Lift from airfoils, decrease lift as a result of curved shape at large angles?

In summary, at large angles of attack, the curved shape of an airfoil decreases the lift by reducing the dominating dynamic pressure under and increasing it over the airfoil. This is due to the laminar streamlines detaching from the upper side of the airfoil and creating turbulent flow with low pressure centers above the airfoil. However, the net effect is still an increase in lift due to the air being pulled down over the top surface creating more lift than the air being pushed down under the bottom. This is only possible up to a certain angle of attack before stalling occurs.
  • #1
tsimon
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Hi,
in my point of view lift is created as a result of two pressures, the static and the dynamic.

The dynamic pressure is the dominating creator of lift at large angles of attack. For example a flat plane at 45 degrees will create lift.

The static pressure is the dominating creator of lift at small angles of attack. For example the curved airfoil at 0 degrees can produce lift by increasing speed over the airfoil and reducing it under. Increased speed increases the dynamic pressure and lowers the static (as a result of Bernoullis law) but as the static pressure is dominating it results in lift.

But what happends at large angles of attack? is the curved shape decreasing the lift as it decreasing the dominating dynamic pressure under and increases it over the airfoil?

Cheers, Simon.
 
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  • #2
Dynamic pressure isn't actually a pressure, but the potential for pressure. It generates no lift. The two terms entering Bernoulli's equation could just as well have been called the static and dynamic energy density, as Bernoulli's equation is actually an energy balance equation.
 
  • #3
Phrak said:
Dynamic pressure isn't actually a pressure, but the potential for pressure. It generates no lift. The two terms entering Bernoulli's equation could just as well have been called the static and dynamic energy density, as Bernoulli's equation is actually an energy balance equation.

Okay I see.
So the explanation for the lift made by a flat plane at 45 degrees angle is rather explained by that the fluid "collides" into the plane?

Let me rephrase my question then:
But what happends at large angles of attack? is the curved shape decreasing the lift as it decreasing velocity and therefore decreases the lift caused by fluid "colliding" with the airfoil?
 
  • #4
...and unless an airfoil is stalled (and many would be at 45 degrees), the air being pulled down over the top surface creates more lift than the air being pushed down under the bottom.
 
  • #5
What happens at larger angles is fairly dramatic:
The laminar streamlines de-attach from the upper side of the air foil, and in the space between the is filled with turbulent fluid, i.e, lots of vortices. These vortices have their centres of low pressure AWAY from the airfoil (rather than AT it, as when the streamlines are attached). This cannott be modeled by, say, Bernoulli simplifications, since viscous (and dissipative) effects become highly appreciable.

The net effect of all this is that there is a dramatic RISE of pressure on the upper side of the airfoil, thus destroying the airfoil's lift-generating capacity.
 
  • #6
russ_watters said:
...and unless an airfoil is stalled (and many would be at 45 degrees), the air being pulled down over the top surface creates more lift than the air being pushed down under the bottom.

Why is the pressure on the top surface dropping? I don't really understand that, please explain! :)
 
  • #7
tsimon said:
Okay I see.
So the explanation for the lift made by a flat plane at 45 degrees angle is rather explained by that the fluid "collides" into the plane?

I don't think a thin flat plate rounded at each end can lift without stalling at more than about 7 degrees. Stalling means the air does not flow smoothly across the top surface but breaks-away in an unsteady flow.

You can certainly get lift at greater than 7 degrees, and 45 degrees as well, but increasingly, this is only due to the force exerted on the bottom of the plate with an accompanying increase in drag. As Russ was pointing out, both surfaces of a wing contribute to lift, with the majority due to the top surface.

A well designed symmetrical airfoil--that means the same shape top and bottom, will by comparison obtain about 16 degrees angle of attach before stalling.

Let me rephrase my question then:
But what happends at large angles of attack? is the curved shape decreasing the lift as it decreasing velocity and therefore decreases the lift caused by fluid "colliding" with the airfoil?

I'm not sure what you mean. Did the above answer this?
 
