - -CHAPTER III. - -THE LOW CENTRE OF GRAVITY. - - -The first thing that occurs to the investigator on the subject of -stability is that nature offers us a sure means of keeping our -machines upright by adopting the simple method of placing all the -heavier parts at the bottom. In all other constructions we have -adopted this plan with perfect success. In boats, yachts, cars, -balloons, everything man uses in fact, the simplest, best and most -obvious method of keeping a thing upright is to utilize the force of -gravity, place the lighter or supporting parts above and the weight -below, and the thing is done. - -This simple method of obtaining stability did not escape the aeroplane -designers, and we have had several machines which embodied this -principle, more or less. Unfortunately, however, they all proved -failures. A machine would be designed, and, with the weight high, -would fly well, though it was unstable. Put the weight low and you got -rid of the instability, and at the same time the machine became -unmanageable. It looked as if flying and instability were -interchangeable terms. So, as it was a machine that would fly the -designers were after, the weight was kept up and the stability was -left to the pilot. The machines were made "sensitive" as it is called, -that is to say, sensitive to a touch of the rudder or the balancers. -They are also, it is true, equally sensitive to a gust of wind or a -slight shifting of weight or pressure, and this has caused the -smashing of a good many machines and some pilots; but after all this -is the fortune of war, and no one is compelled to go up in an -aeroplane. - -The curious thing about it is that it does not seem to have occurred -to our designers that if their pet design would not fly with the -weight low, perhaps it might be possible to alter the design instead -of altering the position of the centre of gravity, and so obtain what -we are all looking for, a naturally stable machine that is yet -sensitive to control. - -There are two chief difficulties in the way of the low centre of -gravity machine. One is that the heaviest portion of the machine being -some distance below its support, it is apt to give rise to a pendulum -or swaying motion. The other is that of tilting, or banking up, in -turning a corner. These are really two developments of the same -difficulty, i.e. pendulum motion. - -If we take a strip of stiff paper to represent a plane and put a small -weight in the centre of the plane, the model on being glided to earth -does not tend to sway (Fig. 1). If we put our weight on a tiny piece -of wire an inch or so below the plane (Fig. 2) and set the model free, -it will probably acquire a swinging motion as it descends. That is -the whole trouble. The trouble is real enough, but the fallacy is in -supposing it to be all the fault of the low centre of gravity. All -ships that were ever designed have a low centre of gravity, yet some -roll dreadfully and others do not, which, in itself, should be proof -sufficient that it is the design of the machine and not the position -of the ballast that is at fault. - -[Illustration: FIG. 1., FIG. 2. AND FIG. 3.] - -Let us now try some experiments. It will be noticed that in the -machines which have employed the low centre of gravity the span of the -wings has usually been 30 feet or more, and the centre of gravity -about 6 feet below the centre. Here is a paper model of the present -aeroplane (Fig. 1). Here is the same machine with a low centre of -gravity (Fig. 2). Now bend the paper upwards as in Fig. 3 and you get -rid of the swaying. Also, of course, you get rid of the supporting -surface. But there is probably some point of greatest efficiency where -you may compromise. If you take model 2 and bend it slightly (Fig. 4) -it will sway, but not much, not so much as Fig. 2. Now with a pair of -scissors clip the wings a bit at a time, and you will find that as the -span gets shorter the swaying decreases, and that when you have the -three points formed by the ends of the two wings and the weight -equidistant from the centre where they meet, the plane is stable (Fig. -5). The reason is that it is not the pendulum with the weight at the -bottom that swings so much, but the long wings that see-saw. By -shortening the wings you have reduced the length of the see-saw, which -is the same as reducing the length of the pendulum, and consequently, -by pendulum law, the oscillations must be much quicker and shorter and -will at once damp out. It is curious that this point seems to have -escaped the designers. It is well known that all pendulum motion tends -to damp out, and the shorter the pendulum the quicker it comes to -rest. Hitherto the idea has been to shorten it vertically, but the -same effect exactly is obtained by shortening it horizontally, and the -low centre of gravity remains to give stability. It was stated by some -sapient objector to the low centre of gravity, that the pendulum -motion once set up, increased till it turned the machine over. A -pendulum which increased its swing at every stroke would be something -new in the scientific world. - -[Illustration: FIG. 4.] - -[Illustration: FIG. 5.] - -Another development