4 ms·

Electric wheel propulsion on aircraft seems like an entirely unnecessary feature. Airplanes have no difficulty using their primary form of propulsion - moving a

by bdamm 7y ago

Electric wheel propulsion on aircraft seems like an entirely unnecessary feature. Airplanes have no difficulty using their primary form of propulsion - moving air - to move on the ground, and it takes very little energy to do it compared to the energy required to move through the air while off the ground.

smileysteve 7y ago
> Airplanes have no difficulty using their primary form of propulsion There are a few things to be weary of; Mostly that thrust is loud and dangerous to air crews. Weight shifts. Individually driven wheels offer some precision without significant forward thrust.
Yizahi 7y ago
I read years ago when it was first announced that price saving should be significant, also less machine hours for engines (longer maintenance intervals), less safety checks on the ground etc.
gnode 7y ago
Propelling air to move on the ground is far less efficient than wheel propulsion. You have to give the air at least as much momentum as the vehicle, and without a very large propeller, this requires accelerating it to a high velocity, giving it much more kinetic energy than the vehicle itself. The energy use while taxiing can be substantial, and the range of electric planes is critical. Wheel propulsion on the other hand can impart momentum at very high efficiency, as well as regeneratively brake.
ac29 7y ago
No, that's exactly backwards -- jet engines are far less efficient at low speed than at cruise speed. Various numbers I found say 100-300 kg of fuel used taxiing for single aisle planes like 737 [0][1] all the way up to two tons of fuel for large aircraft like 747 at large airports like JFK [0]. This can be extremely relevant for shorter flights: "[taxiing] aircraft fuel burn which is estimated to be as high as 27% of total fuel burn for a 90-minute flight where waiting in queue adds to the time on the ground" [2] [0] https://www.airliners.net/forum/viewtopic.php?t=747735 https://www.airliners.net/forum/viewtopic.php?t=747735 [1] https://www.aviationtoday.com/2019/05/01/electric-taxiing-systems-past-present-possible-future/ https://www.aviationtoday.com/2019/05/01/electric-taxiing-sy... [2] https://en.wikipedia.org/wiki/Taxiing https://en.wikipedia.org/wiki/Taxiing
- sokoloff 7y ago
  The taxi fuel is not primarily used for motivation of the airplane around the airfield, but just from idling the engines. (It's about minutes of ground operation more than miles of ground operation.) Electric drive wheels don't eliminate that unless you don't start the engines until near the hold short line (in which case, you don't have air conditioning or electric power, meaning you end up running the APU [itself a small jet engine] to provide that power).
  - ac29 7y ago
    Yes, you'd need at least enough battery power to power not just the drive wheels, but the other things that engine power provides for like air conditioning. I wonder how large a pack would actually be required (of course there are huge differences between airports - some of them youre in the air only a few minutes after leaving the gate, some it can be 20-30 minutes). If it was a large amount of battery, it would be cool if the batteries were in a small autonomous vehicle that disconnected from the plane when it was time to spin up the main engines and returned on its own to be recharged. Seems like a solvable problem.
    - sokoloff 7y ago
      Anecdote, but relevant, we were on the ground in Las Vegas in a 787-400 (an all-electric, no-bleed airplane) and due to a 15 minute ground hold while they sorted an issue with a brake indication, the APU powered pack alone was not able to keep up with the Vegas environmentals. (They couldn't run a main engine because they were doing maintenance in that area.) They were able to bring in ground-based chillers to take some of the edge off, but it was uncomfortably hot in the cabin with an APU running. A 787 may be ideal for an electric conversion (since it's already a no-bleed airplane). Even then, you probably might as well run the APU for electrics on the ground, since you're going to be carrying around that amount of weight anyway.
  - Gibbon1 7y ago
    That's an another win for electric and hybrid electric designs. For the latter if you can safely take off on battery power you don't need to idle the engine before take off. Saves fuel, reduces noise, and localized pollution. You can also turn the turbine off before landing.

by bdamm 7y ago

smileysteve 7y ago

> Airplanes have no difficulty using their primary form of propulsion There are a few things to be weary of; Mostly that thrust is loud and dangerous to air crews. Weight shifts. Individually driven wheels offer some precision without significant forward thrust.

Yizahi 7y ago

I read years ago when it was first announced that price saving should be significant, also less machine hours for engines (longer maintenance intervals), less safety checks on the ground etc.

gnode 7y ago

Propelling air to move on the ground is far less efficient than wheel propulsion. You have to give the air at least as much momentum as the vehicle, and without a very large propeller, this requires accelerating it to a high velocity, giving it much more kinetic energy than the vehicle itself. The energy use while taxiing can be substantial, and the range of electric planes is critical. Wheel propulsion on the other hand can impart momentum at very high efficiency, as well as regeneratively brake.

ac29 7y ago

No, that's exactly backwards -- jet engines are far less efficient at low speed than at cruise speed. Various numbers I found say 100-300 kg of fuel used taxiing for single aisle planes like 737 [0][1] all the way up to two tons of fuel for large aircraft like 747 at large airports like JFK [0]. This can be extremely relevant for shorter flights: "[taxiing] aircraft fuel burn which is estimated to be as high as 27% of total fuel burn for a 90-minute flight where waiting in queue adds to the time on the ground" [2] [0] https://www.airliners.net/forum/viewtopic.php?t=747735 https://www.airliners.net/forum/viewtopic.php?t=747735 [1] https://www.aviationtoday.com/2019/05/01/electric-taxiing-systems-past-present-possible-future/ https://www.aviationtoday.com/2019/05/01/electric-taxiing-sy... [2] https://en.wikipedia.org/wiki/Taxiing https://en.wikipedia.org/wiki/Taxiing

sokoloff 7y ago
The taxi fuel is not primarily used for motivation of the airplane around the airfield, but just from idling the engines. (It's about minutes of ground operation more than miles of ground operation.) Electric drive wheels don't eliminate that unless you don't start the engines until near the hold short line (in which case, you don't have air conditioning or electric power, meaning you end up running the APU [itself a small jet engine] to provide that power).
- ac29 7y ago
  Yes, you'd need at least enough battery power to power not just the drive wheels, but the other things that engine power provides for like air conditioning. I wonder how large a pack would actually be required (of course there are huge differences between airports - some of them youre in the air only a few minutes after leaving the gate, some it can be 20-30 minutes). If it was a large amount of battery, it would be cool if the batteries were in a small autonomous vehicle that disconnected from the plane when it was time to spin up the main engines and returned on its own to be recharged. Seems like a solvable problem.
  - sokoloff 7y ago
    Anecdote, but relevant, we were on the ground in Las Vegas in a 787-400 (an all-electric, no-bleed airplane) and due to a 15 minute ground hold while they sorted an issue with a brake indication, the APU powered pack alone was not able to keep up with the Vegas environmentals. (They couldn't run a main engine because they were doing maintenance in that area.) They were able to bring in ground-based chillers to take some of the edge off, but it was uncomfortably hot in the cabin with an APU running. A 787 may be ideal for an electric conversion (since it's already a no-bleed airplane). Even then, you probably might as well run the APU for electrics on the ground, since you're going to be carrying around that amount of weight anyway.
- Gibbon1 7y ago
  That's an another win for electric and hybrid electric designs. For the latter if you can safely take off on battery power you don't need to idle the engine before take off. Saves fuel, reduces noise, and localized pollution. You can also turn the turbine off before landing.