7 ms·

Why Cant Commercial Airplanes Go Faster? (2012)

35 points by jasonwilk 11y ago

velox_io 11y ago
"Planes fly by throwing air down." This is wrong. Planes actually fly by stretching air over their wings (creating low pressure) and the higher pressure air below pushes the plane up (to fill in the void) - So planes fly by moving air up. See Bernoulli Principle (https://en.wikipedia.org/wiki/Bernoulli%27s_principle https://en.wikipedia.org/wiki/Bernoulli%27s_principle) PS: The reason most commercial jets cruise at ~585mph (or 85% of Mach one) is because of the huge amount of energy required to break the speed of sound. Edit: I've you're going to downvote, you can at least comment.
- ThrustVectoring 11y ago
  Air pressure is an abstraction over movement of air particles. What a low pressure area is is fewer air molecules hitting the surface, while high pressure is more air molecules hitting the surface. So when you have more air molecules hitting the downward-facing surface, the wing pushes more air molecules down than it pushes upwards. The air-pressure explanation and the "throwing air" explanation is basically the same thing.
- mikeyouse 11y ago
  The vast majority of lift for flight is achieved by pretty much throwing air down, the Bernoilli Principle is outdated as a theory behind flight (as can be demonstrated by flat-winged flight or inverted flight which shouldn't work without the pressure differential) See: http://home.comcast.net/~clipper-108/lift.pdf http://home.comcast.net/~clipper-108/lift.pdf Or: http://physics.stackexchange.com/questions/290/what-really-allows-airplanes-to-fly http://physics.stackexchange.com/questions/290/what-really-a...
  - nether 11y ago
    There's plenty of pressure differential in a lifting plate in a positive alpha. There must be in order to impart a net downward acceleration on the flow, thereby turning it. Here, have some CFD: https://www.youtube.com/watch?v=KldLqShzCRM https://www.youtube.com/watch?v=KldLqShzCRM The problem in these discussion is "Bernoulli" is used to refer to many things, some legitimately physical (mass continuity) and some not (equal time of passage). I like FYFD's point that what we're discussing is how to explain lift to laypeople, not how to calculate and analyze it (which is done every day in order to design aircraft): http://fuckyeahfluiddynamics.tumblr.com/post/121024503205/everyonelikespotatissallad-asks-so-how-is-lift http://fuckyeahfluiddynamics.tumblr.com/post/121024503205/ev.... We (mechanical/aerospace engineers) know the equations that govern fluid motion and how to solve them, whether you abstract that with Bernoulli/Newton/Kutta-Joukowsky/etc. is up to you and not that significant.
  - tzs 11y ago
    All of your references are discussing and debunking misapplication of Bernoulli. In fact, Bernoulli applied correctly explains all of the lift. The Newton approach also explains all of the lift. They are just two different ways of looking at the same thing. Bernoulli comes from applying conservation of energy. Newton comes from applying conservation of momentum. See this discussion from NASA: https://www.grc.nasa.gov/www/K-12/airplane/bernnew.html https://www.grc.nasa.gov/www/K-12/airplane/bernnew.html What's puzzling is that it is not at all uncommon in other areas of physics for there to be different ways to explain something, and this causes no problems. If one person calculated the velocity of a dropped object by integrating the acceleration, and another person did it by using conservation of energy on the conversion from gravitational potential energy to kinetic energy, people might argue over which approach works better, but no one would argue that the other approach does not work and does not explain 100% of the velocity. Or one person might calculate the response of a linear circuit to a sinusoidal signal by working in the frequency domain, and another person might work in the time domain. People might argue over which approach is easier, but again no one would say that only their approach works. But with lift, people seem to like to latch onto one approach to the point of rejecting the very idea that there might be a different way to get the same result. It makes no sense to me. The Newton approach is easier to teach reasonably correctly, so that's probably best up through, say, first year college physics and for pilot training.
- dozzie 11y ago
  Wrong. Laymen typically overestimate importance of force coming from Bernoulli's principle. You could fly on a barn door if you had enough speed, because attack angle is more important. Hint: why can planes fly upside down? We still don't understand areodynamics fully, so you can't arbitrarily state that difference between pressures above and below wings is the main force. And no, I didn't downvote you.
