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Airfoil stall

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Jun. 5th, 2009 | 06:52 pm

I rarely discuss airplanes and fixed-wing things but this is related to the Air France crash and bar-room questions I've answered so now it's getting written up :)

The word aerofoil (also spelled "airfoil") refers to a surface which "foils" the flow of air. Foils are typically found in the form of "spoilers" which disturb the flow of air, thereby causing the air not to adhere to a surface (and therefore reducing its contact with that surface) or wings which have a larger surface area on one side of the travel plane, so that the smaller-surface side gets a high pressure and generates lift.

Spoilers are used in performance street cars to disturb the flow of air over the top of the car. This causes a lack of adhesion (or lift) on the top, while the air flowing underneath the car -- and usually with some ground-effect skirting -- causes adhesion and hence downforce is exerted sticking the car to the pavement, which is really useful in turns and braking.

Wings are also used in formula race cars, but are more prominently known for their effect in keeping planes in the air. Wings operate on Bernoulli's Principle, which says that air which enters a restricted area will reduce in pressure and accelerate. The wing typically has a flatter surface (the bottom) and a curved surface (the top). Air must pass along both sides, but in having to pass a larger area on top, it accelerates (to cross the larger surface area in the same amount of time the rest of the air is passing the smaller surface area below) and in accelerating its pressure is reduced and lift is created.

Lift is a function of three things, one of which relates to the fluid characteristics of the air itself (gas composition, compression ability, viscosity, etc.) but since we're only talking about air we can narrow the factors to the other two: The difference in area between the two sides of the aerofoil, and the angle at which it impacts the air ("angle of attack").

In most modern airplanes there is no way to adjust the angle of the wing. However, by extending wing segments to make the wing longer, bigger, and angled down more, the effective wing angle of attack is changed. Further, by extending those same segments the larger surface area on top is extended, also extending lift.

So if you've followed all this, you know that airplanes do not stay up because the wing deflects air down. Nope. If it did that, the wing would be tipped up at the edges and tight the whole time. Instead we see it as relatively flat, and sometimes shaking with no apparent loss of lift.

Okay, here's an aside. Our helicopters also use this principle, but we don't extend flaps nor change the wing angle size. We change that angle of attack.

Well here's the rub. For every shape of aerofoil and climatic conditions there exists at angle at which the air will no longer adhere to the top of the wing. When that happens, that "low pressure" area will turn into a regular area, and all of a sudden the lift generated by that section of the wing will be gone. Worse yet, it causes such a disturbance when it goes from "low pressure zone" to "normal pressure zone" that it can disturb areas around it which may not yet have reached this bad angle.

This bad angle is the stall angle. A wing that stalls doesn't mean "The plane went too slow and stalled". No, it means "The wing angle as compared to the relative wind no longer allowed the air to adhere to the low pressure area and there was therefore no lift."

Recovering from a stall is difficult. It's not just a matter of aiming the stick down and going in one direction. It's difficult because it's important to get the air layer to adhere to the wing again -- and there's a disturbed vortex of stalled air in the way. There are recovery techniques for small planes, recovery techniques for large planes, but it's taught to all pilots for a reason -- it's critical to avoid, critical to fix, and requires pilot accumen.

In our helicopter we don't have "stall" per se because our power isn't high enough to keep increasing the angle of attack until we get a stall. However, we do have what's called "Retreating-blade stall." That's a different kind of stall (insufficient forward airspeed on the retreating blade to provide sufficient lift).

So this Airbus plane was flying along and apparently its speed was reduced and its wing angle of attack increased and once the air broke loose from the wing, that was pretty much it.

There's the technical explanation.

Best wishes to the families.


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Comments {1}


from: anonymous
date: Jun. 10th, 2009 05:58 pm (UTC)

Thanks for your bar room explanation. I think I like knowing less for all of the flying I do on Airbus 330's. Such as ignorance is bliss, planes just don't fall out of the sky. Auto pilot is the safety to the experienced pilot, etc.

Ironically, the reality of safety and travel, is so irrational in my brain. I am happy to be going cross country and back in an RV with my entire family and friends but not so happy about all of our flights scheduled for the rest of this year.

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