An Electric Airplane That Makes Sense

Hybrid is the word we use now to define a vehicle that combines different propulsion technologies. But we wouldn’t think of calling the most successful hybrid of all time a hybrid. We call it the Diesel engine, or more formally the diesel-electric locomotive.

It was clear more than a century ago that the internal combustion engine had many advantages over steam power. But in a railroad locomotive it was very difficult to transfer the output of a piston engine to the drive wheels. The necessary gears, transmissions, clutches and other hardware just didn’t hold up. And the piston engine had to spin at comparatively high rpm to produce the necessary torque so that made it very difficult to get the heavy train moving.

The solution was to connect the piston engine to a generator. The piston engine turns the generator without gears and clutches and can operate at a constant and efficient rpm. The generator produces electricity that is wired to drive motors on the locomotive axles. Electric motors can generate gobs of torque from zero rpm making them perfect for getting a train rolling and then operating over a range of speeds.

Nobody would think of replacing the Diesel engine in a train locomotive with a battery. But that’s what most people are thinking when it comes to electrically powered airplanes. Just as in a locomotive, an electric motor has many advantages in powering an airplane. But batteries just don’t cut it as an energy source. And I don’t believe battery technology will advance far enough in my lifetime to power an airplane with speed, payload and range similar to our current piston fleet. And who wants to go backward on those qualities?

But if we install an electrical power generating source in the airplane we are freed from the low energy density of a battery but can still gain the advantages of electric motor propulsion.

Electric motors are much more compact than a piston engine of similar output so a nacelle could be designed for low drag, not simply to accommodate the engine.

Electric motors produce torque at low rpm and propeller efficiency benefits greatly from low rpm, particularly when operating at high airspeeds.

A battery could also be used to supply a brief burst of power for takeoff so the actual generating engine would need be sized for cruise, not takeoff and initial climb or go-around. The battery would act like the water injection systems that upped the power in some pistons and turbojets years ago. A couple minutes worth of extra energy would be enough.

With hybrid power the weight and mass of the propulsion system could be distributed ideally within the airframe. Many have tried to locate piston engines midship but the compromise of drive shafts has not worked out well. Even turbine engine efficiency suffers when long ducts are used to feed induction air.

With hybrid power the power producing engine could be positioned in the best spot for weight and balance, and to minimize drag of the necessary structure around it.

Tesla has made lots of headlines with its all-electric battery fueled car. But Porsche has blown Tesla into the weeds with its new 918 Spyder that is a true hybrid with both battery and gasoline engine power. The electric motors that drive the 918 wheels are extremely efficient and powerful and the car has beat all of Porsche’s conventional cars in lap time around the famous Nurburgring track in Germany. The piston engine in the 918 is connected to the wheels along with electric motors, but it seems certain a Porsche with electric motor propulsion only can’t be far in the future.

I think battery-only power is going to be restricted to vehicles–including airplanes–with very limited applications. But a true hybrid holds great promise. The power generating source can be a gasoline or diesel piston engine, a turbine, or perhaps a hydrogen fuel cell, or even some technology not yet invented.

An electric airplane could be great, as long as it’s making its own electrical power. I don’t want to watch my range go out the window just because I turned the heater up or the air conditioning down. However, if you’re goal is to avoid burning fossil fuel entirely and you are willing to sacrifice speed, range and payload there is already an airplane that suits that mission–a glider.

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56 Responses to An Electric Airplane That Makes Sense

  1. Bill Berson says:

    Glad to see an experimental topic.
    Burt Rutan announced his hybrid electric “Bipod” at Oshkosh a few years ago, what happened to that? And the hybrid Panthera?
    I think adding an electric motor and a generator just adds weight. Weight is almost always bad for aviation.
    It might work well for surface cars and trains that cruise at 20% power. But most airplanes cruise at 75% or more, so very little could be gained with hybrid.

    • Bill Tomlinson says:

      Quite a lot of that extra weight can be clawed back, because you can use a small-light high-revving engine (perhaps a motor-cycle engine?) which will weigh a good deal less than our “standard” Lycomings and Continentals. Also, the gearing effect will mean that propellors can be run at much more efficient speeds than our current direct-drive arrangements.

      Best of all, being able to use cheap automotive-derived engines will lower the cost of flying. I for one would rather be flying an overweight aircraft than standing with my nose against the shop window looking at a lighter aircraft I cannot afford to fly.

      • Bill Berson says:

        Hybrids are not “affordable”. The engine/generator/motor/battery/controller combination is far more expensive. (And more parts to fail)
        That’s why a Prius costs double .
        My old Honda CRX gets the same 50mpg…. And cost me $700 used.

        • Roger Halstead says:

          The Prius does not cost double. Maybe 15 to 25% more. The battery typically has been lasting over 100,000 miles. Ours has reached that point and is still going strong. Mechanic said they are between $2,000 and $3,000 to replace. Some Conventional engined cars are are capable of matching the cost per mile. At a 100,000 miles and the max of 3000 that works out to 3 cents per mile (and just gets less), so figure the gas use/cost + 3 cents per mile. Then subtract what you get for the used car from the new price and divide by the miles. That gives you the actual cost per mile.

          All of the cars I’ve seen that get better than 35 mpg be they hybrid or conventional, are built like beer cans.

          As for the gas engine and electric. I think some have forgotten that we cruise at 75% power. That means, you are not going to be able to use a small gas, or diesel engine. IOW, you will need one that through the generator and electric motors hits 75%. You can basically add at least 5% each for motor and generator losses, which takes us to 85%. That means to get 75% to the prop, you need an engine capable of running continuous at 85% of current. IOW, a larger engine than we have now. Yes, we can turn the prop slower, but in general that means a longer and larger prop, not more blades.

          What do you want to do with it? Like the flying car. There are several and one that is legal in both realms. Now, we need one that is really practical for the general public. Do you want it for training, play, or travel?