of the pendulum difficulty is the probable fore -and aft sway, but this may be overcome by increasing the supporting -surface of the tail. Many machines do not lift with the tail at all, -and those that do employ lifting tails, have them with very small -surface. Consequently, the centre of gravity comes nearly under the -centre of the main plane, and the whole machine, turning on its centre -of gravity in all directions as on a pivot, is liable to swing fore -and aft. If the supporting surface of the tail be increased and the -centre of gravity carried further aft, this pendulum motion is also -rendered impossible, and the machine is stable both ways. - -A few illustrations may serve to make the advantages of the low centre -of gravity more clear, and to avoid complications we will suppose the -planes to be still and in still air. Let Fig. 6 represent an ordinary -flat plane having its centre of gravity coincident with its centre of -pressure, the centre of pressure of each half or wing being at A A. -The plane is in equilibrium. Now allow it to tilt (Fig. 7), and it -will be seen that it is still in equilibrium, since the weight is in -the centre and the wing tips equidistant from it. Let it tilt still -more till it is vertical (Fig. 8), and the balance is still the same. -It is evident, therefore, that such a plane would travel equally well -in any of the positions shown, and that it can only be kept in -position (Fig. 6) by the skilful manipulation of the pilot. - -[Illustration: FIG. 6., FIG. 7., AND FIG. 8.] - -In the same way, the machine having no lifting tail is longitudinally -unstable, for, being balanced on its centre of pressure which would be -coincident with its centre of gravity and probably about 2 feet from -the trailing edge of the plane--it may assume any position (Figs. 9, -10, 11 and 12), and still be in equilibrium, when it is evident that -the proper position (Fig. 9) is only maintained by the constant -control of the tail elevator. - -[Illustration: FIG. 9., FIG. 10., FIG. 11., FIG. 12., AND FIG. 13.] - -Now take the case of a machine having a low centre of gravity. Its -natural position is shown at Fig. 13, and it is at once evident that -any other position such as Figs. 14 and 15 could not be maintained for -a moment, since the weight being at an angle, must inevitably drag the -machine back to its natural position (Fig. 13). In the same way with -regard to longitudinal balance, a machine with two lifting surfaces -such as Fig. 13, is in its natural position with the centre of gravity -perpendicularly under the centre of pressure, any other position, such -as Fig. 17, A, is impossible, as the gravity pull must drag the -machine along the dotted line till it resumes its proper and natural -position (B). - -[Illustration: FIG. 14., FIG. 15., AND FIG. 16.] - -The next difficulty is in the banking or tilting caused by the turning -of the machine in going round a curve. In a very interesting -discussion carried on in the "Aero," it was stated that a low centre -of gravity machine could not bank up, as the pull of gravity acting on -the low weight would prevent it. It was also stated by another writer -that the machine would bank up too much and slide down sideways, -because the greatest weight having the greatest momentum would swing -out too much. There is evidently some confusion here. Let us consider -the question. - -In turning there are three forces to take into consideration: - -(1) The centrifugal force, which tends to make the machine fly off at -a tangent to the curve at which it is turning. - -(2) The action of gravitation. - -(3) The extra lift given by the wing on the outside of the curve, -owing to the fact that it travels faster through the air. - -[Illustration: FIG. 17.] - -The centrifugal force acts strictly in proportion to the mass it acts -on, but, at the same time it must be remembered that the greater force -acting on the greater mass has the greater mass to move. That is to -say, that if the top part of the machine was very light and the -bottom part very heavy, the force acting on the light part would be -sufficient to send that part swinging out when rounding a curve, and -the greater force acting on the greater mass at the bottom would be -sufficient to send that out to exactly the same degree. Consequently, -if only centrifugal force is considered, the whole machine would swing -out without any tilting at all, retaining its upright position. But -here we must take another factor into consideration, the resistance of -the air. This resistance would be greater on the greater surface of -the light top part than on the heavy bottom part, and consequently the -bottom part would, automatically, swing out most, giving the banking -effect. This would be increased by the extra lift given to the outer -wing by reason of its greater speed. If we then take the force of -gravitation into the problem we shall see that we have two -factors--unequal speed and unequal air resistance--tending to bank up -the machine, and one force--gravity--tending to pull it straight -again. At a certain angle due to the amount of force exerted by each -of these, the two opposing factors would balance, and