Performance of a Turboshaft Engine for Helicopter Applications Operating at Variable Shaft Speed

An off-design steady state model of a generic turboshaft engine has been implemented to assess the influence of variable free power turbine (FPT) rotational speed on overall engine performance, with particular emphasis on helicopter applications. To this purpose, three off-design flight conditions were simulated and engine performance obtained with different FTP rotational speeds were compared. In this way, the impact on engine performance of a particular speed requested from the main helicopter rotor could be evaluated. Furthermore, an optimization routine was developed to find the optimal FPT speed which minimizes the engine specific fuel consumption (SFC) for each off-design steady state condition. The usual running line obtained with constant design FPT speed is compared with the optimized one. The results of the simulations are presented and discussed in detail. As a final simulation, the main rotor speed Ω required to minimize the engine fuel mass flow was estimated taking into account the different requirements of the main rotor and the turboshaft engine.