How Does Wing Shape Affect Airplane Lift and Pressure Differences?

[member 529879]

I'm confused about how an airplane generates lift. If I'm correct the wing of the plane is bent so air can flow over the top of the wing faster than the bottom of the wing, the faster fluids somehow apply less pressure resulting in a net upward force. Why though does a bent wing allow air to flow faster over the top, and why do fast moving fluids apply a lesser pressure than slow ones?

 

[phinds]

There have been approximately 6,420 threads on this forum discussing it, so I suggest a forum search.

 

[rootone]

The simplest way to describe it is that wing's angle of attack forces air downward, so (Newton's third law) - reaction of the wing is to be forced up.
This also causes 'drag', so the aerofoil is contoured to minimise the element of drag.
The details of it can get get surprisingly complex, but that's the basic idea.
You COULD have a complete flat wing, but the induced drag would make it very inefficient

 

[russ_watters]

Why though does a bent wing allow air to flow faster over the top , and why do fast moving fluids apply a lesser pressure than slow ones?

Both of those are largely related to the Venturi effect (just a one-sided venturi):
http://en.wikipedia.org/wiki/Venturi_effect

 

[russ_watters]

The simplest way to describe it is that wing's angle of attack forces air downward, so (Newton's third law) - reaction of the wing is to be forced up.

While that's basically true it can be enough of a simplification that people lose sight of the fact that the top surface is a bigger contributor to the lift than the bottom surface. And in this case, the OP was asking about what's going on with the top surface anyway.

 

[boneh3ad]

Both of those are largely related to the Venturi effect (just a one-sided venturi):
http://en.wikipedia.org/wiki/Venturi_effect

This is in no way similar to a so-called "one-sided" Venturi effect. The Venturi effect describes a system with finite boundaries. The flow over a wing has effectively infinite boundaries. The two situations are not the same.

 

[boneh3ad]

While that's basically true it can be enough of a simplification that people lose sight of the fact that the top surface is a bigger contributor to the lift than the bottom surface. And in this case, the OP was asking about what's going on with the top surface anyway.

This is at best misleading and at worst untrue. You can't decouple the top and bottom surfaces. Without one, the flow over the other does not develop the same way. Also, the pressure on a wing's surface is meaningless without the pressure on the other side of the wing since the net force only comes from factoring in both. If you only consider the top surface, you will get negative lift, so there's no way the upper surface can be the "bigger contributor".

 

[russ_watters]

This is in no way similar to a so-called "one-sized" Venturi effect. The Venturi effect describes a system with finite boundaries. The flow over a wing has effectively infinite boundaries. The two situations are not the same.

The air provides its own boundary in low-speed flow when it is essentially incompressible. The velocity increase/pressure drop happens because it is being "squeezed", just like in a venturi. This is similar to the principle behind an aerospike engine, where air pressure helps constrain the flow:
http://en.wikipedia.org/wiki/Aerospike_engine

This is at best misleading and at worst untrue. You can't decouple the top and bottom surfaces. Without one, the flow over the other does not develop the same way.

I'm aware they can't be completely decoupled. Nevertheless, it is typical for the top surface to produce more of the lift and the OP was asking about the top surface, whereas the other answer implied it was about the bottom. The reality is that for most people what is going on with the bottom surface is intuitively obvious but what is going on with the top surface is not.

Also, the pressure on a wing's surface is meaningless without the pressure on the other side of the wing since the net force only comes from factoring in both. If you only consider the top surface, you will get negative lift, so there's no way the upper surface can be the "bigger contributor".

That isn't true. Both the top and bottom surface have pressure profiles that are measured/expressed as gauge pressure because the default is atmospheric pressure. As a result, the pressure on the top surface is measured to be negative. For a simplified/idealized example, a flat-bottom airfoil with the bottom parallel to the airflow would essentially just have atmospheric pressure below it and all of the lift generated by the top surface. Here's a sample graph of a pressure profile (not a flat bottom but still showing more of the lift derived from the top surface):

http://www.wfis.uni.lodz.pl/edu/Proposal/image093.gif

 

[boneh3ad]

The air provides its own boundary in low-speed flow when it is essentially incompressible. The velocity increase/pressure drop happens because it is being "squeezed", just like in a venturi. This is similar to the principle behind an aerospike engine, where air pressure helps constrain the flow:
http://en.wikipedia.org/wiki/Aerospike_engine

The fact that it is incompressible makes it less likely to "provide its own boundary," which does not accurately reflect what actually occurs. The velocity increase happens because he design of the trailing edge sets a certain separation point and the conservation laws then require the velocity to increase greatly over the upper surface to maintain physical equilibrium. For an explanation of why the Venturi approach is wrong, see this link:
https://www.grc.nasa.gov/www/k-12/airplane/wrong3.html

Also, that is not how an aerospike engine works. Aerospikes are, first and foremost, compressible flow devices and the Venturi effect is irrelevant to the phenomenon. The constraint by the outer atmosphere here (and importantly, variable constraint) is due to some of the unique features of compressible flows.

