NASA
Technical Memorandum
85895
NASA-TM-85895 19840011266
In-Flight
Acoustic
Testing
Techniques
Using
the
YO-3A
Acoustic
Research
Aircraft
J.L. Cross and M.E. Watts
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February 1984
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NASA Technical Memorandum 85895
In-Flight
Acoustic
Testing
Techniques Using
the
YO-3A
Acoustic
Research
Aircraft
J.
L.
Cross
M.
E.
Watts, Ames
Research
Center,
Moffett
Field, California
NI\S/\
National Aeronautics and
Space Administration
Ames Research
Center
Moffett Field, California
94035
IN-FLIGHT
ACOUSTIC
TESTING
TECHNIQUES
USING
THE
YO-3A
ACOUSTIC
RESEARCH
AIRCRAFT
Jeffrey
L.
Cross*
and
Michael
E.
Watts*
Ames
Research
Center,
NASA,
Moffett
Field,
California
Abstract
This
report
discusses
the
flight
testing
techniques
and
equipment
employed
during
air-to-
air
acoustic
testing
of
helicopters
at
Ames
Research
Center.
The
in-flight
measurement
tech-
nique
used
enables
acoustic
data
to
be
obtained
without
the
limitations
of
anechoic
chambers
or
the
multitude
of
variables
encountered
in
ground
based
flyover
testing
•.
The
air-to-air
testing
is
made
possible
by
the
NASA
YO-3A
Acoustic
Research
Aircraft.
This
"Quiet
Aircraft"
is
an
acoustical-
ly
instrumented
version
of
a
quiet
observation
aircraft
manufactured
for
the
military.
To
date,
tests
with
the
following
aircraft
have
been
con-
ducted:
YO-3A
background
noise;
Hughes 500D;
Hughes AH-64;
Bell
AH-lS;
Bell
AH-lG.
Several
system
upgrades
are
being
designed
and
implemented
to
improve
the
quality
of
data.
This
report
will
discuss
not
only
the
equipment
involved
and
air-
craft
tested,
but
also
the
techniques
used
in
these
tests.
In
particular,
formation
flying,
po-
sition
locations,
and
the
test
matrices
will
be
discussed.
Examples
of
data
taken
will
also
be
presented.
Nomenclature
C
t
thrust
coefficient
g
gravitational
constant
L
rotor
lift
M
t
advancing
tip
Mach
number
r
rotor
radius
R
gas
constant
T
outside
air
temperature
V
aircraft
velocity
y
specific
heat
ratio
~
advance
ratio
p
density
·cr
rotor
solidity
Q
rotational
velocity
Introduction
Flight
testing
is
an
inherent
part
of
aero-
space
engineering.
Every
facet
of
aircraft
design
is
at
one
time
or
another
flight
tested
to
deter-
mine
its
true
"real
world"
characteristics.
Many
of
these
facets
are
relatively
simple
to
measure
in
a
flight
environment
while
others
provide
ob-
stacles
which
are
only
recently
being
overcome.
One
such
area
is
acoustics
of
helicopters.
While
it
is
true
that
air-to-ground
and
internal
noise
analysis
have
been
performed
for
many
years,
it
is
only
recently
that
air-to-air
acoustics
tests
have
been
attempted.
These
have
been
in
response
to
the
increasing
concern
over
helicopter
noise
levels.
In
response
to
these
concerns,
NASA
and
the
Army
have
had
an
ongoing
acoustic
research
program
in
the
area
of
helicopter
noise
for
many
years.
As
research
progressed,
it
became
evident
that
to
fully
understand
the
various
noise
genera-
*
Aerospace
Engineer,
Member
AIAA.
1
tion
processes
involved,
the
methods
used
in
ob-
taining
the
noise
data
were
inadequate
and a
dif-
ferent
method
of
testing
would
be
necessary.
This
realization
was
brought
about
because
the
two
main
sources
of
impulsive
noise
in
helicopters
are
blade
vortex
interaction
and
shock
effects.
As
both
of
these
occur
almost
entirely
in
forward
flight,
a means
of
accurately
measuring
these
gen-
eration
processes
was
needed.
Methods
used
to
measure
helicopter
noise
in-
cluded:
ground
based
microphone
arrays;
exter-
nally
and
internally
mounted
microphones;
and
wind
tunnel
tests.
Each
of
these
methods
provided
good
data
in
certain
areas,
but
they
all
have
drawbacks
which
point
t~
the
necessity
of
meas-
uring
noise
in
an
airborne
fixed
relative
refer-
ence
frame.
Drawbacks
for
the
other
methods
include:
1.
reflected
noise
interference
from
the
ground
or
other
surfaces
2.
doppler
effects
caused
by
relative
speeds
between
source
and
microphone
3.
airframe
attenuation
for
internally
mounted
microphones
4.
near
field
effects
interfering
with
far
field
measurements
5.
atmospheric
effects
caused
by
altitude
differences
in
flyover
testing.
To
eliminate
the
other
effects,
a method
of
measurement
was
needed
which
caused
the
micro-
phones
to
be
traveling
in
the
far
field
with
the
same
velocity
as
the
aircraft
being
measured,
and
away from any
reflective
surfaces.
The
answer
was
to
mount a
microphone
on
an
aircraft
which
would
fly
at
the
same
speed
as
the
helicopter.
This
was
done on
the
OV-l
Mohawk
aircraft
in
1975 by
the
Army
(references
land
2).
While
this
eliminated
the
problems
listed
above,
it
added
the
problem
of
background
noise
overwhelming
the
noise
being
measured.
In
general,
the
background
noise
of
the
Mohawk
would mask
the
helicopter
noise.
However,
if
the
specific
frequencies
being
investigated
were
known,
the
engines
could
be
tuned
to
seperate
the
Mohawk
blade
passage
and
engine
noise
for
those
frequencies.
This
worked
satisfactoraly
if
specific
noise
data
were
being
sought,
but
was
cumbersome
if
general
noise
data
was
the
objec-
tive.
Also,
with
a
stall
speed
of
80
knots,
the
Mohawk
was
not
well
suited
for
measuring
blade
vortex
interaction
effects
which
are
most
promi-
nent
in
slower
flight.
This
brought
about
the
in-
strumentation
and
use
of
a
YO-3A
"Quiet"
aircraft
belonging
to
the
Federal
Bureau
of
Investigation
in
1976
for
air-to-air
acoustic
testing.
The
success
of
this
testing
led
to
NASA's
acquisition
of
a
YO-3A
dedicated
to
in-flight
acoustic
re-
search.
This
report
deals
with
the
present
and