          Training and play could settle for reduced power, except for takeoff, climb, and go arounds.
          Whose willing to settle for 30% (give or take) for travel?
          The range in the Deb would go to nearly 1700 miles @ 120 mph, but you’d be up there for a very long day. Go to 75%, burn 14gph (give or take) and hit 190 mph. Then it becomes a serious traveling airplane.

          I don’t see this approach as practical except for training or local play.

          BTW, You have cells and a battery. A battery is nothing more than 2 or more cells hooked together. Don’t confuse the two. Most Hybrids do have two batteries. A really big one for the electric motors and a tiny little thing to start the engine. One of their great advantages is putting power back in the big battery every time you slow down, or use the brakes. You do not drive a constant speed in hilly country, nor for best mileage in stop and go traffic, without being run over. Wear warm clothes if you live in the North!

          • Bill Berson says:

            Roger- I meant the Prius cost to purchase is about double, I suppose you were talking about operating cost.
            I am guessing that a simple new Toyota Corolla is $12-15k vs. a Prius at $$25-30 k.

            I see you agree about my 75% power cruise comment
            You made some additional good comments.

          • Bill Tomlinson says:

            “That means to get 75% to the prop, you need an engine capable of running continuous at 85% of current. IOW, a larger engine than we have now.”

            Not necessarily. Our existing engines are pretty heavy. A Lycoming IO-320 produces 200bhp and weighs a tad over 400 lbs, i.e. roughly half-a-horsepower per pound. Back in WW2 the Germans had aircraft diesels that gave close to one horsepower per pound.

            I do agree with the implication of your comment “Wear warm clothes if you live in the North!” In fact, also if you want to fly at any height in the south. All combustion engines produce waste heat, which can easily be diverted to provide cabin heating – i.e. essentially free cabin heating. Obviously this is not possible for battery-powered vehicles.

  2. Robert Jans says:

    CTLS had a hybrid for a while: what happened to that one? Pure electric power would give you approximately 10 minutes of flight time. Perfect if you have an (piston) engine out: the 10 minutes are probably enough to stretching the descent to a suitable field or a go around if you’re too high for that emergency field. During take-off the 100 hp Rotax would be assisted by the 40 hp electric power; thaty’s 40% more power for initial climb. OK, weight would be higher, but so does a whole-plane parachute. Safety has its price and weight.

  3. Bill Tomlinson says:

    Mac, you have said what I have been saying for a long time now.

    Two more things:

    First, when you say “batteries just don’t cut it as an energy source” there is a reason rooted in fundamental physics and chemistry why they never will. We all learned at school that oxygen is required to support combustion, but we take for granted the oxygen that supports the combustion of our avgas.

    In fact, the oxygen we use weighs considerably more than the avgas; it just doesn’t feature in our flight-planning because it is freely available from the surrounding atmosphere. (Consider the weight of the oxidiser vs. the fuel in a rocket and you will see what I mean.)

    Batteries are like rockets: they have to carry their oxidiser along with them. And that is why they can never reach the efficiency of even a primitive air-breathing engine.

    Second, the ability of an electric transmission to act as gearing – which you mention in connection with diesel-electric locomotives – will get us around the biggest single obstacle to using cheap automotive engines rather than expensive aero-engines.

    • Nikolas says:

      It would actually be theoretically possible for batteries to use the oxygen from the air too. There are lithium-air batteries in development, but they won’t be commercially feasible for some time, if ever. For now, fuel certainly has a far higher energy-to-weight ratio, and is therefore far more efficient (strictly weight-wise) than batteries. However, electric engines are much more efficient than combustion engines. (They are about 90% efficient, whereas combustion engines have an upper limit on efficiency of around 40%.) This means that an electric airplane could carry half as much “fuel” energy to provide the same amount of energy to the props, which will help make them practical much sooner, although they are unlikely to fully match internal combustion in the next few decades.

      • Thomas Boyle says:

        I’ve wondered about lithium-air batteries. Do they get heavier as they discharge?

        • Bill Tomlinson says:

          Just had a look at Li-air batteries on Wikipedia and it is as I suspected. As I suspected, discharge of the battery causes formation of Lithium oxides, which are ‘stored’. “Under discharge [atmospheric] oxygen is reduced and the products stored in the pores of the carbon electrode.”

          So, unless some way can be found to get rid of these Lithium oxides, the whole thing becomes analagous to an IC engine whose exhaust is stored on board.

          • Nikolas says:

            That is certainly true. And unless another method of storing the energy is developed, those oxides will always be there after discharge. (Until you recharge the battery.) However, they would still be far more efficient than current batteries. Coupled with the high efficiency gains over IC, it could still make for a very attractive power system. Not a perfect replacement for everything of course, (I’m not fundamentally opposed to IC or anything) but it would certainly be a viable option and possibly a better one for some applications.

          • Battery technology is improving and the are batteries on the horizon with very high energy density that would be possible of powering long distance flights, “possibly” even transcontinental flights. The one thing common to all batteries, or cells with high energy density is safety. The higher the energy, the lower the internal resistance. So we are raising the joules stored and the speed at which it can be discharged at the same time. The two go hand and hand and so far are inseparable. As I said earlier. What separates gas (high energy) from dynamite (low energy) is the speed at which we can release that energy. The same is true with batteries, but in their case the energy is going wayyyy up. This(so far is a law of physics) “The higher the energy density, the more dangerous a battery becomes!” They have little to go before you are carrying a load of 40%, soon to become 60% and how far before it becomes the equivalent of C4? Somewhere in there the government is going to regulate batteries because of their explosive potential in a crash, or ability to release all that energy, quickly (internal resistance)

            Super conductor batteries have been built tat worked, but they required liquid N2 for cooling and were kept in a heavy bunker (just in case). So, at present, they are not practical. OTOH if a super conductor can be developed that will maintain superconductivity at or above room temperature, we are on the way to long distance, battery powered travel.

