By Eric Berman
These are the early days of manned electric flight.
Ampaire’s six-seat modified Cessna 337, Bye Aerospace’s forthcoming two- and four-seat all-electric trainers, and the recently certified (in Europe) Pipistrel Velis Electro show imagination and promise, and a wide variety of eVTOLs are attracting major new investments.
I’m an amateur in this space, but I think I’m qualified to weigh in here. I am a (piston) pilot, I have one degree in physics and two in electrical engineering, so I’m something of a geek on the technology side. I’ve spent the last 14 years as an “angel investor” in early-stage companies, so I have a bit of experience trying to spot technology trends. And I’ve been driving electric for almost eight years, so I’ve experienced the liquid-fuel-to-electrons transition directly, learning a few things in the process.
As the saying goes, though: It’s hard to make predictions, especially about the future. So below I’m going to make observations based on the trends I see, with some extrapolation, rather than making outright predictions.
Why electric aviation?
Why is electric aviation even interesting to consider, when existing fossil/liquid fuel-based aviation works pretty darned well? There are many reasons.
The first is reliability. Electric motors have few moving parts, and very few failure modes. They deliver torque across a wide rpm range. You can’t over- or under-lean; the engine doesn’t need air to operate so it can deliver full power at any altitude. There is no fuel contamination, no detonation or preignition, and the TBO is generally...well, forever.
A related advantage of electric aviation is maintainability. With so few moving parts, there just isn’t a lot to inspect or maintain: no belts to break, hoses to burst, no oil to check/change/drain, no starter motor, no fuel filters or air filters. And the “fuel” has no weight at all—you can always apply all of your useful load to payload, take off with “full-tanks” even with full load, and you have no fuel-related center-of-gravity issues to manage in-flight.
Electric motors also are significantly more efficient. Your car’s internal combustion engine turns only about 20 to 35 percent of the gasoline’s chemical engineering into forward motion. A jet engine has higher thermodynamic efficiency, turning about 55 percent of the fuel’s energy into useful work. Electric motors, on the other hand, are commonly north of 85 to 90 percent efficiency.
Electric motors are lightweight and can allow for more aerodynamic designs. For example, for a single-engine propeller airplane you can have a narrow cowling because you don’t need airflow through the engine; this also allows more of the propeller’s lift to become thrust.
Electric motors have low operating costs. This obviously varies depending on a lot of factors, but a joule of energy delivered electrically is often significantly cheaper than a joule of energy in liquid form. When coupled with low maintenance costs, the impact on cost-per-mile can be significant.
Electric aircraft are quieter. Sure, a propeller makes noise, but take the engine out of the equation and the noise level can drop considerably. Switch from one or two high-rpm propellers to more low-rpm propellers, and the noise level drops more.
But perhaps the biggest advantage of electric aviation is the subtlest: The engine is abstracted from its power source. In other words, it doesn’t know or care from where its energy is derived. A huge part of the engineering of traditional aviation engines is about getting a very specific liquid fuel to the right place at the right time with the right fuel/air mixture and only then converting it to energy from which to produce thrust. Think of carburetors and fuel injectors, air filters and ducts, mixture controls, spark plugs, valves, exhaust systems, all of which makes the traditional aviation engine intimate with its energy source.
That abstraction carries at least five under-appreciated but significant benefits:
With all of the above, why does fossil/liquid fuel-based aviation continue to dominate? Energy density.
It’s the same reason that gasoline/diesel cars continue to dominate automotive transportation. For all of the many advantages of electric engines, having a dense power source is critical, and today, liquid fuels provide that. This is changing more rapidly on the ground than in the air, but the transition in ground transportation is driving innovations that will benefit electric aviation.
First let me distinguish two kinds of energy density, both of which are important: Volumetric density is a measure of how much energy you can store per unit volume. Specific energy or gravimetric density is a measure of how much energy you can store per unit of mass (weight). Obviously, for aviation, both space and weight are at a premium, so you want both a high volumetric density and a high specific energy.
Batteries have improved dramatically over the past 20 years, to the point that cars with more than 300 miles of useful range are economically viable. But aviation is an energy-intensive endeavor. How do current batteries stack up against liquid fuels?
Liquid fuels are about 58 times denser on a weight basis, and 25 to 30 times denser on a volumetric basis. Those are truly daunting ratios. For electric aviation to compete head-to-head with fossil/liquid fuel-based aviation, it needs to make up a deficit of almost 60 times. Game over? No. Significantly more energy is wasted as heat with liquid fuels than in an electric system. Going electric means going from 30 to 50 percent efficiency to potentially over 90 percent. It’s not quite a doubling, but it’s close. So, let’s say that the hurdle for electric aviation comes down from roughly 60 times to roughly 30 times—it’s still an order of magnitude.
