We’re thinking about electric airplanes all wrong

Bill Gates has famously said, “We always overestimate the change that will occur in the next two years and underestimate the change that will occur in the next ten.” Thus we overhyped the internet in 2000, but failed to recognize how completely it would change life by 2010. The same could be said for electric aircraft, an emerging technology that seems to have been on the “coming soon” list for decades.

To take just one example, a 2009 Air & Space article asked, “Shouldn’t we be flying these by now?” The technology seemed so close, but some seven years after that article we aren’t flying them — and it’s not likely we will be in the near future. Why the delay?

Pipistrel Alpha Electro
Pipistrel’s Alpha Electro, a 2-seat electric trainer, can fly for one hour plus a 30 minute reserve.

Some of this is merely a result of our outrageous expectations. Technological breakthroughs always take longer to get from concept to finished product than we think. In the case of electric airplanes, battery technology seems to be dictating the pace of development more than anything. We simply need batteries to get smaller, lighter but also more powerful (cheaper would be OK too). It’s a lot to ask.

Beyond technology, much of the blame lies with the FAA, who has literally made it impossible to certify an electric airplane. In its zeal to dictate the smallest details of aircraft design, it has created rules that do not even conceive of electric propulsion. A long-awaited Part 23 rewrite offers some hope, by moving from prescriptive rules to goal-oriented standards. Instead of talking about the cylinders on a piston engine, the FAA will require the powerplant (whatever it is) to meet certain reliability and safety standards. The method of compliance with these standards will be left open. There are lots of positive rumblings about this reform effort; let’s hope it comes soon.

Questions of cost and regulation are important, but they obscure a far more interesting question: what will electric airplanes actually look like when they finally arrive en masse? What are we underestimating about the 10-year timeframe in our complaining about the 2-year timeframe? It’s here that most pilots, for once, ask too little of technology. We impose our current understanding of airplanes on what could end up being a radical rethinking of aircraft design.

Early car
A lack of imagination?

Pilots are hardly unique in this regard, as the early history of automobiles proves. As early as 1910, it was clear that horse-drawn carriages would eventually fade in popularity, but few observers imagined the second order effects that would fundamentally change American life. An expansion of current technology always seems possible, but a dramatic cultural shift is often impossible to imagine. As Carl Sagan said, “It was easy to predict mass car ownership but hard to predict Walmart.”

Are we making the same mistake with electric airplanes? Pipistrel, for example, is doing some exciting work on electric airplanes and may be the first company to market with a practical electric airplane. Yet I can’t help but think we’re doing the equivalent of the “horseless carriage” for airplanes: we’re bolting electric motors onto existing designs, adding new technology to old platforms.

For a look further into the future, consider NASA’s Leading Edge Asynchronous Propeller Technology (LEAPTech) project. This experiment goes beyond just replacing a piston engine, and reimagines aircraft design by using distributed electric propulsion across the entire leading edge of the airplane’s wing. The array of motors can directly create lift by moving air over the wing, increasing lift as power is increased. And since each motor can operate at a different speed (the asynchronous part), aircraft control and performance could be dramatically better than traditional aircraft.

This may all sound like science fiction, but imagine an airplane that couldn’t stall. Getting slow on final? The autopilot could increase power just enough to keep the wings – or even one wing – flying. Need to take off from a short field? Use those motors to create instant lift, without waiting for 60 knots of airspeed. It’s a whole new kind of fly by wire.

DEP airplane
The future may look different.

Other concepts involve ducted fans, tilt-wings and modified helicopters. Many of these won’t progress any further than the pages of Popular Science, but they show what’s possible when you design the aircraft around the powerplant, instead of vice versa.

Electric propulsion is really just one part of the equation. Add in the possibilities of automation (and some day artificial intelligence) and you have a flying machine that can do things regular airplanes simply cannot. The LEAPTech demonstrator has so many motors that it demands fairly sophisticated computers to keep them doing the right thing at the right time.

