Can someone give me a simplified explanation of the force "lift"

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It's for a project my gf is doing with little kids. What we don't understand is, does the lift primarily come from the pressure of the air flowing under the wing or does it have something to do with the way the air flows over the curve of the wing?

― Abbadavid Berman (Hurting), Tuesday, 31 January 2006 02:32 (twenty years ago)

I suspect wiki or similar may be of help? (not being snarky, it really might!)

― Trayce (trayce), Tuesday, 31 January 2006 02:33 (twenty years ago)

No, I tried that and find it unclear!

― Abbadavid Berman (Hurting), Tuesday, 31 January 2006 02:34 (twenty years ago)

The confusion comes from the fact that it refers to the "downward deflection" of the air (which presumably means the force comes from under the wing), but also that lift is created by "air flowing over the wing" (do they mean "over" as in over the top of it or just "over" as in around?)

― Abbadavid Berman (Hurting), Tuesday, 31 January 2006 02:37 (twenty years ago)

by "it" I mean the Wiki article.

― Abbadavid Berman (Hurting), Tuesday, 31 January 2006 02:38 (twenty years ago)

complicated subject needing URL beyond "wikipedia.org" shockah

my derision conceals my ignorant shame

― Tracer Hand (tracerhand), Tuesday, 31 January 2006 02:41 (twenty years ago)

Wait, I half-remember this one! Yeah, it's not because of "deflection" -- i.e., more complicated than the air just "bouncing" down off the bottom of the wing. If I remember right, it has to do with air taking longer to run over the top of the wing than to run along the bottom? Something like that -- the air has to move at different rates (top and bottom) to move around the wing, which is what creates the acceleration of the air/water/whatever. This is why actual plane wings have that complex curvy shape, which I guess maximizes that difference in speed.

― nabisco (nabisco), Tuesday, 31 January 2006 02:46 (twenty years ago)

Hows this?

http://www.pbs.org/wgbh/nova/naziprison/glid_airfoil.html

(I have no effin idea why this is in a directory labelled "naziprison" but it is a nice simple thingy on aerodynamic lift)

― Trayce (trayce), Tuesday, 31 January 2006 02:48 (twenty years ago)

My primary school understanding of it is that the same amount of air goes above and below the wing, but because the air flowing over the top has further to go (due to the shape of the wing), that air is forced to expand, and is therefore at lower pressure than the air below the wing. And low air pressure creates suction...so the wing is sucked up into the air, basically.

― JimD (JimD), Tuesday, 31 January 2006 02:56 (twenty years ago)

I think that's all part of the same thing. The air over the top moves more quickly; the wing's shape and tilt direct that fast-moving air downward. So yeah, it's just displacing air downward in all senses.

― nabisco (nabisco), Tuesday, 31 January 2006 02:59 (twenty years ago)

http://www.av8n.com/how/img48/flow-past.png

― nabisco (nabisco), Tuesday, 31 January 2006 03:00 (twenty years ago)

That pbs link is pretty good. Thanks. I get it now :)

― Abbadavid Berman (Hurting), Tuesday, 31 January 2006 03:02 (twenty years ago)

Lift comes from a mix. THere's a little bit of the "deflection" bit, which comes from what they call "the angle of attack," but it's mainly the air over top going faster than the air underneath. Faster air = less pressure = wing gets "sucked upwards."

http://drbonesshow.com/images-a1/airfoil-lift1.jpg

There's a bunch of other factors involved, too. (wing shape, camber, etc).

Things get more interesting in supersonic air flows.

― kingfish kuribo's shoe (kingfish 2.0), Tuesday, 31 January 2006 03:06 (twenty years ago)

The important thing here is that its the pressure differential between the top and bottom of the wing thats in action. Its wrong and misleading to talk about the faster moving air causing anything...its not a causal thing, its that all other things being equal, faster air is at a lower pressure.

― Rhodia (Rhodia), Tuesday, 31 January 2006 03:23 (twenty years ago)

I think maybe we should all stop getting on planes until we've got this one properly sussed.

― JimD (JimD), Tuesday, 31 January 2006 03:27 (twenty years ago)

Wait, really? It's just the pressure difference, and not the air displacement? (I mean I guess these are parts of the same process, but for some reason I thought the displacement alone created lift force.)

― nabisco (nabisco), Tuesday, 31 January 2006 05:08 (twenty years ago)

Just tell them it's magic. Kids love magic.

― Hikaru Genji (Mingus Dew), Tuesday, 31 January 2006 05:14 (twenty years ago)

I can't come up with a simple way of unifying the pressure difference and the displacement (aside from "fluid dynamics ain't easy"), but I think its best to think of the overall process as the thing that is driving the lift, and the pressure difference and air displacement as equally valid ways of explaining the process.

Too many years since I've studied physics.

― Rhodia (Rhodia), Tuesday, 31 January 2006 05:52 (twenty years ago)

I wonder if my bootleg copies of the feynman lectures has anything on lift...

― Rhodia (Rhodia), Tuesday, 31 January 2006 05:53 (twenty years ago)

Yeah, it's a mix of those actions.

God, and I got my degree is Aerospace Engineering, so many years ago, too...

― kingfish kuribo's shoe (kingfish 2.0), Tuesday, 31 January 2006 06:05 (twenty years ago)

"in"

― kingfish kuribo's shoe (kingfish 2.0), Tuesday, 31 January 2006 06:05 (twenty years ago)

Its when I feel better

― Mr. Latham Green (hanle y 3000), Tuesday, 31 January 2006 07:36 (twenty years ago)

http://www.straightdope.com/mailbag/mairplanesfly.html

― Casuistry (Chris P), Tuesday, 31 January 2006 08:30 (twenty years ago)

That straightdope is right in that it's not true that the air moves faster over the top of the wing because it has further to go - i.e. what they told you in primary school is wrong. However it seems to be mistaken about the coanda effect - this site
http://www.av8n.com/how/htm/airfoils.html that nabisco linked an image from says it's all to do with angle of attack, which produces a circulation pattern around the wing, which speeds up the air over the top, and retards the air underneath - see http://www.av8n.com/how/htm/airfoils.html#sec-circulation in particular.

― ledge (ledge), Tuesday, 31 January 2006 09:42 (twenty years ago)

Also this bit about business cards and tennis balls is illuminating - http://www.av8n.com/how/htm/airfoils.html#sec-spinners - basically if you flick a business card away from you with backspin, it should float to the ground, because the spin will increase the airflow speed on the top, and decrease it on the bottom, and the difference in airspeed will produce lift, even though the air is not moving relative to the surface of the card.

― ledge (ledge), Tuesday, 31 January 2006 09:51 (twenty years ago)

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