SAPETY DEVICES IN BIRDS ' WINGS — GRAHAM 289 



the place of the elevators of a normal tail when they are moved in 

 conjunction. 



No doubt nature, having feathers to work with in place of the 

 sheets of metal or fabric which we use, finds it more economical 

 to employ a number of small surfaces for controlling than a single 

 large one, such as the controller of the Pterodactyl; but it is just 

 possible that investigation of the matter might reveal something of 

 use to aircraft designers. A comparison between Figures 24 and 6 

 is illuminating in this respect. 



Figure 25 illustrates another way of looking at this antistalling 

 effect of the multislot wing tip. It should be considered in con- 



* *'" 



^_ J^t r-ectton. o/ 





< — 



Figure 25. — Section showing tbe probable flow of the air 

 stream through tlie wing-tip of the eagle in Figure 1 " 



nection with Figure 1, as it is meant to be a diagrammatic sketch 

 of a section taken through the separated wing tip feathers of the 

 left wing of the eagle ^ shown in Figure 1. The dotted arrow^s rep- 

 resent the probable flow of the air stream. They are drawn by 

 guessing, in the light of our present knowledge of the behavior of 

 air, at the way in which one would expect the air stream to behave 

 on meeting such an obstacle as this slotted wing-tip. 



Working backwards from the first feather, each blade in turn 

 deflects the air stream in a downward direction, so that the one 

 behind it does not have to twist through such a large angle to set 



" See footnote 2 on page 272. 



^ Some years ago Mr. Handley Page, without Ijnowing that this type of slot was to be 

 found in birds' wings, designed and tested a model, having 7 slots, arranged in much the 

 same way. He found that it increased the maximum lift by 250 per cent at an angle 

 of 42°, as compared with the unslotted wing. 



