Flight control mechanisms in birds of prey

Here we present the first measurements of 3D acceleration and turn rate obtained from a bird in wide-ranging free flight. These data on body kinematics are accompanied by simultaneous onboard video sequences of the head and tail kinematics, and by video of the bird taken from the ground. The bird, a Steppe eagle Aquila nipalensis, carried a miniature inertial measurement unit outputting data on body orientation, acceleration and rate of turn, together with a data logger and two wireless video cameras pointing forward over the head and aft over the tail. The complete instrumentation package and harness weighed <0.25kg, which is <10% of the bird's body mass. Measurements were made in soaring flight over coastal cliffs under windy conditions (up to 32 knots). We describe the body kinematics during two sequences of maneuver involving a banked turn, a wing-tuck maneuver, a sharply banked pull-up maneuver, and a sharp gust response. The total load experienced during these maneuvers ranged from 0 to 2.5g. Turn rates were typically ±60°s-1 in any axis, but were often much higher. We use the onboard video to identify tail and head kinematics, and use these to analyze the role of head and tail movements in flight control. The angle and spread of the tail is continually adjusted in soaring flight, with tail bank angle changing changing through up to 30° in just a few seconds of flight. The fast response times of the tail relative to the motions of the body may indicate a role in active gust response. Head movements are used to stabilize gaze during deliberate turns, in a manner analogous to the nystagmic eye movements of humans. Implications of these results for the design of small autonomous air vehicles are briefly discussed.