S1-4.8 Jan. 5 Slow pigeon flight shows a compromise between aerodynamic and inertial power minimisation USHERWOOD, J.R.; The Royal Veterinary College email@example.com
When most birds fly slowly or hover, they do not use a horizontal stroke plane: the analogy of a hovering helicopter is limited. Pigeons, like many birds but unlike hawkmoths and hummingbirds, are capable of hovering for only brief periods, presumably achieving this anaerobically. This behaviour may therefore be considered, to a large extent, power limited, and the kinematic strategy adopted would be expected to minimise the required muscle power. I describe a new experimental set-up with which to measure the lift-drag polars of revolving bird wings, and show that, from purely aerodynamic considerations, the wings 'should' operate at a near-horizontal stroke plane, with low angles of attack and high wingbeat frequency. However, hovering pigeons use high aerodynamic angles of attack and high stroke-plane angles: they do not minimise aerodynamic power. A second power requirement for flapping wings is the inertial power - that associated with repeatedly accelerating massive wings. This power is minimised with low wingbeat frequency, and so is in conflict with the requirements of aerodynamic power minimisation. I suggest that understanding the kinematics of slow bird flight is impossible using aerodynamic considerations alone, and requires an appreciation of the balance between aerodynamic and inertial powers. Further, I consider the possibility of using the relatively easily measured inertial power as an indicator of aerodynamic and total power for slow flight.