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

Abstract

Variable speed rotors represent an innovative research field for the development of new rotorcraft designs. Issues related to employing a main rotor variable speed are numerous and require an interdisciplinary approach. For this reason, a preliminary 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 both the helicopter and engine model is necessary to adequately determine the correct rotational speed corresponding to minimum fuel consumption. More than 10% fuel saving is shown to be feasible. The results obtained employing a variable speed main rotor are broadly discussed, and future possible applications of the methodology are suggested. Nomenclature

Variable rotor speed and active blade twist for civil rotorcraft: Optimum scheduling, mission analysis, and environmental impact

“…Miste et al [2] investigated numerically the performance implications of variable rotor speed for the UH-60A Black Hawk helicopter including the impact of rotor-engine integration. Miste et al noted that incorporating the effect of engine coupling is essential for the determination of the optimum rotor speed for minimum fuel consumption.…”

“…Several authors have examined the topics of variable rotor speed and active blade twist in the existing literature [1,2,3]. However, previous researchers have approached these concepts either strictly from an aero-mechanical standpoint [6,9,14,19], or solely from an "isolated-rotor" point of view [5,11,13].…”

“…Although some recent references have tackled the problem from an "aircraftengine integration" perspective [2,3], the effectiveness of these technologies at "mission level" where assessments are carried out under realistic operational procedures instead of idealized trim conditions, has not been investigated. Thus, the implications associated with the optimum scheduling of variable rotor speed and active blade twist during a complete mission when the aircraft trim-state and gross weight vary continuously with time, have not been examined in the literature.…”

Performance improvement of variable speed rotors by Gurney flaps

“…Reducing rotor speed in hover and forward flight, has been extensively investigated to save helicopter power and improve helicopter performance [1][2][3][4][5][6][7][8][9]. Kang et al analyzed several rotor morphing concepts, and the comparisons showed that a reduced rotor speed could yield more power savings than other investigated concepts [10].…”

Study on the Mechanism of the Variable-Speed Rotor Affecting Rotor Aerodynamic Performance

“…Misté, Benini and Garavello et al analyzed the variable-speed rotor with a turboshaft engine and optimized the rotor speed based onthe minimal required power and minimal fuel consumption. The results showed that in hover and fast flight, these two optimizations lead to different optimal rotor speeds, and the one based on the required power could not reduce the entire vehicle fuel consumption due to the deterioration in engine efficiency [12]. Then, a continuously variable transmission concept was proposed to solve this problem [13,14].…”