A wind-tunnel investigation of a 15-foot-diameter rotor was made to explore rotor operation at tip-speed ratios near 1.00. A two-blade teetering-type rotor system was tested in the Langley full-scale tunnel for tip-speed ratios from O.65 to 1.45. Representative data were compared with theoretical calculations to indicate the predictability of the experimental trends obtained.nThe results indicate that rotor efficiencies as high as 13 can be attained at the highest tip-speed ratio investigated. Although high rotor efficiencies are thus shown to be possible, a problem requiring further study was noted. The slope of the variation of rotor thrust with collective pitch was negative for a constant disk attitude at tip-speed ratios greater than 1.00. The negative slope resulted primarily from the increased sensitivity of the rotor longitudinal flapping response with respect to collective pitch compared with the sensitivity of the flapping response with respect to angle of attack. The theoretical calculations of the rotor characteristics also indicated this reversal phenomenon.
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