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Old Mar 24, 2012, 03:56 PM
Joe_Crash
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Symetrical airfoils same as plate wings?

I just read something that makes sense, but the more I think of it, wonder if it's actually true or not. I don't recall the source, but I read that a symetrical airfoil in fact generates NO lift - which would be true of plate wings also, if it was flying with an angle of attack of 0 degrees.

I don't know much about aerodynamics - you can probably tell from this ? - but is true that both wings at a zero angle of attack, or neutral, generate no lift, and therefore the angle of attack is what determines the lift of a wing with either style airfoil? (I guess in the case of a depron plane w/ no airfoil at all, it seems more logical.) If this is the case, why bother with a symmetrical airfoil on a wing? Why does my 3D profile Aeroworks plane if flown level, not sink to the ground? With wings with a Clark airfoil, I can understand the theory of it generating an area of negative and positive pressure, resulting in lift - but what about symmetrical airfoils? (And yes, I've read all the threads in this forum the low/high pressure idea being but one theory of what creates lift.)\

Thanks in advance for the explanation - please help my mathematically/physics challenged brain understand this ....looking forward to hearing an explanation!!!

Joe
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Old Mar 24, 2012, 04:12 PM
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Yes, a flat plate is a symetrical airfoil with 0% thickness. Simple as that.

The angle that is needed for level flight is so low for normal cruise speed flying that they LOOK level. But in reality they are not.

NO plane with a flat or symetrical airfoil will fly level with a 0 AoA. The only time they are truly at a 0degree AoA is when they are going straight up or straight down. If you set the model level then they "fall" or transition into a descending glide where the level attitude then generates a positive angle of attack. It's only when such models slow down to near the stall speed that they have a more obvious nose up attitude to their line of flight when flying at a constant height.

Just don't confuse the "harrier" style of flying with the flat foamies with "near stall". Flat plate wings on 3D flat foamies will stall at around 5 to 7 degrees. Once past that angle they are flying in a post stall mode. That doesn't mean the lift goes away. Simply that the top surface has a complete airflow breakdown and is generating high drag which needs to be compensated for by the use of excessive power to remain airborne.
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Old Mar 27, 2012, 12:29 PM
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From a Science Fair at the AV Mall a few years back...
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Old Mar 27, 2012, 02:34 PM
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Ah, yes, "lies to children" at work again. Hopefully someone will explain Elizabeth that things are not as simple as they seem once she is old enough to understand it. But the vast majority of children will not grow up to be an aerodynamics expert, and will just stick to what they were taught.
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Old Mar 27, 2012, 04:27 PM
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Well to be fair the basic conclusion of the experiment (the shape of an airfoil does makes a difference) was absolutely correct. It just went a teeny bit wrong with the "why" . And I'd guess someone a lot older than Elizabeth told her to put that bit in. Still it is amazing what poor old Bernoulli, who never saw an aeroplane in his life, gets blamed for .

And of course joe_crash is relying on fallible eyesight for his conclusions. If your plane was truly flying with the wing precisely level it would indeed lose height or"sink to the ground". The fact that it doesn't means that it's actually flying with the wing at a slight angle....but it doesn't need to be a lot, one or two degrees is enough to work but is really difficult to spot.

Steve
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Old Mar 28, 2012, 08:26 AM
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The well intended but incorrect "fact" of airfoils required for producing lift , really bugged me ,as a teenager .
No one bothered to note that these finely developed curves were simply attempts at maximizing EFFICIENCY at some given condition.
Change the conditions> the best shape also changes.

Fast forward to today.
For most powered models in smaller sizes the refinements in shapes is really not important
perfectly flat,squared edgewings work extremely well at very low wing loadings and small sizes
Those who doubt this, should actually build and fly more and cogitate less.
My second law of aerodynamics :
if the loadings are low enough - the shape does not matter ,
If they are too high - it still does not matter.
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Old Mar 28, 2012, 10:33 AM
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There's actually nothing wrong with that project except the implied reference to the 'equal transit time' fallacy in the Why part. It just happens to be a study of lift at zero angle of attack, not a complete study of the subject but not 'wrong' as such. The pink and yellow sections are symetrical hence no lift and the cambered foil does rely on pressure differential created by curved surfaces (well camber) to produce lift at zero alpha.
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Old Mar 28, 2012, 10:41 AM
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Quote:
Originally Posted by joe_crash View Post
I just read something that makes sense, but the more I think of it, wonder if it's actually true or not. I don't recall the source, but I read that a symetrical airfoil in fact generates NO lift - which would be true of plate wings also, if it was flying with an angle of attack of 0 degrees.

