416 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



Nevertheless, there cannot but be some doubt about the validity of 

 data taken in turbulent air. For this reason, making a measurement 

 during the early hours of the morning when the air is very still sug- 

 gests itself. For this test, several wild captured buzzards would be 

 carried aloft in a two-seater sailplane. On tow, behind the same 

 towplane, would be the measuring sailplane, of light, maneuverable 

 design, fitted with radio communication equipment. After the two 

 sailplanes reach an altitude of 1,500 meters, they will be released 

 from the towplane, which will descend to the airport. The bird- 

 carrying two-seater will move ahead of the measuring sailplane, 

 headed toward the airport where the birds have been cooped. On a 

 signal from the measuring sailplane, the bird handler will release a 

 bird from the two-seater by dropping it out in an open-ended bag, 

 to which is attached a line. At the end of the line, the bird will fall 

 out of the bag, head first, and will start gliding toward its coop. 

 "Whether every bird will cooperate in this manner is yet to be deter- 

 mined. However, if the birds merely glide in any direction, useful 

 data can be obtained, for the measuring sailplane is capable of land- 

 ing in any small field and can be disassembled for return to its base by 

 trailer [6a]. 



During the glide of the bird, the measuring sailplane will record 

 data in the manner ascribed for the comparison-flight method. 



The precision of this method should be much greater, for, in this 

 case, both the bird and the sailplane will be flying in smooth air, that 

 in which the sailplane has been calibrated. 



The results of these measurements in still air should either confirm 

 the measurements given in figure 5 or perhaps, under certain flight 

 conditions, especially at the lower speeds with the bird's slotted wing- 

 tips open, reveal a disparity. If the difference is significant and if 

 the sinking speed measurements made in turbulent air are lower than 

 those made in still air, then we must look to the mechanism of dynamic 

 soaring for an explanation. As a matter of fact, the investigation of 

 the nature of this energy extraction will yield valuable information 

 on the little-known science of dynamic soaring, of which some aspects 

 are discussed below. 



BraO AND AIRPLANE 



Up to now, all our comparisons of bird aerodynamics have been 

 "within the family." The question naturally arises. How good is the 

 bird compared with modern aircraft? Obviously, trying to compare 

 a bird cruising at 30 to 60 miles per hour with a supersonic airplane 

 would be absurd. Even if we compare the bird with some of our 

 subsonic airplanes, we still have the problem of scale and speed differ- 

 ences. Fortunately, we can rely on the well-known Keynolds number 



