拂曉時分，一輛大型牽引卡車轟鳴著駛過干涸的湖床，車頂上載著一片飛機機翼。你可能覺得這樣一副景象與航空業(yè)的未來沒什么關(guān)系，但是當(dāng)這片機翼上的18個風(fēng)車狀小型推進器像深紅色的蝴蝶一樣在第一束陽光中閃閃爍爍，想要將機翼帶入荒涼而平靜的天空中，這副略顯奇特的畫面向我們揭示了它的真正含義——下一代飛機推進器很可能就是這個樣子的。
位于加州的愛德華茲空軍基地正上演著一場黎明前的沖刺，這是NASA對一項新技術(shù)首次進行的真實環(huán)境測試。這項技術(shù)就是均布電氣化飛機推進技術(shù)（distributed electric aircraft propulsion technology），它有可能在未來十年內(nèi)，為包括輕型飛機到地域性飛機在內(nèi)的航空界帶來一場革命，NASA蘭利研究中心聚斂電氣推進技術(shù)子項目首席研究員Mark D. Moore表示。
傳統(tǒng)飛機使用的是一個、兩個或者數(shù)個大型推進發(fā)動機，但是在NASA混合動力與電動綜合系統(tǒng)試驗臺（HEIST）上接受測試的這片“吹氣機翼”，卻搭載了大量小型電動推進器，它們能在機翼上表面上方推送多股高速氣流，產(chǎn)生前所未有的強大升力。
“如果你能在整個機翼上方推送高速氣流，就能提升整個機翼的動壓力，從而在低速飛行時大大提高升力，” Moore解釋道。在這種全新的原理下，機翼的尺寸可以縮小，但起飛和降落時的升力反而會增加。這不僅可以提高駕駛員的安全裕度，縮短起飛時間，而且整體駕駛質(zhì)量都能得到提升。不僅如此，該設(shè)計還能減少巡航階段的拖拽和燃耗，提高里程，并且能夠降低噪音級別。
HEIST測試中使用的是一片31英尺長的碳復(fù)合材料機翼，它被安裝于一個裝有測壓頭的支撐架上，所有這些都漂浮在一個吸收振動的空氣袋上，NASA阿姆斯特朗飛行研究中心項目工程師Sean Clark表示。這18個推進器總共能產(chǎn)生大約300hp的功率，而這片機翼能提供約3500磅的升力。地面測試臺成了一個“移動的風(fēng)洞”，成本要比大型風(fēng)洞低不少。在70-80 mph速度下進行的測試能夠以較低成本獲得極有價值的數(shù)據(jù)。
航空技術(shù)的躍升
這些地面測試是NASA尖端異步推進器技術(shù)（LEAPTech）項目的一部分，該項目耗資1500萬美元，歷時三年完成。項目的主要目標(biāo)是為了驗證一個原理——借助電力讓推進系統(tǒng)與機身實現(xiàn)更密切的整合，并且能夠提高效率和安全性能、甚至環(huán)保與經(jīng)濟性也能得到提升。
為了開發(fā)并搭建HEIST，蘭利中心和阿姆斯特朗中心的NASA工程師們與“雖然個子不大，但充滿熱情與效率的兩家公司”展開了合作。第一家是項目主承包商，位于加州的San Luis Obisbo航空航天實驗系統(tǒng)公司。第二家是位于圣克魯茲的Joby Aviation，負(fù)責(zé)測試臺、機翼、電機和推進器的搭建。
NASA團隊希望在未來幾年內(nèi)開發(fā)出一架LEAPTech飛行驗證機，將Tecnam P2006T輕型雙子飛機的機翼和發(fā)動機替換成經(jīng)過改良的均布推進吹氣機翼（distributed-propulsion blown wing）。使用現(xiàn)有機身，可以方便研究人員在標(biāo)準(zhǔn)配置與修改版之間進行比較。Moore表示，研究人員正在為新機翼測試機申請“X-Plane”資格。
全新的交通解決方案
該研究項目開始于2011年，當(dāng)時Moore及其同事開始研究一個理念的可行性，該理念為“將小型飛機改造成中程交通解決方案，而非一個用于娛樂的新事物，” Moore回憶道。“汽車的里程最多為100英里左右，而商用飛機可達500-1000英里。但對于那些里程為100-500英里的廉價高速飛機，卻沒有很好的交通運輸解決方案。”
“我們的研究表明，均布式電動推進器對600英里以下的里程成本效益最高，”他繼續(xù)說道。這個新科技加上改進的自主系統(tǒng)（控制/安全系統(tǒng)），可將行駛速度與連接便利性提升至遠遠超過汽車的水平，但使用的方便程度卻能與汽車媲美。“它可以為整個航空業(yè)創(chuàng)造一個全新的市場。”
“隨著我們的研究不斷展開，我們意識到均布式推進器也能用在更大的飛機身上，甚至是那些階段里程可達600英里的商用飛機。因此，該技術(shù)有可能徹底改變當(dāng)今的渦輪螺旋槳飛機和噴氣式支線飛機。”
“我們正在考慮將Tecnam的X-Plane用作小尺寸驗證機，”Moore指出。“我們計劃先在GA水平上發(fā)展這個概念，然后再把它做大。”
沒有比例限制的推進器
這一整個方案能夠?qū)崿F(xiàn)的關(guān)鍵之處在于“沒有比例限制的推進器，”Moore表示。“現(xiàn)在的推進發(fā)動機無法很好地按比例改變性能。全尺寸渦輪發(fā)動機的效率在40%左右，但如果你把功率降到100 hp，它能產(chǎn)生的效率僅為24%。前后對比為6 hp/lb和0.5 hp/lb。”
而相反，電動推進器非常緊湊可靠，并且效率很高。“它們的功率重量比值極高，比渦輪發(fā)動機高2倍，比往復(fù)式發(fā)動機高3倍。”
此外，“你不僅可以將電動推進器縮小到你喜歡的任何尺寸，而且你還可以在整個機翼邊緣上隨意挑選擺放位置。因此，這個緊湊耦合式安裝的控制與提升機翼表面設(shè)計便能帶來各種協(xié)同優(yōu)勢。”他認(rèn)為這項創(chuàng)新技術(shù)可以帶來“絕好的機遇。”
性能優(yōu)勢
傳統(tǒng)的輕型飛機需要一片巨大的機翼面積來滿足FAA認(rèn)證的失速要求，但在巡航階段，這種設(shè)計便顯得非常低效。LEAPtech飛行驗證機將機翼尺寸降至三分之一以減小拖拽，并將機翼負(fù)載提升至原來的三倍（50 lb/ft²以上，普通小型飛機為20 lb/ft² ），以提高駕駛質(zhì)量和對陣風(fēng)的反應(yīng)。但與此同時，成排推進器卻將低速時的最大升力系數(shù)翻了一番。
為低速階段專門改良的小尺寸推進器擁有更低的葉端速，以降低噪音。此外，它們的轉(zhuǎn)速也有些許不同，這樣可以將發(fā)出的聲頻擴散出去，因此社區(qū)噪音最多有望降低15分貝。起飛和降落階段，所有發(fā)動機都將開啟，但在巡航過程中會收起一些發(fā)動機以減小拖曳。位于翼尖的推進器是專門為高速階段改良的，它們可在翼尖內(nèi)側(cè)生成翼尖渦流，以提升效率。
