8 ms·

Wings generate lift primarily by deflecting air downward. There is an area of low pressure over the wing where air is taking a longer path than under the wing,

by Zak 10y ago

Wings generate lift primarily by deflecting air downward. There is an area of low pressure over the wing where air is taking a longer path than under the wing, but that is only responsible for a small fraction of the lift.

nether 10y ago
"Longer path" has no bearing on reality. A flat plate can generate plenty of lift even with both sides the same. It's also hard to define "path," as the air starts convecting well ahead of the wing in subsonic flow.
- hexane360 10y ago
  Additionally, the paths don't meet up/join at the trailing edge of the wing. There's no reason air above and below needs to separate and meet up into the same flow.
- xelxebar 10y ago
  While you're on point about flat wings, in wind tunnel tests on profiled wings, you do in fact see a velocity difference between top flowing and bottom flowing streams. It's not enough of a difference, however, to cause coincident streams on the leading edge to line back up at the trailing edge.
- teapot01 10y ago
  NASA has even published on this issue https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html
- ajross 10y ago
  Flat plates generate lift even in near vacuum conditions where the whole idea of Bernoulli flow is inapplicable, even.
- simion314 10y ago
  If I remember right the "longer path" thing was in Encarta, there was a guy that used Encarta to prove to me that the theory is right and I believed it m it made sense I am wrong and not the encyclopedia.
- cyberferret 10y ago
  Sorry, but I don't subscribe to the 'flat plate' theory. There is a reason that almost every commercial (and most military) aircraft have leading edge slats to generate lift at lower speeds (by making the top of the wing more convex and the bottom of the wing more concave). If pure 'flat plate' theory was valid, then all those aircraft speeding down the runway with the leading edge of their wings canted downwards 20 or so degrees would result in the airplanes simply being pushed towards the ground and never lifting off... And Boeing, Airbus et al would build planes with flat slab wings mounted at a 45 degree angle to the airflow, because that would give the maximum lift by the 'flat plate' theory, wouldn't it? Not that I disbelieve that a flat slab cannot generate lift - but that it is probably a very inefficient way to generate lift compared to the standard aerofoil shape. [0] - https://upload.wikimedia.org/wikipedia/commons/thumb/3/3b/Boeing_727-2S7_Advanced_Champion_LAX.jpg/1200px-Boeing_727-2S7_Advanced_Champion_LAX.jpg https://upload.wikimedia.org/wikipedia/commons/thumb/3/3b/Bo... [1] - https://www.metabunk.org/sk/20141127-081301-jrgr5.jpg https://www.metabunk.org/sk/20141127-081301-jrgr5.jpg
  - dorfsmay 10y ago
    The shape of the wing makes a huge difference, so a plate or a piece of wood wouldn't make a good wing, but it is still able to fly. Proof? Inverted flight on low power aircraft and gliders.
    - cyberferret 10y ago
      And as I have mentioned elsewhere on here - I have flown inverted a few times, and to do so take a tremendous amount of forward pressure on the control stick to even try and attempt to hold altitude while inverted, in order to counteract the wings natural (slight) positive AoA and the tendency for the wing to move towards it upper surface. In fact, most of the aircraft I have flown would not be able to sustain inverted flight at all. The fact that full aerobatic and military jets do, is because they (as I explained earlier) usually don't have that curved 'fish' shape cross section, but are usually symmetrically shaped on the top and bottom of the wing. To whit, I've had the fortune to fly an old DH Tiger Moth biplane - on that little baby, when you approach the stalling point, you can actually see the the canvas on top of the bottom wing bulge and contort with pressure differential, and you can hear the sucking sounds as the airflow struggles to 'stick' to the wing. There is a little movement on the bottom surface of the top wing too, but not as pronounced. I'd be interested to see in this thread, who here has actually studied aeronautical engineering, or flown actual aircraft, and who is relying on YT videos or a pure theoretical approach to come up with these theories? Also interestingly, I believe most of the textbooks I used at flight school were filled with data from NASA and other US military branches with regards to flight dynamics etc., and here on this thread we see articles from NASA (albeit aimed at K-12 audience rather than trainee pilots) basically disproving their earlier academic research.
