Soaring and gust response in the Steppe Eagle

Thesis

This thesis explores how onboard instrumentation can be used to study how birds interact with the atmosphere, in particular to understand their soaring strategies and gust response mechanisms. The instrumentation unit consists of integrated sensors, including a GPS, IMU (inertial measurement unit) and pressure transducer, which provide accurate, high frequency measurements of the aerodynamics and kinematics of a free flying bird. The equipment development and derivation of parameters are initially presented. The data recorded from multiple flights of a steppe eagle (Aquila nipalensis) are then used to analyse the different aspects of its soaring flight throughout the rest of the thesis.

First, the occurrence and mechanism of the wing tuck manoeuvre is investigated and found to be a gust response, precipitated by a drop in aerodynamic loading. It is suggested that this reduction in loading results in a moment imbalance on the wings, and that ultimately the wings are pulled down by an internal musculoskeletal moment. The bird's slope soaring behaviour is then studied by using CFD analysis to model the local updrafts for comparison with the bird's performance. The analysis shows that the bird predominantly flew in the regions of strongest updraft along the ridge but did not attempt to maximise gravitational potential energy. This leads to gliding flight and a review of the theoretical derivation of the classic glide polar. A sensitivity analysis of the polar to different drag coeffcients highlights the need for more research into the selection of suitable values. The development of a new 'soaring optimisation' chart to include the effect of wind conditions on the selection of best glide speed is presented. This is then compared with experimental glide data from the bird and used to determine the optimal drag coeffcients for the polar model. Finally, a technique is developed for automatically identifying sections of thermalling flight and for removing the wind drift component to allow us to test thermalling behaviour. This exploratory analysis highlights the complexity of the flow structure within the thermal and the distinctly unsteady circling of the bird.

Authors: Reynolds, K

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: soaring; turbulence; birds; flight