在高效超音速飞行器（ESAV）的设计过程中，必须格外重视以整体化的方式进行推进系统的设计。在建立飞行器的性能模型时必须考虑各方面的因素，包括推进系统的安装效果。在近期的一项有关ESAV推进系统的项目中，理想飞行科学有限公司（Optimal Flight Sciences LLC）和美国空军研究实验室（AFRL）对一种三气流可变循环发动机（VCE）展开了研究。

即便在传统性能设计中，发动机对飞机整体性能的重要性都无须赘言。若能在飞行器性能分析中掌握机身与推进系统的互动机制，那么整个飞行器的设计前景便更加光明。

当前主流的“机身-推进系统”设计方法是将高保真、单一专门学科的的推进系统建模结果转化为低保真的表格格式，方便机身制造商在传统的性能建模中使用。机身制造商可能会被要求在这个经过弱化的发动机模型上加入安装后的效果，但是其结果可能与原模型截然不同。在未来，一体化整合将成为包括ESAV在内的飞行器推进系统的特征，而之前的方法将不能满足这样的要求。

在设计的早期阶段，机身制造商将信息传递给发动机制造商时，其要点往往集中于任务计划中几个节点的净推力与推力燃料消耗率（TSFC）。如果项目中使用的是现有的发动机或发动机内核，那么传递给发动机制造商的信息就是上述参数的比例系数。

因此可以这样认为，如果在概念设计阶段，发动机制造商与机身制造商之间就能进行高保真衔接，那么最后设计出来的飞机系统也会更好。相反，如果缺少这种衔接，很可能导致物理交互机制的设计出现失误，而这一机制对于最终设计效果是非常重要得。事后进行补救不仅耗费成本，而且往往会导致系统性能的损失。

在本次研究中，研究人员使用“数字推进系统模拟（NPSS）”软件建造了一个计算模型，用于进行发动机分析。这一发动机模型是由AFRL涡轮发动机部门在一款通用型适配涡轮发动机模型的基础上进行开发的。除了可变循环NPSS模型外，研究人员还为概念设计开发了一款三坡道外部压缩进气口模型，以详细了解进气口的安装效果，包括超音速飞行状态下进气口对攻角变化的影响。

这些模型都被整合进以服务为导向的计算环境（SORCER）中，在该环境中，NPSS模型和上述模型可以同时运作，从而实现对真实飞行性能的快速评估。通过SORCER环境中的NPSS模型，研究人员搭建了一个可扩展的发动机研究平台，比起标准的概念级发动机平台，在这个新平台上可以选择并改变更多的参数，包括进气口攻角、来流损失百分比与气流留存率等。这些多元化的发动机参数被用于ESAV系统模型性能的评估。评估结果显示，新加入的非传统可变参数对飞机设计非常重要，值得认真对待。

研究人员搭建了一个概念级的三坡道外部压缩进气口模型，并将其与通用型适配涡轮发动机（GATE）NPSS模型整合起来。该进气口模型采用二维可压缩流方程建立，其结果与采用保真度更高的欧拉代码CART3D得出的气流结果吻合，因而证实了它的有效性。这个用于多学科设计与分析优化（MDAO）的进气模型与参数化、一体化的GATE，统称为MSTC-GATE推进模型（注：MSTC为AFRL的多学科科技中心）。

为了在概念设计的阶段便能实现推进器的多参数计算，研究人员将进气代码整合进了NPSS的GATE模型中。借助一个基于物理的方法，进气模型还能完成泄漏拖拽的计算。此外，在飞机设计的空间中，研究人员还将更多的效果和参数考虑在内，包括攻角的效果和各种发动机部件的设定值。

研究人员将MSTC-GATE模型整合进SORCER环境中，目的是促进不同与飞机设计相关的学科之间的衔接，并使该模型在计算上实现可追溯性，便于MDAO的应用。因此，在单一领域进行的变更能够传达至飞机的整个系统，而所有受影响的其他学科领域也可以及时进行更新。通过这种方法，不同子系统之间复杂的物理交互（如推进系统与空气动力学研究的结合）在概念设计阶段就能得到明确规划和探讨。

该项目使用SORCER环境建立MSTC-GATE模型，以研究飞机攻角、发动机逸散损失百分比与气流留存值的改变对飞机系统性能的影响。为理解这些参数的影响，研究人员将发动机安装在一架超音速兰姆达机翼平台试验飞机上，对MSTC-GATE发动机的使用方式，或是对该发动机各种特性的改进方法进行评估。结果显示，对研究中涉及的所有特性（攻角连接、超音速溢流拖拽、气流错配溢流拖拽、带有TSFC最小化客观函数的VCE特征、溢流拖拽最小化、SEP最大化等）的影响，都能进行量化。

除了传统的马赫数值和海拔高度任务参数外，其他新加入的参数可以让研究人员更深刻地理解并进行高性能飞机的设计。因为通过多参数性能分析，可以在早期阶段的设计中更接近真实的物理效果。传统的概念设计在预估飞机的最终造价时，只能借助极少量的设计阶段的知识。而这个项目通过对设计知识的增加，大大改善了这一情况，可以实现最终造价的降低或系统性能的提高，甚至二者兼得。

本次研究还发现，确定VCE的优化使用，是一个多目标问题，比单目标问题更为复杂。

另外，研究展示，增加拖曳确实可以提高发动机运行效率，这为提高“机身-推进系统”这一水平的性能打开了新的思路，并且又一次强调了同时推进系统和机身设计相结合，对实现性能的最佳水平是非常重要的。

最后，研究人员还展示了将单位剩余功率（SEP）调至最大值，或将错配溢流拖曳调至最小值时（这只是众多目标参数中的两个例子），怎样使用VCE来操作同一台飞行器。标准的飞机性能分析只能针对一架飞行器生成一个SEP图，而多参数性能方案则可以根据不同目标，为设计师提供不同的飞行方案，从而全面展现飞机性能。为了说明这一点，研究人员针对上述两个目标设计了可量化的飞行方案。

本文基于SAE International技术文章2014-01-2133改编而成。后者由理想飞行科学有限公司的Darcy Allison与美国空军研究实验室的Edward Alyanak联合撰写。

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.