Dispelling the
Myth of the MV-22
By Lieutenant Colonel Kevin Gross, U.S. Marine
Corpsv
Proceedings, September 2004
 |
| In 2000, two tragic accidents grounded
the Marine Corps’ newest and most innovative aircraft, miring it
in controversy and casting doubt on its future. Rigorous testing
since then, however, has resulted in measures that should prevent
the unusual aerodynamic condition that caused the first accident
from happening again. (Photo by Mike Jones, U.S. Navy) |
The
MV-22
Osprey is a very capable medium-lift military transport aircraft the
Marine
Corps has needed for a long time. It is twice as fast, can carry three
times as much, and goes six times farther than the
CH-46E,
the aircraft it is replacing. It is no reach to say that if the MV-22
continues its current run of success in testing and is fielded as planned,
it will change everything about how maneuver warfare is conducted.
The Osprey, however, also is an airplane with an image problem, primarily
resulting from two highly publicized mishaps that killed 23 Marines
four years ago. The investigation into these accidents resulted in the
discovery and subsequent resolution of an aerodynamic condition affecting
all rotorcraft but unduly linked with the MV-22: vortex ring state (VRS).
On the evening of 8 April 2000, a flight of four MV-22s was conducting
a night assault mission to a small airfield in Marana, Arizona, when
the second airplane (or “Dash 2”) rolled nearly inverted on short final
and crashed, killing all on board. During the subsequent investigation,
it was discovered that the lead aircraft was almost 2,000 feet higher
than planned at the initial point (the location where the conversion
from airplane mode to VTOL [vertical take-off and landing] mode for
landing begins). The lead aircraft entered a steep approach profile
with a high rate of descent while it rapidly decreased speed for landing.
During the rapid deceleration, Dash 2 no longer could remain in trail
as briefed but came abeam of the lead’s right side. To return to the
trail position, Dash 2 flew slower and with a higher rate of descent
than his lead. At approximately 300 feet above ground level, with a
more than 2,000-feet-per-minute rate of descent and with less than 30
knots forward airspeed, the mishap aircraft started a right roll that
could not be corrected by the pilot.
The mishap investigation, having ruled out all other possibilities,
soon focused on the extremely high rate of descent at low altitude as
the primary cause of the accident. It was concluded that during the
descent, the aircraft entered an aerodynamic condition called vortex
ring state.
Vortex Ring State
Our search for the VRS boundary started with a thorough
review of analytical and wind tunnel research and slow-speed, high-rate-of-descent
flight testing. We soon discovered that the body of known actual flight-test
data for VRS in other rotorcraft was very small. There were only two other
rotorcraft flight research projects known to us at the time we began our
initial flight testing, one by NASA Langley in 1964 and one more recently
by the ONERA organization in France with a Dauphin helicopter. There was
a larger amount of theoretical data available, however, from the private
and academic sectors of flight-test research. The principal work we reviewed
was from a paper published in 1965 by Kyuichiro Washizu and Akira Azuma
of the University of Tokyo.
[1]
VRS is an aerodynamic condition in which the tangential airspeed at the
rotor is small (associated with low forward airspeed) and the airspeed
perpendicular to the rotor is high (associated with powered rate of descent).
VRS typically becomes a concern below 40 knots forward airspeed at high
rates of descent. To reach this condition, power must be applied during
the steep descent. Some might think that during VRS the rotor stalls,
but that is not the case. VRS is a reingestion state, not a stalled state.
Rotor lift creates a down flow of air, called induced velocity (Vi), and
the up flow created by the nearly vertical rate of descent is called vertical
velocity (Vv). When the induced velocity equals the vertical velocity,
VRS may occur, causing a reduction in rotor lift or increased sink rate.
VRS can occur as a rotorcraft settles down through its own vortex field
at slow forward airspeeds.
This condition is not peculiar to the tiltrotor; in fact, every rotary
winged aircraft is susceptible to it. A helicopter pilot flying a single
rotor system exits VRS by lowering his collective (reducing power with
his left hand) and pushing forward the cyclic (tilting his rotor disk
forward to accelerate) with his right hand. A tiltrotor pilot has another,
more-effective, option: he can move the nacelles into clean air.
Test Objectives
In developing the plan to understand VRS, Naval Air Systems Command directed
the MV-22 Integrated Test Team at Patuxent River, Maryland, to conduct
flight-test exploration of the Osprey tiltrotor’s high-rate-of-descent/low-airspeed
boundary. The initial test plan was developed to conduct partial- and
minimum-power descents to investigate the low airspeed descending flight
characteristics and determine the effects of the thrust-control lever
and cockpit-control inputs on handling qualities in this flight regime.
