An Experimental Investigation of Ground Effect on a Quad Tilt Rotor in Hover and Low Speed Forward Flight
An Experimental Investigation of Ground Effect on a Quad Tilt Rotor in Hover and Low Speed Forward Flight
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Date
2006-11-06
Authors
Radhakrishnan, Anand M
Advisor
Schmitz, Fredric H
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Abstract
The performance of a Quad Tilt Rotor (QTR) in helicopter mode was experimentally
studied in ground effect (IGE) and out of ground effect (OGE). A 0.03
geometrically scaled fuselage/wing model of the QTR was tested in hover and very
low speed forward flight. Fixed-pitch propellers were used to model the rotors. In
order to avoid the boundary layer problems associated with wind tunnel testing of
rotorcraft IGE, a unique moving setup was developed for testing in forward flight.
The effect of ground proximity was tested by varying the height of the model above
the ground. Download on the airframe; thrust, torque and rpm of the rotors, and
pressures along the centerline of the bottom of the fuselage were measured. The
downwash distributions of the rotors were measured and found to compare well
with V-22 rotor measurements. Tuft flow visualization was used to identify the
physical processes causing changes in the download and pressure measurements. An
uncertainty analysis was performed on the measured quantities to determine the
95% confidence levels.
A strong download (9% of the rotor thrust) was observed in hover, OGE. The
download reduced substantially IGE and become an upload (9% of the rotor thrust),
when the wheels of the QTR were on the ground. The upload IGE was found to be
caused by the entrapment of the rotor wakes under the fuselage. The upload was
observed to persist in forward flight IGE, but reduced slightly at certain low skew
angles. The measured downloads, coupled with power measurements, indicate that
for a given power, the available vehicle thrust greatly increases IGE. Therefore, the
QTR displays a potential for significant increase in payload carrying capacity by
operating IGE.