As the aeronautics industry and aviation startups design and test zero-emissions aircraft, they are solving problems beyond just adapting to fuel sources that cut greenhouse gas emissions. Problems of weight, noise, redundancy, refueling, cost, and turnaround time are being tackled one airline seat at time.
Powerful tools can help aircraft designers look at the aircraft system as a whole, ferreting out issues that can affect other parts of the aircraft. At they same time, they are breathing life into some long-imagined use cases, such as air taxis and flying cars.
Zero emissions vehicles use fuel that will not produce two greenhouse gases — carbon dioxide (CO2) and nitrogen oxides (NOx, which includes nitric oxide [NO] and nitrogen dioxide [NO2]). Battery-powered, hydrogen-powered, and the combination of both are zero emission fuels for planes.
“One of the big areas of interest for electrification of aircraft is the energy density of batteries versus traditional fuels,” said John Chawner, senior group director for CFD product management at Cadence. “It’s quite a bit lower. Think of aviation fuel and batteries and hydrogen as energy storage devices. Energy density impacts how much payload you can carry for a given platform size. For electric aircraft, right now we’re looking at smaller platforms and shorter routes. Other issues are aircraft noise, and logistical issues or MRO (maintenance, reliability, overhaul). Add to that recharge time, replacement, some weird electrical phenomenon that can occur with big batteries when operating at high altitudes, engine-out operations, etc.”
So what exactly makes electric planes different from traditional gas-powered aircraft?
“The simplest answer is just the sheer amount of electrical power that you need,” said Dale Tutt, vice president of industries strategy at Siemens Digital Industries Software. “When you have a gas-powered airplane, the primary source of power comes from the gasoline. You carry your fuel, and that powers the engine. And on the engine, you have a generator that drives all the electrical systems on the aircraft, so you might need three or four kilowatts of power to drive most of the electrical systems on an airplane. When you start looking at these electric aircraft, now you need hundreds of kilowatts of power.”
All that power has ramifications. “Now you have bigger wires,” said Tutt. “You have to manage your thermal management. There’s a lot of heat load going through your batteries and through the electrical systems to drive the motors. In addition to all the electrical systems on the airplane, when you have a lot of electromagnetic effects with all that high power that’s going around the airplane, you have to think about how you’re shielding your systems. And if you have critical systems, you need to shield them from the electromagnetic effects. You also have to think about how do you have separation between all of those systems so that you don’t lose multiple systems on the airplane when a single system goes down.”
“The electric powertrain, consisting of electric motors, inverters and batteries, also requires thermal management to keep operating temperatures within limitations, throughout the operational envelope,” said Geoffrey Bower, chief engineer at Archer, an eVTOL company. “There are lots of interesting design tradeoffs in these systems (e.g., air vs. liquid cooling, self-contained vs. distributed) that have a big impact on performance and overall system reliability.”
Redundancy is always critical in avionics, but it’s more complex in electric planes. “With any airplane that you are designing, whether it’s powered by jet fuel or gas, one of the challenges you always have is that if you lose a generator on one engine, you don’t want to lose all the electrical systems on your airplane,” said Tutt. “You really have to think about the level of redundancy that you have on those airplanes in order to make sure you can safely operate them if you have a component failure. And it’s the same thing on an electric airplane. You just can’t have a single failure that would cause you to lose all your systems. So there are a lot of safety considerations to think about with how to separate those systems and make sure that you can always safely operate the aircraft.”
“The overall system reliability requirements drive us to use redundancy throughout the powertrain (i.e., 12 motors and multiple battery packs connected in an intelligent way) to ensure a very high degree of powertrain reliability,” said Archer’s Bower. “If there is a component failure in an automotive powertrain you can pull over to the side of the road and it is an inconvenience, while for an eVTOL aircraft we have to maintain flight and land safely.”
To break down the separation concept even further, a lot of aircrafts have a left- and a right-hand system. “You also have a mission-critical system and emergency bus, depending on what different companies call it,” he said. “You have an emergency bus that is on-battery only, so in theory that if you lost all of your generators you still have a battery back-up powering the most critical systems. When you start thinking about an all-electric aircraft, they may have two, three, or even four different electrical systems that are connected together when everything is operating normally. You can lose one without losing the other, or in the end you may still have just a single kind of this emergency bus. This emergency bus is a critical system that would help you get home safely.”
Use cases, aircraft types
The distance the plane can go based on its fuel defines its use case, and there are four major ones:
Fuel power density, weight, size, and refueling
Battery and fuel cell weight and size can require seats to be removed because the weight and size of fuel containers are fixed. This is in contrast to jet fuel, which is used up during flight, making the plane lighter as it continues on its course. The fuel tanks, in the wings of the planes, do not physically get smaller in flight, but they are not interfering with sellable space. The tradeoff, according to Universal Hydrogen, is cheaper fuel costs using swap-out H2 cells (as shown on in this Bloomberg video). Existing turbo prop planes can switch over to the hydrogen system, as the H2 tanks load up through cargo doors.
