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Airfoils and Wing Shapes

11/07/2007 1:13 PM

I was doing a search on Google about, airfoils and lift, and came across this site. http://amasci.com/wing/airfoil.html I don't know much about wings. I was taught that the shape of the wing is what caused lift, but after reading the article the attract angle makes since. Any comments?

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#1

Re: Airfoil

11/07/2007 2:02 PM

Both are partially right........ and wrong.

The air, after it has left the wing tip, has no mechanical effect on the wing.

Both the attack angle and the shape create a pressure difference between the top and the bottom of the wing.

The compression of air in under the wing creates a positive pressure.

The speeding up of the air on the back side or top creates a low pressure.

The pressure difference between the two areas causes the lift. Any turbulence in either area will reduce the pressure difference.

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#2
In reply to #1

Re: Airfoil

11/07/2007 4:15 PM

"The air, after it has left the wing tip, has no mechanical effect on the wing."

Correct, but the wing altering the flow of air is the key to lift.

"Both the attack angle and the shape create a pressure difference between the top and the bottom of the wing."

The shape of the wing is negligible as far as developing lift. Aerobatic planes do not have an airfoil (their wing curve is symmetrical on the top and bottom. Attack angle (AOA) of the wing is really what provides lift. But why?

My favorite demonstration to people is getting them to hold a spoon from the far end of the spoon and gently move the spoon's convex surface toward a stream of water running from a faucet. I ask them what they think will happen when the spoon's backside contacts the falling water. The answer that people give is that the spoon will get kicked out away from the water as water strikes the backside of the spoon.

In fact, the spoon gets sucked into the water stream with a good bit of force! Why? Because the water flow on the backside of the spoon follows the curve of the spoon's back and this diverts some water away from the falling stream at an angle. Since there is force required to divert the water away from the stream (the water has mass and momentum), an opposite (and equal) force is applied to the spoon, which pushes it into the water stream. Try it for yourself (alone), then get the family to try it and bet what will happen. You will look like a hero of wisdom. :-)

Good article.

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#12
In reply to #2

Re: Airfoil

11/08/2007 1:20 PM

Arobatic wings do not have the same shape because they also fly upside down. To remove the shape causes a partial loss of lift when upright, however, it causes an increase in lift while upside down.

The most efficient wing for normal flight still has the airfoil shape. The only correct answer is that both contribute to the lift.

Your demonstration with the spoon confirms the shape is not "negligable"

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#14
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Re: Airfoil

11/08/2007 2:02 PM

The most efficient wing for normal flight still has the airfoil shape.

A symmetrical airfoil is a perfectly good airfoil. In the NACA 4 digit numbering scheme, the first digit is the percentage of camber (camber/chord), the second is the location of the camber peak (in tenths of chord length), and the remaining two digits are the thickness (in percent of chord). So a 2410 airfoil has 2% camber located at 40% back, and is 10% thick. A 0010 foil is the same airfoil without camber. A 0015 airfoil is on the boat in my avatar, with a 0010 foil underwater.

You're right -- the shape is anything but negligible. In practice, a designer, given a wing span to work with, tries to find an airfoil section shape that operates most efficiently at a particular coefficient of lift. He'll play around with span and chord and section shapes and eventually zero in on a particular lift coefficient, and then, typically find a section shape and camber that provides that lift coefficient at a point fairly close to the center of the low drag "bucket" in the lift curve. Usually this whole process goes back and forth until the designer gets a headache, at which point the design is considered finalized.

For a typical general aviation small plane, in which you are not overly concerned with fuel efficiency, you could mount a symmetric wing with an angle of incidence of about 3 degrees relative to the rest of the airframe (so that when the plane is flying at cruise, with a 3 degree angle of attack, the fuselage is horizontal). Instead, a cambered foil might be mounted at 1 degree incidence, and would develop the required lift coefficient at that angle of attack. Other things being equal, the difference in L/D might be 1 or 2 percent? Obviously, in airliners, the optimizations get quite serious, because 1 or 2 percent might be a zillion gallons of fuel.

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#20
In reply to #14

Re: Airfoil

11/09/2007 9:25 AM

Well said!

So, while both contribute, the airfoil shape is a very small fraction of the lift component.

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#21
In reply to #12

Re: Airfoil

11/09/2007 9:27 AM

"Your demonstration with the spoon confirms the shape is not "negligable""

Actually, it proves quite the opposite. Only the convext edge of the spoon touches the water, yet it is pulled into the stream.

AH

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#3

Re: Airfoils and Wing Shapes

11/07/2007 6:28 PM

both newton and Bernoulli come into play, there is a decrease in pressure on the top surface of the wing and there is a force down against the air with an equal and opposite force up against the wing, also the tendency of fluids (air) to follow the shape of the wing causes a venturi of down ward air behind the wings (even if the angle of attack is zero)... it all magical lol

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#4

Re: Airfoils and Wing Shapes

11/07/2007 6:51 PM

It's neither one nor the other alone, but several effects together. Quite a while ago, I started a thread on this subject -- the gist being that you can prove that Bernoulli effect can only account for a small portion of total lift, and that actual airflow is not as shown in old textbooks.

I used to fly aerobatics, and my plane had a conventional wing and flew OK upside down. A friend had a plane with a symmetrical airfoil (same shape top and bottom when viewed from the side) and his plane flew fine right side up and upside down.

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#5

Re: Airfoils and Wing Shapes

11/07/2007 6:54 PM

The aerofoil shape begins to be really important when you're trying to get as much as you can out of a situation.

