Learning to fly in the U.S. Army

by E. N. Fales

Fig. 27.—Washout in left-wing tips.

Washout.—Due to torque of the motor, the airplane tends to rotate in the opposite direction to the propeller. This tendency may be neutralized by giving one wing tip a smaller angle of incidence, called “washout,” so that the machine normally tends to neutralize the torque-effect.

PRINCIPLES OF AIRPLANE EQUILIBRIUM

Fig. 28.—Balances of forces in an airplane.

Weight forward of lift, thrust below resistance. Thrust equals resistance, weight equals lift.

Introductory.—Under this head will be discussed: (a) features of airplane design which tend to maintain equilibrium irrespective of the pilot; (b) matters of voluntary controlling operations by the pilot. As regards (a) the tendency of the airplane toward inherent stability acts to oppose any deviation from its course whether the pilot so desires or not. The more stable is a machine, the less delicately is it controlled, and the present consensus of opinion among pilots is that a 50-50 compromise between stability and controllability is the best thing.

In questions of airplane equilibrium the starting point is the center of gravity; obviously, if the center of gravity were back at the tail or up at the nose there would be no balance; the proper place for it is the same spot where all the other forces such as thrust, lift and resistance act; there it is easy to balance them all up. But it is not always easy to bring the line of thrust and the line of total resistance into coincidence, because the line of thrust is the line of the propeller shaft and when this is high up as in the case of some pushers it may be several inches above the line of resistance. And as the thrust is above the resistance there is a tendency to nose the machine down; to balance which the designer deliberately locates the center of gravity sufficiently far behind the center of lift so that there is an equal tendency to tip the nose upward; and all four forces mentioned completely balance each other. But things may happen to change the amount or position of these forces during flight, and if this does happen the first thing to do is to restore the balance by bringing in a small new force somewhere. In an actual airplane this small restoring force is supplied at each critical moment first, by the tail, etc., of the airplane and second, by voluntary actions of the pilot. The center of gravity of any airplane may be determined easily by putting a roller under it and seeing where it will balance, or by getting the amount of weight supported at the wheels and tail, according to the method of moments.

Longitudinal Stability.—Longitudinal stability has to do with the tendency of an airplane to maintain its proper pitching angle. It was said above that the four forces of lift, resistance, thrust and weight always exactly balanced due to their size and their position. Now the first consideration about longitudinal stability is that while the centers of gravity and other forces remain in a fixed position, the center of lift changes its position whenever the angle of incidence (that is the speed) is changed. The phenomenon of shift of center of pressure applies only to the wings and to the lift (the position of center of resistance remains practically fixed at all angles).

Note the effect on center of pressure position of a change of wing angle (see Fig. [20]). The wing used on the U. S. training machine has a center of lift which is about in the middle of the wing when flying at a small angle of maximum speed; but if the angle is increased to the stalling angle of 15°, the center of pressure moves from midway of the wing to a point which is about one-third the chord distance of the wing from the front edge. The lift may travel about ½ foot, and it is equal in amount to the weight of the machine (that is, nearly a ton), and the mere effect of changing the angle from its minimum to its maximum value therefore tends to disturb the longitudinal equilibrium with a force which may be represented as 1 ton acting on a lever arm of ½ ft. Suppose that the airplane is balancing at an angle of 2° so that the center of gravity coincides with the center of lift for this angle; now if a gust of wind causes the angle to increase for an instant to 2¼°, the center of lift will move forward and tend to push the front edge of the wing up, thus increasing the angle further to 2½°. Then the center of lift, of course, moves further forward to accommodate the increase of angle, and in a fraction of a second the wing would rear up unless it were firmly attached to the airplane body and held in its proper position by the tail. Similarly if for any reason the proper angle of 2° were decreased, the same upset would follow, only this time tending to dive the wing violently to earth. This tendency is neutralized in an airplane by the “Penaud Tail Principle.”