“這些改變有望減少30%的運行成本，”如果未來能將混合動力發(fā)動機替換成改良的電池發(fā)動機，“那么實現(xiàn)飛行過程中的零排放也不在話下。”
Moore總結(jié)時表示，“GA飛機的安全性統(tǒng)計數(shù)據(jù)不是特別理想，大多數(shù)事故都是在起飛和降落的時候發(fā)生的，那時候飛機飛得又慢又低。”
均布式推進器提供的大量冗余，可使飛機更好地應(yīng)對發(fā)動機故障，并將駕駛員的控制主動權(quán)大大提升。“吹氣機翼的橫向可操縱性能非常驚人。如果一片機翼在低速階段失去升力并失速，使機身翻轉(zhuǎn)，那你只需給它加大功率就能夠有效控制機身。”
Flight propulsion goes electric
Somehow, the sight of an airplane wing perching incongruously atop a big-rig truck tractor that’s rumbling across a dry lake bed at dawn might not seem particularly significant to the future of aviation. But when the 18 small, pinwheel-like propellers on the wing’s leading-edge flash into the sun’s first rays like a swarm of crimson butterflies trying to lift the wing into the still, desert air, the odd spectacle actually provides a good indication of what may become a new propulsion paradigm for next-generation aircraft.
Those morning speed runs across Edwards Air Force Base in California represent some of NASA’s first real-world tests of distributed electric aircraft propulsion technology, a budding design concept that could revolutionize everything from light planes to regional airliners within a decade or so, said Mark D. Moore, Principal Researcher for NASA’s Convergent Electric Propulsion Technology Sub-Project atLangley Research Center.
In place of one, two, or several large propulsion engines as in conventional aircraft, the unusual “blown wing” that the Hybrid-Electric Integrated Systems Test-bed (HEIST) is evaluating features an array of small electric-powered propellers that send multiple streams of high-speed air over the upper surface of the wing to produce unprecedented lift capabilities, he said.
“If you distribute higher velocity air across the entire wing, you can raise the dynamic pressure over the wing and thus increase lift substantially at low flight speeds,” Moore explained. This novel arrangement allows use of a downsized wing that nonetheless generates greater lift during takeoffs and landings, which not only provides a greater safety margin for the pilot and shorter takeoff runs, but better overall ride quality as well. The design also can deliver less drag and fuel use in cruise operations and longer range, even lower noise levels.
The experimental HEIST test article, a 31-ft-span, carbon-composite wing section, is mounted on a supporting truss with load cells attached, all of which floats on a vibration-absorbing airbag, said NASA Armstrong Flight Research Center project engineer Sean Clark. Combined, the 18 propellers generate about 300 hp and the wing provides around 3500 lb of lift. The ground-test rig thus serves as a “mobile wind tunnel” at significantly lower cost than a large-scale wind tunnel. Testing at speeds up to 70-80 mph is providing valuable data at an affordable price.
