Anna C. Carruthers

Automatic aeroelastic devices in the wings of a steppe eagle Aquila nipalensis.

SUMMARY Here we analyse aeroelastic devices in the wings of a steppe eagle Aquila nipalensis during manoeuvres. Chaotic deflections of the upperwing coverts observed using video cameras carried by the bird (50 frames s–1) indicate trailing-edge separation but attached flow near the leading edge during flapping and gust response, and completely stalled flows upon landing. The underwing coverts deflect automatically along the leading edge at high angle of attack. We use high-speed digital video (500 frames s–1) to analyse these deflections in greater detail during perching sequences indoors and outdoors. Outdoor perching sequences usually follow a stereotyped three-phase sequence comprising a glide, pitch-up manoeuvre and deep stall. During deep stall, the spread-eagled bird has aerodynamics reminiscent of a cross-parachute. Deployment of the underwing coverts is closely phased with wing sweeping during the pitch-up manoeuvre, and is accompanied by alula protraction. Surprisingly, active alula protraction is preceded by passive peeling from its tip. Indoor flights follow a stereotyped flapping perching sequence, with deployment of the underwing coverts closely phased with alula protraction and the end of the downstroke. We propose that the underwing coverts operate as an automatic high-lift device, analogous to a Kruger flap. We suggest that the alula operates as a strake, promoting formation of a leading-edge vortex on the swept hand-wing when the arm-wing is completely stalled, and hypothesise that its active protraction is stimulated by its initial passive deflection. These aeroelastic devices appear to be used for flow control to enhance unsteady manoeuvres, and may also provide sensory feedback.

Aerodynamics of aerofoil sections measured on a free-flying bird:

Abstract Birds are adapted to a wide range of flight conditions, from steady fixed-wing glides to high angle of attack manoeuvres involving unsteady separated flows. They naturally control and exploit the transitional Reynolds number regime of Re≈ 105 that is currently of interest in unmanned air vehicle technologies. This article presents a reconstruction of the inner portion of a wing of an eagle in free flight, during a rapid pitch-up manoeuvre at the end of a shallow glide to an elevated perch. Photogrammetric techniques were used to map the identified points on the wing and these were used to fit a mathematical model of the upper and lower surface topography using polynomial regression techniques. The surface model accounts for spanwise twist, spanwise bending, and varying chord distribution, as well as for the shape of the aerofoil. The aerodynamics of the two-dimensional aerofoil sections were analysed using XFOIL and were compared against two technical aerofoils, namely the Selig S1223 and Clark Y aerofoils, at 1×105≤Re≤2×105. The bird aerofoil maintains a robust, near-constant drag coefficient over a wide lift coefficient range.

Mechanics and aerodynamics of perching manoeuvres in a large bird of prey

This paper reviews recent results on the mechanics and aerodynamics of perching in a large bird of prey, the Steppe Eagle Aquila nipalensis . Data collected using onboard and high-speed video cameras are used to examine gross morphing of the wing planform by the flight muscles, and smaller-scale morphing of the wing profile by aeroelastic deflection of the feathers, Carruthers et al . High-resolution still images are used to reconstruct the shape of the wing using multi-station photogrammetry, and the performance of the measured wing profile is analysed using a panel code, Carruthers et al . In bringing these lines of research together, we examine the role of aeroelastic feather deflection, and show that the key to perching in birds lies not in high-lift aerodynamics, but in the way in which the wings and tail morph to allow the bird to transition quickly from a steady glide into a deep stall.

Use and Function of a Leading Edge Flap on the Wings of Eagles

*† ‡ § In this paper we present video evidence of unsteady maneuvers in which leading edge feather deflections occur on the wings of an Eagle in free flight. High-speed video is used to analyze the use of these feathers in greater detail during landing sequences. The results suggest that the feathers are being used as a means to stabilize the wing during these unsteady maneuvers. A novel method of measuring wing profiles of a bird in free flight is outlined.

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 clis 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.