如今，燃油效率以及与之密切相关的经济性和生态效益仍是全球航空业所追求的“猎物”。在此背景之下，发动机制造商开始积极向复合材料和电气化设计靠拢，机身设计师也开始着眼于轻量化和空气动力性能设计。最近，在美国国家航空航天局（NASA）位于俄亥俄州克利夫兰市的格伦研究中心（GRC）中，工程师完成了一项创新概念的测试：边界层吸入（BLI）推进器。

这款概念BLI推进器采用将风扇与进气口部分内置在机身之中的设计，这与传统的亚音速固定翼飞机设计有较大不同，后者的吊舱式发动机通常安装在远离机身的位置。通过这种嵌入式风扇与进气口的设计，BLI推进器可在飞机飞行过程中，吸收沿机身缓慢流动的边界层空气，这与传统发动机设计极力回避边界层空气的思路截然不同。

据了解，BLI推进器是由美国NASA宇航局与联合技术研究中心（UTRC）合作研发的而成，研发期间也得到了弗吉尼亚理工大学和州立大学的研究支持。

由于边界层气流存在扭曲，这将给风扇的性能与操作带来一定影响，因此这种设计存在一定挑战。为了实现这种设计，研发人员必须开发一款可以容忍扭曲边界层气流的高性能风扇，从而实现加速慢速边界空气的目的。

NASA高级航空交通运输技术项目经理Jim Heidmann表示，“本研究测试的主要工作之一就是了解这些风扇叶片的空气动力性能，观察叶片在扭曲气流下的表现，从而探索延长叶片有效使用寿命的途径。”

尽管BLI推进器对操作环境的要求很高，但NASA的工程师认为，与CFM国际公司的LEAP等现行高能效发动机相比，BLI推进器可以取得4-8%的能效提升。

“大量详细研究分析表明，BLI推进器有望显著提升飞机的燃料经济性。”NASA格伦实验室的BLI推进器专家DavidArend表示，“如果新设计及其实现技术可以成为现实，BLI推进器则可以更低的推进功率输入，为飞机提供所需推力。”

此外，由于这种设计可以减少尾流、拖曳，并降低机翼与发动机舱的自身重量，飞机的燃料经济性本身也可以实现一定提升。

据了解，去年12月9日完成的BLI测试为同类首创。为了配合推进器的巨大尺寸与配套边界层控制系统，NASA还专门改造了局里的GRC 8×6的风道。NASA的工程师们测试了BLI推进器在不同风速、边界层厚度和风扇运行条件下的表现，并对推进器的性能、可操作性和结构进行了监测。本实验涵盖飞机航行中的所有阶段，并大量模拟了一系列复杂的飞机操作过程，包括起飞、最大负荷飞行、巡航和下降等。

一旦所有实验数据的分析完成，BLI推进器的风扇和进气口设计将达到TRL（技术成熟度）4级水平，采用BLI推进器的完整飞行系统也将达到TRL 3级水平。

目前，有计划使用BLI推进器的飞机包括一些“N+3”设计，比如极光飞行科学公司（Aurora Flight Sciences）的D8双气泡客机和NASA的涡轮发电STARC-ABL（采用ABL推进器的单轴电涡轮飞行器），这两款飞机均预计将在2030到2035年间上市。

据Heidmann博士称，BLI推进器的验证机（或成为“X-飞机”）可能会在未来五到十年成为现实。

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