Aerodynamics - why wings create lift - current vs historical discussions

[zanick]

My son and i were discussing aerodynamics and he brought up a paper from https://phys.org/news/2012-01-wings.html It seems that the latest discussions seem to completely discount the differential velocity of air flow as a cause of differential pressure, but point to a differential pressure (pressure gradient) caused by the force acting on the air being accelerated to cause the differential pressure. in other words, some of these papers and i don't know if they are outliers, are insinuating that it is the differential pressure causing the speed variance above and below an airfoil, and not the other way around.
thoughts??

 

[jbriggs444]

I do not think that cause and effect reasoning leads you anywhere useful here. You know that there is differential pressure. You know that there is a relationship with differential velocity. Why is it important that we label one as the cause of the other? In my view, both are caused by the fact that the engines are dragging the craft forward through the free stream, the fact that gravity is pulling the craft downward and the circumstance of the shape of the wing.

 

[russ_watters]

Summary:: What is the real cause of list said to be true by current aerodynamics

My son and i were discussing aerodynamics and he brought up a paper from https://phys.org/news/2012-01-wings.html It seems that the latest discussions seem to completely discount the differential velocity of air flow as a cause of differential pressure, but point to a differential pressure (pressure gradient) caused by the force acting on the air being accelerated to cause the differential pressure. in other words, some of these papers and i don't know if they are outliers, are insinuating that it is the differential pressure causing the speed variance above and below an airfoil, and not the other way around.
thoughts??

"Seem to."

The problem we've encountered here is that some sources, such as the one you linked, overly strongly attack a very narrowly focused misconception and as a result "seem to" take down the entire Bernoulli explanation with it, and this little crusade has done a lot of damage.

The misconception is "the equal transit time" fallacy and if you take it away you are still left with airflow speeding up as it flows over the top of the wing, causing the pressure drop (difference) that is lift.

These sources should be making clear that the Bernoulli's explanation is true/works, and then move on to the separate, next level down issue of why the airflow speeds up.

Edit: I'll also note that conservation of momentum and acceleration also work, and which explanation you use should depend on the context and information available. Conservation of momentum is much more useful for describing helicopters, for example.

Edit2; One also has to be careful with the "air is curved down" explanation because it carries it's own misconceptions.

 

[berkeman]

Summary:: What is the real cause of list said to be true by current aerodynamics

My son and i were discussing aerodynamics and he brought up a paper from https://phys.org/news/2012-01-wings.html

Check out this Insights article to see if it helps...

https://www.physicsforums.com/insights/airplane-wing-work-primer-lift/

 

[zanick]

"Seem to."

The problem we've encountered here is that some sources, such as the one you linked, overly strongly attack a very narrowly focused misconception and as a result "seem to" take down the entire Bernoulli explanation with it, and this little crusade has done a lot of damage.

The misconception is "the equal transit time" fallacy and if you take it away you are still left with airflow speeding up as it flows over the top of the wing, causing the pressure drop (difference) that is lift.

These sources should be making clear that the Bernoulli's explanation is true/works, and then move on to the separate, next level down issue of why the airflow speeds up.

Edit: I'll also note that conservation of momentum and acceleration also work, and which explanation you use should depend on the context and information available. Conservation of momentum is much more useful for describing helicopters, for example.

Edit2; One also has to be careful with the "air is curved down" explanation because it Carrie's it's own misconceptions.

Thanks for your explanation. This the position i usually take as well. as Riggs mentioned above, much of the argument is almost analogous to "the chicken and egg". It seems that when some of these "experts" are talking about medium velocity, there is a disconnect if the medium is not moving and the body is... is one way to look at this by way of a reference point. whether the body is moving through the medium or the medium is moving over the body, it is all about the reference point. the air seems to have a velocity from the reference point of a position on the airplane's surface. those molecules have a velocity , mostly to the rear of the plane, so if the medium has a pressure, its pressure moving past the port on an aircraft (for example, ) would seemed to have to have less pressure due to the conservation of momentum. in other words, if the air has a velocity (or relative velocity), it can't have the same pressure on a surface that it would have on it if the body , or the air , wasnt moving. the angle of impact would be changing ... like trying to punch a moving train, you will make much less of a dent.

 

[Lnewqban]

This video has helped me to see what happens.
It seems to me that most of the magic happens around the leading edge and stagnation point.

 

[rcgldr]

The relationship between pressure and velocity is co-existent, not cause and effect.