A leap in aviation
The ground tests are part of NASA’s $15-million, three-year Leading Edge Asynchronous Propeller Technology (LEAPTech) program, which aims to evaluate the premise that the tighter propulsion-airframe integration that is enabled by electric power will yield improved efficiency and safety, as well as environmental and economic benefits.
To develop and build HEIST, NASA engineers at Langley and Armstrong partnered with specialists and engineers at “two small, nimble, enthusiastic firms,” Empirical Systems Aerospace of San Luis Obisbo, CA, the prime contractor, and Santa Cruz-based Joby Aviation, which built the test rig, wing, motors, and propellers.
Within a few years, the NASA team hopes to develop and test a LEAPTech flight demonstrator by replacing the wings and engines of a Tecnam P2006T light twin airplane with an improved version of the distributed-propulsion blown wing. Using an existing airframe will allow the researchers to compare the performance of the modified vehicle with that of the standard configuration. Moore says that the researchers are applying for “X-Plane” status for the re-winged test aircraft.
New transportation solutions
The research project got its start in 2011, when Moore and his colleagues began studying the possibility of “turning the small airplane into real mid-range transportation solution instead of a mostly recreational novelty,” Moore recalled. “An automobile works great up to about 100 miles, while a commercial airliner works great for 500 to 1000 miles, but for affordable, high-speed speed mobility between 100 to 500 miles, there’s no great transportation solution.
“Our studies say that distributed electric propulsion would be cost-effective for distances less than 600 miles,” he continued. The new technology plus improved autonomy (control/safety) systems could provide dramatically higher-speed, more affordable access than cars but with car-like ease of use, he contended. “It could create a whole new market for general aviation aircraft.
“As our analysis went forward, we realized that distributed propulsion is applicable to larger aircraft as well, even commercial transports flying stage lengths of around 600 miles. It could therefore also be a game-changer for turboprop and regional jets that the airlines fly today.
“Think of the Tecnam X-Plane as a subscale demonstrator,” he noted. “We want to incubate the concept at the GA level and then scale it up.”
Scale-free propulsors
The key to the entire approach is that the electric motor is “a scale-free technology,” Moore asserted. “Current propulsion engines just don’t scale well. A full-size turbine engine can be around 40% efficient, but if you take it down to 100 hp, it’s only 24% efficient—6 hp/lb vs. 0.5 hp/lb.”
In contrast, electric motors can be very compact, very reliable, and highly efficient, he said. “They provide extremely good power-to-weight ratio—two times better than turbine engines and three times better than any reciprocating engine.”
And not only is “electric propulsion happy to scale to any size, you can place them anywhere you want, for instance, along the entire leading edge of a wing to attain synergetic benefits from close-coupled control and lift surfaces.” This innovation, he stated, offers “exciting opportunities.”
Performance benefits
A traditional light aircraft needs a large wing area to meet the low stall-speed requirement for FAA certification, but it is inefficient in cruise. The LEAPtech flight demonstrator would feature a wing that is one-third the size for reduced drag and have nearly three times the wing loading (more than 50 lb/ft² vs. 20 lb/ft² for a typical small aircraft) for improved ride quality and better response to gusts. Meanwhile, the propeller array should double the maximum lift coefficient at low speeds.
Optimized for low speed, the small-diameter propellers have low tip speeds for reduced noise. In addition, they all rotate at slightly different velocities to spread out the sound frequencies they emit, cutting community noise, it is hoped, by as much as 15 dB. All the props blow the wing for takeoff and landing operations, but some fold back to reduce drag in cruise, when wingtip propellers that are optimized for high velocities provide propulsion, operating inside the wingtip vortices to boost efficiency.
"Such changes are expected to deliver a 30% reduction in operating costs, not to mention zero in-flight emissions" if improved batteries replace hybrid power at some point, he said.
Moore concluded by noting that “the safety statistics for GA aircraft are not all that great, with most accidents happening during takeoffs and landings, when planes are flying low and slow.”
Distributed propulsion provides redundancy against engine failure and maximizes control authority. “With the blown wing, you have incredible lateral control. If one wing loses lift and stalls at low speed causing the plane to roll off to one side, you can just power out of it.”
NASA正在使用LEAPtech吹氣機翼來評估飛機的均布式電動推進器。
NASA計劃將Tecnam輕型飛機的機翼和發(fā)動機替換成改良后的成排推進器吹氣機翼。
NASA混合動力與電動綜合系統(tǒng)試驗臺（HEIST）正在橫穿莫哈韋沙漠干涸的湖床。
NASA阿姆斯特朗飛行研究中心的技術(shù)人員正在組裝吹氣機翼及18個電動推進器。
NASA的LEAPTech飛行驗證機可能在整個航空業(yè)掀起一場革命。
NASA的LEAPTech均布式電動推進原型機，有可能成為下一架X-Plane。