44 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 62 



The following table facilitates comparison : 



Stable High Speed Unstable Low Speed 



Curtiss Clark Curtiss Clark 



1 1° 0° 14° 12° 



Xu — .128 - .158 — .223 — .162 



X,o + .162 + .356 - .132 O 



Zu - -557 - -57 - -993 - i • 19 



Z,v - 3-95 - 5-62 - .555 - i.o 



Mw + 1-74 + Z-^ + 1-99 + 1-41 



Mq —150.0 —192.0 —108.0 —60.5 



Kl 340 21.6 34.0 21.6 



A-^ 34.0 21.6 34.0 21.6 



B^ 289.0 317.0 134.0 85.1 



Ci 834-0 1492.0 213.0 150.0 



D^ 1 15.0 266.0 28.0 22.1 



£, 31-2 59.2 63.6 54.0 



Routh's discr. 18x10'' 117x10'' —.37x10" -.12x10''' 



P sec 34.0 34.7 II. 5 10.6 



f sec 1 1.0 ^ 8.1 — 24.7 —24.7 



U, ft.-sec... -II5-5 -112. 5 - 64.8 -54-0 



We may infer in general that : 



1. Any ordinary aeroplane is likely to be unstable longitudinally 

 below a certain critical speed. 



2. Stability is improved by large wing area, i. e., light load per 

 square foot. 



3. Stability is improved by large horizontal tail surfaces. 



4. Stability is improved by high speed. 



5. Stability is improved by great head resistance or a poor lift drift 

 ratio. 



6. Stability is improved by a small longitudinal moment of inertia. 



7. Stability is improved by wings with slight center of pressure 

 motion.^ 



There appears to be no reason to depart from the normal type of 

 aeroplane in a search for longitudinal stability. A steady motion in 

 flight is to be obtained by careful adjustment of surfaces in the ordi- 

 nary type aeroplane, and the invention of freak types to accomplish 

 great stability at the expense of speed or climb is to be discouraged. 



Furthermore, the ordinary type of aeroplane may be made dy- 

 namically stable longitudinally without material sacrifice of desirable 



^ For a biplane combination giving a stationary center of pressure without 

 material loss in other desirable features, see " Stable Biplane Arrangements," 

 by J. C. Hunsaker, Engineering, London, Jan. 7 and 14, 1916. 



