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.

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.