Drones are a perfect example of how electric propulsion and automation change core design assumptions. The ubiquitous quadcopter design, with four small electric motors on arms extending from the center of the aircraft, would be almost impossible with traditional gas engines. And the precise, hands-off flying they are famous for is only attainable with significant computer input to constantly vary the speed of each motor. This technology may not scale to the size of a Boeing, but it certainly can work in a two place trainer. It’s already in use by large military drones.

The first step for general aviation will be hybrid gas/electric airplanes, then probably a movement to retrofit electric motors onto existing airplanes – a comforting transition phase for those of us used to burning fossils. Eventually though, when electric airplanes move beyond research projects, they won’t look much like a Cessna 172. This may not happen in two years, or even five. But it’s likely we are all underestimating the possibilities, especially in light airplanes, for 2030 and beyond. The business model of flight schools, FBOs and even airframe manufacturers could change, too – the Walmart effect.

Does that mean Cherokees and Skylanes are doomed? Hardly. Just as we have an army of dedicated restorers who work on Ceconite fabric and Jacobs radials, piston airplanes will have their fans for decades to come. But they might have to share the sky with some unusual aircraft.

15 Comments

  • Electric propulsion is highly desirable for the reasons John listed above. Electric storage in chemical batteries is not desirable, however – they’re just too inefficient a means of storing energy such that the aircraft has insufficient range and must be excessively small and lightweight to be practical traveling machines. The incremental improvements being made in battery technology over many decades are not nothing, but do we really want to wait another 100+ years for an airplane with more than one hour of useful flying range, and hope that somehow technology will produce a result when real alternative solutions are already available? I think not.

    Consequently, electric-internal combustion hybrids (not so much gas but diesel-electric) make a great deal of sense for the moment. Hydrogen fuel cells are also not far behind, having been demonstrated extensively in automobiles but lacking the necessary liquid hydrogen fuel distribution system to support mass acceptance (a much lower challenge for airports to meet).

    The FAA certification process is, just as with conventional aviation technology, hopelessly entangling, expensive, time consuming, and innovation-killing. I am afraid that the Federal government and aviation community won’t get what we need from Part 23 reform – it needs to boot the FAA entirely out of the business of certifying private aircraft and components altogether, leaving it solely to industry performance standards. Then Katie bar the door – innovation and advancement will flood the market very quickly!

    • Have to say I can’t disagree more with Duane.

      Battery power has been in an accelerated phase of development for for the last 20 years with high demand for better performance in cell phones, computers, tablets, etc. New developments, like graphene batteries, are now demonstrating amazing performance in the lab. Once one battery breakthrough arrives at the commercial level, electric aviation becomes immediately possible. Oil lamps existed a long time before gas powered cars; the development that made automobiles and airplanes possible occurred very rapidly once a few key technologies reach maturity and then advanced at an alarming pace–Orville Wright witnessed the jet age. The conditions for electrically powered cars and airplanes are at the same point as gas powered cars and engines were in 1900.

      Hybrids cars make sense only from energy harvesting of their own inertia, and might have a commercial aviation application (using surplus power from turbofans to electrically supercharge a fan stage or power a ducted fan), but won’t have an application on a small airplane. Gas airplanes have heavy engines and are powered by light weight high energy gas. Electric airplanes have lightweight engines and are powered by heavier batteries. You loose the advantage of electric when have to haul around an internal combustion engine.

      Hydrogen has to be stored in a pressurized tank that safety dictates cannot be made very light unless Hindenburg (the last hydrogen powered aircraft) landings somehow become desirable. Also, hydrogen has a problem with scalability, diffuse hydrogen is light and contains low energy, concentrated hydrogen contains higher energy, but must be compressed in heavy containers for storage. Unless and until hydrogen power can be extracted from water, hydrogen will continue with a insurmountable storage problem.

