如今，燃油效率以及與之密切相關(guān)的經(jīng)濟(jì)性和生態(tài)效益仍是全球航空業(yè)所追求的“獵物”。在此背景之下，發(fā)動(dòng)機(jī)制造商開(kāi)始積極向復(fù)合材料和電氣化設(shè)計(jì)靠攏，機(jī)身設(shè)計(jì)師也開(kāi)始著眼于輕量化和空氣動(dòng)力性能設(shè)計(jì)。最近，在美國(guó)國(guó)家航空航天局（NASA）位于俄亥俄州克利夫蘭市的格倫研究中心（GRC）中，工程師完成了一項(xiàng)創(chuàng)新概念的測(cè)試：邊界層吸入（BLI）推進(jìn)器。
這款概念BLI推進(jìn)器采用將風(fēng)扇與進(jìn)氣口部分內(nèi)置在機(jī)身之中的設(shè)計(jì)，這與傳統(tǒng)的亞音速固定翼飛機(jī)設(shè)計(jì)有較大不同，后者的吊艙式發(fā)動(dòng)機(jī)通常安裝在遠(yuǎn)離機(jī)身的位置。通過(guò)這種嵌入式風(fēng)扇與進(jìn)氣口的設(shè)計(jì)，BLI推進(jìn)器可在飛機(jī)飛行過(guò)程中，吸收沿機(jī)身緩慢流動(dòng)的邊界層空氣，這與傳統(tǒng)發(fā)動(dòng)機(jī)設(shè)計(jì)極力回避邊界層空氣的思路截然不同。
據(jù)了解，BLI推進(jìn)器是由美國(guó)NASA宇航局與聯(lián)合技術(shù)研究中心（UTRC）合作研發(fā)的而成，研發(fā)期間也得到了弗吉尼亞理工大學(xué)和州立大學(xué)的研究支持。
由于邊界層氣流存在扭曲，這將給風(fēng)扇的性能與操作帶來(lái)一定影響，因此這種設(shè)計(jì)存在一定挑戰(zhàn)。為了實(shí)現(xiàn)這種設(shè)計(jì)，研發(fā)人員必須開(kāi)發(fā)一款可以容忍扭曲邊界層氣流的高性能風(fēng)扇，從而實(shí)現(xiàn)加速慢速邊界空氣的目的。
NASA高級(jí)航空交通運(yùn)輸技術(shù)項(xiàng)目經(jīng)理Jim Heidmann表示，“本研究測(cè)試的主要工作之一就是了解這些風(fēng)扇葉片的空氣動(dòng)力性能，觀察葉片在扭曲氣流下的表現(xiàn)，從而探索延長(zhǎng)葉片有效使用壽命的途徑。”
盡管BLI推進(jìn)器對(duì)操作環(huán)境的要求很高，但NASA的工程師認(rèn)為，與CFM國(guó)際公司的LEAP等現(xiàn)行高能效發(fā)動(dòng)機(jī)相比，BLI推進(jìn)器可以取得4-8%的能效提升。
“大量詳細(xì)研究分析表明，BLI推進(jìn)器有望顯著提升飛機(jī)的燃料經(jīng)濟(jì)性。”NASA格倫實(shí)驗(yàn)室的BLI推進(jìn)器專(zhuān)家DavidArend表示，“如果新設(shè)計(jì)及其實(shí)現(xiàn)技術(shù)可以成為現(xiàn)實(shí)，BLI推進(jìn)器則可以更低的推進(jìn)功率輸入，為飛機(jī)提供所需推力。”
此外，由于這種設(shè)計(jì)可以減少尾流、拖曳，并降低機(jī)翼與發(fā)動(dòng)機(jī)艙的自身重量，飛機(jī)的燃料經(jīng)濟(jì)性本身也可以實(shí)現(xiàn)一定提升。
據(jù)了解，去年12月9日完成的BLI測(cè)試為同類(lèi)首創(chuàng)。為了配合推進(jìn)器的巨大尺寸與配套邊界層控制系統(tǒng)，NASA還專(zhuān)門(mén)改造了局里的GRC 8×6的風(fēng)道。NASA的工程師們測(cè)試了BLI推進(jìn)器在不同風(fēng)速、邊界層厚度和風(fēng)扇運(yùn)行條件下的表現(xiàn)，并對(duì)推進(jìn)器的性能、可操作性和結(jié)構(gòu)進(jìn)行了監(jiān)測(cè)。本實(shí)驗(yàn)涵蓋飛機(jī)航行中的所有階段，并大量模擬了一系列復(fù)雜的飛機(jī)操作過(guò)程，包括起飛、最大負(fù)荷飛行、巡航和下降等。
一旦所有實(shí)驗(yàn)數(shù)據(jù)的分析完成，BLI推進(jìn)器的風(fēng)扇和進(jìn)氣口設(shè)計(jì)將達(dá)到TRL（技術(shù)成熟度）4級(jí)水平，采用BLI推進(jìn)器的完整飛行系統(tǒng)也將達(dá)到TRL 3級(jí)水平。
目前，有計(jì)劃使用BLI推進(jìn)器的飛機(jī)包括一些“N+3”設(shè)計(jì)，比如極光飛行科學(xué)公司（Aurora Flight Sciences）的D8雙氣泡客機(jī)和NASA的渦輪發(fā)電STARC-ABL（采用ABL推進(jìn)器的單軸電渦輪飛行器），這兩款飛機(jī)均預(yù)計(jì)將在2030到2035年間上市。
據(jù)Heidmann博士稱(chēng)，BLI推進(jìn)器的驗(yàn)證機(jī)（或成為“X-飛機(jī)”）可能會(huì)在未來(lái)五到十年成為現(xiàn)實(shí)。
Fuel efficiency—and the economic and ecological benefits associated with it—continues to be the white rabbit of the global aviation industry. While engine builders look toward composites and electrification, and airframe designers toward lightweighting and aerodynamics, engineers at NASA’s Glenn Research Center(GRC) in Cleveland, OH, recently completed testing of a novel concept: the boundary layer ingesting (BLI) propulsor.
