Experimental Investigation into Unsteady Force Transients on Rapidly Maneuvering Wings

Abstract

Small-scale aircraft, such as biological fliers and micro air vehicles, typically operate in a low Reynolds number flight regime, Re ~ O(10^2 - 10^5), that is known to contain massive flow separation and unsteady force production. This work conducts comprehensive experimental campaigns on a flat plate undergoing three simple wing motions typical of low Reynolds number flight: surging from rest, pitching in a constant free stream, and deflection of a large trailing edge flap. Water tunnel tests were performed in collaboration between the University of Maryland (UMD) and the Air Force Research Laboratory (AFRL). Unsteady force measurements and time-resolved velocity fields were obtained for a wide range of incidence angles and motion rates, spanning cases of fully attached flow to those of massively separated flow. Experiments were conducted at Reynolds number Re = 20,000 over an extensive breadth of reduced frequencies (0.06 < k < 3) representative of the conditions found in small biological fliers. Detailed investigations into rapidly surging and pitching wings illustrated the direct relationship between observable vortex dynamics and force/moment coefficients during the transient acceleration and subsequent relaxation to steady state. It was shown that circulation strength is proportional to motion rate, and faster acceleration transients produce stronger, more coherent leading edge vortices. Experiments were also performed on a hinged wing with a 50%-chord trailing edge flap. Dynamically pitching the wing was shown to generate instantaneous lift response upon motion onset regardless of initial flow attachment. Additionally, direct measurement of wing component forces and numerical simulations using an unsteady panel method confirmed the production of considerable unsteady forces on the stationary fore element of the hinged wing. Using a modified aerodynamic model that accurately predicts force histories on the hinged wing, it was determined that the largest discrepancy between Theodorsen's classical solution and the measured forces is due to its over-prediction of steady circulatory lift.