"Flowfield of a Swept Blade Tip at High Pitch Angles".
Komerath, N.M., Liou, S-G., Hyun, J-S.
AIAA Paper 91-0704, Aerospace Sciences Meeting, Jan. '91.
Abstract: Alleviation of retreating blade stall is crucial to reducing rotorcraft
vibrations. The flow around a swept blade tip configuration is studied using
laser sheet flow visualization and laser Doppler velocimety under conditions
which simulate the high angles of attack encountered on the retreating blade.
The first set of measurements was conducted under axial flow conditions
at high blade pitch settings of 15 and 30 deg. The blade was then installed
in a 7' x 9' wind tunnel and the measurements were repeated at a freestream
velocity that matched the tip speed in the first set. The issue of centrifugal
effects on the flowfield is studied. The vortex system of the rotor becomes
unsteady even at 15 degrees, and is difficult to discern at 30 degrees,
although thrust is produced. Vortex formation is observed at the tip, but
no strong vortex is seen at the inboard forward-swept "notch"
of the blade. Flow reversal is minimal over the rotor at 15 deg., while
it is substantial at 30 deg. inboard of the notch of the blade tip. Flow
separation and recirculation at 30 deg. angle of attack are much stronger
in the fixed-wing case than in the rotor case. Flow visualization confirms
the velocity data. Cross-flow vector plots provide evidence that the difference
in severity of flow separation is accompanied by the expected reduction
in inboard-directed spanwise flow at the trailing edge in the rotor case,
thus strengthening the hypothesis of stall aleviation by centrifugal pumping.