“Energy density impacts how much payload you can carry for a given platform size,” said Cadence’s Chawner. “So for electric aircraft, right now we’re looking at smaller platforms and shorter routes.”
An alternative approach is to use different types of fuel and propulsion systems, which can be different types of batteries, one optimized for high power and the other for high energy. “If you can have the same device do both, you’ll probably end up doing that,” said Venkat Srinivasan, director of the Argonne Collaborative Center for Energy Storage Science and deputy director of the Joint Center for Energy Storage Research. “And that’s what has happened today. A Tesla can go 0 to 60 in three seconds. And it can give you 300 miles or more range. So turns out, we don’t have a problem there, and there are very few places where having two technologies in the same device seems to make sense. But that could change. If we take electric planes, you might be seeing taking off requires a lot of power. So I’m going to have one device for takeoff, and then I’m going to run the plane using a second device as it goes along, and I’ll charge the first device for landing because I need a lot of power to stop.”
Electric motors also have lower maintenance costs. But the big issue has been charging time, and that is improving, particularly for airplanes where unloading and loading of passengers takes significantly more than the time it takes to pump gas into a car.
“Part of this is how fast can you charge up the batteries, and that technology is really advancing,” said Perry Rothenbaum, senior staff power electronics design and applications engineer at Infineon Technologies. “The state of the art now is they’re getting down to a seven-minute charge on a car. I don’t know how much that would be on a plane, but the point is, that’s a lot of energy transferring at one time. If you can charge your car in seven minutes, that makes electric vehicles, eVTOL, taxis, or anything electric very viable. Now it’s equivalent to going to a gas station and filling up with gas. And these charging stations are being put across the United States. We know of companies that are going full forward into putting these high energy charging stations. That’s going to be a game changer.”
Charging electric planes also can be spread out over the day. With air taxis/eVTOLs, a 20-minute flight might be the average. “It would fly from hub to hub. Maybe it’s landing on top of buildings or something, but there would be a charging station on the top of that building and the eVTOL would get a quick charge,” aid Siemens’ Tutt. “They literally are sizing the batteries so that they start the day fully charged, and then they use a little bit, and then they recharge a little bit but not back to full charge. And then the next point they use some more, so by the end of the day they’re at the end of the battery, and then overnight you charge it up.”
Charging strategies would depend upon the use model for the planes. “On the bigger aircraft, maybe like a regional aircraft, they would be looking at some quick-turn charging that does the same thing, and then overnight you do what I call a normalization charge to help stabilize the battery,” Tutt said. “You can give the plane a quick injection of power so it can recharge, but that’s sub-optimal. You can do that kind of quick turn — maybe takes you 45 minutes to an hour. But at most of these airports, it takes about 45 minutes to an hour to turn most of the airplanes anyway, so that can give you enough power and energy to do that quick charge. Overnight, you put it more on a sustainment or optimization charge. Curiously enough, your iPhone does the same thing. It starts to learn how you charge and your charging patterns, and when it thinks it normally is going to be plugged in for several hours, that will go into an optimization charge and it will take longer to go from 50% to 100%. But if you plug it in at an abnormal time, it goes ‘Oh, normally not plugged in at this time’ and it will recharge in 15 minutes. That is happening today, and most people don’t even realize it. It’s the same thought process on airplanes and electric cars.”
Higher efficiency motor control
Another option is to make motors more efficient, which in turn can extend battery life further. This is where field-oriented controls (FOCs) fit into the picture. By improving efficiency, they can help reduce weight, lower cost, and increase distance.
“A FOC is a very high efficiency type of control for a motor,” Rothenbaum said. “It also happens to be a fairly complex control for a motor. And although we’d like to think that all companies are technology-savvy, having experts in field-oriented control is very useful. There are a lot of motors in an e-plane. It’s not just a propeller spinning around.”
Hybrid planes that still use gas engines along with electric also are in the works. Airbus is working with Rolls Royce and Siemens on a 100-seat medium-haul aircraft, called the e-Fan X. That plane is expected to be commercially available sometime between 2025 and 2030.
Noise
Noise is a frequent topic among companies working on electric plane/zero emissions aircraft. Electric planes aren’t silent, and lowering the sound pollution from airplanes is a big consideration.
