Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/236

§ 154 planes four inches apart, at the average speeds, the stratum of air disturbed during its passage over it is, at any rate, less than four inches thick. In other words, the plane is sustained by the compression and elasticity of an air layer not deeper than this, which we may treat for all our present purposes as resting on a solid support less than four inches below the plane.” “(The reader is again reminded that this sustenance is also partly due to the action of the air above the plane.)”

In the author's opinion the whole of this inference is unsound. Professor Langley appears to have overlooked the possibility of a superposition of the two systems of flow, such as is plotted for the Eulerian fluid in Fig. 73, Chap. IV. It is evident that such a superposition is not only possible but highly probable, each plane affecting the other profoundly so far as the actual stream-line system is concerned; but the combined supporting power of the planes, that is, the sum of the two systems, being substantially unaffected. Fig. 73 illustrates, from the case of the perfect fluid, the manner in which the two cyclic systems proper to the two planes may react on one another.

Beyond this it is evident that the speed of propagation of the compression and rarefaction within the fluid will be equal, or approximately equal, to that of sound, so that if this were really the determining factor as affecting the layer of fluid involved,