the machine -would be in equilibrium. - -It would appear that most of the difficulties connected with the low -centre of gravity machine are the result of hazy thinking and -slip-shod reasoning, and that they do not exist in fact. And let it be -remembered that the low centre of gravity machine with short span has -not yet been tried except by the writer, who has succeeded in making a -paper model on this plan turn in its own length without in any way -losing its stability, swaying, banking too much, turning over, sliding -sideways, or doing any of the frightful things which some people -declare it must do. What it does do is to recover its balance though -started from the most impossible positions and always land on its -feet. - - - - -CHAPTER IV. - -SHORT SPAN AND AREA. - - -Both on account of speed, and also on account of stability, with a low -centre of gravity, we are forced in the direction of the short span -machine. How are we to construct a machine with a span short enough to -damp out swaying and yet with sufficient lifting surface to raise the -machine and its load? - -The position is somewhat simplified, as already pointed out, by the -fact that though the lift is decreased by the decrease in span, it is -to a great extent compensated by the increase in speed. Also another -compensation is offered by the fact that fore and aft stability -requires a lifting tail. - -Lift is largely in proportion to the length of the entering edge of -the plane, but it does not always follow that this entering edge must -be at right angles to the direction of flight. The Dunne machine -obtains its lift with an entering edge that is entirely at an angle of -some 45 degrees, and its shape is an exact replica of the arrow head -of prehistoric man and the paper darts of our schooldays, a design, by -the way, that was patented in 1860. - -At first sight it would seem that the lift on a plane shaped thus -(Fig. 18), would only be equal to the lift given by a plane with an -edge as long as the distance between A and C, thus (Fig. 19), but this -is not so. Although the lift is not so great as it would be if the -edge was straight in one line (Fig. 20), it is very much greater than -it would be on Fig. 19. The probability is that it is about half-way -between (19) and (20), but probably nearer to (20) than (19). There -are no exact data to go on, but the efficiency of the Dunne machine -would seem to show this. - -[Illustration: FIG. 18.] - -[Illustration: FIG. 19. AND FIG. 20.] - -Again, in seeking for planes that offer the least resistance to the -air, one of the best that suggests itself is the T-shape (Fig. 21), -and this may be improved by cutting off useless corners (Fig. 22). A -plane of this shape lends itself to great strength of construction -owing to its small extending parts. It is compact, it gives an -entering edge half as long again as its span, and gives a lift in -proportion to that edge, and it is in itself stable. Having thus -evolved a suitable plane for the front of the machine, the best thing -to do is to base the back plane on the same design, and join the two -planes together to form the supporting surface of the machine, -allowing sufficient space between them to avoid any interference or -overlapping. The design then stands thus (Fig. 23), when the back -plane is a slightly smaller copy of the front one. The position of the -centre of gravity in this design would be coincident with the centre -of pressure longitudinally and laterally, and would be situated about -at A. A paper model on these lines with a low centre of gravity may be -easily constructed and will prove useful in illustrating the different -points here stated. The paper should be cut out sufficiently wide to -allow of a central longitudinal fold (Figs. 24 and 25), and a roll of -paper should be made for ballast and pushed through the fold as shown -in Fig. 26 at the point marked A. - -[Illustration: FIG. 21. AND FIG 22.] - -[Illustration: FIG. 23.] - -[Illustration: FIG. 24. AND FIG. 25.] - -[Illustration: FIG. 26.] - -The writer, when exhibiting at Olympia this year, distributed 500 of -these paper models, and the almost uncanny way in which they righted -themselves when started from all sorts of impossible positions -greatly interested the visitors. In fact, numbers of persons spent -considerable time and ingenuity in trying to force the little glider -to turn over or dive, but quite without success. - -In order to test the turning capacity of this design, a rudder should -be fixed to the tail, and the model launched at a moderate speed, when -it will be found that it turns quickly and without any pendulum -motion, and without any perceptible tilt. And although the writer's -experiments with the paper model and with many larger ones on the same -plan have run into thousands, none of the models have ever been -induced to come down in any other position but on their feet. The -largest model, which measured 6 ft. 6 in. in length, was launched both -upside down and with its head pointing vertically to earth from a -height of 30 ft., and in each case righted before it reached the -ground and landed on its skids. - -As a further lifting surface, a