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SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 62 



a steep curve of yawing moments A^, may cause the aeroplane to be 

 unmanageable in gusty air. It may " take charge " and, due to 

 excessive " weather helm," be difficult to keep on any desired course. 



Fig. 15. — Curves of lateral force, rolling moment, and yawing moment, as 

 angle of yaw changes. 



It wall be shown later that the so-called " directional "' stability is not 

 only undesirable in gusty air, but is the determining factor in " spiral 

 instability." Indeed, " directional stability " is very nearly incom- 

 patible with inherent dynamical stability in roll, yaw, and side slip 

 considered together. 