            Yes, the Germans developed a diesel that could develop 1 HP per pound, or close to it, but how reliable was it. In wartime many things were developed to last long enough to get the job done. Look at the rocket planes they developed. They were more a danger to the pilot than combat.

            We are now seeing diesel engines built for aircraft. Smaller, higher revving ones should be possible. Just pure diesel can be made more efficient than what we have been using. An electric motor driving the prop is smooth and would simplify the prop as it would not have the engine’s power pulses. Diesel engines by nature must be more beefy than gas engines.

            Slow revving 2 blade props are more efficient than more blades and higher RPM, but the HP requirements are high for a compound engine, generator, electric motor because of the loss at each stage. Long blades mean more ground clearance. A lot more clearance and that means a taller landing gear with more drag.

            So, the question is with an engine that must develop more than a conventional aircraft engine in a compound set up would, be capable of driving a more efficient prop at lower RPM, save fuel and provide a practical distance for travel? Is the added complexity even worth the effort.

          • Nikolas says:

            I agree that the idea of simply using a diesel engine to directly produce the necessary electricity to power the electric motors is more or less useless for conventional applications. (I could see more use with VTOL systems like the Joby S2 that have many small electric motors.) It is inefficient and complex, with few or no benefits. The only reason trains use it is because they actually need the performance characteristics of the electric motors, something that can be done without in a plane. (The diesel is just as good at powering the prop as an electric motor, and if we want lower RPM we can use a gearbox.) A small diesel would be a good range extender for a primarily electric plane, but it shouldn’t be used to replace a battery. (At that point it should just be the primary engine.) As to explosive potential, there is definitely a danger that needs to be taken seriously. But a properly set up battery pack with sufficient safety mechanisms is fairly stable and unlikely to physically explode at any rate, although in a catastrophic crash it may well burn very nicely, just like jet fuel does. (Albeit likely much hotter than jet fuel.) It would just have to be a new safety consideration; it isn’t impossible to design around.

          • Bill Tomlinson says:

            Roger: Of course you are right that the higher the energy density of an energy source the more dangerous it becomes. That also holds true for a tank of avgas, which makes a pretty impressive bang if mishandled.

            Indeed the most powerful non-nuclear explosive used by the military is FAE – Fuel-Air Explosive – whereby ordinary gasoline is sprayed as an aerosol to form exactly the right mix with air. The tricky part is getting the mix right, but the bang is much bigger than a similar weight of TNT.

            Those German diesels were fairly light because they were 2-strokes. (They approached the specific ouput of a Rolls-Royce Merlin, of 1bhp/lb.) Because, in a diesel engine, the fuel is injected during the power stroke, they avoid the problem with petrol 2-strokes, of having to suck the mixture into the crankcase.

            They were used mainly in bombers and were totally reliable. They were much favoured by the crews because of the reduced risk of fire if hit by anti-aircraft defences.

            If you are ever in England you can see one at the RAF Museum at Cosford, a few miles north-west of Birmingham.

  4. DEL says:

    Conventional electric generators and motors are compact and efficient, but they are heavy. Their compact volume is close to 100% stuffed with copper and iron. And the more powerful they are, the heavier they are—unless you constuct them of some exotic and exotically expensive magnetic materials. And their weight adds up to that of the primary power plant that you cannot do without. If you save something, it’s the weight of the gear box. But judging from my Rotax 912 ULS gearbox, that’s a trifle.

    Locomotives are a different matter: the heavier they are, the greater the traction available between wheels and tracks and the greater the torque that can be applied to the wheels without causing slippage and abrasion. For locomotives weight is an asset and therefore they cannot serve as an example for aviation.

  5. Grant Smith says:

    I agree with Del. Applying Locomotive technology to aircraft does not make a better aircraft unless there is a good reason or explanation. I do not see that in McCellan’s article. The Electrics in the Diesel Electric are used because of their good starting torque, and as Del pointed out weight is an advantage. Starting torque means little to aircraft performance. Propeller RPM at crusie and takeoff is what matters.

    The supplimental battery power for takeoff is a good point but the high amperage, short duration output is a battery killer. A high tecnology flywheel or other energy storage might be better suited.

    However, a small single shaft turbine coupled to a motor/generator could be used to lower propeller speed and improve part power fuel consumption. It could also get arround the fixed pitch propeller limitations of Light Sport Aircraft except that turbines are not permitted in Light Sport Aircraft. The same technology could be applied to ultralights if the batteries were considered fuel and not counted in the aircraft empty weight limitation.

  6. Jeff Boatright says:

    Agreed, Mac, for your type of flying, pure electrics will probably not work in your lifetime. For the type of flying that the majority of EAA members enjoy, I’d say it’s still an open question.

  7. P.Palmer says:

    Neither internal combustion engines, nor electric motor technology, has advanced much since the 1930′s. Slightly better materials, such as metal alloys and rare earth magnets, and electronic controls, are really all that has improved for quite some time. If IC/electric “hybrid” engines made any sense for planes, it could have been done 75 years ago, as it was for locomotives. These vehicles have no need for batteries; the batteries in a hybrid car are for the storage of the energy recovered by braking, and used during idling and low speed where IC engines are less efficient; or for storing low-cost electricity from home for the “plug-in” variety.

    The technology that has advanced quite a bit is battery technology, not driven by vehicle demand, but by demand for portable electronic devices like laptops and smart phones. This technology advance has allowed the development of a large electric car with a suitable range for most people -the Tesla – at a $69K price, far under the $845K price of the Porsche 918 “hybrid” car.

    Already, electric ultralights are superior to gas-powered versions for safety, noise, and even cost – though more of that cost is upfront for battery purchase than over time for fuel costs. If battery technology continues to improve – driven by demand for other things, not vehicles – the “cross-over” point where electric planes make more sense than gas ones will continue to happen at increasing power levels. Though this may not happen for the 100+horsepower range for quite some time, agreed.

