如今，燃油效率以及與之密切相關的經濟性和生態效益仍是全球航空業所追求的“獵物”。在此背景之下，發動機制造商開始積極向復合材料和電氣化設計靠攏，機身設計師也開始著眼于輕量化和空氣動力性能設計。最近，在美國國家航空航天局（NASA）位于俄亥俄州克利夫蘭市的格倫研究中心（GRC）中，工程師完成了一項創新概念的測試：邊界層吸入（BLI）推進器。
這款概念BLI推進器采用將風扇與進氣口部分內置在機身之中的設計，這與傳統的亞音速固定翼飛機設計有較大不同，后者的吊艙式發動機通常安裝在遠離機身的位置。通過這種嵌入式風扇與進氣口的設計，BLI推進器可在飛機飛行過程中，吸收沿機身緩慢流動的邊界層空氣，這與傳統發動機設計極力回避邊界層空氣的思路截然不同。
據了解，BLI推進器是由美國NASA宇航局與聯合技術研究中心（UTRC）合作研發的而成，研發期間也得到了弗吉尼亞理工大學和州立大學的研究支持。
由于邊界層氣流存在扭曲，這將給風扇的性能與操作帶來一定影響，因此這種設計存在一定挑戰。為了實現這種設計，研發人員必須開發一款可以容忍扭曲邊界層氣流的高性能風扇，從而實現加速慢速邊界空氣的目的。
NASA高級航空交通運輸技術項目經理Jim Heidmann表示，“本研究測試的主要工作之一就是了解這些風扇葉片的空氣動力性能，觀察葉片在扭曲氣流下的表現，從而探索延長葉片有效使用壽命的途徑。”
盡管BLI推進器對操作環境的要求很高，但NASA的工程師認為，與CFM國際公司的LEAP等現行高能效發動機相比，BLI推進器可以取得4-8%的能效提升。
“大量詳細研究分析表明，BLI推進器有望顯著提升飛機的燃料經濟性。”NASA格倫實驗室的BLI推進器專家DavidArend表示，“如果新設計及其實現技術可以成為現實，BLI推進器則可以更低的推進功率輸入，為飛機提供所需推力。”
此外，由于這種設計可以減少尾流、拖曳，并降低機翼與發動機艙的自身重量，飛機的燃料經濟性本身也可以實現一定提升。
據了解，去年12月9日完成的BLI測試為同類首創。為了配合推進器的巨大尺寸與配套邊界層控制系統，NASA還專門改造了局里的GRC 8×6的風道。NASA的工程師們測試了BLI推進器在不同風速、邊界層厚度和風扇運行條件下的表現，并對推進器的性能、可操作性和結構進行了監測。本實驗涵蓋飛機航行中的所有階段，并大量模擬了一系列復雜的飛機操作過程，包括起飛、最大負荷飛行、巡航和下降等。
一旦所有實驗數據的分析完成，BLI推進器的風扇和進氣口設計將達到TRL（技術成熟度）4級水平，采用BLI推進器的完整飛行系統也將達到TRL 3級水平。
目前，有計劃使用BLI推進器的飛機包括一些“N+3”設計，比如極光飛行科學公司（Aurora Flight Sciences）的D8雙氣泡客機和NASA的渦輪發電STARC-ABL（采用ABL推進器的單軸電渦輪飛行器），這兩款飛機均預計將在2030到2035年間上市。
據Heidmann博士稱，BLI推進器的驗證機（或成為“X-飛機”）可能會在未來五到十年成為現實。
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
為了配合推進器的巨大尺寸與配套邊界層控制系統，NASA還專門改造了局里的GRC 8×6的風道。
STARC-ABL實驗方案的設計基礎是NASA之前的混動飛機設計，以及NASA-波音合作項目蘇格計劃（Sugar Program）中的電渦輪飛機概念。
極光飛行科學公司的D8“雙氣泡”概念飛機以一款改進版“機身加機翼”設計為基礎，采用了超寬機身以提供額外的升力，還采用了低翼設計以減少拖曳和重量。此外，飛機還將內置發動機安排在機翼尾部的位置。