- stereo 10y ago
  Yes, it’s hard to visualise for some people, but it’s the same reason why a small sailboat can turn with a flat rudder.
deleted 10y ago
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cyberferret 10y ago
Well, the same Venturi principle applies if you look at it in terms of laminar airflow over the wing. The top of the wing (via curvature and downward deflection) is essentially creating a widening venturi tube, whereas the bottom of the wing is creating a closing venturi tube. (Oh, ok, it is planar rather than a tube, but the principle is essentially the same). Hence the high pressure under the wing and the low pressure on top of the wing. The very act of 'deflecting' millions of cubic metres of airflow generates high/low pressure points. Hence at the end of the day, we could argue that the pressure differential is what is causing the lift? Disclaimer: Not an aeronautical engineer, just a former commercial pilot.
- termain 10y ago
  Yes to pressure part and no to the venturi part. Well, it's not enough. You still have to generate a downdraft to conserve momentum and the venturi idea doesn't explain that.
na85 10y ago
>There is an area of low pressure over the wing where air is taking a longer path than under the wing, but that is only responsible for a small fraction of the lift. The "equal transit time" theory that you appear to be referencing here (air over the top takes a longer path, thus it must go faster and air under the bottom takes a shorter path thus it goes slower) is a complete fallacy. There is no mechanism in physics that requires the air to "meet up" at the trailing edge of the wing.
- billforsternz 10y ago
  Thanks for this. I read a million explanations of how airfoils worked as a boy and they all said the air had to travel faster over the top surface to catch up. Why does it have to catch up? I would scream internally. I'm stunned to find it was never true.
  - rhizome 10y ago
    Probably like the Bohr model in Chemistry, it's an easy way to introduce the concept. Frankly I was under the impression it was true, too, and my dad was a small-plane pilot.
    - halomru 10y ago
      The Bohr model at least has simplicity going for it, and it's close enough to explain most things that are relevant to chemistry. For aircraft lift, "equal transit time" is not easier to understand than "an angled wing pushes air down, because of conservation of momentum that pushes the aircraft up".
    - deleted 10y ago
      [deleted]
  - luca_ing 10y ago
    If you're interested in the topic, maybe you'd like to read http://www.allstar.fiu.edu/aero/airflylvl3.htm http://www.allstar.fiu.edu/aero/airflylvl3.htm
  - 10y ago
deleted 10y ago
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jeffdavis 10y ago
The only thing that can possibly lift a normal aircraft (i.e. not a rocket) is if the air is higher pressure on the bottom than the top. Deflecting air downward is the way they create that pressure difference.
ajross 10y ago
That's a meaningless correction, and technically not correct. Aircraft are lifted up by air molecules hitting the underside of the rigid body with more force than the top side, by definition. The net integrated instantaneous pressure at the surface of the aircraft IS the lift. The fact that air gets deflected downward is a necessary (c.f. conservation of momentum) effect too. So it's not incorrect to do the analysis that way. But it's absolutely wrong to correct someone saying that "lift is pressure" with "lift is actually deflected air".
- chrisbennet 10y ago
