4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 62 



The Curtiss aeroplane shows the same sort of spiral instability at 

 high speeds. This aeroplane had no dihedral angle of wings and 

 had a large rudder and deep body. 



The second type of motion has been called a "Dutch roll" from 

 analogy to a figure in ice skating. The aeroplane takes up an oscilla- 

 tion in yaw and roll simultaneously. It swings to the right banking 

 for a right turn, then swings back to the left banking for a left turn. 

 The combined yaw and roll has a fairly rapid period. The Clark 

 model at all speeds shows that this motion is heavily damped and 

 hence stable. At high speed, the period is 6 seconds and an initial 

 amplitude is damped to half value in less than 2 seconds. At low 

 speed the period is 12 seconds, damped to half amplitude in 6 seconds. 



It appears from an approximate calculation that the " Dutch roll " 

 may become unstable if an aeroplane has too much high fin surface 

 and if there be not sufificient " weather helm " or rear fin surface. It 

 is seen that these conditions are the reverse of those for spiral insta- 

 bility. The conflicting nature of the requirements for stability in 

 these two kinds of motion suggests that an aeroplane is unlikely ever 

 to be unstable in each sense. It also indicates the difficulty of obtain- 

 ing lateral stability by raised wing tips. 



In confirmation of theory, we found the Curtiss spirally unstable 

 at high speed and stable in the " Dutch roll," while at low speed the 

 spiral motion was stable and the " Dutch roll " unstable. The period 

 was 6 seconds and an initial amplitude doubled itself in 8 seconds. 



It is believed that the majority of modern aeroplanes of ordinary 

 type are spirally unstable because of excess of fin surface aft. When 

 attempts have been made to remedy this fault by use of a large 

 dihedral angle upwards for the wings, matters have been made 

 worse. It is only to be expected that in overcorrecting spiral in- 

 stability a " Dutch roll " of more or less violence may be introduced. 

 Especially in gusty air would one expect high fin surface to produce 

 violent rolling. 



The Clark aeroplane has very slight rise of wings, about i°6, 

 and a small rudder. It is shown that at ordinary speeds this aero- 

 plane is stable in every sense, both longitudinally and laterally. 

 Whether this stability is excessive can only be determined by actual 

 flight experience in turbulent air. However, it has appeared possible 

 to secure a degree of stability in every sense in an aeroplane of con- 

 ventional type. 



The object of the research has been to show how various features 

 of design may afifect the motion of the aeroplane and only incidentally 



