| Popis: |
Variable speed rotors (VSRs) represent an innovative research field for the development of new rotorcraft designs. The possibility to change the rotational speed of a helicopter rotor, as a function of the specific flight condition, makes it possible to achieve optimum performance and relevant fuel consumption reductions. However, issues related to employing a main rotor variable speed are numerous and require an interdisciplinary approach. The present study represents the first effort in the open literature aimed at understanding the performance implications of coupling helicopter trim and turboshaft engine simulations for a VSR model. A gas turbine simulation code, TSHAFT, and a helicopter performance tool, TCOPTER, have been implemented in order to be able to carry out the coupled analysis. Following this, the VSR concept has been tested on a real helicopter case: the models of a UH-60 Black Hawk helicopter and a GE T700 turboshaft engine have been implemented and validated against experimental data. Subsequently, an optimization routine has been employed to find the optimal main rotor speeds which minimize fuel consumption in different helicopter flight conditions. Two different approaches to VSRs have been analyzed: the former employing a traditional fixed ratio transmission (FRT), the latter assuming a continuously variable transmission (CVT). In the FRT case, since the turbine and main rotor speeds are in a fixed ratio, the optimal speed has been calculated as the best compromise between optimal main rotor speed and optimal turbine speed. In the CVT case, the study has been carried out assuming that main rotor and turbine speeds were free to rotate at their optimal speeds. Different simulation cases have been analyzed in order to quantify the benefits related to the optimal main rotor speed depending on flight conditions, altitude and helicopter gross weight. It has been found that coupling both the helicopter and engine model is important to adequately determine the correct rotational speed corresponding to minimum fuel consumption. More than 13% fuel saving has been shown to be feasible. Finally, possible improvements to the VSR concept have been studied. In particular, a trial has been made to redesign the power turbine of the GE T700 in order to improve the efficiency of the engine in a wide speed variation range. |
