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Because high levels of helicopter vibration are
bothersome to pilots, passengers, and on-board
equipment, efforts are under way to develop
improved active rotor concepts for vibration control.
In an effort to explore the benefits of on-blade
controls, a small-scale active rotor was previously
tested in the Army/NASA 7- by 10-Foot Wind Tunnel.
The rotor shown in the figure contained one elevon
(or control surface) per blade which was actuated by
piezoceramic bimorph actuators to reduce vibratory
blade loads. Previous reports have described the test
in detail along with preliminary 2GCHAS (Second
Generation Comprehensive Helicopter Analysis
System) calculations used for data correlation and for
explaining the observed aeroelastic phenomenon.
More recently, CAMRAD II (Comprehensive Analytical
Model of Rotorcraft Aerodynamics and Dynamics)
calculations were made to study several model
features, including tip loss and elevon dynamics, and
to allow further forward flight vibratory loads comparisons.
In all cases, the control consisted of elevon
deflection, and the response consisted of blade root
bending and torsion moments.
In hover, the predictions captured the basic
aeroelastic effects evident in the data, including
elevon reversal and aeroelastic resonant peaks. For
some cases, however, the magnitude of the predictions
significantly differed from the experimental
measurements. For example, the calculated peak
torsion moment response (to elevon deflection) was
only 60% of the experimental peak response for a
nominal hover condition. A parameter found to be
rather effective in changing the torsion resonant peak
was tip loss - ignoring the aerodynamic loads for the
outboard 2% of the blade increased the torsion peak
by 13%. A tip loss for lift can be predicted by conventional
lifting-line theory when nonuniform inflow
exists. This approach, however, does not work for
pitching moment, suggesting that the incorporation of
an advanced lifting-line model may be warranted.
Less significant was the inclusion of a model of
elevon dynamics, which increased the torsion peak
response by 6% and also caused a coupling between
blade response and elevon motion. Results from
forward flight tended to confirm the hover results, but
suffered from the conventional problems of vibratory
load prediction for helicopter rotors - the general
trends were captured, but significant differences
existed. For example, a correlation plot of the
2-5 per-revolution flap bending moment harmonics
was produced for four wind tunnel speeds. A least-squares
curve fit yielded a correlation coefficient of
0.56, indicating a low level of correlation between
the analysis and the test data.
Point of Contact: M. Fulton
(650) 604-0102
mfulton@mail.arc.nasa.gov
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Fig. 1. Active rotor with on-blade elevons in the Ames 7- by 10-Foot Wind Tunnel.
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