Development of an Open Rotor Cycle Model in NPSS Using a Multi-Design Point Approach

Abstract

NASA’s Environmentally Responsible Aviation Project and Subsonic Fixed Wing Project are focused on developing concepts and technologies which may enable dramatic reductions to the environmental impact of future generation subsonic aircraft [1][2]. The open rotor concept (also referred to as the Unducted Fan or advanced turboprop) may allow the achievement of this objective by reducing engine emissions and fuel consumption. To evaluate its potential impact, an open rotor cycle modeling capability is needed. This paper presents the initial development of an open rotor cycle model in the Numerical Propulsion System Simulation (NPSS) computer program which can then be used to evaluate the potential benefit of this engine. The development of this open rotor model necessitated addressing two modeling needs within NPSS. First, a method for evaluating the performance of counter-rotating propellers was needed. Therefore, a new counter-rotating propeller NPSS component was created. This component uses propeller performance maps developed from historic counter-rotating propeller experiments to determine the thrust delivered and power required. Second, several methods for modeling a counter-rotating power turbine within NPSS were explored. These techniques used several combinations of turbine components within NPSS to provide the necessary power to the propellers. Ultimately, a single turbine component with a conventional turbine map was selected. Using these modeling enhancements, an open rotor cycle model was developed in NPSS using a multi-design point approach. The multi-design point (MDP) approach improves the engine cycle analysis process by making it easier to properly size the engine to meet a variety of thrust targets throughout the flight envelope. A number of design points are considered including an aerodynamic design point, sea-level static, takeoff and top of climb. The development of this MDP model was also enabled by the selection of a simple power management scheme which schedules propeller blade angles with the freestream Mach number. Finally, sample open rotor performance results and areas for further model improvements are presented.

Performance and Weight Estimates for an Advanced Open Rotor Engine

“…These modeling improvement efforts have concentrated in four main areas and were presented in a previous paper by the author (Ref. 6). The following sections briefly summarize the relevant modeling developments produced during the first 2 years of this research effort.…”

“…Therefore, a control strategy was created during the initial model development in which the blade pitch was varied with freestream Mach number according to a fixed schedule as shown in Figure 4 (Ref. 6). This schedule was selected based on observing the impact of several pitch angles on the engine operation at various flight conditions.…”

Initial Assessment of Open Rotor Propulsion Applied to an Advanced Single-Aisle Aircraft

“…As detailed in Ref. 16, the rotor performance maps used to develop these engine models were based on the F7/A7 rotor set designed in the 1980s and used on the GE36 UDF demonstrator. These performance maps may not be representative of the performance of newer blade designs.…”

Modeling Contra-Rotating Turbomachinery Components for Engine Performance Simulations: The Geared Turbofan With Contra-Rotating Core Case

“…Since these early attempts, the technological improvements achieved with respect to design and hardware implementation have resulted in a revival of the contra-rotating propeller concept (see, e.g., Refs. [3,4,[9][10][11][12]). …”

“…Regarding performance modeling of engine eonfigtirations containing contra-rotating turbomachinery components, there are only a few studies available in the public domain for contra-rotating open rotors and ducted contra-rotating fans for high bypass turbofan engines [10][11][12][29][30][31]. In these studies, though, either:…”