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In an effort to expand helicopter flight envelopes,
this analytical study explores the potential of using
higher harmonic blade pitch to reduce the adverse
effects of dynamic stall on rotor blades. Since excessive
stall-induced loads can damage rotor structural
components, stall severely restricts helicopter maximum
speed and loading capabilities. On the other
hand, successful control of stall can enhance the
utility of helicopters.
The rotorcraft analysis code UMARC (University
of Maryland Advanced Rotorcraft Code) was modified
for a stall suppression investigation of the
UH-60A rotor. At a severe stalled condition, the
analysis predicts three distinct stall events spreading
over the retreating side of the rotor disk. Prescribed
2-per-rev input can reduce stall moderately, as
shown in the figure, where the lift excess is used as a
measure of stall; the other input harmonics are less
effective. Stall responses to individual input harmonics
exhibit highly nonlinear behaviors, rendering the
closed-loop controller ineffective in suppressing stall
and the combined effects of individual harmonics
non-additive.
Point of Contact: K. Nguyen
(650) 604-5043
knguyen@mail.arc.nasa.gov
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Fig. 1. Effect of higher harmonic input on rotor stall: baseline (left), with 2-per-rev input (right), 102 knots, blade
loading C T / s 0.13.
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