Thompson, T. L., "Velocity Measurements Near the Tip of a Hovering Model Rotor and a Preliminary Investigation of the Flow in the Wake", Ph.D. Thesis, Georgia Institute of Technology, Dec. 1986.

Prediction of the complex flow-field near a hovering helicopter rotor is strongly linked to a knowledge of the vortex shed from the tip. The performance of the rotor is sensitive to the geometry of the tip-vortexand its induced velocity. This research focussed on acheiving a greater understanding of the formation of the tip-vortex and the structure of its core in an effort to ultimately remove the empiricism from the analyses.

Detailed measurements with a Laser-Doppler Velocimeter ( LDV ) have been performed in the region of the blade tip and in the tip-vortex core of a single-bladed model rotor. The rotor blade is untwisted and of constant chord with a NACA0012 airfoil section and a square tip. The testing was conducted at a rotor tip-speed of 32 m/s , a chord-based Reynolds number of 280,000 and at two values of the thrust coefficient 0.0022 and 0.0057.

The data from the measurements near the blade tip were used to determine the tip-vortex inception point and study the subsequent growth. Velocity measruements at locations uneffected by a tip-vortex passage were generally in good agreement with predictions from a hovering rotor lifting surface code developed at Georgia Tech. A flow-visualization study was conducted to study the steadiness of the flow-field and the flow-field was found to be devoid of facility recirculation effects for the rotor geometry and the thrust coefficient. Velocity in the inner core was measured using an remote controlled off-axis device . The self-induced velocity components in the core were isolated from the data and used to arrive at a model for the core self induced motion. Expressions for both core circulatory and axial velocity components are presented. A method for including the previously neglected axial velocity component in the core in the analysis is discussed.

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