NO. 5 STABILITY OF AEROPLANES HUNSAKER AND OTHERS 35 



Apparatus and Model, Incidence of Wing, 12" 



Velocity 35 



t 23.5 



fi 0074 



fJ.Q 0016 



jXy} 0002 



[Xm (net) 0066 



Values computed as above for /.i,,;, net, for the three cases are plotted 

 in figure 13. The points appear to lie along straight lines in justifica- 

 tion of the assumption that the damping coefficient varies as the first 

 power of the velocity of flight, To convert to full-speed full-scale, 

 we use the formula, 



Mg=^;;^(26)* 



m 



Velocity aeroplane] 

 Velocity model J ' 



for i=o°, Mq=( — )ig2. 0=1. yiU, 

 i=6\ M,= (-) 93.7=1-43^. 

 i=I2°, Mq={-) 6o.5=i.i2f/. 



The marked decrease in damping at slow speed must impair 

 stability. For the Curtiss Tractor JN2, with a somewhat shorter 

 tail, we found Afg=i.32^ at i=2°, and Mq=i.66U at i=i5°5. 

 Bairstow found for the Bleriot, Mg— 1.84^/ at i = 6° . We should 

 expect greater damping to be shown there, since the horizontal tail 

 surface is verv lar2:e. 



§14. LONGITUDINAL STABILITY, DYNAMICAL 



We have now determined the resistance derivatives needed for 

 the three equations of the longitudinal motion in the plane of sym- 

 metry with the exception of A^; and Zq. From a consideration of 

 various terms in the criteria for stability it is concluded that both 

 Xq and Zq enter into products which are small and relatively unim- 

 portant. They are consequently neglected. 



The bicjuadratic has been calculated, following the formulae given 

 above, for several speeds and attitudes of flight. The results are 

 summarized in the following table. The curves of figures 8, 9, 10. 

 and II were used to obtain the resistance derivatives. 