The objectives of the test effort were to define the boundary of VRS,
derive a fleet operational envelope, define the recovery technique from
VRS, determine applicability for warning systems, and document the condition
in pilot training ground school simulations and the NATOPS (Naval Air
Training and Operating Procedures Standardization) flight manual for the
tiltrotor. The first two objectives initially were approached as two phases,
to validate the current fleet low-airspeed rate-of-descent limit and document
the VRS boundary, and to evaluate the capability to expand the current
fleet limit. As the testing progressed, however, we soon realized that
breaking the test effort into two phases was not necessary, and we added
multiaxis control inputs and dynamic maneuvers to our test effort.
For the third objective, we knew that recovery from VRS, like the poststall
departure recovery of a fixed-wing aircraft, requires proper and timely
procedures. For the Osprey, the most powerful flight control is the pilot’s
ability to change nacelle angle (thrust vector) at up to 8° per second
through a thumb switch on the thrust-control lever. This ability to change
nacelle angle (and hence inflow angle at the rotor disk) would lead to
an effective and immediate recovery tool.
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For the fourth objective, several crew-alerting methods were discussed
to determine the feasibility of active VRS avoidance. A few of the mechanical
and tactile methods considered were a mechanical stick shaker, seat
shaker, or rudder pedal shaker, but these were quickly rejected because
of complexity, weight, and systems integration concerns. Instead, a
visual and aural warning system was developed to alert the aircrew when
the aircraft exceeded the NATOPS flight manual’s rate-of-descent limit.
For the fifth objective, data obtained from flight testing were used
to develop pilot training courseware, update the simulation model to
replicate VRS if deemed necessary, and update the NATOPS flight manual
with a comprehensive narrative description of VRS to include pilot cueing
to aid avoidance and emergency procedures for recovery should VRS occur.
Our first six-month period of flight testing started
within two and a half months of the Marana mishap, and only weeks after
preliminary mishap board results had been reported. On 11 December 2000,
a second MV-22 mishap not related to vortex ring state occurred, taking
the lives of all on board and resulting in the grounding of the Osprey
fleet. Several investigative bodies evaluated the MV-22, the most significant
of which was the blue ribbon panel.[2] The cause
of the second accident was found to be the combination of a software
anomaly and a hydraulic line failure. The grounding allowed the test
team to analyze flight data collected to date, refine the test plan,
and develop instrumentation, including a new low-airspeed measurement
system. After evaluating several low-airspeed sensors, the test team
selected the R. M. Young Model 81000 Ultrasonic Anemometer to provide
the desired airspeed to as low as ten knots forward velocity. When integrated
into the aircraft, this sensor provided the low-airspeed confidence
required to complete our flight test. In addition to the normal aircraft
display of flight information on the pilot primary flight display, we
added two flip-down liquid crystal displays that indicated sideslip
in one-degree increments and calibrated airspeed with one-knot accuracy
from the ultrasonic anemometer. The addition of the flip-down digital
displays, within the pilot’s central field of view, greatly reduced
workload and increased test-point efficiency.
Flight Test
Our test-plan strategy was simple: approach the unknown boundary from
higher air speed and down from above with increasing sink rate increments.
For the purpose of pilot build up and standardization, only one pilot,
chief test pilot Tom Macdonald of Boeing, flew each test point with
a small group of copilots assisting. The aircraft was configured with
nacelles full aft at 95°, flaps auto, and landing gear down. Each descent
track was completed at the same target airspeed, with a package of data
points at each 500-feet-per-minute descent increment, where we checked
stability, handling qualities, ride quality, aural signatures, and descent
arrest and recovery effectiveness.
Our testing was conducted at altitude with a target test band of 8,000-
to 7,000-feet altitude to allow for recovery well before the hard deck
of 3,500 feet above ground level. Each aircraft input at the target
airspeed and rate of descent required one dedicated descent. With the
addition of the Young Model 81000 low-airspeed system, we gained flight
efficiency by omitting the sawtooth climbs to determine true winds in
the test band for the next descent profile. By the end of our VRS test
effort, we had flown 62 flights for 104 flight hours and exceeded 5,600-feet-per-minute
rate of descent and flew as slow as ten knots calibrated forward airspeed.
