SAFETY DEVICES IN BIEDS WINGS — GRAHAM 



293 



All this air that is moving sideways and upwards constitutes a 

 waste of energy, because the only way a Aving can obtain lift is by 

 causing air to move " downwards." The broader a wing tip is, the 

 greater will be the amount of air that thus tends to circulate around 

 it, and the less efficient the wing will be. 



In Figure 29 the rectangular shapes A and B represent two wings 

 of equal area, but A is three times as long as B, and therefore one- 

 third of its breadth. Suppose that a particle of air strikes the lead- 

 ing edge of wing A at point X. It endeavors to escape sideways from 

 the pressure, but fails to do so before reaching the trailing edge at Y. 



V. 



J< z 



A 



B 



Q 



^ 



FiGDRB 29 



}r,jkt 



That means that the wing has got full lifting value out of it ; but any 

 particles that strike the leading edge outside point X will make good 

 their escape without completing their job, so we can suppose that the 

 area affected by wing-tip air spill is the triangle XYZ. 



In wing B we might reasonably expect this area to be far larger 

 (the triangle RQP) with a correspondingly greater loss; but if the 

 tip is split up into a number of narrow winglets (keeping the total 

 area the same), as in wing C, the affected area will consist only of the 

 sum of the little shaded triangles in wing C, and that is a good deal 

 smaller than RQP. That is what nature appears to have done to the 

 short, broad wings of birds that can not afford to have long, narrow 

 ones. The actual result is that circulation of air from the lower to 

 the upper surface of the wing-tip is reduced. 



