Measuring Aerodynamic Interference Drag Between a Bird Body and the Mounting Strut of a Drag Balance

ABSTRACT The drag of a bird body mounted on the strut of a drag balance in a wind tunnel is more than the sum of the drags of the isolated strut and the isolated body. The strut changes the air flow around the body and generates additional drag, known as interference drag. This paper describes practical methods for measuring the drag of bird bodies: a strain-gauge drag balance, dimensions for struts made with machine or hand tools, and a procedure for correcting drag measurements for interference drag. Interference drag can be measured by extrapolating a relationship between the drag of isolated struts with different cross-sectional sizes and shapes and the drag of a body mounted on those struts. The interference length -the length of an isolated strut that produces drag equal to the interference drag -is a useful quantity for predicting interference drag. The relationship mentioned above is a straight line for a model peregrine falcon (Falco peregrinus L.) body mounted on smooth struts -struts with convex cross-sectional shapes ranging from streamlined to circular. This finding simplifies the determination of interference drag in three ways: (i) the line can be found from measurements with just two struts -a standard strut with low drag and a calibration strut with high drag; (ii) the two struts need not have the same shape -for example, the standard strut can be changed to a calibration strut by attaching a spoiler without disturbing the body mounted on the strut -and (iii) a single value of interference length (33.1mm) describes smooth struts with a range of shapes and sizes. These struts had drag coefficients between 0.33 and 0.91 at Reynolds numbers between 2100 and 10800. The interference length of a strut supporting the actual falcon body with a feathered surface is not significantly different from that of the strut supporting the model body with a rigid surface. As a hypothesis, interference length (h1, in metres) of a smooth strut varies with the size of the body mounted on it: where m is the body mass (in kg) of the intact bird. 95 % of the interference drag appears to arise from a 15 mm length of the strut nearest the body.