SAEETY DEVICES IN BIKDS WINGS — GEAHAM 



273 



for the reduced downward velocity his wings are giving to it.' 

 This he does by increasing their incidence. That is all very well, 

 and it works beautifully, but only up to a certain limiting angle, 

 which, unless it is increased by some special means, is in the region 

 of 15°. (These special means take the form of certain peculiar ar- 

 rangements of the feathers akin to the Handley-Page slotted-wing 

 device. They are of particular interest because they vary very much 

 in different species of birds, and are therefore of great help to 



diTCttion oJ- ^li^Sr 



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FiGDRB 2. — The flow of the air-stream past a wing seen end 

 on, in normal flight 



anyone trying to arrive at an understanding of the differences in 

 their flight.) 



As the limiting angle of incidence is approached, the upper of 

 the two air streams, being deflected more and more sharply down- 

 wards in its effort to fill in the furrow cut by a wing, finds increasing 

 difficulty in turning the corner, until finally, and quite suddenlj'^, it 

 gives up the struggle and instead, just rushes on for a short distance 

 and then turns, and, as it were, follows the wing. Thus the smooth 



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"D.'Tecr.or 



c\ nc.o,, 



Figure 2a. — The flow of air past a wing seen end on, in 

 " stalled " flight 



flow of air over the top of the wing is broken down and the air- 

 stream begins to form into little whirls, a process known as " bur- 

 bling." The burbling is accentuated by the lower air stream, which, 

 being no longer kept in place by the even flow of the upper stream 

 over the trailing edge of the wing, is able to flow up and join in filling 

 up the partial vacuum. (One can get a very good idea of what bur- 

 bling is, by dragging one's hand at an angle through water.) Figure 

 2a shows more or less what would be seen if the air stream were visible. 



' Speed is of the greatest importance, for by halving the speed a bird reduces the 

 value of the force his wings are producing to one-quarter, unless he increases the inci- 

 dence. In the same way he caJi quadruple the amount of force by doubling the air speed. 

 The law in accordance with which this happens is that at such speeds as birds attain the 

 value of the total resultant force varies as the square of the air speed. 



