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A different kind of range anxiety

Discussion in 'Electric Vehicles' started by Patrick W, Sep 29, 2015.

  1. Patrick W

    Patrick W Member

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    This has nothing to do with Tesla but I post it because of the similarities I see between the problems mentioned in the article with electric aircraft today and those Tesla was up against in the early days.

    Actually, reading through the piece I was half expecting some reference to Tesla but they only mentioned electric cars in general.

    Pipistrel Alpha Electro: The trainer of the future? - AOPA
     
  2. wdolson

    wdolson Active Member

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    Though electric aircraft have to intend with a major factor cars don't have to worry about as much: weight. Weight is a critical factor in aircraft design and batteries are heavy. Weight can be a factor in car design, but it isn't as critical. The Model S is a very heavy car, but still has good performance for the most part.
     
  3. deonb

    deonb Active Member

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    #3 deonb, Sep 30, 2015
    Last edited by a moderator: Mar 13, 2016
    Also a good read on why there aren't more electric airplanes around:
    http://www.wired.com/2015/09/blame-faa-blunder-lack-electric-airplanes/


    I guess the Icon A5 would be the Tesla equivalent of light sports aircraft. If Icon ever offers an electrical A5, I'll be first in line to buy it :love:. It shouldn't be too difficult. Both the Pipistrel Alpha and A5 uses a Rotex 912 engine, and the A5 has enough weight capacity for those 270lbs of batteries from the Alpha, once you take the engine and gas out.

    Oh. And you can tow an A5 behind a Model X. Easily :).

     
  4. wdolson

    wdolson Active Member

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    I note the Pipestrel only has a range of 81nm. Fine for short jaunts around the local airport, but you aren't going to go anywhere in it. Batteries are significantly heavier per KWh than avgas. Avgas is 13 KWh/kg and Li-Ion batteries are 0.25 KWh/kg. Electric motors are more efficient, so you don't need as many KWh of energy for the same range, but you still get more energy out of a Kg of AvGas. Another added bonus is the plane gets lighter as you burn the fuel and an electric plane doesn't get any lighter, so you have to carry the extra weight around all the time.

    There will be experiments in electric planes if the FAA decides to change the rules, but I doubt they will be much more than local airport playthings until batteries get a better energy per Kg.
     
  5. deonb

    deonb Active Member

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    Sure, but keep in mind that 81 nm usage is partly because of the 30min safety. If you double the batteries (another 270lbs) you can increase the range to 200nm, which is somewhat usable.

    The main obstacle that prevents progress to this isn't the current technology - it's the FAA rules for what you have to do to certify as an LSA. 270lbs extra isn't a huge extra payload, but added to an LSA it will exceed the LSA gross weight max cap. And the tech itself violates the FAA regulations.


    If you get into real GA territory and you take a plane like a Cirrus SR22 and throw out that 430lbs Continental engine and 552lbs of gas and all of the other gas support, you can get yourself a 90kWh battery + inverter + motor instead. That should get you 400nm of range. Sure, it's not the 1200nm you get from the avgas engine (yet), but your TBO will be something insane like... what, 10'000 hours before that electric motor needs to be rebuilt? And your energy price per nm would be peanuts. So now your $180/hr plane suddenly becomes a $30/hr plane, which is hugely appealing.


    But manufacturers can't just take a larger airframe and start building an electric version just to see if there is demand - the licensing for that is in the 10's of millions of dollars. So new technology needs to come in from the LSA side where it has a much smaller barrier of entry. But in order to do that, the FAA needs to loosen up on the weight and fuel restrictions first.
     
  6. wdolson

    wdolson Active Member

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    Adding 270 lbs to a small plane which only has a 440 lb payload is a big change. You just reduced the plane from two people to a single seater. Kind of a bad deal in a plane intended as a trainer.

    With conventional aircraft, like the Crrus SR22, you take off with 552 lbs of fuel, but you land with significantly less. You're plane gets more efficient as you travel. With an electric plane you take off with 500 pounds of batteries and they are still there when you land. Electricity is significantly cheaper than Avgas, which is more expensive than gasoline for cars, so the cost per nm would be cheaper, that's true. The second article above talks about the initial use for electric planes being as trainers, which don't need long ranges and for a flight school, it's probably OK if the plane sits for an hour between flights as the instructor can check out the next student on the ground while the plane is charging.

    I was an engineer at Boeing back in the late 80s and early 90s. I'm an electrical engineer and my group was involved in doing engineering test on the avionics for commercial aircraft. However at all levels of an airplane's design, there is a constant focus on making things as light as possible. Shaving even a couple of pounds off a part is favored, save a hundred pounds and you're a hero. Even in the electronics side of things there is a constant push to save weight.

    One major factor in range is the fact the plane gets lighter the longer it flies. If the weight of the fuel stays constant, that's going to cut into the range in a rather dramatic way. I don't think you'd see 400nm from 552 lbs of batteries, which would be about 60-65 KWh of batteries. 90KWh of batteries is about 750 lbs. The Model S also has about 250 lbs of non-battery material in the battery pack such as the structure and internal partitions. As the weight of the batteries goes up, you quickly start reaching the point of diminishing returns.

    You may be able to save a little weigh with a 90 KWh aviation pack by eliminating the armor the Model S pack carries, but if you make it too thin, you would be vulnerable to bird strikes. You also have to consider where you are carrying the batteries and need to build the structure of the plane to support them. If the batteries are in the wings, the wings may need to be reinforced to carry the batteries which are probably denser than fuel which has a density of 0.72. Li-Ion batteries have a density of 2.1, just about 3X avgas. Reinforced wings adds weight to the plane, which reduces range and performance.

    The engineering trade offs with land vehicles are a lot more forgiving than with planes. The Physics bites you in the backside at every turn.
     
  7. sandpiper

    sandpiper Active Member

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    It would be wonderful. I've looked at the SR22 and thought about how great it would be as an electric plane: dirt simple, quiet and reliable. And no need for a turbo since there's no power loss with altitude.

    But I don't think 400 NM is in the cards for quite a while. Average power load on an SR22 over the course of a full flight including climb out and approach is something around 225HP (55kw) with a planning speed of around 170 knots. Assuming 80kwh usable that gives you something like 250 NM range to dead battery with no reserve. And that assumes that you don't need to use cabin heat at all - which isn't realistic.

    If something happened to make 300+kwh batteries available, then you've got a totally different game. But you're going to need one heck of a supercharger unless you want your turnarounds to be 3-4 hours!
     

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