    • Bill Tomlinson says:

      “more of that cost is upfront for battery purchase than over time for fuel costs” ======= You are ignoring the fact that most batteries have a life of about 1000 charge/discharge cycles. (If you charge/discharge once a day that’s about 3 years worth; the way a typical flying school aircraftnis used it will be a lot less.) Then you have to replace the batteries – the most expensive part of an electric aircraft.

      • P.Palmer says:

        I’m not ignoring it, the cost of batteries continues to decline as their performance improves. Are you ignoring the cost of IC engine overhaul every 1500-2000 hours? The reg’s aren’t written for electric aircraft motors, but I am assuming it will be vastly better on both time and cost, as there is all of one moving part for an electric motor.

  8. Finbar Sheehy says:


    You missed a lot of potential for magic, right up to your last sentence. Gliders have been compared to sailboats, and it’s not a bad comparison: sailboats are slow and quiet, and essentially impractical as a means of transportation (although usable if you’re not in a hurry). But for all their impracticality, sailboats are very popular, broadly in two ways: racing, which is exciting and athletic; and cruising, which is peaceful and soothing.

    Larger sailboats carry auxiliary engines to get them home when the wind stops (or the sailor gets tired). Gliders have used 2-stroke pop-up engines for years, but 2-stroke engines do not have the one thing that is an absolute must for gliders: reliable starting when cold. Electric motors do. And, the ability to put the weight (batteries) between the wings while the motor/prop is on the nose, greatly simplifies the whole system compared to a pop-up motor system. Search for “Front Electric Sustainer” to see how it works. These aircraft can fly for an hour on electric power (less if the batteries are used for the initial launch) – not a lot, but almost always enough to get to an airport. As a solution to the get-the-glider-home problem, it’s already arguably superior to internal combustion.

    But, there’s another interesting possibility. Gliders currently offer no parallel to the cruising sailboat: the glider pilot is constantly searching for lift, constantly “on”, unable to relax and just let the nautical miles scroll by silently. But, keep the glider light and put solar cells on the wings, and things change. Now you have a new kind of aircraft, one that flies slowly, like an ultralight or sailplane; silently (or nearly), like a sailplane; and able to maintain level flight, hour after hour without using any fuel. Of course, if you want, you can fly it like a sailplane, pick up energy from thermals and go faster; or you can use the solar power to climb above the clouds, above the thermals, and just cruise for the joy of it. Practical? Hardly – cruising speeds are likely to be 50kt or so. But sailboats cruise at 6kt, which is about as impractical as it gets – and they’re quite popular. Such “cruising gliders” are becoming a reality: search for Sunseeker for a fully-developed example that has been flying in Europe for years and has crossed the Alps, and you’ll also find an in-development 2-seater (Sunseeker Duo) that is already in flight test; and search for PC-Aero to see a mixed solar-battery design capable of flying at about 75kt all day (while slowly consuming the energy in the battery).

    But – there’s more! Often the real potential of new technologies is found in new applications, rather than existing ones. Multicopters are an ideal application of electric power, and one where 18 little internal combustion engines are just the wrong answer. Here, running an engine/generator combination may indeed make sense. To save weight, the engine might need to be a turbine or a very high-revving small 2-stroke – possibly not the most fuel-efficient, but for the right applications it might not matter. I’m not even sure what a multicopter is good for that a helicopter isn’t – perhaps damage tolerance in combat zones – but it’s a genuinely new type of aircraft. Search for e-volo to see what one might look like.

    Hybrid power for conventional light airplanes doesn’t seem to make a lot of sense, to me. If batteries get better, so that they would usually – but not always – provide enough range with plenty of payload, a lightweight (turbine?) range extender for occasional use might make sense as the equivalent of “long-range tanks”. Or, if fuel cells get (much!) cheaper and lighter, and capable of consuming jet fuel (all requiring a lot of progress from where we are now), we could see jet-fueled electric airplanes, which could be much more reliable, with far fewer moving parts, little vibration and dramatically less noise.

    • Mac says:

      Well, Finbar, I’ve been a lifelong sailor. A racer, really. Never did cruise. But after decades of spending almost every weekend racing, along with the weeknight beer can events, Stancie and I have gone over to power. We bought an old Dyer 29 Downeast cruiser and we plan to go directly upwind at 12 knots and see what we have been missing.
      Gliders are fun, but maybe it’s just me, the reason to sail is to race and try and beat everybody else around. Same for gliders. Unless you are working for the stones and records I lose interest.
      But, maybe that we’ve gone over to power on the water, gliders will hold new attraction.
      Mac Mc

      • Finbar Sheehy says:

        So… you have seen the value of transportation that goes at 12kt.



        (I myself was a dinghy racer. I tried cruising and I got cold, seasick, and bored. But I notice that in the years since, my old yacht club has a lot of large expensive cruisers at the marina, and very few dinghies in the yard.)

  9. Michael Rosing says:

    “Nobody would think of replacing the Diesel engine in a train locomotive with a battery”

    It is surprising what some people would think. Norfolk Southern has been experimenting with a battery-powered switching locomotive for a few years, and their website claims that they are working on a prototype battery-powered road locomotive that would move freight over long distances.

    Though you could simply couple in a few boxcars full of batteries behind a locomotive, and easily swap them out for new charged ones at the end of the line, that is not so easy on a Baron. It will likely remain so during our lifetimes. But with most pilots these days eligible for AARP, that might not be that long.

    Today though, for ultralites, light-sport aircraft and glider sustainers, battery power could make a lot of sense.

  10. Josh Johnson says:

    Driving a generator is an excellent application for a small turbine engine – a hybrid small aircraft is an intriguing idea.

    • That’s a good idea. Get a “cheap” APU, at least cheap compared to a primary propulsion turbine engine. Most APU designs are a point design optimized for a single RPM and thus don’t need the extras that a propulsion turbine needs to handle the wide range of loads and speeds demanded in flight. Probably using a design that thats widely used already could allow for leveraging some development and testing costs.

