Electric Airplane

Hi there,

I'm just getting into homebuilding as well, but have been following developments on the electric front, because I believe it's the only way I'm going to be able to fly in an affordable manner.  (Also, it's "green," which I'm all for.) 

First, your motor requirement is going to add some pretty impressive weight.  The "typical" thing to do in that kind of power range is look at motors used on electric vehicles.  However, your HP requirement is pretty dang high.  Google reveals that 200HP is equal to about 150 kilowatts.  If you only use that amount of horsepower for very short bursts, then you can "overdrive" a lesser motor with more electricity to get it there, but at the cost of having potential motor meltdown.  Note that existing electric motors will typically be listed at their maximum continuous output, rather than peak.  At any rate, there's a lot to look at there.

From what I've seen, battery tech isn't quite there yet either, and is largely the limiting factor.

My opinion is that there is tech out there, but it's not feasible with today's tech.  The Sonex folks are working on an electric aircraft with their ESA and associated motor and battery tech, found here: http://www.sonexaircraft.com/research/e-flight/

I'm thinking that is probably has the most promise of things still coming around the bend, but unfortunately you can't even start building one right now due to the airframe still being worked on.  The weight characteristics will almost surely be different than a traditional aircraft, so some thought has to be put into this.  Also, efficiency is KEY.  For the Sonex project, they have two main ways of dealing with it, from what I've seen: make the airframe more like a glider, and tune the (brushless, computer-controlled) motor.

Sure its most likely doable.  I would think a smaller motor with some type of gear drive allowing it to swing the prop at proper RPM would work.   Power duration I think would be the limiting factor, especially flip-flop flying, due to battery drain.  Battery weight will most likely be the killer here.

Hi Richard,

Thanks for the response, but I did want to comment that in the electric world, redrives don't make sense due to efficiency reasons.  You use more power at higher RPMs, and your motors are typically 93%+ efficient already at turning power into movement.  This means that the gears only introduce friction losses to the equation, and, as you suspect, battery weight is a much greater issue than motor weight.  (Within reason) 

Chris C.,

Yeah, I get what you're saying.  However, consider that battery charging efficiency for Li-Ion is 99%+ efficient, and the electric motor will also be about that high, so the only losses I need to accept are transmission related.  As internal combustion engines top out around 20% efficient for fuel burnt, I still end up pretty green for energy spent.  Even more so if I pay the electric bill and select a "green" supplier, thereby avoiding the petroleum trap.  (Granted, that's not available everywhere, but it is a choice in many states.) 

I also wanted to mention that I found it interesting that Sonex decided they needed to make their own motor, which suggests "off the shelf" EV motors didn't have what they wanted.

As someone still extremely interested in building electric, I think my vote is still "wait and see," like many others.

Hi Christopher,

I too have been looking into building an electric powered plane.  Mine is a little less powerful than the one you mentioned, but I am currently limited to Light Sport Aircraft due to lack of a medical certificate.  I have started building a Wittman Buttercup and plan to install electric power in it when I get to that point - about two years from now.

I went to Oshkosh last Summer to learn more about electric power options.  I attended the Symposium on the subject and also spoke to vendors of actual components for this project.  The most promising vendor was Yuneec (sp?).  They are a Chinese based company with a history in manufacturing and selling Li-poly batteries to the R/C marketplace.  They showed a complete electric airplane at Oshkosh and also showed motors and motor controllers to be shipped soon to builders.

The biggest motor was 85 horsepower.  I judged this to be enough for my project.  The Buttercup was originally designed to use 65 hp and currently calls for 100.  I think electric motors don't need to be as big as reciprocating engines for the same application because of the more favorable torque curve.  Electrics provide high torque at low rpm while reciprocating engines need to spin up to high rpms to get high torque.

I suggest you purchase a complete motor and controller combination rather than trying to roll your own power controller.  This is a complicated technology that has already been invented and perfected by many product producers.  You just don't need to do it over again.  The battery pack is the area that still needs development.  Yuneec promised me a battery pack sufficient to my requirements for around $20,000.  This would power the 85 hp motor for two hours of cruise on a single charge.  It sounds like a lot, but really isn't much more than a new Jabiru or Rotax installation.

