STABILITY, 25 



body rises and falls, its centre of gravity travels 

 onward almost in a straight line.* 



Points that make for Automatic Stability. 

 Though the position of the centre of gravity is 

 of no avail, the general build of the bird and the 

 elasticity of the feathers make for automatic stabil- 

 ity. To begin with, there is the question of curves. I 

 have already pointed out that a curved surface gives 

 more lift and less drift than a flat one. But anyone 

 who experiments with gliders soon finds out that 

 an excessive curve is fatal to stability. In the 

 preface to the Aero Manual (1910) it is stated that 

 the curve (the depth of the concavity) should not 

 be more than one-twelfth of the breadth. Imagine 

 a surface with a more considerable curve. The wind 



W — — ^ 



Fig. 11. 



Diagram to show the disadvantage of an excessive curve. 



will impinge upon its upper side and the glider will 

 duck and descend rapidly to earth. But if the curve 

 is only slight, though the tendency to duck and dive 

 may, no doubt, arise, yet it tends to correct itself. 

 Imagine the glider launched on its way and develop- 

 ing a tendency to lower its head and raise its tail 

 unduly. In proportion as the tendency develops, in 



* There is no need to discuss the question of pendulum stability. 

 It is possible to suspend a heavy weight from an aeroplane, and 

 there are theorists who hold that automatic stability may be thus 

 attained. It is obvious that in the case of a bird nothing of the 

 nature of a pendulum is possible. See Flight, Feb. 25, March 18, 

 and March 25, 1911. 



