XV-15 litho - NASA's History Office

Figure 59.
Hover acoustics tests
during low wind conditions
at sunrise.
(Ames Photograph
AC90-0448-31)
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A further series of noise measurements
was made during hover tests at
Ames in December 1990, and during
terminal area and flyover tests at the
Crows Landing NALF in August and
September 1991, with the new composite
blades installed on XV-15
N703NA. These were the first such
experimental measurements from
flight data with a proprotor blade configuration
other than the original metal
blades. The data were acquired to validate
acoustics analyses being developed
by researchers at the Langley
Research Center, under the NASA
Short-Haul Civil Tiltrotor (SHCT) program.
These tests were a joint effort between the Langley acoustics engineers
and technicians and the Army/NASA TRRA team at Ames. Operations were conducted
just after sunrise (shown in figure 59) to ensure low wind conditions (usually
less than 3 knots) during noise data measurements.
Additional investigations of the terminal area noise generated by the XV-15
with metal blades were conducted by Bell at a remote site near Waxahachi,
Texas, in October and November of 1995. The relatively level, undeveloped terrain,
far from major roads and undesirable background noise, provided an ideal
environment for this work. A large microphone array was set up around the target
landing point while a mobile laser tracker from Ames was placed nearby to
measure the position of the XV-15 during the tests. This study focused on the
effect of approach profile on the intensity of the noise propagated to the ground,
and utilized approach conditions examined earlier during simulation evaluations
of terminal area operations in the Ames Vertical Motion Simulator. Bill Decker,
the NASA Ames principal investigator for the simulation studies, participated in
the terminal area test planning and test operations. To provide flight path guidance,
the XV-15 used a Global Positioning System (GPS) monitoring research
flight director which was developed by Mark Stoufflet and Colby Nicks of Bell.
A Langley team acquired acoustic data from an array of 33 microphones covering
an area of five miles long and 1.25 miles wide. The test results confirmed
that appropriate combinations of aircraft configuration and flight path profile
could be used to significantly reduce the noise level and footprint area during
tilt rotor approaches.
In December 1995, with plans being developed for an acoustics test of the XV-
15 metal-bladed proprotor in the acoustically treated test section of the Ames 80-
by 120-foot wind tunnel, a special flight investigation was required to obtain
comparable free flight noise data to determine the effect of the wind tunnel walls
on the measured sound. The evaluation involved flying the XV-15 behind, and in