272 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 193 2 



downward. Secondly, it should be remembered that a bird seldom, 

 if ever, beats the air downward, even in flapping flight. Instead, 

 he makes his wings slice through the air and deflects it downwards, 

 thereby obtaining an upward reaction. 



Really, a wing acts on air in much the same way as a plough 

 deals with earth; a curious simile perhaps, but true in so far as it 

 cuts a furrow and piles up the displaced material on one side — of a 

 wing, the under side. 



The air which is displaced beneath a wing accounts for approxi- 

 matelj'' one-third of the total force derived. The remaining two- 

 thirds are generated on the top of the wing in a way that is not 

 quite as simple. Taking again the simile of the plough : the furrow 

 it cuts is, practically speaking, filled with air as soon as it is made; 

 but the furrow cut by a bird's wing is made at such a speed that the 

 air is unable to fill it immediately. There is nothing else to do the 

 job, because the only other possible object, the wing itself, is being 

 prevented from moving upward by the muscles that control it. So 

 the furrow remains as a partial vacuum (for the air does manage 

 partly to fill it) and follows the wing wherever it goes, so long as the 

 speed and the angle of incidence are suitable. If the air were suf- 

 ficiently fluid to fill the furrow immediately it formed, there would 

 be no suction remaining to exert an upward force on the wing, so 

 it is really the slowness of the air in moving down that is responsible 

 for the force derived on the top of a wing. 



The combined force, one-third from below and two-thirds above, 

 is known as the total resultant force. It has been found to act at 

 about 90° to the surface of the blade of a normal wing; therefore, 

 by setting his wing at any particular angle, a bird can make it pro- 

 duce a reaction in whatever direction he requires, provided always 

 that the air speed and the incidence are suitable. It is by holding 

 the wing against this combined reaction of the air that a bird can 

 defy gravity and, by suitable inclination of the surface, obtain 

 forward movement. 



Figure 2 shows roughly the lines which the two streams of air 

 follow as they pass the wing. ^Vliere the lines are close together 

 the air is under pressure, and where they open out there is tension 

 between the air and the wing ^ ; in other words, the pressure is less 

 than that of the atmosphere. Observe that the flow of the upper 

 stream is quite smooth, and that it flows at high speed close over 

 the trailing edge of the wing. 



When a bird wishes to glide more slowly, he must make his wings 

 cut a deeper furrow in order to make up in quantity of air displaced 



^ Though it is technically inaccurate to consider the air pressures in this way, it is the 

 simplest means of getting a clear view of what happens. 



