Read Ebook: Natural Stability and the Parachute Principle in Aeroplanes by LeMaitre W

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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.

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 , which is, perhaps, the strongest of all devices, being an M strut girder, familiar to us in numberless bridges.

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 , 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.

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.

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.

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 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:--

Speed. Stability. Strength of construction. 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.

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