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Recent airframe noise studies at Ames Research
Center and elsewhere have identified flap side-edge
noise as an important component of aircraft landing
noise. During aerodynamic tests of an Air Force
stealth fighter design (called SHARC) in the Ames
40- by 80-Foot Wind Tunnel, it came to our attention
that stealth technology might have acoustic attributes.
Specifically, the Continuous Moldline Technology
(CMT) developed for SHARC eliminates flap side
edges by blending the flap and wing trailing edges
with a flexible material that is load bearing. This led
to a 7- by 10-foot wind tunnel aeroacoustic test of a
two-dimensional airfoil with a simple hinged flap
with and without a CMT modification (figure 1).
To investigate CMT performance, a 70-element
phased microphone array technology (PMAT)
acoustic system was installed in the Ames 7- by
10-Foot Wind Tunnel test section wall. The array is
designed for visualization of noise sources and
determination of their individual strengths. Noise
radiation from the vertically mounted wing/flap
model revealed a virtual elimination of the side-edge
noise in the flyover direction. Because of the large
noise reduction, it was necessary to recess the
microphones in order to reduce flow-induced noise
and increase the signal-to-noise ratio. A strong Kevlar
cloth covered the recess. With this array geometry,
noise levels that were 20 decibels below the wind
tunnel background noise could be identified.
With the new array, it was found that the CMT
flap was at least 6-8 decibels quieter than the simple
hinged flap, as shown by the phased array images of
figure 2. Background noise made it impossible to
identify noise sources below that level. Previous work
indicates that the simple hinged flap is quieter than
the conventional Fowler flap used on current air
transports. Hence, new aircraft designs such as the
Blended Wind Body, which can operate with a
simple high lift system, would benefit from the low
noise of simple hinged or CMT flap systems. Aerodynamic
performance data were acquired simultaneously
with the acoustic data.
Point of Contact: P. Soderman
(650) 604-6675
psoderman@mail.arc.nasa.gov
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Fig. 1. Upstream view of airfoil model with simple
flap (upper white) blended to wing by CMT section.
Phased microphone array is mounted on test section
right wall.
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Fig. 2. Noise source images at simple flap tip (upper)
eliminated by CMT flap (lower).
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