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洛克希德馬丁公司完善翼身融合混合佈局概念
2014-02-24
[據美國《航空週刊》網站2014年2月18日報導]通常情況下，飛機性能驅動軍用飛機設計決策，能源消耗對飛機設計的影響是次要的。但隨著燃料成本的提高以及預算的減少,這種狀態正在發生轉變。能源正迅速成為限制飛機設計的關鍵約束，這可能重塑飛機設計觀念。
目前,美國空軍努力降低燃料消耗，為此他們專注運輸機和空中加油機隊，因為運輸機和空中加油機每年消耗航空燃油占總量的三分之二。儘管近期飛機改型（如編隊飛行,翼梢小翼以及其他的減阻裝置）能夠降低燃油消耗，但這不是長遠之計。
空軍研究實驗室(AFRL) 的RCEE （Revolutionary Configurations for Energy Efficiency）高能效顛覆性佈局專案表明:顯著降低燃油消耗將可能是飛機設計觀念的最大變化。
RCEE專案的第1階段於2009 年11啟動，該階段的目標是下一代空中運輸隊的燃油消耗比現在降低90%（原文如此）。2011年啟動了RCEE專案的第二階段，該階段將持續到2015年,在這階段，各公司將研究特殊的飛機佈局來降低燃油消耗。
在第一階段,波音公司提出了混合運輸編隊，這種編隊能達到燃油消耗減少90%的目標:有效載荷為20噸的全電絎架翼型設計;有效載荷為40噸的分散式推力混電設計;有效載荷為100噸的翼身融合混電設計。在第二階段,波音公司密切關注分散式推力、混合燃料推進設計。
洛克希德•馬丁公司為了達到燃油消耗減少90%的目標，在第一階段對飛機佈局以及各種技術進行了大量的研究，研究表明翼身融合混合佈局(HWB)可能對降低燃油消耗具有最大潛能。在第二階段,洛克希德馬丁公司進一步細化了HWB概念,HWB概念是翼身融合佈局和傳統佈局的結合，機體前部採用翼身融合佈局，這種佈局具有高效率的空氣動力和結構，後部採用機身加尾翼傳統佈局，這種佈局有利於運輸機的空運特別是空投。
採用雙發的HWB佈局的飛機可以攜帶220000磅（100噸）的有效載荷（包括C-5運輸機可以運輸的所有特大貨物），起飛距離不到6500英尺（1981米），飛行距離可達3200海裏（5926公里）。由於HWB採用了新發動機，具有高效的空氣動力學和更輕的結構，與波音公司研製的C-17運輸機相比HWB佈局飛機將可以降低70%的燃油消耗。洛克希德•馬丁公司航空工程師裏克•胡克說,“如今我們的技術已經成熟，我們可以製造出這種飛機並且經濟上可承受”。
HWB研究特點是利用計算流體動力學(CFD)工具進行高度的氣動優化。最初巡航馬赫數為0.7的飛機,利用CFD進行外形優化後巡航速度可增加到0.81馬赫，並且跨聲速阻力可以減少45%。洛克希德馬丁公司估計，與C-17運輸機相比，HWB佈局飛機的空氣動力學效率要高出65%。與C-5運輸機相比，HWB佈局飛機空氣動力學效率要高30%,與波音787相比，HWB佈局飛機即使在低馬赫數下氣動效率要高出5%。
飛機氣動效率高有以下幾個原因。首先，翼身融合前機身提供了25%的升力，因為翼根弦外移可以在沒有增加機翼重量的前提下增加翼展並減少了阻力，可以改善氣動力沿翼展方向的分佈，展弦比可以從常規佈局的9增加到12。其次,後機身可確保HWB佈局飛機空運和空投與現有運輸機相容，這對BWB飛翼佈局來說是一個挑戰。為了能夠短距起降並且防止重心在空投時發生重心突變，飛翼佈局需要開發一種新的控制作動器和演算法，這將帶來風險和費用，雖然與飛翼佈局相比傳統T型尾翼會增加5%的誘導阻力，但可以提供魯棒控制並且可以避免開發新控制作動器和演算法所帶來的費用和風險。類似於C-5運輸機，HWB後機身設計可以在投放傘兵部隊時在門和舷梯周圍提供一個平穩的氣流流動區域，尾翼可以保持重心在20%平均氣動弦內，並且巡航時尾翼可以避免產生誘導阻力。
HWB設計的不尋常之處是翼身融合機身前部有一個圓形的增壓機身。裝載在外部非增壓艙裏的貨物可以放在後斜板上通過傳送滾筒往前移，再通過機身側門進入外部貨倉。這就可以使得HWB佈局的增壓艙機身在貨倉體積相同的情況下比C-5運輸機機身更小、重量更輕。據洛克希德馬丁公司計算HWB佈局的結構比傳統設計輕18%。HWB佈局飛機的另一個非傳統特點是發動機安裝在機翼後緣上方。一直以來，飛機設計都避免採用這種安裝方式，因為這種安裝方式會在跨聲速時引起機翼不利干擾,但採用這種安裝方式的本田噴氣公務機很好的優化了這種設計。
洛克希德馬丁公司對發動機安裝在機翼前緣,機翼後緣以及前機身位置時的巡航干擾阻力進行了研究。研究結果表明,發動機短艙安裝在機翼後緣有利於提高升阻比,不論什麼型號的發動機，發動機安裝在機翼後緣上方的空氣動力學效率要比翼下安裝的常規佈局高5%。
目前大家公認的低油耗發動機有三款。第一款是GE公司目前可投入使用的GEnx發動機，第二款是2030可投入使用的羅羅公司的超級風扇概念發動機，第三款是2025年可投入使用的GE公司的開式轉子發動機。與C-17和C-5M運輸機的發動機相比，第一款、第二款和第三款發動機的燃油消耗率分別要低25%、30%、35%。據洛克希德馬丁公司計算，HWB佈局的飛機由於空氣動力學效率高以及品質輕，比C-17分別安裝GEnx發動機、超級風扇發動機、開式轉子發動機的燃油消耗要低70% 、75%、 80%。雖然GEnx發動機、超級風扇概念發動機、開式轉子發動機的直徑大小各異，但通過優化同一架飛機可以根據需要模組化安裝不同的發動機。
分析表明發動機安裝在機翼後緣上方還有其他的好處。首先短艙前面長長的翼弦可以起到氣流導向器的作用，從而減少進氣道畸變以及阻擋發動機雜訊向地面傳播。其次發動機安裝在機翼後緣上方可以方便發動機的維修和拆卸，甚至可以在發動機短艙上安裝一個小尾翼。最後還有利於升力的產生，發動機吸入氣流可以為機翼提供很大的吸升力，類似於安裝在翼下的發動機噴出氣流打到下偏的襟翼上在翼下形成高壓區域，C-17運輸機就採用了這種方法。這可使最大升力係數增加15%。
為了提供短距起降能力，類似於洛克希德馬丁公司在美國空軍研究實驗室的速度敏捷專案中研究的短距運輸機概念，過剩的燃油容積可以用來進行襟翼吹風產生環流控制。另外還可以像F-35向量噴管一樣，產生垂直升力。
如果C-17按計劃在2033年開始退役，那麼美國空軍將要開始研究下一代戰略運輸機，因為C-17運輸機的研究歷時了21年。(中國航空工業發展研究中心 張斌)
http://www.aviationweek.com/Article.aspx?id=/article-xml/AW_02_17_2014_p40-662419.xml
Lockheed Martin Refines Hybrid Wing-Body Airlifter Concept
By Graham Warwick
Source: Aviation Week & Space Technology
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February 17, 2014
