Jet Propulsion Applied to Helicopter Rotors

Some of the factors affecting jet-driven rotor performance are discussed and it is shown that the relatively high tip speeds required in order to realize a reasonable jet efficiency result in a considerable reduction in the aerodynamic efficiency of the rotor. Although this aerodynamic efficiency may be improved by decreasing the rotor solidity, a practical limit is reached, in the case of jet drives involving airflow through the blades, beyond which the specific fuel consumption increases almost asymptotically with decrease in solidity. However, the weight-saving possible with such drives, and their simplicity, may in certain cases offset their low aerodynamic and propulsive efficiencies. A rough comparative performance and weight estimate are made between conventional and jet-driven helicopters in an at tempt to evaluate their relative merits.