From the reference frame of the aircraft, the relative flow is diverted downwards (lift), and somewhat forwards (drag). How this is done depends on the wing, but for a typical wing at sub-sonic speed, it's more efficient to have most of the lift due to the flow tending to follow the upper curved surface to fill in what would otherwise be a void as the wing sweeps out a volume of air. Curvature of the air coexists with a pressure differential perpendicular to the flow, and the lower pressure near the upper surface of a wing coexists with a higher velocity (with respect to the wing).

Using the aircraft or wing as a frame of reference, given the velocity of flows around a wing allows the co-existent pressures to be calculated with reasonable accuracy. Using the air as a frame of reference, Bernoulli is violated, because work is performed on the air by the wing (similar to a propeller). The velocity of the affected air is increased from zero to some non-zero velocity along with a pressure jump (from lower to higher pressure) as the air flows downwards and forwards behind the trailing edge of a wing.

As a simple example, consider a 1500 lb Nimbus 4 glider with a 60 to 1 glide ratio at about 60 mph (horizontal component of velocity). In a no-wind condition, the glider's downward component of velocity is 1 mph (~1.46667 fps), and it's gravitational potential energy decreases at the rate ~ 2200 foot pounds / second == 4 horsepower. All of that energy is going into the air, almost all of it related to lift, with some of it due to drag and thermal energy.

 

[sophiecentaur]

This is a very old chestnut and seems to polarize minds at least as much as Brexit.
There can be no doubt that conservation of momentum must be at work. When a heavy object stays up in the air there must be air leaving downwards to produce the necessary force. The issue is the mechanism that causes this flow. Bernouli is convenient but the assumptions about the conditions are questionable when people want it to be a ‘reason’ for lift.
Evidence from a wind tunnel ignores the details of a downwards air current. The tunnel sits on the floor and is small compared with the region of air that’s disturbed during normal flight

A helicopter (hovering) can fit the momentum model as the downwards air flow is easy to identify but hovering requires more fuel so it’s not so ‘simple’.

Aeronautical Engineers use models of flight that produce good aircraft. They (good ones) don’t need to ask the ‘why’ question. Non- experts seem to be over occupied with that question.

 

[cjl]

Bernouli is convenient but the assumptions about the conditions are questionable when people want it to be a ‘reason’ for lift.

Bernoulli's relation is absolutely correct, and a pressure differential between the lower and upper surface of a wing fully accounts for the lift (and is associated with a velocity difference). Conservation of momentum is also absolutely correct, and the downwash from the wing also fully accounts for the lift. These are not independent, they're just two different ways of looking at the situation.

Now, Bernoulli says absolutely nothing about why the flow over the top surface is faster, and the common "longer path length" explanation is completely bogus, but that doesn't mean that the Bernoulli relation is at all wrong.

 

[Klystron]

JFTR some (subsonic) wind tunnels are specifically designed to test 'downward air flows' and their complements; e.g., ground effects. Case in point: the 12x12' wind tunnel at NASA ARC redesigned 1983.

True, I have seen several models meant to test downward air flow mounted inverted or otherwise contrived to take advantage of the most sensitive strain/stress gauges embedded in the model mounts but also to reduce effects from gravity and, as @sophiecentaur states, to reduce interference from wind tunnel structures.

Thousands of lines of wind tunnel systems code tuned to specific structural geometry help researchers compensate for differences between an aircraft in free flight and scale models mounted within a wind tunnel. Non-professionals might not distinguish flight data from wind tunnel.

 

[sophiecentaur]

@cjl and @Klystron The bigger the wind tunnel the better and, not surprisingly, the downward air flow is there if the tunnel looks for it. I would not, for a second, contest Bernoulli but Momentum Conservation can never ‘not apply’ so the two approaches do not disagree with other. It’s only the disciples of one or the other who disagree.
Engineers are pragmatic and make things work by using the best model. Talk to any RF Antenna expert and they freely admit to some awkward paradoxes in the analysis. Antennae still work predictably tho’.

 

[Master1022]

Summary:: What is the real cause of lift, said to be true by current aerodynamics

Hi, there is a good explanation by Professor Holger Babinsky in this paper he wrote here.

From fluids, we know that the pressure increases radially outwards from the centre of curvature (an intuitive explanation is that the pressure needs to be greater on the outside to cause the net pressure acting radially inwards required for centripetal acceleration). A proof is shown on page 503 of the paper above.

