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I’m a pilot and this is… a vast oversimplification. Jet aircraft are significantly more efficient at higher speeds and at higher altitudes precisely because the

I’m a pilot and this is… a vast oversimplification. Jet aircraft are significantly more efficient at higher speeds and at higher altitudes precisely because there’s less drag up there and the engines can operate more efficiently. They are wildly inefficient when low and slow. A lot of the assumptions people make about efficiency don’t hold water in aerospace. There are no laws of physics preventing supersonic aircraft from being efficient provided they can fly high enough and we figure out how to build better engines.

sillysaurusx 4y ago
I don’t think people will believe you even though you’re right. Pure math has a strange way of swaying one’s opinions; ML in particular suffers from this. But thank you for trying to point out that engineering efficiencies matter a great deal. Hopefully the message will get through.
- witheld 4y ago
  I thought you were saying that Marxism-Leninism suffers from naivety of pure math and I was really twisting my brain working that one out
  - sillysaurusx 4y ago
    Economic theory isn’t free from this phenomenon either. :) Though I wonder if it’s possible to model the probability of a communist revolution.
  - ithkuil 4y ago
    Ah finally I understand why everybody is so worked up about Anarchist Ideology lately
ramraj07 4y ago
Please unsimplify this then? Can you give a scenario where flying faster would be more economical fuel-wise?
- mdmglr 4y ago
  This should enlighten you: https://aviation.stackexchange.com/questions/1609/why-do-jet-engines-get-better-fuel-efficiency-at-high-altitudes https://aviation.stackexchange.com/questions/1609/why-do-jet...
  - selectodude 4y ago
    It’s not a linear relationship but unless you’re flying in space, you’re going to deal with the fuel vs speed trade off.
- TylerE 4y ago
  If you fly too slow the wing will start to stall, so you have to pull the nose up to raise the angle of attack…. Which vastly increases drag.
robopsychology 4y ago
Only a recreational pilot but I agree, when we calculate fuel usage (because people want you to pay your share this is a serious topic) there’s so many more ‘real’ variables
sandworm101 4y ago
While true at cruise speeds/altitudes, any supersonic transport still has to climb up to those altitudes. On any american flight (not to alaska/hawaii) i doubt the greater efficiencies at 60+ thousand feet would be worth the fuel to get there with a supersonic wing. These would only be 2 or 3-hour flights at most anyway. If we really want time savings, fix the entire airport experience. Thats where time can be most saved. When travelling domestic I ussually end up spending far more time waiting for flights/bags/security than actually flying.
- rkagerer 4y ago
  If you're old enough, there was a time before the airport experience was so broken when you could show up 15-20 mins before your flight.
  - londons_explore 4y ago
    An plane could be like a bus... Show up, hop on, tap your credit card to pay, and the plane takes off while you're still wandering down the aisle looking for a seat.
    - fallingmeat 4y ago
      I like where your head is at! make that the vision. /fire suit on
  - noduerme 4y ago
    TTP/Global Entry sort of fixes this. What really fixes it is flying private. Unfortunately, that's doesn't do much to help the carbon problem. The real difference now is that flying is too affordable. It used to be, only the rich could fly. What we need to do is make air travel much, much more expensive, out of the reach of most people except on special occasions, and we can go back to having (1) less unnecessary carbon emissions from people flying to Disneyland or whatever shitty resort they spend their vacations at, and (2) very short waits at the airport for people who need to get somewhere in a hurry and can afford it. Environmental and efficiency problem solved.
russdill 4y ago
Ok, then limit airspeed and not true airspeed. Done.
djaychela 4y ago
>Jet aircraft are significantly more efficient at higher speeds and at higher altitudes precisely because there’s less drag up there and the engines can operate more efficiently. This is true, but isn't the flip side of this that the high altitude emissions are much more damaging?
Timshel 4y ago
It's an oversimplification but as far I understand it's not wrong at least with past/current projects. Efficiency for a given aircraft is nice but it's not the point. I believe Concord was most efficient at cruising speed (~Mach 2) but it does not change the fact that it burned like 4x more per seat/km than a b737 (Cf: https://en.wikipedia.org/wiki/Fuel_economy_in_aircraft https://en.wikipedia.org/wiki/Fuel_economy_in_aircraft). Of course Concorde was only first class but that's part of the point of the parent. As for boom, some guesstimate are around at least 4x too (https://theicct.org/new-supersonic-transport-aircraft-fuel-burn-parity-or-environmental-parody https://theicct.org/new-supersonic-transport-aircraft-fuel-b...). No idea how biased this is but I believe it's quite telling that there is only mention of SAF on https://boomsupersonic.com/sustainability https://boomsupersonic.com/sustainability. At best in some 2017 article they compare themself to a lay-flat bed in subsonic business class, but since they only manage to secure an engine maker last year it was firmly in the guesstimate too.
- FuriouslyAdrift 4y ago
  New York to London round trip ticket price was $14,522 in 2023 dollars...
Gravityloss 4y ago
Yes, from basic principles, energy is force times distance. This force on an airplane is drag. Since on an airplane, both the drag and lift are proportional to the square of velocity, if (thought experiment) you assume a fixed lift to drag ratio, speed cancels out. It doesn't affect energy usage. Basically a faster airplane can have smaller wings which produce less lift and less drag. If we in spherical cow tradition ignore body drag, you can halve wing size, increase speed by 44% and have the same drag. You need more power but for a shorter duration, for the same energy usage per trip.
- dredmorbius 4y ago
  Most commercial aircraft take off and land well below their cruise speed. My understanding is that this limitation applies to supersonic aircraft as well. Applied power is typically restricted due to noise control requirements near airports. That wing lift/drag relationship may be less fungible than you're supposing in order to address lower-speed flight segments.
  - Gravityloss 4y ago
    Yes, it was just a thought experiment from first principles to get a feel for the problem. In reality, planes fly higher where the air is less dense, and faster to keep the lift and drag equal. In a car, lift is not needed. Higher speed doesn't have compensating effects. Higher speed more clearly causes more fuel burn for the distance.
    - dredmorbius 4y ago
      Fair enough. If anything, automobiles frequently utilise negative lift, as with a Formula 1 or Indy Car's inverted wing which generates increased downforces. On street cars you'll find spoilers and similar factors. Aircraft can fly low and fast, though that's typically associated with combat aircraft evading radar or air-defence systems. Such missions are known as fuel-burners precisely because of the greatly increased drag.
- albrewer 4y ago
deleted 4y ago
[deleted]

