Mahalingam, R., and Komerath, N. M., “ Measurements of the Near-Wake of a Helicopter Rotor in Forward Flight”, AIAA Paper 98-0692, 36th AIAA Aerospace Sciences Meeting, January 1998, Reno, NV.

This paper describes initial measurements of the near wake of a 2-bladed teetering, untwisted, square-tipped rotor in forward flight.  Issues of periodicity and repeatibility of the core are studied using laser velocimetry and flow-visualization on the front of the rotor wake where cycle to cycle variations are expected.  Core passage uncertainty from cycle to cycle is less than 1deg of rotor azimuth. Velocity measurements on the advancing blade side show wake-like core-axial velocities higher than the core circumferential velocity for the first 180deg of vortex age.  Both the axial and circumferential velocities approach 50% of blade tip speed. The inboard vortex sheet has substantial wake-like velocities and rolls up into a concentrated circulatory region of high wake-like axial velocity rotating opposite to the tip-vortex, within 30deg from the blade.  The fully-developed values of the vortex core circulation, radius and axial velocity remain constant over 180deg of age as current measurements indicate. Measurements on the front, rear and retreating blade side  of the rotor are in progress.

Complete paper in PDF format