NO. 5 STABILITY OF AEROPLANES IIUNSAKER AND OTHERS 



49 



Low-speed attitude, 1=12°, 1 = 26 inches, 

 ^ = 6.91 inches, a= — 1.71 inch. 



The variation, with angle of yaw, ip, of the rolHng moment L, yaw- 

 ing moment N, and lateral force Y, are shown by the curves of figure 

 15. In its symmetrical position, the aeroplane has no tendency to roll, 

 yaw, or slide slip, and Lq, A^(„ and Y^ are zero, as stated in connection 

 with the discussion of equilibrium conditions. 



As the aeroplane yaws from its course, the plane of symmetry 

 swings through an angle ij/, measured positive to the pilot's right 

 hand. The momentum tends to carry the center of gravity forward 

 in its original direction of motion. As a result, the apparent wind 

 seems to strike the left cheek of the pilot. The curves of A^ show 

 that, if this aeroplane yaw to the right, a negative yawing moment is 

 produced which tends to turn the aeroplane to the left and hence to 

 put it back on its course. The aeroplane is hence " directionally " 

 stable, having a preponderance of fin surface behind the center of 

 gravity, and the pilot need not use his rudder to stop the yaw. Nu- 

 merically, we see that for a yaw of 10° at high speed, the value of 

 A^ is — 13.5 units, or about 670 pounds-feet. For a perfectly neutral 

 aeroplane, to produce an equal yawing moment the pilot must exert 

 a force of about 34 pounds on a vertical rudder 20 feet to the rear of 

 the center of gravity. 



When flying straight ahead, if the direction of the wind suddenly 

 shifts so as to bring the apparent wind 10° to the left of the fore and 

 aft axis of the aeroplane, the aeroplane tends to head over into the 

 wind. An excessive amount of " directional " stability, indicated by 