very simple expedient offers itself in -the shape of a duct built on the box-kite principle. The -diamond-shaped box has been proved over and over again to be a very -efficient lifting device, but it has not yet been tried on an -aeroplane (Fig. 27). It is also a great stabilizer, since the air -entering into the diamond-shaped opening is collected and compressed -into the top angle there, and the whole box is thus practically -suspended from its apex line in absolute stability. The lifting -efficiency of such a box--or rather the top portion of the box, for -the bottom part is not needed on our machine--is considerably -greater than the value of the entering edge, and if run the whole -length of the machine it forms a triangular girder of great strength, -giving rigidity to the whole structure. The lifting efficiency is -doubled by allowing the centre third of the girder to be open, as the -dead air from the front part escapes, and the back part forms a new -entering edge. - -[Illustration: FIG. 27. AND FIG. 28.] - -There is also another lifting factor to be considered, and this is the -car. If the car is formed with a flat bottom, this at once becomes an -efficient lifting plane, and if the car is suspended with an open -space between it and the under surface of the plane, the loss caused -by the negative angle of the upper portion of the car front is -compensated for by the lift given by the deflected air to the under -surface of the main plane (see Fig. 28). - -It will be recognized that in the design here being gradually evolved, -the great lifting surfaces of the ordinary machine have not been -largely reduced, they have simply been broken up into several smaller -surfaces, each of which retains its efficiency. Something of the same -nature happened in prehistoric days, when our first navigators at last -made up their minds to abandon the flat-bottomed raft with its huge -supporting surface, for the new-fangled and dangerously narrow boat. - -When all the different surfaces here mentioned are taken into -consideration, it will be found that the lifting surface in a -monoplane machine of this design, with a span of 20 feet, is equal to -the lifting surface of an ordinary bi-plane with a span of 40 feet. -And as the head resistance is less than half that of the bi-plane the -speed should be very much greater. At the same time the increased -speed renders the planes more efficient, area for area, than the -planes of the slower machine. - - - - -CHAPTER V. - -VARIABLE SPEED AND THE PARACHUTE PRINCIPLE. - - -Hitherto, on the score of efficiency and also of stability, our -investigations have led us to seek for speed as the grand panacea. But -there are usually two sides to a question, and though, while in the -air, speed may be most desirable, it becomes a source of considerable -difficulty at both starting and landing. A machine built to fly at 80 -miles per hour would have to get up something like 60 miles per hour -before it could rise. And this difficulty is nothing like the problem -that presents itself when we consider how it is to land in safety from -a flight at such a speed. It becomes evident that some provision must -be made for starting and landing at some more practicable rates; we -must have a variable speed machine. - -To convert a high speed machine into a low speed machine means either -variable surface area, variable camber, or variable angle of -incidence. Any of these is possible, but the choice must be decided by -simplicity of action. To spread extra wings when rising or landing is -a cumbersome suggestion full of pitfalls and liable to accidents -through the failure of mechanical devices, which, experience shows, -always have a way of failing at inopportune moments. To vary the -camber of the planes is easier, but having decided on using flat -planes it would be loss of strength to make these flexible, and an -increase of mechanical complications to have to flex them. It would be -easy to alter the angle of incidence by having the leading edge -capable of a rotary movement, and machines have been constructed -employing this principle. But the easiest plan of all, since it does -away with all moving parts whatever, would be to alter not the planes -themselves, but the whole machine. Thus suppose the angle of -incidence, in order to get an efficient lift, to be 1 in 6, the -lifting plane, all in the same line, would be set on its chassis so -that it presented an angle of 1 in 5. The machine would then lift at a -much slower speed. Naturally, the tail being the furthest from the -centre of gravity would lift first, and as soon as the speed was -sufficient the pilot would alter the elevator, send down the tail on -to the ground, thereby raising the leading edge of the front plane, -and the machine would rise. As the speed increased the tail would -continue to rise, till, at the maximum speed, the plane would be at -the minimum angle with the horizontal, i.e. at its lowest angle of -incidence. - -This solves the problem of starting and to some extent of landing, but -we have not yet come to the end of our resources. Most landings are -effected by shutting off the engine and planing down. All flying -machines will glide if put at the proper angle, and it is the -business of the pilot to attend to this when he stops the engine. But -to glide with the same wing area as is used in flying, means to glide -at the same rate. In order to descend slowly it is necessary to have -more area. Is it possible to increase the area used for descent -without interfering with the area used for flight? In the design we -are engaged in considering, it is possible, and without any mechanical -devices. There is a large space between the front plane and the back -plane which is at present unused. It is of very little value in -flight, being in apteroid aspect and having practically no entering -edge. But if this space is covered in it gives no resistance in -flight, and in descent it becomes a very efficient parachute. Further -than this, if openings be cut in this plane immediately under the -centre of the two box-kite ducts, the air under the longitudinal -plane, having offered its resistance to the vertical passage of that -plane, will escape into the duct and again offer considerable -resistance to the descent of this closed-in surface before it escapes -finally out of the end of the duct. - -A model made on these lines will not need putting at any angle. It -will assume its proper angle when left to itself by reason of its -design and the way the weight is balanced between the supporting -planes, and it will descend by partly gliding and partly parachuting -at a steep angle but quite slowly. While, if the pilot so choose, he -can, by raising the tail, increase the speed to a glide, which he can -turn into a parachute action at any moment. - - - - -CHAPTER VI. - -THE DESIGN WHICH FULFILS THE CONDITIONS. - - -In constructing any sort of machine it is usual to first obtain the -most important device and then to build up the accompanying parts to -that. We have now succeeded in evolving the thing we set out to look -for, i.e., a plane which will fly and lift with the minimum of head -resistance, and which is absolutely stable laterally and -longitudinally by reason of its construction and without any -interference from the pilot or the employment of balancing devices of -any description. We have now to fit the propelling apparatus, car, and -chassis on to this. - -Fortunately, the design is one that lends itself easily to -manipulation, which is not always the case with models. The short span -of the planes, for instance, with the dihedral angle, at once suggests -girder construction (see Figs. 29, 30), which is, perhaps, the -strongest of all devices, being an M strut girder, familiar to us in -numberless bridges. - -[Illustration: FIG. 29. AND FIG. 30.] - -The photo which forms the frontispiece of this book, and which, by the -way, makes the car look much too large owing to its position nearest -the camera, represents a 6-foot model which was exhibited at the -Olympia Show, in order to show the construction of a full-sized -machine made to the design of the paper model. This has since been -considerably simplified, though the broad lines have been retained, by -doing away with the struts and supports at the rear. The whole of the -back plane is now supported by two curved members, which start from -the girder of the leading edge and curve down to the T-section -longitudinals which form the rigid part of the chassis. These -longitudinals and the skids end at the leading edge of the back plane -and the laminated skids and wheels are placed there. The machine is -built without a wire and without a casting. It was made entirely of -wood, but is so designed that it can be made entirely out of steel -tube by using the ordinary screw connexions. If built of timber, the -joints are made with strips of steel bolted and screwed on to the -wood. The girders forming the leading edge of each plane have sockets -formed in the upright struts of the M into which the ribs fit (see -Fig. 30), and these are solid pieces on edge tapering to the trailing -edge, where they are clipped to a slight spar which holds them -together. This construction, while very strong, is yet sufficiently -flexible to bend considerably before it reaches breaking point. -Longitudinal rigidity is secured by means of the triangular duct which -forms a complete girder from end to end. A sufficient number of -uprights fill the space between the plane and the two T-section -longitudinals which form the rigid bottom of the machine. On these -latter the floor is placed and the car is built up, enclosing all the -obstructions and putting the pilot in a place of safety, enclosed on -all sides in the middle of the strongest part of the machine, with the -strongest portion of that part between him and the ground. The centre -of gravity is situated behind the pilot in the back of the car, near -the floor, and here is space for the oil and petrol tanks. The engine -is in front of the pilot, who is thus able to control it and watch it, -and at the same time is free from the danger of having it fall upon -him in case of an accident. As the machine turns horizontally and -vertically on its centre of gravity, the front part of the car forms a -sort of baffle or blinker for the rudder and elevator