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This formula is applied to surfaces parallel to the wind such as the sides of the body, rudder, stabilizer, and elevator surfaces (when in neutral). A second important feature of the friction formula is that it illustrates the law of "similitude" or the results of a change in scale and velocity, hence it outlines what we must expect when we compute a full size aeroplane from the results of a model test.

The Inclined Plane

Fig. 13. Flow About, Inclined Plane and Forces Produced by Stream. Fig. 14. Normal Plane with C.P. at center of Plate. Fig. 15.. C.P. Moves Toward Entering Edge When Plate Is Inclined to Wind.

A line OR is the resultant of the lift and drag forces L and D, this resultant being the force necessary to balance the two forces L-D. It is on the point of application O that the plate balances, and this point is sometimes known as the "Center of pressure." The center of pressure is therefore the point at which the resultant intersects the surface of the flat plate. The resultant OR is approximately at right angles to the surface at small incident angles, and the point O is nearer the front or "leading edge" (A) of the plane. The smaller the angle of incidence the nearer will the point O approach the leading edge A. By drawing OL to scale, representing the lift, and OD to scale representing the drag, we can find the resultant OR by drawing LR parallel to the drag OD and DR parallel to the lift line OL. All lines drawn through the intersection of LR and DR will give the resultant OR to scale. All of the lines must be started from the center of pressure at O.