Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/181

Rh influence upon the dissipation of energy, irrespectively of the aspect ratio. It is evident that if, as a provisional assumption, we suppose the pressure distribution to be uniform over the whole area, the circulation will be much more rapid in the immediate vicinity of the edge than at some distance away; and since the fluid in circulation in the stray field represents energy lost, we can minimise this to a considerable extent by adopting a pointed, or acutely rounded, extremity, as in Fig. 69; so that the stray field is not contiguous to the edge of the aerofoil except in one spot at each extremity. If we neglect other factors that have weight in practice in determining wing form, and endeavour to rationalise on purely an aerodynamic basis, we can lay it down that for uniform load distribution, if we take the

. 69.

extreme wing tip as origin, the form of the wing extremity will be a surface that can be generated by a straight line passing through the origin. This law may be taken as holding good for such a length of the aerofoil at each end as may be regarded as inconsiderable in comparison to the total length, and follows from the absence of any scale factor in the problem; a surface as above defined may be regarded as a segment of the surface of an irregular cone. It is possible that in a viscous fluid some departure from the form above prescribed may be anticipated from the fact demonstrated in the previous chapter, that the existence of viscosity is sufficient to give a scale to a fluid.

In practice, it will be shown later in the work, the question of wing-form, especially with regard to the extremities, is not