      Wonder how much a well matched APU, motor, and electric constant speed prop would weigh putting out equivalent thrust at cruise to a conventional 300 horsepower flat six cylinder with a constant speed prop… The APU probably also has a much longer TBO than the reciprocating engine. I don’t know enough about the electric motor to say anything there, but I bet a brushless DC motor has to be pretty reliable with only one moving part. And the APU burns jet fuel so no worries about leaded fuel.

      A design study on this might make a good senior project for a mechanical or aerospace engineering student. Kind of like what Arnold Ebneter did in his E-1.

      • John Patson says:

        Fuel-cell APU’s have been the “Next Big Thing” for years now — a fuel cell / electric motor hybrid might be the way forward. Some are being developed to use methane instead of hydrogen too.
        The locomotive analogy also plays — some hospitals now have fuel cell APU’s for when the grid goes down instead of diesel ones which have proved surprisingly unreliable, usually due to the diesel being syphoned off on the QT because the people who do so think the electricity will never go off grid.
        At the moment these “industrial” land based fuel-cell machines are huge and heavy, but the lessons learnt may filter down to small and light units ready to take to the air.

  11. Harold Bickford says:

    As batteries discharge they have less and less ability to supply current; at about 80% of capacity they are effectively discharged. Then it is time to recharge. Thus any battery has to have about 125% of rated capacity for a given run time. A good analog is to look at a rubber powered model airplane to visualize the effect of the battery having less and less energy.

    Using an engine to supply current for a motor (hybrid) is intriguing as illustrated by the Porsche 918. In it’s own way the concept harks back to the first Porsche design over a hundred years ago.

    In any aircraft we operate at a given gross weight (or less) and hope to have the highest payload at that gross. A hybrid would be a tough sell if that payload is materially impacted and/or effective fuel efficiency or performance is not improved.

    Within the experimental arena we might find folks with the desire and wherewithal to develop a hybrid system. The 918 as a bellweather suggests that cost would be a significant factor. That doesn’t mean dismiss the idea; only that the outcome could be quite different than perception.

    • Mac says:

      Hi Harold,
      There is already electric power being applied to airplanes–to the landing gear. Honeywell is demonstrating that using electric motors on the main gear to taxi is much more efficient than pushing the airplane around with the jet engines. The APU is already there and can supply some taxi electric power, and a battery supplement will weigh less than the fuel burned to taxi and hold for takeoff.
      So we will see a hybrid jet soon. All the major companies are looking at the technology, and I believe Airbus is already in the testing stage.
      Mac Mc

      • Thomas Boyle says:

        Good point!
        So far, it appears that electric landing gear will make sense on short-haul, but not long-haul flights. An electric motor that can move an airliner is pretty beefy, and the weight penalty adds to fuel burn in flight. Narrowbodies spend a greater percentage of their time on the ground, and the savings there easily outweigh the flight penalty. Long-haul widebodies… maybe not.
        Hybrid jets… They’ve been discussed, but the power density of an electric motor doesn’t come close to that of a gas turbine. I’d put that one a long way off.

      • Harold Bickford says:

        Hi Mac,
        Thanks for the reply. Yes, there are developments albeit at a fairly high price point and the outcomes are different than folks typically think about. Since I don’t have a big R&D budget others will have to do the engineering and feasibility work. It is interesting to watch nonetheless.

  12. wsbriggs says:

    With today’s power sources electrically powered aircraft aren’t really practical. A practical 900 KW electric drive would work – 900 KW is ~1200 shp. I’ve racked my brain trying to see how some ultra-powerful unobtainium magnets could allow such a drive to be made, I just can’t see it – major sigh!

  13. Howard Riley says:

    I don’t believe what I am hearing! Throw physics, conservation of energy out the window! Add a generator and a motor in stead of a PRU (gearbox)? Weight savings? Hardly. There are losses at every transfer of energy-engine to generator, generator to motor, motor to prop. If you want the engine in the rear, use a diagonally oriented carbon filament wrapped shaft from the crank (high RPM low torque) to a PRU at the prop. GM did something like this in and Olds many years ago-front engine, rear transmission and differential.

    • Bill Tomlinson says:

      Reduction gearing in light aircraft does not have a happy history. It is expensive and trouble-prone. Remember the Cessna 175 (a 172 with a geared prop)? I do, and I shudder whenever I think of it.

      It seems to work OK up to about the size of a Rotax 912. You may argue that really big engines, like the Rolls-Royce Merlin, also have satisfactory gearboxes. But they come at Rolls-Royce prices.

      The idea of a front engine with rear transmission and differential, has been tried many times – it’s called a trans-axle – and likewise has a history of being troublesome.

      • Howard Riley says:

        There are probably only 15,000 to 20,000 trans-axels produced per day in North America. Every front wheel drive vehicle has one! So maybe we would have a problem producing just a reduction gear? I think not! As an example, give it to Borg Warner-they will not produce a defective gear box.

        • Bill Tomlinson says:

          The term trans-axle is conventionally used to describe vehicles with the engine in front and the transmission and diff at the back. Obviously that means that the prop-shaft turns at engine speed, which seems to be the problem.

          I’m no longer sufficiently interested in cars to know what, if any, trans-axles are made in America, though I recall that Pontiac had one decades ago. This layout clearly helps weight distribution and therefore roadholding, so has been used over the years by a number of exotic (and exotically priced) European sports cars.

          A reduction gear in a car has the weight of the vehicle to act as a damper. A reduction gear in a plane has only the weight of the propeller to act as a damper, which is not enough.

          The problem is not insurmountable, but the solutions are so expensive that most light aircraft continue to bolt the prop directly to the crankshaft, and put up with the inefficiency of a 2500 rpm prop. If gearing was practical, all things considered, Cessna and Piper would do it. Cessna tried with the 175, burned their fingers, and learned their lesson.

  14. Bill Tomlinson says:

    I think Mac unintentionally muddied the waters by referring to a diesel-electric locomotive as a “hybrid”, as this term has come to mean a system that includes a battery, and of course diesel-electric locomotives do not have batteries.

