228 SYMPOSIUM ON AERONAUTICS. 



not certain but the finite gust represents more nearly actual condi- 

 tions in the air when flying is at all possible. 



An article by Brodetsky, with an introduction by Bryan, has re- 

 cently reached this country,- in which impulsive gusts are considered, 

 relative to Bryan's skeleton aeroplane consisting of a forward main 

 plane and rear tail plane. The discussion is both interesting and 

 important as is everything to which Bryan, the great pioneer in this 

 subject, sets his name, but it does not seem to help me, so far as I 

 have yet been able to examine it, in regard to the effect of an im- 

 pulsive gust upon a machine whose properties are actually de- 

 termined in the wind tunnel. I have therefore decided to let stand 

 the brief general considerations above. 



The Action of the Air Screw. 



27. In the work to this point, I have made for the discussion of 

 gusts the same assumption concerning the action of the propeller 

 that Hunsaker, Bairstow, and others have made for discussions of 

 stability, namely, that under varying conditions the motor speeds up 

 or slows down so as to deliver a constant thrust along the .^--axis. 



It would be equally reasonable, from some points of view more 

 reasonable, to assume that under changing conditions of relative air 

 velocity a motor speeds up or slows down so as to deliver the same 

 effective horsepower. We should then have the power P equal to 

 the thrust H (taken positive) multiplied by the velocity — U '. 



P = - IIU= - {H-^dH){U+u), 

 UdH + uH = o, 



dH= -H^= -P^2- (10) 



This is an additional force which is directed along the X-axis if 



the propeller shaft is horizontal for the velocity of flight — U. If 



in the standard condition the shaft is not horizontal there would be 



components 



u u . 



— P — cosa, -{- P—sm a 



~ Aeronautical Journal, London, 20, 1916, 139-156. 



