ideal aerofoil

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Topaz

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As someone else said -- it is too broad. And usually not correct. Especially not usually correct if looking at stall speed, but it does depend on the airfoil design. You can have airfoils that have laminar flow at cruise, and at the same time handle Clmax nearly same dirty vs clean, and thus get the same stall speed. So back to ... the statement is too broad. I wonder in what context Roncz said this? I'm guessing that some qualifiers were included. He has designed airfoils that are vastly different (not desirable) when 'turbulent' (dirty).
The qualifier was "modern" laminar-flow airfoils. Newer than the NACA 64-66 series. What's "modern"? Roncz made the comment in his 1991 series of articles in Sport Aviation, entitled "Designing Your Homebuilt." So "modern" means: Harry Riblett's laminar-flow sections, the NASA NLF series, and any sailplane airfoil designed after about 1985 or so. Roncz's own airfoils post-1985 or so also qualify, but those tend to be proprietary, and coordinates aren't often readily available.

Older (NACA in particular) laminar-flow sections tend to have lower Clmax when "dirty." One of the great improvements in airfoils in the last 40 years is that the newer ones generally do not.
 

davidjgall

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Newer than the NACA 64-66 series. What's "modern"? .... So "modern" means: Harry Riblett's laminar-flow sections, the NASA NLF series, and any sailplane airfoil designed after about 1985 or so. Roncz's own airfoils post-1985 or so also qualify, but those tend to be proprietary, and coordinates aren't often readily available.
Riblett's airfoils *are* NACA 64-66 series airfoils (actually 63-65 series; and assembled incorrectly) and the NLF airfoils were largely miserable in practice, but I'm with you on the rest of the sentiment.

(Due credit to Riblett: he had a legitimate beef with the leading edge treatment of the "stock" NACA airfoils. Due credit to NLF airfoils: they did point the way.)
 

Topaz

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Riblett's airfoils *are* NACA 64-66 series airfoils (actually 63-65 series; and assembled incorrectly) and the NLF airfoils were largely miserable in practice, but I'm with you on the rest of the sentiment.

(Due credit to Riblett: he had a legitimate beef with the leading edge treatment of the "stock" NACA airfoils. Due credit to NLF airfoils: they did point the way.)
Yes, regarding the Riblett airfoils, I'm fully aware that he's using the NACA camber lines and thickness distributions. The leading-edge treatment is part of why the original NACA 'foils exhibit the characteristic they have of shedding lift coefficient when "dirty." That de-cambering resulting from the NACA lofting method makes them sensitive to disruptions in the flow there, and the disruptions flow and grow "downhill" along the chord, as it were, dumping lift along the way. Riblett's sections have, and he notes this, a relative "leading edge droop" compared to the original NACA sections and that makes them much more tolerant of dirt and bugs, along with Riblett's choice of keeping the maximum camber forward of the 0.40c point.

The NLF(1)-0215F suffered from nothing more than being horrifically mis-marketed as a "general aviation airfoil" instead of the sailplane airfoil it really is. The aft camber that gives it both the long laminar run and low drag it has, and the gigantic pitching moment that made it so awful on short-coupled sport-planes would be perfectly acceptable in a sailplane of the period, with its long tail arm to counter that pitching moment with a relatively small and lightly loaded tail. On a sportplane, it was a trim-drag and control-forces disaster. But the authors couldn't get grant funding for designing a sailplane airfoil, so they pitched the grant as a "general aviation" airfoil and did what they wanted anyway, and published wind-tunnel data at Reynold's numbers too high for even smaller GA airplanes, just to "prove the point." Somewhere, I'd be willing to bet a fair sum that there are unpublished data runs on that airfoil at sailplane-relevant Re values. It's not sensitive to rain or bugs, at least.

The NLF-0417F, later rebranded as the GA(W)-1, was similarly over-cambered, but with a strange thickness distribution and over-thick. Trim drag problems again. It was an attempt at a "real" GA airfoil, but again by sailplane airfoil designers. There's a morality lesson in there somewhere.
 

mcrae0104

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It's interesting that the OP made a single post without return to this thread, and yet we have gone on for 4+ pages about the "ideal" airfoil (not to take anything away from many good points that have been made). We might consider whther we are being responsive to the OP's question/assertion (whichever it was). So again I'll ask: ideal for what?
 
