The Drooped Wings of Gulls: Aerodynamic Optimum or Kinematic Constraint?

In gliding flight, gulls adopt a characteristic drooped wing position, i.e. continual increasing anhedral from root to tip along the span. This project set out to determine whether this position was due to aerodynamic benefits... [ view full abstract ]

In gliding flight, gulls adopt a characteristic drooped wing position, i.e. continual increasing anhedral from root to tip along the span. This project set out to determine whether this position was due to aerodynamic benefits or to kinematic constraints of the gull's physiology [1]. If a drooped wing configuration represented an aerodynamic optimum, it could be advantageous for small unmanned aerial vehicles (UAVs) which operate at similar length and velocity scales as gulls and which require high aerodynamic efficiency to execute long range and long endurance missions.
Previous work has examined the aerodynamic performance of gulls both in the wild [2] and in laboratory conditions [3]. These studies have shown that gulls are able to achieve very high gliding performance, however, such studies were unable to attribute this performance to any factor of the wing geometry such as anhedral. Therefore, this work created a computational model of a gull in gliding flight in order to examine the aerodynamic performance of gulls for a wide range of wing geometries. A kinematic model of how a gull wing moves while flapping [4] was used to allow the wing to be analyzed while constrained to geometries which were compatible with the gull's physiology. The variation of wing planform area and aerodynamic performance throughout this flapping cycle is shown in Figure 1.
Using a gradient-free optimization algorithm, the most advantageous wing geometries, with respect to gliding flight, were determined for cases when the wing was both unconstrained and constrained to the gull's flapping cycle. It was found that the optimal performance achieved in the unconstrained analysis was the same as that obtained in the constrained analysis. This indicated that gulls have developed a drooped wing configuration due to aerodynamic benefit rather than any physiological constraint.
This finding clearly has applications beyond the study of gulls. Many small UAVs are of a similar size and operate in similar flight regimes as gulls and could benefit from the improved aerodynamic performance that a drooped wing has proved able to achieve.

References
1. Andrews, S. A., Perez, R. E., Allan, W. D. E., 2013. “Aerodynamic implications of gull's drooped wing-tips.” Bioinspiration and Biomimetics: 8(4).
2. Raspet A., 1960. “Biophysics of bird flight” Science: 132, pp191–200,
3. Baudinette R. V., 1974. “Energy cost of gliding flight in herring gulls.” Nature: 248(5443), pp 83–84.
4. Liu T., Kuykendoll R., Rhew R. and Jones S., 2006. “Avian wing geometry and kinematics” AIAA J.: 44(5), pp 954–963.