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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 wi

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.
- sigmoid10 5y ago
  >a wing necessarily bends the stream of air downwards This is only true for control surfaces and perfectly symmetric wings, but not for zero angle of attack wings as I explained above. Normal, cambered wings work because of the Bernoulli effect, where pressure is lower above the wing because of the way airflow is designed. Lift is not generated by pushing air down, the wing is literally getting sucked upwards. There are some mass dependent losses due to turbulence on the outer wing edges (which is, incidentally, why you see winglets on all jets nowadays), but those are higher order corrections as I also mentioned above. For an actually gentle introduction regarding how wings work, see e.g. this student level explanation here: https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicsofflight.html https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicso... -> Note that for the shown wingtype, no part of the airflow gets pushed down during level flight. If you still don't believe me and instead want a realistic, highly technical argument, I suggest you take a look at literally any plane's operating handbook - especially the performance section. Here's one for the Cessna172 as an example: https://www.cpaviation.com/images/downloads/Cessna%20172M.pdf https://www.cpaviation.com/images/downloads/Cessna%20172M.pd... -> Note that under Cruise Performance, fuel consumption in terms of gallons per hour is calculated using pressure altitude and outside air temperature. You do not need to take aircraft mass into consideration for this part of trip planning. It is only important during takeoff and climb. >Gliders can stay in the air for very long times because they have very large lift-to-drag ratios You should look up how gliders sometimes carry extra weight in special water tanks to improve speed without sacrificing performance. Their optimal lift-to-drag stays the same, but the speed at which they achieve it becomes higher. How do you explain that? And of course they descend with regards to the surrounding air, I never said anything like that. But in terms of basic physics, updrafts are just a way of getting potential energy back that was lost due to drag lowering the kinetic energy. Also: You are confusing angle of attack and angle of incidence. These things are not a matter of reference, the latter being inherent to any airplane's design. But they are irrelevant to this discussion.
  - roelschroeven 5y ago
    > This is only true for control surfaces and perfectly symmetric wings, but not for zero angle of attack wings as I explained above. Normal, cambered wings work because of the Bernoulli effect, where pressure is lower above the wing because of the way airflow is designed. Lift is not generated by pushing air down, the wing is literally getting sucked upwards Bernouilli is one of the ways to describe the pressure differences around an aircraft wing in an airstream. But the popular Bernoulli-explanation where wings don't deflect the airstream is utterly wrong. Think about it, and consider for a moment the airplane as a black box, or a spherical cow for all I care. How does it stay in the air? Because the air exerts an upward force on it. Therefore, according to Newton's third law, the airplane exerts a downward force on the air. That force deflects the airstream down; there simply is no other way. > Lift is not generated by pushing air down, the wing is literally getting sucked upwards. By which force, exerted by which object? The air! If the air pushes/pulls the aircraft upwards, the aircraft pushes/pulls the air down. > Note that under Cruise Performance, fuel consumption in terms of gallons per hour is calculated using pressure altitude and outside air temperature. You do not need to