Beneath a gray, rainy sky, the normally vibrant business district of Singapore looked listless. The glass skyscrapers didn’t glitter and no sunlight dappled across the waves in the bay. But that didn’t matter much because the crowd gathered amid the tall buildings today had come to gawk at something else.
At the stroke of noon, from a promontory across the bay, a speck of white rose into the air. With a lawnmower-like hum, a flying taxi that looked like the love child of a helicopter and a drone approached, drawing a swell of cheers from the crowd.
Volocopter’s three-minute test flight was not the first time the German aircraft manufacturer has flown its full-scale prototype publicly. But today’s demonstration was momentous in other ways. It marks the first official test flight in Asia, and the first time the aircraft was put through its paces in an urban environment. That’s big news because big cities are the places where the company hopes its air taxis will ultimately find a niche.
“In the next 10 years, we hope to see Volocopter integrated as an addition to existing mobility methods in mega cities,” says Christian Bauer, who is in charge of the firm’s business development. Volocopter is aiming to be the first company in the world to offer commercial air taxi services to the masses.
Air taxis, part of a category called electric vertical takeoff and landing (eVTOL) aircraft, form a rapidly growing market—one that is expected to reach $1.5 trillion by 2040. More than 215 such aircraft are being developed worldwide, and with varying designs. Volocopter operates on drone technology with 18 motors, while others such as Lilium Jet have fixed wings. But only a handful of Volocopter’s competitors have actually built flying prototypes.
Volocopter, which was founded in 2011 and counts Intel, Daimler AG, and the Geely Holding Group (which owns automaker Volvo) among its investors, has raised close to US $95 million to date. That cash and access to a broad array of expertise have allowed Volocopter to present its third generation of lithium battery–operated, two-seater air taxis. Its next prototype, VoloCity, to be launched by 2022, promises improved specs over the current 2x series. The VoloCity expected to debut with an estimated range of 35 kilometers and a top speed of close to 110 kilometers per hour.
“Volocopter is focused on serving the inner-city mission,” says CEO Florian Reuter. With fares expected to be in the “hundreds rather than thousands of dollars,” Reuter says the airborne taxi service’s expected customers fall into three categories: businessmen looking to get quickly from point A to B, commuters seeking ways to beat rush hour traffic, and tourists.
“I believe eVTOLs will play a significant part in the future of mobility,” says Roei Ganzarski, CEO of magniX, an Australian firm developing motors for electric planes. “I don’t think we will see thousands of these flying around each city as some companies would like the public to believe, but I do think we will see shuttle models, movement between nearby airports, movement of cargo between main depots and last mile distribution [hubs], corporate use between campuses, and more.”
However, it could take 10 to 15 years for this to become reality, says Ganzarski, because there are “many other things that need to be solved first.” Among the hurdles he cites are battery power, regulatory issues, and the ability of autonomous aircrafts to handle emergencies. Other experts, such as aviation professor Jason Middleton from the University of New South Wales, voice concerns about hardware and software safety, the need to build supporting infrastructure, the challenges of navigating in bad weather, and how to manage air traffic control.
Pilots act as a fail-safe in many respects, says Middleton, who has been flying for nearly 50 years. “In an urban environment with lots of skyscrapers, you’re going to have gusts and you can’t predict where they’re going to be. Weather is unpredictable; it can quickly develop from nothing into a raging thunderstorm,” he says. “Who’s going to predict where [air taxis] can or can’t fly? And what happens when they’re in the air and can’t go to their destination?”
He adds that, “At least if you have a pilot, they’re going to look out the front and see what’s going on and take necessary action.”
One of the answers to those concerns is unmanned aircraft system traffic management platforms, or UTMs for short. Volocopter is looking to use them to govern its air taxis. “You can take most of the airspace management techniques we use in drones and apply it to air taxis,” says Pamir Sevincel, who leads urban air mobility strategy at AirMap, one of the UTM companies Volocopter is working with. Drones, which usually fly below 400 feet, are subject to different air traffic management protocols than those applied to helicopters and other aircraft.
AirMap has developed numerous UTM capabilities, all of which can theoretically be used for eVTOLs as well. These include the digital submission and approval of flight plans, surveilling an aircraft and sending alerts if it veers off track, monitoring traffic and sending real-time updates, as well as providing dynamic rerouting during emergencies. In the future, the California-based company wants to enable pilots or ground-based fleet managers of drones and air taxis to update flight trajectories based on an automated assessment of risk as a function of pedestrian and car densities, as well as other potential safety issues along planned routes. It also plans to equip flying craft with “sequencing, scheduling, and spacing” capabilities, which would allow the safe and efficient scheduling of operations in and out of vertiports and within the urban air mobility network as a whole.
