在高效超音速飛行器（ESAV）的設(shè)計過程中，必須格外重視以整體化的方式進行推進系統(tǒng)的設(shè)計。在建立飛行器的性能模型時必須考慮各方面的因素，包括推進系統(tǒng)的安裝效果。在近期的一項有關(guān)ESAV推進系統(tǒng)的項目中，理想飛行科學(xué)有限公司（Optimal Flight Sciences LLC）和美國空軍研究實驗室（AFRL）對一種三氣流可變循環(huán)發(fā)動機（VCE）展開了研究。
即便在傳統(tǒng)性能設(shè)計中，發(fā)動機對飛機整體性能的重要性都無須贅言。若能在飛行器性能分析中掌握機身與推進系統(tǒng)的互動機制，那么整個飛行器的設(shè)計前景便更加光明。
當(dāng)前主流的“機身-推進系統(tǒng)”設(shè)計方法是將高保真、單一專門學(xué)科的的推進系統(tǒng)建模結(jié)果轉(zhuǎn)化為低保真的表格格式，方便機身制造商在傳統(tǒng)的性能建模中使用。機身制造商可能會被要求在這個經(jīng)過弱化的發(fā)動機模型上加入安裝后的效果，但是其結(jié)果可能與原模型截然不同。在未來，一體化整合將成為包括ESAV在內(nèi)的飛行器推進系統(tǒng)的特征，而之前的方法將不能滿足這樣的要求。
在設(shè)計的早期階段，機身制造商將信息傳遞給發(fā)動機制造商時，其要點往往集中于任務(wù)計劃中幾個節(jié)點的凈推力與推力燃料消耗率（TSFC）。如果項目中使用的是現(xiàn)有的發(fā)動機或發(fā)動機內(nèi)核，那么傳遞給發(fā)動機制造商的信息就是上述參數(shù)的比例系數(shù)。
因此可以這樣認為，如果在概念設(shè)計階段，發(fā)動機制造商與機身制造商之間就能進行高保真銜接，那么最后設(shè)計出來的飛機系統(tǒng)也會更好。相反，如果缺少這種銜接，很可能導(dǎo)致物理交互機制的設(shè)計出現(xiàn)失誤，而這一機制對于最終設(shè)計效果是非常重要得。事后進行補救不僅耗費成本，而且往往會導(dǎo)致系統(tǒng)性能的損失。
在本次研究中，研究人員使用“數(shù)字推進系統(tǒng)模擬（NPSS）”軟件建造了一個計算模型，用于進行發(fā)動機分析。這一發(fā)動機模型是由AFRL渦輪發(fā)動機部門在一款通用型適配渦輪發(fā)動機模型的基礎(chǔ)上進行開發(fā)的。除了可變循環(huán)NPSS模型外，研究人員還為概念設(shè)計開發(fā)了一款三坡道外部壓縮進氣口模型，以詳細了解進氣口的安裝效果，包括超音速飛行狀態(tài)下進氣口對攻角變化的影響。
這些模型都被整合進以服務(wù)為導(dǎo)向的計算環(huán)境（SORCER）中，在該環(huán)境中，NPSS模型和上述模型可以同時運作，從而實現(xiàn)對真實飛行性能的快速評估。通過SORCER環(huán)境中的NPSS模型，研究人員搭建了一個可擴展的發(fā)動機研究平臺，比起標(biāo)準(zhǔn)的概念級發(fā)動機平臺，在這個新平臺上可以選擇并改變更多的參數(shù)，包括進氣口攻角、來流損失百分比與氣流留存率等。這些多元化的發(fā)動機參數(shù)被用于ESAV系統(tǒng)模型性能的評估。評估結(jié)果顯示，新加入的非傳統(tǒng)可變參數(shù)對飛機設(shè)計非常重要，值得認真對待。
研究人員搭建了一個概念級的三坡道外部壓縮進氣口模型，并將其與通用型適配渦輪發(fā)動機（GATE）NPSS模型整合起來。該進氣口模型采用二維可壓縮流方程建立，其結(jié)果與采用保真度更高的歐拉代碼CART3D得出的氣流結(jié)果吻合，因而證實了它的有效性。這個用于多學(xué)科設(shè)計與分析優(yōu)化（MDAO）的進氣模型與參數(shù)化、一體化的GATE，統(tǒng)稱為MSTC-GATE推進模型（注：MSTC為AFRL的多學(xué)科科技中心）。
