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symmetrical wings - 3/17/2008 1:27:11 PM   
gaRCfield


 

Posts: 1250
Joined: 5/20/2008
From: Burlington, VT, USA
Status: online 		Hello,
I am a new flier and am just curious as to how symmetrical wings work. I have always understood the classic theory of lift relating to an airplane wing, where the longer distance over the top of the wing causes a low pressure area above the wing and therefore creates lift. Based on this principle, a "symmetrical wing" would cause areas of equal pressure above and below the wing, canceling the lift properties. Is there something I'm missing, or does a plane with a symmetrical wing fly using thrust and "attack angles" of the wings and elevator?Cu

Thanks,


FatOraneKat, fueled by 100% curiosity

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RE: symmetrical wings - 3/17/2008 2:38:53 PM   
dieFluggeister



Posts: 665
Joined: 3/10/2004
From: New London, MN, USA
Status: offline 		Symmetrical wings will only generate lift when they are at some angle of attack. At a 0 AOA they are simply a streamlined body!

General principals of lift still apply - you just do not have a shape which creates lift at or below 0 AOA's as cambered wings can.

< Message edited by dieFluggeister -- 3/17/2008 2:42:30 PM >

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RE: symmetrical wings - 3/17/2008 2:41:48 PM   
gaRCfield


 

Posts: 1250
Joined: 5/20/2008
From: Burlington, VT, USA
Status: online 		Hmm, thanks! That's kind of what I'd figured.

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RE: symmetrical wings - 3/17/2008 3:51:02 PM   
dick Hanson



Posts: 10026
Joined: 12/12/2001
From: slc, UT, USA
Status: offline 		when it's all said n done - the old theory of longer distance over the top etc', is really a lousy example

In actual practice
ANY SHAPE which has lower pressure on one side than the other - will provide "lift".
a flat plate works perfectly well in many cases - - the slight tilt required to get pressure difference is very minor in many cases.
If you have not flown very light flat wing foamies - you may say this isn't so- but trust me on this - it is so.

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RE: symmetrical wings - 3/17/2008 4:00:16 PM   
jlkonn



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Status: offline 		http://www.amasci.com/wing/airfoil.html


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RE: symmetrical wings - 3/17/2008 4:13:53 PM   
JCINTEXAS


 

Posts: 404
Joined: 3/23/2007
From: Smithville, TX, USA
Status: offline
quote:

ORIGINAL: FatOrangeKat

Hello,
I am a new flier and am just curious as to how symmetrical wings work. I have always understood the classic theory of lift relating to an airplane wing, where the longer distance over the top of the wing causes a low pressure area above the wing and therefore creates lift. Based on this principle, a "symmetrical wing" would cause areas of equal pressure above and below the wing, canceling the lift properties. Is there something I'm missing, or does a plane with a symmetrical wing fly using thrust and "attack angles" of the wings and elevator?Cu

Thanks,
FatOraneKat, fueled by 100% curiosity

______________________________________________________________

Archimedes' principle, states that a body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid. The principle applies to both floating and submerged bodies and to all fluids, i.e., liquids and gases.

The principle of "buoyancy" applies to boats in water and to airplanes in air. (It also applies to helicopters, blimps, and hot air baloons etc.).
An airplane achieves neutral buoyancy when the wing displaces a volume of air with a weight equal to the weight of the airplane (level flight). If the volume of displaced air is greater than the weight of the aircraft (positive buoyance)....the plane climbs. Archimedes' principle applies regardless of the shape of the airfoil. Symmetrical airfoils produce "lift" (positive buoyancy) via angle of attack. Waterskis also work via angle of attack.

Helicopters operate in accordance with the principle of buoyancy by rotating the wing through the air. Hence the wing is called a "rotor". The volume of air displaced per unit time is controlled by varying the pitch (angle of attack) and speed of the rotor. In a "fixed wing" airplane, the volume of air displaced per unit time is controlled by varying "airspeed" and the "pitch axis". When the pitch axis is changed via the elevator, the pitch (angle of attack) of the wing is increased or decreased. This results in increasing or decreasing the volume of air displaced per unit time.
[This explains why planes "fly better in heavy air". Jetliners require less take-off roll (less airspeed) when air temperatures are low and humidity is high. This also explains why jetliners required longer take-off rolls at Denver Airport than they do at Miami Airport. The density (weight) of the air is higher at sea level than at 5,000 feet.]