I'm aware they can't be completely decoupled. Nevertheless, it is typical for the top surface to produce more of the lift

So which is it? You can believe both things.

The shape of the top certainly contributes to lift. You cannot measure lift an just the top surface, though. The top definitely plays a role, but it does so as a part of the whole shape.

That isn't true. Both the top and bottom surface have pressure profiles that are measured/expressed as gauge pressure because the default is atmospheric pressure. As a result, the pressure on the top surface is measured to be negative. For a simplified/idealized example, a flat-bottom airfoil with the bottom parallel to the airflow would essentially just have atmospheric pressure below it and all of the lift generated by the top surface. Here's a sample graph of a pressure profile (not a flat bottom but still showing more of the lift derived from the top surface):

http://www.wfis.uni.lodz.pl/edu/Proposal/image093.gif

First, I know what gauge pressure is. In fact, I typically would give zero points if a student uses gauge pressure where absolute pressure is required because it's that important to keep straight.

Second, plotted there is the pressure coefficient, the definition of which includes a numerator that is equivalent to gauge pressure, so in that, you are correct. That is where it ends. Just because the pressure coefficient is negative does not mean there is somehow negative pressure on the upper surface. The force on a given surface due to pressure is always dependent on the absolute pressure, not gauge pressure, and absolute pressure is always positive. The pressure on the upper surface is always downward. A more negative pressure coefficient just means there is less downward force on the top to counteract the larger pressure on the bottom.

Third, a flat-bottomed airfoil would not necessarily have atmospheric pressure on its underside. This would generally only be true if the leading and train edges were sharp in a way such that the bottom approximated a flat plate, otherwise the curvature of the leading edge would accelerate the flow.

Even if the bottom is atmospheric, the total lift force is still the integrated sum of the absolute pressure on the bottom minus that on the top. The force due to the pressure on the top is still downward. The importance of the upper surface contour is therefore effectively to make the force on the upper surface less downward for a given upward force on the bottom. This is what I mean by being unable to decouple the two.

 

[A.T.]

the top surface is a bigger contributor to the lift than the bottom surface.

If you push a body with 20N from one side and 15N from the opposite side, which side "is a bigger contributor" to the net force on the body?

 

[A.T.]

Both the top and bottom surface have pressure profiles that are measured/expressed as gauge pressure .

That is just a convention. You can express them relative to any reference pressure you want. But the forces on the two surfaces are unambiguous and a function of absolute pressure.

 

[A.T.]

The force due to the pressure on the top is still downward. The importance of the upper surface contour is therefore effectively to make the force on the upper surface less downward for a given upward force on the bottom. This is what I mean by being unable to decouple the two.

Also visualized here:

 

[rcgldr]

Also visualized here ... (youtube video with basic description of lift)

From the video at about 1:10 into the video, pressure is reduced on the top "partially because it's shielded by the wing" as the wing travels forwards through the air. So the air fills in what would otherwise be a void by following the upper surface of a wing, or in the case of a stall, with turbulent flow consisting of vortices or mostly one very large vortice.

A wing doesn't have to be curved. Flat wings and/or symmetrical wings are reasonably efficient on small model gliders, from the dime store type models to the larger ones like this one with nearly symmetrical airfoil.

http://www.4p8.com/eric.brasseur/glider2.html

Aerobatic aircraft, both full scale and models, use symmetrical airfoils.

 

[A.T.]

A wing doesn't have to be curved.

True, and I don't think anyone implied otherwise.

 

[rcgldr]

... Why though does a bent wing allow air to flow faster over the top ...

A wing doesn't have to be curved.

True, and I don't think anyone implied otherwise.

From the original post, "bent wings", which I thought he meant curved (as opposed to dihedral).

 

[stedwards]

You're all wrong. Lift is due to the bound vortex, in superposition with the airstream, induced be viscous drag. :P

Am I making this up? Curved foils, or a planar foil at a positive angle of attack will not induce lift in an inviscid fluid. Invent any shape you wish. Immerse it in an inviscid stream. It will not produce a component of force perpendicular to the stream.