“This capability is really going to enable scale in a safe way…because if you don’t, you won’t be able to integrate many flights into urban airspace” says Sevincel.
Building infrastructure to support air taxis—vertiports with passenger lounges, check-in and security facilities, as well as battery charging and aircraft maintenance stations—is another issue that must be addressed before air taxis can become a commercial reality. To that end, Volocopter has partnered with Skyports, a British infrastructure firm that has just unveiled the first prototype of its VoloPort— the air taxi equivalent of a helipad—in Singapore.
Volocopter’s Reuter says his firm is also working closely with global aviation authorities to ensure that its next-generation air taxi rises to “the same safety level airliners are built to.” He’s also well aware that gaining public acceptance is key when it comes to autonomous transport, which is why he says Volocopter’s first stage of commercial operations, scheduled within five years, will likely involve piloted flights, with the eventual aim of moving towards full autonomy.
“We, as a global society, have to feel our way into this technology…and try it out in a safe and secure environment,” says Reuter.
“Many people picture the skies becoming dark and aircraft whizzing around the city without any control or rules. That’s a very negative and chaotic image,” he says. “But let’s take it step by step and evaluate how it goes.”
Growing urbanization around the globe is creating increasingly difficult challenges in areas of transportation and energy, but engineers at the University of Maryland (UMD) think there are solutions in the promise of electric vertical takeoff and landing (eVTOL) aircraft.
Just a decade ago, the idea of air taxies and cityscapes equipped with “verti-port” stations may have seemed like the latest science fiction, but with the technical advances and commercial success of electric vehicles, eyes are turning to the sky to see how similar ideas of electric power and propulsion could create a new generation of lightweight air vehicles capable of moving people quietly, safely, and efficiently in dense urban environments.
“eVTOL has many advantages over traditional helicopters,” explains Anubhav Datta, an associate professor in UMD’s A. James Clark School of Engineering. “They don’t cause the pollution of traditional engines, have no engine noise, require fewer mechanical parts, and depending on the design could be easier to fly and more responsive to autonomy.”
While eVTOL technology is in its infancy, Datta has been involved from the start. He published the first peer-reviewed journal article demonstrating the viability of eVTOL by presenting conceptual designs for three all-electric options for a manned ultralight utility helicopter, and anticipating growth of the field. Since then, he has been instrumental in spearheading efforts to expand basic research in eVTOL, create pools of technical knowledge, and develop multidisciplinary education and outreach programs.
At Maryland, Datta and his graduate students are pursuing several projects addressing some of the principal barriers that prevent eVTOL from becoming a day-to-day reality.
One major barrier to eVTOL success is developing lightweight on-board electrical energy storage systems that would allow these aircraft to fly for longer periods with adequate reserves. According to Datta, lithium-ion batteries built for consumer electronics and automobiles are too low-energy to be a long-term solution. Batteries must be built to meet VTOL requirements or alternative sources of power explored, such as the work of Ph.D. student Emily Fisler, who is trying to quantify these requirements and explore more advanced chemistries for future batteries.
Datta, along with students Wanyi Ng and Mrinal Patil, are also exploring the application of hydrogen in fuel cells as a renewable and clean energy source. Hydrogen gas can store four to five times as much energy as current batteries, but the high power fuel stacks are heavy—so Datta’s team is looking at ways to maximize the energy benefits of hydrogen by using supplemental batteries to boost output during high-power loads, such as takeoff and landing.
Since 2017, UMD’s Department of Aerospace Engineering has won two multi-year research tasks on eVTOL funded jointly by the U.S. Army, NASA, and the U.S. Navy. As part of this work, Ph.D. student Brent Mills has built a unique hybrid-electric engine—capable of powering a scaled-down 50-pound VTOL aircraft to test and acquire data on electro-aero-mechanical behavior of the engine. Aircraft designers of the future can use this data in conceptualizing and building vehicles.
A key advantage of electric drives is that they do not require heavy, interconnecting mechanical shafts to drive more than one rotor. While multiple rotors are less efficient, they make an aircraft more stable and maneuverable, which could possibly reduce training times for future pilots and make them safer to operate in urban environments. In addition, being easier to control makes them more receptive to autonomous operation.
According to Datta, one critical advantage to UMD’s eVTOL research is the university’s historic Glenn L. Martin Wind Tunnel. Constructed in 1949, it is one of only a few tunnels its size on a university campus. This facility enables them to acquire truth-data from direct observation that is critical to the safe design of advanced rotorcraft, yet far beyond what the best computational tools can predict. Special-purpose rigs are needed to carry out model tests in the tunnel.
