SAFETY DEVICES IN BIRDS ' WINGS — GRAHAM 297 



During this down beat, the wings, having their bones much nearer 

 the leading than the trailing edges, will automatically tend to turn 

 their blades into line with the air stream; so all that a bird has to 

 do to apply the 10° of incidence is to prevent his wings turning any 

 further when they have reached that incidence. 



So much for the down beat. With regard to the upstroke, it is 

 only necessary to say here that, as a rule, no lifting or driving force 

 is produced; instead, the wings are relaxed and allowed to stream- 

 line themselves so that they offer the minimum of resistance to the 

 downward and backward gradient air stream which they must 

 encounter while moving up. The subject of the detailed working 

 of wings in different phases and forms of upstrokes is such a tre- 



I 



Figure 33. — The action of a section of wing iu the down beat 

 OB, the path of the section through the air ; CB and AB, the down- 

 ward and forward movement of the section while it is travelling from 

 O to B ; R, the direction in which the resultant force acts. 



inendous one that it could, like the question of the down beat, be 

 made to fill a book by itself. 



The action of the slot-forming feathers in the upstroke appears 

 simply to be to join in with the others in effacing themselves as much 

 as possible. 



During a single down beat in straightforward flapping flight, all 

 points on a wing move forward about the same distance, but the 

 distance they move down varies a great deal, from approximately 

 nothing at the shoulder to a maximum at the tip. Therefore the 

 wing tip encounters a much steeper gradient air stream than the 

 wing root, and to get the required incidence along the whole span, 

 the wing itself must be twisted like the blade of a propeller. It 

 seems probable that no incidence is given to the wing root and only 

 a small amount at points inside the wrist; otherwise the reaction at 

 those points would be directed backwards from the vertical — the 



