The Reverse-Velocity Rotor - 300 Knots Plus Hover Capability

The forward-flight regime of present helicopters is limited by the rotor propulsive force and controls capability and by high-speed-lift limitations or loads. All these limitations are a result of the following: - Compressibility on the advancing tip; - Loss of lift due to reverse flow on the inboard end of the retreating blade; - Stall on the outboard end of the retreating blade . An advanced rotor concept has been developed to achieve at least 300 knots that will avoid or eliminate these three problems. A definition of this concept is presented, along with a description of a model that was fabricated to demonstrate the characteristics of the concept. The model-test results are very encouraging and show 2/rev cyclic providing a significant improvement in rotor lift capability with no degradation of lift-to-drag ratio. A model-rotor lift-to-drag ratio of 9 was measured at an advance ratio of 1.2 and a flight speed of over 300 knots. Blade loads and vibration were measured which indicate that the blade must be carefully designed to avoid operating on or close to a resonant condition when reducing the rotor rpm for high-speed flight. Rotor control capability similar to a conventional low-speed helicopter was demonstrated at high speed. The feasibility of this concept has been demonstrated successfully. Estimates of the full-scale rotor performance indicate that a lift-drag ratio of approximately 12 would be achieved at 300 knots, which is very promising. The major technological requirements for follow-on development which establish the next step in the development program of this advanced rotor concept are discussed.