Variable-Speed Rotor Helicopters: Performance Comparison Between Continuously Variable and Fixed-Ratio Transmissions

Abstract

Variable-speed rotor studies represent a promising research field for rotorcraft performance improvement and fuel-consumption reduction. The problems related to employing a main rotor variable speed are numerous and require an interdisciplinary approach. There are two main variable-speed concepts, depending on the type of transmission employed: fixed-ratio transmission and continuously variable transmission rotors. The impact of the two types of transmissions upon overall helicopter performance is estimated when both are operating at their optimal speeds. This is done by using an optimization strategy able to find the optimal rotational speeds of the main rotor and turboshaft engine for each flight condition. The process makes use of two different simulation tools: a turboshaft engine performance code and a helicopter trim simulation code for steady-state level flight. The first is a gas turbine performance simulator (called TSHAFT), developed and validated at the University of Padova. The second is a simple tool used to evaluate the single blade forces and integrate them over the 360 deg revolution of the main rotor, and thus to predict an average value of the power load required by the engine. The results show that the fixed-ratio transmission does not present significant performance differences compared to the continuously variable transmission for a wide range of advancing speeds. However, close to the two conditions of maximum interest (that is, hover and cruise forward flight), the discrepancies between the two transmission types become relevant: in fact, engine performance is found to be penalized by fixed-ratio transmission, proving that significant fuel reductions can be obtained only by employing the continuously variable transmission concept. In conclusion, fixed-ratio transmission is a good way to reduce fuel consumption at intermediate advancing speeds; continuously variable transmission advantages become relevant only near hover and high-speed cruise conditions.