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Foolproof airfoil

Old 11-03-2006, 08:11 PM
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west6008
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Default Foolproof airfoil

After years of fiddling with airfoil shapes on wings I built,
I could never find anything worth all the hassle.
Epplers, NACA, etc,etc
Finally, a more experienced builder suggested I stick with
a "shoe" airfoil.
Step on a piece of paper and trace the outside curve of your shoe,
and use that for the upper surface.
A straight line is good enough for the bottom, but you could trace
the instep for the undercamber if you're ambitious.
Old 11-03-2006, 08:24 PM
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combatpigg
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Default RE: Foolproof airfoil

The bigger and heavier the plane is, the more important the airfoil is. So far, the best flying models that I've EVER seen are those ShockEFlyers with the flat plate airfoils and reversable pitch props. I like flat bottom airfoils for RC combat because they are less work and they get the job done.
Old 11-03-2006, 08:25 PM
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Default RE: Foolproof airfoil

If you're using built up open fram wings then yeah, that's about all you need. The inaccuracies in the covering sag that occurs between the ribs reduces the finest airfoil to chopped liver as far as the airflow is concerned.

For open frames the Florsheim 9% works just fine. Use a flat bottom for floaters and trainers and a double Florsheim with some curve on the bottom for models that will spend more time upside down and fully symetrical double Florsheim.

Also you typically only really see the bigger advantages of the nicer airfoils coupled with more supportive structures on gliders where you've got only a limited amount of "fuel", as in height, and you need to spin it out as much as possible.
Old 11-03-2006, 08:27 PM
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rainedave
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Default RE: Foolproof airfoil

What if you're Bozo the Clown? Wouldn't that make the LE radius to blunt for 1/2A models?
Old 11-03-2006, 08:51 PM
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Default RE: Foolproof airfoil

I like a Clark Y. Semi-symmetrical so it's got nice lift with fair inverted performance, and it's got a nice flat bit on the bottom to make building easy.
Old 11-03-2006, 08:56 PM
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soarrich
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Default RE: Foolproof airfoil

Ditto the Clark Y, it's easy to build and it flies great.
Old 11-03-2006, 09:36 PM
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treedog
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Default RE: Foolproof airfoil

So a clark y would be fine for a racer a warbird and a trainer/park flyer ?Great deal
Td
Old 11-03-2006, 09:38 PM
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burtcs
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Default RE: Foolproof airfoil

All:

Ditto then Clark-Y somemore...

HTH - Steve B.
Old 11-03-2006, 09:46 PM
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soarrich
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Default RE: Foolproof airfoil

ORIGINAL: treedog

So a clark y would be fine for a racer a warbird and a trainer/park flyer ?Great deal
Td
Yep! Even a sailplane.
Old 11-03-2006, 10:39 PM
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dieFluggeister
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Default RE: Foolproof airfoil

I never realized that Dr. Scholl did a lot of work in fluid dynamics... Hmmm.

I had read that the covering sag between the ribs of an open frame structure was even suggested to be helpful in low Re situations (like 1/2A) below 200,000. It was proposed that it may actually reduce drag and increase lift by causing a turbulent boundary layer and delaying separation. So for small and slow models the old-timers built them perfectly!
Old 11-03-2006, 10:53 PM
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nekked_man_2000
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Default RE: Foolproof airfoil

I never say much, usually just read. But here's my 2 cents. Usually I build D-tube type wings with open structures past the spar. And to that end, I've found little or no difference in what I do to the airfoil behind the spar. I've done naca and I've done the "it just looks right method" and made the section from the spar to the TE just a straight line. But I find that the front section from spar to LE makes a big difference. For the way I like to fly, I prefer a relatively thin wing with a small LE radius, fully symmetrical. But I find that the Clark Y works well for a lot of stuff, and I like A good NACA 0018 on a fully aerobatic bird, but I don't get good snaps or spins because of the LE radius and the thickness, but too me the NACA 0018 lands and flies as slow as a Trainer, but is much more crisp on the roll axis, and is better for obvious reasons on outside maneuvers than a flat bottom wing.

So to summarize, I don't care a hole lot about the open bay areas of the wing, I care a great deal about what I do with the sheeted 25% to 30% on the front. And you can't go wrong with a Clark Y.

