Variable speed rotor studies represent a promising research field for rotorcraft performance improvementudand fuel consumption reduction. The problems related to employing a main rotor variable speed areudnumerous and require an interdisciplinary approach. There are two main variable speed concepts,uddepending on the type of transmission employed: Fixed Ratio Transmission (FRT) and Continuously VariableudTransmission (CVT) rotors. The impact of the two types of transmission upon overall helicopter performanceudis estimated when both are operating at their optimal speeds. This is done by using an optimization strategyudable to find the optimal rotational speeds of main rotor and turboshaft engine for each flight condition. Theudprocess makes use of two different simulation tools: a turboshaft engine performance code and a helicopterudtrim simulation code for steady-state level flight. The first is a gas turbine performance simulator (TSHAFT)uddeveloped and validated at the University of Padova. The second is a simple tool used to evaluate the singleudblade forces and integrate them over the 360 degree-revolution of the main rotor, and thus to predict anudaverage value of the power load required by the engine. The results show that the FRT does not presentudsignificant performance differences compared to the CVT for a wide range of advancing speeds. However,udclose to the two conditions of maximum interest, i.e. hover and cruise forward flight, the discrepanciesudbetween the two transmission types become relevant: in fact, engine performance is found to be penalizedudby FRT, stating that significant fuel reductions can be obtained only by employing the CVT concept. In conclusion, FRT is a good way to reduce fuel consumption at intermediate advancing speeds; CVT advantages become relevant only near hover and high speed cruise conditions
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