初看之下，電力推進(jìn)飛機(jī)的未來似乎希望渺茫。由于當(dāng)下電池組的重量過大（即比能過低），電動(dòng)飛機(jī)的發(fā)展受到了極大的限制。然而，得益于相關(guān)技術(shù)的近期發(fā)展，如今的電力推進(jìn)(electric propulsion)系統(tǒng)憑借其特有屬性，將有潛力大幅提高飛機(jī)設(shè)計(jì)的靈活度，突破很多傳統(tǒng)燃料動(dòng)力飛機(jī)的典型限制，從而帶來在過去看來并不實(shí)際，或根本不可能出現(xiàn)的新型飛機(jī)。這點(diǎn)在短途飛機(jī)的設(shè)計(jì)中尤為明顯，因?yàn)檫@類飛機(jī)的尺寸通常相對(duì)較小，多為活塞式機(jī)型。
由于尺寸和重量限制，以及活塞發(fā)動(dòng)機(jī)的保養(yǎng)需求，多數(shù)活塞式機(jī)型在設(shè)計(jì)時(shí)僅能選用為數(shù)不多的幾種發(fā)動(dòng)機(jī)（很多時(shí)候甚至只有1種可選），而且發(fā)動(dòng)機(jī)的安裝位置也相對(duì)固定。如今，絕大多數(shù)的當(dāng)代通用航空飛機(jī)和直升飛機(jī)看起來仍與上世紀(jì)50年代的機(jī)型非常相似，這就是其中的原因。通常來說，電驅(qū)動(dòng)的動(dòng)力總成系統(tǒng)的體積更小、質(zhì)量更輕，而且結(jié)構(gòu)非常簡(jiǎn)單，有的系統(tǒng)甚至僅需要一個(gè)活動(dòng)部件；而傳統(tǒng)的活塞發(fā)動(dòng)機(jī)則復(fù)雜很多，至少需要冷卻系統(tǒng)、電氣系統(tǒng)、液壓系統(tǒng)，以及燃油系統(tǒng)等不同配置。
采用電動(dòng)系統(tǒng)可以降低飛機(jī)的復(fù)雜度，從而減少維修保養(yǎng)的需求。一般來說，內(nèi)燃機(jī)的體積越小，其功率質(zhì)量比(power to weight)和能效就越低，而電機(jī)的性能則基本與大小沒有直接關(guān)系。這就意味著1-kW電機(jī)與1000-kW電機(jī)的功率質(zhì)量比與能效幾乎沒有太大差別。
其次，電驅(qū)動(dòng)的動(dòng)力總成系統(tǒng)能效基本可以達(dá)到90%-95%，而傳統(tǒng)內(nèi)燃機(jī)動(dòng)力系統(tǒng)的能效在30%-40%之間，差距大約在3倍。此外，電機(jī)可以在更廣的轉(zhuǎn)速范圍內(nèi)工作，而且轉(zhuǎn)速的改變也相對(duì)較快。最后，電動(dòng)動(dòng)力系統(tǒng)在運(yùn)行時(shí)比內(nèi)燃機(jī)系統(tǒng)安靜的多，任何接觸過電動(dòng)車的人都能證明這一點(diǎn)。
雖然直接用電機(jī)替換現(xiàn)有飛機(jī)上的內(nèi)燃發(fā)動(dòng)機(jī)也可以帶來很多好處，比如降低噪聲、提高動(dòng)力能效等，但從一開始設(shè)計(jì)飛機(jī)時(shí)就考慮采用電力推進(jìn)系統(tǒng)則可以帶來更多優(yōu)勢(shì)。由于這種系統(tǒng)的特殊屬性，工程師在設(shè)計(jì)時(shí)可以大量采用相對(duì)較小的電機(jī)，這并不會(huì)大幅增加系統(tǒng)的復(fù)雜度和保養(yǎng)成本，而且也不會(huì)犧牲電機(jī)的重量和性能。由于重量和體積都相對(duì)較小，這些電機(jī)在飛機(jī)上的安裝位置也非常靈活。此外，雖然電機(jī)僅會(huì)在飛行中的特定階段（比如起降階段）發(fā)揮作用，但由于其重量非常輕，因此額外配備這些電機(jī)幾乎不會(huì)帶來任何影響。
傳統(tǒng)的燃燒推進(jìn)系統(tǒng)經(jīng)常會(huì)犧牲飛機(jī)的性能，比如，牽引式螺旋槳造成的擦洗阻力(scrubbingdrag)就會(huì)增加機(jī)身上的反向速度。與此形成對(duì)比的是，靈活的電力推進(jìn)系統(tǒng)不但不會(huì)犧牲飛機(jī)的性能，反而還能提升機(jī)身的空氣動(dòng)力性能，其中一個(gè)做法是在翼尖位置安裝螺旋槳，回收翼尖渦流(wingtip vortices)損失的部分能量。
美國飛機(jī)制造商Joby Aviation已經(jīng)設(shè)下目標(biāo)，將利用電力推進(jìn)技術(shù)開發(fā)數(shù)款新型飛機(jī)，預(yù)期可提供前所未有的超高性能。然而，由于飛機(jī)各個(gè)部分之間的互動(dòng)非常復(fù)雜，而且缺少可以借鑒的現(xiàn)有設(shè)計(jì)，Joby公司在設(shè)計(jì)過程中，不得不進(jìn)行大量高階空氣動(dòng)力分析(aerodynamicanalysis)。在這場(chǎng)不同尋常的設(shè)計(jì)研發(fā)中，Joby公司通過工業(yè)軟件開發(fā)商CD-adapco公司的STAR-CCM+軟件，進(jìn)行了大量CFD（計(jì)算流體動(dòng)力學(xué)）分析。
Joby公司的主要研發(fā)對(duì)象是一款S2垂直起降(VTOL)飛機(jī)。由于噪聲大、運(yùn)行成本高、飛行速度慢，而且安全系數(shù)也相對(duì)較低，直升飛機(jī)大小的傳統(tǒng)VTOL飛機(jī)的發(fā)展受到了嚴(yán)重的限制。在起降階段，S2飛機(jī)會(huì)同時(shí)啟用多個(gè)螺旋槳，通過冗余增加安全性。在巡航階段，S2飛機(jī)上絕大部分展開的螺旋槳會(huì)重新折疊收起，以減少飛行阻力。事實(shí)上，螺旋槳葉片的設(shè)計(jì)就是不斷在提升螺旋槳性能和降低飛行阻力之間做出平衡，這個(gè)過程需要高階工具進(jìn)行合理的分析。Joby公司利用STAR-CCM+軟件，評(píng)估了多種不同運(yùn)行環(huán)境下的螺旋槳設(shè)計(jì)，還對(duì)巡航階段的發(fā)動(dòng)機(jī)艙進(jìn)行了分析。結(jié)果顯示，改變螺旋槳葉片的形狀可以提高層流(laminar flow)，并降低巡航阻力。
