Body drag, feather drag and interference drag of the mounting strut in a peregrine falcon, Falco Peregrinus

Abstract

ABSTRACT The mean, minimum drag coefficients (CD,B) of a frozen, wingless peregrine falcon body and a smooth-surfaced model of the body were 0.24 and 0.14, respectively, at air speeds between 10.0 and 14.5 ms−1. These values were measured with a drag balance in a wind tunnel, and use the maximum crosssectional area of the body as a reference area. The difference between the values indicates the effect of the feathers on body drag. Both values for CD,B are lower than those predicted from most other studies of avian body drag, which yield estimates of CD,B up to 0.41. Several factors must be controlled to measure minimum drag on a frozen body. These include the condition of the feathers, the angle of the head and tail relative to the direction of air flow, and the interference drag generated by the drag balance and the strut on which the body is mounted. This study describes techniques for measuring the interference drag generated by (a) the drag balance and mounting strut together and (b) the mounting strut alone. Corrections for interference drag may reduce the apparent body drag by more than 20 %. A gliding Harris’ hawk (Parabuteo unicinctus), which has a body similar to that of the falcon in size and proportions, has an estimated body drag coefficient of 0.18. This value can be used to compute the profile drag coefficients of Harris’ hawk wings when combined with data for this species in the adjoining paper (Tucker and Heine, 1990).