 

[A.T.]

You're all wrong.

I don't think that Newtons Laws are wrong. The momentum transferred by the air molecules hitting the bottom side must be greater than by those hitting the top side. The "why?" of this fact can be explained and discussed (almost endlessly) in various ways. Here are discussion of the point you make:
http://physics.stackexchange.com/questions/46131/does-a-wing-in-a-potential-flow-have-lift

 

[Delta2]

The lift can be explained either by using bernoulli's principle (which principle explains why in a fluid like air, in regions of higher velocity the pressure is lower) or using Newton's laws.

According to wikipedia the explanations are equivalent https://en.wikipedia.org/wiki/Bernoulli's_principle#Misunderstandings_about_the_generation_of_lift.

Bernoulli's principle can be derived from Newton's 2nd law. https://en.wikipedia.org/wiki/Bernoulli's_principle#Derivations_of_Bernoulli_equation

However i prefer bernoulli's principle explanation because it is not so crystal clear how Newtons law aplly in fluid's particles that flow, we are used to apply Newton's laws in point particles or rigid bodies.

As to why the velocity becomes higher in the upper surface of the wing it is because the air has to travel bigger distance in the same time.

 

[A.T.]

the air has to travel bigger distance in the same time.

Why?

 

[Delta2]

Why?

It is basically because of the equation of continuity and because we assume incompressible flow.
In speed's well below the speed of sound the flow of air is incompressible.

 

[russ_watters]

It is basically because of the equation of continuity and because we assume incompressible flow.
In speed's well below the speed of sound the flow of air is incompressible.

While Bernoulli's equation is correctly applied (change in speed of the airstream creates a pressure change), the equal transit time idea is not correct. The different paths result in different times reaching the trailing edge (counterintuitively, air from the top surface reaches the trailing edge first). Continuity applies along a single streamline, not when comparing two different streamlines and in order for the wing to do anything, the air behind it can't look the same as the air in front of it, otherwise nothing happened when the wing passed.

 

[HuskyNamedNala]

You're all wrong. Lift is due to the bound vortex, in superposition with the airstream, induced be viscous drag. :P

Am I making this up? Curved foils, or a planar foil at a positive angle of attack will not induce lift in an inviscid fluid. Invent any shape you wish. Immerse it in an inviscid stream. It will not produce a component of force perpendicular to the stream.

The lift can be explained either by using bernoulli's principle (which principle explains why in a fluid like air, in regions of higher velocity the pressure is lower) or using Newton's laws.

According to wikipedia the explanations are equivalent https://en.wikipedia.org/wiki/Bernoulli's_principle#Misunderstandings_about_the_generation_of_lift.

Bernoulli's principle can be derived from Newton's 2nd law. https://en.wikipedia.org/wiki/Bernoulli's_principle#Derivations_of_Bernoulli_equation

However i prefer bernoulli's principle explanation because it is not so crystal clear how Newtons law aplly in fluid's particles that flow, we are used to apply Newton's laws in point particles or rigid bodies.

As to why the velocity becomes higher in the upper surface of the wing it is because the air has to travel bigger distance in the same time.

"Curved" means cambered.

While Bernoulli's equation is correctly applied (change in speed of the airstream creates a pressure change), the equal transit time idea is not correct. The different paths result in different times reaching the trailing edge (counterintuitively, air from the top surface reaches the trailing edge first). Continuity applies along a single streamline, not when comparing two different streamlines and in order for the wing to do anything, the air behind it can't look the same as the air in front of it, otherwise nothing happened when the wing passed.

1) Inviscid fluids don't exist in reality. What you are describing is potential flow, who's solution to a complicated shape like an airfoil is the superposition of sources and sinks. With that said, you can superimpose a source solution with a vortex solution or simply use a doublet, solve a system of equations (for the source and vortex strengths) such that the flow is tangent to the airfoil. This will give you a pretty good approximation of the lift generated prior to significant separation. Further, you can couple this solution with a code that resolves the boundary layer to get some pretty accurate "rough" estimations of airfoil performance. I think it would be a real interesting experiment to try to generate (or halt) turbulence in a superfluid, since this is as lose as we can get to an inviscid flow.

2) The "equal transit time" theory is crap. I don't know who came up with it or why it is even considered valid. Get it out of your head.

3) I personally don't like the "Bernoulli principle" explanation of lift. For one, it doesn't really answer anything with detail (what about airfoil shape, what about delta wings, how do slotted flaps work, etc). Bernoulli's principle is only valid for inviscid flow and does not take into account irreversible losses due to viscosity, but it is a good explanation for low angles of attack.

