Do Pilots Understand How An Airfoil Generates LIft? (Survey Says...)

Which (if any) of the following are true?

  • I understand how a wing generates lift

    Votes: 42 52.5%

  • Airfoils generate lift because they are curved on top and flat on the bottom

    Votes: 19 23.8%

  • Air flowing over the top and bottom gets to the trailing edge at the same time.

    Votes: 21 26.3%

  • Newton's laws do not explain the low pressure on top of an airfoil.

    Votes: 24 30.0%

  • There are two componants of life - Newton on the bottom, and Bernoulli on the top

    Votes: 44 55.0%

  • Bernoulli's principle provides an adequate explanation for the low pressure on top of a wing.

    Votes: 30 37.5%

  • Blowing over a sheet of paper demonstrates Bernoulli's principle.

    Votes: 29 36.3%

  • Air bouncing off the bottom of an airfoil creates "Newtonian" lift.

    Votes: 25 31.3%

  • Pressure does not explain all of lift.

    Votes: 38 47.5%

  • A wing works like a venturi.

    Votes: 24 30.0%
  • Total voters
    80
Capt. Geoffrey Thorpe

Capt. Geoffrey Thorpe

Touchdown! Greaser!
Joined
Jun 7, 2008
Messages
16,021
Location
DXO124009
Display Name
Display name:
Light and Sporty Guy
I the past, I have made the assertion that most pilots don't understand how a wing generates lift. And that the common "explanations" have evolved like a bad game of telephone...

But, I really don't know how many is "most" - if you could be so kind as to answer above...
And, if none of the above seem to be true, what is your understanding?

T.I.A.
 
Capt. Geoffrey Thorpe said:
I the past, I have made the assertion that most pilots don't understand how a wing generates lift. And that the common "explanations" have evolved like a bad game of telephone...

But, I really don't know how many is "most" - if you could be so kind as to answer above...
And, if none of the above seem to be true, what is your understanding?

T.I.A.
Click to expand...

I understand lift. What I don’t understand is what T.I.A. means
 
I went to school for aeronautical engineering and have worked in the aerospace industry for a number of years now. I’m pretty convinced nobody knows how lift is generated. Sure we’ve developed analytical tools that allow us to predict it and how it will behave, but I’m not sure anyone really understands the physics of what causes it other than pressure builds up below the airfoil and is reduced on top of the airfoil.

In school they teach us about calculating it with vortex sheets and discuss abstract mathematical concepts like circulation. We look at empirical plots for different airfoils. We use CFD to predict behaviors of more abstract shapes.

I believe someone on the homebuilt airplanes forum has as their signature, “engineering is just educated guessing worked out to 3 decimal places”. I like that way of looking at it.
 
Many pilots may believe they understand lift, but without a relatively involved knowledge of fluid dynamics, I am very skeptical of that claim. Even with formal training, it is difficult to appreciate the subtleties and reality of lift generation, and fluid flow in general. There continues to be academic discussion by people far more knowledgable than myself on what is the most fundamental explanation of lift generation. Even after 3 degrees in aerospace engineering (BS, MS, and PhD) with a focus on turbomachinery, I feel my knowledge is inadequate to claim that I fully understand lift generation around an airfoil.
 
You’re missing an answer - K. Some of, but not all of the above.

Newton and Bernoulli are two expressions of the same principle. Lift is the equal and opposite reaction to air moving down. Some of that movement happens because of the wing banging air downward, some of the movement comes because of pressure differential.

the air flowing over the wing does not arrive at the same time as the air under the wing.
 
Airflow, angle of attack, and airfoil shape can all be adjusted to fit your requirements. A flat on top, curved bottom supercritical airfoil can still create lift at lower speeds. It's all in the efficiencies you are trying to achieve, in which flight regime it matters to your specific mission.
 
AKiss20 said:
Many pilots may believe they understand lift, but without a relatively involved knowledge of fluid dynamics, I am very skeptical of that claim. Even with formal training, it is difficult to appreciate the subtleties and reality of lift generation, and fluid flow in general. There continues to be academic discussion by people far more knowledgable than myself on what is the most fundamental explanation of lift generation. Even after 3 degrees in aerospace engineering (BS, MS, and PhD) with a focus on turbomachinery, I feel my knowledge is inadequate to claim that I fully understand lift generation around an airfoil.
Click to expand...

