Elastically tailored composite rotor blades for stall alleviation and vibration reduction

An aeroelastic analysis has been developed to study the effects of elastic couplings on blade response, loads, and dynamic stall. Low and high speed conditions are examined at high thrust and cruise-level thrust for both hingeless and articulated rotor configurations. The blade cross-sectional model is based on Vlasov theory for multi-cell closed sections with thick walls. The structural model includes non-classical effects of transverse shear, torsion-related warping, and two-dimensional inplane elasticity. For the aeroelastic analysis, the blade is modeled as an elastic beam undergoing deflections in flap, lag, and torsion. The blade governing equations are approximated by a finite element in space model. A twelve degree of freedom specialized finite element is employed in the structural model which accounts for torsion-related warping. The analysis includes both a quasisteady and a time-domain unsteady aerodynamic model including the effects of non-linear separation and dynamic stall. The nonlinear periodic response is calculated through a finite element in time procedure with displacement and velocity continuous elements. The blade and hub loads are calculated using the force summation method and the reaction force method. The two loads calculation methods are compared for articulated and hingeless rotor configurations. The reaction force method is shown to more accurately predict blade bending moments in articulated rotors. The blade is modeled as a NACA airfoil section consisting of a D-spar and skin. Elastic couplings are introduced by anisotropy of plies in the D-spar to produce pitch-flap couplings. Results indicate that by coupling elastic twist to the second flap mode, the angle of attack on the retreating side of the rotor disk can be reduced up to two degrees, resulting in a significant reduction in blade stall for both articulated and hingeless rotors. Sensitivity studies are conducted to determine the effects of flap frequency on the induced twist. The third harmonic of twist is shown to be sensitive to flap frequency and can be tuned to reduce certain 4/rev vibratory loads.