  The Coanda [or "coma daily" as my iPad helpfully autocorrects] Effect also contributes to lift. The air "sticks" to the surface of the wing and leaves the trailing edge in a partially downward direction i.e the air is deflected downward by the top of the wing not just the bottom. For a quick demo of this, hold the bowl of a spoon under a running faucet and see how it pulls the spoon into the stream.
- E6300 10y ago
  No, the correction is valid. The main component of lift is an upwards reactive force produced by pushing a mass of fluid downwards, in accordance with Newton's third law.
  - foldr 10y ago
    That is the only component of lift --- unless you think that Newtin's laws can be violated on a macro scale. But owing to the way that pressure is defined, the same phenomenon can also he understood in terms of a pressure differential.
  - wayn3 10y ago
    this is just an argument in whether you want to explain the phenomenon mechanically or in terms of thermodynamics. pushing a mass of fluid downward is equivalent to creating a net downward pressure. its the same thing, just expressed in different terms. the unit of pressure is force per square inch, for you americans. by integrating over all the area, you get the force that you are talking about.
cyberferret 10y ago
Or we can just go to the demonstration that our flight instructor did in the first 5 minutes of our Aerodynamics 101 lectures. Get a piece of A4 or Letter sized paper and try this experiment with it [0]. For best effect, hold both the edges closest to you in each hand and twist the front edge downwards to keep it straight and prevent the paper twisting which could inadvertently straighten it. (i.e. Not with one hand like he is doing in the video). The Bernoulli principle with postulates the Venturi effect is about the only theory that can explain why the trailing edge of the paper moves UPWARDS when you blow across the top of the curved paper. [0] - https://www.youtube.com/watch?v=R3VVbajPqc0 https://www.youtube.com/watch?v=R3VVbajPqc0
- snowwrestler 10y ago
  Sure but now try the same thing with a piece of metal. No one questions whether the Bernoulli and Venturi effects actually exist. But airplanes are heavy and it's pretty obvious that the Bernoulli effect does not produce sufficient lift by itself. As proof that most of the lift comes from deflecting air, I offer the fact that airplanes can fly upside down.
  - 0xfaded 10y ago
    Haha this is a fantastic answer. Also, not most of the effect, all of the effect. Do the same experiment with the paper, but rest the trailing edge on a table. The air can no longer be deflected downwards, so the paper will not rise. Another one to think about, at the air metal boundary the air is stationary on both sides of the wing. So why is there a pressure difference? Bernoulli's law is valid only along flow lines and is a consequence pressure difference required to accelerate (or decelerate) a fluid. Since in the laminar situation flow does not occur from the top surface to the bottom (minus around the tips), direct application is nonsensical. If you integrated all flow lines from all surfaces however, you would get the correct answer.
    - cyberferret 10y ago
      Well, I just instantly disproved your hypothesis by holding a piece of paper against the front edge of my desk with it curving up and the trailing edge resting on the flat of the desk, then leant down and blew over the top of it... and the paper lifted at the back. QED. Not sure why people are looking at the situation as ONE Venturi plane. What I am trying to explain is that there is effectively TWO - and expanding one over the top of the wing leading to a low pressure effect, and a constricting one below the wing leading to a high pressure situation. I wouldn't say that both provide equal amounts of lift, but the paper experiment, and the 747 in the boneyard proves it to a point.
Daviey 10y ago
You should listen to Cabin Pressure, best radio series ever: Explanation of why aeroplanes fly to airhost: https://youtu.be/AaE9j7u3XJA?t=642 https://youtu.be/AaE9j7u3XJA?t=642 From Captain to Airhost: https://youtu.be/AaE9j7u3XJA?t=1031 https://youtu.be/AaE9j7u3XJA?t=1031 From First Officer to Airhost: https://youtu.be/AaE9j7u3XJA?t=1458 https://youtu.be/AaE9j7u3XJA?t=1458
deleted 10y ago
[deleted]

