5 ms·

> As for CO2, the plane flying slower is more fuel-efficient. Isn't this lower CO2 emissions? Possibly lower emissions per minute, but if they are in the air f

> As for CO2, the plane flying slower is more fuel-efficient. Isn't this lower CO2 emissions? Possibly lower emissions per minute, but if they are in the air for significantly longer, then that may be cancelled out by the extra time. Often, although fuel efficiency drops off sharply as you go above a certain speed in a vehicle, going slower than that speed is not significantly more efficient per minute, and can therefore be less efficient per distance. Driving a car for 100 miles at 30MPH is typically less efficient than doing so at 60MPH. Driving that distance at 90MPH is even less efficient.

cletus 8y ago
Think of it this way: if you need to drive from Kansas City to Boise the distance is constant (barring route changes). If you drive at 120mph vs 60mph you will get their faster but you will also use a lot more fuel in total even though when you're going slower you're on the road for twice as long. Fuel expenditure falls in to two buckets: 1. Moving the vehicle. Accelerating to cruising speed and fighting the deceleration of friction and air resistance. 2. Keeping the vehicle running. Lights, climate control, that sort of thing. My guess would be that the vast majority of fuel usage for planes is (1) not (2). As evidence of this, airlines care deeply about costs. It's not a high margin business (any more). My argument is that if it wasn't overall more fuel efficient to fly slower airlines just wouldn't do it. More evidence, the latest planes (A350, B787) have a lower cruising speed than, say, the 747. This also wouldn't be the case (IMHO) if it didn't decrease fuel costs.
- SketchySeaBeast 8y ago
  Isn't there's a 3. for airplanes - the constant force required to keep the plane in the sky? Cars don't have that, and there's going to be a particular speed at which that speed / time ratio changes for a plane compared to a car. Also, is it necessarily fuel costs behind the decision to steer away from 747, or could it be other costs, like plane utility and capacity, parts and service cost, and the requirements of cross training?
  - logfromblammo 8y ago
    Cars do have that. It shows up as "rolling resistance" in the tires when the vehicle moves, which varies depending on tread depth and inflation pressure. That drops to zero when lateral motion stops, though, and even the worst possible operational configurations won't cost you more than about 3% of your fuel. For boats, greater mass to float means deeper draft, and more resistance on the hull when it is moving. All that gravity-fighting energy is basically nothing for a ground-based vehicle when it is at rest, but manifests as a percentage loss when it moves. You can only escape it with a perfectly rigid, perfectly spherical object, on a perfectly flat surface. If an aircraft even has the capability to hover in place, it usually has to expend even more fuel to do so than flying an orbit or holding pattern. The ideal way for an aircraft to handle the counter-gravity force is to push an infinite amount of air downward at infinitesimal speed. A helicopter would want to have infinitely long rotor blades, rotating at an imperceptibly slow rate. Obviously, we can't do that, so very long blades spinning slowly enough to see them move is the best compromise we can manage. The output of a jet engine is a relatively small amount of air, moving very fast, but the jets push the craft though the atmosphere, and the wings translate that forward motion into pushing a larger amount of air downward just a little bit, for more efficient lift. Vectoring the engine thrust downward to hover in place means you lose that efficiency boost from the wings. Also, rotary-wing aircraft that are heavily-laden experience "coning", which reduces hover efficiency by making the force from each wing slightly off vertical, and some of the lifting energy goes into canceling out a lateral wobble. Keeping a plane in the air for another 15 minutes of fighting gravity just isn't significant compared to the effects from changing its speed away from the aircraft's efficiency optimum in order to arrive 15 minutes later, or from fighting air resistance by doing a holding pattern at optimal speed and altitude for enough extra distance to expend another 15 minutes. As long as the flaps are not extended, the plane is just not very concerned with gravity. Also, once you hit about 11 km/s, you don't need to expend any more energy on counteracting gravity, anyway. At that point, it's all about atmospheric drag.
    - dredmorbius 8y ago
      "You can only escape it with a perfectly rigid, perfectly spherical object, on a perfectly flat surface." An approximate description of a train.
- frobozz 8y ago
  3. Heat given off by the engine and propulsion mechanism when running at that speed. For many vehicles, the difference in that factor between running at ideal cruising speed (the 60MPH car in both of our examples) and a lower speed is minimal, and could well be offset or overtaken by the extra spend in (2). This is my 30MPH car example. Running at a higher than ideal speed is inefficient in this factor and increases fuel usage a lot. This is your 120MPH (my 90MPH) example. As you say, airlines care deeply about costs. It is highly unlikely that they were mostly running their planes inefficiently above design cruise speed, and are now slowing down to at or below the design cruise speed. It is more likely that they have always been at around the optimal speed, and are now going slower than that to reduce the risk of fines.

