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

[Capt. Geoffrey Thorpe]

Did you read it?

yes

 

[AlleyCat67]

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.

Thank you for the clarification/correction.

 

[rk911]

magic performed by the Keebler elves on their off time.

 

[Capt. Geoffrey Thorpe]

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.

I would argue that an explanation that violates the laws of physics is not a "simplification" - it's more of a fairy tale.
Yes, the math is complicated if you want to do detailed calculations of pressures and velocities. Way complicated.
But...
The basic concept of air flowing along the top and bottom surfaces which causes a net acceleration of the air downwards requires low pressure on top and a relatively higher pressure underneath to make the air accelerate is pretty darn straightforward and takes less than 10 minutes. Without violating the laws of physics.

Really crappy video that explains this but is shorter than the one previously linked (but the other is better done and involves an actual expert)...

 

[David Megginson]

One of the worst ideas in pilot training was deciding to force student pilots to try to understand Bernoulli. A cambered wing is interesting optimization for aeronautical engineers to discuss, but has zero value for simply learning to fly a plane — better to teach about pitch stability, yaw-roll coupling, etc. if you're going to dive into theory, because at least they have some practical application to flying a plane.

And for the original question, no, of course camber isn't what makes a wing fly. If that were the case, then inverted flight wouldn't be possible.

 

[brcase]

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.

Above is the longer way of saying the wing deflects or throws air downward. The OP lists some details about how it does that but the LIFT is generated as the reaction of throwing the air molecules downward.

Brian

 

[Piperonca]

For fun, google Flettner rotor. Lift without airfoil. Has various nautical applications and could work on aircraft except for safety concerns.

 

[EdFred]

I just checked all the options because I knew they were wrong or incomplete, and I always like to mess up a poll.

 

[Palmpilot]

I understand lift better than I did ten minutes ago!

 

[MBDiagMan]

yes

Then please explain where the Scientific American is incorrect.

 

[wsuffa]

1/3 Bernoulli and 2/3 angle of attack.

I read that as 1/3 bourbon.

Time to go to bed.

 

[Timbeck2]

I read that as 1/3 bourbon.

Time to go to bed.

2 oz bourbon
.5 oz sweet vermouth
.5 oz dry vermouth
dash of bitters
garnish with a maraschino cherry

perfect Manhattan

NOW its time for bed

 

[EdFred]

2 oz bourbon
.5 oz sweet vermouth
.5 oz dry vermouth
dash of bitters
garnish with a maraschino cherry

perfect Manhattan

NOW its time for bed

I like mine with SoCo

 

[Timbeck2]

SoCo is waaaay too sweet for me. I omit the cherry most the time.

Oh, and going to bed for real. 0430 comes early.

 

[TxJim]

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.

My first job at Hughes Aircraft in 1982 as a brand new engineer working the F-14 program, we had coffee cups that said "Engineers: We do precision guesswork."

 

[JimBirk]

Dr. Alexander Lippisch's "The Secret of Flight" short movies do a great job (at least they did for me) of showing and explaining how wings work...thankfully without going into any math. It's a multi-part movie series made available from the University of Iowa and is available on youtube.

I'm guessing some of you have already see the series. They are definitely worth a view if you want get an idea about how wings work. Also, I found it fun to get to see the cool 1950s-ish technology.

 

[TxJim]

• 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 mean this?

 

[Everskyward]

Now explain how a sailboat works. People say the sails work just like a wing on an airplane.

They even say this to non-pilots.

I always found this explanation a little hilarious since you also need to account for the water, the keel, and what point of sail you are on.

 

[PeterNSteinmetz]

I would argue that an explanation that violates the laws of physics is not a "simplification" - it's more of a fairy tale.
Yes, the math is complicated if you want to do detailed calculations of pressures and velocities. Way complicated.
But...
The basic concept of air flowing along the top and bottom surfaces which causes a net acceleration of the air downwards requires low pressure on top and a relatively higher pressure underneath to make the air accelerate is pretty darn straightforward and takes less than 10 minutes.

Agree that such simplifications should agree with the laws of physics.

I think even this video has a few issues. Firstly the embarrassing correction of Newton’s third law to first law, but that is not a big deal.

The bigger issue in my view is his statement that the pressure differences arise from the fact that the air is accelerated (@5:52). The acceleration is caused by the force, which he does describe elsewhere. And the force is simply the integral of the pressure difference over an area, so neither causes the other, they are the same thing. So his statement of causality in that order , acceleration -> force is not technically correct. Rather, pressure difference integrated over area = force -> acceleration and lift.