Here is a good example: how do delta wings generate lift? The Bernoulli principle would state that the air is accelerated over the top of the wing relative to the bottom of the wing. Well, that is partially true. The air is accelerated but it behaves nothing like a straight wing. On a delta wing the air separates near the leading edge and develops a strong vortex over the upper surface. The vortex maintains a low pressure region which is responsible for phenomenal gains in lift at moderate and high angles of attack. This is why the concord needed to land at almost a 45 degree angle - at low angles of attack and low speeds the delta wing does not maintain a vortex over the upper surface and thus will not generate enough lift.

Here is my favorite example: the dragonfly wing. At low Reynolds numbers (low velocities with a small wing, all else being equal) the dragonfly wing (which looks like a crushed piece of cardboard) has superior performance to a smooth wing that you might see on an airliner or drone! Why is this? Well it turns out that the grooves on the dragonfly wing force the smooth air (we call this laminar flow) over the wing to become chaotic and generate turbulent rotational motion. The rotational motion (small vortices not entirely unlike those on the delta wing) gets "trapped" in the grooves, maintaining low pressure on the upper surface, thus creating lift.

Here is a paper with pretty pictures. The jargon gets pretty technical so don't worry about reading it if you don't have a solid background in fluids
http://www.aere.iastate.edu/~huhui/paper/journal/2008-JA-Corrugation.pdf

If you want to explain lift you have to take into account these and other odd cases.

To tell you the truth, I can't think of a very simple explanation for why an airfoil generates lift. I want to say that the geometry of the airfoil develops a pressure gradient across the wing via acceleration and deceleration of the flow. Depending on your geometry you can have mild accelerations which can be explained roughly by Bernoulli's principle, or you can have significant changes in the flow field such as the development of a vortex structure which also lead to the development of a pressure gradient across the wing. Thin airfoil theory states lift is proportional to (and can be calculated from) circulation, which is basically a measure of the rotation of the fluid around the airfoil.

 

[A.T.]

the equal transit time idea is not correct

As demonstrated here at 0:15:

 

[russ_watters]

2) The "equal transit time" theory is crap. I don't know who came up with it or why it is even considered valid. Get it out of your head.

I don't know who first came-up with the idea, but it sticks around because it makes intuitive sense to some people and there is a void of explanation of the issue. The idea is basically that the air must look the same after the wing passes as before, so air going over the top must meet the air going over the bottom. As another easy consequence, all of the extra speed comes from adding a Y-component of speed to the air. But once you recognize that the air behind the wing can't look like the air in front of it (otherwise the wing and air didn't exchange any momentum), it becomes easier to discard it.

3) I personally don't like the "Bernoulli principle" explanation of lift.

Well, like it or not it is an essential component of lift both in principle and in practice. But it seems like what you actually don't like is that it isn't 100% of the picture. But there's certainly nothing wrong with needing more than one physical principle, simultaneously, to explain a phenomena.

Moreover, when first explaining a concept, you can't just jump right in with the most complicated example, you have to start with the simple/basic/typical examples and build from there. So:

If you want to explain lift you have to take into account these and other odd cases.

No you don't, at least not unless you need for some reason to write a complete history/course on aerodynamics. The OP asked about lift on an airplane and there is no need to bring up the completely separate issue of how insects fly when describing how a basic airplane flies. What you describe is akin to someone asking a simple question about Newtonian gravity and instead answering it by bringing in General Relativity.

To tell you the truth, I can't think of a very simple explanation for why an airfoil generates lift. I want to say that the geometry of the airfoil develops a pressure gradient across the wing via acceleration and deceleration of the flow.

That isn't quite right. Differences in velocity have to be accompanied by accelerations that made them happen, but it is the velocity difference, not the (across the airfloil) acceleration itself that generates the lift.

Otherwise, personally I don't see why such a simple explanation isn't good enough for someone's first introduction to lift. It is highly applicable to most every-day situations where one encounters (non-living-thing) lift.

What I think people do a bad job of, though, is explaining how/why the shape of the airfoil causes the speed change in the air. That was exactly the question the OP asked and no one else actually attempted to answer it. That void is what let's the "equal transit time" fallacy creep in.

 

[insightful]

I found this really interesting:

http://en.wikipedia.org/wiki/User:J_Doug_McLean/sandbox#Momentum_balance_in_lifting_flows

Pull quote:

"When a ground plane is present, there is a pattern of higher-than-ambient pressure on the ground below an airplane in flight, as shown on the right[109] For steady, level flight, the integrated pressure force associated with this pattern is equal to the total aerodynamic lift of the airplane and to the airplane's weight."