If we look at the deflection of streamlines around a wing (images on pages 501 and 498 in paper above) and using the fact that pressure increases with radius, we can first look at the top and recongnise that it must be at a higher pressure than atmospheric pressure (which the pressure assumed for the air above the wing) as the top of the wing is closer to the centre of curvature. Thus $p_{top} < p_{atm}$. If we now look at the bottom and compare it to the streamlines below the wing, we can see that it must be at a higher pressure than atmospheric pressure $p_{bottom} > p_{atm}$. Thus, we have $p_{bottom} < p_{top}$, which provides the pressure difference which causes lift.

I hope this made some sort of sense.

 

[Swamp Thing]

Aeronautical Engineers use models of flight that produce good aircraft. They (good ones) don’t need to ask the ‘why’ question.

Shut up and calculate, eh?

 

[sophiecentaur]

Shut up and calculate, eh?

Straw Man here. The idea that you need to know the ‘fundamental truths’ in order to produce worthwhile work is preposterous.
You’d have to confirm that they actually exist in the first place.

 

[A.T.]

Shut up and calculate, eh?

Not in the same sense as for quantum mechanics. But for any system, where A affects B, while B also affects A, tracing back the fundamental reason why A changes, is pointless.

 

[Master1022]

Shouldn't that be lower pressure at the upper surface (boundary layer), due to upper surface being closer to center of curvature? ... and also it should be pbottom > ptop .

Yes sorry, I just put the sign the wrong way around

 

[rcgldr]

Shouldn't that be lower pressure at the upper surface (boundary layer), due to upper surface being closer to center of curvature? ... and also it should be pbottom > ptop .

Yes sorry, I just put the sign the wrong way around

You can edit your prior post if you want.

 

[FactChecker]

Bernoulli's relation is absolutely correct, and a pressure differential between the lower and upper surface of a wing fully accounts for the lift (and is associated with a velocity difference). Conservation of momentum is also absolutely correct, and the downwash from the wing also fully accounts for the lift. These are not independent, they're just two different ways of looking at the situation.

Now, Bernoulli says absolutely nothing about why the flow over the top surface is faster, and the common "longer path length" explanation is completely bogus, but that doesn't mean that the Bernoulli relation is at all wrong.

Bernoulli is absolutely correct but does not apply well here (CORRECTION: It applies but it leaves a lot unexplained). Any simple explanation is either terribly incomplete or incorrect.
This series of NASA pages discuss the logic from Newton's 3'rd law (https://www.grc.nasa.gov/www/k-12/airplane/Newton3.html ) to flow around a spinning ball (https://www.grc.nasa.gov/www/k-12/airplane/bball.html ) to the lift of a spinning ball (https://www.grc.nasa.gov/www/k-12/airplane/beach.html ). It is all closely related to the Coanda Effect (I think).

Wikipedia has a good summary of the shortfalls of simplified explanations. The same article explains some of the CFD techniques.

You will see many intuitively simple explanations of lift. They are all wrong or at least incomplete. The real explanation of the lift force is unsatisfyingly complicated.

If small packets of air are traced along so that they satisfy all the physics equations (a very complicated process of computational fluid dynamics (CFD)), they end up with a net downward motion after the wing has passed. They also travel faster over the top and reach the trailing edge sooner than the packets beneath the wing. The net downward motion of the air is an action for which the equal and opposite reaction is a lift force on the wing.

 

[russ_watters]

Using the air as a frame of reference, Bernoulli is violated, because work is performed on the air by the wing (similar to a propeller). The velocity of the affected air is increased from zero to some non-zero velocity along with a pressure jump (from lower to higher pressure) as the air flows downwards and forwards behind the trailing edge of a wing.

Strictly speaking it is true that Bernoulli's equation does not deal with energy input, but if the broader context is omitted, that statement can be misleading.

Bernoulli's equation is a slightly modified conservation of energy statement and like any conservation of energy statement, it of course can deal just fine with input or output energy, just by adding a term for it.

Specifically, Bernoulli's equation is in terms of pressure, which is also energy per unit volume. Multiply through by volume, and now it's energy. Then you simply insert a term for the energy gain/loss. Then you can apply it as needed. Again, strictly speaking it's not Bernoulli's equation anymore, but it would still be recognizable as being just a couple of easy steps removed.

Point being, I don't want to give the erroneous impression that Bernoulli's principle/equation is not applicable to airfoils. The net effect for airfoils is that velocity and static pressure still have the same relation per Bernoulli's principle/equation, but the change in velocity over the wing is less than it would be if there was no drag.

