5 ms·

A plane is not a satellite. You need to constantly expend energy to keep it in the air. The more weight is on the plane, the more lift you need. To get a bigger

A plane is not a satellite. You need to constantly expend energy to keep it in the air. The more weight is on the plane, the more lift you need. To get a bigger lift, you need to angle the control surfaces steeper, which increases drag. Therefore transporting heavy things by air (like fuel) is extremely inefficient.

tehjoker 5y ago
Does this apply to cars? I always wonder if there's a mileage bonus for riding with a half empty tank.
- ac29 5y ago
  Sure, it takes less fuel to move a car that moves less, but fuel is an nearly irrelevant fraction of total weight in most cars. A gallon of petrol weighs about 6 lbs.
  - fragmede 5y ago
    But what's the math on mpg difference for moving an extra 60 lbs? It's probably pretty negligible but without actual numbers, can't actually say.
  - tehjoker 5y ago
    Thanks, I wasn't sure how heavy the gas was. So it's like carrying an extra child in the car.
- gruez 5y ago
  A heavy car takes more energy to get up to speed. The extra weight also "stores" more energy once you get up to speed, but that's useless when you don't have regenerative braking. Also I'd imagine that rolling resistance would be higher when the car is heavier.
- deleted 5y ago
  [deleted]
- roelschroeven 5y ago
  As gruez says, a heavier car needs more fuel to accelerate and has more rolling resistance. It also needs more fuel to drive up a slope. Some of these can be regained in a car with regenerative braking, but even than only partially because there are always losses. But the difference is not nearly as large as it is for airplanes. Having your fuel tank only half full means more trips to the gas station, which might even negate all gains you try to make. It seems a good idea though not to drive around with things live heavy tools all the time when you don't need them.
- olliej 5y ago
  For general cars it’s negligible, but for formula 1 it matters: I think it’s in the order of 1/10th a second per lap or something - the car get multiple seconds per lap faster over the course of a race (despite the lower grip as tires wear)
sigmoid10 5y ago
That's a common misconception. In level flight, when you align the lift and gravity vectors correctly, they do not expend energy as work. Only 100% symmetric wings (as seen e.g. on acrobatic planes) require a constant positive angle of attack to generate lift. By using "normal" flat-bottomed wings and minimizing air resistance from forward motion, you can effectively glide huge distances using only the energy you get from slightly lowering your altitude in the gravitational field (downward motion component parallel to the gravitational vector equals work done on the plane to keep its forward velocity constant and balance losses from drag). Mass gets cancelled out there, because forward drag doesn't act on it and you also get more energy back from the gravitational field while descending. With an infinitely thin frame (=zero air resistance) or by using thermal updrafts, you could even stay in the air indefinitely. That's how glider planes work without any engine and why commercial jets at cruising altitude (very low drag up there) have some of the best fuel economy per person of all forms of travel. That's also how airlines can offer tickets at prices from which you wouldn't even be able to pay for gas when you drive the trip by car. Ryanair for example can fly you from Rome to London and back for ~40€. You'd be hard pressed to find a car that uses less than 40€ worth of fuel for just one leg of this trip, even if it could drive in a straight line. Flying itself is really cheap - it's everything else around aviation that's expensive. Edit: I should probably add that this is different for cars. Cars don't just have aerodynamic drag, they also have rolling drag, mostly from small displacements of the tires where they contact the ground. If you ever tried to ride a bike with low tire pressure you'll know what I'm talking about. A heavy car that has the same aerodynamic cross section as a lighter car (think two cars of the same make/model, but one fully loaded and one empty) will have more rolling drag and thus use more energy/fuel. Two planes of the same model but with different mass will roughly consume the same fuel during level flight (there are slight higher order effects that I've glanced over here, but they are neglectable in the big picture).
- roelschroeven 5y ago
  Your explanation is misguided and half-truth at best. Every airplane needs wings that generate lift to support its weight, and heavier airplanes need more lift. That lift does not come for free, except (if I'm not mistaken) in the theoretical case of infinitely long wings. To generate lift, a wing necessarily bends the stream of air downwards; that's it reaction mass. Different references are used for angle of attack: some say zero angle of attack corresponds to zero coefficient of lift (called the absolute angle of attack), others try to align the reference with the chord line of the wing (geometric angle of attack). Obviously for symmetric wings the two are the same. In the case of geometric angle of attack, you could say a wing generates lift even with zero angle of attack. That doesn't mean that that lift comes for free! To generate more lift, the wing still needs a higher angle of attack (or more airspeed). Higher angle of attack means more drag. There's just no way around it. Gliders can stay in the air for very long times because they have very large lift-to-drag ratios, and hence very shallow glideslopes. They descend slowly, and the pilot tries to find rising air (thermals, ridge lift, lee waves) that rises faster than the plane descends. But make no mistake, even it those cases the plane still descends compared to its surrounding air. So no, two identical planes but with different mass will most certainly not consume the same amount of fuel during level flight. The heavier plane will consume much more than the lighter one (depending of course on the difference in weight between the two), and the difference will be much larger than the difference between to identical cars carrying different loads. See "See How It Flies" (https://www.av8n.com/how/ https://www.av8n.com/how/) for a relatively gentle introduction to the physics of flight.