- ori_b 11y ago
  F=MA still holds for airplanes. If there's no downwards acceleration of air, then there's no force to push the airplane upwards. The way that this happens comes from a number of effects, from vortexes at the tips of the wings to the angle of attack, but in the end it all comes down to F=MA. And the only thing that can be accelerated by a plane is air. Ergo, air is moving downwards so that the plane will move upwards.
  - deciplex 11y ago
    It's better to just remember that energy is conserved. Bernoulli's principle is a real thing and will add to the lift a wing generates without contributing to any net motion of the air upward or downward. The work done to compress the air below the wing and expand it above helps keep the plane in the air. But, as everyone else responding to the parent has pointed out, the forces generated in this way are not nearly enough to keep the plane in the air on their own.
    - ori_b 11y ago
      > Bernoulli's principle is a real thing and will add to the lift a wing generates without contributing to any net motion of the air upward or downward. The compression of the air below the wing moves it downwards, more or less. There are no ways to get around Newton's laws of motion (barring relativity, and planes aren't fast enough for that).
      - deciplex 11y ago
        After the wing passes, the air is no longer compressed, and the net motion of the air is unchanged from before it encountered the wing. What then? Anyway, I'm not saying you're wrong, I'm just saying that thinking about the individual forces involved leads to more complexity than necessary (this is usually the case). It's much simpler to just consider the energy going into the system and derive the net work produced from that.
        ori_b 11y ago
- tghw 11y ago
  There's nothing that says that two air particles that start together at the front edge of the wing and split up have to arrive at the tail edge of the wing at the same time. So why would it be that the air on top of the wing has to go faster than the air below the wing?
  - deciplex 11y ago
    The idea is that the wing is slicing through still air, so the molecules arrive back at the same spot much like you'd expect a slice of cheese to line back up with itself after cutting it. Friction on the surface of the wing means this isn't quite going to happen, but that's the idea.
    - tghw 11y ago
      Again, this assumes the air is already connected to the air around it in the same way cheese is. You can't lift a layer of air and have it connected to the air it used to be connected to. Likewise, you don't have turbulence in cheese. There's a significant difference between solids and fluids.
  - lutorm 11y ago
    There's nothing that says that two air particles that start together at the front edge of the wing and split up have to arrive at the tail edge of the wing at the same time. Exactly. This is one of the fallacies that are explained in the best reference I know about this subject: http://www.av8n.com/how/ http://www.av8n.com/how/ It's even illustrated in the title picture... ;-)
- jameshart 11y ago
  "planes fly by moving air up" simply fails the sniff test - every action has an equal and opposite reaction, after all, so anything that throws air up must be pushing itself down. Since lift is a net upward force on a wing exerted by air, there has to be a net downward force on the air. Then, to your PS, If a huge amount of energy is required to break the speed of sound, you've explained why commercial jets don't exceed mach one, not why they don't get within 100mph of it.
- rdtsc 11y ago
  > Planes actually fly by stretching air over their wings (creating low pressure) and the higher pressure air below pushes the plane up (to fill in the void) So why can planes fly upside down then?
  - PhasmaFelis 11y ago
    I just posted a brief answer above that may explain it better: https://news.ycombinator.com/item?id=10218403 https://news.ycombinator.com/item?id=10218403
  - yellowapple 11y ago
    Doesn't extended upside-down flight require pointing the nose up (as in away from the ground) in order to compensate for the decrease in lift? I've yet to try this (I suppose I could; I've got an X-Plane-running flight sim rig about 10 feet away from me), but my intuition and understanding suggests that if the only difference between two planes flying in the same exact atmospheric conditions is that one of them is upside-down, the upside-down one will start losing altitude.
    - rdtsc 11y ago
      Yeah give it a try if you have a minute out of curiosity. X-Plane, if I remember correctly, prized itself on a pretty good airfoil model.
