Wednesday, October 21, 2020

Electric aviation

Over the past few years, we've seen a flood of drones - almost all electrically powered - take flight in our skies, making up the vast majority of electric flight today.  But we're also starting to see more and more manned electric aviation in our skies, such as the recent test flight of Ampaire's 6-seat modified 337, Bye Aerospace's forthcoming 2- and 4-seat all-electric trainers, and the recently certified (in Europe) Pipistrel Velis Electro.  I think there's an interesting trend here that is well worth watching, and I'd like to weigh in here with some of my thoughts.

At a high level, I think it's worth stating up front: these are early days, there are significant challenges to solve, and fleet turnover is a very very slow process.  While I do think it's very possible we could see an electric equivalent to the Cessna 172 within a decade, I suspect that an electric transport aircraft (i.e., more than 30 people, more than 1,000nm range) is unlikely within even 20 years.  But 30?  There I'm not so sure.

I'm an amateur in this space, but I think I'm qualified to weigh in here nevertheless.  I am a (piston) pilot myself, so I know a little about aviation. 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 at least a bit of experience trying to spot technology trends.  And I've been driving electric for almost 8 years ago, 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 make observations based on the trends I see, with some extrapolation, rather than making outright predictions.

Why Electric Aviation?

Why is electric aviation ("EA") even interesting to consider, when existing fossil/liquid fuel based aviation (I'll refer to this as "LFA") works pretty darned well?  There are many reasons.

But first, a disclaimer: for the sake of this discussion - I'm going to make a simplifying assumption that the environmental impact of both LFA and Electric Aviation (EA) are both identically zero.  Why?  Three reasons: (a) I don't want to get into a policy/political discussion; I think the technology/economic trends are fascinating enough, (b) you don't need environmental arguments for EA to be an attractive proposition, and (c) EA is still not real enough that any statements about its environmental impact pro or con are meaningful.  Perhaps in a later post I can talk a bit about that, but for this post I will not make any argument involving emissions/environment/etc., nor about any policy to try to push EA over LFA: I'm going to assume here that policy is frozen as it is today, with the small exception of fixing obviously broken things like requiring an oil gauge for each engine in order to be more technology agnostic.

Where was I?  Oh, right: what benefits does EA offer over LFA?  Many, I think (obviously assuming that the technological and economic issues can be addressed).

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 pre-ignition, and the "TBO" is generally...well, forever.  (OK, my lack of turbine expertise is clearly showing through here; but I am comfortable asserting that electric engines are much simpler.)

Related to reliability is safety, but don't think I can make a definitive call either way on that.  Batteries have obviously had issues with thermal runaway, but equally obviously all liquid fuels are by definition highly flammable.  New battery designs have improved safety, but its hard to say whether future battery technologies will be safer - or will have different dangers to be aware of.  Something to watch...

A related advantage of EA 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, etc. 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) 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 are also significantly more efficient. Your car's internal combustion engine turns only about 20-35% of the gasoline's chemical engineering into forward motion, a jet engine has higher thermodynamic efficiency, turning about 55% of the fuel's energy into useful work; electric engines, on the other hand, are commonly north of 85-90% efficiency.  These latter numbers are energy-in/energy-out, so net propulsive efficiency (= energy-in / kinetic-energy of the aircraft that results) is a bit lower due to propulsion inefficiencies, but here I'm just comparing the power plant.

Electric engines have low operating costs.  This obviously varies 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.  By analogy, my experience with my EV has been that it's about $0.03/mile (I go about 3.3 miles on a kWh, which costs me about $0.10 where I live), whereas my previous car, a 25-mpg sedan (a lighter car, believe it or not), would cost $0.08/mile at $2/gallon (a price I think I never saw while I owned it), nearly 3x as much; as they say, your actual mileage may vary, but the principle stands.

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 (see below), and the noise level drops significantly more.

But perhaps the biggest advantage of electric aviation is the most subtle: the engine is abstracted from its power source.  In other words, it doesn't know or care from where its energy source is derived.  All conventional LFA engines are intimately connected with their fuel source.  (Try putting Jet-A into your piston aircraft and see how that works out for you.) A huge part of the engineering of LFA engines is all about getting a very specific liquid fuel to the right place at the right time with the right mixture with the air 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 (again I'm displaying my piston familiarity and turbine ignorance here...), all of which makes the LFA engine intimate with its energy source.  

