Bats in Gliding Flight

Bats in Gliding Flight: A comparative wind tunnel investigation of the aerodynamics of gliding bats and a bat inspired gliding wing model

Stuiver, M.

Bijl, H. (mentor)
van Oudheusden, B.W. (mentor)
Muijres, F.T. (mentor)
Remes, B.D.W. (mentor)

Aerospace Engineering

2011-02-18

Due to the high cost of flight, there is a high evolutionary selection pressure for energy efficient flight patterns, such as using external natural forces for soaring or flying intermittently. Some bats at time soar, glide or flap glide. Bounding flight is not possible as their membranous wings will go slack, and soaring is not common amongst bats, as most bats are nocturnal and during night thermals are usually of insufficient strength. From an aerodynamic point of view, gliding flight is less complex than flapping flight, however in bats undulating flight patterns are less observed than in birds. So, why should bats glide? Flight performance studies on live bats have revealed a part of the complexity of hovering and steady flapping flight, but gliding flight in these animals is poorly studied. To get insight in how bats glide and in their gliding flight performance, gliding flight of bats is studied from two points of view; gliding flight of real bats and gliding of a flexible, bat inspired wing model, in a low speed, tiltable wind tunnel. The kinematics of both the bats and the model are filmed by two synchronised high speed cameras, and the flow field in a transverse plane behind the wings is visualized by means of a PIV system. Three medium sized bats Leptonycteris yerbabuenae, are trained to glide at a feeder in the test section of the wind tunnel at a know, fixed glide angle. This known glide angle enables to calculate the aerodynamic forces, which are fixed properties in steady gliding flight. A gliding wing model, based on a bat’s wing, with an adjustable leading edge flap, is designed, build, and tested at different angles of attack. The wing model is tested with both a smooth and a structured top surface to see what the effect of ’turbulators’ can be. Additionally the wing model is mounted onto a balance in order to measure the aerodynamic forces. By means of experiments with the wing model, wake structures of gliding flight can be connected to a single changing morphology parameter to explore the parameter space, and the wake structures can be compared to the wake structures of the gliding bats. The bats are observed to glide for some seconds in the test section, but only the parts of the glides at the feeder where the tip vortex strength and position were stable are analysed. From the PIV data, an average wake is constructed per glide sequence of the bats, and for each leading edge setting and speed combination of the model wing. From the average wake the flight forces and the resulting flight performance properties are derived. The wing model approaches the glide behaviour of the bats. Deploying the leading edge flap increases the span efficiency and the lift coefficient at low angles of attack. Also the structure on top of the wing is beneficial for flight performance at low angles of attack.

http://resolver.tudelft.nl/uuid:4b95f124-7ea5-49ea-af65-dfb436d6bdc7

Student theses

master thesis

(c) Delft University of Technology