An Approximate Unsteady Aerodynamic Model for Flapped Airfoils Including Improved Drag Predictions

摘要

Actively controlled trailing-edge flaps (ACFs) have been extensively studied for vibration and noise controlin rotorcraft using various approximate aerodynamic models. In this study, two-dimensional unsteadyairloads due to oscillating flap motion obtained from computational fluid dynamics (CFD) are comparedwith approximate unsteady loads. The approximate loads are obtained from the Rational Function Approximation(RFA) model developed for use with comprehensive rotorcraft simulation codes, which is astate-space, time-domain model that accounts for unsteadiness, compressibility and time-varying freestreameffect. Unsteady compressible Reynolds-averaged Navier-Stokes computations are based on an overset meshthat accounts for oscillatory flap motion. The comparison is conducted over a wide range of unsteady flowconditions representing combinations of parameters such as airfoil angle of attack, flap deflection amplitudes,reduced frequencies, and freestream Mach numbers. The comparison between the RFA model andCFD based calculations illustrates the limitations of the approximate theory, particularly at transonic Machnumbers and high angles of attack where nonlinear effects dominate. Nevertheless, the RFA model yields agood approximation for the unsteady effects of the blade section/trailing edge flap combination, for conditionsrepresentative of rotorcraft aerodynamic environment. An improved drag model for flapped airfoils isdeveloped using surrogate based approximation. This new drag model can be implemented in comprehensiverotorcraft simulation codes to predict performance penalty associated with active flap deflections.