Traditionally, performance drives military-aircraft design decisions and the energy implications of those choices are secondary. But as fuel costs eat into reduced budgets, the balance is shifting. Energy is fast becoming a critical constraint on operations, and the results could reshape aircraft design.
For now, the U.S. Air Force's efforts to cut fuel bills are focused on its transport and tanker fleet, which consumes two-thirds of the aviation fuel the service burns each year. While near-term retrofits—such as formation flying, winglets and other drag-reduction devices—can reduce the fuel consumption of existing aircraft, they will not provide the scale of savings sought in the long term.
The name of the Air Force Research Laboratory's (AFRL) Revolutionary Configurations for Energy Efficiency (RCEE) program says it all: Dramatic changes in aircraft design may be required to achieve significant reductions in fuel consumption.
The goal of RCEE Phase 1, which ran from 2009-11, was to define a next-generation mobility fleet that would use 90% less fuel than today's transports and tankers. Under Phase 2, which began in 2011 and will run until 2015, companies are taking a closer look at specific configurations.
In Phase 1, Boeing defined a mixed fleet that met the 90% savings target: an all-electric truss-braced-wing design with 20-metric-ton payload; a 40-ton-payload distributed-thrust hybrid-electric design; and a 100-ton payload hybrid-electric blended wing-body (BWB). In Phase 2, the company is taking a closer look at the distributed-thrust, hybrid-propulsion design.
Lockheed Martin, meanwhile, studied a wide range of configurations and technologies in Phase 1 in search of the 90% goal, concluding a hybrid wing-body (HWB) offers the most potential. In Phase 2, the company is further refining the concept, which combines a blended wing and forebody for aerodynamic and structural efficiency with a conventional aft fuselage and tail for compatibility with current airlift missions, including airdrop.
The twin-engine HWB is designed to take off in less than 6,500 ft. and fly 3,200 nm carrying 220,000 lb. of payload, including all the outsize cargo now airlifted by the Lockheed C-5. Lockheed calculates the aircraft will burn 70% less fuel than the Boeing C-17 through a combination of better aerodynamics, newer engines and lighter structures. “We use mature technologies to be affordable and could build it today,” says Rick Hooker, an aeronautical engineer at Lockheed Martin Aeronautics
The HWB study is marked by a high degree of aerodynamic optimization using computational fluid dynamics (CFD) tools not available when today's airlift fleet of C-17s and Lockheed C-130s and C-5s was designed. Starting with a cruise Mach number of 0.7 as originally lofted, extensive shape optimization using CFD increased cruise speed to Mach 0.81 and reduced transonic drag by 45%, says Lockheed aeronautical engineer Andrew Wick.