 

[russ_watters]

When a heavy object stays up in the air there must be air leaving downwards to produce the necessary force. The issue is the mechanism that causes this flow.

This view always troubled me because if it were exactly true, it would mean that as airplanes flew, they would eventually force the atmosphere down into a liquid ocean on the surface of the Earth. And that's not a joke; the air has to end up back where it started.

But the main reason I don't prefer the conservation of momentum model for wings is that the flow-field can't be clearly defined:

• It is infinitely tall so doesn't have a definable cross sectional area.
• Its velocity profile isn't uniform.

Please don't misunderstand; it's not wrong, it's just difficult to use.

It works great for helicopters (or fans), though, because oftentimes you don't look at the blades of the rotor, you just look at the disk, and the air flowing across it. It's an exact area and volume, and a uniform velocity, so it is easy to use for calculations.

Now, Bernoulli says absolutely nothing about why the flow over the top surface is faster...

It's also worth noting that the conservation of momentum model also doesn't say why the flow is bent downward, and oddly people don't seem to get bent out of shape over this limitation or its obvious associated misconception:

Often the first time people think about lift is when they stick their hand out the window of a car and make it fly. The simple explanation: the air hits the bottom of your hand and is deflected downward. Heck, you can FEEL it. Momentum transfer! But it is a lot harder to deal with the fact that the air moving over the top of the wing is much more important to lift than the air bouncing off the bottom. The reasons for the air accelerating and curving down over the top of the wing present the same problems/complications for Bernoulli and Newton.

 

[FactChecker]

Point being, I don't want to give the erroneous impression that Bernoulli's principle/equation is not applicable to airfoils. The net effect for airfoils is that velocity and static pressure still have the same relation per Bernoulli's principle/equation, but the change in velocity over the wing is less than it would be if there was no drag.

It applies, but there is very little that it explains. There are very important changes of airflow and streamlines around a wing that are not explained just by Bernoulli.

 

[russ_watters]

It applies, but there is very little that it explains. There are very important changes of airflow and streamlines around a wing that are not explained just by Bernoulli.

Agreed... Just like they aren't explained for the conservation of momentum model. The reason why flows change as they interact with the wing isn't explained in either model. Indeed, the conservation of momentum model doesn't address the airflow over the wing at all. It only tells you what the airflow looks like (in a qualitative way) after it passes the wing.

We kick around cause and effect of velocity vs pressure change for Bernoulli, but the conservation of momentum model only avoids that by having an effect with no cause at all!

 

[russ_watters]

JFTR some (subsonic) wind tunnels are specifically designed to test 'downward air flows' and their complements; e.g., ground effects. Case in point: the 12x12' wind tunnel at NASA ARC redesigned 1983.

True, I have seen several models meant to test downward air flow mounted inverted or otherwise contrived to take advantage of the most sensitive strain/stress gauges embedded in the model mounts but also to reduce effects from gravity and, as @sophiecentaur states, to reduce interference from wind tunnel structures.

Do you have any examples of either of those that you could share? I'm having trouble picturing how that would work. I've seen drag measured using a vertical array of horizontal pito-static tubes just behind a wing to measure the change in speed of the air. Can they do the same for lift by rotating the tubes 90 degrees? Without them interfering with each other?

 

[Klystron]

During my tenure at Ames (1984-1995) wind tunnel experiments using the Standardized Wind Tunnel System (SWTS) software often collected air flow data from surfaces using an array of pressure sensor pitot valves called scanivalves. I found this eponymous company and from a 1973 PDF instruction set:

Pressure tap - A small orifice on a model surface at which apressure is measured. Pressures are generally measured using a "port" on a "scanivalve".

Scanivalves designed compactly to reduce interference, provide ports, ducts and sensors in a tidy package, usually with the ability to compare multiple ports for error correction. Similar embedded sensors provide control data for wind tunnel operations and to correct for the tunnel architecture on simulated flight data, as previously discussed. Scale models often had small holes connected by flexible plastic tubes to the scanivalves in turn wired to the front-end-processors (FEP).

I cut my teeth at NASA improving scanivalve and related real-time data collection software. We presented scientists a broad selection of data collection measurements while configuring the experiment and then dynamically during operations in near-time.

Sophisticated error detection and correction code kept data flowing during crucial and expensive operation windows; re-configuring around failure points in nearly-real-time. Wind tunnel startup required direct contact with utilities and configuring the grid for the enormous loads. So, time was precious.