by Zak 10y ago

nether 10y ago

"Longer path" has no bearing on reality. A flat plate can generate plenty of lift even with both sides the same. It's also hard to define "path," as the air starts convecting well ahead of the wing in subsonic flow.

hexane360 10y ago
Additionally, the paths don't meet up/join at the trailing edge of the wing. There's no reason air above and below needs to separate and meet up into the same flow.
xelxebar 10y ago
While you're on point about flat wings, in wind tunnel tests on profiled wings, you do in fact see a velocity difference between top flowing and bottom flowing streams. It's not enough of a difference, however, to cause coincident streams on the leading edge to line back up at the trailing edge.
teapot01 10y ago
NASA has even published on this issue https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html
ajross 10y ago
Flat plates generate lift even in near vacuum conditions where the whole idea of Bernoulli flow is inapplicable, even.
simion314 10y ago
If I remember right the "longer path" thing was in Encarta, there was a guy that used Encarta to prove to me that the theory is right and I believed it m it made sense I am wrong and not the encyclopedia.
cyberferret 10y ago
Sorry, but I don't subscribe to the 'flat plate' theory. There is a reason that almost every commercial (and most military) aircraft have leading edge slats to generate lift at lower speeds (by making the top of the wing more convex and the bottom of the wing more concave). If pure 'flat plate' theory was valid, then all those aircraft speeding down the runway with the leading edge of their wings canted downwards 20 or so degrees would result in the airplanes simply being pushed towards the ground and never lifting off... And Boeing, Airbus et al would build planes with flat slab wings mounted at a 45 degree angle to the airflow, because that would give the maximum lift by the 'flat plate' theory, wouldn't it? Not that I disbelieve that a flat slab cannot generate lift - but that it is probably a very inefficient way to generate lift compared to the standard aerofoil shape. [0] - https://upload.wikimedia.org/wikipedia/commons/thumb/3/3b/Boeing_727-2S7_Advanced_Champion_LAX.jpg/1200px-Boeing_727-2S7_Advanced_Champion_LAX.jpg https://upload.wikimedia.org/wikipedia/commons/thumb/3/3b/Bo... [1] - https://www.metabunk.org/sk/20141127-081301-jrgr5.jpg https://www.metabunk.org/sk/20141127-081301-jrgr5.jpg
- dorfsmay 10y ago
  The shape of the wing makes a huge difference, so a plate or a piece of wood wouldn't make a good wing, but it is still able to fly. Proof? Inverted flight on low power aircraft and gliders.
  - cyberferret 10y ago
    And as I have mentioned elsewhere on here - I have flown inverted a few times, and to do so take a tremendous amount of forward pressure on the control stick to even try and attempt to hold altitude while inverted, in order to counteract the wings natural (slight) positive AoA and the tendency for the wing to move towards it upper surface. In fact, most of the aircraft I have flown would not be able to sustain inverted flight at all. The fact that full aerobatic and military jets do, is because they (as I explained earlier) usually don't have that curved 'fish' shape cross section, but are usually symmetrically shaped on the top and bottom of the wing. To whit, I've had the fortune to fly an old DH Tiger Moth biplane - on that little baby, when you approach the stalling point, you can actually see the the canvas on top of the bottom wing bulge and contort with pressure differential, and you can hear the sucking sounds as the airflow struggles to 'stick' to the wing. There is a little movement on the bottom surface of the top wing too, but not as pronounced. I'd be interested to see in this thread, who here has actually studied aeronautical engineering, or flown actual aircraft, and who is relying on YT videos or a pure theoretical approach to come up with these theories? Also interestingly, I believe most of the textbooks I used at flight school were filled with data from NASA and other US military branches with regards to flight dynamics etc., and here on this thread we see articles from NASA (albeit aimed at K-12 audience rather than trainee pilots) basically disproving their earlier academic research.
stereo 10y ago
Yes, it’s hard to visualise for some people, but it’s the same reason why a small sailboat can turn with a flat rudder.

deleted 10y ago

[deleted]

cyberferret 10y ago

Well, the same Venturi principle applies if you look at it in terms of laminar airflow over the wing. The top of the wing (via curvature and downward deflection) is essentially creating a widening venturi tube, whereas the bottom of the wing is creating a closing venturi tube. (Oh, ok, it is planar rather than a tube, but the principle is essentially the same). Hence the high pressure under the wing and the low pressure on top of the wing. The very act of 'deflecting' millions of cubic metres of airflow generates high/low pressure points. Hence at the end of the day, we could argue that the pressure differential is what is causing the lift? Disclaimer: Not an aeronautical engineer, just a former commercial pilot.

termain 10y ago
Yes to pressure part and no to the venturi part. Well, it's not enough. You still have to generate a downdraft to conserve momentum and the venturi idea doesn't explain that.

na85 10y ago

>There is an area of low pressure over the wing where air is taking a longer path than under the wing, but that is only responsible for a small fraction of the lift. The "equal transit time" theory that you appear to be referencing here (air over the top takes a longer path, thus it must go faster and air under the bottom takes a shorter path thus it goes slower) is a complete fallacy. There is no mechanism in physics that requires the air to "meet up" at the trailing edge of the wing.

billforsternz 10y ago
Thanks for this. I read a million explanations of how airfoils worked as a boy and they all said the air had to travel faster over the top surface to catch up. Why does it have to catch up? I would scream internally. I'm stunned to find it was never true.
- rhizome 10y ago
  Probably like the Bohr model in Chemistry, it's an easy way to introduce the concept. Frankly I was under the impression it was true, too, and my dad was a small-plane pilot.
  - halomru 10y ago
    The Bohr model at least has simplicity going for it, and it's close enough to explain most things that are relevant to chemistry. For aircraft lift, "equal transit time" is not easier to understand than "an angled wing pushes air down, because of conservation of momentum that pushes the aircraft up".
  - deleted 10y ago
    [deleted]
- luca_ing 10y ago
  If you're interested in the topic, maybe you'd like to read http://www.allstar.fiu.edu/aero/airflylvl3.htm http://www.allstar.fiu.edu/aero/airflylvl3.htm
- 10y ago

deleted 10y ago

[deleted]

jeffdavis 10y ago

The only thing that can possibly lift a normal aircraft (i.e. not a rocket) is if the air is higher pressure on the bottom than the top. Deflecting air downward is the way they create that pressure difference.

