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Tilt-rotor aircraft have great potential to relieve
air traffic congestion by ferrying passengers directly
to and from vertiports located near urban areas. Since
these aircraft operate like helicopters during landing,
the tilt rotors produce highly impulsive noise owing
to blade-vortex interactions (BVI). Thus, reducing BVI
noise is a key enabling technology that will allow tilt
rotors to operate in populated areas. Higher harmonic
control (HHC) was shown to be highly
effective in reducing BVI noise on tilt rotors. For a
three-bladed rotor, an HHC system generates low-amplitude
blade-pitch oscillations two, three, and
four times per rotor revolution that are superimposed
with the primary control input for trim. In addition,
practical applications of HHC to tilt rotors require the
development of suitable signal processing techniques
to identify the radiated BVI noise for feedback
control. A method using pressure sensors mounted on
the blades for identification and control of BVI noise
was demonstrated in an 80- by 120-foot wind tunnel
test of a full-scale XV-15 tilt rotor. The controller
successfully reduced the BVI noise level by more
than 5 decibels, as indicated by the measured noise
contours under the rotor shown in the figure on the
following page.
Point of Contact: K. Nguyen
(650) 604-5043
knguyen@mail.arc.nasa.gov
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Fig. 1. Noise controller reduces noise measured on a plane under the XV-15 rotor: tunnel speed 69 knots, rotor thrust 5,500 pounds.
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