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WINGITIS

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It's interesting that the OP made a single post without return to this thread, and yet we have gone on for 4+ pages about the "ideal" airfoil (not to take anything away from many good points that have been made). We might consider whther we are being responsive to the OP's question/assertion (whichever it was). So again I'll ask: idea for what?
Perhaps because we made no comments on his initial "ZERO DRAG" airfoils he posted!? Just a thought.
 

PiperCruisin

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The qualifier was "modern" laminar-flow airfoils. Newer than the NACA 64-66 series. What's "modern"? Roncz made the comment in his 1991 series of articles in Sport Aviation, entitled "Designing Your Homebuilt." So "modern" means: Harry Riblett's laminar-flow sections, the NASA NLF series, and any sailplane airfoil designed after about 1985 or so. Roncz's own airfoils post-1985 or so also qualify, but those tend to be proprietary, and coordinates aren't often readily available.

Older (NACA in particular) laminar-flow sections tend to have lower Clmax when "dirty." One of the great improvements in airfoils in the last 40 years is that the newer ones generally do not.
I was looking at airfoils for my design and compared the NACA and the Riblett versions. The NACA seemed to do better and does not seem to have an poor stall characteristics. They also seemed as good as a lot of the other airfoils I looked up. Just a couple points:
1. It would be nice to see the Riblett airfoils tested.
2. What airfoils do you like? I read through your design work on the motorglider (I like).
3. I saw in interview with Dick Rutan complaining about the canard on Voyager losing lift in rain, it was a Roncz airfoil I believe, but I think he fixed it.
4. I read a paper awhile back on "Universal Airfoil Parametrization Using B-Splines" which I thought would be useful to do a monte carlo optimization of an airfoil for an application (parametrize a 64-415, for example, and optimize). I've lost too many brain cells to do this myself.
 

Topaz

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1. The NACA camber lines and thickness distributions have tested extensively. The computer simulations of Riblett's modified sections show pretty much what you'd expect the modifications to do, and there's no legitimate reason to doubt the computer-run data and polars shown in his book. He's not claiming any magic, and not getting any magic. It's pretty straight-ahead stuff.

2. What airfoils do I like? For which application? Airfoils are just another thing to select to achieve a desired outcome, from my point of view. For my various design studies (which hopefully will someday result in a flying airplane!) the methods I've learned and developed result in me designing the wing first, figuring out the weight and performance of the overall airplane, and then choosing an airfoil that matches (as closely as possible) the characteristics (Clmax, pitching moment, Re sensitivity, drag coefficient and extent of the drag "bucket", build material, etc.) I need for that design. I don't think there's any generic "best" airfoil for a homebuilt. I think the Riblett series provides a really broad range of useful solutions for the amateur sportplane designer. Designers of sailplanes and motorgliders have less available, since most competitive sections are proprietary and unavailable. It's all well and good to copy the section off a sailplane on the airfield but, unless you know the characteristics of that airfoil (a polar, basically, and pitching-moment at zero-lift), it's useless for designing your own.

3. Voyager used Roncz sections exclusively, yes. I don't recall that airplane having pitch-trim issues with dirty airfoils but, since Roncz designed the replacement airfoil that "fixed" that problem on the EZ and Quickie designs, he's quite capable of doing so.

4. Heh. I feel your pain. Airfoil design is a black art, as far as I'm concerned. I've played a little, but never learned enough to design a 'foil from scratch that would accomplish a particular task. The farthest I've gotten in that quest was a simple camber-percentage modification I made to the Eppler 361 for the current version of my motorglider, due to Reynolds number concerns with the Wortmann FX79. That was a very simple change. But really, IMHO, there are so many sections available out there for so many different purposes that for most any "normal" design in our homebuilt world, the designer has more than enough material to choose from.
 

Vigilant1

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I have heard this method of choosing an airfoil for a new design, and it appeals to me.
1) Work the conceptual design and have a good idea of the sizes and realistic performance you might achieve.
2) Look for an existing plane that has similar performance and which has a long and successful track record of good flying performance.
3) Strongly consider using that airfoil as it is.

There have been so many talented designers that rolled out planes with crummy stall manners, etc and which required an airfoil change. In this area, I think achieving good, safe flight performance the first time may be better than reaching for the gold ring with an exotic airfoil.
 

