Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/179

Rh aerofoil is to receive a current of air in upward motion and impart to it a downward velocity, the whole air being dealt with possessing relatively to the aerofoil a superposed motion of translation. It would appear that any appropriate smoothly curved form, whose leading and trailing angles (Fig. 68) are conformable to the lines of flow, might be regarded as fulfilling the necessary conditions, the essential feature evidently being that neither edge shall give rise to a surface of discontinuity.



Since the amplitude of the motion may be regarded, for a fluid of given density as a function of the load on the aerofoil and its velocity of travel, the steepness of the lines of flow must also be a function of these variables, and for a given sectional form of aerofoil there is some critical velocity at which the advancing edge may be taken as conformable. When the aerofoil is supposed of infinite lateral extent, then if the sectional form be made symmetrical, at the velocity at which the leading edge becomes conformable, the trailing edge will also be conformable. If, however, the aerofoil be of finite lateral extent, we do not know what the relation ought to be between the angles $$\alpha$$ and $$\beta$$,