Austin
Old 11-03-2006, 11:26 PM
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Default RE: Foolproof airfoil

Freehanding a Clark Y works well, too - yiou don't need to worry about getting a true plotted airfoil. Close 'nuff is good 'nuff.
Old 11-03-2006, 11:52 PM
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MJD
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Default RE: Foolproof airfoil


ORIGINAL: digital_trucker

I like a Clark Y. Semi-symmetrical so it's got nice lift with fair inverted performance, and it's got a nice flat bit on the bottom to make building easy.
Just so long as you are talking about a TRUE Clark-Y, i.e. plotted carefully from the coordinates, versus what so many people call a Clark-Y that is merely a ruler line with a french curve stuck on top, and the LE rounded off with sandpaper until it looks right.

I learned the lesson about the value of a true airfoil in graphic fashion in around 1992, when I was building small R/C gliders of various sorts (power pod, high start, rocket boosted..). When I flew the first of a series of similar models converted to, for a change, a pretty accurate Clark-Y airfoil, i.e. CAD plotted ribs, D-box and capstrips - man O man, did I ever get my eyes opened. I don't pretend for a minute that it is the ultimate small glider airfoil, but it works - folks like Dave Thornburg preach it's potential and I see why, it's no slouch - based on the comments in his book I made the experiment. Compared to the beach sandal airfoils I had been using, it stood head and shoulders above - the difference in L/D was astounding to me, it really was. So that lesson taught me a bit about the issue of primitive airfoils versus the real thing. Sure, depending what you expect from an aircraft it may not be a big issue for some, but it was a neat education for me to see first hand. From that point forward I have not lent any credibility to the arguments that airfoil does not matter, i.e. on small models it can be taken lightly. To me it was as easy to build that wing as any other and the benefit of a good airfoil was evident.
Old 11-04-2006, 03:31 AM
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combatpigg
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Default RE: Foolproof airfoil

That is a point well taken. I've never taken gliders seriously enough to appreciate what a difference real airfoils can make. The smaller the model, the more difficult it is to be true to form, down to the thickness of a hair. Even the overlap at a covering seam represents a significant deviation from the form that was proven in a wind tunnel. We have to be realistic about what we strive to achieve here. I don't really bother with stack sanding a batch of ribs anymore, I just want the notches to be uniform so that the spars fit. AMA fast combat planes might be the highest performing examples of planes in our hobby, if you combine speed with manueverability, and the best examples of these planes don't use real airfoils, they emphasise lightness and strength. They also have a 6:1 power to weight ratio, but that's for another topic.
Old 11-04-2006, 07:35 AM
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Default RE: Foolproof airfoil

Obviously, for a high-performance aircraft, one can accept no substitutes. For most sport applications you can eyeball it - but a good thing to do is have a reference to eyeball to. A lot of people probably don't know what a true Clark Y looks like to begin with. The attached are true plotted Clark Y foils, rotated just a tad to show off the flat section on the bottom. One of the main problems with airfoil construction in models (especially itty-bitty ones) is the trickiness of getting the trailing edge sharp enough to match a plot. More real-world compromises...
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Old 11-04-2006, 05:55 PM
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MJD
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Default RE: Foolproof airfoil


ORIGINAL: digital_trucker

Obviously, for a high-performance aircraft, one can accept no substitutes. For most sport applications you can eyeball it - but a good thing to do is have a reference to eyeball to. A lot of people probably don't know what a true Clark Y looks like to begin with. The attached are true plotted Clark Y foils, rotated just a tad to show off the flat section on the bottom. One of the main problems with airfoil construction in models (especially itty-bitty ones) is the trickiness of getting the trailing edge sharp enough to match a plot. More real-world compromises...
And the other feature often missed on crudely copied "Clark Y" airfoils is that the bottom section is only flat up until the 25% point.

I probably got the trailing edge on my glider wings down to about 1/32" or so.

The way I look at it, sure on small wings it is indeed to make a 100% faithful copy of the selected airfoil, but why not give it the best effort and hope for the best, rather than tossing the idea out the window?

MJD
Old 11-04-2006, 06:23 PM
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combatpigg
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Default RE: Foolproof airfoil

MJD, my only answer to that question is to know where it is important to spend your time, and to know where that effort is a waste. On a 40 mph plane that weighs 14 ozs, will never get flown in any kind of competition or have anything else to compare its' performance to, I would worry more about just building it straight. A local aerodynamicist used to design and build all his own F2D and AMA combat planes with airfoils that were plotted down to the .001". He did this for many years and his planes performed as well as anyone elses', usually better. Then along came RTF planes that stressed strength and lightness over aerodynamic perfection and these planes gave the pilots as many scoring opportunities as the home brewed planes of "perfection". To make a long story short, he now flies the RTFs from eastern Europe.











