2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 62 



Curtiss Aeroplane Company gave their full cooperation with a desire 

 to learn what improvements in the design might be suggested by our 

 Stability calculations. Dr. A. F. Zahm of the research department 

 of that company made careful tests to locate the center of gravity 

 and to determine the moments of inertia of the actual aeroplane. 



The Curtiss machine is a practical aeroplane with powerful con- 

 trols, which does not pretend to possess any particular degree of 

 stability. The Clark aeroplane, on the other hand, was designed to 

 be inherently stable while departing as little as possible from the lines 

 of the ordinary military aeroplane as typified by the Curtiss JN2. 



The comparison of these two aeroplanes is interesting and leads to 

 the conclusion that inherent dynamical stability, both longitudinal 

 and lateral, may be secured in an aeroplane of current type by careful 

 adjustment of its surfaces and without material effect on controlla- 

 bility or performance. 



The discussion in detail is confined to the Clark model, for brevity 

 of presentation, and the results only of the parallel calculations for 

 the Curtiss model are introduced where a comparison is suggested. 



In Part I the general equations of motion are deduced in a simpli- 

 fied form which applies to horizontal flight in still air. The longi- 

 tudinal motion is then considered separately and the necessary 

 aerodynamical constants determined from wind tunnel tests. It is 

 found that the longitudinal motion, if disturbed by any accidental 

 cause, is a slow undulation involving a rising and sinking of the 

 aeroplane as well as a pitching^ motion. This undulation is stable for 

 high aeroplane speeds since it is rapidly damped out. At lower 

 speeds, the undulation is less heavily damped imtil at a certain critical 

 low speed the damping vanishes. For speeds below this critical 

 speed, the undulations tend to increase in amplitude with each swing 

 and the longitudinal motion is, therefore, unstable. 



Similar calculations for the Curtiss aeroplane show a similar 

 critical speed below which the longitudinal motion is unstable. It is 

 believed that the existence of instability at low speeds has not been 

 indicated before, and it is hoped that the recommendations made to 

 reduce the critical speed may be of assistance to designers. 



It appears a simple matter to secure any desired degree of longi- 

 tudinal stability by the use of properly inclined tail surface, and by 

 the use of light wing loading. It is pointed out that excessive statical 

 stability, as indicated by strong restoring moments, is vmdesirable 

 and may cause the motion to become violent in gusty air. This vio- 



