Learning to fly in the U.S. Army

by E. N. Fales

There are certain shapes of wings in which the center of pressure travels in the reverse direction; a flat plate, for example; or a wing having its rear edge turned up so that the general wing shape is like a thin letter “S.” Such wings as these would not tend to lose their proper angle, because when the angle is changed for any reason the center or pressure in these wings moves in just the manner necessary to restore them to their proper position; but these wings are inefficient and are not in present use on airplanes.

Fig. 29.—Diagrams illustrating theory and application of longitudinal dihedral angle.

The Penaud Tail Principle.—Rule.—The horizontal tail must have a smaller angle of incidence than the wings. The upsetting force above mentioned must be met by a strong opposite righting force, and this latter is furnished by the horizontal tail surface. In the angle of equilibrium of 2° above mentioned, the flat horizontal stabilizer will perhaps have no force acting on it at all because it is edgewise to the air and its angle of incidence is zero. When the angle of the wing increases to 2¼° and the lift moves forward tending to rear it up, the wing being rigidly fastened to the body pushes the tail downward so that the tail now begins to have a small lift force upon it due to its angle of ¼°; and this newly created force, though small, acts at such a long lever arm that it exceeds the rearing force of the wing and will quickly restore the airplane to 2°. This action depends upon the principle of the Penaud Tail or longitudinal “Dihedral” which requires that the front wings of an airplane make a larger angle with the wind than the rear surface. This principle holds good even when we have rear surfaces which actually are lifting surfaces in normal flight, the requisite being that the wings themselves shall in such cases be at an even greater angle than the tail. No mention has been made of the elevator control, because its action is additional to the above-mentioned stability. The elevator is able to alter the lift on the tail; such alteration requires, of course, immediate change of angle of the wings so that equilibrium shall again follow; and this equilibrium will be maintained until the lift at the tail is again altered by some movement of the elevator control. Thus the elevator may be considered as a device for adjusting the angle of incidence of the wings.

The air through which the wings have passed receives downward motion, and therefore a tail which is poised at zero angle with the line of flight may actually receive air at an angle of -2° or -3°. In the above case we would expect an actual downward force on the tail, unless this tail is given a slight arch on its top surface (for it is known that arched surfaces have an angle of zero lift which is negative angle).

Longitudinal Control.—Steering up or down is done by the elevator, which as explained above is merely a device for adjusting the angle of incidence of the wings. The elevator controls like all the other controls of an airplane depend for their quick efficient action upon generous speed; they can not be expected to give good response when the machine is near its stalling speed. The elevators like the rudder are located directly in the blast of the propeller and in case the speed of motion should become very slow, the elevators may be made to exert considerable controlling force if the motor is opened up to blow a strong blast against them. This is good to bear in mind when taxying on the ground because if the motor is shut off at the slow speed of motion the elevator and rudder will lose their efficacy. The propeller blast, due to a 25 per cent. slip, adds 25 per cent. of apparent speed to those parts which are in its way, and therefore the tail forces are affected as the square of this increase, that is, the forces may be 50 per cent. greater with the propeller on than off.

Lateral Stability.—This depends upon the keel surface or total side area of an airplane. The keel surface includes all the struts, wires, wheels, wings, as well as body, against which a side wind can blow. Skidding and side-slipping have the same effect as a side wind, and the resulting forces acting against the side of the machine should be made useful instead of harmful. This is done by properly proportioning the keel or side surface. If keel surface is low, the side force will rotate the airplane about its axis so that the windward wing sinks; if high, so that it rises. But if the keel surface is at just the right height (i.e., level with the center of gravity) the side forces will not rotate the machine at all and will simply oppose the skidding without upsetting equilibrium.

Fig. 30.—Diagram showing effect on lateral stability of dihedral angle and non-skid fins.

(a) Machine flying level. (b) Machine tips and side-slips: excess pressure is created on windward wing and fins, (c) Machine has side-slipped and rotated back to level.