One such rig is the Maryland Tiltrotor Rig (MTR). Designed to study the aeromechanics of advanced prop-rotors and wing combinations, the MTR has a direct electric drive on the pylon so that data collected in the course of the project can also be applied to eVTOL. The MTR can test up to 4.75-foot diameter Mach-scaled rotors, features interchangeable blades and hubs turning at 2,500 revolutions per minute, and has interchangeable spars that can change the wing behavior.
“It is the only test rig of its kind on a university campus,” says Datta, “and the Ph.D. students who developed it are laying the foundations of the future of tiltrotor and eVTOL research at Maryland for the next decade.”
Datta was a member of the American Helicopter Society’s (AHS) inaugural eVTOL workshop in 2014, chaired the NASA Aeronautics Research Institute’s (NARI) Transformative Vertical Flight working group on intra-city Urban Air Mobility in 2016, and led the AHS in establishing eVTOL as a distinct technical discipline by founding the eVTOL Technical Committee in 2019. Chaired by Datta, this committee includes technical leaders from across industry, government, and several UMD alumni who have become leaders in the field of rotorcraft.
As part of these efforts, Datta, with support from the Vertical Flight Society (VFS, formerly AHS) and NASA, created the first formal education course in eVTOL now taught annually at the VFS Forum and the American Institute of Aeronautics and Astronautics (AIAA) Aviation forum.
Datta believes that the promise of better utilization of airspace through eVTOL advancements could bring about more energy efficient transportation solutions, but there is a lot of research and expertise that still needs to be developed to propel this new field forward.
“Through research efforts here at Maryland, we are not just building the future of eVTOL,” Datta says, “but we are providing opportunities for students to become the next generation of engineers that will have the knowledge and hands-on expertise to go out and be major contributors to that field.”
The world is electrifying fast. Manufacturing processes, cars, trucks, motorcycles, and now airplanes are making the move to electrons that Edison predicted more than a century ago. And they are all doing so for much the same reasons: quieter operation, reduced maintenance requirements, better performance and efficiency, and a more flexible use of energy sources.
At the heart of this great process of electrification stands the electric machine, filling either the role of a generator, for turning mechanical energy into electricity, or that of a motor, for doing the opposite.
For a long time, electric machines have hewed to a standard design, which has had the advantage of being very easy to manufacture. However, our startup, Magnax, based in Belgium, has taken another design that in theory can wring much more power and torque from a given mass and has made it commercially practical. We believe this new design can supplant the old one in many applications, notably in electric vehicles, in which it is now being tested.
One of our designs has a peak power density of around 15 kilowatts per kilogram. Compare that with today’s motors, such as the one in the all-electric BMW i3, which delivers a peak power density of 3 kW/kg—or just one-fifth as much. And the Magnax machine is also more efficient.
We believe that we can scale the design to whatever size carmakers (and other customers) may demand. If so, then there is every reason to believe that this design will push aside the traditional one. If it does, it will help to improve performance, save on energy and overall operating costs, and reduce carbon emissions for a better world.
The concept of an electric machine is simple. You start with a housing, which is called a stator because it remains stationary. Then you add a rotor, which spins, usually inside the stator but sometimes outside, an idea we’ll discuss later. When the machine is functioning as a motor, the magnetic fields of the stator and the rotor interact: Strategically placed magnets around the circumference of the rotor and stator repel or attract each other in a sequence to sustain the rotor’s spin and create torque. In this way, the machine converts electrical energy to mechanical energy. When the machine functions as a generator, the process operates in reverse.
Such a rotating machine today generally uses permanent magnets rather than electromagnets in the rotor and is thus called a permanent-magnet synchronous machine (PMSM). When operating as a motor, it passes alternating current to structures in the stator known as teeth. The result is a rotating magnetic field in the stator that acts on the permanent magnets of the rotor, spinning it.
The big advantage here is that permanent magnets don’t need energy to create a magnetic field. That makes this design more efficient and more powerful for a given weight and volume than a machine that uses electromagnets in the rotor.
There are many compelling reasons why PMSMs began to dominate in the 1980s, but the most important one was the development of a much more powerful breed of permanent magnet, based on neodymium. Nevertheless, because there was no change in the overall layout of the machine, the new magnet could provide only an incremental improvement. To further reduce the weight, size, and cost of the machine, the electromagnetic interaction had to be fundamentally rethought. That’s what we’ve done. We call our product a yokeless axial-flux permanent-magnet machine.