Chemistries like lithium-sulfur and lithium-air are on the horizon and can increase gravimetric density by approximately a factor of four to five. This alone would reduce the hurdle from 30 times to about six. Six times is not remotely adequate for long-haul flights, for example, but it’s also no longer a full order-of-magnitude problem. And given the opportunities to play with the aerodynamics of the aircraft themselves, fine-tune L/D ratios without being constrained by one or two big engines, and adjust operating speeds for efficiency, it’s more than sufficient for shorter-range flights.
One advantage of fossil/liquid fuel-based aviation is that you can turn around quickly after a flight. While fast-charging technology exists for batteries today, it is still slower than filling a tank with a liquid, and is harder on the battery than a slower charging rate; these are both areas of active research. There is already one solution, though, to electric “fast charging”: battery swapping, where you keep a cache of fully charged batteries, and when an airplane lands you remove the depleted battery and replace it with a fully charged one, which you then recharge at your leisure. There is no technology challenge to this at all; rather, it presents a logistical and economic challenge.
Even if the technological challenges above are addressed, electric aviation will be confined to niche applications unless it can compete economically with fossil/liquid fuel-based aviation.
It’s the capital costs that are high today—in particular, batteries are expensive. And they’re life limited, so while an electric aircraft engine might have a TBO well beyond that of a traditional aviation engine, the batteries likely will need to be replaced every few years. Newer, denser battery technologies will undoubtedly enter at the higher end of the price spectrum, but they will also benefit from efficiencies in manufacturing, safety, supply chain, packaging, distribution, and more.
Electric aviation also will be able to draft off of the tailwinds from increasing electric vehicle (EV) penetration. EV growth is driving battery management sophistication, charging infrastructure and methods of payment, investment in lightweight materials like carbon fiber, and so forth. All of the benefits of traversing those learning curves will accrue to electric aviation.
The other big challenge for electric aviation is charging infrastructure. Fossil/liquid fuel-based aviation has a massive installed infrastructure to ensure that fuel is available to a lot of airports; today, at least, there is very little comparable charging infrastructure. The good news here is that this is not a technology problem, or a distribution problem. There’s nothing really to invent, and just about every airport today that has fuel (and many that don’t) already has electricity. The distribution of “fuel” is easier for electric aviation than for liquid fuels. 100LL, gasoline, and Jet A all need distinct distribution pipelines, refineries, storage, and pumps.
EVs provide a great template for how to do this, and the scale of the problem for aviation is significantly smaller—both in terms of fleet size and in terms of the number of charging stations needed. For example, the United States currently has about 168,000 gas stations, but only about 4,000 airports that have fuel services of one sort or another, so the scale of the problem is significantly smaller than that for EVs.
I certainly don’t want to be a Pollyanna about electric aviation, particularly for long-range or high-capacity aviation. There are significant and very real technological and economic challenges to overcome.
Technology development is never a linear extrapolation of prior trends. New technologies rarely are able to compete as drop-in replacements for existing missions. The first PCs were effectively toys; the first cellphones didn’t displace landlines; cars didn’t merely replace the horse and buggy.
What’s fascinating about Bye, Pipistrel, and Ampaire is that they are showing that electric manned flight is possible. That’s actually the critical step—after that, it’s a matter of slow but steady iterative improvements that expand the set of achievable missions. We’re going to be surprised, not just by how soon we’ll be seeing electric aircraft in our skies, but perhaps even more by their application to missions we aren’t even thinking about today. Fifteen years ago nobody predicted the widespread adoption of drones; today, they outnumber manned aircraft by a good margin.
Electric aviation could open up small-capacity medium-range point-to-point markets that are simply impractical today with traditional turbine aircraft, providing congestion relief from major airports and a better passenger experience (no TSA lines, shorter travel between airport and home/destination). And when you realize that the main NIMBY opposition to new airports or expanded operations at existing ones is typically noise based, you can imagine the potential advantages of electric aviation playing out in unexpected ways.
We’re now seeing a lot of discussion of eVTOL short-hop autonomous concepts and other applications that can be ideal for electric aviation, none of which were in the public consciousness 10 years ago. Indeed, many of these simply aren’t practical with fossil/liquid fuel-based aviation and can provide new niches in which electric aviation can grow, possibly complementing rather than replacing traditional aviation engines.
Not all endeavors will work, of course. Indeed, most will fail—it’s a super hard problem, after all. But that’s a healthy and an inherent part of the “creative destruction” of capitalism. At a high level, all of the trends are pointing in the right direction to enable this next generation of flight.
Eric Berman is an instrument-rated commercial pilot who lives in Woodinville, Washington. He is a software developer and an angel investor, and flies a Mooney.
Email: [email protected]