Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/239

Rh of $$R$$ and the angle $$\beta,$$ we have:—

The quantity $$S$$ is in many cases very small and difficult to measure directly, so that it is usually, for small values of $$\beta ,$$ deduced from the normal reaction, values of which have been given in Figs. 93 and 96.

It is evident that if the form of aerofoil under investigation be other than plane no such simple relationship as the foregoing exists; the vertical and horizontal components require to be measured independently.

The propriety of neglecting tangential forces has sometimes been questioned; such forces certainly cannot be neglected on the grounds of their negligibility (an error actually fallen into by Langley), and it is desirable to inquire into their exact nature in order that the consequences may be clearly understood and a correction provided.

If we had to deal with a plane devoid of thickness, so that its whole boundary surfaces might be said to lie in one plane, then there is only one possible kind of tangential force, i.e., that due to the viscosity of the fluid ; there must not only be viscosity within the fluid, but also physical continuity between the fluid and the plane itself capable of transmitting viscous stress. We could imagine this source of tangential force disposed of, either