It’s a mouthful, and we’ll explain it in a moment. First, though, it’s important to understand that people already knew that the axial-flux topology had intrinsic advantages. It’s just that there seemed to be no way to exploit those advantages commercially, mainly because a design based on them would be hard to mass-produce using automated procedures.
Before we could begin designing our motor, we had to overcome a fundamental problem: There was no commercially available software that could accurately and simultaneously model the electromagnetic and thermodynamic properties of an axial-flux motor. However, Peter Sergeant and Hendrik Vansompel of Ghent University, in Belgium, have been working on this problem since 2008. Their efforts, combined with several years of R&D and prototyping by Magnax, led to our design and our manufacturing methods.
A traditional, radial-flux machine puts the rotor inside the stator. Here the stator consists of a supporting part, called the yoke, which is fitted with teeth that contain electromagnet coils. The teeth thus function as magnetic poles. As the rotor turns, its own poles transmit flux every time they sweep past a stator tooth, and the stator carries the flux elsewhere—closing what’s called the flux loop. The flux is routed from the rotor’s permanent magnet through the air gap and the stator teeth, taking a 180-degree bend through the yoke and back to another magnet. Meanwhile, of course, the interaction between the permanent magnets and the rotating electromagnetic field in the stator teeth keeps the rotor spinning.
For highest efficiency, the design should minimize the distance—the air gap—between the rotor and the stator teeth, because air transports magnetic flux poorly.
Our axial-flux machine turns that traditional arrangement inside out. It uses not one but two rotors, on either side of the stator, bracketing it. In this arrangement, the stator merely functions as the bearer of the electromagnetic teeth, not as the support—or yoke—for the rotor. In other words, it creates the possibility of a stator that is yokeless—hence the inclusion of this word in the name.
Eliminating the yoke—basically a steel cylinder that composes about two-thirds of the stator iron—saves an enormous amount of weight. As a result, yokelessness more than doubles the machine’s power density, compared with that of the older, yoked axial motors, and quadruples it compared with that of a traditional motor (like the one in the BMW i3). It also improves efficiency by reducing a bane of electric machines: iron loss.
Iron loss is mainly the result of two phenomena. First, there is the energy consumed when alternating current repeatedly magnetizes and demagnetizes cores in the stator—a process called hysteresis loss. Second are the losses to eddy currents, which are created by the varying magnetic flux through the cores.
There are other reasons why the design is so power dense. In this design, the magnetic flux goes from the permanent magnets on the first rotor disk, through the stator core to the permanent magnets on the second rotor disk—a relatively short and straight path.
Thanks to that unidirectionality, Magnax can further decrease the flux losses in the iron by 85 percent by using a material that’s perfect for conducting flux in one direction only—grain-oriented steel. Such steel couldn’t go into a traditional, radial-flux motor or generator because such machines route the flux from the rotor through the stator and back to the rotor—a multidirectional route. Magnax closely collaborated with Thyssenkrupp Electrical Steel on the design of the laminated grain-oriented cores.
Other advantages: In our yokeless axial-flux design the stator needs only about 60 percent as much copper and the rotor needs about 80 percent as much magnetic material than would a radial-flux motor of comparable power and torque.
In theory, all of these advantages make possible a relatively inexpensive and lightweight machine that delivers a lot of torque. But actually building such a machine meant facing down several serious engineering challenges.
The most obvious involve finding ways to replace the traditional functions of a yoke. In a conventional motor, the yoke holds the stator teeth in place and provides a thermal path for transporting the heat from the coils to the motor casing. It also serves as a path that closes the loop along which the magnetic flux flows when returning to its original source.
First, Magnax had to solve the mechanical challenges. Because there is no yoke to connect the individual stator teeth, another solution had to be found to create a stator with sufficient strength and stiffness to hold the teeth firmly in place even as they are wrenched by powerful electromagnetic forces.
Next came the thermal challenges. Because the windings are buried deep inside the stator and between the two rotor discs, the heat they generate can be hard to disperse. Better cooling lets you increase a machine’s nominal power—that is, the actual mechanical power it puts out. Older axial-flux concepts—those that use a yoke—cool the coils by integrating a cooling channel in the yoke. However, that arrangement makes the heat flow through the yoke, and iron is not particularly good at transporting heat. Because the Magnax design has no yoke, we needed to find another way to directly cool the coils.
Manufacturing was yet another challenge. Existing axial-flux machines have always been hard to manufacture because the stator and the windings are complex. That’s why until now such machines generally didn’t lend themselves to automated production. These challenges translate to higher cost and very poor scaling, which can be seen in most of the axial-flux designs that are now commercially available.
