Helicopter Performance Improvement with Variable Rotor Radius and RPM

This paper examines rotor power reductions achievable through a combination of radius and RPM variation. The study is based on a utility helicopter similar to the UH-60A and considers +17% to –16% variation in radius and ±11% variation in RPM about the baseline, over a range of airspeed, gross weight, and altitude. Results show that decreasing RPM alone effectively reduced power at cruise velocities in low-and-light conditions, but the power reductions diminished at increasing altitude and/or gross weight, and in low-speed flight. Increasing radius alone, on the other hand, had greatest effectiveness in power reduction in high-and-heavy operating conditions and at lower flight speeds. When radius and RPM variation is used in combination, minimum RPM is always favored, along with radius increases at increasing altitude and gross weight, and in low-speed operation. At low-to-moderate gross weight, the significant power reductions seen in cruise and at low altitude with RPM variation alone are obtained even at higher altitude, and over the airspeed range, using radius and RPM variation in combination. In high-and-heavy conditions, the combination of RPM reduction and radius increase yields very large power reductions of over 20% and up to 30% over the baseline. Power reduction in low-and-light conditions comes almost entirely from profile power reduction due to RPM decrease. In cruise and high-speed flight, the profile power reductions progressively give way to induced power reductions at increasing gross weight and altitude. At low speeds, reduction in induced power due to increased radius and decreased disk loading dominates.