Symetrical airfoil and lift  
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Symetrical airfoil and lift - 10/16/2003 6:42:54 AM   
3DFanatic


 

Posts: 2669
Joined: 7/22/2003
From: Tacoma, WA, USA
Status: offline 		I don't keep up o nthe going on in this forum, so if this question has been asked a thousand times before, please forgive.

Suppose you have a perfectly symetrical airfoil. I.E. the air going over the top pushes down with the same force as the air on the botom pushes up. And suppose the wing has no incidence.
What keeps the plane in the air? Shouldn't the top and bottom pressures cancel out and do nothing???

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3Dfoamy.com
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RE: Symetrical airfoil and lift - 10/16/2003 8:43:03 AM   
BMatthews



Posts: 9015
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		Yes the pressures would cancel out in such a case and the model would not be flying normally in such a case. What you described would only happen when the model did not require any lift at all to be produced. On a funfly type this can happen in many situations be they steady state or momentary. Like a straight line vertical dive or climb or knife edge flight where the wing does not produce any lift (tips don't count for this ).

But for normal flight what you described can never be the case. To support it's weight in level flight the wing MUST have an angle of attack for symetrical airfoils. If you tried to force it to zero angle the model would react by pitching over into a vertical dive if you hold to the zero angle and keep controlling it to maintain that angle. Even if you go to zero and then hold that new attitude the model will react by pitching down into a steady descent at a constant rate to restore the angle it needs to maintain flight at that new fuselage angle.

Smart little things aren't they...

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RE: Symetrical airfoil and lift - 10/16/2003 12:30:55 PM   
ksechler



Posts: 506
Joined: 9/19/2002
From: Frederick, MD, USA
Status: offline 		I think some terms are getting mixed up here. So let me clarify.
Incidence is the angle between the wing and the centerline of the fuselage.
Angle of attack is the angle of the wing to the relative wind.

A wing can have zero incidence and still have a positive (or negative) angle of attack. It just depends on what the model is doing.

The lift a wing produces is determined by the pressure distribution on the wing which is further determined by the angle of attack, shape and the airspeed. For a symetrical wing no lift is produced at zero angle of attack. That means your model will accelerate toward the ground at 9.8 m/s2. Leave pitch out of it. The model will fall. period. That is why you often see models with symmetrical airfoils built with 1-2 degrees of positive incidence.

(in reply to 3DFanatic)
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RE: Symetrical airfoil and lift - 10/16/2003 2:11:20 PM   
LouW



Posts: 806
Joined: 1/1/2003
From: Moreland, GA, USA
Status: offline 		BMatthews is correct. In level flight the airplane would be going through the air with the nose pitched up enough so that at that angle of attack the lift would equal the weight. If the airplane were flying level inverted, the nose would still be pitched up at the same angle such that the lift would again equal the weight. Therein lies the only real advantage of a symetrical airfoil. It produces lift at the same angle of attack upright and inverted. For normal upright flight a cambered airfoil* is always more efficient, providing its lift with less drag. The cambered airfoil is normally set at an angle of incidence (which or may not be zero) such that at the design speed, the fuselage is parallel to the direction of motion. For an airplane designed to excel in inverted flight, not only is the airfoil symetrical, but the incidence of both wing and horizontal stabilizer is always set at zero. To do otherwise would negate the only advantage of using a symetrical section.




* A slightly cambered airfoil is sometimes incorrectly called a "semi-symetrical" airfoil. An airfoil is either symetrical or cambered, with the degree and shape of the meanline (camber) chosen for particular section characteristics.

(in reply to 3DFanatic)
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RE: Symetrical airfoil and lift - 10/16/2003 6:06:53 PM   
Tall Paul



Posts: 4701
Joined: 6/23/2002
From: Palmdale, CA, USA
Status: offline 		Please do a search on the subject. It gets beat to death monthly!

(in reply to LouW)
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RE: Symetrical airfoil and lift - 10/17/2003 6:34:04 AM   
3DFanatic


 

Posts: 2669
Joined: 7/22/2003
From: Tacoma, WA, USA
Status: offline 		So is that why some 3D models that aren't tailheavy apear to fly with the tail sagging a bit? It's because the nose is up to get a positive angle-of attack?

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-Aaron
3Dfoamy.com

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RE: Symetrical airfoil and lift - 10/17/2003 7:22:00 AM   
BMatthews



Posts: 9015
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		The angles required to fly a lightweight model like a fun fly are so slight you wouldn't be able to see them at normal speeds. Probably in the order of one or two degrees. Now when they start trying to hang around or just get real slow then you'll see the angles begin to show. But then you can see that sort of nose high stuff on any model when you slow it down even with cambered airfoils.

If you want to check all this out a bit more in a manner you can adjust to go over to Foilsim and play with cambered and symetrical airfoils in their virtual windtunnel. Set up the numbers for a typical model and play with the airfoil shape. Adjust the angles and speeds so the lift generated is in the 4 pound range of most of the fun fly types.

FoilSim online windtunnel

_____________________________

Witty saying to be plagarized shortly.....

(in reply to 3DFanatic)
       Post #: 7

RE: Symetrical airfoil and lift - 10/18/2003 4:00:37 AM   
3DFanatic


 

Posts: 2669
Joined: 7/22/2003
From: Tacoma, WA, USA
Status: offline 		COOL! Thanks for the link!

_____________________________

-Aaron
3Dfoamy.com

(in reply to 3DFanatic)
       Post #: 8

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