為了在概念設(shè)計的階段便能實現(xiàn)推進器的多參數(shù)計算，研究人員將進氣代碼整合進了NPSS的GATE模型中。借助一個基于物理的方法，進氣模型還能完成泄漏拖拽的計算。此外，在飛機設(shè)計的空間中，研究人員還將更多的效果和參數(shù)考慮在內(nèi)，包括攻角的效果和各種發(fā)動機部件的設(shè)定值。
研究人員將MSTC-GATE模型整合進SORCER環(huán)境中，目的是促進不同與飛機設(shè)計相關(guān)的學(xué)科之間的銜接，并使該模型在計算上實現(xiàn)可追溯性，便于MDAO的應(yīng)用。因此，在單一領(lǐng)域進行的變更能夠傳達至飛機的整個系統(tǒng)，而所有受影響的其他學(xué)科領(lǐng)域也可以及時進行更新。通過這種方法，不同子系統(tǒng)之間復(fù)雜的物理交互（如推進系統(tǒng)與空氣動力學(xué)研究的結(jié)合）在概念設(shè)計階段就能得到明確規(guī)劃和探討。
該項目使用SORCER環(huán)境建立MSTC-GATE模型，以研究飛機攻角、發(fā)動機逸散損失百分比與氣流留存值的改變對飛機系統(tǒng)性能的影響。為理解這些參數(shù)的影響，研究人員將發(fā)動機安裝在一架超音速蘭姆達機翼平臺試驗飛機上，對MSTC-GATE發(fā)動機的使用方式，或是對該發(fā)動機各種特性的改進方法進行評估。結(jié)果顯示，對研究中涉及的所有特性（攻角連接、超音速溢流拖拽、氣流錯配溢流拖拽、帶有TSFC最小化客觀函數(shù)的VCE特征、溢流拖拽最小化、SEP最大化等）的影響，都能進行量化。
除了傳統(tǒng)的馬赫數(shù)值和海拔高度任務(wù)參數(shù)外，其他新加入的參數(shù)可以讓研究人員更深刻地理解并進行高性能飛機的設(shè)計。因為通過多參數(shù)性能分析，可以在早期階段的設(shè)計中更接近真實的物理效果。傳統(tǒng)的概念設(shè)計在預(yù)估飛機的最終造價時，只能借助極少量的設(shè)計階段的知識。而這個項目通過對設(shè)計知識的增加，大大改善了這一情況，可以實現(xiàn)最終造價的降低或系統(tǒng)性能的提高，甚至二者兼得。
本次研究還發(fā)現(xiàn)，確定VCE的優(yōu)化使用，是一個多目標(biāo)問題，比單目標(biāo)問題更為復(fù)雜。
另外，研究展示，增加拖曳確實可以提高發(fā)動機運行效率，這為提高“機身-推進系統(tǒng)”這一水平的性能打開了新的思路，并且又一次強調(diào)了同時推進系統(tǒng)和機身設(shè)計相結(jié)合，對實現(xiàn)性能的最佳水平是非常重要的。
最后，研究人員還展示了將單位剩余功率（SEP）調(diào)至最大值，或?qū)㈠e配溢流拖曳調(diào)至最小值時（這只是眾多目標(biāo)參數(shù)中的兩個例子），怎樣使用VCE來操作同一臺飛行器。標(biāo)準(zhǔn)的飛機性能分析只能針對一架飛行器生成一個SEP圖，而多參數(shù)性能方案則可以根據(jù)不同目標(biāo)，為設(shè)計師提供不同的飛行方案，從而全面展現(xiàn)飛機性能。為了說明這一點，研究人員針對上述兩個目標(biāo)設(shè)計了可量化的飛行方案。
本文基于SAE International技術(shù)文章2014-01-2133改編而成。后者由理想飛行科學(xué)有限公司的Darcy Allison與美國空軍研究實驗室的Edward Alyanak聯(lián)合撰寫。
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.
為了進行這一項目，研究人員帶有兩臺完全相同的發(fā)動機的蘭姆達機翼概念確定為ESAV。ESAV概念上的進氣口不是二維的外部壓縮進氣口，而只能代表ESAV的配置。
NPSS是一種基于部件的、以目標(biāo)為導(dǎo)向的發(fā)動機循環(huán)模擬器，專為實施與推進系統(tǒng)相關(guān)的普通任務(wù)而設(shè)計。圖上為一臺三氣流可變循環(huán)發(fā)動機。