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

[Phrak]

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.

 

[cjl]

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.

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.

 

[K^2]

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.

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.

 

[tsimon]

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.

 

[Phrak]

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.

 

[K^2]

I've experienced the opposite explanation, a net force is giving an acceleration.

That's the same thing. Force causes acceleration.

You can compute it either way you want, because mathematically, it's an equivalence relation. If you know force, you know acceleration. If you know acceleration, you know force. But in terms of causality, it's a one way implication.

 

[tsimon]

That's the same thing. Force causes acceleration.

Reading error, my bad.

 

[arydberg]

I think the airfoil shape merely allows increased angle of attack without creating turbulence. I think that the majority of the lift comes from air hitting the bottom of the wing and being deflected downward. Gliders are towered aloft about 200 feet behind a powered planes and while being towed can execute a maneuver called "boxing the wake" where the glider is directly behind the towing plane but about 30 degrees below it. The glider is then subject to turbulence from the air from the wings of the towing aircraft. The wake is so localized that the glider can determine it's top and bottom and left and right sides.

 

[cjl]

The majority of the lift actually comes from the top surface. Interestingly, you are correct that one of the main purposes of the shape of the airfoil is to allow higher angle of attack without stalling, (the lift slope is actually very similar for most airfoils as long as they haven't stalled), but at all normal flight conditions, the reduction in pressure on the top surface is much more significant than the increase on the lower surface. If you look at the pressure coefficient around an airfoil, this becomes apparent. For example, here's a fairly typical plot of Cp around an airfoil: http://adg.stanford.edu/aa241/airfoils/images/AirfoilCp.gif

Note that a Cp of 0 means that the pressure is the same as freestream - effectively, that part of the airfoil is making no lift. Near the leading edge, there is a significant high pressure region on the lower surface, but across most of the airfoil, the lower surface is near 0 Cp, while the upper surface has a strongly negative Cp (much lower pressure than ambient). Interestingly, this is even true with a flat plate at a nonzero angle of attack - the negative Cp on the upper surface will make a much more significant contribution to lift than the positive Cp on the lower surface will.

 

[arydberg]

Ok if you say so but I am in a club that flies a Schweitzer 2-33. When i am boxing the wake are you telling me that the turbulence directed downward from the tow plane does not have a upward component exerted on the tow plane. If it does how do we find the different contributions to lift. 1) lift due to reduced pressure on top of the wing. 2) lift due to air directed downward due to angle of attack.

 

[cjl]

I don't really know what you mean when you say "the turbulence directed downward from the tow plane". As for your two "contributions to lift", you can't really separate it out like that. Air above the wing is directed downwards by the wing just like air below the wing, and it isn't possible to make lift without redirecting air. If you look at the streamlines around an airfoil (http://upload.wikimedia.org/wikipedia/commons/f/f7/Streamlines_relative_to_airfoil.png), you can see that the flow just behind the wing is aimed downwards both above and below the wing. In fact, if you look carefully, you may notice that the region of affected flow is larger above the wing than it is below it.

Basically, no matter how you make lift, it will be visible both as a pressure contribution at the surface of the airfoil, and as a downwash behind the airfoil. Both are completely correct.

 

[rcgldr]

How do we find the different contributions to lift. 1) lift due to reduced pressure on top of the wing. 2) lift due to air directed downward due to angle of attack.

These are not different contributions. Lift can be calculated as the result of pressure differential, or lift can be calculated as the change in momentum of the affected versus time, aerodynamic force = mass of affected air x Δvelocity / Δtime (similar to force = mass x acceleration). The reduced pressure above a wing accelerates the air downwards from further above the wing, while the higher pressure below a wing also accelerates air downwards. The two effects combine to produce the total downwash.

I think there are some very efficient wings that under certain conditions much of the pressure under those wings is also below ambient, but higher than the pressure above those wings, so the end result is still downwash and lift.

 

[cjl]

I think there are some very efficient wings that under certain conditions much of the pressure under those wings is also below ambient, but higher than the pressure above those wings, so the end result is still downwash and lift.

That's true on some supercritical airfoils - they aren't extremely efficient at lower speeds - rather, they are designed for maximum efficiency at high subsonic speeds (so they are very common on passenger jet aircraft). They are kind of a strange shape if you're used to the more standard subsonic airfoils.