Lockheed estimates the aircraft is 65% more aerodynamically efficient than the C-17, which is penalized by its 1980s design and the requirement for short-takeoff-and-landing (STOL) capability. The HWB is 30% more efficient than a C-5, and Lockheed says it is even able to achieve an aerodynamic efficiency 5% better than the Boeing 787, albeit at a lower Mach number.
That efficiency comes from several sources. To start, the blended forward fuselage carries 25% of the lift and moves the wing roots outboard, extending span and reducing drag without increasing wing weight. The spanwise lift distribution is improved and wing aspect ratio increased to 12 for the weight of a conventional aspect-ratio 9 wing,
The aft fuselage, meanwhile, ensures the aircraft is compatible with current loading and airdrop operations—a challenge for pure flying-wing designs like the BWB, says Hooker. The conventional T tail incurs a 5% drag penalty relative to a pure BWB, but provides robust control and avoids the cost and risk of developing new control effectors and algorithms for a flying wing to enable STOL and manage the abrupt center-of-gravity (CG) shift when airdropping heavy loads.
The aft fuselage is designed to provide a smooth flow field around the aft paratroop doors and cargo ramp, similar to a C-5, says Hooker. The tail is sized to handle a CG range of 20% mean aerodynamic chord, the same as a C-5. And the aircraft is designed so the tail is not needed for trim in the cruise, avoiding a drag penalty.
An unusual aspect of the HWB design is that the blended forebody encloses a circular pressurized fuselage. Some cargo is carried in unpressurized outer bays—pallets are loaded via the rear ramp, moved forward on floor rollers, then sideways through fuselage doors and into the outer bays on ball mats. The result is a pressurized fuselage that is smaller and lighter than the C-5's despite the similar cargo capacity. Lockheed calculates the HWB's structure is 18% lighter than a conventional design.
Another unconventional element of the configuration is the engine location above the wing trailing edge. Over-wing nacelles have long been avoided in aircraft design because of adverse transonic interference with the wing, but careful optimization by Honda of the engine location on the HondaJet has given the configuration new credibility.
Lockheed studied cruise interference drag with engines mounted in several locations—under and over the wing leading edge, over the trailing edge and on the aft fuselage—and generated more than 15,000 Navier-Stokes CFD solutions. The results showed that mounting the nacelles over the inboard trailing edge improved lift-to-drag ratio, regardless of engine type, for an aerodynamic benefit of up to 5% over a conventional under-wing location.
Three potential powerplants have been identified. General Electric's GEnx is available today, providing a 25% reduction in specific fuel consumption (sfc) over the C-17 and C-5M engines. Rolls-Royce's conceptual Ultra Fan has a 30% lower sfc and could be available by 2030. Third is a GE open rotor that could be available after 2025 with a 35% lower sfc. Combined with the improved aerodynamic efficiency and lighter weight, lower sfc results in the HWB burning 70% less fuel than a C-17 with GEnx engines, 75% with Ultra Fans and 80% with open rotors, Lockheed calculates.
Interestingly, despite diameters ranging from the GEnx's 11.8 ft. to an open rotor's 21 ft., “the wing optimized out to the same shape for all three engines,” says Wick. “The same wing for all three allows the engine installation to be modular. We could build it today and it would be designed to be able to be reengined.”
Analysis showed the over-wing installation offers other benefits, he says. The long wing chord ahead of the nacelle acts as a flow straightener to reduce inlet distortion and also shields fan noise from the ground. The overhang from the trailing edge means the engine is still accessible for maintenance and removal. And a smaller tail is possible with over-wing engines, says Hooker.
There is a powered-lift benefit from placing the engine nacelles over the trailing edge of the wing. “The inlet flow provides a large amount of suction lift on the wing,” says Hooker. This has a similar effect to the high-pressure area generated by under-wing engines blowing over deflected flaps, as happens in the C-17, and allows the over-wing engines to achieve a similar 15% increase in maximum lift coefficient.
To provide STOL capability, excess fuel volume could be traded for flap blowing to create a circulation-control wing, as in the STOL airlifter concept developed by Lockheed for AFRL's Speed Agile program. Another possibility is deflecting thrust downward, using flaps aft of the engine, core flow vectoring with an F-35B-style swiveling nozzle, or rotating the engines when the flaps deploy, “so they go along for the ride,” Hooker says.
Although RCEE is just a study effort, the Air Force will have to begin work on its next strategic airlifter in the near future if the C-17 is to be retired as planned starting in 2033. Noting it took 21 years to field the C-17, Hooker says ,“We need to start today to avoid a future gap.”
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