NO. 



STABILITY OF AEROPLANES — IIUXSAKER AND OTHERS 



Case I, with tail at — 2°75, shows very small pitching moments and 

 may be said to be neutral for ordinary angles of incidence. Thus 

 if the aeroplane be flown at +2° incidence, in order to maintain 

 balance at this attitude the pilot must impre'ss a diving moment of 

 — .037 pound-inch (on the model) to overcome the stalling moment 

 + .037 given above. Then if the aeroplane be accidentally tilted up to 

 + 12° by a wind gust or other cause, in the new attitude the net pitch- 

 ing moment is still positive, and hence tends to tilt the machine still 

 more. It is, therefore, unstable unless the pilot intervenes with the 

 horizontal rudder. 



For case II, tail at —7°, there is 'a strong righting moment always 

 acting to prevent stalling or diving. The machine is very stable, in 

 fact excessively so. For instance, flying at 2° incidence, the moment 

 to be held by the pilot is very small. Suppose, however, he wishes 

 to fly at +12° corresponding in the full-scale aeroplane to about 36 

 miles per hour. To maintain a balance at + 12° incidence, he must 

 exert a stalling moment by use of the horizontal rudder equal to about 



' — ^x (26)^P j =1,970 pounds-feet. The arm of the elevator is 



about 20 feet (distance aft of center of gravity), requiring a lift of 

 100 pounds, on the elevator flaps. The elevator is able to exert this 

 force if turned up about 10°. The elevator motion available for con- 

 trol in gusty air is thus largely used up in maintaining balance. The 



