of hardware to accurately measure the
Osprey’s forward speed at less than 40
knots. Then, from November 2002 to
July 2003, Macdonald and his copilots
did sixty-two HROD tests, flying a total
of 104 hours over a restricted flight range
between the towns of Cambridge and
Salisbury on Maryland’s eastern shore.
Macdonald would climb to 10,000 feet,
then tilt the rotors up into helicopter
mode and slow to a target airspeed. As
Macdonald held it there for a bit, a chal-
lenge in itself, the copilot would put his
hands on his own set of controls, just in
case. To duplicate the way the Osprey that
crashed at Marana had been flying, Mac-
donald would lower the landing gear, tilt
the nacelles back to 95 degrees, and start
reducing power. The Osprey would begin
to drop. Macdonald would let it fall until
they hit a target rate of descent, then adjust
ally trying to lose control of the Osprey,
they were just trying to find the edge of
the envelope, the point where they felt
vortex ring state coming on. They got
there a number of times without incident.
“The thrust would begin to oscillate,”
Macdonald told me. “A little up and down
motion would settle into the aircraft that
we weren’t commanding.” The Osprey
would rock a little bit in some cases, and
at the lowest speeds they tested, as slow
as 10 knots forward speed while plunging
toward the ground, the pilots would hear
“this eerie howling and audio sound of
the air rush,” Macdonald said. “We’d just
get really silent and quiet.” Eleven times
during the tests Macdonald flew, one of
the rotors went into vortex ring state and
the Osprey did a sudden, uncommanded
roll—seven times to the right, four times
to the left. Each time, Macdonald was able
The Osprey suddenly snap-rolled right more
violently than Macdonald had ever seen it
do. Instinctively, he pushed the stick left as
the roll began but the Osprey didn’t respond.
the power to hold the Osprey at a steady
sink rate. At that point, still descending,
Macdonald and his copilot would talk
into their microphones about the Osprey’s
stability and handling, the quality of the
ride, and whether they heard anything
unusual. They were looking for ways a
pilot might detect the onset of vortex ring
state before getting into it. They repeated
the flight profile over and over during
those months, taking their measurements
at every 500-foot increment of sink rate at
a series of ever slower airspeeds.
As predicted, they found it hard to
put the Osprey into vortex ring state. The
boundary for a helicopter was a sink rate
somewhere around 800 feet per minute
at 40 knots or less of forward speed. Mac-
donald had to let the Osprey sink at least
2,500 to 2,600 feet a minute at 40 knots
before it would near vortex ring state. Even
at slower forward speeds, the boundary
was a sink rate of around 1,700 feet per
minute. He and his copilots weren’t actu-
to recover by pushing the thumbwheel
switch to tilt the nacelles forward and
put the rotors into undisturbed air. Two
seconds was usually all it took. Those
seconds, though, gave Macdonald and his
copilots some hairy moments.
Sink Rate, Sink Rate
The hairiest was on July 17, 2003. The
tests already had achieved their primary
goal—charting the Osprey’s envelope for
vortex ring state—a couple of months
earlier. Based on that envelope, the pro-
gram had developed warning devices for
the Osprey to alert pilots when they were
flirting with vortex ring state. One was
visual, a red light on the control panel in
front of each pilot that would flash SINK,
SINK if an Osprey exceeded a safe rate
of descent. The other was a recording of
a woman—a device pilots call a “bitch-
ing Betty”—who would be heard in the
pilot’s headset saying “sink rate, sink rate”
in an urgent monotone if the limit were
exceeded. The HROD tests were continu-
ing, though, because Navair was trying to
satisfy key critics that the tests had been
adequate. That July 17, Macdonald and
Grohsmeyer were flying 7-10 knots and
descending at more than 2,300 feet per
minute, well beyond the vortex ring state
envelope already established. They were
dropping like a rock while a special test
device alternated the lateral tilt of each
rotor to see if that would prevent vortex
ring state. Macdonald was on the radio
with an engineer on the ground when the
Osprey suddenly snap-rolled right more
violently than Macdonald had ever seen
it do. Instinctively, he pushed the stick
left as the roll began but the Osprey didn’t
respond. By the time Macdonald recog-
nized what was happening, the Osprey
was flying on its side, left wing up and
right wing down, and spiraling toward the
ground ever faster. Macdonald pushed the
thumbwheel to tilt the nacelles forward
as fast as they would go, all the way to air-
plane mode. The seconds it took seemed
like minutes, but finally he regained con-
trol and straightened the Osprey out. For
a moment, there was silence in the cockpit
and over the radio. Macdonald, Grohs-
meyer and the engineers on the ground
knew the pilots had just had a close call. If
the Osprey had rolled all the way upside
down in helicopter mode, there was no
telling what damage they might have done
to the aircraft—or whether they could
have regained control at all.
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APRIL 2010 | ROTOR & WING MAGAZINE
