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

[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.