56 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 62 



of the drift coefificient in pounds per square foot per foot-second is 

 then 



A - =A.' ,. -j-aa. 

 where Ka-^ is the coefficient when i is the normal angle, a is the slope 

 of the tangent line and a the small change in incidence defined above. 

 The slope o- is conveniently measured in units of Kx change per degree 

 angle. If the subtangent or projection of the tangent line is ; degrees, 



J 



and 



K^ = K„ +K„ *^ . 



X Jo •'O ,1 



The head resistance of an element of the right wing is 

 - hdyK.W = - ('a;,^+ A;,,^ ": \ hdyU\ 

 and the vawing moment on the aeroplane due to it is 



But tana= T?' or a — ^y.-^j^', if a is small. Then the total yawing 

 moment on a single plane is 



The integral of the first term is zero, and the second term reduces to 



_ 57-3UKJ 



where / is moment of inertia of one plane. For a biplane of two 

 rectangular wings, the total yawing moment in pounds-feet is 



mN= — ^^ ^^. -^ p. 



Hence : 



To calculate Np, we have : 



i U Ka; j b S 



O — 1 1 2 . 5 .... GO 5 . 62 40. 2 



6 — 65.3 .0000443 6.0 5.62 40.2 

 12 — 54.0 .0001047 6.9 5.62 40.2 



Since U is feet per second. /C^, must be in pounds per square foot 

 per foot-second velocity. Values for the drift coefficient were taken 



