AEROMECHANICAL BEHAVIOR OF TWIST-MORPHING, HIGH-SPEED, SLOWED RPM ROTORS
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This thesis describes the first comprehensive analysis of a composite coupled edgewise rotor in high-speed forward flight. The design objective was to use composite coupling, namely extension-torsion coupling, to morph the built-in twist of a UH-60A-like rotor in slowed RPM flight. As a part of this work, this study included the first analysis of a morphing rotor using full 3-D analysis coupled with aeromechanics. The use of 3-D FEM along with an integrated trim solver and aerodynamic modeling was shown to have been key in developing a fundamental understanding of how composite coupling effects rotor performance and the aerodynamics in different flow conditions. This research shows that extension-torsion composite coupling in the spar of a UH-60A-like rotor can provide a significant increase in the efficiency when the RPM is reduced. This was achieved through a combination of delayed stall drag along the retreating side of the rotor and reduced negative lift along the advancing side, providing an overall improvement in rotor efficiency. A comprehensive analysis was performed using a full 3-D FEA based aeroelastic computational structural dynamics (CSD) solver with the inclusion of a freewake aerodynamics model. A reduction of RPM down to 85% of the nominal hover RPM (which is well within the operational capacity of current turboshaft engines) showed an improvement in the lift-to-drag ratio, 𝐿/𝐷𝑒, over all blade loadings, 𝐶𝑇/𝜎. The maximum improvement in efficiency occurred at the peak blade loading, 𝐶𝑇/𝜎≈0.1. A further RPM reduction to 65NR (65% of nominal RPM), an RPM that future rotorcraft could potentially achieve with improvements in variable drive train design, showed general efficiency improvement at blade loadings below 𝐶𝑇/𝜎=0.08, with no change in the peak efficiency when compared to an uncoupled rotor. A hygrothermally stable Winckler layup was shown to perform just as well as a nominal coupled layup at 85NR, and marginally better at 65NR, in addition to contributing to practical manufacturability of the rotor design. Close study of the strains in the rotor showed that a rotor with an extension-torsion coupled composite spar would be within the realm of practical manufacturability as the axial strains around the azimuth fell well within IM7/8552’s allowable tensile strain of 6000 𝜇𝜀. Tensile strain is directly related to the amount of twist change in the rotor and is reduced when the RPM is slowed and the rotor untwists towards its original cold shape.