Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/390

§ 240 great moment in relation to the dynamics of flight, and the determination of $$\xi$$ with a reasonable degree of accuracy is therefore a matter of prime importance.

Incidentally, data are obtained from which other aerodynamic constants may be deduced, though in this respect the method of free flight experiment has not furnished as reliable data as may be expected in the future when more suitable apparatus has been elaborated.

The various methods employed by the author do not give results that are altogether in accord, but in view of the extent of the general disagreement in the work of other investigators, and in the difficulties of determining $$\xi$$ in particular, this is in no way surprising.

Experiment apart, there is a primâ facie case for the existence of a coefficient skin-friction of some considerable magnitude in the fact that the similar coefficient, as determined by Froude and others, is, in the case of water, a matter of one per cent, or thereabouts, and in the fact that the kinematic viscosity of air is fourteen times as great as that of water.

'''§ 241. Author's Experiments. Method.'''—Three modifications of the free flight method of experiment have been employed; these may be enumerated as follows:—

(1) The Added Surface Method.—In this an aerodone is first constructed on the lines laid down in patent specification 17935 of 1905 (Fig. 151), the auxiliary surface being made about the minimum necessary for stability. The natural gliding angle and velocity are very carefully measured from trial “glides.” The auxiliary surface is then increased by gumming extension laminae on to the fins, care being taken not to alter the total weight or the position of the centre of gravity. Further glides are then made and the angle and velocity are again measured; the added resistance is then calculated from the data obtained, and so the value of the $$\xi$$ is determined.

(2) The Total Surface Method.—An aerodone is constructed of