ENGINEERING: J. C. HUNSAKER 
283 
Aeroplane 
Rise of wings 
Angle of incidence 
Velocity, miles 
S S S U U 
1?63 — — 0 — 
0? 6? 12? 1? 15?5 
76.9 44.6 36.9 78.9 43.6 
Spiral motion 
Damp 50 per cent, seconds 
Double, seconds 
10.4 2.7 
— — 3.3 
7.2 28.0 — 
Dutch roll 
Period, seconds 
Damp, 50 per cent seconds 
Double, seconds 
5.9 10.7 12.0 5.2 5.7 
1.4 1.3 6.0 1.8 — 
— — — — 7.7 
General Conclusions. — It is believed that the majority of modern aero- 
planes are spirally unstable but stable in the Dutch Roll. Furthermore 
it appears to be a simple matter so to adjust surfaces that any aeroplane 
can be made completely stable without sacrifice in speed or climb. At 
extreme low speed an aeroplane must be unstable in its longitudinal 
motion but need not be unstable laterally. 
The degree of stabiKty to provide in a given case cannot be determined 
from mechanical considerations alone. For example, the comfort of the 
pilot must be a first consideration and for this reason the righting mo- 
ments giving statical stability should be small; the period of the aero- 
plane can then be made relatively slow, and if the damping is adequate, 
the free oscillations will be stable. 
The theory is applied here only to flight in still air. Obviously the air 
is never still, and the aeroplane must finally be judged from its behavior 
in gusts. An inherently stable aeroplane tends to preserve its normal atti- 
tude with relation to the relative wind, and if the velocity and direction of 
the relative wind change in an irregular manner, the stable aeroplane 
will tend to follow. The result will be to force on the aeroplane a mo- 
tion which will be more violent the greater the statical stability. Conse- 
quently in rough air an aeroplane very stable statically is unsuitable as a 
gun platform and for many other military purposes. 
Considerations of theory indicate that a slight degree of statical sta- 
bility combined with the maximum of damping give an aeroplane slow 
periods of oscillation and a dynamically stable motion, with little ill ef- 
fect upon performance or controllabiHty.'^ 
^ G. H. Bryan, Stability in Aviation, Macmillan, 1910. 
2L. Bairstow, Technical Report of the Advisory Committee for Aeronautics, London, 1912-13. 
^ A description of the wind tunnel and the results of some experiments therein may be 
found in Smithsonian Inst., Misc. Coll., 62, No. 4, 1-92 (1916). 
4 Full details of this investigation will be offered for publication in a forthcoming number 
of Smithsonian Misc. Coll. 
