Helicopter Aeroelastic Analysis in Forward Flight Using Dynamic Wake Model

Abstract

In this study, the aeroelastic analysis of rotorcraft in forward flight has been performed using dynamic wake model to handle unsteady aerodynamics. The quasi-steady airload model based on the blade element method has been coupled with dynamic wake model developed by Peters and He. The nonlinear steady response to periodic motion is obtained by integrating the full finite element equation in time through a coupled trim procedure with a vehicle trim for stability analysis. The aerodynamic and structural characteristics of dynamic wake model are validated against other numerical analysis results by comparing induced inflow and blade tip deflections(flap, lag). In addition, mechanism of aeroelastic instability will be investigated by evaluating the aeroelastic stability using different linear inflow and dynamic wake models at a low advance ratio. As the influences of nonlinearity result in significant differences in the blade deflection behavior and aeroelastic characteristics, an appropriate analysis is required to capture these attributes. The dynamic wake model is more accurate than the linear inflow model is at low advance ratios (0.0 ≤ µ ≤ 0.15) yet more time-effective than 3-D aerodynamic models. Thus it is most suitable for aeroelastic analysis in the preliminary design of a helicopter. NomenclatureC T = thrust coefficient e = strain at the datum line m = harmonic function number n = shape function number r = nondimensional blade radial coordinate, r =r/R t = nondimensional time, t =Ωt w = warping deformation α s = longitudinal shaft tilt angle, degree α n m , β n m = induced inflow expansion coefficients δT = variation of the strain energy 2 δU = variation of the kinetic energy δW = variation of the virtual work θ 0= collective pitch angle, degree θ 1c = lateral cyclic pitch angle, degree θ 1s = longitudinal cyclic pitch angle, degree κ i = difference in curvature before and after deformation λ 0 = mean induced inflow ratio λ = induced inflow ratio µ = advance ratio ϕ s = lateral shaft tilt angle, degree ϕ j r = radial expansion function ψ = azimuth, angle, rad ( )' = differentiation in the axial direction ( ), i = differentiation in 2 and 3 directions at a cross-section