tehjoker 5y ago

Does this apply to cars? I always wonder if there's a mileage bonus for riding with a half empty tank.

ac29 5y ago
Sure, it takes less fuel to move a car that moves less, but fuel is an nearly irrelevant fraction of total weight in most cars. A gallon of petrol weighs about 6 lbs.
- fragmede 5y ago
  But what's the math on mpg difference for moving an extra 60 lbs? It's probably pretty negligible but without actual numbers, can't actually say.
- tehjoker 5y ago
  Thanks, I wasn't sure how heavy the gas was. So it's like carrying an extra child in the car.
gruez 5y ago
A heavy car takes more energy to get up to speed. The extra weight also "stores" more energy once you get up to speed, but that's useless when you don't have regenerative braking. Also I'd imagine that rolling resistance would be higher when the car is heavier.
deleted 5y ago
[deleted]
roelschroeven 5y ago
As gruez says, a heavier car needs more fuel to accelerate and has more rolling resistance. It also needs more fuel to drive up a slope. Some of these can be regained in a car with regenerative braking, but even than only partially because there are always losses. But the difference is not nearly as large as it is for airplanes. Having your fuel tank only half full means more trips to the gas station, which might even negate all gains you try to make. It seems a good idea though not to drive around with things live heavy tools all the time when you don't need them.
olliej 5y ago
For general cars it’s negligible, but for formula 1 it matters: I think it’s in the order of 1/10th a second per lap or something - the car get multiple seconds per lap faster over the course of a race (despite the lower grip as tires wear)

sigmoid10 5y ago

That's a common misconception. In level flight, when you align the lift and gravity vectors correctly, they do not expend energy as work. Only 100% symmetric wings (as seen e.g. on acrobatic planes) require a constant positive angle of attack to generate lift. By using "normal" flat-bottomed wings and minimizing air resistance from forward motion, you can effectively glide huge distances using only the energy you get from slightly lowering your altitude in the gravitational field (downward motion component parallel to the gravitational vector equals work done on the plane to keep its forward velocity constant and balance losses from drag). Mass gets cancelled out there, because forward drag doesn't act on it and you also get more energy back from the gravitational field while descending. With an infinitely thin frame (=zero air resistance) or by using thermal updrafts, you could even stay in the air indefinitely. That's how glider planes work without any engine and why commercial jets at cruising altitude (very low drag up there) have some of the best fuel economy per person of all forms of travel. That's also how airlines can offer tickets at prices from which you wouldn't even be able to pay for gas when you drive the trip by car. Ryanair for example can fly you from Rome to London and back for ~40€. You'd be hard pressed to find a car that uses less than 40€ worth of fuel for just one leg of this trip, even if it could drive in a straight line. Flying itself is really cheap - it's everything else around aviation that's expensive. Edit: I should probably add that this is different for cars. Cars don't just have aerodynamic drag, they also have rolling drag, mostly from small displacements of the tires where they contact the ground. If you ever tried to ride a bike with low tire pressure you'll know what I'm talking about. A heavy car that has the same aerodynamic cross section as a lighter car (think two cars of the same make/model, but one fully loaded and one empty) will have more rolling drag and thus use more energy/fuel. Two planes of the same model but with different mass will roughly consume the same fuel during level flight (there are slight higher order effects that I've glanced over here, but they are neglectable in the big picture).

roelschroeven 5y ago
Your explanation is misguided and half-truth at best. Every airplane needs wings that generate lift to support its weight, and heavier airplanes need more lift. That lift does not come for free, except (if I'm not mistaken) in the theoretical case of infinitely long wings. To generate lift, a wing necessarily bends the stream of air downwards; that's it reaction mass. Different references are used for angle of attack: some say zero angle of attack corresponds to zero coefficient of lift (called the absolute angle of attack), others try to align the reference with the chord line of the wing (geometric angle of attack). Obviously for symmetric wings the two are the same. In the case of geometric angle of attack, you could say a wing generates lift even with zero angle of attack. That doesn't mean that that lift comes for free! To generate more lift, the wing still needs a higher angle of attack (or more airspeed). Higher angle of attack means more drag. There's just no way around it. Gliders can stay in the air for very long times because they have very large lift-to-drag ratios, and hence very shallow glideslopes. They descend slowly, and the pilot tries to find rising air (thermals, ridge lift, lee waves) that rises faster than the plane descends. But make no mistake, even it those cases the plane still descends compared to its surrounding air. So no, two identical planes but with different mass will most certainly not consume the same amount of fuel during level flight. The heavier plane will consume much more than the lighter one (depending of course on the difference in weight between the two), and the difference will be much larger than the difference between to identical cars carrying different loads. See "See How It Flies" (https://www.av8n.com/how/ https://www.av8n.com/how/) for a relatively gentle introduction to the physics of flight.