I am hoping the battery situation will improve in a couple of years.  Nearly every automotive manufacturer is spending millions of development dollars to deal with this issue and some of them will probably make at least a little progress.  Indeed we might see a real major improvement in batteries for transportation use soon.

Good luck,

Paul Mulwitz

Camas, WA

Tkank you. i'm learning for my job.

Although electrics are coming, they are not here yet.  Motors and controllers are not the issue; energy storage is.  The power per pound ratio of avgas is roughly 50 times greater than that for current battery technology.  In other words if your airplane currently holds 60 lbs of gas (10 gal), it would need 3000 lbs of batteries! ... at the same efficiencies.  Yes, electric motors are more efficient (but not 50 times more).  As for needing less horsepower with electrics, I don't think the laws of physics change just because we want to be "greener" using electrons.

As for the electric motor versus gas engine torque curves, they don't really matter; the propeller is the limiting factor (yes, even if it is a "constant speed" propeller).  Except for the aerobatic guys, propeller RPM is set somewhere around 2500 RPM and left there for the entire flight.  The aerobatic guys are not using the propeller for maximum efficiency ... in fact, sometimes they use it for increased drag.  Short duration aerobatics is a great area for electric applications, though.

Also, comparing cars and airplanes is worse than apples and oranges.  A car requires low end torque to accelerate away from a stop light and uses very little horsepower in cruise (around 15HP or so).  On the other hand, an airplane is always running at "high" rpm (and fairly constant) but is also using 75% (or more) horsepower. 

Bottom line:  This whole arena is way cool and needs to be developed.  It could be the future of general aviation!

Jittawit:  What is your job?  I am looking for a fun, exciting and challenging job like designing an electric airplane!  You can email me personally at fly-in-home @att.net if that is better.  Thanks, Ron 

Hi Ron,

I almost feel like I'm hijacking the thread, but I did want to respond to this.  Note that in electric terminology we're not bending physics, but rather there is a lot of confusion regarding HP/kW ratings of electric motors.  The motors are often (and should be!) specified in maximum continuous output.  However, the same motors can have a 2x-3x "boosted" maximum output when overdriven with electricity.  This means that, should you have the means to keep the motor from melting down(!), you can temporarily run the motor in an overdriven state for things that need a lot of power.  (Takeoff comes to mind.)  After that, you reduce the power to a comfortable continuous HP for cruise or cruise-climb.  Of course, this requires some cleverness in managing temperatures, and requires careful engineering to be safe.  For non-engineers, I would think it might be wise just to stay in the maximum continuous range.  However, this explains why you can technically "go smaller," and potentially get away with it using some clever thermal engineering.

Thanks for all of the replies.  I was hoping for some out of the box thinking.  From what little research I have done, the weight of the electric motor isn't an issue until you get up into the 300 hp or 200KW range.  I have seen 150 KW motors in the 80 lb range on the internet.  What seems to be the main issue is the power supply.  All of the discussion I have heard centers on battery use but it seems that this technology is probably not going to help us for the foreseeable future. 

The idea of a battery is fine but when I think of a battery I think power storage over a long period of time with little leakage and immediate access.  From emperical evidence on how I fly, however, I tend to refuel before each flight and then enroute.  Left over fuel is an afterthought once I land.  So my initial thinking is why bother with a battery if there were a way to make use of capacitors.  There is a lot of open space all over the plane to make use of.  Unlike fuel, a capacitor's weight doesn't change when charged or discharged similar to a battery.  If I am only going to need the energy for the short duration of the flight that I am going to make, then why worry about the long term storage afterward.  Then the next question is how much charge to I need for each flight, how much capacitance do I need and how long can the capacitors maintain that charge.  My goal was 1 hour of flight time, with a 10 percent leakage during that hour.

IThoughts?  Pipe dream?

