Analysis of Bird and Bat Flight Using a Wind Tunnel

Model Development

Overview

Bird and bat wings did not evolve from the same evolutionary origin; as such their wing shapes and flight styles vary. While bats continually flap, birds alternate between flapping and gliding. I hypothesized that one possible reason for this was due to differences in lift-to-drag ratios between the two wing types, making gliding favorable to birds and disadvantageous to bats. To test this, I modeled a bird and bat gliding and calculated their lift and drag coefficients(CL/CD) using a wind tunnel.

To make the comparison more equitable, I selected a bird and bat species with similar sizes, flight speeds, diets, and migratory distances, the resultant comparison was between a hoary bat (Lasiurus cinereus) and barn swallow (Hirundo rustica), with primary interest in how wing shape affects CL/CD.

I created models using a foam body and wire ‘skeleton’ to create scaffolding for an accurate wind shape. I covered the body in modeling clay, and made the wing shape using masking tape. Because the primary concern was shape, I removed textual differences between the species. I creaetd the models at half size.

Each model was tested three times at speeds between 15.2 and 35.3 m/s. This range roughly correlates from the bat’s lowest speed to the bird's fastest speed, doubled to satisfy the Reynolds number. The bird had an angle of attack (AOA) of 6.15, and the bat had an AOA of 7.1; both are the best replicas of the actual animals' AOAs.

At all speeds, the bird's CL/CD was greater than the bats, meaning the bird has greater gliding efficiency.

Possible causes for the bat’s increased drag...

1. Bat’s greater AOA leading to increased flow separation at the trailing edge.

2. Section of the bat wing with greater chord length and longer trailing edge creating a larger boundary layer.