Rotor design changes intended toimprove tiltrotor whirl-flutter stability margins Were analyzed. A baselineanalytical model similar to the XV-15 (23% thick wing) Was established, and then a 15% thick wing Was designed to he representative of a high-speed tiltrotor. While the thinner wing has lower drag, it also has lower stiffness, reducing whirl-flutter stability. The rotor blade design was modified to increase the stability speed margin for the thin-wing design. Small rearward offsets of the aerodynamic-center locus with respect to the blade elastic axis created, large increases in the stability boundary. The effect was strongest for offsets a t the outboard part of the blade, where an offset of the aerodynamic center by 10% of tip chord improved the stability margin by over 100 knots. Forward offsets of the blade center of gravity had similar but less pronounced effects. Equivalent results were seen for swept-tip blades. Combinations of tip sweep, control-system stiffness,and delta-three werealsoinvestigated. Alimited investigationofbladeloadsin helicopter and airplaneconfiguration indicated that proper choice of parametric variations can avoid excessive increases in rotor loads. Notation blade section aerodynamic center, positive afI of EA blade chordwise center of gravity, positive forward of EA thrust coefficient, divided by solidity elastic axis blade quarter chord, positive aft of EA rotor radius wing thickness-to-chord ratio change in blade chordwise QC or CG position kinematic pitch-flap coupling ratio advance ratio (flight speed divided by tip speed)

Optimization of tilt-rotor systems requires the consideration of performance at multiple design points. In the current study, an adjoint-based optimization of a tilt-rotor blade is considered. The optimization seeks to simultaneously maximize the rotorcraft figure of merit in hover and the propulsive efficiency in airplane-mode for a tilt-rotor system. The design is subject to minimum thrust constraints imposed at each design point. The rotor flowfields at each design point are cast as steady-state problems in a noninertial reference frame. Geometric design variables used in the study to control blade shape include: thickness, camber, twist, and taper represented by as many as 123 separate design variables. Performance weighting of each operational mode is considered in the formulation of the composite objective function, and a build up of increasing geometric degrees of freedom is used to isolate the impact of selected design variables. In all cases considered, the resulting designs successfully increase both the hover figure of merit and the airplane-mode propulsive efficiency for a rotor designed with classical techniques.

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Tiltrotor whirl flutter in cruise flight is investigated using comprehensive rotorcraft analysis codes Comprehensive Analytical Model of Rotorcraft Aerodynamics and Dynamics (CAMRAD) II and Rotorcraft Comprehensive Analysis System (RCAS). A generic tiltrotor model with a three-bladed gimballed rotor was systematically developed starting with a simple rigid rotor mounted on a rigid pylon and a more sophisticated model was built up by adding one design variable at a time. The rotor is also coupled with a flexible wing/pylon modeled from NASTRAN for aeroelastic stability analysis. The effects of pitch-flap coupling (δ 3), blade elasticity, precone, undersling, yoke chord and flap stiffness, pitch link stiffness, rotor rotational speed, density, speed of sound, inflow modeling, unsteady aerodynamics, and realistic airfoil tables on whirl flutter speed are thoroughly examined. With careful and thorough modeling/analysis, aeroelastic stability (frequency and damping) calculated by CAMRAD II and RCAS shows consistently excellent agreement with each other for wide variations of design variables and operating conditions. For the configurations investigated in this study, blade pitch-flap coupling, rotor lag frequency, rotor rotational speed, and density have an important influence on whirl flutter speed. Nomenclature a speed of sound k x , k y , k z pitch bearing translational stiffness k θx , k θy , k θz pitch bearing rotational stiffness R blade radius V speed X, Y, Z translational NASTRAN mode shape at rotor hub β flap angle θ blade pitch angle δ 3 pitch-flap coupling θ X , θ Y , θ Z rotational NASTRAN mode shape at rotor hub ν ζ blade fundamental lag mode frequency ρ freestream density rotor rotational speed