The BLI propulsor comprises a fan and inlet partially nested into the airframe—a departure from conventional subsonic fixed-wing aircraft arrangement where podded engines are positioned away from the fuselage. By embedding the inlet and fan, the BLI propulsor ingests the slow-moving boundary layer air that develops along aircraft surfaces during flight, the opposite goal of conventional engine placement.
The design is a cooperative effort between NASA and United Technologies Research Center, with research support from Virginia Polytechnic and State University.
The approach comes with challenges, as turbulent boundary layer air flow is distorted and affects fan performance and operation. A high-performance, distortion-tolerant fan capable of accelerating slow-moving boundary air needed to be developed.
“A key part of this research and testing was understanding the aeromechanics of how these fan blades react to a distorted flow and how to maintain their effective service lifespan,” said Jim Heidmann, manager of NASA’s Advanced Air Transport Technologies project.
Although the operational environment for the BLI propulsor is demanding, NASA engineers believe that the BLI propulsor is capable of achieving a 4-8% efficiency increase over current high-efficiency engines, such as the CFM International LEAP engine.
“Studies backed by more detailed analyses have shown that [BLI] propulsors have the potential to significantly improve aircraft fuel efficiency,” said David Arend, a BLI propulsion expert at NASA Glenn. “If this new design and its enabling technologies can be made to work, the BLI propulsor will produce the required thrust with less propulsive power input.”
The elimination of wake, drag, and weight of wing or pylon mounted engine nacelles can contribute to additional aircraft efficiency.
The BLI testing, which completed on December 9, was the first of its kind and required modifications to the NASA GRC 8’ x 6’ wind tunnel to accommodate and power the large propulsor model and boundary layer control system. NASA engineers varied wind speed, boundary layer thickness, and fan operation and monitored propulsor performance, operability, and structure. The experiment covered all phases of the flight envelope and simulated a wide range of operations including takeoff, max load, cruise, and descent.
Once all experiment data has been analyzed, the BLI propulsor fan and inlet arrangement will achieve technology readiness level (TRL) 4 status and the fully BLI propulsion-incorporated aircraft system will achieve TRL 3 status.
Planned applications for the BLI propulsor include “N+3” designs such as Aurora Flight Sciences’ D8 “Double Bubble” and NASA’s turboelectric STARC-ABL, both slated for 2030/2035.
According to Heidmann, a BLI demonstrator aircraft (or “X-plane”) may be possible within the next five to ten years.
Author: William Kucinski
Source: SAE Aerospace Engineering Magazine
為了配合推進(jìn)器的巨大尺寸與配套邊界層控制系統(tǒng)，NASA還專(zhuān)門(mén)改造了局里的GRC 8×6的風(fēng)道。
STARC-ABL實(shí)驗(yàn)方案的設(shè)計(jì)基礎(chǔ)是NASA之前的混動(dòng)飛機(jī)設(shè)計(jì)，以及NASA-波音合作項(xiàng)目蘇格計(jì)劃（Sugar Program）中的電渦輪飛機(jī)概念。
極光飛行科學(xué)公司的D8“雙氣泡”概念飛機(jī)以一款改進(jìn)版“機(jī)身加機(jī)翼”設(shè)計(jì)為基礎(chǔ)，采用了超寬機(jī)身以提供額外的升力，還采用了低翼設(shè)計(jì)以減少拖曳和重量。此外，飛機(jī)還將內(nèi)置發(fā)動(dòng)機(jī)安排在機(jī)翼尾部的位置。