Seems like this is the right answer. (Disclaimer... I took graduate classes in fluid mechanics, but that was 30 years ago so my knowledge is pretty spotty at this point). If you solve the 2-d Navier-Stokes equations for incompressible turbulent flow around an airfoil, you'd find the force imbalance that generates lift. But it's a combination of local conservation of mass and momentum, not some simple global rule like "the particle at the top has to meet the particle at the bottom". Maybe the question itself is ill poised - you can write down the governing equations, but does that mean you "understand" the solution?
 
AlleyCat67 said:
Seems like this is the right answer. (Disclaimer... I took graduate classes in fluid mechanics, but that was 30 years ago so my knowledge is pretty spotty at this point). If you solve the 2-d Navier-Stokes equations for incompressible turbulent flow around an airfoil, you'd find the force imbalance that generates lift. But it's a combination of local conservation of mass and momentum, not some simple global rule like "the particle at the top has to meet the particle at the bottom". Maybe the question itself is ill poised - you can write down the governing equations, but does that mean you "understand" the solution?
Click to expand...
It's easy to get lost in the math.
 
The real question is how does a cape produce lift? I’m pretty sure the wings not there to provide lift. It’s just there so you don’t corkscrew through the air. Least that’s what I heard.
 
There was a wonderful article in Scientific American about six months or a year ago. I will try to find it and post the year and month in this thread.
 
Some pilots understand lift better than others. Stalling in the pattern proves that you don’t understand lift. Flying safely for decades proves that you do understand lift, and that’s what’s important.
 
AlleyCat67 said:
Seems like this is the right answer. (Disclaimer... I took graduate classes in fluid mechanics, but that was 30 years ago so my knowledge is pretty spotty at this point). If you solve the 2-d Navier-Stokes equations for incompressible turbulent flow around an airfoil, you'd find the force imbalance that generates lift. But it's a combination of local conservation of mass and momentum, not some simple global rule like "the particle at the top has to meet the particle at the bottom". Maybe the question itself is ill poised - you can write down the governing equations, but does that mean you "understand" the solution?
Click to expand...

No offense intended at all but I am struggling a bit to parse this statement/question so I will try and clarify a few things.

1) I think you meant to write the "incompressible, viscous Navier-stokes equations" as turbulent/laminar flow is not a classification of the fundamental physics being included or not but rather a description of the resultant flow for a specific situation. The most general possible set of governing equations is the "Navier-Stokes equations" which do not inherently assume an incompressible flow and include the effects of viscosity via the viscous stress term. Whether or not the flow is turbulent or laminar (which themselves are not fundamental definitions but rather rough descriptions of identifiably different flow regimes) falls out of the specific situation at hand, but it is all described by the governing Navier-Stokes equations.

2) Only in very select cases can you analytically solve the full viscous Navier-Stokes equations, and such situations require making some simplifying situations about the nature of the flow (e.g. Poiseulle flow in a pipe). In any other case, in order to solve the Navier-Stokes equations you must do so numerically through some form of computational method (furthermore you can almost never directly solve them as that is far too computationally expensive, instead you use averaged or filtered forms and empirical turbulence models).

3) As to the actual question at hand about lift, there are far better treatises on this topic than I could ever produce here (and also far longer), but I will say simply put that ultimately lift is due to a pressure difference between the suction and pressure sides of the wing. Why that pressure difference manifests comes down to the coupling of flow turning due to the inability of the air to penetrate the wing, and how that flow turning in turn changes the pressure of the air locally through the momentum equation. This concept of "some lift is due to pressure, some is due to the air "bouncing" off the wing" is a misnomer as it is fundamentally mixing two things: particle and continuum mechanics. People tend to think of air as particles bouncing of the metal surface, imparting some force. This is fundamentally true, but this is a particle mechanics view of the world, one in which a pressure force is not defined as pressure is a continuum mechanics concept that represents this exact force. The only way the fluid, in a continuum mechanics sense, can impart a force normal to the surface, is via pressure. Integrate the pressure force over the entire surface of the wing and you get the net force (typically known as lift). There are a lot more involved here, but this is a high level view of it.
 
It's not the wing that produces lift it's the fuel contained within the wing. Everyone knows that avgas is magic and under certain conditions, will create a phenomenon that resembles anti gravity.
 
My take on this is that the following:

The actual explanation is that the wing changes the pressure distributions in a way best described by fluid mechanics.

That is complicated so as instructors we have simplifications such as the Bernoulli effect or momentum changes which are valid in different regimes of flight.