take aircraft mass into consideration for this part of trip planning. It is only important during takeoff and climb. The tables and charts used for the calculations clearly indicate the conditions for which they're valid, one of which is a weight of 2300 pounds. I don't know how you're supposed to make calculations for other takeoff weights, but the fact that the weight is clearly stated as a condition shows that it's important. > You should look up how gliders sometimes carry extra weight in special water tanks to improve speed without sacrificing performance. Their optimal lift-to-drag stays the same, but the speed at which they achieve it becomes higher. How do you explain that? As you say, it doesn't change their lift-to-drag ratio. When they go faster they also meet more rising air in the same time. What matters is that they can only stay aloft when they find sufficient rising currents, which is completely impractical for most kinds of airplane. Airliners for example can glide to an extent and they sometimes do, but they can not "effectively glide huge distances using only the energy you get from slightly lowering your altitude in the gravitational field" as you originally stated. > Also: You are confusing angle of attack and angle of incidence Angle of incidence is the angle at which the wing is mounted to the airplane. It is not in itself relevant to the lift the wing produces, which is why I didn't mention it. I'm not confusing anything. For more practical considerations, see e.g. this video: "How much does it cost to FUEL an airliner?!" by Mentour Pilot (https://www.youtube.com/watch?v=dU8Ah1VC2Wg https://www.youtube.com/watch?v=dU8Ah1VC2Wg). Some excerpts: At 1:47: > The cost of fuel is one of the single biggest costs any airline out there has. My airline that I work for spent about 11 billion dollars in fuel last year. And this is why keeping a check on the costs, as in both the price of the fuel and also how much fuel is being used is extremely important. It is the difference between being profitable and non-profitable in the airline world. At 4:26 > The total cost of fuel for the airline is about 40% of the cost of the whole flight. At 10:01: > We only carry as much fuel as we need for the flight that we need to do with [(couldn't understand this word)] reserves and diversion fuel. The reason we don't carry more fuel than that is because it costs fuel to carry fuel. The weight of the fuel is going to make the aircraft burn more fuel. So the cost of fuel is very important to the airline industry, and carrying fuel costs fuel and hence money, because of the weight of that fuel. Yes, flying is still relatively cheap because the fuel cost is shared by all passengers, and because airlines for reasons that I don't quite understand pay no or almost no taxes on their fuel; certainly much less than what you pay to fill up your car.
jonsen 5y ago
> Mass gets cancelled out there, ... Two equal airplanes but with different weight can glide the same distance, same glide angle, but the heavier plane will have higher airspeed to give the extra lift needed for the extra weight.
- roeles 5y ago
  This is mostly true, but due to a small Reynolds effect the heavier plane should actually perform a little bit better. What you describe is why gliders sometimes carry water as ballast in their wings. It enables the same lift/drag ratio at a higher speed, at the cost of a higher stall speed.
mlyle 5y ago
Induced drag is a thing. https://en.wikipedia.org/wiki/Lift-induced_drag https://en.wikipedia.org/wiki/Lift-induced_drag
- sigmoid10 5y ago
  This is one of the higher order corrections I was referring to. Note how the article describes that this is not a dominant factor under normal conditions and actually gets lower the faster the plane flies. So if you can keep parasitic drag down by designing your aircraft nicely, you can get pretty far to the right in the plot at the end of the page.