Yokeless concepts, however, have a simpler winding scheme, which saves on labor. So cooling emerged as one of the biggest challenges. YASA, in England, another developer of yokeless axial-flux motors, has a manufacturable motor concept; the company uses oil cooling and is building its own factory for volume production in the United Kingdom. Magnax’s design uses a different, and more flexible, cooling scheme.
Magnax has one that can use a number of coolants, notably air, water-glycol, and oil. Air cooling is preferred for use in drones and in two- and three-wheel electric vehicles (popular in India, for instance). It’s also good in big machines, such as wind-turbine generators. Liquid cooling is better for maximum power densities, in combination with gearboxes. Thus, it is often used in automotive applications.
We start by laminating aluminum or copper heat sinks in close thermal contact with the windings. The heat sinks transport the heat to the outer perimeter, where it can be carried away by cooling fins or a water-cooling jacket. This not only gives the machine a much higher capacity to evacuate heat, making it possible to produce greater nominal torque and power, it also allows for a very stiff and completely solid stator construction. That means that the machine can handle a lot of torque and still last for a long time.
At the moment, our focus is on custom motor designs for automotive original-equipment manufacturers and their suppliers. Because axial-flux motors have a short axial length, they can help keep the power train short. That proves useful to automakers that integrate the motor, the transmission, and the electronics into an electric vehicle’s axle, an assembly called an eAxle. These motors are also very useful in a hybrid design, where the combination of an engine and an electric drive system usually leaves little room for the motor.
Our design is also suited for in-wheel applications, where the motor goes right inside the wheel assembly. That configuration has many advantages—for instance, you can help to steer the car by varying the torque at each wheel, a trick known as torque vectoring. However, putting the motor in the wheel increases the unsprung mass—the part of a car that’s between the suspension and the road—and that can make the ride bumpier. Every gram of weight saved on an in-wheel motor is therefore golden.
A European carmaker is now track-testing an in-wheel car concept that uses four Magnax motors, all made in the “outrunner” configuration. That’s where the spinning part of the motor is on the outside (rather than on the inside, on a shaft), making the machine ideal for integration inside the very tight spaces within a wheel assembly. Here, too, the result is a power density that’s twice as high as a conventional motor’s, with higher efficiency to boot.
Although most car designs don’t put motors right inside the wheels, many do use more than one motor in the vehicle. In fact, any car that uses multiple motors will benefit particularly from our product. The more motors you carry, the more important it is that they be light and compact. We have calculated that the absence of a yoke and its associated iron losses can increase the range an EV could travel by 7 percent in a car with a single motor and up to 20 percent in a car with two motors. Imagine the further effects on the battery, which is the most expensive part of an EV.
The main challenge now is to bring the concept into series production; Magnax will organize this together with production partners. We have invested a lot of time in the design for manufacturing our machines. As a result, we can prove that our machines can be produced. This capability, together with the savings we can realize on materials, makes our concept competitive on price—a key point for graduating from the niche markets to the original-equipment manufacturers.
The assembly line we are building will be capable of producing motors of several diameters. We plan to begin producing 25,000 motors per year by 2022 and to scale to hundreds of thousands later on.
Over the past two years we’ve had inquiries from hundreds of companies that are interested in motors of widely varying diameters for use in electric motorcycles, trucks, and other EV applications. In addition, we still receive requests from makers of wind turbines and industrial equipment. These particular markets are not our highest priority, but the widespread demand shows that our technology has what many companies need: compactness, power, and efficiency.
Our design can cut costs substantially in a high-volume business—for instance, the production in China of millions of motors of between 1 and 10 kW. When producing in large quantities, what counts is limiting the cost of the raw materials, which as we’ve shown is significantly lower than for traditional motors.
Tens, even hundreds of millions of electric motors were sold in 2017, for a total of some US $97 billion. Their average efficiency remains below 90 percent.
In tests at the University of Ghent on the first prototype, our yokeless axial-flux motor reached efficiencies from 91 to 96 percent. And that was just the prototype.
Motors and motor systems account for approximately 53 percent of global electricity consumption. We estimate that improving the efficiency of all the world’s motors by just 1 percent would reduce the motors’ power consumption by 94.5 terawatt-hours and shrink their carbon dioxide footprint by the equivalent of 60 million metric tons.
If yokeless axial-flux machines replaced only a fraction of the older machines, we would save our customers some money and make the planet more livable while we’re at it.
This article appears in the October 2019 print issue as “Turning the Electric Motor Inside Out.”
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