Understanding the High Speed Stability of a Hingeless Hub Proprotor through Wind Tunnel Testing and Analysis

Abstract

A 4.75-ft diameter Froude scaled hingeless-hub proprotor model was designed, fabricated, andwind tunnel tested on the Maryland Tiltrotor Rig (MTR) up to a high speed of 205 knots; loosely corresponding to 480 knots full-scale. Stability data was measured in the wing beam, chord, and torsion modes. Testing revealed that the hingeless hub overcame the key speed barrier faced by gimballed-hub tiltrotors – whirl flutter. Whereas a traditional gimballed hub encountered whirl flutter at 160 knots, the hingeless hub remained stable up to the maximum wind tunnel speed of 205 knots with identical blades, wing spar, and pylon. A number of important parametric variations were explored: straight versus swept-tip rotors, stiff versus soft wing spars, and fore and aft pylon center-of-gravity locations relative to the wing. The new hingeless hub and rotor was modeled with the in-house University of Maryland AdvancedRotorcraft Code version 2 (UMARC-II) to predict and validate against this data, and to understand the underlying mechanisms of the stability. It appears the hub pitch moment keeps the beam mode free of whirl-flutter, and the hub roll moment increases chord mode stability. The highly-damped regressive lag mode has an important role in chord and torsion mode stability. It affects the chord mode for the straight blades, and torsion mode for the swept-tip blades. The regressive lag effect was more important at lower speeds, whereas the hub moments were more important at higher speeds. The most stable configuration was a hingeless-hub with swept-tip blades with the pylon centerof gravity placed aft of the wing spar. Predictions captured the general trends over all parametric variations, but fell short for swept-tip blades particularly in the chord damping mode. It is hoped that a suite of stability test data acquired by this dissertation would provide a good database for validation of advanced analysis and pave the way to the development of next-generation tiltrotors that will fly twice as far and twice as fast.