I assume this is true for any altitude, i.e., not just low-level (ground effect) flight.

 

[HuskyNamedNala]

Yes, pressure ultimately drives lift. Another way to compute the lift on an airfoil is to sum the variation of pressure along the surface and multiply by the chord.

 

[russ_watters]

Mod note: Argumentative posts and responses deleted. If you didn't receive a warning, your posts were fine. Sorry for the lost effort.

 

[russ_watters]

A couple of instructive quotes from my copy of Anderson's "Introduction to Flight":

"No matter how complex the flow field, and no matter how complex the shape of the body, the only way nature has of communicating an aerodynamic force to a solid object or surface is through the pressure and shear stress distributions which exist on the surface. These are the basic funamental sources of all aerodynamic forces." [italics by author]

"...as emphasized in the previous chapters, the funamental source of lift is the pressure distribution over the wing surface...
An alternate explaantion is sometimes given: The wing deflects the airflow downward...From Newton's third law, the equal and opposite reaction produces a lift. However, this explanation really involves the effect of lift, and not the cause."

"A third argument, called the circulation theory of lift,, is sometimes given for the source of lift. However, this turns out to be not so much an explanation of the lift per se, but rather more of a mathematical formulation for the calculation of the lift..."

It also notes that the definition of lift coefficient includes the dynamic pressure term from Bernoulli's equation.

It also discusses the different "types" of pressure, an argument often had on PF:
"When an engineer or scientist uses the word "pressure," it always means static pressure unless otherwise identified...
A second type of pressure is commonly utilized in aerodynamics, namely, total pressure." Later: "dynamic" pressure (I usually say "velocity" pressure).

Unfortunately, it does not discuss the mechanism for the airflow speeding-up along the top surface of the airfoil.

 

[russ_watters]

Ah, found it. It's hidden in the discussion of continuity. The word/name "Venturi" never appears in the book, but continuity is discussed using a thought experiment concept called "stream tubes". Stream tubes are the missing link between the Venturi tube and the flow over the airfoil.

A stream tube is an imaginary collection of streamlines, that form a tube. A Venturi tube is a stream tube, but you can also grab a bundle of streamlines flowing over a wing as a stream tube. The flow through the stream tube follows the same area/velocity ratio as the Venturi. And (from a source more descriptive than Anderson:

These streamlines form a tube that is impermeable since the walls of the tube are made up of streamlines, and there can be no flow normal to a streamline (by definition). This tube is called a streamtube. From mass conservation, we see that for a steady, one-dimensional flow, the mass-flow rate is constant along a streamtube. In a constant density flow, therefore, the cross-sectional area of the streamtube gives information on the local velocity...

As an example, consider steady, constant density flow over a cylinder...

In this region the streamlines come closer together, and the area between them decreases. Since the density is constant, the velocity must increase according to the principle of mass conservation. For constant density flow, wherever the area between streamlines decreases, the velocity increases. This is exactly similar to what happens with constant-density flow through a duct or a pipe --- if the area decreases, the velocity increases so that the volume flow rate is maintained constant (volume flow rate out must equal volume flow rate in by continuity). [emphasis added]

http://www.princeton.edu/~asmits/Bicycle_web/streamline.html

So: the velocity on the top surface increased because the streamlines got closer together. Why did the streamlines get closer together? Because they encountered an object that deflected them away from their previous direction of motion, but the streamlines further above didn't move out of the way enough to maintain the spacing. And this is "exactly similar" to what happens in real tube such as a Venturi tube.

 

[boneh3ad]

So: the velocity on the top surface increased because the streamlines got closer together. Why did the streamlines get closer together? Because they encountered an object that deflected them away from their previous direction of motion, but the streamlines further above didn't move out of the way enough to maintain the spacing. And this is "exactly similar" to what happens in real tube such as a Venturi tube.

Almost. The difference is that with a variable-area duct, you are enforcing the outer boundaries of the streamtube. With a cylinder or a wing or other object with air moving over it, only one side of that streamtube is enforced, so it is not a perfect analogue. What Smits means in the page you cite is that given a set of streamlines over an object, you can note that as they pass over the object they get closer together, so using those streamlines, you can treat them using the same basic rules as a variable-area duct. You can't determine the actual streamlines by drawing an arbitrary box around the flow area and treating it like a streamtube, though, since there is nothing enforcing that upper boundary as a streamline.