None of that is true in an EA engine.  The wires are carrying electric current.  It simply doesn't matter if the source of that energy is dead dinosaurs (yeah, I know it's probably plant-based, but that's a less interesting metaphor),  nuclear, solar, or humans-in-vats a la "The Matrix".  

That abstraction carries at least five very under-appreciated but significant benefits:

  • The engine is simpler, more reliable, more maintainable, higher efficiency, etc. precisely because it can throw out all of the stuff related to the liquid fuel and work with electrical energy in a much simpler manner.  
  • There's no notion of having the right grade of fuel.  (Look at how difficult it is to come up with a drop-in replacement for 100LL, for example!)  Electrons are electrons.
  • Because the engine is simpler/lighter, and wires are much easier to route than fuel and exhaust, non-traditional designs that improve performance and safety become more feasible.  For example, instead of just one or two big engines, you could have 10 small engines.  An engine failure in such a design is effectively a non-event.  You could even imagine designs where you - the pilot - could snap-in or remove engines on the fly (no pun intended), removing engines for efficiency in a lightly loaded flight and adding more when you need more thrust, perhaps allowing you to trade off range and useful load on a mission-by-mission basis.
  • It enables software (rather than hardware) control, which means updates and improvements are easy and inexpensive to deploy widely.  (Over-the-air updates is one of my favorite features of my EV, by the way - and many of the enhancements simply could not be done with internal combustion).
  • Most importantly, it means that the engine - and thus the aircraft - can enjoy any improvements to the energy source.  So if you buy an electric airplane in year 1 and in year 6 a new battery technology is available that is lighter, cheaper, and higher capacity, then your aircraft can suddenly have longer range and more useful load.
Of course, allowing the engine to be ignorant of its power source just pushes that problem to the power source itself.  Which makes a great segue to...

Technology trends

With all of the above, why does LFA continue to dominate?  Two words account for almost the entire reason: energy density.  There are a few other factors I'll discuss, but this is the big one.

It's actually the exact 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 EA.

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 energy you can store per unit of mass (weight).

Obviously, for aviation, both space and weight are at a premium, so you want a very high volumetric density, in order to pack as much energy as possible into whatever space you have, and a very high specific energy, to carry as much energy as possible for every bit of weight you can carry.

Batteries in 2020 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?


Gravimetric Density
Volumetric Density
100LL44 MJ/kg
31.6 MJ/L
Jet Fuel43 MJ/kg
35 MJ/L
"Standard" Lithium-Ion
(actual numbers vary
based on technology)
~750 KJ/kg
~1.2 MJ/L

Wow.  So liquid fuels are about 58x more dense on a weight basis, and 25-30x more dense on a volumetric basis.  Those are truly daunting ratios.  For EA to compete head-to-head with LFA, it needs to make up a deficit of almost 60x.  Game over?

No.  And here's where I'm going to do some back-of-the-envelope math and extrapolate various trends.  I'm going to switch now to rough numbers, since we're talking trends, various technologies, and long timelines.

The first thing I'd observe is that significantly more energy is wasted as heat with liquid fuels than in an electric system.  Going electric means going from 30-50% efficiency to potentially over 90%.  It's not quite a 2x improvement, but it's close. So for the sake of rough math, let's say that the hurdle for EA to overcome comes down from roughly 60x to roughly 30x.  Still an order of magnitude.

Where is battery technology headed over the coming years?  The first place to look is to extrapolate trends to date.  They're encouraging, having more or less tripled over the last 10 years:

Of course, as they say "past performance is no guarantee of future performance": battery advances are dealing with difficult physics and chemistry, and each advance is hard fought and hard won, but the incredible growth of solar and EV's have produced a tailwind of R&D here that does not show signs of abating any time soon.

It's hard to say what that means for our 30x multiple: the first 3x is certainly easier than the next 3x.  But there are encouraging signs that there may be some step functions ahead of us.

The picture below is from a webinar in which I was a speaker in a few months ago for my angel investing group, showing various technologies on the horizon.  Current commercial Lithium-Ion Batteries (LIB) are the dark blue.  (For comparison, lead-acid, which is in your car and in your aircraft's starter motor, is the red dot in the lower left corner; the aqua oval is Nickel Metal Hydride).