      The FAA isn’t the only regulator in a global industry and will keep up or America will be left behind. Moreover, the FAA certainly was willing to certify li-ion batteries the 787s prior to maturity. In a world where counterfeits and cheaply made Chinese products (like hover-boards which catch on fire) are entering the United States through online sources, and the army has trouble buying non-defective helicopter parts, I pray that we increase our regulation vigilance.

      • Yes, Orville viewed the jet age. However, development to reach the jet age was driven by warfare.

        GA is not big enough to drive rapid progress. Transporting large numbers of people could be, but that’s much longer than 10 years out.

        There will be a few botique electrics out there in 10 years, but market acceptance could be less than the LSA market.

      • Garrett – you are welcome to disagree with me, of course, but you’re wrong. Exhibit A for the folly of battery powered aircraft is that after 113 years of aviation development, we have managed to produce a battery-powered electric aircraft with all of one hour of useful endurance. That’s it.

        That’s “failure” by any rational measure, to produce a practical battery powered aircraft.

        Hybrids aren’t useful solely for regenerating power during electromotive braking. Hybrids make sense mostly because they produce reliable power with temporary battery backup, and provide an overall increase in fuel economy with the ability to support short term bursts of power (for cars, during acceleration … for aircraft, during climb).

        Hybrids make perfect sense for aircraft because the internal combustion engine can be placed anywhere within the airframe that makes practical sense, and can, as John pointed out, produce electrical power that is not limited to a single prop on the nose or the tail of the aircraft. The “power burst” capability from the relatively small onboard battery bank has obvious utility during climbs, as well as providing a safe landing margin should the pilot allow fuel exhaustion to occur. Also, you actually can produce regenerative power in an aircraft – every time the aircraft descends that energy recapture can be fed back into the battery .. but it won’t be a major contributor to efficiency.

        Hybrid technology doesn’t need to be developed .. it exists and is extremely well developed.

        Ditto with hydrogen fuel cell technology – also exists and has developed and improved vastly more quickly than has battery technology over the same timeframe (i.e, over the last decade or so).

        Your comments about unsafe hydrogen fuel storage are extremely off base. Hydrogen is actually a much safer fuel to store on board an aircraft than is gasoline. We all know that the single biggest factor in causing fatalities in aircraft crashes after human body absorption of kinetic energy is the post-crash fire. Such fires cannot occur with pressurized hydrogen gas storage. Pressurized H2 gas will simply disperse vertically into the atmosphere if the tank ruptures, and will not cause the airframe and its occupants to become an inferno. High pressure H2 tanks are also much more resistant to kinetic rupture than are unpressurized gas tanks in aircraft, which virtually always fail in a crash.

        As for weight, it is true that the high pressure hydrogen tanks are relatively heavy, but they are getting much lighter now using lightweight carbon composites .. and you are ignoring the fact that the weight of the hydrogen fuel is vastly lower than the gasoline equivalent weight of hydrocarbon fuel, by an order of magnitude. A few pounds of H2 can replace hundreds of pounds of gasoline or jet fuel, for equivalent range performance. And that H2 gas is a heckuva lot cheaper than avgas or jet fuel.

        Just for comparison sake, consider the following facts:

        The latest iteration of Honda’s Clarity FCV, just announced last month, as compared to its previous version of just three years ago, provided an increase of 50% in the energy production density of its fuel cell stack; the new version also delivers a 30% reduction in the volume of the fuel cell stack, while providing a 30% increase in the tractive power output to 174 HP. Additionally, Honda increased the range of its FCV from 300 miles to 434 miles – a 45% increase! All that improvement in just three years, mind you.

        And unlike a battery powered aircraft, a FCA (fuel cell aircraft) like the Honda Clarity can be completely refueled in 3 minutes! A battery recharge under even the most aggressive charging would take hours if not overnight. How could that possibly be practical for any kind of cross-country travel?