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.

sigmoid10 5y ago
>a wing necessarily bends the stream of air downwards This is only true for control surfaces and perfectly symmetric wings, but not for zero angle of attack wings as I explained above. Normal, cambered wings work because of the Bernoulli effect, where pressure is lower above the wing because of the way airflow is designed. Lift is not generated by pushing air down, the wing is literally getting sucked upwards. There are some mass dependent losses due to turbulence on the outer wing edges (which is, incidentally, why you see winglets on all jets nowadays), but those are higher order corrections as I also mentioned above. For an actually gentle introduction regarding how wings work, see e.g. this student level explanation here: https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicsofflight.html https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicso... -> Note that for the shown wingtype, no part of the airflow gets pushed down during level flight. If you still don't believe me and instead want a realistic, highly technical argument, I suggest you take a look at literally any plane's operating handbook - especially the performance section. Here's one for the Cessna172 as an example: https://www.cpaviation.com/images/downloads/Cessna%20172M.pdf https://www.cpaviation.com/images/downloads/Cessna%20172M.pd... -> Note that under Cruise Performance, fuel consumption in terms of gallons per hour is calculated using pressure altitude and outside air temperature. You do not need to take aircraft mass into consideration for this part of trip planning. It is only important during takeoff and climb. >Gliders can stay in the air for very long times because they have very large lift-to-drag ratios You should look up how gliders sometimes carry extra weight in special water tanks to improve speed without sacrificing performance. Their optimal lift-to-drag stays the same, but the speed at which they achieve it becomes higher. How do you explain that? And of course they descend with regards to the surrounding air, I never said anything like that. But in terms of basic physics, updrafts are just a way of getting potential energy back that was lost due to drag lowering the kinetic energy. Also: You are confusing angle of attack and angle of incidence. These things are not a matter of reference, the latter being inherent to any airplane's design. But they are irrelevant to this discussion.
- roelschroeven 5y ago
  > This is only true for control surfaces and perfectly symmetric wings, but not for zero angle of attack wings as I explained above. Normal, cambered wings work because of the Bernoulli effect, where pressure is lower above the wing because of the way airflow is designed. Lift is not generated by pushing air down, the wing is literally getting sucked upwards Bernouilli is one of the ways to describe the pressure differences around an aircraft wing in an airstream. But the popular Bernoulli-explanation where wings don't deflect the airstream is utterly wrong. Think about it, and consider for a moment the airplane as a black box, or a spherical cow for all I care. How does it stay in the air? Because the air exerts an upward force on it. Therefore, according to Newton's third law, the airplane exerts a downward force on the air. That force deflects the airstream down; there simply is no other way. > Lift is not generated by pushing air down, the wing is literally getting sucked upwards. By which force, exerted by which object? The air! If the air pushes/pulls the aircraft upwards, the aircraft pushes/pulls the air down. > Note that under Cruise Performance, fuel consumption in terms of gallons per hour is calculated using pressure altitude and outside air temperature. You do not need to take aircraft mass into consideration for this part of trip planning. It is only important during takeoff and climb. The tables and charts used for the calculations clearly indicate the conditions for which they're valid, one of which is a weight of 2300 pounds. I don't know how you're supposed to make calculations for other takeoff weights, but the fact that the weight is clearly stated as a condition shows that it's important. > You should look up how gliders sometimes carry extra weight in special water tanks to improve speed without sacrificing performance. Their optimal lift-to-drag stays the same, but the speed at which they achieve it becomes higher. How do you explain that? As you say, it doesn't change their lift-to-drag ratio. When they go faster they also meet more rising air in the same time. What matters is that they can only stay aloft when they find sufficient rising currents, which is completely impractical for most kinds of airplane. Airliners for example can glide to an extent and they sometimes do, but they can not "effectively glide huge distances using only the energy you get from slightly lowering your altitude in the gravitational field" as you originally stated. > Also: You are confusing angle of attack and angle of incidence Angle of incidence is the angle at which the wing is mounted to the airplane. It is not in itself relevant to the lift the wing produces, which is why I didn't mention it. I'm not confusing anything. For more practical considerations, see e.g. this video: "How much does it cost to FUEL an airliner?!" by Mentour Pilot (https://www.youtube.com/watch?v=dU8Ah1VC2Wg https://www.youtube.com/watch?v=dU8Ah1VC2Wg). Some excerpts: At 1:47: > The cost of fuel is one of the single biggest costs any airline out there has. My airline that I work for spent about 11 billion dollars in fuel last year. And this is why keeping a check on the costs, as in both the price of the fuel and also how much fuel is being used is extremely important. It is the difference between being profitable and non-profitable in the airline world. At 4:26 > The total cost of fuel for the airline is about 40% of the cost of the whole flight. At 10:01: > We only carry as much fuel as we need for the flight that we need to do with [(couldn't understand this word)] reserves and diversion fuel. The reason we don't carry more fuel than that is because it costs fuel to carry fuel. The weight of the fuel is going to make the aircraft burn more fuel. So the cost of fuel is very important to the airline industry, and carrying fuel costs fuel and hence money, because of the weight of that fuel. Yes, flying is still relatively cheap because the fuel cost is shared by all passengers, and because airlines for reasons that I don't quite understand pay no or almost no taxes on their fuel; certainly much less than what you pay to fill up your car.

jonsen 5y ago

> Mass gets cancelled out there, ... Two equal airplanes but with different weight can glide the same distance, same glide angle, but the heavier plane will have higher airspeed to give the extra lift needed for the extra weight.

roeles 5y ago
This is mostly true, but due to a small Reynolds effect the heavier plane should actually perform a little bit better. What you describe is why gliders sometimes carry water as ballast in their wings. It enables the same lift/drag ratio at a higher speed, at the cost of a higher stall speed.

mlyle 5y ago

Induced drag is a thing. https://en.wikipedia.org/wiki/Lift-induced_drag https://en.wikipedia.org/wiki/Lift-induced_drag

sigmoid10 5y ago
This is one of the higher order corrections I was referring to. Note how the article describes that this is not a dominant factor under normal conditions and actually gets lower the faster the plane flies. So if you can keep parasitic drag down by designing your aircraft nicely, you can get pretty far to the right in the plot at the end of the page.