I helped port SWTS to the Army 7x10' wind tunneL: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160007399.pdf

This PDF file describes typical structure interference corrections: https://www.nasa.gov/sites/default/files/643655main_Wall_Interference_Overview.pdf

This NASA Ames website lists these documents and links to images including model supports. Late here now. I will try to locate the data sheets for the model mounts and sensors soon. Need to remember the specialized jargon.

 

[sophiecentaur]

I am confused by the Bernouli adherents’ total attention to the effect on the Plane by the air flow and the curves. The pressures on the wing work in all directions. I know it’s a lot harder to predict the overall flow of the atmosphere (and the losses due to viscosity). But the net downward momentum will meet up with the ground.

 

[A.T.]

Bernoulli's equation is a slightly modified conservation of energy statement ...

To me, conservation of energy is just as abstract as conservation of momentum. I don't see how invoking one "explains" more than the other.

 

[A.T.]

What is the real cause of lift, said to be true by current aerodynamics

Aerodynamics is there to predict how much lift there will be. If you want just to know why, you can use probability:

If a body is symmetrical and aligned with the flow, there will be zero lift by symmetry.

If a body is not symmetrical or not aligned with the flow, there is an infinite number of possible values for lift. So the probability for exactly zero lift is zero.

 

[FactChecker]

Aerodynamics is there to predict how much lift there will be. If you want just to know why, you can use probability:

If a body is symmetrical and aligned with the flow, there will be zero lift by symmetry.

If a body is not symmetrical or not aligned with the flow, there is an infinite number of possible values for lift. So the probability for exactly zero lift is zero.

I don't think this is answering the real question. People are not asking why there is a non-zero force; they are really asking why there is a sustained, significant lift force of such large magnitude that it can lift very heavy objects. Your argument might only say that there are fluctuating, negligible forces in random directions.

 

[A.T.]

... they are really asking why there is a sustained, significant lift force of such large magnitude ...

"Large" is rather subjective. I am always disappointed in the amount of lift my arms create, no matter how fast I run.

 

[FactChecker]

"Large" is rather subjective. I am always disappointed in the amount of lift my arms create, no matter how fast I run.

Ok, a less subjective question -- Why is there a predictable, sustained lift force large enough to lift a 400 ton Boeing 747 airplane? (Max takeoff weight nearly 500 tons)

 

[russ_watters]

I am confused by the Bernouli adherents’ total attention to the effect on the Plane by the air flow and the curves. The pressures on the wing work in all directions.

That sounds wrong the way you said it: pressure at a surface acts perpendicular to the surface. So as long as you know the shape and orientation of the wing and where you put the pressure taps, you can easily calculate the forces by measuring the pressures. It's a common practice in wind tunnel testing.

 

[russ_watters]

During my tenure at Ames (1984-1995) wind tunnel experiments using the Standardized Wind Tunnel System (SWTS) software often collected air flow data from surfaces using an array of pressure sensor pitot valves called scanivalves. I found this eponymous company and from a 1973 PDF instruction set:

Pressure tap - A small orifice on a model surface at which apressure is measured. Pressures are generally measured using a "port" on a "scanivalve".

Pitot or static? I'm familiar with an array of static ports on a wing surface, and that's a distinctly "Bernoulli" approach. I suppose measuring the velocity at the surface would be as well, but I don't see how that would work, because it changes rapidly as distance from the surface changes (and at the surface itself should be zero due to friction in the boundary layer). Previously you mentioned measurement of the downward airflow - for Newton - and that's what I was asking about.

 

[russ_watters]

To me, conservation of energy is just as abstract as conservation of momentum. I don't see how invoking one "explains" more than the other.

I agree. My point (and the OP's confusion/impression over the attacks on Bernoulli) is that Bernoulli and Newton are both equally correct. Both describe "what", and neither explain "why". But depending on the situation or approach, either can be more useful. The concern the OP brought to us - which I have seen many times on PF and around the web - is the erroneous impression that Bernoulli's principle is of little or no use at all.

 

[A.T.]

Why is there a predictable, sustained lift force large enough to lift a 400 ton Boeing 747 airplane?

Why lift is predictable? That's more of a philosophical question. Why should anything be predictable?

Why it has the magnitude it has? That's again a quantitative question about how much not about why. Run a CFD stimulation, look at the results, that's your "why".

 

[sophiecentaur]

Doesn’t the word “principle” imply the status of Bernoulli’s calculations? It doesn’t appear to claim to be a Law. But the OP seems to want a ‘why’ reply and that is always difficult when it’s just a mechanism.