Joby公司另一個(gè)項(xiàng)目的研發(fā)對(duì)象是Lotus飛機(jī)，公司會(huì)通過一款重約55磅的無人機(jī)，探索創(chuàng)新VTOL飛機(jī)配置。在項(xiàng)目配置下，飛機(jī)的每個(gè)翼尖上均配備2個(gè)螺旋槳，可為飛機(jī)的垂直起飛提供推力。一旦飛機(jī)累積到一定程度的前向速度，足夠提升機(jī)翼，2個(gè)一組的葉片就會(huì)相互交叉起來，而每個(gè)獨(dú)立葉片也會(huì)成為翼尖的延續(xù)，形成一種剪刀型分列式翼尖。而機(jī)尾傾斜的尾槳?jiǎng)t可以在起降過程中提供節(jié)距控制，并推動(dòng)飛機(jī)向前方行駛。
可以想象，設(shè)計(jì)翼尖葉片要考慮跨度、翼型選擇、扭轉(zhuǎn)分布、弦分布及節(jié)距等各種參數(shù)，整個(gè)過程就是在螺旋槳和翼尖性能之間做出平衡。在巡航配置下，Joby公司通過交叉組合這些設(shè)計(jì)參數(shù)，進(jìn)行了十多項(xiàng)CFD模擬，以期在滿足配置要求的前提下，取得最大化飛機(jī)巡航性能。同時(shí)，公司還通過CFD工具分析了螺旋槳配置中的葉片性能，以驗(yàn)證低階設(shè)計(jì)方法的結(jié)論。
Joby公司的第三個(gè)項(xiàng)目是與美國國家航空航天局(NASA)和Empirical Systems Aerospace公司合作進(jìn)行的LEAPTech（Leading Edge Asynchronous PropellerTechnology，前緣異步螺旋槳技術(shù)）研究。該項(xiàng)目的目標(biāo)是研究電力推進(jìn)系統(tǒng)可能給傳統(tǒng)固定翼飛機(jī)帶來的潛在提升。
研發(fā)人員會(huì)在機(jī)翼前緣安裝一組小型螺旋槳，從而在起降過程中增加機(jī)翼的速度，提升動(dòng)壓(dynamicpressure)。這種設(shè)計(jì)可以增加機(jī)翼產(chǎn)生的提升，從而在相同的失速速度要求下，使用更小的機(jī)翼。很多小型飛機(jī)為了達(dá)到失速速度要求，而不得不采用較大的機(jī)翼，但較大的機(jī)翼會(huì)在巡航過程中影響飛機(jī)的性能，因此由于允許使用更小的機(jī)翼，這種設(shè)計(jì)能夠提高飛機(jī)的巡航階段的能源效率。
此外，由于機(jī)翼的負(fù)載更大，機(jī)上人員的駕乘體驗(yàn)也能得到大幅提升。不過，低階工具很難分析這種吹氣機(jī)翼的性能，特別是多數(shù)所需的分析均發(fā)生在失速階段。因此，Joby公司在設(shè)計(jì)過程中進(jìn)行了大量CFD模擬，分析研究了不同螺旋槳尺寸、功率、展弦比和沖角等參數(shù)的組合。為了縮減計(jì)算的成本，公司在使用STAR-CCM+工具時(shí)，將螺旋槳處理為具有螺旋槳體積力的驅(qū)動(dòng)盤，從而減少葉片實(shí)際幾何形狀對(duì)分析的影響，大幅降低了所需的網(wǎng)格尺寸。
在該配置的測(cè)試中，Joby公司首先要建立一套全尺寸機(jī)翼、螺旋槳和電機(jī)，并將這些部件安裝在一個(gè)改裝的拖掛車上。接著，這輛拖掛車將在NASA阿姆斯特朗飛行研究中心(Armstrong Flight ResearchCenter)的跑道上，以飛機(jī)起飛的速度行駛。
與S2上的螺旋槳一樣，該配置下的前緣螺旋槳在起降階段之外，也會(huì)保持折疊收縮的狀態(tài)，而上文中提到的翼尖螺旋槳?jiǎng)t會(huì)負(fù)責(zé)給飛機(jī)提供推力。雖然公司在評(píng)估這些螺旋槳可能對(duì)翼尖渦流造成的阻力和能效影響時(shí)，的確采用了低階分析法。但事實(shí)證明，CFD仍是迄今最可靠的分析法。按照計(jì)劃，一架演示機(jī)將在2017年起飛。
作者：Alex Stoll
來源：《SAE 航空工程雜志》
翻譯：SAE上海辦公室
Promise for an electric propulsion aircraft future
At first glance, it may seem that the excessive weight (i.e., low specific energy) of today’s batteries limits electric aircraft to, at best, a few trivial niches. However, the different properties of electric propulsion compared to traditional combustion power, coupled with recent technology advances, promise to significantly relax typical design constraints for many aircraft configurations, which will allow for new types of aircraft that were previously impractical or impossible. This is particularly true for shorter-range designs, which have traditionally been relatively small and piston-powered.
Because of the size, weight, and maintenance requirements of piston engines, most piston aircraft designs are limited to a small number of engines (often just one) located in a small number of practical locations. This is why most modern general aviation airplanes and helicopters look very similar to designs from the 1950s. In contrast, electric powertrains are much smaller and lighter, and they are incredibly simple—some having only a single moving part—compared to the relatively extreme complexity of piston engines, which include a coolant system, an electrical system, an oil system, a fuel system, and so forth.