 

[Southpaw]

Model airplane "pattern ship". Top and bottom of wing symmetrical. Air flow different because of angle of attack?
Huge engine on front providing lots of thrust pulls it .

How does Snoopy's Dog House fly ?

 

[Initial Fix]

Top posting, sorry

@Capt. Geoffrey Thorpe you did a great job explaining this.

• 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".

 

[Capt. Geoffrey Thorpe]

For fun, google Flettner rotor. Lift without airfoil. Has various nautical applications and could work on aircraft except for safety concerns.

Circulation.

 

[Capt. Geoffrey Thorpe]

Now explain how a sailboat works. People say the sails work just like a wing on an airplane.

They even say this to non-pilots.

I always found this explanation a little hilarious since you also need to account for the water, the keel, and what point of sail you are on.

I'm missing something here...

A sail generates lift just like the wing on airplane. Sailing downwind, sails tend to be at least partially stalled and that is why downwind is slow. Trimming a spinnaker you want to keep it eased as much as possible to maintain as much attached flow as possible to maximize "lift".
The keel generates lift just like the wing on an airplane.
The fundamental physics are the same. One just trims the sails to get the maximum forward component of force.

 

[Capt. Geoffrey Thorpe]

Rather, pressure difference integrated over area = force -> acceleration and lift.

And, where does the pressure difference come from?

To accelerate a bit of air around a curve, you have to apply a force. That force is expressed as a pressure when dealing with fluids. So, to "pull" the air over the top of an airfoil, there has to be a low pressure at the surface that results from the fact that the air follows the surface.

 

[Capt. Geoffrey Thorpe]

How does Snoopy's Dog House fly ?

Same as anything else. Laws of physics do not change.

 

[Doug Reid]

Lift is directly proportional to dollars...jets fly faster and higher because they cost more...

 

[luvflyin]

I'm missing something here...

A sail generates lift just like the wing on airplane. Sailing downwind, sails tend to be at least partially stalled and that is why downwind is slow. Trimming a spinnaker you want to keep it eased as much as possible to maintain as much attached flow as possible to maximize "lift".
The keel generates lift just like the wing on an airplane.
The fundamental physics are the same. One just trims the sails to get the maximum forward component of force.

Hmm. In the spinnaker thing, is ‘lift’ really the right word to even be using in the first place? Let’s go airborne with that. Does a parachute generate lift? Canopy parachutes, not those airfoil things. Yeah, they slow your descent down. But I’m having trouble getting my head wrapped around they are generating lift.

 

[bflynn]

• There are two components of lift - 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.

Newton and Bernoulli are derived from the same principle. If A=B, then to say top is A and bottom is B invokes the transitive property, so Top is Newton and Bottom is Newton. Or, if you prefer, Top is Bernoulli, bottom is Bernoulli. They are the same thing.

Either way you name the rose, air moves downward and an equal and opposite force called lift is created.

• 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.

I suppose if you want to get technical, this could be called wrong. Air doesn't "bounce", it is directed downward by the airfoil. To me, that's picking nits. The air is directed downward by the wing, it's an English sematic debate whether or not that is "bouncing"

 

[Capt. Geoffrey Thorpe]

Then please explain where the Scientific American is incorrect.

https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/

OK, re-reading it wasn't quite as bad as I recall. More of a mix of good and bad.

The click-bait title is nonsense.

'What Anderson said, however, is that there is actually no agreement on what generates the aerodynamic force known as lift. “There is no simple one-liner answer to this,”'
The lack of a one liner is not an inability to explain.

Also misleading (at best) is the claim that Newton's laws don't explain the lower pressure on the top of the wing - their illustration of "Newton" shows just the deflection of the flow under the wing.

The problem isn't alack of understanding. The problem is explaining it without getting tangled up in the math and staying consistent with reality.

My guess as to where all this went wrong:
If you go back to the very early 1900's, you have the Kutta–Joukowski theorem which tells us that lift is caused by circulation - the superposition of two flows - the flow past a foil as it moves through the air and a flow that circulates around the airfoil. Where the definition of circulation is, of course, the line integral of the component of the flow velocity along a closed curve. The theorem tells us that the magnitude of lift is proportional to the magnitude of the circulation and that the magnitude of the circulation is determined by the Kutta condition - the velocity of the flow at a sharp trailing edge must be finite. I suspect that this is less than intuitive for a non-technical (math, physics, engineering) person.



