Fundamental Understanding, Prediction and Validation of Rotor Vibratory Loads in Steady-Level Flight
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This work isolates the physics of aerodynamics and structural dynamics from the helicopter rotor aeromechanics problem, investigates them separately, identifies the prediction deficiencies in each, improves upon them, and couples them back together. The objective is to develop a comprehensive analysis capability for accurate and consistent prediction of rotor vibratory loads in steady level flight. The rotor vibratory loads are the dominant source of helicopter vibration. There are two critical vibration regimes for helicopters in steady level flight: (1) low speed transition and (2) high speed forward flight. The mechanism of rotor vibration at low speed transition is well understood - inter-twinning of blade tip vortices below the rotor disk. The mechanism of rotor vibration at high speed is not clear. The focus in this research is on high speed flight. The goal is to understand the key mechanisms involved and accurately model them. Measured lift, chord force, pitching moment and damper force from the UH-60A Flight Test Program are used to predict, validate and refine the rotor structural dynamics. The prediction errors originate entirely from structural modeling. Once validated, the resultant blade deformations are used to predict and validate aerodynamics. Air loads are calculated using a table look up based unsteady lifting-line model and compared with predictions from a 3-dimensional unsteady CFD model. Both Navier-Stokes and Euler predictions are studied. By separating aerodynamics from structural dynamics, it is established that the advancing blade lift phase problem and the problem of vibratory air loads at high speed stem from inaccurate aerodynamic modeling, not structural dynamic modeling. Vibratory lift at high speed is caused by large elastic torsion deformations (-8 to -10 degrees near the tip) driven by pitching moments and wake interactions on the advancing blade. The dominant phenomenon at the outboard stations (86.5\% R to 99\% R) is the elastic torsion. Vibratory lift at these stations are dominantly 3/rev and arise from 2/rev elastic torsion. At the inboard stations (67.5\% R and 77.5\% R), the vibratory lift is impulsive in nature and is not captured by elastic torsion alone. An accurate rotor wake model is necessary in addition to accurate elastic torsion. Accurate elastic torsion requires accurate pitching moments. Lifting-line models, with airfoil tables, unsteady aerodynamics, near wake and far wake do not capture the unsteady transonic pitching moments at the outboard stations (86.5\% R to 99\% R). A 3-dimensional CFD analyses, both Navier-Stokes and Euler, significantly improve pitching moment predictions at the outboard stations. The 3D Navier-Stokes CFD analysis is then consistently coupled with a rotor comprehensive analysis to improve prediction of rotor vibratory loads at high speed. The CFD-comprehensive code coupling is achieved using a loose coupling methodology. The CFD analysis significantly improves section pitching moment prediction near the blade tip. because it captures the steady and unsteady 3D transonic effects. Accurate pitching moments drive elastic twist deformations which together with a refined rotor wake model generate the right vibratory airload harmonics at all radial stations. The flap bending moments, torsion bending moments and pitch link load predictions are significantly improved by CFD coupling.