    Diesel-electric aircraft and hybrid aircraft are really two different concepts.

  15. John Hunt says:

    Rather than a battery an ultra capacitor would be better as a takeoff and climb booster. Lighter weight and high power output for short periods.
    Also I think that turbo-compounding, using a generator rather than the mechanical transmissions that the B-29 s used, would be a natural fit for this type of power unit.

    • Roger Halstead says:

      Currently, a battery is the ultimate capacitor.

      As batteries continue to become more efficient, the internal resistance goes down the ultimate would be a superconductor battery. Unfortunately the internal resistance of a superconductor is essentially zero.

      Now for a comparison, by weight or volume, gas has more energy than dynamite. It’s just that the dynamite can release all those joules in a tiny fraction of a second. Take all the energy in an electric car battery. Release it in a millisecond or two. There wouldn’t be enough of the car left to sweep up.

      We are approaching going a point beyond which we will not be allowed to go, because of the potential (no pun intended) for the battery to be used for something other than its intended purpose. However, by my estimation, that still leaves room for something like 50 to a 100% increase in power per pound before we reach that point. Just look at the difference in weight or power density between the old lead, acid battery and the current lithium. There is a sulfur and Aluminum (Not sure about the metal) that has a higher power density. It has a problem with stability though which makes it unsafe.

      There are potentially (again, no pun intended) advances in technology being worked on, or not even thought of … yet that “might” make battery power feasible., or very light weight, high RPM, internal combustion engines combined with ultra efficient generators, or fuel cells that would make for a relatively inexpensive to power, airplane.

      Electricity, produced “in bulk” is relatively inexpensive compared to gas or diesel engines. Power produced on a small scale is expensive. Purchase a generator to power your house and see how close it comes to matching the power company.

      BTW base price of the Carola (MSRP) is nearly $17,000 and it’s easy to hit $20,000 for a well equipped one. The Prius is about $24K and can go to $34K
      It would be better to compare a Hybrid/gas version of the same car if such is still offered.

  16. Dave Passmore says:

    A serial-electric aircraft already exists in prototype form:

  17. Greg W says:

    The Walker Dynamotive delivery trucks used a drive system much like a diesel/electric locomotive, perfect for the consent start/stop cycle of on street deliveries.
    There have been several recent attempts at a “hybrid” engine that used an electric motor to help drive the prop/crankshaft for added take-off power. The systems were said to work well with little weight gain. A flywheel type motor replaced the conventional starter motor and starting batteries were used for power thus little net gain in weight from the conventional set up. It is not thought of like this by many but, a conventional modern engine is a hybrid system in that an electric motor is often used to start the combustion engine and can turn the drive-train in the process, ie. propeller. It is likely that politics and the politics of market inertia are preventing these systems from entering production. Despite the cries of “we are held back by the regs.” most do not want change. If they did many more that currently can would be using autogas, as a glaring example of an approved alternative and yet it is “different” so many defame it with no first hand knowledge. Other propulsion systems will be the same people and thereby the markets do not like change. People like to say that something needs to be done but will stay with what they know, it is simply human nature.

  18. Glade Montgomery says:

    I agree that a diesel-locomotive should not be included within the definition of what is considered a “hybrid” (at least in terms of how that expression has come to be used within the automotive world). Most typically, the term seems to indicate a vehicle that has more than one source of power (e.g., concurrently burned gasoline plus stored electrical potential). The diesel-electric locomotive has only a single power source (diesel). Electricity is at no point the source of power as used. It is only a transfer agent.

    I also agree that the diesel-locomotive is a very poor model for what is practical in an airplane. You’re comparing an application where weight is beneficial to one where it is anathema. You’re comparing between applications where, on the one hand, the diesel-electric must produce a high-torque output with out-the-door rpms ranging from zero to more than 30,000 — to applications where shaft output (i.e., for an aircraft) needs a working range between only, what: 400 to 2500? These are huge differences in application and need. And the differences continue. An aircraft’s power plant spends the majority of its real working life producing something relatively close to full power output. A diesel-electric works regularly within all “oomph” ranges (from mighty-pushing to barely nudging).

    I also think the main reason true hybrids find occasional benefit in road vehicles is because the needed power output, for this context, varies so greatly and regularly. A hybrid allows the electrical source to provide momentary boosts for such frequent but incidental needs as accelerations from stop, hill climbs and passing. These events need a lot of power. Plain cruising needs a great deal less. And, of course, coasting (another very frequent drive mode) needs none at all. By using a hybrid setup, a much smaller gas engine can be used, and fully suffice for standby/cruising needs, while the electric provides such incidental boosts as are occasionally needed. Those dynamics do not apply in an airplane, where we need something pretty close to full power output (usually 75 percent or more) for the entire duration of most any flight.

    So, in an airplane we don’t need the extreme rpm range with full-torque power output that a locomotive does (or anything even near it), and we certainly don’t need the weight. Nor do we need the large efficiency losses that result from converting chemical energy into rotational energy, then rotational energy into electricity, then electricity back into rotational energy once again. Each such conversion produces a significant loss, meaning a great deal more energy must be put into the front end, for a given production out the back end.

    Nor does as an airplane, like a car, have a normal cruise state that requires a relatively small fraction of the output power, as compared to what’s needed in occasional short bursts. So, the “true” hybrid model as seen in some cars (and the true benefit that inheres there) does not sensibly transfer — to the airplane world — either.

    It’s a plain fact. Where we want sustained high power and long endurance, it will remain true that nothing will better fit the bill than a system that carries with us one portion of a very concentrated and lightweight energy source (e.g., gasoline or diesel) and scavengers the greater portion (i.e., oxygen) as we fly along. It will also remain true, moreover, that the best methods for doing this will be those that involve the least inefficiency (i.e., the least possible losses via repeated conversions from one form of energy to another). In other words, for airplanes that cover significant distances and at speed, something pretty close to a pure internal combustion engine will continue to be optimum.

