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Flight and gravity

CurtC said:
Why does the air on top of the wing have to move faster?
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Imagine two "parcels" of air, one just above the other. They're sitting there, just touching, when a wing slams into them and separates them. The one that was on top travels over the curved top of the wing, the other along the bottom, straight face. After traveling along their opposite faces the two parcels rejoin at the trailing edge.

In order for the top parcel to just meet its neighbor it would have to move faster to travel the farther distance.

But, why does it HAVE to move faster? Because if it didn't there would be less air above the wing than below it. Imagine they moved at the same speed and the bottom parcel got to the back of the wing while the top one was only 90% of the way there. What would happen then? We need as much air entering the wing as leaving it, but if both parcels didn't get to the back at the same time we'd be "losing" air on that top surface.

Couldn't the air from the bottom just race around to the top and rejoin it up there? No, because then it would be going against the direction of the flow and there's nothing pushing it that way. It would have to move WITH the wing, in the same direction, but faster than the wing. Remember, the air in this illustration is really only getting pushed out of the way by the moving wing, it is trying to stay stationary.

Real wings generate all sorts of turbulence and real parcels of air almost never neatly rejoin their former neighbors at the back. This rejoining would require laminar flow, a type of flow rarely seen in real applications, but the effect is the same whether there's turbulent flow or not. To conserve mass the top air has to speed up a little to get to the back when it has to.

Did this help?
 
Not really, because wind tunnel tests always show that the parcel of air going over the top of the wing does not rejoin the parcel that went under the wing. Not even close.
 
CurtC said:
Not really, because wind tunnel tests always show that the parcel of air going over the top of the wing does not rejoin the parcel that went under the wing. Not even close.
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Right. Remeber when I said this:
Real wings generate all sorts of turbulence and real parcels of air almost never neatly rejoin their former neighbors at the back. This rejoining would require laminar flow, a type of flow rarely seen in real applications, but the effect is the same whether there's turbulent flow or not. To conserve mass the top air has to speed up a little to get to the back when it has to.
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It's a mass conservation issue -- the total amount of air passing over the wing cannot be changed, and neither can the total amount of air passing under it. The only way those totals can be conserved is to have the air going over the top, by the longer route, is to go faster. It does this and generates lift.

OK?
 
I seem to have come late to this conversation.

swellman: Question from a third grader's science homework - does an airplane need gravity to fly? There seem at least three possible answers:

1. An atmosphere would not form or stay contained if gravity was absent. (the wisea$$ answer)

2. No, because lift is a function of only velocity and air density. (this is the book answer)

3. Yes, because a standard aircraft balances lift, drag, thrust, velocity and pitching moments to maintain stable flight. Remove the gravity vector and the aircraft becomes unstable without radical redesign/trim.

Any predictions on the stability margin of an aircraft in condition number 3? Or is this answer out to lunch?
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Answer #3 is totally out to lunch. But how many third graders would know that? Or how many adults, for that matter?

I'd like to add my vote that the question is ill-formed, specifically in the use of the word "fly". But I think answer #2 gives some insight into the intent of the question. The lift generated by an airplane wing is not a function of gravity, but that wasn't how the question was phrased.

Nonetheless, I would go with "no" (answer 2), as being the most reasonable of the three, given my interpretation of the intent of the question. However, the justification given in answer 2 is incomplete (as was already pointed out.) Lift is a function of more than just velocity and air density.

However, to complicate the issue, a glider cannot "fly" without gravity. It uses gravity to produce forward motion, without which there would be no lift.

One thing I didn't see mentioned in this thread was a comparison with submarines (which could be thought of as "flying" in a "zero g" environment).

As a pilot, I must concur with Curt that there is a lot of misinformation in this thread. I would be curious to know how anyone reading this thread can identify the bogus from the real if they didn't already know which was which. However, some of the misinformation is countered by the excellent link posted by Roger.
 
xouper said:
I seem to have come late to this conversation.

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However, to complicate the issue, a glider cannot "fly" without gravity. It uses gravity to produce forward motion, without which there would be no lift.
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Very true. Though the problem doesn't specify if the "airplane" is powered or not.


One thing I didn't see mentioned in this thread was a comparison with submarines (which could be thought of as "flying" in a "zero g" environment).
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Guess I would think of a submarine as being more analogous to a blimp with control surfaces than a winged aircraft.


As a pilot, I must concur with Curt that there is a lot of misinformation in this thread. I would be curious to know how anyone reading this thread can identify the bogus from the real if they didn't already know which was which. However, some of the misinformation is countered by the excellent link posted by Roger.
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By misinformation, do you mean the old Bernoulli vs. Newtonian explanations of lift? The performance of stunt aircraft? Or the predictions of how a standard aircraft would fly in zero g?

I think the best part of this problem is it's inate pointlessness. :D
 
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