Computational Study of Microflaps with Application to Vibration Reduction in Helicopter Rotors

Abstract

A comprehensive study on unsteady effects of oscillating Gurney flaps, or microflaps, has been conducted. Two-dimensional unsteady airloads, lift, moment and drag, due to an oscillating microflap were computed using a compressible Reynolds-Averaged Navier-Stokes (RANS) flow solver. The CFD results were generated with an overset mesh approach that captures oscillatory microflap motion. Three microflap configurations were examined so as to determine the type most suitable in terms of actuation efficiency and practical implementation. Furthermore, a reduced order model (ROM) for the unsteady microflaps was developed based on CFD simulations, using the Rational Function Approximation (RFA) approach. The resulting RFA model is a state-space, time-domain aerodynamic model that accounts for unsteadiness, compressibility and time-varying freestream effect, suitable for use with comprehensive rotorcraft simulations. The agreement between the ROM and direct CFD calculations was found to be excellent even in presence of strong nonlinear flow effects. The approximate model is suitable for incorporation in a comprehensive code, from which the potential of microflaps for active control of vibrations in rotor can be determined. Preliminary studies with open loop control showed that the microflap produces substantial vibration reduction (52% reduction in vertical shear) on a hingeless rotor configuration resembling the MBB BO-105, confirming the control authority of this novel technique.

Linear Reduced-Order Model for Unsteady Aerodynamics of an L-Shaped Gurney Flap

“…The corresponding curve for the clean baseline NACA 0012 geometry is also shown. It can be noticed that, in these cases, for attached flow conditions, the slope of the C L − α curve for the L-tab-equipped section is almost equal to the slope of the clean airfoil curve, consistent with what has been observed by other authors; see [5,8,13,25,32,54,55]. When the L-tab is deflected at β L-tab 0, the lift curve shifts vertically with respect to the clean section, whereas the baseline angular configuration of the L-tab recovers almost the same lift of the clean airfoil.…”

“…Figure 10 shows the same comparison with the experiments of Li et al [28] on the NACA 0012 section equipped with the 1.5%c GF, and here an agreement was found in the range of angle of attack where the flow separation is not involved. The disagreement in terms of the stall onset prediction, for both the clean airfoil as well as the section with the trailing-edge device, also found by other authors [32], may be because of the assumption of fully turbulent flow and to limitations of the turbulence models of the present computations.…”

“…This behavior has been shown by AIAA Early Edition / MOTTA AND QUARANTA other authors; see [32,40]. Figure 19 points out how lift and moment from pitch and plunge motions may sometimes show significant oscillations at frequencies higher than that of the forcing movement.…”

“…Min et al [31], by using CFD-CSD simulations, showed significant reduction on the classical HART-II test case with deployable GF. Liu et al [32] investigated by numerical simulations the effect of several Gurney flap-like devices on a NACA 0012 airfoil. They used numerical CFD computations as a reference to develop a reduced-order model (ROM) for the unsteady loads developed by the airfoil section equipped with his movable device.…”

Actuator Saturation in Individual Blade Control of Rotorcraft

“…High levels of blade-vortex interaction (BVI) noise are a primary hindrance to the community acceptance of a civilian helicopter. During the last two decades, various active control approaches, such as the higher harmonic control, 1 individual blade control (IBC), 2, 3 actively controlled conventional plain trailing-edge flaps (ACF), 4-7 the active twist rotor (ATR), 8,9 and the microflaps [10][11][12] have been explored and shown to have potential for vibration and noise reduction in rotorcraft. Approaches such as the IBC and the ATR which rely on deflecting or twisting the entire blade require high actuation power.…”