Reducing tilt-rotor noise by 12 decibels was a
major goal of the Short Haul Civil Tiltrotor (SHCT)
program. Blade vortex interaction (BVI) noise reduction
was considered a key enabling technology that
would allow tilt rotors to land in populated areas
such as city centers, thereby relieving congestion at
major airports. One of the SHCT objectives was to
investigate BVI noise reduction techniques using an
isolated, full-scale XV-15 tilt rotor in approach flight
conditions in the Ames 80- by 120-Foot Wind
Tunnel.
Figure 1 shows two contour plots of noise levels
measured on a horizontal plane below and forward
of the advancing side of the rotor. This measurement
area was chosen to capture the highest BVI noise
levels. The center of the rotor is located at the origin,
and the arc in the lower left corner represents the
location of the rotor blade tips. The left-hand plot
shows noise measurements at a high BVI noise
condition. The highest noise level, near the center of
the measurement area, is 118.5 decibels. The right-hand
plot shows the result achieved by tilting the
rotor tip-path-plane forward from +3 degrees (aft tilt)
to -3 degrees (forward tilt) and applying higher
harmonic control (HHC). An HHC system provides
high-frequency inputs to the rotor controls, resulting
in blade-pitch oscillations at two, three, and four
times per revolution on the three-bladed rotor. By
making these inputs at the proper phase relative to
the blade azimuth position, a large reduction in the
rotor's noise signature is produced. The highest noise
level for this condition is 102.0 decibels, at a point
located near the upper left corner of the measurement
area.
Therefore, the peak noise within the measurement
area was reduced by 16.5 decibels, surpassing
the SHCT program goal of 12 decibels. This noise
reduction was achieved by utilizing a combination of
HHC and by changing the rotor's tip-path-plane
angle-of-attack. The rotor tip-path-plane angle of a
tilt-rotor aircraft can be changed in flight by varying
the nacelle tilt angle, wing flap position, and
approach glideslope, thus producing a slightly
different approach flight condition.
Point of Contact: M. Betzina
(650) 604-5106
mbetzina@mail.arc.nasa.gov
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