YU, B*; CLARK, C.J.; DUDLEY, R; Univ. of California, Berkeley; Univ. of California, Berkeley; Univ. of California, Berkeley: Using a Physical Model to Study the Aerodynamic Cost of Hummingbird Tails
Hummingbird tails exhibit remarkable diversity in morphology and size. Approximately one fifth of the hummingbird species possess tails that measure longer than their bodies. These exaggeratedly long tails are important in sexual selection, but their aerodynamic roles remain to be investigated. In aerodynamic cost studies, drag is a common indicator employed. The parasite drag of a flying animal, generated by the body including the tail but excluding the wings, is the component of drag singly most affected by the tail. The hypothesis is that longer tails will incur greater drag during flight because of the increased fluttering amplitude of the tail. In previous studies, we measured the drag on a live Anna�s hummingbird (Calypte anna). However, those results showed fluctuations due to the birds� continuously turning their heads and altering their body orientations in the wind. To get around this problem, we employ a torpedo shaped model that is dynamically similar to the bird’s body. This also allows us to easily manipulate the length and stiffness of the tails. In the setup, a force transducer is mounted in the wind tunnel which controls the wind speed. The model is clamped onto a beam extending from the transducer, which registers the drag. The total drag experienced at various wind speeds are calculated from the parasite drag combined with estimations of the profile drag and induced drag. Our experimental treatments include the attachment of tails ranging from 3cm to 15cm, and steel rods of different stiffness are glued along the 15cm tail to reduce the fluttering amplitude. Results to date show that across the speeds, drag increases with tail length when tails longer than 6cm are attached. In particular, at 12m/s, with 15cm tails attached, there is an 18% increase in parasite drag, which corresponds with the 22% increase measured on live hummingbirds.
