Development and Testing of Unconventional Micro Air Vehicle Configurations

Abstract

The Naval Research Laboratory (NRL) is developing a group of micro air vehicle (MAV) concepts intended for operation in urban or other confined environments. Primary flight requirements for these MAVs are low speed maneuverability, vertical or near vertical takeoff and landing, the potential for hovering, and compatibility with ground operation as a “perched” sensor platform. The additional requirement for such MAVs to be small enough (wingspans of less than 150 mm) to operate unobtrusively has focused attention on flapping foil propulsion and other unconventional configurations. In the low Reynolds number (2,000 ≤ Re ≤ 30,000) and low speed (2-5 m/s) regimes applicable to such tiny aircraft, airfoil performance degradation significantly reduces propeller efficiency as size decreases. Flapping wings can be dynamically advantageous for propulsion under these conditions. While many flapping wing vehicles such as ornithopters attempt to mimic the flight of birds or insects, it is possible to use flapping foil propulsion in ways that are inspired by biological flight but do not copy it directly. “Aerial swimming” vehicles use a flapping wing that resembles a boat’s sail with the mast running horizontally, driven by a central oscillating beam. The wing’s camber reverses on each half cycle. Three configurations have flown successfully: a vehicle with a fixed forward wing and a rear flapping wing; a tandem fixed-wing design with a third flapping wing that claps down atop the rear fixed surface on each stroke; and a vehicle with tandem pairs of biplane configured flapping wings, in which each pair moves in opposition so as to alternately clap together and separate. This latter vehicle, which does not employ fixed lifting surfaces, has the advantage of being dynamically balanced in flight so that its center of mass