      - yellowapple 11y ago
        So I tried it in some simulated Cessna, and as I predicted, upon rotating so that my head pointed at the ground, the plane started to descend. I'm a pretty terrible pilot, though :) I think this has a lot to do with the wing's angle of attack (most planes' wings have a slight upward tilt; when upside-down, that tilt goes downward instead). It would be more interesting to see the effect on something without an AoA bias.
  - NolF 11y ago
    Whenever I've played a flying simulator, if I've put the plane upside down, to maintain altitude I need to push the joystick forward. In an upright plane it would cause a nose dive, on an upside down plane, it creates lift. Due to it's not optimal aerodynamics, it also requires more thrust to maintain. Basically you are changing the shape of the wing to compensate for the non-conventional position.
- PhasmaFelis 11y ago
  > Planes actually fly by stretching air over their wings (creating low pressure) and the higher pressure air below pushes the plane up (to fill in the void) - So planes fly by moving air up. Even if your premise is accurate (I'm not qualified to say 100%), your conclusion does not follow. Water holds up the hull of a boat, but you wouldn't say that a boat floats by moving water up.
  - x0ra 11y ago
    some do: https://en.wikipedia.org/wiki/Hydrofoil https://en.wikipedia.org/wiki/Hydrofoil
- PhasmaFelis 11y ago
  A few people have posted links already, but here's another that I found simpler and punchier: https://danielmiessler.com/blog/why-planes-fly-what-they-taught-you-in-school-was-wrong/ https://danielmiessler.com/blog/why-planes-fly-what-they-tau... Basically, the idea that the Bernoulli Principle is the primary factor in airplane lift is a popular lie. For some reason it continues to be broadly taught, so you can't be blamed for not knowing better. The main factor is actually angle of attack--when you hold your hand flat out a car window, tilting the leading edge up drags your hand upwards. That's the core principle of powered flight. Most conventional planes have the wings installed with an upward angle, so they can achieve lift in level flight. Fighter and aerobatic planes have wings parallel with the fuselage, which means they have to keep the nose slightly up to fly level, but in exchange they can fly upside down almost as well as rightside up.
  - rdtsc 11y ago
    It makes sense. Can stick you hand out of a window of a moving car and you can feel this effect.
- MrBra 11y ago
  https://news.ycombinator.com/reply?id=10218616&goto=threads%3Fid%3DMrBra https://news.ycombinator.com/reply?id=10218616&goto=threads%...
ryanburk 11y ago
the article includes the link to the previous HN discussion on this here: http://news.ycombinator.com/item?id=4644712 http://news.ycombinator.com/item?id=4644712
ricardobeat 11y ago
This is in direct contradiction to this article (also posted today) on new developments in tech: http://www.compositesworld.com/articles/aeroengine-composites-part-1-the-cmc-invasion http://www.compositesworld.com/articles/aeroengine-composite...
- goodcanadian 11y ago
  Yes and no. The article acknowledges that the key to improving aircraft efficiency comes in improving engine efficiency. While it says, "Engines are pretty darn efficient already," it doesn't really quantify that or do any analysis of the efficiency limits of engines.
- iofj 11y ago
  Just because you can't gain efficiency by changing the speed of the plane, with no other changes allowed, doesn't mean you have no options : 1) reduce weight (fuselage, engine, wings, have the passengers diet first, ...) 2) increase engine fuel efficiency, displace more air for fewer hydrocarbons 3) (in the article) reduce drag on surfaces that don't generate lift 4) lower cross section (a longer, thinner tube), of course, still needs structural soundness The lower cross section would have the additional advantage that it would make the plane fly faster. So in a way, the article is "wrong". You can make a faster plane, assuming you can solve all the issues that the obvious modifications bring : longer, thinner and lighter fuselage and wings, combined with similar efficiency engines (of course they'd have to have that similar efficiency at higher speeds). Another way would be to fly (much) higher. But then the cost of climbing high would start to offset the gains from flying faster.
grecy 11y ago
I really like the simplified explanation of how much energy to takes to move the air out of the way (v)^3 - I've heard this stated for vehicles before which is why roof racks are so bad for mileage. Does anyone know of a similarly simple explanation of how much energy a vehicle must use relative to it's weight?
- IIAOPSW 11y ago