        • A fuel cell is, at is core, an electric battery powering an electrical motor. However, hydrogen matches gas assuming a four-fold increase in energy density over current 700 bar–the same pressure the FCX clarity currently uses. Assuming a 100% increase in fuel cell efficiency, you still need to double the energy storage (1400 bar storage tank) to reach gas efficiency. Increasing pressure while decreasing weight is a daunting challenge. But a fuel cell is ultimately limited to 100% efficiency in hydrogen conversion into electricity and it still needs a pressurized storage tank, an electric motor, and a battery. This is a ton of complexity. Since a fuel cell has to have battery in the loop, the demand will be to increase the battery capacity and reduce its weight which then logically leads to eliminating the complexity and weight of the fuel cell. So why make massive investments to develop a technology that will inevitably be surpassed by a battery?

          Current fuel cells cars also don’t carry the same amount of energy as EVs so its already behind on the energy density curve. http://insideevs.com/hydrogen-fuel-cell-toyota-mirai-energy-efficiency-compared-to-bevs/

          Hydrogen has several second order challenges as well: the optimal storage container is a sphere or cylinder and the optimal shape of energy storage in an airplane is the shape of the wing; container fatigue from repeated pressurization; risk catastrophic failure (rupture plus spark=boom, http://www.seattletimes.com/seattle-news/greenwood-explosion-levels-buildings-injures-firefighters/); and there is practically no hydrogen infrastructure nor a feasible way to get hydrogen to GA airports.

          Proper energy management in an airplane makes using a hybrid a challenge since an airplane already is a hybrid that stores gas energy expended into kinetic energy–altitude. Battery regeneration comes at the expense of glide performance.

          One of the advantages of graphene batteries is a quick recharge rate, calculated in minutes and not hours. http://www.electric-vehiclenews.com/2015/05/graphene-supercapacitor-equals-li-ion.html

          Another is safety and simplicity: multiple engines + multiple separate power sources = no single source failure problem.

          Batteries can also double as weight bearing structure (like the Tesla Model S battery pack providing floor structure) reducing their weight penalty and are flexible in their shape and placement location allowing efficient packaging.

          Battery technology is also spurred by multiple highly competitive industries desiring better efficiency and insatiable worldwide demand. Each increase in efficiency spurs a new industry (from notebook computers, to cell phones, to tablets, to GPS, to cameras, to EVs, to watches containing CPUs). These are similar conditions as wartime.

          Finally, NASA (greased lightening, Helios, etc.), Boeing (787–no bleed air, Watts Up airplane) and Airbus (efan) are all actively investigating electrically-powered aircraft. I know of no one investigating fuel cell powered aircraft.

          I can be wrong, but I doubt the giants in aerospace are.

          • Garrett – You’re obviously a battery guy, but sorry, despite the tech talk you provide you reveal your fundamental lack of understanding of fuel cell technology.

            A fuel cell is NOT a battery.

            A “fuel cell” (to be specific, the “fuel cell stack”) does not “store” any energy at all. A fuel cell is a reactor – a specialized container that promotes an electrolytic reaction between oxygen in the air and hydrogen fuel which is stored in a separate container. The hydrogen can be stored as high pressure gas, or at medium pressure as liquified hydrogen. Hydrogen can also be stored in the form of liquified ammonia gas at relatively low pressures and then catalytically cracked to form free hydrogen gas.

            A battery stores energy chemically in electrolyte. All batteries are extremely inefficient storage mechanisms on the basis of specific energy density (i.e., units energy per unit mass). Batteries may be improving slightly over time and with R&D, but fundamentally they are still limited by the physics of electro-chemical energy storage. Hydrocarbon fuels are vastly more efficient in specific energy storage than are all chemical batteries, and hydrogen fuel is much more specific energy efficient than hydrocarbon fuel.

          • You’re also completely off base on your wild claims that hydrogen storage is unsafe or that it’s infeasible to provide hydrogen fuel at airports.

            Again, as I stated above, hydrogen gas is vastly safer than gasoline or jet fuel when stored on aircraft because it eliminates post crash or even in-air fires. Hydrogen is much lighter than air and when released through a breach it immediately disperses upwards and into the atmosphere, not sloshing around inside the airframe where it promotes an inferno.

