Electric Flight Technology: The Unfolding of a New Future

By Ravi Rajamani

The environmental impact of hydrocarbon-burning aircraft is one of the main motivations for the move to electric propulsion in aerospace. Also, cars, buses, and trucks are incorporating electric or hybrid-electric propulsion systems, reducing the pressure on hydrocarbons and lowering the costs of electrical components. The economies of scale nec

• Language: English
• Publisher: SAE International
• Release date: May 28, 2018
• ISBN: 9780768084726

Book preview

Electric Flight Technology: The Unfolding of a New Future

CHAPTER 1: Introduction

Print ISBN: 978-0-7680-8469-6

eISBN: 978-0-7680-8472-6

DOI: 10.4271/T-135

CHAPTER 1

Introduction

The need to lessen the environmental impact—both from gas emissions and from noise—of an aircraft burning hydrocarbon fuel is one of the main motivations for the move to electric propulsion. The day-to-day cost of operating an aircraft on electricity would also be lower, but since that would be offset by the higher acquisition cost, it is not clear whether the overall cost of ownership of an electric aircraft would be lower than that of a conventional aircraft. Increased use of electric aircraft would help lower the consumption of fuel, which is a nonrenewable resource. While the world will ultimately run out of fossil fuels, this is not going to be anytime soon. Innovations in the oil and gas industry like shale oil and fracking have helped extend this deadline. Also, cars, buses, and even trucks are incorporating electric or hybrid-electric propulsion systems, further reducing the pressure on hydrocarbons. The added benefit from this shift to electric propulsion is that it has resulted in lowering the costs of electrical components such as motors, power electronic circuits, and batteries that are essential to this technology. The economies of scale engendered by the automotive industry will help contain costs in the aviation sector as well, even though the need for added safety and certification will take its toll on system development costs. However, it is not just changes in technology that are helping fuel this growth in electrification. Business reasons are just as important. New business models involving electric aircraft, such as autonomous air-taxi services, are providing the financial incentives for companies to invest in this mode of transportation. While these are all plausible explanations for the rapid growth in the development of electric propulsion, the real reason for this growth probably has more to do with the zeitgeist of our times than anything else. The automotive sector has embraced electric propulsion in a big way and this is having an impact on the aviation sector as well. For aviation, however, the technology associated with electric propulsion still has a long way to go before it would make financial sense. This is not restricted to on-board technology alone. It includes off-board elements as well. A sophisticated ground-based infrastructure will be needed to support electric propulsion. This includes not only the manufacturing, operations, and maintenance infrastructures, but also those associated with the generation, transmission and storage of electric power. For electric propulsion to grow and prosper in a sustained manner, progress will have to be made in all these aspects of technology and infrastructure.

Electric propulsion is not a new phenomenon; its history goes back more than a century stretching back, indeed, to the nineteenth century when lighter-than-air aircraft were powered by electrically driven propellers. However, it is only recently that it has taken off in a concrete manner with a viable commercial future. Major research organizations such as the National Aeronautics and Space Administration and airframers like Airbus have helped develop and demonstrate electric aircraft, but even smaller companies, and a few startups, in the general aviation sector are doing their bit to push this technology forward.

In this book, we review the history of electric propulsion, discuss the key underlying technologies, and describe how the future for these technologies will likely unfold. We will distinguish between all-electric and hybrid-electric architectures. From what we can see today, all-electric propulsion systems may be technically and commercially viable in the general aviation category, up to about 5 occupants; possibly 10. The moment we consider larger aircraft serving the commercial aviation sector, the numbers for all-electric propulsion just do not make sense, so the next best hope is the use of hybrid solutions.

A note on terminology: When required, we will differentiate explicitly between energy and power, but in common parlance they are used interchangeably, so we will mean the same thing when we say electric energy or electric power. But, as engineers know, these are very different entities. Energy, measured in joules (J), watt-hours (Wh), foot-pound (ft.-lb), or British thermal unit (Btu), is a measure of how much capacity a system has to perform work (which has the same units as energy). On the other hand, power, measured in watts (W, which is the same as J/s), horsepower (hp), or Btu per hour (Btu/h), is a measure of the rate at which a system is expending energy or doing work. Electric power delivered to a system (assuming DC supply) is measured in watts, which is equal to the voltage drop [in volts–(V)] across the system multiplied by the current [in Amperes or Amps-(A)] flowing through the system. This is perfectly valid for battery systems that deliver direct current. For AC electrical supply, things become more interesting. We have to consider the total impedance (consisting of the resistance as well as the reactance, i.e., the inductance and capacitance) to the flow of current, not just the resistance. The nonreal part of the impedance determines how out-of-phase the current and the voltage are, and has a direct effect on the ability of the source to deliver power to the load. Suffice it to say that the calculations are more complicated, and we direct the reader to any basic textbook on electrical engineering for a more coherent explanation. Because these power calculations make electrical power different from the more traditional mechanical power, it is customary to give electrical power the units of Volt-Amp (VA) rather than watts. For a purely resistive circuit, the two are the same.

The book is organized into five chapters, including this introduction.

In the next chapter, the history of electric propulsion is presented. Electricity was used in the late nineteenth century to power lighter-than-air airships, but it was not until later in the twentieth century (1973) that the first heavier-than-air electric aircraft, powered solely by a battery, successfully demonstrated sustained flight, albeit for less than a quarter-hour. The focus then shifted to solar-powered aircraft, and the field of electric aircraft was dominated by these for the next several decades. While solar power is an important means of generating electric energy, an aircraft powered solely by solar panels is not very practical, because the surface area needed to generate enough electricity to be able to lift a decent amount of payload is too large. For example, Solar Impulse 2, which completed the circumnavigation of the world in 2016, has a wingspan the size of a Boeing B747 jumbo while carrying just a single pilot. While the work on battery-powered aircraft continued in the latter part of the twentieth century, it is only since the beginning of this century that progress in more practical all-electric and hybrid-electric vehicles has accelerated.

Chapter 3 lists the various architectures that are being considered for electric vehicles. This will necessarily break down into different weight and power classes. Because of fundamental constraints, the same solution will not work across different aircraft classes. We will discuss two major classes of aircraft while framing our argument: smaller general aviation aircraft and business jets, and larger single-aisle and twin-aisle commercial aircraft. This chapter also examines how various aircraft systems can be integrated to deliver more optimal performance at the vehicle level. In the future, it is clear that the existing tube-and-wing configuration will not be the only available architecture; instead, we will be more likely to find the architecture in which the propulsion system is embedded within the airframe. This will result in aircraft that will use the propulsors as active flight control devices, thereby reducing the need for conventional control surfaces. The resulting vehicle would be lighter and more reliable with fewer moving parts.

Chapter 4 discusses the various systems and subsystems of an electric aircraft along with some description of the current state of the art. The main components of an electric aircraft are the motors, the generators, and the various power-electronic circuits controlling these components. For all-electric architectures, batteries form the main source of electric power. We will discuss the state of the art of battery storage systems. Even today, more-electric aircraft architectures have started to employ many electrically-powered components that are replacing hydraulically-powered components. With electric propulsion