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NPSS is a component-based object-oriented engine cycle simulator that is designed to perform many common tasks related to a propulsion system. Shown is a three-stream variable cycle engine.

Propulsion performance model for efficient supersonic aircraft

For the design process of the class of aircraft known as an efficient supersonic air vehicle (ESAV), particular attention must be paid to the propulsion system design as a whole including installation effects integrated into a vehicle performance model. The propulsion system assumed for the ESAV considered in a recent study done by Optimal Flight Sciences LLC and the Air Force Research Laboratory was a three-stream variable cycle engine (VCE).

The importance of engine performance on overall aircraft performance, even when using traditional performance methods, is hard to overstate. The ability to capture airframe-propulsion system interactions during air vehicle performance analysis promises great insights into the air vehicle design process.

Prevailing airframe-propulsion design methods involve high-fidelity, single-discipline propulsion modeling translated to a low-fidelity table format for an airframer's use in traditional performance modeling. The airframer may be required to add installation effects to this reduced engine model that are not coupled to the propulsion model that originally generated the table. This approach is not sufficient for the integrated nature of propulsion systems envisioned for future aircraft, including an ESAV class.

When information is passed from the airframer to the engine manufacturer in the early design stages, it is generally limited to net thrust and thrust specific fuel consumption (TSFC) requirements at some few points in a mission envelope. If an engine or engine core that already exists will be used to power the aircraft program, the data passed to the engine manufacturer are scale factors of the above parameters.

It can be argued that a better aircraft system could be produced if a high-fidelity interface between the engine manufacturer and the airframer existed during conceptual design stages. Without this coupling, real physical interactions that are key to the eventual design that might otherwise possibly be capitalized on through design work will be missed, and will of necessity be dealt with later on costing money and usually aircraft system performance.

In this study, a computational model was built with the Numerical Propulsion System Simulation (NPSS) software to analyze the engine. This engine model was based on the generic adaptive turbine engine model developed at the turbine engines division of the AFRL. Along with this variable cycle NPSS model, a three-ramp external compression inlet model meant for conceptual design was developed. This model was used to capture inlet installation effects, including those attributable to angle of attack changes at supersonic Mach numbers.

Those models were integrated into the Service ORiented Computing EnviRonment (SORCER), which enables parallel execution of the installed NPSS model to rapidly evaluate a full flight envelope. The SORCER-enabled NPSS model was used to produce an engine deck with an expanded selection of variable state parameters compared to a standard conceptual level engine deck. These parameters were the inlet angle of attack, inlet flow bleed percentage, and flow holding percentage. This multiparameter engine data was used to evaluate the performance of an ESAV system model. The results of the evaluation showed that the additional nontraditional variable parameters included in the engine deck are significant and are worthwhile to consider in aircraft design work.

A conceptual design level, three ramp, external compression inlet model was constructed and integrated with the Generic Adaptive Turbine Engine (GATE) NPSS model. The inlet model was built using the two-dimensional compressible flow equations, and it has been verified in that it agrees well with flow results using the higher fidelity Euler code, CART3D. This inlet model and the parameterization and wrapping of GATE to be used in a multidisciplinary design and analysis optimization (MDAO) context is collectively called the MSTC-GATE installed propulsion model. (MSTC is the Multidisciplinary Science & Technology Center with AFRL.)

The inlet code was integrated with the GATE model in NPSS for the purpose of being able to calculate the installed propulsion multiparameter performance at the conceptual design level. The inlet model enabled the calculation of spillage drag using a physics-based approach. In addition, further effects and parameters were exposed to the aircraft design space including angle of attack effects and variable engine component settings.

The MSTC-GATE model was incorporated into the SORCER environment to facilitate the coupling of physics between different aircraft disciplines and to make the MSTC-GATE model computationally tractable for MDAO applications. Therefore, changes in one discipline can propagate into the whole aircraft system so that all affected disciplinary analyses can be properly updated. In this way, the complex physical effects that occur between different aircraft subsystems can also be accounted for, and possibly exploited, during the conceptual design phase, such as coupling propulsion and aerodynamics disciplines.

This effort utilized SORCER to exercise MSTC-GATE so as to study the effect of aircraft angle of attack and varying the engine diffuser bleed percentage and the flow holding value on aircraft system performance. To understand the impact of these parameters, the engine was coupled to a supersonic-capable lambda wing planform aircraft. Different performance methods that either utilize or fix various features of the MSTC-GATE engine model were evaluated. It was found that the impact of the features explored in the study such as angle of attack linking, supersonic spillage drag, flow mismatch spillage drag, and VCE features with objective functions of TSFC minimization, spillage drag minimization, and SEP maximization all have a measurable effect.

These extra parameters, beyond the traditional Mach number and altitude mission envelopes, permit deeper insights into high-performance aircraft design by bringing more realism and physical effects earlier into the design process through multiparameter performance analysis. Conceptual design traditionally sets the majority of the eventual aircraft cost with the least amount of knowledge during the design process. This work has improved the situation by increasing the level of knowledge available at this stage of the design process, thus ideally reducing the eventual cost of the final aircraft and/or increasing the final system performance.

This investigation found that determining the optimal use of a VCE is a multiobjective optimization problem that is more complicated than the single objective problem envisioned.

Additionally, the potential to improve overall airframe-propulsion system level performance was demonstrated by showing that increasing drag improved the engine operational efficiency. This emphasized the importance of designing the propulsion system and airframe simultaneously for best performance.

Finally, researchers showed how a VCE could be used to operate the same air vehicle for either maximum specific excess power (SEP) or minimum mismatch spillage drag (only two of the many possible objectives). A standard aircraft performance analysis produces one SEP plot per air vehicle, whereas the multiparameter performance method offers designers an expanded view of many different flight envelopes based on different objectives for a complete picture of aircraft capability. These two objectives and their effect on the flight envelope were quantified as an example.

This article is based on SAE International technical paper 2014-01-2133 by Darcy Allison, Optimal Flight Sciences LLC, and Edward Alyanak, Air Force Research Laboratory.

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