Quasi-static rotor morphing applications in flight mechanics and active track-and-balance

Abstract

Conventional rotorcraft generally operate with a fixed-geometry rotor and perform sub-optimally over the flight envelope. Rotor morphing, as a concept, allows planform variation that can overcome this limitation to improve performance over diverse flight conditions. However, despite the promising potential, the technology readiness level is still quite low. In light of this assessment, a fundamental understanding of the impact of rotor morphing on two areas is examined in this study: flight mechanics and active track-and-balance.

In assessing the impact on flight mechanics, span and chord-extension morphing are considered on a simulation model of the UH-60A Black Hawk helicopter. For span morphing, retraction to 22.8 ft (15%) and extension to 31.5 ft (17%) was considered over the baseline radius, whereas for chord-extension morphing, an effective increase of 20% chord between 63-83% of the blade span was considered. Span morphing considerably affected the dynamic flap and inflow modes but not the rigid-body modes. In level flight at 40 knots, main rotor torque requirements increased during the course of span morphing, which was attributed to an increase in either induced (for retraction) or profile (for extension) power. The collective pitches on the main and tail rotor were primarily driven by changes in main rotor thrust and torque, respectively. The combination of change in tail rotor thrust with wake impingement effects on the empennage drove trends in cyclic controls, flapping and aircraft attitudes. In contrast to span morphing, chord-extension morphing produced minimal changes in aircraft states or controls at all flight conditions tested.

For rotor track-and-balance, the effectiveness of chord-extension morphing was investigated as an active trailing-edge tab mechanism on a UH-60 rotor model developed in RCAS. Imbalance loads were seeded onto one blade to simulate dissimilarity and the resulting 1/rev (1P) loads were minimized using a weighted least-squares optimization method over a range of airspeeds. Results indicated that the tab is effective in reducing 1P in-plane forces in hover and 1P vertical forces in cruise. Best reductions in 1P vibration were achieved by varying tab settings over airspeeds, favoring an active version of the mechanism. The tab was also compared to an active pitch link mechanism and showed comparatively much smaller reductions in 1P lateral forces in cruise, with slightly smaller reductions in 1P in-plane moments.