Wind Tunnel Experiments to Assess the Effect of Back-Mounted Radio Transmitters on Bird Body Drag

Abstract

To whom reprint requests should be addressed. The aerodynamic drag of bird bodies was measured in a wind tunnel, with and without back-mounted dummy radio transmitters. Flight performance estimates indicate that the drag of a large transmitter can cause a substantial reduction of a migrant's range, that is, the distance it can cover in non-stop flight. The drag of the transmitter can be reduced by arranging the components in an elongated shape, so minimizing the frontal area. The addition of a rounded fairing to the front end, and a pointed fairing behind, was found to reduce the drag of the transmitter by about onethird, as compared with an unfaired rectangular box.

Hydrodynamic Aspects of Design and Attachment of A Back-Mounted Device in Penguins

“…Measurements of logger f were compared with those for a device (logger i ) shaped following the suggestions of Obrecht et al (1988) for telemetric devices in flying birds. Because stream velocity varied slightly between series of measurements, comparisons were made by using a square function approximation.…”

“…Because stream velocity varied slightly between series of measurements, comparisons were made by using a square function approximation. We then used the procedure described by Obrecht et al (1988) to determine, for each of the two devices, the streamvelocity-dependent incremental drag ⌬D (N) by subtracting the measurements made without the device from those made with the device. This value was converted into incremental effective flat-plate area (⌬A, m 2 ) using:…”

“…Although the device described here (logger f ) and that built according to Obrecht et al (1988) (logger i ) had the same frontal area and, if placed at the posterior end of the body, did not enlarge the cross-sectional area of the model penguin, logger i increased body drag by 52-71 % at medium speeds (1.5 m s Ϫ1 <v<2.5 m s Ϫ1 ) and by up to 100 % at higher speeds (v=4 m s Ϫ1 ). In contrast, logger f caused increases of only 15-25 % and 41 % respectively (Fig.…”

“…A caudal position, determined here to be favourable in penguins, may not be applicable to flying animals because it will change their centre of gravity (Kenward, 1987). In flying birds, devices should be placed between the shoulder blades to be optimally balanced (see Obrecht et al 1988). In aquatic animals, device mass is considered to be less important (see Wilson et al 1991), but attachment to the caudal part of the body may impede the generation of thrust by animals that use their tail for propulsion (e.g.…”

“…A pioneering study on the reduction of drag caused by devices mounted on the back of flying birds was carried out by Obrecht et al (1988). We have used penguin models in wind tunnel and water flume experiments in an attempt to optimise the shape of a new logger to be attached externally to pygoscelid penguins (Wilson et al 1993).…”