NO. 5 STABILITY OF AEROPLANES HUNSAKER AND OTHERS 5 



to produce a stable aeroplane. The discussion has been confined to 

 motion in still air. If an aeroplane be unstable in still air it is 

 obviously worse ofif in gusts. The converse is, unfortunately, not 

 true, for an aeroplane which is very stable in still air may be so stiff 

 that in turbulent air it will be violently tossed about. 



It is conservative to conclude that aeroplanes should not be un- 

 stable and that they need not be, since slight changes in the nature 

 of adjustments suffice to correct such instability of motion. 



In view of the military use of aeroplanes inside the zone of fire the 

 probability of controls becoming inoperative is ever present. An 

 inherently stable aeroplane, with controls abandoned or shot away, 

 could still be operated by a skilful pilot by manipulation of the motor 

 power alone. 



Any sort of automatic (or gyroscopic) stabilizer which operates 

 on the controls is of no use when those controls fail, and it should 

 be judged as an accessory to assist a pilot rather than as a cure-all 

 for the inherent instability of an aeroplane's motion. 



The ordinary type of aeroplane readily lends itself to adjustments 

 which make for inherent stability of motion and there is no reason 

 to seek radical changes of type to insure stability. Freak aeroplanes 

 of great "stability" may be excessively stable in some ways and 

 frankly unstable in others. It is likely that the most satisfactory 

 aeroplane will be only slightly stable and that this aeroplane will in 

 any possible attitude be easily controlled by the pilot. 



Controllability and statical stability are to some extent incompatible. 

 Dynamical stability recj[uires some amount of statical stability and 

 considerable damping. It appears to be of advantage to provide the 

 minimum of statical stability and the maximum of damping. Then 

 the aeroplane's motion will be of very long period but heavily damped. 



It is believed that the methods of investigation here described may 

 be applied to any type of aeroplane, and, by the systematic variation 

 of one feature of design at a time, a full understanding may be had 

 of the effect on the motion of each change. The process is of 

 necessity laborious, but compared with the diflficulty of full-scale 

 experiment in the open air, the model method is rapid and inex- 

 pensive. It is rarely possible in actual flying to obtain any idea of the 

 effect of slight changes in design. Weather conditions, motor 

 troubles, personal peculiarities of pilots, etc., tend to add to the com- 

 plexity of an otherwise very simple problem. 



Furthermore, experimental flying is dangerous. For example, I 

 knew a pilot who, to determine whether a new aeroplane was spirally 



