by making the fluid inviscid or by supposing the surfaces of the plane frictionless and not attached to the fluid in any way. Whichever be the assumption, the quantity that is being ignored is that known as “skin-friction,” the general principles relating to which have been discussed in Chap. II.
In actual planes it is impossible to do away with thickness, so that in addition to skin friction there must be the possibility of a longitudinal pressure component due to the shape of the plane. Thus, if the plane be of “fair” form, i.e., a stream-line solid based on an axis plane (Fig. 104), the pressure distribution, not being in any sense symmetrically disposed, may conceivably possess a longitudinal component of quite considerable value; or if the plane be of uniform thickness and square edges, as in the planes of Langley,
Fig. 104. we have no means of computing the edge pressure resultant, for it is by no means certain that it can be represented by the resistance of the edge equivalent divested of its associations. There might, for example, in the types of motion illustrated in Fig. 98, be a region of negative pressure or suction on the front edge of the plane such as would entirely invalidate any ordinary computation.
§ 157. The Coefficient of Skin-Friction.—The hypothetical case of an aeroplane of zero thickness in edgewise motion offers the simplest possible case of skin-friction. The magnitude of the resistance due to this cause has been variously estimated, but at present is not known with any great degree of certainty. The value of skin-friction can be conveniently expressed as a coefficient, this coefficient being the resistance of a plane moving edgewise in terms of the resistance of the same plane when normal to the direction of motion. Reasoning from the facts
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