Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/241

Rh known in connection with skin-friction in the case of water, we may infer that this form of resistance will vary approximately as the square of the velocity, but more accurately, proportionately to some power of the velocity rather less than the square, the index being lower than in the case of the normal plane. A consequence of this is that the “coefficient” will be greater for small planes at low velocities and less for larger planes at higher velocities.

Langley in his Memoir (“Experiments in Aerodynamics,” pp. 9 and 25), and Hiram Maxim (Century Mag., xlii., 829 and 836, 1891) have both stated explicitly that the influence of skin-friction in its relation to flight is negligible. Langley gives this result as a deduction from certain of his experiments, also as a matter of calculation based on Clerk Maxwell's value of the viscosity of air. He concludes from the latter that the frictional resistance is “less than 1/50 of one per cent, of that of the same plane moving normally,” that is to say, he arrives at a coefficient of skin-friction of less than .0002.

The author finds that Langley' s deduction in this matter is not justified by the experiments upon which it is founded, and, further, that his calculation is based upon inadequate data and is in error. The author has further shown, in Chap. VII, that skin-friction is a dominating factor in the economics of flight.

The direct measurement of skin-friction is a matter of considerable difficulty, so much so that experiments specially devised merely to detect its presence (as in the disc experiment of Dines) have proved abortive. The author, by means of experiments (described in a subsequent chapter), has succeeded in measuring approximately the value of the coefficient of skin-friction $$\xi$$ the following conclusions may be stated:—

(1) For smooth planes of a few square inches area at low velocities (about 10 feet per second), $$\xi =$$ .02 to .025.