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  • #8
tsimon said:
But what happends at large angles of attack?
A wing at an angle of attack introduces what would be a void on it's upper surface if the air didn't fill in the space aft of the upper surface of a wing as it passes through a volume of air. If the angle of attack is low enough, the air fills in that void with relatively smooth flow (somewhat turbulent is ok), and is diverted downwards, resulting in reduced pressure above the wing, and contributing to lift. If the angle of attack is too large, then very turbulent flow occurs, and the end result can be a large vortice above the wing that moves at the same speed as the wing, increasing drag and decreasing lift.

I'm not aware of wings that operate at 45 degrees, but delta type wings can operate with turbulent flow in the form of numerours small vortices that result in downwards and outwards flow above the wing to generate lift at lower speeds and high angles of attack around 20 degrees. There's a lot of drag, but the wing isn't "stalled".

http://en.wikipedia.org/wiki/Delta_wing
 
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  • #9
If we consider lift is made by two "effects":

1. Angle of attack & Speed

2. Difference in static pressure

The curved shape will do following: Increase speed at top and reduce under the airfoil.

IMO this should give following contribution to the lift:
A=Difference in static pressure gives a positive contribution
B=Higher speed over the airfoil and same angle of attack should give a positive contribution
C=Lower speed under the airfoil and same angle of attack should give a negative contributionIs it possible that A+B+C is negative? If so, I guess it would be at "large" angles of attack were the 1st effect is contributing more ("Angle&speed effect").
 
  • #10
tsimon said:
If we consider lift is made by two "effects":

1. Angle of attack & Speed

2. Difference in static pressure

The curved shape will do following: Increase speed at top and reduce under the airfoil.

IMO this should give following contribution to the lift:
A=Difference in static pressure gives a positive contribution
B=Higher speed over the airfoil and same angle of attack should give a positive contribution
C=Lower speed under the airfoil and same angle of attack should give a negative contribution


Is it possible that A+B+C is negative? If so, I guess it would be at "large" angles of attack were the 1st effect is contributing more ("Angle&speed effect").

Lower speed under the airfoil increases the static pressure under the airfoil and this gives a POSITIVE contribution to lift. If the pressure on the upper surface did not change at all from the atmospheric pressure but the pressure on the lower surface increased relative to atmospheric pressure you would still get a net force that creates lift. This lift would be significantly less than normal because as has been mentioned before more of the lift is due to the decreased pressure on the top.
 
  • #11
RandomGuy88 said:
Lower speed under the airfoil increases the static pressure under the airfoil and this gives a POSITIVE contribution to lift. If the pressure on the upper surface did not change at all from the atmospheric pressure but the pressure on the lower surface increased relative to atmospheric pressure you would still get a net force that creates lift. This lift would be significantly less than normal because as has been mentioned before more of the lift is due to the decreased pressure on the top.

Thank you for your answer.

Altough I've already written that the change in speed results in a difference in static pressure which results in a positive contribution to lift (I called this "A"),
 
  • #12
tsimon said:
Thank you for your answer.

Altough I've already written that the change in speed results in a difference in static pressure which results in a positive contribution to lift (I called this "A"),

Well that is what causes lift. So what exactly is your question?
 
  • #13
RandomGuy88 said:
Well that is what causes lift. So what exactly is your question?

tsimon said:
If we consider lift is made by two "effects":

1. Angle of attack & Speed

2. Difference in static pressure

The curved shape will do following: Increase speed at top and reduce under the airfoil.

IMO this should give following contribution to the lift:
A=Difference in static pressure gives a positive contribution
B=Higher speed over the airfoil and same angle of attack should give a positive contribution
C=Lower speed under the airfoil and same angle of attack should give a negative contribution


Is it possible that A+B+C is negative? If so, I guess it would be at "large" angles of attack were the 1st effect is contributing more ("Angle&speed effect").

My question in bold font.
 
  • #14
tsimon said:
IMO this should give following contribution to the lift:
A=Difference in static pressure gives a positive contribution
B=Higher speed over the airfoil and same angle of attack should give a positive contribution
C=Lower speed under the airfoil and same angle of attack should give a negative contribution


Difference in velocity over the different surfaces causes the difference in static pressure. A results from B and C.
 
