Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/237

Rh the latter would be much greater than the observed four inches; it would, in fact, at the average velocity employed, amount to something like eight feet in either direction, that is to say, some sixteen feet in all.

The employment of a series of superposed members for the support of a load in flight was not new at the date of Langley's experiments. This system appears to have been well known to, if not actually employed by, Horatio Phillips, being foreshadowed in his specification of 1884, and further in 20,435 of 1890, and very thoroughly developed in his captive machine at Harrow about the same date. The supporting members adopted by Phillips were rightly of curvilinear section (see Fig. 60), but the critical distance of separation is evidently much the same for such a form as for a plane; at least Phillips appears to have independently adopted for his aerofoil spacing substantially the proportions subsequently proved by Langley to be admissible for the aeroplane.

§ 155. The Centre of Pressure as affected by Aspect.—The general behaviour of the centre of pressure as a function of the angle has been discussed in respect of the square plane in § 148. It remains for us to examine the subject in its relation to aspect.

So far as the author is aware, the only experimental determinations other than for the square plane are those of Kummer (Berlin, Akad. Abhandlungen, 1875–6), from which it appears that the displacement of the centre of pressure from its normal position is less in planes in apteroid aspect than in the square plane, and is greater in planes in pterygoid aspect. This is substantially what might be anticipated, for in the case of the infinite lamina in apteroid aspect the pressure distribution along its length is uniform, so that the centre of pressure for a very long plane will be sensibly undisturbed by its change of angle. On the other hand, in planes in pterygoid aspect the cyclic motion results in an increased pressure region under the leading edge, and in a partial vacuum in the region above. If the cyclic