“If you’ve ever heard of drone flying, especially the smaller ones, it’s a high-pitched, shrill whine,” said Cadence’s Chawner. “Now imagine not just the autonomous ones, but an air-taxi–sized one that is maybe the size of an SUV with four props spinning, and multiple of these flying all over the cityscape. It could create quite a buzz. It takes all sorts of mitigation to figure out how we’re going to deal with that noise environment. If you look at regular aircraft around airports, noise abatement is still a huge concern. And that’s everything from just the exhaust from the jet engines — the nozzles do all sorts of interesting things to try to mitigate the noise signature. But then something very fundamental that you can’t avoid — lowering the landing gear into the stream — causes all sorts of turbulence in the flow, which then causes noise. So that’s been an ongoing area of research.”
Computational fluid dynamics (CFD) is used to calculate not only air flow and thermal, but the level and signature of an aircraft’s sound. Increasingly CFD is offered by the EDA and design companies to run simulations.
Electric planes sound different. “Electric changes the nature and the signature of that sound significantly. It’s a different frequency,” said Chawner. “There are different levels of it. The flight profiles of the aircraft might be different, like air taxi in a quadcopter design. Those will not be overflying neighborhoods. A typical jet aircraft comes in for landing on a runway and it’s got to fly over somebody’s house to get to the airport and land. It is those overflights where the aircraft is low to the ground and in a noisy configuration — because the gear is down, the leading-edge slats are down, the flaps are down — that generate all of that noise.”
Sometimes noise is addressed by the flight path because engineering has not solved the problem yet. “For example, John Wayne Airport [Santa Ana, Calif.] has some strict noise abatement policies that can’t get addressed by engineering. They are addressed instead by the flight profile on takeoff. We can avoid that noisy configuration if you’ve just got a quadcopter that you can bring in and drop down straight over because it’s not flying over everyone. You go straight up, you take hard turns to get away from places. But that’s one class of aircraft — small drones and quadcopter-type air taxis — that you would have for urban air mobility,” said Chawner.
What about larger electric planes with a multitude of propellers? “When you look at larger short-haul type of aircraft, they’ll have distributed propulsion. Some of those configurations have 10 propellers down the leading edge of each wing, rather than trying to run one big prop off of all the batteries. You distribute that propulsion across the wing, and that changes everything because you have multiple power plants running,” said Chawner, “It also disturbs the flow in a different way when it goes over the wing. All of that flow creates the noise. It’s not just that the roaring of the jet engine. It’s what’s happening to the air and how it’s being disturbed. It’s just like if you put your hand out a car window, you can hear that flutter at highway speeds.”
The challenge is to figure out the aeroacoustics, which is difficult to compute because it requires fine detail and long runtimes to get that wave propagation.
High altitude
No matter the fuel type, electronics cooled by convection on solid ground have a heat problem at high altitude.
“Because you’re generating all this heat with the high power electronics and the batteries, or pushing power to your propulsion system, the first challenge you run into at high altitude is the air is thinner, and so it’s much less effective at cooling the electronics,” said Cadence’s Tutt. “So you have to size your air flows and your openings and your heat exchangers for the high-altitude operations.”
Other electromagnetic effects show up at high altitude, as well. “As you start talking about high-power electronics, there are some electromagnetic effects and corona effects that can happen,” said Tutt. “It’s just another consideration. All of those are pretty well understood and a lot of our solutions are able to help you model those and solve for those problems. But solving that thermal challenge is one of the most complex aspects of this.”
Conclusion
As batteries improve and the industry prototypes get more air time, technical issues with electric and hydrogen-electric aircraft may be the easier part of problem for the air taxis and short hop deliveries.
Aircraft design requires looking at the system as a whole and verifying its requirements.
“Those requirements might be the performance requirements for your customer, the regulatory requirements from the FAA, and how you need to certify it,” Tutt said. “Those requirements drive the design of the different systems on the aircraft. The systems tend to be electro-mechanical systems, because there are mechanical elements or sensors. There are also the electrical system connections and the software that drives them, and you continue down into the chips, it’s a very iterative process. That traceability goes from requirements to airplane to electrical system number one, two, PC boards, to chips, and all of those work together in a process to really evolve the design of the vehicle. All of the parts have to work together. And then they’re verified at the component level, like a chip or PC board, and again at the system level. It’s really just the ability to take from that very first ‘I have an idea,’ and then how it manifests itself in ‘Okay, I need this performance out of this chip.’ That’s the criticality of the system.”
And finally, safely managing all that low-altitude air traffic is a necessity for avoiding nightmare situations. The drone industry and the FAA in the U.S. have been working on air-traffic control systems for unmanned aerial vehicles for years. Air taxis may be able to save travel time and be convenient, but if everyone and everything travels by air taxi, friendly skies rapidly will become frenzied skies.