  Airplanes can just fly a bit higher so the atmosphere is less dense and the drag is less of a problem. Non-flying vehicles do not have this luxury.
- upofadown 11y ago
  That's actually a bit oversimplified for highly aerodynamic objects like aircraft. It is known that such shapes can approach (v)^2. A (v)^3 characteristic is more representative of shapes that actually do have to accelerate all the air up to the speed of the vehicle such as the proverbial flat plate. This doesn't change the conclusion; you still end up with a low spot on the graph. It is just that the graph looks more like a parabola.
- jcranmer 11y ago
  The best way to think of simple back-of-the-envelope calculations is this: To go from point a to point b at a cruising speed of v, you need to speed up to v, maintain v, and brake to a complete stop. Analyze each separately: 1. Speeding up. Since this takes so little distance (compared to the total length), let's ignore resistance. This speedup takes KE = ½·m·v² of energy to do. 2. Maintain speed. This means spending energy to resist air friction (drag) and ground friction. Drag is ½·ρ·v²·Cd·A [0]; friction is (at most) μ·m·g. Energy is force × distance, and since the distance at top speed is effectively constant, it's going to be O(m + v²) in this case. [1] 3. Braking. You can theoretically recover -½·m·v² here, but most of the energy is going to be lost to heat. So, when you sum these up, the energy it takes to drive a vehicle a certain distance is linear in mass and quadratic in speed. [0] The drag coefficient, Cd, is really weird. At low speeds, it's actually proportional to 1/v instead of a constant (i.e., drag is linear in velocity, not quadratic). [1] Yeah, the article says that energy is proportional to v³. It's actually wrong--the power is proportional to v³. Whether or not the total energy is proportional to v³ or v² depends on what you hold constant: if you hold total time constant, it's v³, but if you hold distance, it's v².
dmishe 11y ago
Well then, how about charging more for faster flights, to account for fuel? I mean, domestic business class isn't that great, but people who pay others people money for it would presumably go for a faster flight.
- stevewilhelm 11y ago
  > Well then, how about charging more for faster flights... They did with limited success. https://en.wikipedia.org/wiki/Concorde https://en.wikipedia.org/wiki/Concorde
- jcranmer 11y ago
  The majority of people who fly do so in cattle class, where basically the headline ticket price is the biggest factor in choosing prices (which is why low-cost airlines offer cheap tickets but charge steep prices for doing things like bringing luggage or getting a drink on board the plane). You can't fit enough people in first or business class to fill a plane on most routes, and you simply can't go fast enough to make speed premiums really worth it. Note that going to an airport usually requires on the order of 2 hours of hassle before accounting for flight time, which means that you've already lost a day of work in practice. No, unless it's a long-haul flight, there's little benefit to faster speeds, and the Concorde was never really profitable.
yellowapple 11y ago
I'm not sure if I agree with the conclusion of the article; if the problem is that the effect of drag increases with speed, then the answer here seems to be to decrease drag (perhaps with a more efficient flight profile). Or would that interfere with lift?
markbnj 11y ago
Ah, nothing like a good old-fashioned brouhaha over angle of attack vs. the Bernoulli principle. This same argument was going on when my Dad was training as a private pilot back in the 60's. Tastes great! Less filling! Personally, I think they're facets of the same thing, but I don't know anything about it. Beyond that the piece seemed a bit of a mess to me. The content has not aged well. I usually expect Wired to do a better job of conserving the state of what they publish.
MrBra 11y ago
I remember how happy I was when I firstly made this consideration myself :) --> as racing cars have a back "aileron" which forces air to push them down (and stick to the road), airplanes have the same thing but inverted, which pushes them up. This becomes exceptionally clear when thinking about how flaps change the shape of the wing.
tantalor 11y ago
Please fix the title! Should be something like "Can We Build a More Efficient Airplane? Not Really, Says Physics", not "Why Cant Commercial Airplanes Go Faster?"
tzs 11y ago
OK, so the plane shoves air out of the way, and that takes energy. But when the plane passes, air will fill the air tube. Is there any way in theory to recover from that air filling the air tube some of the energy that was expended in making the tube?

