PIV and force measurements on the flapping-wing MAV DelFly II

Abstract

Recent years have seen an increasing interest in Micro Air Vehicles (MAVs). MAVs are small (micro sized) aircraft and find their application in a multitude of commercial, industrial and military purposes. To perform their missions MAVs should be small sized, have good manoeuvrability, be well controllable and have a broad flight envelope. When flying in small confinements, the ability to fly at low airspeed and to have good manoeuvrability is critical. One type of MAVs, the flapping-wing MAV, particularly has attractive characteristics for flight in confined spaces. DelFly is a biplane flapping-wing MAV designed and built at Delft University of Technology. DelFly is able to hover and has an onboard camera for observation and vision-based control. For the DelFly project a top-down approach is followed, where from the study of a relative large model experience and theoretical insights can be gained, that can assist to create smaller, functional versions of the DelFly. The ultimate aim of the DelFly project is to improve the design to a very small full autonomous aircraft. For the current experimental investigation, force and flow field measurements were performed on a hovering DelFly II, since this model has a broad flight envelope and proven flight performance. The flow field is studied using particle image velocimetry. Due to the flexible wings there is a strong fluid structure interaction, therefore also the in-flight wing deformation is determined. The aerodynamic mechanism generating forces on the DelFly are related to those found in insect flight. Since leading edge vortices (LEVs) in insect flight are identified as the most important unsteady aerodynamic mechanism enhancing lift generation for insects, the development of these for the DelFly are very interesting. The vortex development is studied for various wings, at various flapping frequencies and at various spanwise positions. For the DelFly wing a conical LEV is developed, starting at out-board spanwise positions, approximately halfway during the translation. This LEV grows larger and is shed along the chord and at this time a new LEV starts to grow at the leading edge. This second LEV is dissipated at the end of the out-stroke during wing rotation, but at the end of the in-stroke this LEV moves above the wings and interacts with the counter-rotating LEV from the mirror wing. Inside the vortex tube a spanwise velocity component out-board is present. The shedding of the initial vortex and start of a second LEV is not completely consistent with LEV development for insect flight (which typically operate at a lower Reynolds number).