  • #15
RandomGuy88 said:
Difference in velocity over the different surfaces causes the difference in static pressure. A results from B and C.

A curved airfoil at a positive angle of attack gets lift from two effects:
1. Angle of attack & speed results in a high pressure beneath and a low pressure above.
An example of this is a flat plane at an positive angle of attack will produce lift even if it is not curved.

2. The curved shape increases the velocity above and decreases it under the airfoil which results in a difference in static pressure. This means that a curved profile produces lift even at 0 degrees angle of attack.

BUT! A curved airfoil at a positive ange of attack (NOT 0 degrees) will get lift from both of those "effects".

If we just takes a look at the surface under the airfoil:
As the curved shape reduces the speed under the airfoil it increases (or rather creates) the second effect but this should ALSO reduce the 1st effect. Do you understand what I mean? Sorry for my poor english.
 
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  • #16
tsimon said:
2. The curved shape increases the velocity above and decreases it under the airfoil which results in a difference in static pressure. This means that a curved profile produces lift even at 0 degrees angle of attack.

Wing sections with a generally concave surface are not too useful, and it's difficult to see what you mean. If you really want to know what happens, look for pressure profiles vs. angle of attack data with flaps deployed. The deployed flaps should give you an approximation of a convex bottom surface.

For a typical cambered wing section, an increase in angle attach will increase the overall pressure on the bottom surface.
 
  • #17
Phrak said:
Wing sections with a generally concave surface are not too useful, and it's difficult to see what you mean. If you really want to know what happens, look for pressure profiles vs. angle of attack data with flaps deployed. The deployed flaps should give you an approximation of a convex bottom surface.

For a typical cambered wing section, an increase in angle attach will increase the overall pressure on the bottom surface.

When I say curved I mean a profile that is not flat, let's say a naca 2312.
 
  • #18
At 0 angle of attack the curvature of a non-symmetrical airfoil causes the velocity to be higher on the upper surface and lower on the bottom surface. Increasing the angle of attack increases the velocity on the upper surface even more. So curvature and angle of attack have very similar results in terms of lift.
 
  • #19
Using the air as a frame of reference, as a wing passes through the air, it disturbs the air, and as the affected air's pressure returns to ambient, it ends up with a non-zero "exit" velocity, mostly downwards and somewhat forwards. This "exit" velocity represents the work done on the air, which is due to a non-Bernoulli like interaction between wing and air, although it's contribution to lift is small compared to the Bernoulli related (no work done) effects, depending on the lift to drag ratio of a wing.

Thin airfoils such as classic hang gliders and early aircraft are cambered, with the bottom surface following the same curve as the upper surface, and they can work well at low speeds.

Getting back to the original post, regardless of the airfoil, if the angle of attack is excessive, the wing becomes "stalled", and instead of air flow being diverted downwards above a wing, a signficant amount of turbulent flow in the form of vortices moves at the same speed as the wing (above the wing), the wing ends up diverting a lot of air forwards as opposed to downwards, generating much less lift and a lot more drag
 
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  • #20
rcgldr said:
Using the air as a frame of reference, as a wing passes through the air, it disturbs the air, and as the affected air's pressure returns to ambient, it ends up with a non-zero "exit" velocity, mostly downwards and somewhat forwards. This "exit" velocity represents the work done on the air, which is due to a non-Bernoulli like interaction between wing and air, although it's contribution to lift is small compared to the Bernoulli related (no work done) effects, depending on the lift to drag ratio of a wing.

Thin airfoils such as classic hang gliders and early aircraft are cambered, with the bottom surface following the same curve as the upper surface, and they can work well at low speeds.

Getting back to the original post, regardless of the airfoil, if the angle of attack is excessive, the wing becomes "stalled", and instead of air flow being diverted downwards above a wing, a signficant amount of turbulent flow in the form of vortices moves at the same speed as the wing (above the wing), the wing ends up diverting a lot of air forwards as opposed to downwards, generating much less lift and a lot more drag

Great post, thank you.