35 points by jasonwilk 11y ago

"Planes fly by throwing air down." This is wrong. Planes actually fly by stretching air over their wings (creating low pressure) and the higher pressure air below pushes the plane up (to fill in the void) - So planes fly by moving air up. See Bernoulli Principle (https://en.wikipedia.org/wiki/Bernoulli%27s_principle https://en.wikipedia.org/wiki/Bernoulli%27s_principle) PS: The reason most commercial jets cruise at ~585mph (or 85% of Mach one) is because of the huge amount of energy required to break the speed of sound. Edit: I've you're going to downvote, you can at least comment.

ThrustVectoring 11y ago
Air pressure is an abstraction over movement of air particles. What a low pressure area is is fewer air molecules hitting the surface, while high pressure is more air molecules hitting the surface. So when you have more air molecules hitting the downward-facing surface, the wing pushes more air molecules down than it pushes upwards. The air-pressure explanation and the "throwing air" explanation is basically the same thing.
mikeyouse 11y ago
The vast majority of lift for flight is achieved by pretty much throwing air down, the Bernoilli Principle is outdated as a theory behind flight (as can be demonstrated by flat-winged flight or inverted flight which shouldn't work without the pressure differential) See: http://home.comcast.net/~clipper-108/lift.pdf http://home.comcast.net/~clipper-108/lift.pdf Or: http://physics.stackexchange.com/questions/290/what-really-allows-airplanes-to-fly http://physics.stackexchange.com/questions/290/what-really-a...
- nether 11y ago
  There's plenty of pressure differential in a lifting plate in a positive alpha. There must be in order to impart a net downward acceleration on the flow, thereby turning it. Here, have some CFD: https://www.youtube.com/watch?v=KldLqShzCRM https://www.youtube.com/watch?v=KldLqShzCRM The problem in these discussion is "Bernoulli" is used to refer to many things, some legitimately physical (mass continuity) and some not (equal time of passage). I like FYFD's point that what we're discussing is how to explain lift to laypeople, not how to calculate and analyze it (which is done every day in order to design aircraft): http://fuckyeahfluiddynamics.tumblr.com/post/121024503205/everyonelikespotatissallad-asks-so-how-is-lift http://fuckyeahfluiddynamics.tumblr.com/post/121024503205/ev.... We (mechanical/aerospace engineers) know the equations that govern fluid motion and how to solve them, whether you abstract that with Bernoulli/Newton/Kutta-Joukowsky/etc. is up to you and not that significant.
- tzs 11y ago
  All of your references are discussing and debunking misapplication of Bernoulli. In fact, Bernoulli applied correctly explains all of the lift. The Newton approach also explains all of the lift. They are just two different ways of looking at the same thing. Bernoulli comes from applying conservation of energy. Newton comes from applying conservation of momentum. See this discussion from NASA: https://www.grc.nasa.gov/www/K-12/airplane/bernnew.html https://www.grc.nasa.gov/www/K-12/airplane/bernnew.html What's puzzling is that it is not at all uncommon in other areas of physics for there to be different ways to explain something, and this causes no problems. If one person calculated the velocity of a dropped object by integrating the acceleration, and another person did it by using conservation of energy on the conversion from gravitational potential energy to kinetic energy, people might argue over which approach works better, but no one would argue that the other approach does not work and does not explain 100% of the velocity. Or one person might calculate the response of a linear circuit to a sinusoidal signal by working in the frequency domain, and another person might work in the time domain. People might argue over which approach is easier, but again no one would say that only their approach works. But with lift, people seem to like to latch onto one approach to the point of rejecting the very idea that there might be a different way to get the same result. It makes no sense to me. The Newton approach is easier to teach reasonably correctly, so that's probably best up through, say, first year college physics and for pilot training.
dozzie 11y ago
Wrong. Laymen typically overestimate importance of force coming from Bernoulli's principle. You could fly on a barn door if you had enough speed, because attack angle is more important. Hint: why can planes fly upside down? We still don't understand areodynamics fully, so you can't arbitrarily state that difference between pressures above and below wings is the main force. And no, I didn't downvote you.
ori_b 11y ago
F=MA still holds for airplanes. If there's no downwards acceleration of air, then there's no force to push the airplane upwards. The way that this happens comes from a number of effects, from vortexes at the tips of the wings to the angle of attack, but in the end it all comes down to F=MA. And the only thing that can be accelerated by a plane is air. Ergo, air is moving downwards so that the plane will move upwards.
- deciplex 11y ago
  It's better to just remember that energy is conserved. Bernoulli's principle is a real thing and will add to the lift a wing generates without contributing to any net motion of the air upward or downward. The work done to compress the air below the wing and expand it above helps keep the plane in the air. But, as everyone else responding to the parent has pointed out, the forces generated in this way are not nearly enough to keep the plane in the air on their own.
  - ori_b 11y ago
    > Bernoulli's principle is a real thing and will add to the lift a wing generates without contributing to any net motion of the air upward or downward. The compression of the air below the wing moves it downwards, more or less. There are no ways to get around Newton's laws of motion (barring relativity, and planes aren't fast enough for that).
