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Airliners travel in air that is >60% less dense than the air at sea level, and usually travel over or around energy-wasting terrain features and weather.

by nickff 6y ago

titzer 6y ago
That's not a citation, it's a random factoid, which doesn't make for a fruitful discussion. Other random factoids include: * trains are long * trains travel slower than airplanes, and drag is proportional to the square of speed * making conclusions solely based drag coefficients from the density of the air is sound reasoning One of these statements is false. I get that hackernews is just a forum for casual discussion of various technical topics, but overly categorical statements based on a reasoning in a vacuum (no pun in intended) just force the conversation into unproductive threads like this one. I'm kinda sorry I took the bait at this point.
- deepnotderp 6y ago
  Trains are long, but skin friction drag is real. Traveling slower is a bug, not a feature. If you want to go slower, airplanes can be more efficient as well!
- kerkeslager 6y ago
  > I get that hackernews is just a forum for casual discussion of various technical topics I hear this sentiment a lot, and I don't understand it. Trying to speak the truth isn't just optional outside formal contexts. There's no situation so casual that I it suddenly becomes appropriate to make up bullshit and present it as facts. That is to say, I agree with your larger point: if nickff wants to make the claim that trains consume more energy than aircraft, he'd better come up with a reasonable source for that information.
newacct583 6y ago
Yes, but they travel 5-10x faster than ground vehicles. Drag losses go linearly with air density and quadraticly with speed. Likewise the distance traveled is going to be higher on a rail network but probably in the "2x as far" realm. You're not going to make those two orders of magnitude up with this. I think if you want to make this analysis come out you need to include starting/stopping losses (and maybe fuel hauling, empty car overhead, etc...) to the train trip or somesuch. Naively trains are going to win and it's not even close.

Airliners travel in air that is >60% less dense than the air at sea level, and usually travel over or around energy-wasting terrain features and weather.

by nickff 6y ago

Airliners travel in air that is >60% less dense than the air at sea level, and usually travel over or around energy-wasting terrain features and weather.

titzer 6y ago

That's not a citation, it's a random factoid, which doesn't make for a fruitful discussion. Other random factoids include: * trains are long * trains travel slower than airplanes, and drag is proportional to the square of speed * making conclusions solely based drag coefficients from the density of the air is sound reasoning One of these statements is false. I get that hackernews is just a forum for casual discussion of various technical topics, but overly categorical statements based on a reasoning in a vacuum (no pun in intended) just force the conversation into unproductive threads like this one. I'm kinda sorry I took the bait at this point.

deepnotderp 6y ago
Trains are long, but skin friction drag is real. Traveling slower is a bug, not a feature. If you want to go slower, airplanes can be more efficient as well!
kerkeslager 6y ago
> I get that hackernews is just a forum for casual discussion of various technical topics I hear this sentiment a lot, and I don't understand it. Trying to speak the truth isn't just optional outside formal contexts. There's no situation so casual that I it suddenly becomes appropriate to make up bullshit and present it as facts. That is to say, I agree with your larger point: if nickff wants to make the claim that trains consume more energy than aircraft, he'd better come up with a reasonable source for that information.

newacct583 6y ago

Yes, but they travel 5-10x faster than ground vehicles. Drag losses go linearly with air density and quadraticly with speed. Likewise the distance traveled is going to be higher on a rail network but probably in the "2x as far" realm. You're not going to make those two orders of magnitude up with this. I think if you want to make this analysis come out you need to include starting/stopping losses (and maybe fuel hauling, empty car overhead, etc...) to the train trip or somesuch. Naively trains are going to win and it's not even close.