sillysaurusx 4y ago

I don’t think people will believe you even though you’re right. Pure math has a strange way of swaying one’s opinions; ML in particular suffers from this. But thank you for trying to point out that engineering efficiencies matter a great deal. Hopefully the message will get through.

witheld 4y ago
I thought you were saying that Marxism-Leninism suffers from naivety of pure math and I was really twisting my brain working that one out
- sillysaurusx 4y ago
  Economic theory isn’t free from this phenomenon either. :) Though I wonder if it’s possible to model the probability of a communist revolution.
- ithkuil 4y ago
  Ah finally I understand why everybody is so worked up about Anarchist Ideology lately

ramraj07 4y ago

Please unsimplify this then? Can you give a scenario where flying faster would be more economical fuel-wise?

mdmglr 4y ago
This should enlighten you: https://aviation.stackexchange.com/questions/1609/why-do-jet-engines-get-better-fuel-efficiency-at-high-altitudes https://aviation.stackexchange.com/questions/1609/why-do-jet...
- selectodude 4y ago
  It’s not a linear relationship but unless you’re flying in space, you’re going to deal with the fuel vs speed trade off.
TylerE 4y ago
If you fly too slow the wing will start to stall, so you have to pull the nose up to raise the angle of attack…. Which vastly increases drag.

robopsychology 4y ago

Only a recreational pilot but I agree, when we calculate fuel usage (because people want you to pay your share this is a serious topic) there’s so many more ‘real’ variables

sandworm101 4y ago

While true at cruise speeds/altitudes, any supersonic transport still has to climb up to those altitudes. On any american flight (not to alaska/hawaii) i doubt the greater efficiencies at 60+ thousand feet would be worth the fuel to get there with a supersonic wing. These would only be 2 or 3-hour flights at most anyway. If we really want time savings, fix the entire airport experience. Thats where time can be most saved. When travelling domestic I ussually end up spending far more time waiting for flights/bags/security than actually flying.

