Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/210

§ 137 These figures must be regarded as a mere illustration. Not only are the data unreliable but there is considerable doubt attaching to the accuracy of a curve drawn through three points only. Observations are wanted made with planes of one standard size and shape, and at a standard velocity in fluids of different $$\nu$$ value, in order that this indirect method of estimating the index values should be really effective.

§ 138. The Nature of the Pressure Reaction.—The resistance experienced by an aeroplane in motion is due to the difference of pressure between its anterior and posterior faces, and it is the integration of this difference into the area that has hitherto been referred to as the pressure on the plane and denoted by the symbol $$P.$$

By the theory of Helmholtz, the pressure difference in a fluid of zero viscosity is entirely due to the excess of pressure on the front face, the “dead-water” being supposed to carry the ordinary hydrostatic pressure of the fluid.

In real fluids there is a viscous drag at the surface of discontinuity, or stratum of turbulence, across which a continuous communication of momentum takes place. This constitutes a