Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/242

(2) For larger planes, .5 to 1.5 square feet area, at higher velocities (about 20 to 30 feet per second), $\xi =$ .009 to .015.

(3) A plane of about ${\tfrac {1}{2}}$ square foot area, coated with No. 2${\tfrac {1}{2}}$ (Oakey's) glass paper gave, $\xi =$ .02 (approx.).

(4) For single surfaces (as the surface of a stream-line body) the half value of $\xi$ must be employed.

The experiments upon which the above results are based were made with planes of from 3 : 1 to 4 : 1 ratio in pterygoid aspect; the values are probably lower for square planes or planes in apteroid aspect. These experiments are still in progress.

§ 158. Edge Resistance in its Relation to Skin-Friction.—There is a subtle interaction between direct edge resistance and skin friction which merits discussion. Where the plane is bounded by square cut edges, or edges of bluff form, a certain amount of direct resistance is experienced. The work done from this cause is largely employed in setting in motion the air that impinges on the leading edge of the plane, and which afterwards “washes” its two surfaces. This has for a consequence the lessening of the skin- or surface- friction, for the air in contact with the plane, having already a velocity imparted to it, does not exercise so great a viscous drag. The influence of this edge effect is comparable to the diminution of the coefficient, as the distance from the “cut-water” is increased (discovered by Froude in the case of water); here the fluid, having been set in motion by the first part of the plane, does not exercise so great a drag on the part that follows. In a plane such as we are considering the total resistance will not be the sum of the edge resistance and skin-friction separately assessed, but will be less than this amount, and may be very little greater than the one or the other of the resistances measured separately.

It is probable that for planes of less than a certain proportionate thickness the augmentation due to the edge area is imperceptible, and that for such thin planes edge effect can be ignored. Equally it is probable that for planes of rectangular section of

222