The U.S. Navy is currently conducting an
extensive land-based and shipboard flight test
program to evaluate the V-22 Osprey tilt-rotor
aircraft. During shipboard tests aboard USS SAIPAN
(LHA 2), V-22 test pilots experienced a roll anomaly
while descending from hover. This anomaly consisted
of uncommanded roll oscillation of approximately
a 90-degree amplitude. Postflight analysis
revealed that the roll was consistent with factors that
might include an aberration in the aircraft's flight
control systems logic, or air-load asymmetries caused
by vehicle/ship interactional aerodynamics. Based on
related ongoing shipboard air-wake measurement
efforts at the Fluid Mechanics Lab (FML), the Navy
requested FML support in investigating possible
aerodynamic causes for the incident.
In an attempt to identify the cause of this
anomaly, a 1/120-scale model of the V-22 Osprey's
side-by-side, three-bladed, counterrotating twin rotor
system was designed, constructed, and installed in
the FML 32- by 48-inch wind tunnel (figure 1).
Significant full-scale incident conditions were
duplicated in the wind tunnel, including relative
wind speed and direction, and deck spotting
location, and the presence of the upwind parked
H-46 helicopter. To facilitate smoke (laser sheet and
ambient illumination) and tuft-flow visualization, the
tunnel was operated at approximately 40 feet per
second. To ensure the correct ratio of rotor
downwash to incoming ambient wind velocities, the
rotors were spun at approximately 20,000 revolutions
per minute, producing average downwash velocities
of 60 feet per second. A 1.5-horsepower universal
motor, speed-governed by a feedback controller,
powered the rotors via a system of gears and belts.
The rotors and motor were mounted to an
instrumented roll pivot, which was able to record roll
moment variations, by means of a strain gage, as the
model was moved to various locations over the deck.
This assembly was mounted on an automated
traversing and data collection system, enabling
precise positioning and measurements around the
ship deck.
The tests investigated flow patterns in the vicinity
of each rotor disk. Flow-field pattern variations with
height demonstrated clearly that a large ground
vortex (figure 2) forms upwind of the inboard rotor at
very low wheel heights above deck. The presence of
this ground vortex is consistent with prior full-scale
tests and with video from the actual incident. When
the rotor system was moved laterally, the vortex
tended to vary in size and location, depending on
model proximity to the deck edge. The effects of ship
superstructure on flow asymmetries showed that the
port-side superstructure tends to trap the upwind
ground vortex, whereas superstructure removal
produced smaller upwind vortices and flow
asymmetries.
The tests also investigated the effect of deck
proximity on the vehicle roll moment. When the
rotor system was moved laterally from a position
outboard of the deck edge inwards to a position over
the desired landing spot (figure 3), the roll moment
was found to vary significantly, further substantiating
the flow-visualization findings. The roll moment was
found to also vary with traverse height, providing
further insight into the complicated shipboard
rotorcraft interactional environment.
Point of Contact: G. Zilliac
(650) 604-0613
gzilliac@mail.arc.nasa.gov
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