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Old 06-10-2003, 06:44 AM
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twoturnspin
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I was thinking about wing shapes recently (no I don't really know why) and began wondering how a symmetrical wing actually flies given my understanding that the shape of the top and bottom of the wing is the same and so there is no difference to create lift?

Can anyone help me better understand this?
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Old 06-10-2003, 01:54 PM
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Aussie Flying,

If a symmetrical wing is generating lift, there IS a difference between the upper and lower surfaces, at least from the perspective of the oncoming air. The reason is that a lifting symmetric airfoil will be at a non-zero angle of attack. This means that the air will meet the wing at a point slightly below the front. As a result, the air that travels over the top surface will 'see' a different shape than the air that travels under the bottom surface does.

Think of it this way. Any roughly flat body will generate some lift if it is at a positive angle of attack relative to the oncoming flow. Having a good airfoil shape only makes the generation of lift more efficient, it is not a requirement. The bottom line is lower pressure on the top than on the bottom. However you achieve that, you will get lift.

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Old 06-10-2003, 03:56 PM
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Ask this question in the aerodynamics forum.

Then sit back and prepare to be amazed!
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Old 06-10-2003, 04:58 PM
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BillHarris
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And toss in the "downwind turn" and the "downwash _vs_ Bernoulli" questions and we'll all be amazed... for a long time.

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Old 06-10-2003, 10:50 PM
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Thankyou for the responses, much appreciated. The answer from banktoturn makes sense - ie. there is a positive angle of attack (I assume 'angle of attack' means the same thing as 'wing incidence') and therefore the air meets the wing below the leading edge of the wing.

Ok, so how do these aircraft with a symmetrical wing and a positive angle of attack fly inverted?
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Old 06-11-2003, 01:34 PM
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Aussie Flying,

When you fly inverted, or upright for that matter, you do not directly control the angle of attack, you control the position of the elevator. Whether inverted or upright, you look at your plane and move the elevator stick until it levels off. When your plane is inverted, this happens when the wing adopts an angle of attack that generates enough lift to carry the weight of the plane. Theoretically, if your plane were completely symmetrical, the angle of attack when flying upright would be the same as when flying inverted. In either case, the plane would be a little bit nose-up in order to generate enough lift. If your wing is not symmetrical, then the angle of attack required to generate sufficient lift will be higher for inverted flight than for upright flight, since the wing is shaped for upright flight.

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Old 06-11-2003, 09:41 PM
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Banktoturn,

Thankyou for the explanation, much appreciated & makes a lot of sense.

Aussie Flying
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