rkagerer 4y ago
If you're old enough, there was a time before the airport experience was so broken when you could show up 15-20 mins before your flight.
- londons_explore 4y ago
  An plane could be like a bus... Show up, hop on, tap your credit card to pay, and the plane takes off while you're still wandering down the aisle looking for a seat.
  - fallingmeat 4y ago
    I like where your head is at! make that the vision. /fire suit on
- noduerme 4y ago
  TTP/Global Entry sort of fixes this. What really fixes it is flying private. Unfortunately, that's doesn't do much to help the carbon problem. The real difference now is that flying is too affordable. It used to be, only the rich could fly. What we need to do is make air travel much, much more expensive, out of the reach of most people except on special occasions, and we can go back to having (1) less unnecessary carbon emissions from people flying to Disneyland or whatever shitty resort they spend their vacations at, and (2) very short waits at the airport for people who need to get somewhere in a hurry and can afford it. Environmental and efficiency problem solved.

russdill 4y ago

Ok, then limit airspeed and not true airspeed. Done.

djaychela 4y ago

>Jet aircraft are significantly more efficient at higher speeds and at higher altitudes precisely because there’s less drag up there and the engines can operate more efficiently. This is true, but isn't the flip side of this that the high altitude emissions are much more damaging?

Timshel 4y ago

It's an oversimplification but as far I understand it's not wrong at least with past/current projects. Efficiency for a given aircraft is nice but it's not the point. I believe Concord was most efficient at cruising speed (~Mach 2) but it does not change the fact that it burned like 4x more per seat/km than a b737 (Cf: https://en.wikipedia.org/wiki/Fuel_economy_in_aircraft https://en.wikipedia.org/wiki/Fuel_economy_in_aircraft). Of course Concorde was only first class but that's part of the point of the parent. As for boom, some guesstimate are around at least 4x too (https://theicct.org/new-supersonic-transport-aircraft-fuel-burn-parity-or-environmental-parody https://theicct.org/new-supersonic-transport-aircraft-fuel-b...). No idea how biased this is but I believe it's quite telling that there is only mention of SAF on https://boomsupersonic.com/sustainability https://boomsupersonic.com/sustainability. At best in some 2017 article they compare themself to a lay-flat bed in subsonic business class, but since they only manage to secure an engine maker last year it was firmly in the guesstimate too.

FuriouslyAdrift 4y ago
New York to London round trip ticket price was $14,522 in 2023 dollars...

Gravityloss 4y ago

Yes, from basic principles, energy is force times distance. This force on an airplane is drag. Since on an airplane, both the drag and lift are proportional to the square of velocity, if (thought experiment) you assume a fixed lift to drag ratio, speed cancels out. It doesn't affect energy usage. Basically a faster airplane can have smaller wings which produce less lift and less drag. If we in spherical cow tradition ignore body drag, you can halve wing size, increase speed by 44% and have the same drag. You need more power but for a shorter duration, for the same energy usage per trip.

dredmorbius 4y ago
Most commercial aircraft take off and land well below their cruise speed. My understanding is that this limitation applies to supersonic aircraft as well. Applied power is typically restricted due to noise control requirements near airports. That wing lift/drag relationship may be less fungible than you're supposing in order to address lower-speed flight segments.
- Gravityloss 4y ago
  Yes, it was just a thought experiment from first principles to get a feel for the problem. In reality, planes fly higher where the air is less dense, and faster to keep the lift and drag equal. In a car, lift is not needed. Higher speed doesn't have compensating effects. Higher speed more clearly causes more fuel burn for the distance.
  - dredmorbius 4y ago
    Fair enough. If anything, automobiles frequently utilise negative lift, as with a Formula 1 or Indy Car's inverted wing which generates increased downforces. On street cars you'll find spoilers and similar factors. Aircraft can fly low and fast, though that's typically associated with combat aircraft evading radar or air-defence systems. Such missions are known as fuel-burners precisely because of the greatly increased drag.
albrewer 4y ago

deleted 4y ago

[deleted]