NO. 5 STABILITY OF AEROPLANES HUNSAKER AND OTHERS 43 



secure a slow motion in pitch. It will be remembered that Mw is a 

 measure of statical stability or " stiffness " and was mentioned as 

 somewhat analogous to metacentric height for a ship. 



By adjustment of Z,„, Afq, and Mu, it appears possible to combine 

 heavy damping with a fairly long period and so obtain great steadi- 

 ness in normal flight. 



§15. CONCLUSIONS (LONGITUDINAL DYNAMICAL STABILITY) 



Stability calculations are of greater interest when they can be com- 

 pared for diff'erent aeroplanes. At present, information is scanty but 

 we may obtain by inference some general conclusions by comparing 

 the Clark type aeroplane just described with a Curtiss type aeroplane 

 previously tested by us. 



The two aeroplanes are designed to have about the same perform- 

 ance. The principal difference at first sight is the greater wing area 

 of the Clark — about 3.45 pounds per square foot against about 4.7 

 pounds per square foot for the Curtiss. In consequence of the lighter 

 wing loading, the Clark type should have a steeper curve of Z giving 

 Zw large, which is favorable to stability. 



The Clark aeroplane has a smaller horizontal tail area than the 

 Curtiss, but the fixed part is inclined at — 5° to the wing chord against 

 — 3° 5 in the Curtiss. The Clark tail is only a trifle longer than the 

 Curtiss and we may conclude that the pitching moment due to air 

 pressure on the tail surfaces is about the same in the two machines. 

 However, the Clark model uses a wing section on which the center of 

 pressure motion for small angular changes is very slight. The 

 Curtiss has a s.ection described as R. A. F. 6 ' in wdiich this motion is 

 considerable. For equal tail moments we may then expect Mw to be 

 larger for the Clark machine. This is favorable to stability. 



Due to the smaller tail, the damping of the pitching for the Clark 

 model might be less than for the Curtiss. However, we find Mq at 

 high speed —150 for the Curtiss against —192 for the Clark model. 

 The increase must be due to the greater wing area of the latter since 

 a calculation of the damping due to the tail alone gives a result less 

 than one-half that observed for the whole machine. 



The greater stability of the Clark model at high speeds is then due 

 principally to greater values of Z^ and Mio. At low speeds, the 

 resistance derivatives of these two aeroplanes are not greatly differ- 

 ent. Both become very slightly unstable in their longitudinal motion. 



^ See Technical Report Advisory Committee for Aeronautics, 1912-13. 



