A MODERN AEROMECHANICAL ANALYSIS OF HINGELESS HUB TILTROTORS WITH MODEL- AND FULL-SCALE WIND TUNNEL VALIDATION

Abstract

A new aeromechanics solver was developed, verified, and validated systematically to explore how whirl flutter might be eliminated to achieve significantly higher cruise speeds with future tiltrotor aircraft. The hub explored is hingeless, more advanced than the gimballed hub of current generation tiltrotors. The major finding is that whirl flutter is not the barrier at all for hingeless hubs, instead air resonance, which is another fascinating instability particular to soft in-plane rotors. A possible design change to achieve high cruise speeds with thin, low-profile wings is blade tip sweep. The key mechanism is the aerodynamic center shift. The trade-off is the increase in blade and control system loads. A fundamental understanding of the physics for soft in-plane hingeless hub stability was provided. The induced flow model showed no effect on high-speed stability, as the wake is quickly washed away and insignificant for airplane mode flight. Predictions in powered mode are necessary. At least the first rotor flap, lag, and torsion modes need to be included. Rotor aerodynamics should use airfoil tables; wing aerodynamics is not essential for air resonance. Periodic solution before stability analysis is necessary for powered mode flight. Details of the mathematical model were reported. The solver was built to study high-speed stability of hingeless hub tiltrotors; hence the verification and validation cases were chosen accordingly. The stability predictions were verified with U.S. Army's CAMRAD II and RCAS results that were obtained for hypothetical wing/pylon and rotor models. Soft in-plane, stiff in-plane, hyper-stiff in-plane, and rigid rotors were studied with a simple and a generic wing/pylon model. A total of nine cases were investigated. A satisfactory agreement was achieved. Validation was carried out with Boeing Model 222 test data from 1972. This rotor utilized a soft in-plane hingeless hub. Good agreement was observed for performance predictions. Trends for the oscillatory blade loads were captured, but differences in the magnitudes are present. The agreement between the stability predictions and test data was good for low speeds, but some offset in the damping levels was observed for high speeds. U.S. Army also published stability predictions for this rotor, which agreed well with the present predictions. A further parametric validation study was carried out using the University of Maryland's Maryland Tiltrotor Rig test data. This is a brand new rig that was first tested for stability in October – November 2021. Eight different configurations were tested. Baseline data is gimbal-free, freewheeling mode, wing fairings on with straight and swept-tip blades. Gimbal-locked, powered mode, and wing fairings off data was also collected, all with straight and swept-tip blades. Wing beam mode damping showed good agreement with the test data. Wing chord mode damping was generally under-predicted. The trends for this mode for the gimbal-locked, straight blade configurations (freewheeling and powered) were not captured by the analysis. Swept-tip blades showed an increase in wing chord mode damping for gimbal-locked, freewheeling configuration. Locking the gimbal increased wing chord damping, which was picked up by the analysis. Powered mode also increased the wing chord damping compared to freewheeling mode, but the analysis did not predict this behavior. Wing beam mode damping test data showed an increase at high speeds due to wing aerodynamics, and the analysis agreed.