This reduced complexity translates to much lower maintenance requirements. While smaller combustion engines suffer from lower power-to-weight and efficiency, electric motors are relatively scale-free. This means that the power-to-weight and efficiency will be similar between, for example, a 1-kW motor and a 1000-kW motor.
An electric powertrain is also about three times as efficient (around 90%-95% compared to around 30%-40%). Electric motors can operate well on a much wider range of rpms, and they can change rpm relatively quickly. Finally, electric powertrains are significantly quieter than combustion powertrains, as anyone who has heard an electric car can attest.
While simply replacing a combustion engine with an electric motor will see the benefits of lower noise and higher powertrain efficiency, much greater advantages can be gained by designing an aircraft with electric propulsion in mind from the start. The different properties of electric propulsion mean that aircraft can effectively employ a large number of small motors without incurring an undesirable amount of complexity (and maintenance costs) and without sacrificing on motor weight or performance. These motors can be located in a much larger range of positions on the aircraft, due to their relatively low weight and small size. Additionally, the compromises of carrying motors that are only used in some portions of the flight (e.g., takeoff and landing) are relatively minor, since the motors themselves are so light.
While traditional propulsion installations often compromise aircraft performance—for example, the scrubbing drag caused by a tractor propeller increasing the velocity over the fuselage—the flexibility of electric propulsion allows for propulsion installations that actually result in beneficial aerodynamic interactions. One such example is locating propellers on the wingtips, where they can recapture some of the energy lost to the wingtip vortices.
Joby Aviation has set a goal to capitalize on the promise of this new technology to develop several aircraft providing capabilities that were never before possible. However, due to the complex nature of these interactions and the lack of previous designs to extrapolate from, a large amount of high-order aerodynamic analysis had to be performed in the design process, and Joby leaned heavily on CFD analyses using CD-adapco's STAR-CCM+ in the development of its unconventional designs.