During our testing, we experienced 12 roll-off events, 8 to the right
and 4 to the left. The direction of roll off was not predictable from
the cockpit. In fact, the cockpit characteristics approaching VRS were
not as well defined as in single-rotor helicopters. We noticed a slight
increase in vibration, rotor noise, and flight control loosening that
would not in every instance foretell of an impending roll off. Each
roll off, however, was characterized by a sudden sharp reduction of
lift on one of the two proprotors, resulting in an uncommanded roll
in that direction. We also noted that roll offs required nearly steady-state
conditions to trigger them. Any dynamic maneuvering tended to delay
or prevent a roll off from occurring. On many occasions, we entered
the VRS boundary during dynamic maneuvers and then exited the boundary
without encountering a roll off.
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Improvements to the Osprey’s pilot display
include an expanded rate-of-descent scale and a red line added behind
the vertical sink scale. Both features should help prevent future
encounters with vortex ring state. (Photo couresty of author)
|
Pilot recovery procedures from a VRS roll off are easy, immediate,
and effective. The pilot fixes the thrust-control lever and simultaneously
pushes the nacelle-control thumb wheel forward for two seconds. This
two-second beep forward moves the nacelles 12° to 15° lower, which immediately
takes the rotors out of the VRS condition as the aircraft accelerates
rapidly. If required, the pilot then uses lateral stick to level the
wings and then adds power to stop the rate of descent.
Avionics Improvements
Now that we knew where the VRS boundary was located, how the aircraft
responded during VRS, and the proper recovery procedures and techniques,
our focus turned toward avoidance of VRS. Boeing and Naval Air Systems
Command avionics and crew systems engineers developed a simple yet eloquent
method of keeping pilots away from VRS. We made two changes to our avionics
displays that increased the pilot’s situational awareness during low-speed,
high-rate-of-descent flight. First, we expanded the rate-of-descent
scale from 1,000 feet per minute to 2,000 feet per minute in 200-feet-per-minute
increments. Second, we added a red line behind the vertical sink scale
at the rate-of-descent limit along with a visual and aural “sink” warning
when the airspeed and rate of descent exceed the limits. The flight
display used by the pilot as the primary performance instrument in the
Osprey is shown above. The scale on the right side is the vertical speed
indicator. The indicator’s arrowhead is pointing to an 800-feet-per-minute
rate of descent. The airspeed box is on the left of the display and
indicates 39 knots. With this combination of airspeed and rate of descent,
the pilot has exceeded the existing rate-of-descent limit and now hears
“sink rate, sink rate” in his headset and sees the red “sink” warning
in the display near the top and to the right of center. It is important
to note that this warning system is not a predictor of VRS, but a rate-of-descent
limit to keep the pilot away from VRS.
Where We Go from Here
Our ultimate goal for this flight-test effort was to understand fully
the aerodynamic effects of vortex ring state on the tiltrotor, to define
the recovery procedures should the pilot encounter VRS, and, most important,
to develop warning signals to keep pilots away from this condition.
Pilot awareness of VRS and avoidance with rate-of-descent limits are
the only tools available to prevent a high-rate-of-descent mishap from
taking more lives. Our current rate-of-descent limit is 800 feet per
minute below 40 knots calibrated airspeed (KCAS), increasing linearly
to 1,600 feet per minute at 80 KCAS. Presently, Naval Air Systems Command
is evaluating the potential to expand the rate-of-descent envelope in
the 30-to-50-knot airspeed range to provide the user communities more
capability during flight tests and approaches to landing.
So what have we found in our 14-months of low-airspeed, high-rate-of-descent
testing in search of VRS?
- Above 40 KCAS, VRS will not occur, regardless of sink-rate magnitude.
- The lower the disk loading (the ratio between an aircraft’s weight
and rotor size), the smaller the sink rate where VRS might occur.
Conversely, in the case of the MV-22 with higher disk loading, VRS
may occur at a much larger sink rate.
- VRS requires a nearly steady-state condition. Any maneuvering tends
to delay or prevent a roll off.
- As both sink rate increases and airspeed decreases, periodic rotor
thrust fluctuations increase.
- As the VRS boundary is approached, handling qualities degrade because
of unsteady flows at the rotor(s).
- Entry into fully developed VRS may be characterized by a sudden,
sharp reduction of net thrust at the rotor.
- Recovery from VRS is immediate and effective using two seconds of
forward nacelle tilt.
- There is a large margin of safety between rate of descent limit
and VRS boundary below 40 KCAS.
With our test effort behind us, the Integrated Test Team at Patuxent
River is confident we fully understand the location of the VRS boundary
for the tiltrotor, the aircraft roll-off characteristics during steady
maneuvers within the boundary, and the immediate and effective recovery
procedures. We have developed avionics warnings to aid pilots in avoiding
high rates of descent at low airspeed. The fleet now has a better understanding
of the capabilities of the MV-22 and will be confident to fly in harm’s
way knowing vortex ring state never will be encountered again.
