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Exploratory Study of the Nature of Helicopter Rotor Blade Boundary Layers
OHIO STATE UNIV RESEARCH FOUNDATION COLUMBUS
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An experimental study was conducted using flow visualization techniques to investigate the nature of the boundary layer on a helicopter rotor. Hovering and forward flight data were obtained however, efforts were concentrated on hovering unanticipated boundary layer behavior was revealed. The primary flow visualization technique involved the use of ammonia injected into the boundary layer at the leading edge. The blade surface was chemically coated, and as the ammonia moved with the local airflow, it formed a trace on the surface indicative of the boundary layer flow. The hovering traces initially moved chordwise along the surface and then continued aft along the blade in a somewhat diffuse pattern. Similar traces were found over wide ranges of pitch angles and rotor speeds. It is hypothesized that a standing laminar separation bubble exists on the blade surface aft of the peak pressure position. No indication of any separation bubbles could be found on the forward flight traces. An exploratory study of the possible uses of ordinary claw-type pressure probes was undertaken to determine whether such probes could provide an indication of the flow in the boundary layer of a model rotor.
APPROVED FOR PUBLIC RELEASE