Christopher,

What (I think) you're looking to compare is energy density, which unfortunately points to "pipe dream" - the energy density of Li-Ion is 150-250Wh/kg compared to 30Wh/kg for the best capacitor available now.  That's quite a spread!  What it comes out to is that you can store far more power in a battery for the weight, and batteries are only barely just "good enough" as-is.  (In my opinion, anyway)

Thanks.  That is what I was looking for and why I asked.  So here was the other thought.  LP gas through a PEM fuel cell.  I think there is going to be a lot of waste there, though.  I believe those fuel cells are less than 50% efficient but would that efficiency be better than the efficiency from combustion? 

I know I am adding a variable weight in the LP gas but I am just exploring the options of how to start out with an electric motor and how to possibly get it powered until such a time that the storage technology reaches a suitable level.

PEM cells are pretty neat.  Unfortunately, I can't say much about them because I don't have much experience with the actual design of the things.  I do know that you are correct that they are far more efficient than burning the material, as you're sapping the energy of the chemical bonds being formed instead of capturing heat or expansion.

It's a developing field for sure, as indicated in the wikipedia article on the devices.  (Found here https://secure.wikimedia.org/wikipedia/en/wiki/Proton_exchange_membrane_fuel_cell )  I know some schools had efforts to try a fuel cell aircraft, but I don't know how they turned out.

You can do the same thing with IC engines.  The engine on my airplane is rated at 300 HP for takeoff.  However, it cannot maintain that power for long, so that is a METO rating that is valid for 5 minutes.  The maximum continuous rating is only 285 HP.   By the way, the 300 HP is obtained at 2300 RPM and the continuous power rating of 285 HP is at 2200 RPM.  The recommended high efficiency cruise RPM is 1850.  Below that the generator was liable to go offline. 

The standard equivalence in terms of force, distance, and speed between HP and Watts is 745 Watts equals 1 HP.  Of course,  1 HP worth of input to the motor will not give you 1 HP worth of output.   When I size electric motors for industrial applications I always allowed 1 kilowatt of electrical input for each horsepower I wanted for output.  This has worked for me for the last fifty years quite well. 

Electrically, Power equals Volts times Current.  ( P=EI ) That is why vehicles like the Prius use high voltage battery packs.  A fifty horsepower electric motor would require about 50 kilowatts of electricity.  That is 50,000 watts.  With a 12 volt battery that would be over 4000 amperes of current.   My 36 HP electric golf cart will draw 1000 amps from its 36 volt battery pack when I tromp on the pedal.   I understand the Prius used a 250 volt battery.  That lowers the current  for a 50 HP motor to about 200 amps. 

Batteries are generally rated by their ampere/hour capacity.  For example, a standard G35 Aircraft battery is 35 to 37 ampere/hours of capacity.  That means it can supply 1 ampere for 35 hours or 35 amperes for 1 hour.  It would supply 200 amperes for between 9 and 10 minutes! 

This says that a 250 volt battery driving a 50 HP electric motor will require a continuous current flow of about 200 amperes.  If we can fly for 4 hours that would be 800 ampere/hours at 250 volts.  That is about 200 Kilowatt hours.  That is the energy equivalent of a tank full of AVGAS.   A fifty HP gasoline engine will burn about 4 gallons per hour of AVGAS.  You can figure about 1 gallon of gasoline for every 12 horsepower hours from just about any gasoline engine.  At the current price for AVGAS of $5 per gallon it will cost you $20 to run your engine for 4 hours.   At our local price for electricity of 10 cents per kilowatt hour, our 50 HP electric motor will cost us $20 to run for 4 hours.   Your local electric rates may vary, as may your local cost for gasoline! :-)  I don't know about the "TBO" of the batteries. 

John Johnson

EAA 9135,  Technical Counselor and Flight Advisor

John,  Thanks for the great explanation.   That fills things in a little bit more.  I know that a lot of research is going on by people that have a lot more education than I but it doesn't hurt to do my own research and experimentation.  I may just end  up learning something like now.  It is interesting, though, I just haven't seen this sort of discussion anywhere and I thought by now I would see discussions much more in-depth than this one.