Avian Wing Joints Provide Longitudinal Flight Stability and Control

Abstract

Uncrewed aerial vehicle (UAV) design has advanced substantially over the past century; however, there are still scenarios where birds outperform UAVs. Birds regularly maneuver through cluttered environments or adapt to sudden changes in flight conditions, tasks that challenge even the most advanced UAVs. Thus, there remains a gap in our general knowledge of flight maneuverability and adaptability that can be filled by improving our understanding of how birds achieve these desirable flight characteristics. Although maneuverability is difficult to quantify, one approach is to leverage an expected trade-off between stability and maneuverability, wherein a stable flyer must generate larger moments to maneuver than an unstable flyer. Bird’s stability, and adaptability, has previously been associated with their ability to morph their wing shape in flight. Birds morph their wings by actuating their musculoskeletal system, including the shoulder, elbow and wrist joints. Thus, to take an important step towards deciphering avian flight stability and adaptability, I investigated how the manipulating avian wing joints affect longitudinal stability and control characteristics.

First, I used an open-source low fidelity model to calculate the lift and pitching moment of a gull wing and body across the full range of flexion and extension of the elbow and wrist. To validate the model, I measured the forces and moments on nine 3D printed equivalent wing-body models mounted in a wind tunnel. With the validated numerical results, I identified that extending the wing using different combinations of elbow and wrist angles would provide a method for adaptive control of loads and static stability. However, I also found that gulls were unable to trim for the tested shoulder angle.

Next, I developed an open-source, mechanics-based method (AvInertia) to calculate the inertial characteristics of 22 bird species across the full range of flexion and extension of the elbow and wrist. This method allowed a detailed investigation of how manipulating the elbow and wrist angle changed the center of gravity and moment of inertia tensor. Leveraging the previous aerodynamic results, I developed a method to estimate the neutral point of any bird wing configuration and derived a novel metric for pitch agility. With the neutral point and center of gravity, I found that the majority of investigated species had the ability to shift between stable and unstable flight. Further, I implemented an evolutionary analysis that revealed evidence of evolutionary pressures maintaining this capacity to shift, which transforms our understanding of avian flight evolution.

Finally, I combined the aerodynamic and inertial results to investigate the dynamic stability of a gull across a range of shoulder, elbow, and wrist angles. This analysis revealed that a positive dihedral and forward-swept wing allowed a trimmed flight condition. For trimmed configurations, I found that high wrist angles were statically unstable and exhibited a non-oscillating, divergent response to disturbances. Lower wrist angles were both statically and dynamically stable and exhibited a short period and phugoid mode like traditional aircraft. I found that most trimmed configurations exhibited short period characteristics that would be flyable by a human pilot, although with a heavily damped phugoid mode.

In summary, I found that the avian elbow and wrist joints can act as adaptive controls and permit birds to shift between stable and unstable flight. Identifying these characteristics provides a starting point for future UAV designs that hope to incorporate avian-like maneuverability and adaptability.