Variable-speed rotor helicopters: Performance comparison between continuously variable and fixed-ratio transmissions

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 (FRT) and Continuously Variable Transmission (CVT) rotors. The impact of the two types of transmission 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 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 (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 degree-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 FRT does not present significant performance differences compared to the CVT for a wide range of advancing speeds. However, close to the two conditions of maximum interest, i.e. hover and cruise forward flight, the discrepancies between the two transmission types become relevant: in fact, engine performance is found to be penalized by 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 condition

Turbojet Engine Performance Tuning with a New Map Adaptation Concept

Gas turbine off design performance prediction is strictly dependent on the accuracy of compressor and turbine map characteristics. Experimental data regarding component maps are very difficult to find in literature, since it is undisclosed proprietary information of the engine manufacturers. To overcome this limitation, gas turbine engineers use available generic component maps and modify them to reach the maximum adherence with the experimental measures. Different scaling and adaptation techniques have been employed to this aim; these methodologies are usually based upon analytic regression models which minimize the deviation from experimental data. However, since these models are built mainly for a specific compressor or turbine map, their generalization is quite difficult: in fact, regression is highly shape-dependent and therefore requires a different model for each different specific component. This paper proposes a solution to the problem stated above: a new method for map adaptation is investigated to improve steady-state off design prediction accuracy of a generic gas turbine component. The methodology does not employ analytical regression models; its main principle relies in performing map modifications in an appropriate neighborhood of the multiple experimental points used for the adaptation. When using gas turbine simulation codes, component maps are usually stored in a data matrix and are ordered in a format suitable for 2-D interpolation. A perturbation of the values contained in the matrix results in component map morphing. An optimization algorithm varies the perturbation intensity vector in order to minimize the deviation between experimental and predicted points. The adaptation method is integrated inside TSHAFT, the gas turbine prediction code developed at the University of Padova. The assessment of this methodology will be exposed by illustrating a case study carried out upon a turbojet engine

Turboshaft Engine Performance Comparison Between CVT and Fixed Ratio Transmission for a Variable Speed Rotor

In a usual turboshaft engine for helicopter applications, the free power turbine (FPT) rotational speed is normally governed ensuring that the rotor speed is as constant as possible for each type of flight maneuver. The main reason for choosing a constant rotational speed is linked to the supposed decrease in engine efficiency at part load (typical variations in speed do not exceed 15% [1]), as well as because variable speed drives trains of resonant frequencies into the airframe [2]. However, recent studies [3],[4] and real implementations bringing up composites into the airframe structure make it possible to successfully accommodate varying speed rotors without hitting resonant frequencies and with even better engine performance [3]. A recent example is the variable speed Pratt & Whitney Canada PW207D turboshaft, a single-stage shrouded power turbine which powers Boeing's A160 Hummingbird and Bell Helicopter's Eagle Eye UAV

A New Methodology for Determining the Optimal Rotational Speed of a Variable RPM Main Rotor/Turboshaft Engine System

Variable speed rotors represent an innovative field of research for the development of new rotorcraft designs. The problems related to employing a main rotor variable speed are numerous and require an interdisciplinary approach. For this reason, a first effort has been made to understand the performance implications of coupling helicopter trim and turboshaft engine simulations. Following this, two different models of a UH-60 Black Hawk helicopter and a GE T700 turboshaft engine are implemented and validated against experimental data. Then, an optimization algorithm is employed to find the optimal main rotor speed with the aim of minimizing fuel consumption. Different simulation cases are analyzed to quantify the benefits related to the optimal main rotor speed depending on flight condition, altitude and helicopter gross weight. It is found that coupling the helicopter and engine model is necessary to adequately achieve the correct rotational speed corresponding to minimum fuel consumption. More than 10% fuel saving is shown to be feasible. The results obtained by means of numerical simulations are widely discussed and future possible applications of the methodology are suggested

A methodology for determining the optimal rotational speed of a variable RPM main rotor/turboshaft engine system

Variable speed rotors represent an innovative field of research for the development of new rotorcraft designs. The problems Issues related to employing a main rotor variable speed are numerous and require an interdisciplinary approach. For this reason, a first effort has been made in the present paper to understand the performance implications of coupling helicopter trim and turboshaft engine simulations. Following this, two different models of a UH-60 Black Hawk helicopter and a GE T700 turboshaft engine are implemented and validated against experimental data. Then, an optimization algorithm is employed to find the optimal main rotor speed with the aim of minimizing fuel consumption. Different simulation cases are analyzed to quantify the benefits related to the optimal main rotor speed depending on flight condition, altitude and helicopter gross weight. It is found that coupling the helicopter and engine model is necessary to adequately achieve the correct rotational speed corresponding to minimum fuel consumption. More than 10% fuel saving is shown to be feasible. The results obtained by means of numerical simulations are widely discussed and future possible applications of the methodology are suggested