    The above is true, and will remain true, absent some development far more revolutionary than anything since Wilbur and Orville.

    On the other hand, I very strongly look forward to battery-powered electric motors for use in recreational ultralights and gliders. Both can use motors for relatively short periods, and with relatively small power outputs. The enhanced simplicity, reliability, safety and quiet, for those contexts, may be very meaningful.

    • Roger Halstead says:

      Don’t forget that in stop and go as well as in hilly terrain the hybrid motor reclaims power and puts energy back into the battery. The only reclamation would be in slowing and descending. A very small percentage of the flight where it could reclaim power.

      With motor gliders, you only need power for take offs and periods of insufficient lift. Like sail planes they can come down with out power, a skill that would be handy for most pilots. In my years of flying it was a skill I needed 3 times and landed without incident.

  19. Tom says:

    It seems like a motorglider would be a good application for a battery-powered electric motor. It doesn’t need a big, powerful motor (motorgliders are light to begin with, and high speed is not the point) nor does it need power for long stints. A duration of, say, fifteen or twenty minutes would allow a glider pilot to takeoff and climb for maybe ten or twelve minutes, then shut down and soar, while still keeping a few minutes reserve to assure a return to an airport. Then, swap out the battery and go flying again.

  20. Glade Montgomery says:

    That indeed sounds like a superb application for electric, Tom. Very superb!

    But of course, it’s not a hybrid (unless you count soaring as a secondary/other source of power). And, to emphasize my prior point, it’s so in contrast to an application that’s intended to produce sustained speeds over long distances.

    I was thinking further about hybrid cars. If am not mistaken, any benefit in a hybrid automotive design tends to inhere in situations where stop-and-go driving predominates, or perhaps when driving in areas that have many short hills to navigate up and down. I think the design tends generally to be a liability when the owner wishes to drive cross-country, especially and particularly if the trip happens to be over mostly flat terrain (or even over terrain with long, long grades). It is of course the sustained cross-country that is most analogous to the primary task most airplanes are called upon to perform (at least the kind of airplanes Mac seems to have interest in).

    Again, I do not think that is a place any true “hybrid” concept will ever be beneficially deployed. Thus, I think Mac’s dreams on this are pie-in-the-sky.

    Nonetheless, Mac, I thank you for dreaming.

  21. Lincoln says:

    Someone (sorry, don’t remember who) was saying that electric motors haven’t advanced. Well, if you disallow the new materials, then maybe that’s true. But rare earth motors such as most brushless types seem to be more efficient and lighter than older motors. Anyone who started flying electric model planes with ferrite motors some years ago knows this by now.

    Someone else (sorry!) noted the losses from engine to prop, but those are probably on the order of 25 percent or less in cruise. If you can then use an engine optimized for one RPM, you may save some fuel and perhaps have a lighter engine too, possibly even enough to make up for the 25 percent.

    Many people said that hybrids don’t have much of an advantage in airplanes, but that depends on the airplane. For small GA airplanes, we already have to have fairly large engines to get climb, so there’s no need for tremendously (read expensively) sophisticated aerodynamics to get a decent cruise speed at 75 percent power. On the other hand, a sophisticated small airplane is a different story. An extreme example would be the Stemme S-10. According to Wikipedia, it is able to climb at only 790 fpm, but cruises at only 6 knots under Vne. (Presumably the top speed would be over Vne if it didn’t tear itself apart first.) If you want to put a more comfortable margin between cruise and Vne, you’re going to be operating at fairly low power.

    Roger Halstead mentioned longer props requiring longer landing gear. However, small electric motors are efficient and cheap*, so one could use several instead of one. (look up NASA’s Pathfinder) The props would still have a low rpm for their size, which is what matters. Several small props instead of one large one also weigh less. And you could use SHORTER landing gear, thus cutting drag, weight, and perhaps eliminating the need for complicated and heavy retracts. Another advantage of a bunch of small motors is that you could place them to cut drag on the airplane. See some of the Goldschmied papers at the CAFE Foundation web site. For instance, if you had a pod fuselage, a properly set up prop or ducted fan at the back of it could be much more efficient by controlling the airflow around the fuselage. The aircraft would, of course, have to be designed a bit differently. The rest of the motors could be spread out along the wing, thus reducing the load on the spar and providing a handy way to balance the wing at 25 percent MAC to prevent flutter. With a bunch of small motors, the loss of a prop blade is not necessarily a catastrophe. With some of the motors far out on the wing, the ailerons might remain effective much further into stall than on a conventional aircraft, or perhaps the stall would just be delayed to lower speeds.

    Glade Montgomery wrote that hybrid cars only have an advantage in the city. That’s not true unless you’re driving on the autobahn. Most cars in the USA only need 20 to 40hp to cruise on the highway, but they need much larger engines for acceleration or going up hills. So unless you’re going to drive up large mountains at high speed, the hybrid can have a much smaller engine which operates more efficiently since it’s at a higher percentage of its rated power. You don’t get as much advantage as in the city, but you still have an advantage.

    Anyway, I have no idea if hybrid or pure electric airplanes will catch on, but we won’t know if it’s feasible until all the advantages have been designed into the aircraft.


    *I just looked on the BP Hobbies web site. There is a motor there for about $200 that outputs about 5hp. It weighs 3 lbs or so and swings a 26 inch prop. It’s likely over 85 percent efficient in cruise. And ten 26 inch prop disks have as much area as an 82 inch prop disk. So for $2,000 you could have 50hp for a light plane.

    • Electric motors are very efficient. I believe the new ones with exotic materials will run around 95%, bur regular electric motors should be between 85 an 90% efficient so the best doesn’t give a lot of gain.

      I believe you will find that power Vs prop diameter and RPM does not scale well. So, someone who remembers fluid dynamics better than I will have to chime in. Multiple small props are not nearly as efficient as one large one. I believe you will find that 10 small props with the total prop circle to match one large 2 blade prob will have far less efficiency. The solar powered plane uses a bunch of small motors, but it is slow in climb and cruise.

