What Are The Lift and Propulsive Force Limits At High Speed For The Conventional Rotor?

A wind tunnel test has been conducted to determine the characteristics of a conventional rotor in the high-speed regime. The investigation addressed the lift and propulsive force limits at advance ratios between zero and 0.64 and also examined the effect of operating near these limits on performance and control power, as well as blade loads. The impact of propulsive force requirements and advancing-tip Mach number on rotor lift limits was investigated. A final area to be examined was the influence of rotor torsional stiffness on lift limits and blade loads. The lift limit determined from the test data was aerodynamic and was caused by blade stall. There was no corresponding limit for propulsive force, but a model physical restriction limited the testing at 20 degrees of longitudinal cyclic. Performance data indicated that the conventional rotor, even at the advance ratios where the rotor incremental propulsive efficiency was the poorest achieved an efficiency of 85 percent, which is comparable to a propeller. The influence of drag cleanup from the present drag level (X/qd2 σ = 0.10) to that of an advanced helicopter improves the lift limit by 0.01 ΔC’T/σ and reduces the rotor power required by 25 to 30 percent. The test results indicate that a pure helicopter with a conventional rotor has the capability to operate in excess of 200 knots without wings or auxiliary propulsion.