4 ms·

Seems to me that everything is nonlinear in the domain of aircraft, particularly when concerned with "efficiency". There is an equation [1] which makes it clear

Seems to me that everything is nonlinear in the domain of aircraft, particularly when concerned with "efficiency". There is an equation [1] which makes it clear that the power required to maintain flight is a function of air density, drag coefficient, lift coefficient, wing area, and -- velocity cubed. All things being equal (which they never are), double the speed and you need 8 times the power. So in the most simplistic analysis (which is wrong, but instructive), consider that energy is a function of both power and time applied; if you double the speed to fly from say London to NY, would you cut the time to fly to less than 1/8th what it would otherwise be? To be clear, that's wrong, because of other factors; when you go faster in a given airplane, you can be more efficient at higher altitudes - mindful that wing size is dictated more by landing speeds than just how fast you want to go - if anything, you want smaller wings for weight, cost, and other considerations, but landing speed gets in the way. So, if you took the time to solve that equation for an airplane at, say, 30k feet, and for say 50k feet altitude, would you still get efficiencies over say, one of the modern , slower jumbo jets? [1] https://web.mit.edu/16.unified/www/SPRING/systems/Lab_Notes/airpower.pdf https://web.mit.edu/16.unified/www/SPRING/systems/Lab_Notes/... (edited for line spacing)

Moto7451 6y ago
> Seems to me that everything is nonlinear in the domain of aircraft, particularly when concerned with "efficiency". The same physics applies to fast trains and other modes of transit. Fast electric trains have the advantage of not having to lug their fuel along with them at speed. > consider that energy is a function of both power and time applied; if you double the speed to fly from say London to NY, would you cut the time to fly to less than 1/8th what it would otherwise be? I think I understand what you’re trying to say, but note that by definition going twice as fast will get you there in half the time. Work and Energy are in alignment here because of the various efficiency losses you’re talking about. The Concorde did have some reprieve in that it flew 20k feet higher than the usual cruising altitude. https://aviation.stackexchange.com/questions/21881/at-what-altitude-did-concorde-go-supersonic https://aviation.stackexchange.com/questions/21881/at-what-a... Ultimately high speed transport is not about conservation of energy. It’s about conservation of human time. Weighing that against the environment and other factors are important but few people weigh their total use of energy in a given day.
- ClumsyPilot 6y ago
  Fast electric trains are also, well, electric already. Powering planes in a sustainable marrer is not a solved problem.
  - Robotbeat 6y ago
    So I think that’s not really true. Even existing battery chemistry could do 1000km per stop (although it still needs certification for aircraft) and at comparable speed as HSR. That’s much longer than the California HSR project that was canceled, plus you just need a recharge (or swap) to continue on your way. With further efficiency improvements and battery chemistries in development right now like Li-S, we could potentially do 4000km flights (enough to reach Hawaii... and anywhere else using multiple hops, as Boom is planning). And shorter flights at higher than HSR speeds. HSR has its own unsolved problems. Like not being able to work over deep ocean... and insane infrastructure costs.
    - deleted 6y ago
      [deleted]
  - Moto7451 6y ago
    Even if the train’s electric power comes from coal, not carrying fuel with you is a win from an efficiency standpoint. There are some military planes flying on biofuel, so ideally we’re not that far off. https://www.energy.gov/eere/bioenergy/aviation-fuels https://www.energy.gov/eere/bioenergy/aviation-fuels
Someone 6y ago
“if you double the speed to fly from say London to NY, would you cut the time to fly to less than 1/8th what it would otherwise be?” It’s bad, but not that bad. If you double the speed, you need 8 times the power, but only half as long for a given distance, so energy use is ‘only’ 4 times what you had. (Of course, were talking about a very rough approximation that likely is so inexact that 8 might even be a better estimate)

Moto7451 6y ago

> Seems to me that everything is nonlinear in the domain of aircraft, particularly when concerned with "efficiency". The same physics applies to fast trains and other modes of transit. Fast electric trains have the advantage of not having to lug their fuel along with them at speed. > consider that energy is a function of both power and time applied; if you double the speed to fly from say London to NY, would you cut the time to fly to less than 1/8th what it would otherwise be? I think I understand what you’re trying to say, but note that by definition going twice as fast will get you there in half the time. Work and Energy are in alignment here because of the various efficiency losses you’re talking about. The Concorde did have some reprieve in that it flew 20k feet higher than the usual cruising altitude. https://aviation.stackexchange.com/questions/21881/at-what-altitude-did-concorde-go-supersonic https://aviation.stackexchange.com/questions/21881/at-what-a... Ultimately high speed transport is not about conservation of energy. It’s about conservation of human time. Weighing that against the environment and other factors are important but few people weigh their total use of energy in a given day.

ClumsyPilot 6y ago
Fast electric trains are also, well, electric already. Powering planes in a sustainable marrer is not a solved problem.
- Robotbeat 6y ago
  So I think that’s not really true. Even existing battery chemistry could do 1000km per stop (although it still needs certification for aircraft) and at comparable speed as HSR. That’s much longer than the California HSR project that was canceled, plus you just need a recharge (or swap) to continue on your way. With further efficiency improvements and battery chemistries in development right now like Li-S, we could potentially do 4000km flights (enough to reach Hawaii... and anywhere else using multiple hops, as Boom is planning). And shorter flights at higher than HSR speeds. HSR has its own unsolved problems. Like not being able to work over deep ocean... and insane infrastructure costs.
  - deleted 6y ago
    [deleted]
- Moto7451 6y ago
  Even if the train’s electric power comes from coal, not carrying fuel with you is a win from an efficiency standpoint. There are some military planes flying on biofuel, so ideally we’re not that far off. https://www.energy.gov/eere/bioenergy/aviation-fuels https://www.energy.gov/eere/bioenergy/aviation-fuels

Someone 6y ago

“if you double the speed to fly from say London to NY, would you cut the time to fly to less than 1/8th what it would otherwise be?” It’s bad, but not that bad. If you double the speed, you need 8 times the power, but only half as long for a given distance, so energy use is ‘only’ 4 times what you had. (Of course, were talking about a very rough approximation that likely is so inexact that 8 might even be a better estimate)