cletus 8y ago

Think of it this way: if you need to drive from Kansas City to Boise the distance is constant (barring route changes). If you drive at 120mph vs 60mph you will get their faster but you will also use a lot more fuel in total even though when you're going slower you're on the road for twice as long. Fuel expenditure falls in to two buckets: 1. Moving the vehicle. Accelerating to cruising speed and fighting the deceleration of friction and air resistance. 2. Keeping the vehicle running. Lights, climate control, that sort of thing. My guess would be that the vast majority of fuel usage for planes is (1) not (2). As evidence of this, airlines care deeply about costs. It's not a high margin business (any more). My argument is that if it wasn't overall more fuel efficient to fly slower airlines just wouldn't do it. More evidence, the latest planes (A350, B787) have a lower cruising speed than, say, the 747. This also wouldn't be the case (IMHO) if it didn't decrease fuel costs.

SketchySeaBeast 8y ago
Isn't there's a 3. for airplanes - the constant force required to keep the plane in the sky? Cars don't have that, and there's going to be a particular speed at which that speed / time ratio changes for a plane compared to a car. Also, is it necessarily fuel costs behind the decision to steer away from 747, or could it be other costs, like plane utility and capacity, parts and service cost, and the requirements of cross training?
- logfromblammo 8y ago
  Cars do have that. It shows up as "rolling resistance" in the tires when the vehicle moves, which varies depending on tread depth and inflation pressure. That drops to zero when lateral motion stops, though, and even the worst possible operational configurations won't cost you more than about 3% of your fuel. For boats, greater mass to float means deeper draft, and more resistance on the hull when it is moving. All that gravity-fighting energy is basically nothing for a ground-based vehicle when it is at rest, but manifests as a percentage loss when it moves. You can only escape it with a perfectly rigid, perfectly spherical object, on a perfectly flat surface. If an aircraft even has the capability to hover in place, it usually has to expend even more fuel to do so than flying an orbit or holding pattern. The ideal way for an aircraft to handle the counter-gravity force is to push an infinite amount of air downward at infinitesimal speed. A helicopter would want to have infinitely long rotor blades, rotating at an imperceptibly slow rate. Obviously, we can't do that, so very long blades spinning slowly enough to see them move is the best compromise we can manage. The output of a jet engine is a relatively small amount of air, moving very fast, but the jets push the craft though the atmosphere, and the wings translate that forward motion into pushing a larger amount of air downward just a little bit, for more efficient lift. Vectoring the engine thrust downward to hover in place means you lose that efficiency boost from the wings. Also, rotary-wing aircraft that are heavily-laden experience "coning", which reduces hover efficiency by making the force from each wing slightly off vertical, and some of the lifting energy goes into canceling out a lateral wobble. Keeping a plane in the air for another 15 minutes of fighting gravity just isn't significant compared to the effects from changing its speed away from the aircraft's efficiency optimum in order to arrive 15 minutes later, or from fighting air resistance by doing a holding pattern at optimal speed and altitude for enough extra distance to expend another 15 minutes. As long as the flaps are not extended, the plane is just not very concerned with gravity. Also, once you hit about 11 km/s, you don't need to expend any more energy on counteracting gravity, anyway. At that point, it's all about atmospheric drag.
  - dredmorbius 8y ago
    "You can only escape it with a perfectly rigid, perfectly spherical object, on a perfectly flat surface." An approximate description of a train.
frobozz 8y ago
3. Heat given off by the engine and propulsion mechanism when running at that speed. For many vehicles, the difference in that factor between running at ideal cruising speed (the 60MPH car in both of our examples) and a lower speed is minimal, and could well be offset or overtaken by the extra spend in (2). This is my 30MPH car example. Running at a higher than ideal speed is inefficient in this factor and increases fuel usage a lot. This is your 120MPH (my 90MPH) example. As you say, airlines care deeply about costs. It is highly unlikely that they were mostly running their planes inefficiently above design cruise speed, and are now slowing down to at or below the design cruise speed. It is more likely that they have always been at around the optimal speed, and are now going slower than that to reduce the risk of fines.