BIOPHYSICS OF BIRD FLIGHT — RASPET 411 



In some birds, in addition to the usual features of tlie feathers — 

 flexibility, mobility, and porosity — there is also a toothed leading 

 edge. This is true particularly in owls, which must fly silently and 

 stealthily upon their prey in the field. Graham [5] believes that this 

 toothed leading edge reduces the velocity of the flow over the wing. 

 This may be so, but if there is too much loss of velocity in the flow 

 near the leading edge, a wing with a toothed leading edge will not 

 develop as high a lift as one without this edge. This means that the 

 bird with a toothed leading edge to its wing would have to fly faster 

 than one with a smooth-edged wing. Thus, the noise would not be 

 essentially reduced. Yet the owl does fly silently. 



Perhaps we might speculate on the function of the toothed leading 

 edge by drawing on an analogy. If a piece of wire of cylindrical 

 form about 3 millimeters in diameter and 1 meter long is swung 

 through the air in a rotating motion similar to that of a propeller, a 

 distinct tone similar to that of a singing telephone wire is emitted. 

 Now, if instead of a single cylindrical wire, two wires of 1.5-millimeter 

 diameter are twisted together in a tight spiral and then spun, the noise 

 level is much lower in intensity and in frequency. In fact, only the 

 free end emits a noise. 



From this experiment we might say that the toothed leading edge 

 behaves in the same way that the twisted wire does — that is, in a man- 

 ner to reduce the vortex noise emitted by the flow leaving the wing. 

 However, remember that this is merely a hypothesis and not absolute 

 proof of the function of the toothed leading edge of the wings of owls. 



Since the bird possesses little or no inherent stability in pitch, the 

 question of the function of the tail arises. In general, the tail is used 

 as a landing aid similar to the flap on an airplane. Photographs show 

 clearly that the tail of most birds fans out to increase the lifting area 

 just before touchdown and is folded during gliding flight. 



At the same time, during the landing, it will be seen that the alula 

 or false feather, representing the thumb of our hand, opens in order 

 to increase the lifting power of the wing. This same alula is used 

 as a lateral control for initiating rapid turns. 



The reader may wish to try a simple experiment which illustrates 

 the function of the alula. If, while driving at about 50 miles per 

 hour, one puts his hand out of a car window with the hand cupped 

 slightly and at a positive angle to the wind, he can, by simply moving 

 his thumb up or down, cause a large change in the lifting force his 

 arm experiences. Tliis is how the bird applies control in roll about 

 his longitudinal body axis. 



Having seen, in figure 1, that wind-tunnel tests of bird flight are 

 fraught with possible large errors, we are forced to look for new means 

 of determining the aerodynamic properties of bird flight. 