I did a bit of research (many years ago) into wind generator blade shape, and the maths gets pretty hairy!. These blades are particularly difficult, because both the profile and the pitch have to change as a function of the distance from the centre of rotation.

I've also got a fairly strong suspicion that passenger plane manufacturers pay megabucks to have wings designed that will give them the best fuel economy.

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#6
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Re: Airfoils and Wing Shapes

11/07/2007 7:03 PM

These blades are particularly difficult, because both the profile and the pitch have to change as a function of the distance from the centre of rotation.

... and the blasted thing is going around in a circle trying to fling air off the tips!

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#7
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Re: Airfoils and Wing Shapes

11/08/2007 1:41 AM

I think Airfoils and Wing shape is impotent up to a certain speed a plane is designed to fly. Look at fighter jets or the XR 70 no airfoil , why, the saying is , you can make a den door fly if you put enough power behind it.

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#8
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Re: Airfoils and Wing Shapes

11/08/2007 8:26 AM

you can make a den door fly if you put enough power behind it.

True. One of my favorite flight videos is this one. If you watch the smoke trails you can see that the wing is sometimes flying backwards, and often with far more than the "stall" angle of attack. This sort of flight would be impossible without computers on board and a tremendous amount of power.

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#13
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Re: Airfoils and Wing Shapes

11/08/2007 1:27 PM

True. The faster you are going, The more of a pressure difference you are getting. This is why the faster jets do not want too much lift, however this causes a problem when landing and taking off. The jets have a very high minimum speed, or they must adjust the wing span to land at a slower speed.

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#17
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Re: Airfoils and Wing Shapes

11/09/2007 4:44 AM

and the maths gets pretty hairy!

It always amuses me that Australia'a Aborigines can make boomerangs which fly beautifully without recourse to maths or aerodynamic theory.

It's an interesting thread, I remember as a kid not being entirely convinced by the pictures in the books, 'cos we all know that there are forces on a flat plate if the angle of attack is adjusted... after all didn't we all whirl bits of board & stuff aound our heads?

I find that aerodynamics bernoulii and all that is one of the few areas of physics which is counter-intuitive ( ok just don't ask about quantum mechanics )

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#18
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Re: Airfoils and Wing Shapes

11/09/2007 4:52 AM

"It always amuses me that Australia's Aborigines can make boomerangs which fly beautifully without recourse to maths or aerodynamic theory" - but they've had an awful long time to get from chucking sticks to the "modern" boomerang. The empirical + intuitive approach is good, but can be slow.

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#19
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Re: Airfoils and Wing Shapes

11/09/2007 4:54 AM

They'd have got mighty hungry if it had taken all that time...

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#22
In reply to #17

Re: Airfoils and Wing Shapes

11/09/2007 10:30 AM

after all didn't we all whirl bits of board & stuff aound our heads?

... and stick our hands out of car windows, making our arms fly?

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#9

Re: Airfoils and Wing Shapes

11/08/2007 10:52 AM
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#10

Re: Airfoils and Wing Shapes

11/08/2007 12:38 PM

Hey JohnV, On sabbatical for legal school but couldn't help but put my 2 cents in.

Check this out http://www.rexresearch.com/klinfogl/klinfogl.htm#omni

Took me awhile to rap my brain around this foil shape.

Hope you enjoy

Brad

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#11
In reply to #10

Re: Airfoils and Wing Shapes

11/08/2007 12:50 PM

Very interesting. I'll have to play with this one.

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#15
In reply to #10

Re: Airfoils and Wing Shapes

11/08/2007 2:21 PM

Nice design that uses the power and direction of the following vortex to its advantage. T

his same principle is why fish can move through water with such efficiency and why golf balls seem to have less drag then their area should produce, and why avalanches travel faster then gravity.

It does not disprove Bernoulli in any way.

Most turbulence has a negative effect airfoil designs because it disrupts the pressure and the normal flow of the air. In this case the turbulence is controlled to form a continuous vortex and then used. The top of the vortex is forcing the wing forward and up much like ball or pin bearings.

In this case any airflow that has left the notch does have a mechanical effect on the trailing part of the wing.

A better design that would use this principle to the max would have the whole bottom surface with numerous smaller steps like a fish scale.

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#16
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Re: Airfoils and Wing Shapes

11/08/2007 2:59 PM

Very true techno, some of the designs I want to try take it a few generations further.

Even though the article states it disproves Bernoulli, I believe (without more research) that this was only the ignorance of vortices at the time. We have only figured out how a Bumble Bee manages to fly in the last 15 years. Yet it flew long before we could say how it was aerodynamic.

The Kline airfoil produces poor lift, its real attribute is low turbulent drag.

I only dabble in fluid dynamics so if anyone discerns a incorrect concept please let me know.

Brad

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#23

Re: Airfoils and Wing Shapes

11/09/2007 3:41 PM

Regarding symmetrical, or "zero-cambered" airfoils- these actually do have camber, it results from having an angle of attack.

If you look at one of these sections in a photo from smoke tunnel testing, or a good drawing showing the flow of air around it, notice that if there is an angle of attack there will be a "separation point" at the leading edge where some air goes under and some goes over the section.

If you draw a "mean camber line" from this separation point through the center of the section, equal distance from the upper and lower surface at any given point, you will see a curvature in the forward area of this line.

I guess it might be called a "aerodynamically induced camber", or something like that.

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#24

Re: Airfoils and Wing Shapes

08/20/2008 10:32 PM

www.google.com/patents and check out patent 4410806 ... it might have some good related information for you

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