Another question:You wrote that the Bernoulli effect doesn't make any work, why?
I can see that on the upper side, but not under the airfoil where static pressure is rising? That should make s
 
  • #21
tsimon said:
You wrote that the Bernoulli effect doesn't make any work, why?
By definition. The Bernoulli equation is based on the premise that there is no external work done to the affected fluid or gas, except for the case where a gravitational term is included (density x height x acceleration_of_gravity).

I can see that on the upper side, but not under the airfoil where static pressure is rising?
The issue is the direction of the flow. If the wing were oriented vertically, then it would just move the air forwards, all drag, no lift. At 45 degrees, the lift to drag ratio could be around 1 to 1 or even less.
 
  • #22
rcgldr said:
By definition. The Bernoulli equation is based on the premise that there is no external work done to the affected fluid or gas, except for the case where a gravitational term is included (density x height x acceleration_of_gravity).

The issue is the direction of the flow. If the wing were oriented vertically, then it would just move the air forwards, all drag, no lift. At 45 degrees, the lift to drag ratio could be around 1 to 1 or even less.

I don't really understand.

A airfoil can produce lift at 0 degrees of attack right by the "Bernoulli effect"?
If so, then there must possible for it to create work which is made on both the air and the aifoil itself?
 
  • #23
tsimon said:
A airfoil can produce lift at 0 degrees of attack?
Only because angle of attack is normally defined based on the leading and trailing edges and not the average angle of the chord / camber line (a line that would bisect the air foil from front to back). I find it less confusing to use "effective angle of attack", which is defined to be zero when lift is zero.

lift at 0 degrees due to Bernoulli effect
There is no "magical" way to produce lift. Lift, pressure differential above and below a wing, and downwash are all coexistant, you can't have one without the other two, except in the case of ground effects where the downwash is converted into higher pressure horizontal flows below the wing (there can also be a ceiling effect if a plane was flying close to the roof of a large tunnel). Wings aren't 100% efficient, so there's also drag involved.

don't understand
It's not clear to me if you're having an issue with Bernoulli equation based on the premise that no external work is done or something else I mentioned. One example of why a wing performs work on the air is to note that it's surfaces are not parallel to the direction of travel (with respect to the air), and there's a component of distance along these surfaces perpendicular to the direction of travel, and this perpendicular component of distance times the force the wing applies to the air is part of the work done to the air related to lift. Bernoulli is based on the premise no external work is done, so it doesn't cover the component of lift related to the work performed on the air. It's similar to induced drag (the drag associated with the production of lift), but induced drag has a different definition (plus there are variations in how induced drag is defined). When you get into the details, the mathematical model gets really complicated (Navier Stokes equations), and there is no simple explanation.
 
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  • #24
tsimon said:
A curved airfoil at a positive angle of attack gets lift from two effects:
1. Angle of attack & speed results in a high pressure beneath and a low pressure above.
An example of this is a flat plane at an positive angle of attack will produce lift even if it is not curved.

2. The curved shape increases the velocity above and decreases it under the airfoil which results in a difference in static pressure. This means that a curved profile produces lift even at 0 degrees angle of attack.

These two are not independent. You seem to be artificially trying to separate them, when in reality, 1 results from 2. Making the potential flow assumption (no viscosity), the pressure at any point along an airfoil is directly related to the flow velocity at that point. If you know the total pressure of the airflow (which is determined by upstream conditions), and either the velocity or the pressure at some point on the airfoil, you can determine the other property (velocity or pressure). A flat plate at a positive angle of attack will in fact have a slower flow on the bottom side and a faster flow around the top, which causes a higher pressure on the bottom and a lower pressure on top.

tsimon said:
BUT! A curved airfoil at a positive ange of attack (NOT 0 degrees) will get lift from both of those "effects".
As I said, it's really just one effect. A curved airfoil gets lift because the flow over the top is substantially accelerated, which lowers its pressure. Simultaneously, the flow beneath is somewhat slowed, increasing its pressure.

tsimon said:
If we just takes a look at the surface under the airfoil:
As the curved shape reduces the speed under the airfoil it increases (or rather creates) the second effect but this should ALSO reduce the 1st effect. Do you understand what I mean? Sorry for my poor english.

