Quiet, reliable and efficient : Joby Aviation paves the way for an electric future
By   |  December 19, 2015

Electric propulsion is on the verge of causing the biggest changes in aviation since the advent of the jet engine. 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.

Why electric propulsion ?
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 1,000 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.

Electric powertrains are significantly quieter than combustion powertrains, as anyone who has heard an electric car can attest.

Lower noise and higher efficiency
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 compromising 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 drawbacks 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.

With its expertise in electric motor design and fabrication, high-fidelity aerodynamic analysis, and composite airframe design and fabrication, Joby Aviation is fully capitalizing 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 must be performed in the design process. For this reason, Joby Aviation has leaned heavily on CFD analyses using STAR-CCM+ in the development of its unconventional designs.

Project 1 : the Joby S2

A rendering of the Joby S2

Joby Aviation’s main development effort is the S2 Vertical Takeoff and Landing (VTOL) aircraft, shown in Figure 1, 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 using the γ-Reθ transition model. One such nacelle geometry can be seen in Figure 2, where both the unmodified clean nacelle and the same nacelle with the folded blades and spinner gaps are shown. Such results indicate where reshaping the propeller blades may increase laminar flow and reduce cruise drag.

FIGURE 2A: CFD analysis of the S2 nacelles, showing a clean nacelle.

FIGURE 2B: The nacelle with folded blades and spinner gaps.

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