    - deciplex 11y ago
      After the wing passes, the air is no longer compressed, and the net motion of the air is unchanged from before it encountered the wing. What then? Anyway, I'm not saying you're wrong, I'm just saying that thinking about the individual forces involved leads to more complexity than necessary (this is usually the case). It's much simpler to just consider the energy going into the system and derive the net work produced from that.
      - ori_b 11y ago
tghw 11y ago
There's nothing that says that two air particles that start together at the front edge of the wing and split up have to arrive at the tail edge of the wing at the same time. So why would it be that the air on top of the wing has to go faster than the air below the wing?
- deciplex 11y ago
  The idea is that the wing is slicing through still air, so the molecules arrive back at the same spot much like you'd expect a slice of cheese to line back up with itself after cutting it. Friction on the surface of the wing means this isn't quite going to happen, but that's the idea.
  - tghw 11y ago
    Again, this assumes the air is already connected to the air around it in the same way cheese is. You can't lift a layer of air and have it connected to the air it used to be connected to. Likewise, you don't have turbulence in cheese. There's a significant difference between solids and fluids.
- lutorm 11y ago
  There's nothing that says that two air particles that start together at the front edge of the wing and split up have to arrive at the tail edge of the wing at the same time. Exactly. This is one of the fallacies that are explained in the best reference I know about this subject: http://www.av8n.com/how/ http://www.av8n.com/how/ It's even illustrated in the title picture... ;-)
jameshart 11y ago
"planes fly by moving air up" simply fails the sniff test - every action has an equal and opposite reaction, after all, so anything that throws air up must be pushing itself down. Since lift is a net upward force on a wing exerted by air, there has to be a net downward force on the air. Then, to your PS, If a huge amount of energy is required to break the speed of sound, you've explained why commercial jets don't exceed mach one, not why they don't get within 100mph of it.
rdtsc 11y ago
> Planes actually fly by stretching air over their wings (creating low pressure) and the higher pressure air below pushes the plane up (to fill in the void) So why can planes fly upside down then?
- PhasmaFelis 11y ago
  I just posted a brief answer above that may explain it better: https://news.ycombinator.com/item?id=10218403 https://news.ycombinator.com/item?id=10218403
- yellowapple 11y ago
  Doesn't extended upside-down flight require pointing the nose up (as in away from the ground) in order to compensate for the decrease in lift? I've yet to try this (I suppose I could; I've got an X-Plane-running flight sim rig about 10 feet away from me), but my intuition and understanding suggests that if the only difference between two planes flying in the same exact atmospheric conditions is that one of them is upside-down, the upside-down one will start losing altitude.
  - rdtsc 11y ago
    Yeah give it a try if you have a minute out of curiosity. X-Plane, if I remember correctly, prized itself on a pretty good airfoil model.
    - yellowapple 11y ago
      So I tried it in some simulated Cessna, and as I predicted, upon rotating so that my head pointed at the ground, the plane started to descend. I'm a pretty terrible pilot, though :) I think this has a lot to do with the wing's angle of attack (most planes' wings have a slight upward tilt; when upside-down, that tilt goes downward instead). It would be more interesting to see the effect on something without an AoA bias.
- NolF 11y ago
  Whenever I've played a flying simulator, if I've put the plane upside down, to maintain altitude I need to push the joystick forward. In an upright plane it would cause a nose dive, on an upside down plane, it creates lift. Due to it's not optimal aerodynamics, it also requires more thrust to maintain. Basically you are changing the shape of the wing to compensate for the non-conventional position.
PhasmaFelis 11y ago
> Planes actually fly by stretching air over their wings (creating low pressure) and the higher pressure air below pushes the plane up (to fill in the void) - So planes fly by moving air up. Even if your premise is accurate (I'm not qualified to say 100%), your conclusion does not follow. Water holds up the hull of a boat, but you wouldn't say that a boat floats by moving water up.
- x0ra 11y ago
  some do: https://en.wikipedia.org/wiki/Hydrofoil https://en.wikipedia.org/wiki/Hydrofoil
PhasmaFelis 11y ago
A few people have posted links already, but here's another that I found simpler and punchier: https://danielmiessler.com/blog/why-planes-fly-what-they-taught-you-in-school-was-wrong/ https://danielmiessler.com/blog/why-planes-fly-what-they-tau... Basically, the idea that the Bernoulli Principle is the primary factor in airplane lift is a popular lie. For some reason it continues to be broadly taught, so you can't be blamed for not knowing better. The main factor is actually angle of attack--when you hold your hand flat out a car window, tilting the leading edge up drags your hand upwards. That's the core principle of powered flight. Most conventional planes have the wings installed with an upward angle, so they can achieve lift in level flight. Fighter and aerobatic planes have wings parallel with the fuselage, which means they have to keep the nose slightly up to fly level, but in exchange they can fly upside down almost as well as rightside up.
- rdtsc 11y ago
  It makes sense. Can stick you hand out of a window of a moving car and you can feel this effect.
MrBra 11y ago
https://news.ycombinator.com/reply?id=10218616&goto=threads%3Fid%3DMrBra https://news.ycombinator.com/reply?id=10218616&goto=threads%...

