Helicopter Performance Improvement with Rotor Speed Variation and Chord Extension Morphing

This paper examines first the performance benefits associated with rotor speed variation in cruise, followed by the benefits achieved when rotor chord extension morphing is combined with RPM variation. The analysis is conducted on a UH-60A helicopter using the Rotorcraft Comprehensive Analysis System (RCAS). Rotor speed variations up to ±15% NR and a 20% chord extension over a 63-83% span is considered. The performance benefits are examined over a 40-150 kts airspeed range, at low (16,000 lbs), moderate (18.300 lbs) and high (22,000 lbs) gross weight, and altitudes varying from sea level to 12,000 ft. Results show that with RPM variation alone, power reductions up to 19.5% are achievable by reducing the rotor speed to 85% NR for low gross weight, sea level operation. Since RPM reduction requires increase in rotor pitch, both the allowable RPM reductions and the performance benefits diminish with increasing gross weight and altitude. At very high gross weight and altitude, an 8.5% power reduction is obtained by speeding up the rotor to 105% NR and alleviating stall. Results show small increases in rotor collective, longitudinal cyclic and coning with RPM reduction, while main rotor shaft torque increases remain under 10%. Combining chord extension with RPM variation is actually detrimental for low gross weight, low altitude operation due to the profile drag penalty associated with increased solidity. At high altitudes and gross weights, however, the introduction of chord extension morphing results in additional power reductions of up to 4-6% (beyond those achievable with RPM variation alone). In some cases addition of chord extension allows the slowing down of the rotor relative to using RPM variation alone.