Forces on an airfoil I'm building an airplane (Super Baby Great Lakes) and I'm wondering something about airfoils. In particular (this plane is fabric covered), I'm wondering about the lifting forces on the main wings. I've read something about it being very important that the fabric adheres very well on the top of the wing to the ribs so that the fabric doesn't separate when lift is generated.
My question is this: how much lift is generated by direct pressure of the slipstream against the bottom of the wing because of high angle of attack vs. how much "sucking" force is generated due to low pressure on the top of the wing? Is the vacuum on the top of the wing simply a lack of atmospheric pressure, or is it genuinely a sucking force, like a powerful vacuum cleaner which could actually tear the sheet out of a notebook, for example?
Thanks,
Jay
 A: I've never seen actual figures but, in general, articles I've seen about flight state that "most" lift is generated from the angle of attack and relatively little from the Bernoulli effect. I suspect the exact figures are rather variable and probably depend on whether the plane is climbing, descending, banking, etc and will also vary from plane to plane. Maybe this is why exact figures seem not to be quoted.
The pressure difference between the top and bottom of the wing is quite real, though note that on the top of the wing it's not a vacuum as the pressure doesn't decrease that much. The lowered pressure above the wing will indeed tend to pull the skin off the wing, or more precisely the air within the wing that is at normal atmospheric pressure will try to push the skin off. Once again I can't give you exact figures - I must admit I thought ballpark figures would be easy to calculate, but Google has failed me.
Incidentally, there's a good NASA article on this subject at http://www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html and it even includes a Java applet for you to play with the details of the wing. A longer slightly more staid article is at http://www.free-online-private-pilot-ground-school.com/aerodynamics.html
Later:
If an approximate answer would be OK then you could could use Bernoulli's equation as described in http://en.wikipedia.org/wiki/Bernoulli%27s_equation#Incompressible_flow_equation. Although this really only applies to incompressible fluids, and air is obviously compressible, the article suggests it would be a reasonable approximation for low speeds.
Rewriting the equation to make it more useful for our purposes gives:
$$P = \rho A - \rho \frac {v^2}{2} - gh$$
where $A$ is some constant and $h$ is the height. We don't know the constant, but let $P_{bot}$ be the pressure below the wing and $P_{top}$ be the pressure above the wing then we can take the difference between them i.e. the pressure drop between the bottom and top of the wing. If we assume the height is constant i.e. we can ignore the thickness of the wing we get:
$$\Delta P = P_{bot} - P_{top} = 0.5 \rho (v_{top}^2 - v_{bot}^2)$$
I don't know what speed you plane flies at, but let's guess at 30 m/s and let's guess that there's a 10 m/s difference between the air speed at the top and bottom of the wing, so that's $v_{bot} = 30$ and $v_{top}$ = 40. Google gives the density of air at ground level as 1.225 kg/m3.
$$\Delta P = 0.5 \times 1.225 \times (40^2 - 30^2) = 429 Pa$$
429 Pa is 4.29 grams per square cm or 0.06 pounds per square inch, so it's completely insignificant.
A: John Rennie made a pretty good estimate, 0.06 pounds per square inch is 8.6 pounds per square foot.  The Great Lakes Super Baby has a wing loading of 9.6 pounds per square foot at max gross weight.  In the worst case, where all the lift is provided by sucking on the upper surface, the sucking force would therefore be 9.6 lb/sq ft in level flight.  during a 3G maneuver it would be 29 lb / sq ft.  So that is an upper bound.  Your typical vacuum cleaner pulls about 20 kPa suction, or about 400 pounds per sq foot. 
