初看之下，电力推进飞机的未来似乎希望渺茫。由于当下电池组的重量过大（即比能过低），电动飞机的发展受到了极大的限制。然而，得益于相关技术的近期发展，如今的电力推进(electric propulsion)系统凭借其特有属性，将有潜力大幅提高飞机设计的灵活度，突破很多传统燃料动力飞机的典型限制，从而带来在过去看来并不实际，或根本不可能出现的新型飞机。这点在短途飞机的设计中尤为明显，因为这类飞机的尺寸通常相对较小，多为活塞式机型。

由于尺寸和重量限制，以及活塞发动机的保养需求，多数活塞式机型在设计时仅能选用为数不多的几种发动机（很多时候甚至只有1种可选），而且发动机的安装位置也相对固定。如今，绝大多数的当代通用航空飞机和直升飞机看起来仍与上世纪50年代的机型非常相似，这就是其中的原因。通常来说，电驱动的动力总成系统的体积更小、质量更轻，而且结构非常简单，有的系统甚至仅需要一个活动部件；而传统的活塞发动机则复杂很多，至少需要冷却系统、电气系统、液压系统，以及燃油系统等不同配置。

采用电动系统可以降低飞机的复杂度，从而减少维修保养的需求。一般来说，内燃机的体积越小，其功率质量比(power to weight)和能效就越低，而电机的性能则基本与大小没有直接关系。这就意味着1-kW电机与1000-kW电机的功率质量比与能效几乎没有太大差别。

其次，电驱动的动力总成系统能效基本可以达到90%-95%，而传统内燃机动力系统的能效在30%-40%之间，差距大约在3倍。此外，电机可以在更广的转速范围内工作，而且转速的改变也相对较快。最后，电动动力系统在运行时比内燃机系统安静的多，任何接触过电动车的人都能证明这一点。

虽然直接用电机替换现有飞机上的内燃发动机也可以带来很多好处，比如降低噪声、提高动力能效等，但从一开始设计飞机时就考虑采用电力推进系统则可以带来更多优势。由于这种系统的特殊属性，工程师在设计时可以大量采用相对较小的电机，这并不会大幅增加系统的复杂度和保养成本，而且也不会牺牲电机的重量和性能。由于重量和体积都相对较小，这些电机在飞机上的安装位置也非常灵活。此外，虽然电机仅会在飞行中的特定阶段（比如起降阶段）发挥作用，但由于其重量非常轻，因此额外配备这些电机几乎不会带来任何影响。

传统的燃烧推进系统经常会牺牲飞机的性能，比如，牵引式螺旋桨造成的擦洗阻力(scrubbingdrag)就会增加机身上的反向速度。与此形成对比的是，灵活的电力推进系统不但不会牺牲飞机的性能，反而还能提升机身的空气动力性能，其中一个做法是在翼尖位置安装螺旋桨，回收翼尖涡流(wingtip vortices)损失的部分能量。

美国飞机制造商Joby Aviation已经设下目标，将利用电力推进技术开发数款新型飞机，预期可提供前所未有的超高性能。然而，由于飞机各个部分之间的互动非常复杂，而且缺少可以借鉴的现有设计，Joby公司在设计过程中，不得不进行大量高阶空气动力分析(aerodynamicanalysis)。在这场不同寻常的设计研发中，Joby公司通过工业软件开发商CD-adapco公司的STAR-CCM+软件，进行了大量CFD（计算流体动力学）分析。

Joby公司的主要研发对象是一款S2垂直起降(VTOL)飞机。由于噪声大、运行成本高、飞行速度慢，而且安全系数也相对较低，直升飞机大小的传统VTOL飞机的发展受到了严重的限制。在起降阶段，S2飞机会同时启用多个螺旋桨，通过冗余增加安全性。在巡航阶段，S2飞机上绝大部分展开的螺旋桨会重新折叠收起，以减少飞行阻力。事实上，螺旋桨叶片的设计就是不断在提升螺旋桨性能和降低飞行阻力之间做出平衡，这个过程需要高阶工具进行合理的分析。Joby公司利用STAR-CCM+软件，评估了多种不同运行环境下的螺旋桨设计，还对巡航阶段的发动机舱进行了分析。结果显示，改变螺旋桨叶片的形状可以提高层流(laminar flow)，并降低巡航阻力。

Joby公司另一个项目的研发对象是Lotus飞机，公司会通过一款重约55磅的无人机，探索创新VTOL飞机配置。在项目配置下，飞机的每个翼尖上均配备2个螺旋桨，可为飞机的垂直起飞提供推力。一旦飞机累积到一定程度的前向速度，足够提升机翼，2个一组的叶片就会相互交叉起来，而每个独立叶片也会成为翼尖的延续，形成一种剪刀型分列式翼尖。而机尾倾斜的尾桨则可以在起降过程中提供节距控制，并推动飞机向前方行驶。

可以想象，设计翼尖叶片要考虑跨度、翼型选择、扭转分布、弦分布及节距等各种参数，整个过程就是在螺旋桨和翼尖性能之间做出平衡。在巡航配置下，Joby公司通过交叉组合这些设计参数，进行了十多项CFD模拟，以期在满足配置要求的前提下，取得最大化飞机巡航性能。同时，公司还通过CFD工具分析了螺旋桨配置中的叶片性能，以验证低阶设计方法的结论。

Joby公司的第三个项目是与美国国家航空航天局(NASA)和Empirical Systems Aerospace公司合作进行的LEAPTech（Leading Edge Asynchronous PropellerTechnology，前缘异步螺旋桨技术）研究。该项目的目标是研究电力推进系统可能给传统固定翼飞机带来的潜在提升。

研发人员会在机翼前缘安装一组小型螺旋桨，从而在起降过程中增加机翼的速度，提升动压(dynamicpressure)。这种设计可以增加机翼产生的提升，从而在相同的失速速度要求下，使用更小的机翼。很多小型飞机为了达到失速速度要求，而不得不采用较大的机翼，但较大的机翼会在巡航过程中影响飞机的性能，因此由于允许使用更小的机翼，这种设计能够提高飞机的巡航阶段的能源效率。

此外，由于机翼的负载更大，机上人员的驾乘体验也能得到大幅提升。不过，低阶工具很难分析这种吹气机翼的性能，特别是多数所需的分析均发生在失速阶段。因此，Joby公司在设计过程中进行了大量CFD模拟，分析研究了不同螺旋桨尺寸、功率、展弦比和冲角等参数的组合。为了缩减计算的成本，公司在使用STAR-CCM+工具时，将螺旋桨处理为具有螺旋桨体积力的驱动盘，从而减少叶片实际几何形状对分析的影响，大幅降低了所需的网格尺寸。

在该配置的测试中，Joby公司首先要建立一套全尺寸机翼、螺旋桨和电机，并将这些部件安装在一个改装的拖挂车上。接着，这辆拖挂车将在NASA阿姆斯特朗飞行研究中心(Armstrong Flight ResearchCenter)的跑道上，以飞机起飞的速度行驶。

与S2上的螺旋桨一样，该配置下的前缘螺旋桨在起降阶段之外，也会保持折叠收缩的状态，而上文中提到的翼尖螺旋桨则会负责给飞机提供推力。虽然公司在评估这些螺旋桨可能对翼尖涡流造成的阻力和能效影响时，的确采用了低阶分析法。但事实证明，CFD仍是迄今最可靠的分析法。按照计划，一架演示机将在2017年起飞。

作者：Alex Stoll
来源：《SAE 航空工程杂志》
翻译：SAE上海办公室

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