Since the effects are the same, you can't increase one and not the other. As the airspeed beneath the airfoil decreases, the pressure increases.

Here's a picture of the streamlines around a flat plate at a positive angle of attack. Since the flow is assumed incompressible, and from the definition of a streamline, you can estimate the velocity in the flow field by looking at the distance between streamlines. Where the streamlines are close together, the flow speed is fast, while where the streamlines are far apart, the flow speed is slow. You can see that above the plate, the flow speed is very fast, and thus the pressure is low, while below the plate, the flow speed is slow and the pressure is high.

http://www.arvelgentry.com/images/plate2.jpg
 
  • #25

Thank you, great contribution!

Lets see if I got this:
The lift is created by high pressure under and low pressure above the airfoil.

If we take a look at what happends under the aifoil:
The airspeed is reduced and redirected downwards, might be explained out of conservaty of mass(as in http://en.wikipedia.org/wiki/Lift_(force [Broken])) or simply put as the air collides into the surface. The pressure that creates this acceleration is the same thing as the rise in static pressure Bernoullis equation shows because of the change in speed.
 
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  • #26
tsimon said:
The airspeed is reduced
If the wing is used as a frame of reference, the reduction of air speed corresponds to drag. The downwards acceleration of air corresponding to lift corresponds to an increase of air speed.

If the air is used as a frame of reference, then intially it is not moving, and after a wing passes through and after the affected air's pressure returns to ambient, the air is moving downwards (lift) and a bit fowards (drag) corrresponding to the work performed by the wing onto the air.

Although airfoil design helps to make a wing more efficient, the main method used to minimize the work done on the air (to minimize energy consumed) is to make the wingspan longer so more air is affected. Lift is relative to change in momentum, m v, while work done is relative to the change in "exit" velocity, 1/2 m v2. If you double the wing span, and accelerate double the air at 1/2 the rate of acceleration, the change in momentum of the air remains the same, but the energy added to the air is 1/2 of the shorter wing case.

The pressure that creates this acceleration is the same thing as the rise in static pressure Bernoullis equation shows because of the change in speed.
It starts to get confusing when you attempt to define cause and effect. The cause of lift is a wing with an effective angle of attack moving through the air. The effect is the amount of lift and drag created, which are coexistant with acceleration of air, pressure differentials, turbulence (vortices have higher kinetic energy and lower pressure). What determines the amount of the effects are the size and shape of the wing, the wings angle of attack, the wing's speed, and the qualities of the air, density and viscosity.
 
  • #27
rcgldr said:
If the wing is used as a frame of reference, the reduction of air speed corresponds to drag. The downwards acceleration of air corresponding to lift corresponds to an increase of air speed.
Not exactly. This is true if you look only at the wing's wake (after the wing has fully passed), but the air passing by a wing can slow down without causing any drag (due to associated speedups elsewhere). An extreme example of this is the potential flow assumption I mentioned earlier. If you make the assumption that the flow is inviscid, you will still get regions in which the air has slowed down (relative to the wing) or sped up (relative to the wing), but the net drag force will always be zero.

rcgldr said:
If the air is used as a frame of reference, then intially it is not moving, and after a wing passes through and after the affected air's pressure returns to ambient, the air is moving downwards (lift) and a bit fowards (drag) corrresponding to the work performed by the wing onto the air.
True enough, although the problem is usually looked at in the wing's frame of reference. In the air's frame, the problem is an unsteady transient problem, while in the wing's frame, the flow is steady. This corresponds to the wake of the wing though. Locally around the wing, the flow can be doing a wide variety of things that are not necessarily reflected in the wake.