            By the way, batteries have caused numerous fires and explosions, including on aircraft, as it is well known, to where quantities of batteries that may be shipped on passenger aircraft are now strictly regulated.

            As to hydrogen deliveries to airports, it is just as easy and cheap o deliver a tanker truck load of liquified hydrogen to an airport as it is a tanker load of avgas or jet A. The distribution issue for cars is only related to retail delivery at many millions of service stations and C-stores – in other words, it is not a technological matter, it is simply a matter of scale, and the very small scale of airport deliveries virtually nullifies the infrastructure issue.

            Not only that, but hydrogen gas is easily formed from cat cracking of natural gas or propane, which is readily available at most if not all airports in the USA today (either tanker delivery or pipeline). Indeed, Honda provides “home energy stations” to its Clarity buyer … this is a simple system that generates hydrogen fuel from natural gas, easily installed in your garage or basement. The cost is about a buck a gallon – far less than any av gas or Jet A fuel costs anywhere in the USA. The same system is easily available to any individual aircraft owner inside their own hangar, or to an FBO or airport operator on a larger scale. The home energy station even serves duty as a backup electrical generator in the event of power outages.

      • Of course, the Hindenburg wasn’t powered by hydrogen: It wasn’t the last either. I remember that a retired pilot, Bill Conrad, flew a liquid hydrogen fueled Grumman Cheetah down in Florida in 1988. It was a first, but not too practical. The 40 gallon hydrogen tank filled up the back seat and, I believe, only gave about a 1 1/2 hour endurance. Still, a good effort.

  • I’m not so sure about your prediction of a period in which electric motors would be retrofitted onto legacy airplanes. Most of the legacy airplanes are pretty inefficient aerodynamically and unless a huge leap forward was made in electrical storage I would expect a converted airplane to be somewhat disappointing. It sure would be nice to be able to fly along without the engine racket and vibration though. And no oil changes and fuel smell; I could go for that.

  • I would think that all of those small propellers are very inefficient compared to one (or two) large ones.

    You need to move a lot of air to prevent a GA sized aircraft from stalling and those little propellers are not going to do it. They can stall themselves!

    • Ken – small diameter props are not necessarily any less efficient than larger diameter props. Prop efficiency is a function of many design elements, including airfoil shape, area, length, rotational speed, and pitch. Indeed, large diameter props can be less efficient if the result is tip speeds at normal operating RPM that approach Mach 1; supersonic prop tips create a lot of noise which is not welcome either.

      With distributed electric propulsion, as John pointed out, the aircraft designer has a great deal more flexibility to design more maneuverable aircraft that are also less subject to aerodynamic stall, perhaps even stall proof. Small diameter props or shrouded fans mated to electric motors can actually increase the overall propulsion efficiency and provide a much quieter aircraft.

  • Fun to read such a spirited debate between engineering types. You guys are obviously enjoying the dialogue. Me, too, although much of it is way over my head.
    My only personal experience is with my Tesla Model S which is beyond fantastic. Thank you, Elon!

  • I enjoyed the back and forth on electric batteries, fuel cells vs. heat engine (conventional piston and jet turbine) technologies.

    The whole argument hinges around energy density: how much energy does the technology provide per unit of volume and/or mass.

    The electrochemical process doesn’t hold much promise for getting the energy density anywhere close to a pound of 100LL or Kerosene, even considering that the heat engine will waste 60 percent of the energy.

    The fuel cell make the most sense in that it is as dense as a hydrocarbon fuel and converts maybe 90% into electrical energy which a compact motor can convert into thrust with very little energy loss.

    The answer for electric powered aircraft from a density and efficiency standpoint is the fuel cell, not the battery.

    Jack

  • Realy cool stuff, but even the tesla drivers forget that thier electric car is still running on gas, it just comes in a different form… Until the world shifts to solar, wind (atomic?), at the core, everything is still very much running on fossils.

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