motion were perfect, as in Figs. 71, 72, 73, &c, (Chap. IV.), the motion of the fluid would be symmetrical, and the centre of pressure would not suffer displacement; owing, however, to the imperfection of real fluids, the pressure on the region of the following edge is not materialised, the motion becoming discontinuous, as depicted in Fig. 98, so that the centre of pressure is situated towards the forward edge of the plane.
Langley has observed that when the angle of flight exceeds a critical value, the displacement of the centre of pressure is greater for planes in apteroid than in pterygoid aspect, a reversal taking place similar to that discovered by him in the case of the total pressure reaction.
The position of the centre of pressure as a function of the inclination is of most interest in the case of planes of extreme proportion in pterygoid aspect. Under these conditions experiment is most difficult; no reliable data are at present available.
Lord Rayleigh has given the theoretical solution in the case of the infinite lamina in pterygoid aspect, on the Helmholtz hypothesis (§ 97). It is a curious fact that, when plotted, Rayleigh's curve is almost identical with that based on Langley's observations for the square plane, the departure only becoming noticeable at small angles; see Fig. 94 (L = Langley, R = Rayleigh).
§ 156. Resolution of Forces.—It is one of the advantages of the aeroplane as a medium of experiment that, if we neglect any tangential forces acting on its surfaces, the total pressure reaction, the resistance in the line of flight, and the reaction at right angles thereto, are correlated by an ordinary parallelogram of forces. Thus in Fig. 103, assuming the direction of flight to be horizontal, if be the weight supported, be the total normal reaction, and be the force of propulsion, the relative magnitude of these forces will be given by the resolution shown. Expressing and in terms
218