Old 11-05-2006, 01:03 AM
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Default RE: Foolproof airfoil

I cant believe the bad rep the Nike-Y is getting
Old 11-05-2006, 09:22 PM
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Default RE: Foolproof airfoil

If you've got the time and want a delta, try building a Lil Streak off my site. It's a great example of what a good airfoil can do for you. It has a center section that starts as Clark-Y and blends out to a full symmetrical. The result was an airfoil that had built-in washout. (aerodynamic washout? I forget what it's called) It will GLH with an TD .049 and land at a walking pace. With a .15, except the top speed, it's not much different. A well thought out airfoil will make a difference, but like CP says, lighter is better. Less weight makes a MUCH bigger difference.
Old 11-06-2006, 12:17 AM
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Default RE: Foolproof airfoil

Knowing where to locate the thick part of the airfoil, and picking the right thickness for the job at hand seem to matter the most with our little screamers, then just create a smooth shape that doesn't offend those air molecules too badly. With an unpowered glider, even a little HLG, I'm sure it pays to be more scientific.
Old 11-06-2006, 01:32 AM
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burtcs
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Default RE: Foolproof airfoil

Not to offend anyone I'm (yes it's out now) a Clark-Y buff. the Clark-Y is not a flat bottom airfoil as some would think. It is semi-semetrical, check the co-ordinates. I don't recall but I think it is flown with about 2 degrees of positive incidence. Not far removed for eveybody's sweetheart the 2415 (ich). This makes it appear to be flat bottomed. Yeah... The big bonus is that it's easier to build that way. It flys well and it builds well, at least as good as the builder anyway.

If I were going for speed I might use something else but the Clark-Y works well. My mind boggles (doesn't take much these days) at how many inocent models have been stupidly and senselessly killed with a 2415 airfoil.

HTH - Steve B.
Old 11-06-2006, 05:04 AM
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Default RE: Foolproof airfoil

For sport flying, a good approximation of a Clark Y is super easy to freehand. Locate your spars at about 30% of the chord, and make that the thickest part (around 18% thickness works good). Make the bottom flat from the spar to the trailing edge. Make the leading edge radius about 25% of the thickness, and center it at 1/3 the thickness. Draw your curves, making them nice and gradual. Not plotted, I know (and not a true Clark Y) but it's reasonably close and it will perform very well for average knock-about flying. Discussions on accurate airfoils can go on forever, but the fact is that unless your goal is competition there is a fair bit of leeway in airfoil choices. It's also important to note that it's nearly impossible to to build any true airfoil. The only way to come close to a perfect plotted airfoil shape in your wing is to sheet and paint, or foam and glass. Film covering overlaps change the 'foil (as do sags over open bays), and then there's the fact that if you put ANY control surface on it, you've just corrupted it no matter HOW you engineer it.

Bottom line, know what your intended airfoil looks like, come close, and it'll be just ducky.

P.S. I second the notion of having the datum line (I think it's called?) of the fuselage parallel to the flat bit of a Clark Y - that indeed gives you a flying incidence of around 2%. Put the stab parallel to the datum line and give the engine a couple of degrees downthrust to the datum line and you're ready to go - 'twill be OK.

P.P.S. Of course all of this assumes your plane is intended to fly using it's wings and the lift generated from them.
Old 11-06-2006, 08:45 PM
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Default RE: Foolproof airfoil

Bozo gave up model flying years ago.
Old 11-07-2006, 08:38 PM
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Default RE: Foolproof airfoil

Digital t Here's a question for u or any 1 . Would it be better to have two ailerons outboard on the wing rather then 1 long 1 running the whole lenth almost?
Td. I'm talking about for better lift and control purposes.
Old 11-07-2006, 09:26 PM
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Default RE: Foolproof airfoil

I don't know for sure, but I suspect 'barn-door' ailerons as used in full-scale aircraft work better than strip ailerons. The closer to the root of the wing, the less effect the ailerons are going to have, simple principles of leverage. Also, because barn-doors are wider, I would think that there would be an improvement in the overall airflow over the control surface under deflection. I suspect that strip ailerons might cause a lot more overall drag. The one great advantage of strip ailerons is the ease of construction. In model sizes of course, the difference may not be all that significant. I'm trying to think of any full-scale aircraft that use strip ailerons, but I can't come up with any offhand. There's probably a darn good reason for that. Things that work well in model size sometimes don't translate that well in full-scale (I wouldn't want to take a ride in a human carrying flying lawnmower, for example).

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