72 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 62 



The banking moment is appreciable on the aeroplane, as may be 

 shown by the following calculation : 



Incidence 6 degrees, yaw 15 degrees, sweep 10 degrees, moment 

 Lj on model .18 pound-inch at 29.85 M. P. H., moment on biplane of 

 400 square foot area L. at 60 M. P. H. 



The greatest value of swept back wings is to be found on a side 

 slip. If by any accidental cause an aeroplane with swept back wings 

 is heeled over, it begins to side slip toward the low side. The appa- 

 rent wind is no longer from dead ahead, but is a little to one side as 

 compounded from the velocity of side slip and velocity of advance. 

 The effect is the same as if the machine had yawed from its course, 

 and with swept back wings the " natural banking " moment becomes 

 a much greater righting moment than with straight wings. The 

 machine then has a degree of lateral stability. 



For lateral stability, swept back wings can be made to give a 

 righting couple without introducing a lateral force or yawing couple. 

 The absence of a yawing couple and lateral force is of advantage if 

 the machine is to be kept on its course as it rolls. 



Naturally, it is purely a matter of judgment whether too large a 

 righting couple is disadvantageous. In a gusty wind, a machine with 

 swept back wings will tend to roll as side gusts strike it. This may 

 be uncomfortable for the pilot, but if his aileron control be power- 

 ful, he can always overcome the rolling moment due to the wings. 

 Approaching a landing, this is especially necessary. In the air, it 

 would be both unnecessary and fatiguing for him to fight the natural 

 rolling" of his machine. 



A " natural banking '' moment can be obtained by the use of a 

 vertical fin above the center of gravity, or by giving the wings an 

 upward dihedral angle. The equivalence of these methods has not 

 yet been determined. 



These tests bring out simply the fact that with a sweep back of 

 10 degrees an appreciable righting moment may be expected without 

 change in any of the other aerodynamical properties of the straight 

 wing. 



No reference is made here to the " rotary derivatives " or changes 

 in forces and moments produced by angular velocity. The damp- 