And that most people not recognizing the first point think that their preferred simplification is completely valid and all others are invalid.
 
bflynn said:
Really? Please enlighten us.
Click to expand...
  • Airfoils generate lift because they are curved on top and flat on the bottom
The typical "cartoon" airfoils date back to the 1914 though 1930s, but many "modern" airfoils are curved on the bottom as well as the top. Super critical airfoils are pretty flat on top and curved on the bottom. If your explanation of lift depends on the above misinformation, your explanation is wrong.
  • Air flowing over the top and bottom gets to the trailing edge at the same time.
If this were true, aircraft as we know them could not fly. There just is not enough difference in distances. Plus any wind tunnel data shows this to not be true.
  • Newton's laws do not explain the low pressure on top of an airfoil.
If Newton's laws don't explain the low pressure, then Bernoulli's equation can't either. (Contrary to what a Scientific American article mentioned above claims.) It takes about two pages of algebra to derive Bernoulli's equation from Newtons laws (plus the assumptions that make make Bernoulli's equation valid in the first place). So, anything explained by Bernoulli is just a few steps away from being explained by Newton. And, in fact, Newton's laws do a much better job of explaining the low pressure on top than Bernoulli as we will get to below.
  • There are two components of life - Newton on the bottom, and Bernoulli on the top
The laws of physics do not change from one side of an airfoil to the other. Both Bernoulli and Newton apply on both sides. Newton's equations have broad applications, Bernoulli is much more focused.
  • Bernoulli's principle provides an adequate explanation for the low pressure on top of a wing.
Bernoulli's principal describes the conservation of energy along a streamline. In other words, the sum of the potential and kinetic energies is constant along a streamline as long as energy is not added or removed from that streamline. Nothing more, nothing less. This is what leads to the "faster is lower pressure" thing. So, if you want to explain the low pressure, you first have to explain the faster velocity - and that has nothing to do with Bernoulli. Now, one legitimate way way to explain the higher velocity is to turn to circulation - but the concept is not that intuitive for a non math/engineering/physics person. I suspect that a lot of the fairy tales that pass for explanations of lift (including everything on this list) come from attempts to use Bernoulli while attempting to come up with some "plausible" explanation for the velocity difference.
  • Blowing over a sheet of paper demonstrates Bernoulli's principle.
If the paper lifts, then energy from the jet of bad breath was transferred out of the stream to the paper - this violates the conservation of energy that is the basis of Bernoulli's equation. Also, given that the air around the paper is at a lower static pressure than the air inside your mouth then the total pressure around the paper and in the jet are different, so without a bunch of measurement and math, you can't say that the pressure in the jet is lower than the ambient pressure. This does, however, demonstrate that air has viscosity and that people don't understand Bernoulli's principle.
  • Air bouncing off the bottom of an airfoil creates "Newtonian" lift.
Air is a fluid, it does not behave like billiard balls. The air actually flows parallel to the surface and starts to turn before it gets to the surface. If the streamlines "bounced" off the bottom, then you would have streamlines crisscrossing and flowing in multiple directions at the same place and time.
  • Pressure does not explain all of lift.
The very definition of pressure is the normal force (perpendicular) exerted by a fluid on a surface. The force tangent (parallel) to the surface is drag. There is no "transfer of momentum" that does not involve pressure.
  • A wing works like a venturi.
There are more things wrong with this than you can shake a stick at. The idea is that there is "streamline squeezing" that results in a faster velocity / lower pressure on top of the wing and something else is happening on the bottom. Now, it is true that flow past an obstruction will cause the air to accelerate resulting in a lower pressure - but the effect of thickness changes the pressure on both the top and bottom of the wing. Also, were this true, if you put a airplane/bird/helicopter/multicopter in a box sitting on a scale, the weight of the box and device would be reduced as the device flies. I have done this experiment - the weight does not change on liftoff. And, beyond that, consider what happens when you put the flaps down - you are squeezing the streamlines under the flaps which should result in low pressure and a reduction of lift. And, the closer you get to the runway, the stronger this effect becomes (per the Venturi equation) and flaps would cause your airplane to be sucked into the ground an wrecked. I'm pretty sure that doesn't happen. Also, consider your balsa glider - at a positive angle of attack the streamline are squeezed underneath which should result in negative lift so a balsa glider would work exactly opposite to a "real" wing - 100% wrong. I could go on with more examples.

When real life and a "theory" predict the opposite things, one has to suspect the "theory".
 
You must log in or register to reply here.
Back
Top