Anderson, J.D. "Introduction to Flight: Third edition, page 237

Now, given circulation, you can apply Bernoulli's equation to get the pressure differences between the relatively slower lower surface flow and the faster upper surface flow. Life is good - assuming you can wrap your head around the circulation.

Here is where I think the wheels fell off. Taking the above and trying to simplify it. Bernoulli? Simple - faster is lower pressure. But how to explain the velocities. Hmmmm. 1920's airfoils had curved tops and flat bottoms - go with that... And, yeah, a fairy tale is born. And, over the last 100 years, people have latched onto the idea that Bernoulli explains lift, and there just ain't no undoing that.

edit: Fergot
There is a post WW-I paper by Prandtl that has an illustration where it appears that the flow over the top gets to the trailing edge at the same time as the flow under. The paper does not make that claim, but the paper has some reasonably deep math. It would make sense that the illustration may be a source of the equal transit myth. I will try to find the paper again.

 

[bflynn]

Now explain how a sailboat works.

Same thing. Air is directed aft, producing an equal and opposite force to move the boat forward.

The hull and and keel create resistance against sideways motion.

 

[Capt. Geoffrey Thorpe]

Does a parachute generate lift? Canopy parachutes, not those airfoil things.

Yes. The forward velocity of a steerable round chute causes a bit of lift. If you pull the steering risers to close the vents and "flare" you kill the lift and land hard.

 

[Checkout_my_Six]

the wing is a disturbance.....to the flow.

 

[Capt. Geoffrey Thorpe]

I suppose if you want to get technical, this could be called wrong. Air doesn't "bounce", it is directed downward by the airfoil. To me, that's picking nits. The air is directed downward by the wing, it's an English sematic debate whether or not that is "bouncing"

A flight instructor illustrating "Newtonian lift" by throwing ping pong balls at the bottom of a clip board is more than just a semantic problem.

 

[bflynn]

'What Anderson said, however, is that there is actually no agreement on what generates the aerodynamic force known as lift. “There is no simple one-liner answer to this,”'

I disagree. Air moving downward creates an equal and opposite force upward, which we call lift.

Is that a simple sentence? I think so. Does it require an understanding of basic physics? Of course, but if you don't understand physics, then everything is magical.

 

[bflynn]

A flight instructor illustrating "Newtonian lift" by throwing ping pong balls at the bottom of a clip board is more than just a semantic problem.

I agree air doesn't bounce. Are you ready to fund a visible wind tunnel for every flight instructor?

 

[luvflyin]

Yes. The forward velocity of a steerable round chute causes a bit of lift. If you pull the steering risers to close the vents and "flare" you kill the lift and land hard.

Didn't say nuthin bout steerable.

 

[Capt. Geoffrey Thorpe]

Didn't say nuthin bout steerable.

A pure round 'chute with no vents is not going to generate lift. And, as a result it has to be bigger. And, you ain't gonna find one on the market.

 

[Capt. Geoffrey Thorpe]

I agree air doesn't bounce. Are you ready to fund a visible wind tunnel for every flight instructor?

How about a whiteboard?

 

[Piperonca]

Circulation.

One good Flettner deserves another:

 

[PeterNSteinmetz]

And, where does the pressure difference come from?

To accelerate a bit of air around a curve, you have to apply a force. That force is expressed as a pressure when dealing with fluids. So, to "pull" the air over the top of an airfoil, there has to be a low pressure at the surface that results from the fact that the air follows the surface.

I agree that the pressure distribution or difference is in some sense primary. But in the video he mis-speaks a bit and says the acceleration causes it.

Does one normally explain Newton’s laws by saying that the acceleration of an object causes the the force on it? Or the other way around?

If he had said the wing pushes the air up and around it, I think that would have been better as it is fundamentally the presence of the wing in the fluid that gives rise to the changes in the pressure distribution.

To some extent this is a problem with using words to describe fluid mechanics. Which is why I think it best to always precede such explanations with the note that this is fundamentally a fluid mechanics problem, which is just Newton’s laws applied to fluids, and let’s talk about some simplifications.

Overall I think his presentation in the video is a rather good one, but it would have benefited from writing out in advance the exact words to be used at certain points. That would have avoided the whole 3rd law / 1st law correction. Indeed, it would be very nice if he would revoice the video!