      Hybrid cars work well in city traffic and in hilly terrain but you have to learn not to try to maintain a constant speed. That’s why the contests for mileage are run at night. You coast up to stop signs and stop lights, or adjust your speed to make the stoplight on green with the least need to accelerate back up. Going up hill, you let speed decline as much as possible. Once over the top you let it run with the motors charging the batteries. I hat to hurry to make an appointment so drove close to 70 miles @ 70 MPH on the expressway. On the way home I took the old, hilly, highway.
      Going up I managed in the low 40 mpg range. coming back it was over 50.

      Lightweight cars gain mpg which those getting 35 mpg and up are built like beer cans. Like airplanes, they depend on shape for strength. Nothing comes for free.

      BTW I’ve always heard one engine driving another, as gas to electric referred to as a compound set up, not hybrid. These engines were used in WWII. One drove an electric generator, motor combination.

      There is one (or a number of) compound engine that uses the exhaust gases for power and runs an efficiency far greater than the conventional gas or diesel. I’ve forgotten the name though.

  22. Tim Carroll says:

    Use a fuel cell attack as the heat source in a brayton cycle.

  23. Hybrids typically are use for deficiency. The Diesel/electric locomotive is one extreme with no storage and no reclamation of power as most of its time is spent at cruise power as it is in airplanes, but that combination adds literally tons of weight.

    Automotive hybrids “generally” rely on motor/generator to apply power to the wheels in various configurations. The motor/generator usually receives its power from a very high capacity battery and feeds power back into the battery when braking, slowing down, coasting, and going down hill. That is why the type of driving for mileage with an internal combustion engine doesn’t work. You don’t hold a constant speed but let is slow going up hill and speed up going down hill. Taking your foot off the accelerator so you coast to a stop right at the stop sign works well, but will quickly make you unpopular on the road.

    As Airplane, like the locomotive spends most of its time at cruise power (75%), The automotive combination is the least useful with present day technology. The weight gain from the Diesel/electric also renders it unpractical.

    With the gains in battery technology and the potential for future gains in power density, the pure electric makes the most sense, from a weight, cost, and efficiency approach. Cheaper to build, reliable, and cheaper to operate. Add the new prop that changes pitch with RPM designed for the lower RPM of the electric and it’s a winner from the technical side.

    It appears there is now one viable entry in the trainer field. When they become viable for longer trips? Who knows?

  24. Errr efficency…I need a keyboard that can type!

  25. Bill Tomlinson says:

    I refer you to my post way back on April 16: “Batteries are like rockets: they have to carry their oxidiser along with them. And that is why they can never reach the efficiency of even a primitive air-breathing engine.”

    Assume avgas is pure octane and that when it is burned in existing engines all the carbon is converted to carbon- monoxide. (Neither of these assumptions is strictly true, but they are close enough for a back-of-a-fag-packet calculation like this.) That gives you the equation:

    2.C8H18 + 17.O2 = 16.CO + 18.H2O

    Now look at the left side of the equation. The relevant atomic weights are hydrogen 1, carbon 12, and oxygen 16. Therefore the molecular weight of octane is (2*8*12) + 2(18*1) = 52. The molecular weight of oxygen is 17*16*2 = 544.

    Note that the oxygen weghs more than TEN TIMES as much as the octane – but it comes from the atmosphere, so it doesn’t figure in our weight-and-balance calculations. In a battery-powered aircraft the oxygen (or equivalent) cannot come from the atmosphere, so it has to be carried along. That means that, other things being equal, the battery will weigh about ELEVEN times as much as the gas-tank of a conventional aircraft.

    Of course, other things are not equal. For starters, the thermal losses involved in converting fossil fuel to power take place at the power station, long before the electricity enters the aircraft, so the total on-board thermal energy requirement is much reduced.

    (This opens another can of worms: where will cabin heating come from? In a conventional aircraft it comes from the engine’s waste heat – therefore effectively at zero cost – but in a battery-powered aircraft there is no waste heat.)

    Also, electric motors are intrinisically capable of operating at more propellor-friendly speeds so the power requirement will be less. The motor itself will weigh less than a petrol motor of equivalent power. And so on.

    For those willing to wear their overcoats every time they go flying, and to restrict their flights to about 30 minutes each, then battery-powered aircraft may have some attraction. For the rest of us, they don’t.

  26. DEL says:

    Dear Bill Tomlinson, for some reason the carbon-carrying product of your reaction is pure CO. God forbid, You poison the planet! Luckily, the correct ideal pure-octane reaction is
    2C8H18 + 25O2 –> 16CO2+18H2O,
    so that we may live another day.

  27. Michael Rosing says:

    “Batteries are like rockets: they have to carry their oxidiser along with them”

    They do not _have_ to.
    See Lithium-air battery, Iron-air battery, Zinc-air battery, Aluminum-air battery, etc.

  28. Bill Tomlinson says:

    The exhaust usually is CO rather than CO2 as it leaves the exhaust pipe. That is because we run our engines well to the rich of the stochiometric (sp?) point even when they are “leaned out”.

    The reason we don’t poison the planet (this goes for cars too) is that when you blow hot CO into an oxygen-rich environment (i.e. the atmosphere) it rapidly oxidises to CO2.

  29. Bill Tomlinson says:

    Michael: See my post of April 21, 2014 at 08:22, in which I dealt (briefly) with that. To expand slightly, first of all, lithium air batteries are – to put it politely – not yet a fully mature technology.

    There are various types of lithium air batteries, but most seem to end up converting metallic lithium into lithium dioxide (LiO2). The atomic weight of lithium is 7, so the molecular weight of lithium dioxide is (7 + (16*2) =) 39.

    So we take off carrying 7 units of lithium and end up landing carrying 39 units of lithium dioxide. That means that our MALW has to be considerably greater than our MATOW. Possible, I suppose, but in my not-especially-humble opinion, it is not a clever idea.

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