I don't know much about aerodynamics - you can probably tell from this ? - but is true that both wings at a zero angle of attack, or neutral, generate no lift, and therefore the angle of attack is what determines the lift of a wing with either style airfoil? (I guess in the case of a depron plane w/ no airfoil at all, it seems more logical.) If this is the case, why bother with a symmetrical airfoil on a wing? Why does my 3D profile Aeroworks plane if flown level, not sink to the ground? With wings with a Clark airfoil, I can understand the theory of it generating an area of negative and positive pressure, resulting in lift - but what about symmetrical airfoils? (And yes, I've read all the threads in this forum the low/high pressure idea being but one theory of what creates lift.)\

Thanks in advance for the explanation - please help my mathematically/physics challenged brain understand this ....looking forward to hearing an explanation!!!

Joe

Joe. The symmetrical airfoil also creates a difference in pressure between the two surfaces but it requires some angle of attack above zero for this to occur. This happens with any symetrical airfoil or flate plate. Whereas an airfoil with camber will make a small amount of lift at zero angle of attack.

The reason to put a streamlined airfoil on a symmetrical wing is that it will have better drag characteristics and also stall 'later' (at a higher angle of attack) than the flat plate.
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Old Mar 28, 2012, 10:54 AM
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Joe.
The reason to put a streamlined airfoil on a symmetrical wing is that it will have better drag characteristics and also stall 'later' (at a higher angle of attack) than the flat plate.
That sums it up
Add "the fatter the shape the less critical to AOA.
One of my favorites W T F is the diagram of a Clark Y shape with the" zero" AOA listed as parallel with the flat lower surface.
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Old Mar 28, 2012, 11:05 AM
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That sums it up
Add "the fatter the shape the less critical to AOA.
One of my favorites W T F is the diagram of a Clark Y shape with the" zero" AOA listed as parallel with the flat lower surface.
I've actually seen in an old book where they defined the chord to be along the flat bottom instead of connecting the tip of the LE to the TE.
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Old Mar 28, 2012, 11:21 AM
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I have seen some pretty odd wing sections used on full size acrobatic planes. Generally symmetric, because you want the same behavior when upright as when inverted. Fairly thick, because top speed is not essential but keeping the speed down in a dive is, and you can always add a bigger engine. And with the maximum thickness quite forward, at least near the tips. This probably delays the stall to a greater angle of attack, but an acrobatic plane HAS to stall at a reliable AOA, so generally there are stall strips, sharp leading edge protrusions, on the inboard portion of the wing.
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Old Mar 28, 2012, 11:25 AM
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Originally Posted by richard hanson View Post
That sums it up
Add "the fatter the shape the less critical to AOA...
And for the full story it should be noted that this is only holds ture true at 'normal' (higher) Reynolds numbers. Around the Critical Re region (ie small models) the extra thickness of a streamlined section can be a problem (causing seperation at low alphas) and the flat plate starts to look attractive again.
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Old Mar 28, 2012, 12:05 PM
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And for the full story it should be noted that this is only holds ture true at 'normal' (higher) Reynolds numbers. Around the Critical Re region (ie small models) the extra thickness of a streamlined section can be a problem (causing seperation at low alphas) and the flat plate starts to look attractive again.
very true --
The Yaks and Extras use the "ice cream cone" setup - and NO sharp leding edge strips which make for a very efficient wing and one which when stalls does it very crisply
The little bitty flat foils with low loadings have a usable range of 0 to 90 degree (well almost)
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Old Mar 28, 2012, 12:23 PM
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I've actually seen in an old book where they defined the chord to be along the flat bottom instead of connecting the tip of the LE to the TE.
.
That's the "raised entry"... I noted that with this Sukhoi 26 airfoil...
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Old Mar 28, 2012, 12:42 PM
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Originally Posted by Sparky Paul View Post
.
That's the "raised entry"... I noted that with this Sukhoi 26 airfoil...
Back when-- we called that a Phillips Entry ( named by Mrs. Phillips) but in the case of the YAK or an EXTRA, the AOA for level flight has the airfoil at only a slight deviation from "0".
The shape has exellent attachment over the top but lets go instantly when a high AOA is induced Perfect for the purpose
Also the full scale aerobats are made as light as possible (sometimes resulting in a wing departing ) and power to weight is maxed
All different stuff than a Cessna 150----
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