Results of an investigation in the Langley full-scale tunnel of the hovering performance of large-scale twin-rotor-helicopter models are pre¬sented. Measurements of thrust, torque, and rotor flapping are given for overlapped (approximately 76 percent of blade radius) and nonoverlapped configurations and for two different rotor solidities. The measured per¬formance is compared with single-rotor measurements and with available rotor theory. These tests show that the hovering performance of a single rotor and of two rotors without overlap or vertical offset are the same and hence may be calculated by single-rotor theory. These tests in con¬junction with results of previous coaxial-rotor tests show that the per¬formance of highly overlapped rotors can be reasonably predicted by available rotor theory.
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