ryanburk 11y ago

the article includes the link to the previous HN discussion on this here: http://news.ycombinator.com/item?id=4644712 http://news.ycombinator.com/item?id=4644712

ricardobeat 11y ago

This is in direct contradiction to this article (also posted today) on new developments in tech: http://www.compositesworld.com/articles/aeroengine-composites-part-1-the-cmc-invasion http://www.compositesworld.com/articles/aeroengine-composite...

goodcanadian 11y ago
Yes and no. The article acknowledges that the key to improving aircraft efficiency comes in improving engine efficiency. While it says, "Engines are pretty darn efficient already," it doesn't really quantify that or do any analysis of the efficiency limits of engines.
iofj 11y ago
Just because you can't gain efficiency by changing the speed of the plane, with no other changes allowed, doesn't mean you have no options : 1) reduce weight (fuselage, engine, wings, have the passengers diet first, ...) 2) increase engine fuel efficiency, displace more air for fewer hydrocarbons 3) (in the article) reduce drag on surfaces that don't generate lift 4) lower cross section (a longer, thinner tube), of course, still needs structural soundness The lower cross section would have the additional advantage that it would make the plane fly faster. So in a way, the article is "wrong". You can make a faster plane, assuming you can solve all the issues that the obvious modifications bring : longer, thinner and lighter fuselage and wings, combined with similar efficiency engines (of course they'd have to have that similar efficiency at higher speeds). Another way would be to fly (much) higher. But then the cost of climbing high would start to offset the gains from flying faster.

grecy 11y ago

I really like the simplified explanation of how much energy to takes to move the air out of the way (v)^3 - I've heard this stated for vehicles before which is why roof racks are so bad for mileage. Does anyone know of a similarly simple explanation of how much energy a vehicle must use relative to it's weight?

