Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/127

 103 motors, with their propellers, shafting, supplies, &c., weighing less than 20 lb per h.p. The problem, then, is how they should be designed. The knowledge evolved within the last few years has at last made it practicable to give some data for calculating the effects to be expected, with some confidence that they will not be far wrong. It is evident that an apparatus must be designed to be as light as possible, and also to reduce to a minimum all resistances to propulsion. This being kept in view, the strength and consequent section required for each member may be calculated by the methods employed in proportioning bridges, with the difference that the support (from air pressure) will be considered as uniformly distributed, and the load as concentrated at one or more points. Smaller factors of safety may also have to be used. Fig. 4.—Ader’s “ Avion ” in position for flight. (From The Scientific American.) Knowing the sections required and unit weights of the materials to be employed, the was with a machine of the latter type that he was upset weight of each part can be computed. If a model has by a sudden gust of wind and killed in 1896. Pilcher been made to absolutely exact scale, the weight of the improved somewhat upon Lilienthal’s apparatus, but used full-sized apparatus may approximately be ascertained by the same general method of restoring the balance, when the formula endangered, by shifting the weight of the operator’s body. W'= He made several hundred glides in safety, until he too -M!)was upset by a gust of wind and killed in 1899. Chanute confined his endeavours exclusively to the evolution of in which W is the weight of the model, S its surface, and automatic stability, and made the surfaces movable instead W' and S' the weight and surface of the intended apparatus. of the man. He obtained over 1000 glides without accident Thus if the model has been made one-quarter size with two different types of apparatus. The last three in its homologous dimensions, the supporting surfaces experimenters have claimed that it is not impossible for will be sixteen times, and the total weight sixty-four man to imitate the soaring flight of certain birds, wherein times those of the model. The weight and the surface support and propulsion are obtained from the wind alone being determined, the three most important things to under certain favourable conditions. know are the angle of incidence, the “lift,” and the The very first essential for success is safety, which will probably only be attained with automatic stability, and the foregoing account indicates that it is for lack of this that all the experiments have shown such slight success. Safety and stability are therefore to be sought first. The underlying principle is that the centre of gravity shall at all times be on the same vertical line as the centre of pressure. The latter varies with the angle of incidence. For square planes it moves approximately, as expressed by Joessel’s formula: C = (0-2 + (P3 sin a)L, in which C is the distance from the front edge, L the length fore and aft, and a the angle of incidence. The movement is different on concave surfaces, but has not been formulated. The term Aeroplane is understood to apply to flat sustaining surfaces, but experiment indicates that arched surfaces are more Fig. 5.—Ader’s “Avion” with the wings folded. (From The Scientific American.) efficient. Langley has proposed the word Aerodrome, which seems the preferable required speed. The fundamental formula for rectangular term for apparatus with wing-like surfaces. This type is air pressure is well known : P = KY2S, in which P is the the one which results point to as the proper type for further rectangular normal pressure, in pounds or kilograms, K experiments. With this it seems probable that, with well- a coefficient, (0‘0049 for British, and 0T1 for metric designed apparatus, 40 to 50 fb can be sustained per indi- measures), Y the velocity in miles per hour or in metres cated h.p., or about twice that quantity per resistance or per second, and S the surface in square feet or in square “ thrust ” h.p., and that some 30 or 40 per cent, of the metres. The normal on oblique surfaces, at various angles weight can be devoted to the machinery, thus requiring of incidence, is given by the formula P = KY2St/, which pellers (Figs 4, 5). The steam-engines weighed about 7 tb per h.p., but the equilibrium of the apparatus was defective. To Lilienthal belongs the double credit of demonstrating the superiority of arched surfaces over planes, and of reducing gliding flight to regular practice. He made over 2000 glides safely, using gravity as a motivepower, with various forms of apparatus, consisting of concave bat-like wings, in some cases superimposed. It