History books say that old Archimedes was taking a bath when he discovered his "principle". He was so happy that he jumped out of the tub and ran naked through the streets shouting "εύρηκα!" (Eureka I've found it!)

I figure it's a good thing for us that he "found it".
If he hadn't, we would be standing around with our Futaba and JR radios,
trying to figure out how to get them to fly.

Regards
JC

< Message edited by JCINTEXAS -- 3/17/2008 4:19:56 PM >

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RE: symmetrical wings - 3/17/2008 4:35:20 PM   
Nick C


 

Posts: 77
Joined: 5/17/2004
From: Bath, UNITED KINGDOM
Status: offline 		I think your getting your displacements mixed up here, lighter than air airship do occupy a large volume of air and they themselves weigh less than this displaced air. Hence the term bouancy and lighter than air. (same for ships)

You talk of an aircraft displacing air ?? Do mean the work done on a certain volume of air as the craft passes through the medium as displaced air?? Otherwise the amount of air displaced (statically) by a model aircraft will hardly make up the weight of it. Hence an aircraft is a heavier than air, craft. I really hope it is the former, otherwise we're in for popcorn time on this thread.

Nick

(Better check this and put my wings on my plane tonight and watch it float up towards the ceiling !!!)

< Message edited by Nick C -- 3/17/2008 4:38:07 PM >

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RE: symmetrical wings - 3/17/2008 5:04:11 PM   
JCINTEXAS


 

Posts: 404
Joined: 3/23/2007
From: Smithville, TX, USA
Status: offline 		Hi Nick,
Quote: "You talk of an aircraft displacing air ?? Do mean the work done on a certain volume of air as the craft passes through the medium as displaced air??"

Yes.

A "heavier than air" Boeing 747, or an RC Foamie 'Parkflyer' is heavier than the air they displace when at rest. As you say: "the craft passes through the medium (air)" - and the wing displaces (moves) a certain volume of air. The air has mass and "weight". The wing does "work" by pushing on the air....and the air pushes back on the wing with an equal force. The force exerted by the wing on the air is determined by the speed of the wing through the air (relative airspeed), and by the wing's 'angle of attack'. The higher the airspeed, the less angle of attack is required to produce the 'lift' needed to support the weight of the aircraft. What this means in practical terms is that the density of air increases as airspeed increases.

"Think of a paper airplane. Its airfoil is a flat plate --> top and bottom exactly the same length and shape and yet they fly just fine.
Airplanes fly because of Newton's 3rd law (action/reaction forces,) the law of Conservation of Momentum, and the Coanda effect."

For additional information see this NASA Article on "Incorrect Lift Theory":
http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html


Best Regards
JC

PS: If your plane floats up toward the ceiling, please let us know.

< Message edited by JCINTEXAS -- 3/17/2008 5:23:28 PM >

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RE: symmetrical wings - 3/17/2008 5:15:12 PM   
Nick C


 

Posts: 77
Joined: 5/17/2004
From: Bath, UNITED KINGDOM
Status: offline 		"its going to be tricky, all my wings are floating up on the ceiling up out of my reach. " going to have to find something to stand on.....


Ok so we've got STATIC displacement (lighter than air , craft)
and then DYNAMIC displacement (heavier than air , craft)


Nick

< Message edited by Nick C -- 3/17/2008 5:19:29 PM >

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RE: symmetrical wings - 3/17/2008 5:29:11 PM   
JCINTEXAS


 

Posts: 404
Joined: 3/23/2007
From: Smithville, TX, USA
Status: offline 		Quote:
"Ok so we've got STATIC displacement (lighter than air , craft)
and then DYNAMIC displacement (heavier than air , craft)"
________________________________________________________

Roger That!

You're right on.
Well said...You have nailed it.

Best Regards
JC

PS: For additional information see this NASA Article on "Incorrect Lift Theory":
http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html

(in reply to Nick C)
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RE: symmetrical wings - 3/17/2008 5:48:16 PM   
Nick C


 

Posts: 77
Joined: 5/17/2004
From: Bath, UNITED KINGDOM
Status: offline 		alright alright , haar haar....

I just figure that a beginer coming to this thread is going to be confused between lighter than air and heavier than air, craft displacements. For a heavier than air , craft the amount of air that it displaces is a pretty wooly volume of air, (ok example) , just how do you measure that quantity of air, (difficult to get a handle on) i.e the a/c pressure field extends some way out from the surface, by how far??