rcgldr said:
Although airfoil design helps to make a wing more efficient, the main method used to minimize the work done on the air (to minimize energy consumed) is to make the wingspan longer so more air is affected. Lift is relative to change in momentum, m v, while work done is relative to the change in "exit" velocity, 1/2 m v2. If you double the wing span, and accelerate double the air at 1/2 the rate of acceleration, the change in momentum of the air remains the same, but the energy added to the air is 1/2 of the shorter wing case.
That's not really why a longer span is more efficient. All other things equal (airspeed, chord, airfoil shape, etc), a longer span wing will give more lift per unit span than a shorter span wing. This is because one of the main sources of drag and losses of lift in flight is the vortex formed at the tip of the wing. The longer the wing, the less significant this vortex becomes, since it is affecting less and less of the overall wing. The parts near the center of a very long wing can be operating close to the idealized 2-d case, while for a short wing, everywhere is affected by the tip vortices.

rcgldr said:
It starts to get confusing when you attempt to define cause and effect. The cause of lift is a wing with an effective angle of attack moving through the air. The effect is the amount of lift and drag created, which are coexistant with acceleration of air, pressure differentials, turbulence (vortices have higher kinetic energy and lower pressure). What determines the amount of the effects are the size and shape of the wing, the wings angle of attack, the wing's speed, and the qualities of the air, density and viscosity.

This part I agree with though.
 
  • #28
rcgldr said:
Although airfoil design helps to make a wing more efficient, the main method used to minimize the work done on the air (to minimize energy consumed) is to make the wingspan longer so more air is affected. Lift is relative to change in momentum, m v, while work done is relative to the change in "exit" velocity, 1/2 m v2. If you double the wing span, and accelerate double the air at 1/2 the rate of acceleration, the change in momentum of the air remains the same, but the energy added to the air is 1/2 of the shorter wing case.

cjl said:
That's not really why a longer span is more efficient. All other things equal (airspeed, chord, airfoil shape, etc), a longer span wing will give more lift per unit span than a shorter span wing. This is because one of the main sources of drag and losses of lift in flight is the vortex formed at the tip of the wing. The longer the wing, the less significant this vortex becomes, since it is affecting less and less of the overall wing. The parts near the center of a very long wing can be operating close to the idealized 2-d case, while for a short wing, everywhere is affected by the tip vortices.

I find rcgldr's answers, for the most part, agreeable, and find nothing at fault in the above.

However, you seem to be referring to edge effects and cross flow that become dominant in small aspect ratio wings.

We can eliminate trailing vortices and see what happens by imagining a wind tunnel experiment where a sample section spans the width of the tunnel. This has an effective aspect ratio of infinity.

We can do this thing two different ways: Hold the airstream velocity constant and increase the angle attack in the linear region, or hold the angle of attack constant and increase the stream velocity.

In either case the overall delta z momentum imparted to the airstream is equal to the lift.

[tex]F_z=\frac{dp_z}{dt}[/tex]

The energy required to impart this delta p is the square of of the velocity. So in either case, doubling the angle of attack, or doubling the stream velocity imparts four times as much kinetic energy to the air stream to double the lift.

Now, in my mind, the way to examine this problem is to keep the wing area constant and vary the aspect ratio. The trailing vortices are the result of imparting downward momentum to the airstream, and well, the conservation of vorticity ideally true for nonviscous fluids and good enough as conserved well outside the local environs of the wing. The energy and momentum that rcgldr speaks of must show up in the vortices and overall downward change in velocity.
 
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  • #29
tsimon said:
2. The curved shape increases the velocity above and decreases it under the airfoil which results in a difference in static pressure.
It's really the other way around. The pressure differential is the reason for accelerated flow. You shouldn't be trying to use increased velocity above the cambered wing to explain the pressure drop.

Besides, how would you explain the increased velocity of the flow above the wing in the first place? Keep in mind that equal transit time hypothesis is invalid. Under equal-transit, the circulation around the wing is zero.* That means momentum transferred to air is zero, and there is no lift. (Kutta-Joukowski Theorem)

* If you cannot see that, picture a rectangular circulation contour around the wing. If average time at the top and bottom edges are the same, than so are the average velocities, canceling their contributions to circulation.
 