IIAOPSW 11y ago
Airplanes can just fly a bit higher so the atmosphere is less dense and the drag is less of a problem. Non-flying vehicles do not have this luxury.
upofadown 11y ago
That's actually a bit oversimplified for highly aerodynamic objects like aircraft. It is known that such shapes can approach (v)^2. A (v)^3 characteristic is more representative of shapes that actually do have to accelerate all the air up to the speed of the vehicle such as the proverbial flat plate. This doesn't change the conclusion; you still end up with a low spot on the graph. It is just that the graph looks more like a parabola.
jcranmer 11y ago
The best way to think of simple back-of-the-envelope calculations is this: To go from point a to point b at a cruising speed of v, you need to speed up to v, maintain v, and brake to a complete stop. Analyze each separately: 1. Speeding up. Since this takes so little distance (compared to the total length), let's ignore resistance. This speedup takes KE = ½·m·v² of energy to do. 2. Maintain speed. This means spending energy to resist air friction (drag) and ground friction. Drag is ½·ρ·v²·Cd·A [0]; friction is (at most) μ·m·g. Energy is force × distance, and since the distance at top speed is effectively constant, it's going to be O(m + v²) in this case. [1] 3. Braking. You can theoretically recover -½·m·v² here, but most of the energy is going to be lost to heat. So, when you sum these up, the energy it takes to drive a vehicle a certain distance is linear in mass and quadratic in speed. [0] The drag coefficient, Cd, is really weird. At low speeds, it's actually proportional to 1/v instead of a constant (i.e., drag is linear in velocity, not quadratic). [1] Yeah, the article says that energy is proportional to v³. It's actually wrong--the power is proportional to v³. Whether or not the total energy is proportional to v³ or v² depends on what you hold constant: if you hold total time constant, it's v³, but if you hold distance, it's v².

dmishe 11y ago

Well then, how about charging more for faster flights, to account for fuel? I mean, domestic business class isn't that great, but people who pay others people money for it would presumably go for a faster flight.

stevewilhelm 11y ago
> Well then, how about charging more for faster flights... They did with limited success. https://en.wikipedia.org/wiki/Concorde https://en.wikipedia.org/wiki/Concorde
jcranmer 11y ago
The majority of people who fly do so in cattle class, where basically the headline ticket price is the biggest factor in choosing prices (which is why low-cost airlines offer cheap tickets but charge steep prices for doing things like bringing luggage or getting a drink on board the plane). You can't fit enough people in first or business class to fill a plane on most routes, and you simply can't go fast enough to make speed premiums really worth it. Note that going to an airport usually requires on the order of 2 hours of hassle before accounting for flight time, which means that you've already lost a day of work in practice. No, unless it's a long-haul flight, there's little benefit to faster speeds, and the Concorde was never really profitable.

yellowapple 11y ago

I'm not sure if I agree with the conclusion of the article; if the problem is that the effect of drag increases with speed, then the answer here seems to be to decrease drag (perhaps with a more efficient flight profile). Or would that interfere with lift?

markbnj 11y ago

Ah, nothing like a good old-fashioned brouhaha over angle of attack vs. the Bernoulli principle. This same argument was going on when my Dad was training as a private pilot back in the 60's. Tastes great! Less filling! Personally, I think they're facets of the same thing, but I don't know anything about it. Beyond that the piece seemed a bit of a mess to me. The content has not aged well. I usually expect Wired to do a better job of conserving the state of what they publish.

MrBra 11y ago

I remember how happy I was when I firstly made this consideration myself :) --> as racing cars have a back "aileron" which forces air to push them down (and stick to the road), airplanes have the same thing but inverted, which pushes them up. This becomes exceptionally clear when thinking about how flaps change the shape of the wing.

tantalor 11y ago

Please fix the title! Should be something like "Can We Build a More Efficient Airplane? Not Really, Says Physics", not "Why Cant Commercial Airplanes Go Faster?"

tzs 11y ago

OK, so the plane shoves air out of the way, and that takes energy. But when the plane passes, air will fill the air tube. Is there any way in theory to recover from that air filling the air tube some of the energy that was expended in making the tube?