Fundamental Understanding, Prediction and Validation of Rotor Vibratory Loads in Steady-Level Flight
Fundamental Understanding, Prediction and Validation of Rotor Vibratory Loads in Steady-Level Flight
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Date
2004-09-14
Authors
Datta, Anubhav
Advisor
Chopra, Inderjit
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DRUM DOI
Abstract
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.