  • #30
Phrak said:
The energy required to impart this delta p is the square of of the velocity. So in either case, doubling the angle of attack, or doubling the stream velocity imparts four times as much kinetic energy to the air stream to double the lift.
Well, something is definitely incorrect here. Lift is proportional to lift coefficient (which, in the linear region, is linearly proportional to angle of attack), so doubling the angle of attack does indeed double the lift. However, lift is also proportional to dynamic pressure, which is proportional to stream velocity squared. So, doubling stream velocity does not in fact double the lift, as stated here - it quadruples it (all else equal).

Phrak said:
Now, in my mind, the way to examine this problem is to keep the wing area constant and vary the aspect ratio. The trailing vortices are the result of imparting downward momentum to the airstream, and well, the conservation of vorticity ideally true for nonviscous fluids and good enough as conserved well outside the local environs of the wing. The energy and momentum that rcgldr speaks of must show up in the vortices and overall downward change in velocity.

That is indeed one way to look at it. In my mind, it complicates the flow somewhat though, since it involves changing both the span and the chord. However, if you look at it like this, you will find that for a given angle of attack, airfoil cross section, and wing area, the trailing vortices decrease in strength as the aspect ratio is increased, and the lift from the wing slightly increases (due to the increase in efficiency).

I prefer to think about it in terms of a constant chord wing, with a changing span. This does change the area, but I then try to think about it in terms of quantities per unit span, which effectively normalizes the quantities so they can be compared.
 
  • #31
K^2 said:
It's really the other way around. The pressure differential is the reason for accelerated flow. You shouldn't be trying to use increased velocity above the cambered wing to explain the pressure drop.

If you are really interested in cause and effect, the initial cause is viscous drag imparting an imposed circulation around the wing. I can't prove this--just something I read as a concluding statement This should satisfy the chicken or the egg problem. I don't think there is any further cause/effect problem to consider, but that difference in pressure and velocity are a self re-enforcing effect through interference of the downstream air on the upstream air.
 
  • #32
K^2 said:
It's really the other way around. The pressure differential is the reason for accelerated flow. You shouldn't be trying to use increased velocity above the cambered wing to explain the pressure drop.

The two are very closely related, honestly, and you can look at it either way.

K^2 said:
Besides, how would you explain the increased velocity of the flow above the wing in the first place? Keep in mind that equal transit time hypothesis is invalid. Under equal-transit, the circulation around the wing is zero.* That means momentum transferred to air is zero, and there is no lift. (Kutta-Joukowski Theorem)
The kutta condition (imposed by viscosity at the trailing edge) is a decent way to explain the higher velocity on the upper surface. That having been said, you're right that the equal transit time assumption is both very wrong and does not adequately explain the lift on a wing.
 
  • #33
Kutta Condition doesn't explain higher velocity above wing. It predicts a separation layer, which allows for the upper velocity to be higher or lower.
The two are very closely related, honestly, and you can look at it either way.
Is force causing acceleration, or acceleration causing force? Mathematically, they are equivalent, of course. From perspective of modern physics as well, perhaps. But from perspective of classical mechanics, force is the cause of acceleration, so the pressure difference is the cause of the faster flow. Not that it really matters in aerodynamics.
Phrak said:
If you are really interested in cause and effect, the initial cause is viscous drag imparting an imposed circulation around the wing.
Viscosity is just one of the requirements. You would not have lift with zero viscosity, of course. But you also need the continuity, the gas law, the specific internal energy, and so on. It's not anyone of these things. You need all of them to explain why the flow above a cambered wing or wing with positive angle of attack accelerates and generates circulation.
 
  • #34
K^2 said:
Is force causing acceleration, or acceleration causing force? Mathematically, they are equivalent, of course. From perspective of modern physics as well, perhaps. But from perspective of classical mechanics, force is the cause of acceleration, so the pressure difference is the cause of the faster flow. Not that it really matters in aerodynamics.

I've experienced the opposite explanation, a net force is giving an acceleration. Maybe that is a simplified explanation, I'm not a physicist but an engineer.

Or can this be cultural differences? I'f from Sweden.
 
