THE AERIAL ENVIRONMENT 



balance, and that this initiates the various compensatory movements." The 

 whole evolution of the bird, of course, has been toward aerial activity. In 

 this he has certain important advantages over the human pilot, whose her- 

 itage is that of man as a terrestrial being. 



Once the eyes of the human pilot lose sight of the earth, the horizon, and 

 the sun, he can no longer hold his aircraft on a straight and level course. 

 In a cloud the pilot without instruments quickly becomes disoriented with 

 respect to the earth. Believing he is holding a level course, it is typical of 

 him unwittingly to let his craft move into a spiral turn. 



Flight actions are inherent in the bird; man must learn to fly. More- 

 over, tie bird is the flight structure. A man airborne operates by remote 

 control the complex machine in which he sits but of which he is not a part. 

 In bird and aircraft, pressures produce the same forces and stresses upon 

 the flight structure. In the bird, however, every feather springs from a nerve 

 ending, and each flight feather is subjected to varying pressures. Hence the 

 most minute changes in balance create stresses that the bird must per- 

 ceive much more intimately than the human pilot who is in touch with his 

 plane only through the stick in his hands, the rudder pedals at his feet and 

 the "seat of his pants." 



The human pilot is committed to operate his airplane as it was con- 

 ceived by its designer, while a bird may alter its flight structure in a flash 

 as occasion demands. For instance, in both plane and bird stability is closely 

 related to the dihedral angle* of the wing; as this angle is raised, stability is 

 increased. The dihedral is an unalterable feature of the aircraft's design 

 over which the pilot has no control; but the Mallard descending to a 

 wooded pond, or a blindfolded sparrow, may alter wing angle at will. More- 

 over, modern aircraft, although they may be stable in the vertical, longi- 

 tudinal, and lateral axes, are normally designed to be spirally unstable. Such 

 spiral instability is planned for the comfort of pilot and passenger ;f but 

 left to itself, such a plane will eventually fly in an ever tightening spiral 

 with a resultant loss in altitude. Thus a human pilot, when confronted with 

 the problem of flying without reference to the horizon ( and lacking instru- 

 ments), must overcome severe handicaps that do not obtain with birds, and 

 it is not entirely fair to make direct comparisons between a blindfolded bird 



• The angle at which each side of a wing inclines upward or downward from the hori- 

 zontal. 



t The reason that airplanes are not designed to be spirally stable is that such aircraft are 

 very uncomfortable for the passenger, developing lateral oscillations in rough air which the 

 human stomach is incapable of tolerating (G. S. Trimble, Jr.; letter). 



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