412 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



From about 1890 to 1900, S. P. Langley, then Secretary of the 

 Smithsonian Institution, attempted to determine the flight character- 

 istics of buzzards in the neighborhood of Washington, D.C., by 

 photographing the birds with two telephoto cameras, arranged stereo- 

 scopically. Such a technique would certainly have determined the 

 geometry of the bird while the bird was airborne, but it would not 

 have determined the energy losses, unless a time-lapse method had 

 been used, together with triangulation by double theodolite methods. 



In view of the difficulty of studying the flight of wild birds from 

 the ground, George Carter and I, in 1945, started an experiment in 

 which a young wild buzzard was to be trained to carry a small record- 

 ing barograph and anemograph attached to its belly. The bird was 

 trained successfully to do its job and to carry a mockup of the record- 

 ing instrument, which was to weigh 30 grams and have dimensions of 

 2 by 3 by 5 centimeters. However, before the actual measurements 

 could be made, the bird died of an intestinal stoppage. Our success 

 in training this bird was due to the skill and understanding of Greorge 

 Carter. 



Had this experiment been successful, it would have yielded the 

 sinking speed as a function of airspeed — that is, the speed polar, 

 similar to that in figure 1. But for a soaring land bird we would 

 need to observe the mode in which the bird is flying in order to de- 

 lineate the function of the variable geometry of the slotted wing tip. 



Furthermore, the success achieved in training this one bird by 

 Carter clearly supports liis contention that it would be possible to 

 train live birds to fly in a wind timnel whose axis could be inclined 

 to the horizon. Thus, one could force the bird to fly at different 

 gliding angles and at different air speeds, simply by inclining the 

 tunnel and varying the air speed so that the bird would remain 

 motionless in the throat of the tunnel. With this method, one could 

 delineate the function of the slotted wingtip as well as derive drag 

 polars for various changes in geometry which the bird would be 

 compelled to make in order to stay in the tunnel. 



COMPARISON-FLIGHT STUDIES 



Since the technique of using trained birds was so dependent on 

 the training of the birds and so time consuming, the comparison 

 method of flying with birds in a sailplane was developed in 1949 

 [6] as a refinement of the simple one-point comparison test made on 

 the laughing gull, as represented in figure 1. 



In the comparison method for determining the speed polar and con- 

 sequently the drag polar of a bird in free and natural flight, a sail- 

 plane of low sinking speed and low forward speed capability is 

 needed. In addition, the sailplane must be highly maneuverable, since 

 the pilot must follow birds that can turn with extreme rapidity. 