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  • #35
cjl said:
Well, something is definitely incorrect here. Lift is proportional to lift coefficient (which, in the linear region, is linearly proportional to angle of attack), so doubling the angle of attack does indeed double the lift. However, lift is also proportional to dynamic pressure, which is proportional to stream velocity squared. So, doubling stream velocity does not in fact double the lift, as stated here - it quadruples it (all else equal).

Hmm. Right, I should have the stream velocity increase by a factor of 1.414, but it doesn't change anything. The force on the wing then doubles, the imparted momentum doubles and the energy loss increases by a factor of four.
 
<h2>1. What is lift from airfoils and how does it work?</h2><p>Lift from airfoils refers to the upward force that is generated when air flows over a curved surface, such as the wing of an aircraft. This is due to the difference in air pressure above and below the curved surface, with higher pressure below the wing and lower pressure above it. This pressure difference creates a lifting force that allows the aircraft to stay airborne.</p><h2>2. Why does lift decrease as the angle of the airfoil increases?</h2><p>As the angle of the airfoil increases, the curved shape of the wing becomes more pronounced. This results in a larger surface area for the air to flow over, which increases the pressure difference between the top and bottom of the wing. However, at a certain point, the angle becomes too steep and the air can no longer flow smoothly over the wing. This disrupts the pressure difference and decreases the lift force.</p><h2>3. Can the decrease in lift at large angles be compensated for?</h2><p>Yes, the decrease in lift at large angles can be compensated for by using flaps or other devices to change the shape of the airfoil. By altering the shape, the air can flow more smoothly over the wing even at high angles, allowing for a higher lift force to be generated.</p><h2>4. How does the shape of an airfoil affect its lift performance?</h2><p>The shape of an airfoil plays a crucial role in its lift performance. A curved shape, also known as a cambered airfoil, is more efficient at generating lift compared to a flat airfoil. This is because the curved shape creates a longer path for the air to travel over the top of the wing, resulting in a larger pressure difference and a higher lift force.</p><h2>5. Are there any other factors that can affect lift from airfoils?</h2><p>Yes, there are several other factors that can affect lift from airfoils, such as air density, airspeed, and wing size. Higher air density and airspeed can increase the lift force, while a larger wing size can also generate more lift. Additionally, the shape and design of the wing, including the angle of attack and the presence of other aerodynamic features, can also impact the lift performance of an airfoil.</p>

1. What is lift from airfoils and how does it work?

Lift from airfoils refers to the upward force that is generated when air flows over a curved surface, such as the wing of an aircraft. This is due to the difference in air pressure above and below the curved surface, with higher pressure below the wing and lower pressure above it. This pressure difference creates a lifting force that allows the aircraft to stay airborne.

2. Why does lift decrease as the angle of the airfoil increases?

As the angle of the airfoil increases, the curved shape of the wing becomes more pronounced. This results in a larger surface area for the air to flow over, which increases the pressure difference between the top and bottom of the wing. However, at a certain point, the angle becomes too steep and the air can no longer flow smoothly over the wing. This disrupts the pressure difference and decreases the lift force.

3. Can the decrease in lift at large angles be compensated for?

Yes, the decrease in lift at large angles can be compensated for by using flaps or other devices to change the shape of the airfoil. By altering the shape, the air can flow more smoothly over the wing even at high angles, allowing for a higher lift force to be generated.

4. How does the shape of an airfoil affect its lift performance?

The shape of an airfoil plays a crucial role in its lift performance. A curved shape, also known as a cambered airfoil, is more efficient at generating lift compared to a flat airfoil. This is because the curved shape creates a longer path for the air to travel over the top of the wing, resulting in a larger pressure difference and a higher lift force.

5. Are there any other factors that can affect lift from airfoils?

Yes, there are several other factors that can affect lift from airfoils, such as air density, airspeed, and wing size. Higher air density and airspeed can increase the lift force, while a larger wing size can also generate more lift. Additionally, the shape and design of the wing, including the angle of attack and the presence of other aerodynamic features, can also impact the lift performance of an airfoil.

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