NO. 5 STABILITY OF AEROl'LAXES lU'XSAKER AXD OTHERS 5I 



If the aeroplane yaw to the rit;ht, it is practically slartini^' off on a 

 turn to the rii^ht. As is well known, to make such a turn safely an 

 aeroplane should be "banked" to such an angle of roll that the 

 centrifugal force, acting to the left, is about balanced by the hori- 

 zontal component of the normal force Z acting to the right. In other 

 words, the bank proper to a right turn requires a positive angle of roll 

 <f> given by a positive rolling moment L. The curves of L in the figure 

 show that for this aeroplane the natural rolling or banking moments 

 are positive for a positive yaw, and hence tend to bank the aeroplane 

 suitably for the turn. This property is extremely valuable in prevent- 

 ing capsizing. 



As in the case of the yawing moments, an excessive amount of 

 natural banking may be uncomfortable, especially in gusty air. Thus, 

 if the wind shifts to the left, the relative angle of yaw is positive, the 

 aeroplane tends to turn to the left due to its " directional " stability 

 and to bank for a turn to the right due to the natural banking or 

 rolling moment L. The result may be to throw the aeroplane about in 

 a somewhat violent manner, or it may capsize. This motion is dis- 

 cussed later under the heading " Dutch roll." 



Large banking" moments L can be given by vertical fin surface 

 above the center of gravity, by a dihedral angle upwards or a 

 " retreat " or sweep back of the wjngs. All these arrangements are 

 probably equivalent and, though tending to give a stable motion in 

 still air, tend toward violence in gusty air. 



The model under test has, as is shown l)y the drawings, a dihedral 

 angle upwards of the wings made by raising each wing tip i.6°. This 

 amount of dihedral has been found in practice to be not excessive on 

 ordinary aeroplanes. 



The curves of lateral force )' are negative for a positive yaw. This 

 means that if the aeroplane yaws to the right in still air, it is pushed 

 to the right and started off on a right turn. We saw above that the 

 natural banking is suitable for the turn. In gusty air, if the apparent 

 wind shifts io° to the left the lateral force pushes the aeroplane to 

 the right. 



Numerical values are interesting. Suppose a plus yaw of io° in 

 still air. The rolling moment at high speed is 2,000 pounds-feet. 

 This is equivalent to a down load of 55 pounds on the right aileron 

 and an up load of 55 pounds on the left aileron. The pilot with his 

 aileron control can, if he wish, produce a rolling moment over three 

 times this magnitude, so that he can prevent the aeroplane taking 

 charge and hold it level. Approaching a landing, it is most important 