to act against. -Both these are at the tail of the machine, where they have the most -leverage, and these two are controlled by the one lever, which is -pushed forward or pulled backward to raise or lower the elevator, and -turned bicycle fashion to move the rudder. As the machine balances -itself, there is no need for any balancing device either automatic or -controlled. - -[Illustration: FIG. 31. ] - -The propellers may be two or more, and those in front find a very firm -fixing in the intersection of two strong struts, which join the -wingtips to the bottom of the car, and the supports which run from the -centre of these to the strong joint formed by the intersection of the -longitudinal and lateral girder. At the back there may be two -propellers fixed as in the front, or one large one at the rear of the -car. They are all worked from the one engine and the thrust is -slightly above the centre of gravity. Each propeller is placed just -under the leading edge of a plane, Fig. 31, the idea being that a -certain amount of air is always thrown out by centrifugal force all -round a revolving propeller, and this air, which, ordinarily, is lost, -must, when thrown upwards, exert a lift on the under surface of the -plane. Also, when thrown towards the car, it must, by impinging on the -slanting surface of the car, tend to impel it forward, Fig. 32. Where -four propellers are used, the back pair should be of greater pitch -than the front pair, as they must to a certain extent, work in the -stream from the front pair. There are several ways of coupling the -propellers to the engine, but in the model they are shown coupled up -by belts, which seems to be the most efficient and lightest device. - -[Illustration: FIG. 32.] - -In order to cool the engine and keep the air in the car clear, a -ventilating pipe is led from the front of the car to the engine, and -the air, rushing through this at the speed of the machine, plays over -the engine and is conducted out through a large opening and discharged -at the back. - -The whole of this part of the machine is rigid and braced together by -means of struts, though whether made of steel tube or timber, there -must always, from the nature of the construction, be a certain amount -of elasticity which makes for strength, a great advantage over a -construction braced rigidly by non-elastic wires, which snap instead -of giving to a sudden strain. - -Under the two rigid T-section longitudinals there are a number of -elastic laminated wood springs set at an angle, and the lower ends of -these are pivoted on to a long elastic skid. This skid is made in -laminations, with alternate joints, and starts from the point where -the two planes intersect in the front of the machine, which is one of -the strongest joints in the whole construction. From this point it -bends out in a semicircle to protect the propeller and the front of -the machine and car, this portion of it being very elastic by reason -of the laminations having free play one upon the other. At the bottom -of the semicircle the skid is joined to the slanting skids or springs -depending from the bottom of the machine, and here the laminations are -bolted together making the skid stiffer. The skid runs the whole -length of the machine like the runner of a sledge. On this skid the -wheels are sprung with a steel spring lever arrangement, Fig. 33. The -shock of landing is, therefore, taken first on the wheels, and should -it be sufficiently heavy to cause the skids to touch the ground there -is still the series of laminated wood springs to absorb any vibrations -and prevent any possible shock to the car. The car is so secure from -vibration by reason of these precautions that the whole lower half of -the front of it may be made of protected glass, to enable the pilot to -get a clear view of his surroundings. - -[Illustration: FIG. 33.] - -The dimensions of the full-sized machine are estimated to be as -follows:-- - - Span 20 feet - Length 43 feet - Parachuting area 500 square feet - Efficient lifting area 360 square feet - Weight (all up) 800 lb. - -It will be understood that though only 360 square feet is counted as -efficient for lifting, the whole 500 square feet is efficient as -parachuting surface in descending. The weight of the machine compares -very favourably with existing machines, and the load 2-1/4 lbs. per -square foot, gives plenty of margin for passenger carrying. - -The chief advantages claimed for this machine are:-- - - (1) Speed. - (2) Stability. - (3) Strength of construction. - (4) Shock absorbing capacity. - -It is a practical impossibility for the machine to turn over or be -blown over, and it will recover its balance if started at any angle. -If allowed to dive vertically, either tail first or head first, it -will recover its position in six times its own length, purely by its -own balance, without any effort of the pilot. - - -LONDON: PRINTED BY WILLIAM CLOWES AND SONS, LIMITED, - -GREAT WINDMILL STREET, W., AND DUKE STREET, STAMFORD STREET, S.E. - - - - - -End of the Project Gutenberg EBook of Natural Stability and the Parachute -Principle in Aeroplanes, by W. 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