TRAVELS OF WATERFOWL 



and a blind-flying pilot in a standard aircraft. Aerodynamically it is possible 

 to design an airplane spirally stable with high wing dihedral which, like the 

 blindfolded bird, will stay right side up relative to the earth for an indef- 

 inite period of flight. By such design, free-flying model aircraft carry suc- 

 cessful flight, quite obviously without sensory control. 



Just as birds, unlike aircraft, can alter their structural design to increase 

 stability, so, too, their flight structure may instantly achieve a reduction in 

 stability — an advantage, J. M. Smith says (1953:69), "provided there is a 

 parallel increase in the efficiency of control. This can be seen by analogy 

 with aeroplanes. Transport aeroplanes are normally designed with a fairly 

 high degree of stability since safety in steady flight is of greater importance 

 than manoeuvrability. In fighter aircraft, however, manoeuvrability is of 

 first importance, and the stability margin is usually reduced to a minimum." 

 Such control, Smith points out, is greatest in the higher orders of birds 

 where sensory awareness of balance may be more important than structural 

 stability. This variation in inherent structural stability and sensory control 

 may be seen when we compare a loon or a grebe with a Mallard or a Duck 

 Hawk. The loon and grebe, like an arrow sped from its bow, are stable only 

 at a certain velocity; but the Mallard and Duck Hawk are able to shift in 

 a flash from transport-like stability to fighter-plane maneuverability. That 

 a bird is able to compensate for changes in flight structure was shown by a 

 blindfolded Yellow-headed Blackbird which flew 400 yards, then made a 

 successfully gende landing even though it had lost all its tail feathers. 



The "thumb," or winglet, of the bird and the leading edge wing slot in 

 some aircraft are structures that increase stability at low speeds, and thus 

 inhibit stalling, "A bird's winglets he closely against the leading edges of 

 its wings in cruising flight when the wings have a low angle of attack. In 

 fact, they form a part of this neatly rounded edge. But they are usually 

 raised when turning, pitching, and landing, and they are often raised when 

 climbing. To describe this action of the winglet, one might almost use 

 Ludington's words describing the action of an airplane's wing slot — 'it 

 opens automatically as the angle of attack passes a certain point' " ( Queeny, 

 1947:106). 



In the observations made at Delta, hooded birds were given their free- 

 dom by being tossed upward into the air. If there was a wind, the bird 

 usually faced into the air stream the moment it began flight, and this initial 

 adjustment was accomplished regardless of the bird's position relative to 

 the wind when it was cast aloft. This orientation was but momentary, how- 



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