ajross 10y ago

That's a meaningless correction, and technically not correct. Aircraft are lifted up by air molecules hitting the underside of the rigid body with more force than the top side, by definition. The net integrated instantaneous pressure at the surface of the aircraft IS the lift. The fact that air gets deflected downward is a necessary (c.f. conservation of momentum) effect too. So it's not incorrect to do the analysis that way. But it's absolutely wrong to correct someone saying that "lift is pressure" with "lift is actually deflected air".

chrisbennet 10y ago
The Coanda [or "coma daily" as my iPad helpfully autocorrects] Effect also contributes to lift. The air "sticks" to the surface of the wing and leaves the trailing edge in a partially downward direction i.e the air is deflected downward by the top of the wing not just the bottom. For a quick demo of this, hold the bowl of a spoon under a running faucet and see how it pulls the spoon into the stream.
E6300 10y ago
No, the correction is valid. The main component of lift is an upwards reactive force produced by pushing a mass of fluid downwards, in accordance with Newton's third law.
- foldr 10y ago
  That is the only component of lift --- unless you think that Newtin's laws can be violated on a macro scale. But owing to the way that pressure is defined, the same phenomenon can also he understood in terms of a pressure differential.
- wayn3 10y ago
  this is just an argument in whether you want to explain the phenomenon mechanically or in terms of thermodynamics. pushing a mass of fluid downward is equivalent to creating a net downward pressure. its the same thing, just expressed in different terms. the unit of pressure is force per square inch, for you americans. by integrating over all the area, you get the force that you are talking about.

cyberferret 10y ago

Or we can just go to the demonstration that our flight instructor did in the first 5 minutes of our Aerodynamics 101 lectures. Get a piece of A4 or Letter sized paper and try this experiment with it [0]. For best effect, hold both the edges closest to you in each hand and twist the front edge downwards to keep it straight and prevent the paper twisting which could inadvertently straighten it. (i.e. Not with one hand like he is doing in the video). The Bernoulli principle with postulates the Venturi effect is about the only theory that can explain why the trailing edge of the paper moves UPWARDS when you blow across the top of the curved paper. [0] - https://www.youtube.com/watch?v=R3VVbajPqc0 https://www.youtube.com/watch?v=R3VVbajPqc0

snowwrestler 10y ago
Sure but now try the same thing with a piece of metal. No one questions whether the Bernoulli and Venturi effects actually exist. But airplanes are heavy and it's pretty obvious that the Bernoulli effect does not produce sufficient lift by itself. As proof that most of the lift comes from deflecting air, I offer the fact that airplanes can fly upside down.
- 0xfaded 10y ago
  Haha this is a fantastic answer. Also, not most of the effect, all of the effect. Do the same experiment with the paper, but rest the trailing edge on a table. The air can no longer be deflected downwards, so the paper will not rise. Another one to think about, at the air metal boundary the air is stationary on both sides of the wing. So why is there a pressure difference? Bernoulli's law is valid only along flow lines and is a consequence pressure difference required to accelerate (or decelerate) a fluid. Since in the laminar situation flow does not occur from the top surface to the bottom (minus around the tips), direct application is nonsensical. If you integrated all flow lines from all surfaces however, you would get the correct answer.
  - cyberferret 10y ago
    Well, I just instantly disproved your hypothesis by holding a piece of paper against the front edge of my desk with it curving up and the trailing edge resting on the flat of the desk, then leant down and blew over the top of it... and the paper lifted at the back. QED. Not sure why people are looking at the situation as ONE Venturi plane. What I am trying to explain is that there is effectively TWO - and expanding one over the top of the wing leading to a low pressure effect, and a constricting one below the wing leading to a high pressure situation. I wouldn't say that both provide equal amounts of lift, but the paper experiment, and the 747 in the boneyard proves it to a point.

Daviey 10y ago

You should listen to Cabin Pressure, best radio series ever: Explanation of why aeroplanes fly to airhost: https://youtu.be/AaE9j7u3XJA?t=642 https://youtu.be/AaE9j7u3XJA?t=642 From Captain to Airhost: https://youtu.be/AaE9j7u3XJA?t=1031 https://youtu.be/AaE9j7u3XJA?t=1031 From First Officer to Airhost: https://youtu.be/AaE9j7u3XJA?t=1458 https://youtu.be/AaE9j7u3XJA?t=1458

deleted 10y ago

[deleted]