Longitudinal Aerodynamics of Rapidly Pitching Fixed-Wing Micro Air Vehicles

Abstract

Fixed-wing vertical-takeoff-and-landing (VTOL) micro air vehicles (MAVs) can transition between two flight modes: forward-flight and near-hover. This study was conducted to improve the transition-performance of such vehicles. Our test subject consists of a rigid Zimmerman wing with an S-shaped airfoil and a propulsion system with contrarotating propellers. The wing had an aspect ratio of 1.9 and was subjected to freestream and slipstream flows while being rapidly pitched about its aerodynamic center (AC). Data was acquired for non-dimensional pitching rates ranging from -0.031 to 0.031 at an average freestream Reynolds number of 86K. Two elevator deflections were used, 0 and -17 deg, as well as three different propulsive-settings corresponding to propulsion-off and average advance ratios of 0.47 and 0.60. Under steady conditions, with throttle-setting constant, propeller rotation-rate decreases linearly with angle of attack (AOA) from 20 to 70 degrees, while both advance-ratio and thrust-coefficient increase linearly. Normal force coefficient was found to increase with AOA. Under steady conditions, higher throttle-setting results in greater stall delay, which causes maximum lift coefficient to increase significantly. When throttle-setting increases, lift and drag coefficients increase throughout the tested AOA-domain. Interestingly, wing aerodynamic efficiency is virtually independent of throttle-setting between 30 and 70 degrees. Rapid-pitching tests showed that nose-up pitching delays stall and nose-down pitching hastens it. Lift and drag coefficients increase with positive pitching-rate and decreased with negative pitching-rate. Unsteady efficiency curves converge on steady curves near 30 degrees AOA and remained converged thereafter. The pitch-damping observed in flight tests was confirmed by acquired data. The propulsions system was not sensitive to rapid-pitching; thrust, normal force and propulsive moment were relatively unchanged.