Joby’s main development effort is the S2 vertical takeoff and landing (VTOL) aircraft, which addresses the high noise, high operating costs, low speed, and relatively low safety levels that, together, have severely limited the proliferation of conventional VTOL aircraft of this size (helicopters). The S2 employs multiple propellers in takeoff and landing to increase safety through redundancy. In cruise, most of these propellers fold flat against their nacelles to reduce drag. The design of these propeller blades is a compromise between propeller performance and the drag of the nacelles with the blades folded, and higher-order tools were required to properly analyze this tradeoff. A variety of propeller designs were assessed under various operating conditions in STAR-CCM+, and the nacelle was analyzed in the cruise configuration. Results indicated where reshaping the propeller blades may increase laminar flow and reduce cruise drag.
Another Joby project is the Lotus aircraft, which is exploring an innovative VTOL configuration on the 55-lb UAV scale. In this aircraft, two-bladed propellers on each wingtip provide thrust for vertical takeoff. After the aircraft picks up enough forward speed for sufficient wing lift, each set of two blades scissors together and the individual blades become wingtip extensions, forming a split wingtip. A tilting tail rotor provides pitch control during takeoff and landing and propels the aircraft in forward flight.
As one may expect, the design of these wingtip blades—the span, airfoil choice, twist and chord distribution, pitch, and dihedral—was an interesting compromise between propeller and wingtip performance. Dozens of CFD simulations were run on different combinations of these design variables in the cruise configuration, to maximize the cruise performance within the constraints of the configuration. At the same time, the performance of these blades in the propeller configuration was also analyzed with CFD to validate lower-order design methods.
A third project Joby is participating in is LEAPTech (Leading Edge Asynchronous Propeller Technology), a partnership with NASA and Empirical Systems Aerospace. The goal of this design is to investigate potential improvements in conventional fixed-wing aircraft through electric propulsion.
A row of small propellers is located along the leading edge of the wings and, during takeoff and landing, these propellers increase the velocity (and, therefore, the dynamic pressure) over the wings. This increases the lift produced by the wing and allows for a smaller wing to be used for the same stall speed constraint. Since many small aircraft use a wing sized to meet a stall speed constraint but too large for optimal cruise performance, this smaller wing allows for more efficient cruise.
Additionally, the ride quality is significantly improved due to the higher wing loading. However, the performance of this blown wing is difficult to analyze with lower-order tools, particularly since much of the required analysis occurs around stalling conditions. Therefore, a large number of CFD simulations were performed in the design process, looking at various combinations of propeller sizes and powers, wing aspect ratios and sizes, angles of attack, etc. To reduce the computational expense, the propellers were modeled as actuator disks with the body force propeller method in STAR-CCM+, which negated the need to resolve the actual blade geometry, drastically decreasing the required mesh size.
The first phase of testing this configuration was to build the full-scale wing, propellers, and motors, and mount them above a modified semi-truck which was run at takeoff speeds on the runway at NASA Armstrong Flight Research Center.
Outside of takeoff and landing, these leading-edge propellers are planned to fold against their nacelles—similar to the S2 propellers—and wingtip propellers, as mentioned above, will provide propulsion. Although lower-order analysis methods were evaluated for estimating the drag and efficiency impact of operating these propellers concentric with the wingtip vortex, unsteady CFD proved to be the most reliable analysis method. A flight demonstrator is planned for flights beginning in 2017.
This article was written for Aerospace Engineering by Alex Stoll, Aeronautical Engineer, Joby Aviation.
Author: Alex Stoll
Source: SAE Aerospace Engineering Magazine
Joby Aviation公司的S2飛機(jī)。
針對(duì)S2發(fā)動(dòng)機(jī)艙的CFD分析，展示了干凈利落的發(fā)動(dòng)機(jī)艙（如圖所示）與帶有折疊葉片和轉(zhuǎn)盤間隙的發(fā)動(dòng)機(jī)艙（下圖）之間的對(duì)比。
針對(duì)S2發(fā)動(dòng)機(jī)艙的CFD分析，展示了設(shè)計(jì)干凈利落的發(fā)動(dòng)機(jī)艙（前圖）與帶有折疊葉片和轉(zhuǎn)盤間隙的發(fā)動(dòng)機(jī)艙（如圖所示）之間的對(duì)比。
起飛配置下的Lutos形態(tài)。
巡航配置下的Lutos形態(tài)。
針對(duì)巡航配置下的Lutos進(jìn)行CFD分析。
針對(duì)起飛配置下的Lutos翼尖螺旋槳進(jìn)行CFD分析。
NASA阿姆斯特朗中心的LEAPTech實(shí)驗(yàn)測(cè)試設(shè)備。(NASA)
針對(duì)翼尖螺旋槳的CFD模擬。
LEAPTech項(xiàng)目的演示機(jī)。(NASA)