Voidhawk9

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Voyager did indeed have a problem with the canard losing a lot of lift when wet. First time it flew through rain during testing it nose-dived. They only survived because they came out of the rain before reaching 0AGL.
The fix was attaching VGs at about 45% chord along the entire canard. You can see them in close-up photos. After that, there was zero pitch trim change in rain - which they confirmed part-way around the world (not often rain to test in around Mojave!).
 

WINGITIS

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Was there an actual question? I thought it was a test. Or troll, to use the vernacular of the cool kids.
I think that maybe he wanted us to investigate the "forward camber, rearward thickness" airfoil that was included in one of those pictures, basically an airfoil turned backwards.

I thought about doing it but suspected it would not resolve in XFLR5, I could be wrong about that though....!?

If he posted more actual details about what he actually wanted analyzed I would probably do it for him........

K
 

davidjgall

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...Roncz designed the replacement airfoil that "fixed" that problem on the EZ and Quickie designs....
No Roncz airfoil has ever flown on a Quickie or Quickie derivative (Q2, Q200, Tri-Q, Dragonfly, etc.) I've been trying to get somebody else to be *that* guinea pig for a long time. The Quickie/Q-2 used the GU 25-5(11)8 airfoil on the canard, which was replaced by the NLF(1)-0417MOD airfoil on the Q200 and that's all there has ever been. The main wing on each has always been an Eppler section (1212 or 1230 off the top of my head).

...Airfoil design is a black art.... But really, IMHO, there are so many sections available out there for so many different purposes that for most any "normal" design in our homebuilt world, the designer has more than enough material to choose from.
Well said!
 

davidjgall

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I read a paper awhile back on "Universal Airfoil Parametrization Using B-Splines" which I thought would be useful to do a monte carlo optimization of an airfoil for an application (parametrize a 64-415, for example, and optimize). I've lost too many brain cells to do this myself.
There are many such papers. Most get it wrong. Save your sacrificial brain cells for life's truly important sacrifices, like a really good Scotch....
 

PiperCruisin

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There are many such papers. Most get it wrong. Save your sacrificial brain cells for life's truly important sacrifices, like a really good Scotch....
I had a post going over the Riblett and equivalent NACA and never got a satisfactory response. I talked to a number of experts (NASA guys, PhDs, well known designers) and they all thought I should do a custom airfoil. (I'm thinking, "Yeah, ok. No idea what I am doing.") Riblett may be used on some aircraft successfully, but so were the NACA designs which have tunnel data. People also like to say only used airfoils with wind tunnel data (not sure I buy this) One answer I got on the weird calculation results was that not enough significant digits were used so the airfoil data needed smoothing (another algorithm I did not have).

Sorry, this is disjointed. Still find this frustrating. Maybe you're right and a drink would help.

To be fair, the story of the airfoil used on the Avid was that Dean Wilson used a couple splines till it looked "about right". It is also used on a bunch of planes.
 

blane.c

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I think Riblett softened the stall a bit and slowed down the wing a bit, So with straight NACA you will have more abrupt stall and slightly more speed.
 

Pops

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Bob Barrows tried to talk me into using a Riblett airfoil on the JMR. Said it would be a little faster . He uses the Riblett on all except for the first 4 seat Bearhawk. I ended up using the 2414.
 

PiperCruisin

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I think Riblett softened the stall a bit and slowed down the wing a bit, So with straight NACA you will have more abrupt stall and slightly more speed.
I have his book and have read it. Not saying I understood all of it. Cherokees have a
NACA 65-415 and don't have a sharp stall. Maybe it is more of a planform thing, maybe it is "modified". Of course, their wings aren't exactly smooth either.
 

WINGITIS

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Ok Folks, here is a 2412 Cessna 150 Airfoil compared to the "Rear Thickness" airfoil the OP included in his first post, tests are done at 100 Knots with the Reynolds Number matching a 34" Chord.

I closely matched the OP's airfoil as best I could and then additionally made a few mods to see what that would do, neither comes close to the Cessna airfoil but it is not SUPER BAD, it would fly.

There appear to be no drag benefits.

I am open to further suggestions from the OP or perhaps Norman or some others?

The TXT file needs to be renamed to a ".dat" after you download it, its in Selig format.

Cheers
K
 

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