3 ms·

People need to look at the actual numbers, which I think are revealing. Top-end lithium-Ion batteries have an energy density, fully charged, of about 700 Watt-

People need to look at the actual numbers, which I think are revealing. Top-end lithium-Ion batteries have an energy density, fully charged, of about 700 Watt-hours per kg, which converts to 2.5 MJ/kg. Jet-A, and basically all similar refined hydrocarbons, have an energy density of 42 MJ/kg. Is an electric plane impossible? Clearly not. Are batteries getting better and better? Absolutely; a few generations ago their density was < 1 MJ/kg. But the numbers are still bad. 5% of the energy density for electric versus fossil fuels IF you use the latest & best.

FL33TW00D 6y ago
You beat me to it. Any time someone mentions an electric plane the MJ/Kg jumps to mind. Until battery energy density increases by an order of magnitude electric planes aren’t going to happen, at least commercially.
bluGill 6y ago
True, but these seem to be very short haul flights, so they can probably get by. Just don't expect it to scale to the general case.
nickik 6y ago
Very reductionist analysis. - Electric engines are far more efficient. Electric engines have a significantly better trust-to-weight ratio. - Lighter engines on the wing mean lighter overall plane structure - Batteries can be structural components, not just dead weight, unlike fuel - Potentially you can use gravity to recharge the battery You can also do far more with differential thrust and potentially remove the planes tail. Actually you could do that even with normal planes as Chief Scientist at NASA H. Bowers has discovered. Look at the next generation Tesla battery pack. The batteries are literally what is holding together the car and the resulting weight is not much larger then the structure you would have needed without the battery. We are not there yet, it takes a complex vertically integrated approach to build such a plane, but it is not impossible. Just like with EV you need to re-engineer and rethink every aspect of the plane. Your range might still not be as good, but hell of a lot better then 5% and also far, far cheaper to operate.
- na85 6y ago
  >Electric engines have a significantly better trust-to-weight ratio. Aerospace engineer here. This is not accurate, unless you are perhaps discussing cars. But for aircraft the torque supplied by the motor is not nearly so critical. First of all propeller thrust is a function of airspeed, and it falls off significantly as you accelerate, irrespective of whether your powertrain is electric or hydrocarbon. This is a direct consequence of physics that can't really be avoided and it's why we usually talk about propeller aircraft as being "power producers" rather than "thrust producers". Propeller horsepower is much easier to reason about, and more relevant to the performance of the aircraft, than torque. I'm not aware of any electric-powered turbojets/turbofans beyond the prototype phase but they would theoretically have better static thrust properties by virtue of not being props rather than any particular property of electric motors.
  - nickik 6y ago
    I stand corrected. Just to be clear for my understand, would this not only be true during optimal cruise? Outside of optimal operation, specially during the start, this would actually be an advantage of electric motors? I was looking into the potential of electric turbofans. I'm just happy you didn't shoot everything down.
    - na85 6y ago
      >Outside of optimal operation, specially during the start, this would actually be an advantage of electric motors? When you say "this" do you mean "having more low-end torque than a gasoline motor"? In theory, you could tune your prop governor to take a bigger "bite" of the air at low airspeed and thus perhaps get better acceleration and a shorter takeoff roll. But aircraft performance is very rarely limited by the torque output of the engine itself. Usually the driveshafts and gearboxes are the limiting factors in power/torque transmission.
- deepnotderp 6y ago
  Engines on the wings actually reduce weight since they provide a counterweight to the wing’s positive g forces during flight
deleted 6y ago
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FL33TW00D 6y ago

You beat me to it. Any time someone mentions an electric plane the MJ/Kg jumps to mind. Until battery energy density increases by an order of magnitude electric planes aren’t going to happen, at least commercially.

bluGill 6y ago

True, but these seem to be very short haul flights, so they can probably get by. Just don't expect it to scale to the general case.

nickik 6y ago

Very reductionist analysis. - Electric engines are far more efficient. Electric engines have a significantly better trust-to-weight ratio. - Lighter engines on the wing mean lighter overall plane structure - Batteries can be structural components, not just dead weight, unlike fuel - Potentially you can use gravity to recharge the battery You can also do far more with differential thrust and potentially remove the planes tail. Actually you could do that even with normal planes as Chief Scientist at NASA H. Bowers has discovered. Look at the next generation Tesla battery pack. The batteries are literally what is holding together the car and the resulting weight is not much larger then the structure you would have needed without the battery. We are not there yet, it takes a complex vertically integrated approach to build such a plane, but it is not impossible. Just like with EV you need to re-engineer and rethink every aspect of the plane. Your range might still not be as good, but hell of a lot better then 5% and also far, far cheaper to operate.

na85 6y ago
>Electric engines have a significantly better trust-to-weight ratio. Aerospace engineer here. This is not accurate, unless you are perhaps discussing cars. But for aircraft the torque supplied by the motor is not nearly so critical. First of all propeller thrust is a function of airspeed, and it falls off significantly as you accelerate, irrespective of whether your powertrain is electric or hydrocarbon. This is a direct consequence of physics that can't really be avoided and it's why we usually talk about propeller aircraft as being "power producers" rather than "thrust producers". Propeller horsepower is much easier to reason about, and more relevant to the performance of the aircraft, than torque. I'm not aware of any electric-powered turbojets/turbofans beyond the prototype phase but they would theoretically have better static thrust properties by virtue of not being props rather than any particular property of electric motors.
- nickik 6y ago
  I stand corrected. Just to be clear for my understand, would this not only be true during optimal cruise? Outside of optimal operation, specially during the start, this would actually be an advantage of electric motors? I was looking into the potential of electric turbofans. I'm just happy you didn't shoot everything down.
  - na85 6y ago
    >Outside of optimal operation, specially during the start, this would actually be an advantage of electric motors? When you say "this" do you mean "having more low-end torque than a gasoline motor"? In theory, you could tune your prop governor to take a bigger "bite" of the air at low airspeed and thus perhaps get better acceleration and a shorter takeoff roll. But aircraft performance is very rarely limited by the torque output of the engine itself. Usually the driveshafts and gearboxes are the limiting factors in power/torque transmission.
deepnotderp 6y ago
Engines on the wings actually reduce weight since they provide a counterweight to the wing’s positive g forces during flight

deleted 6y ago

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