I much more a mechanical pressure kind of guy, (thats what I tell the women) i.e we've got squeezed flow on the upper surface, (wing top surface , and streamline above) continuity must be satisfied , and hence the velocity increases, static pressure drops, etc.

(Perhaps some reading is in order, I'm not that familar with displaced air from a moving vehicle, that and central differencing and I'll be a happy man)


Nick


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RE: symmetrical wings - 3/17/2008 8:20:58 PM   
gaRCfield


 

Posts: 1250
Joined: 5/20/2008
From: Burlington, VT, USA
Status: online
quote:

ORIGINAL: Nick C
I just figure that a beginer coming to this thread is going to be confused between lighter than air and heavier than air, craft displacements. For a heavier than air , craft the amount of air that it displaces is a pretty wooly volume of air, (ok example) , just how do you measure that quantity of air, (difficult to get a handle on) i.e the a/c pressure field extends some way out from the surface, by how far??




Actually, I got it the first time You could have a totally flat wing, give it an angle of attack and the right amount of thrust and it would fly; I've understood that ever since I outgrew my car seat and stuck my hand out the window! I imagine that, say on a jet liner, that the plane uses these dynamics principles at take-off and landing, by lowering the flaps; this would throw the "low pressure due to longer distance" theory out the window. However, when the plane reaches it's cruising altitude and the flaps are put away, I assume there is the low pressure principle keeping the plane flying level.

I was drawing pictures of a wing in class today; if you draw a symmetrical wing, rounded in front and coming to a point in the back, and position it so that the bottom surface is level, it will indeed have a "longer distance" on the top portion. What I gather from this is that the plane can fly with no attack, if measuring the angle from the horizontal to the bottom surface of the wing. For the plane to fly upside down, only a very small change in pitch would be needed to get the top (now bottom) of the wing even with the horizontal. And of course you could increase attack for lower speeds, etc.

I like this stuff

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RE: symmetrical wings - 3/17/2008 10:44:17 PM   
dick Hanson



Posts: 10026
Joined: 12/12/2001
From: slc, UT, USA
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quote:

ORIGINAL: FatOrangeKat


quote:

ORIGINAL: Nick C
I just figure that a beginer coming to this thread is going to be confused between lighter than air and heavier than air, craft displacements. For a heavier than air , craft the amount of air that it displaces is a pretty wooly volume of air, (ok example) , just how do you measure that quantity of air, (difficult to get a handle on) i.e the a/c pressure field extends some way out from the surface, by how far??




Actually, I got it the first time You could have a totally flat wing, give it an angle of attack and the right amount of thrust and it would fly; I've understood that ever since I outgrew my car seat and stuck my hand out the window! I imagine that, say on a jet liner, that the plane uses these dynamics principles at take-off and landing, by lowering the flaps; this would throw the "low pressure due to longer distance" theory out the window. However, when the plane reaches it's cruising altitude and the flaps are put away, I assume there is the low pressure principle keeping the plane flying level.

I was drawing pictures of a wing in class today; if you draw a symmetrical wing, rounded in front and coming to a point in the back, and position it so that the bottom surface is level, it will indeed have a "longer distance" on the top portion. What I gather from this is that the plane can fly with no attack, if measuring the angle from the horizontal to the bottom surface of the wing. For the plane to fly upside down, only a very small change in pitch would be needed to get the top (now bottom) of the wing even with the horizontal. And of course you could increase attack for lower speeds, etc.

I like this stuff

you just tripped yourself up- forget that flat bottom referrence line - the TRUE zero line is one which drawn thru the airfoil which will show the zero lift angle of that shape.
these fu--ing airfoils can be very confusing - thesy are all just compromise shapes for a given working speed and weight and mostly were done thick enough so they would not break - That's how they evolved.

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RE: symmetrical wings - 3/17/2008 11:14:02 PM   
combatpigg



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Joined: 11/22/2003
From: arlington, WA, USA
Status: online 		This is almost a question for another thread....
why is it that whenever I have asked for advice about the lowest drag airfoils, I am steered towards the symetric ones?
My gut instinct tells me that there is an ideal airfoil shape for whichever aspect ratio and wingloading I might want to work with which will be something other than perfectly symetrical.
Gut instincts or wind tunnel experience.....take your pick.

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RE: symmetrical wings - 3/18/2008 1:16:41 AM