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The least resultant will, of course, occur when the plane is parallel to the air stream. The maximum resultant will occur when the angle of incidence is about 40 degrees, and on a further increase in the angle, the value of the resultant will gradually decrease. When the plane is parallel with the stream, the resultant is parallel to the plate, but rapidly approaches a position at right angles at about an incidence of 6 to 10 degrees. Beyond 10 degrees incidence the angle of the resultant increases past the normal.

The center of pressure (O), or the point where the resultant force intersects the plane, moves forward as the angle of incidence is decreased from 90°. When at right angles to the air current, the center of pressure is exactly in the center of the plane as shown by Fig. 14. In this case the drag (D) is the resultant, and acting in the center, exactly balances the air forces. In Fig. 15 the angle of incidence is reduced, consequently the center of pressure moves nearer the leading edge (A). As the angle continues to decrease, the C. P. moves still further forward until it lies directly on the front edge when the plate becomes parallel with the air stream. The center of pressure movement is due to the fact that more and more work is done by the front part of the surface as the angle is decreased. Consequently the point of support, or C. P., must move forward to come under the load. It should be understood that the plane will balance about the C. P. if a knife edge bearing were applied as at R in Fig. 15.