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Airfoil selection

Old 02-10-2008, 11:53 PM
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hogflyer
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Default Airfoil selection

I’m in the process of rebuilding a model designed/built by one of the illustrious members whom frequents the 1/2A forum. He shall remain anonymous (unless he wishes to make him self known), lets just say he likes very small gliders .

When this little plane with a 21” wing span, 5” chord, and weighing in at about 7.5 ounces, was designed a couple of years ago with an Eppler 1213 airfoil. Since the OEM wing was rendered, shall we say, less than airworthy by some skillful piloting by the above unnamed builder/owner/pilot, I’m in the process of designing and building a new wing. In an effort to achieve a better ratio glide and overall efficiency than that produced by the Eppler airfoil, I’m looking at either the NACA 2412 and NACA 2415.

At this small of scale on this plane I’m leaning towards the NACA 2415 to have a bit more depth for mounting the aileron servo. With about a .15” difference in thickness between the two airfoils I don’t think there will be any noticeable difference performance or stall speed. Does this sound like the right track, or is there a better airfoil to look at?

Hogflyer
Old 02-11-2008, 12:30 AM
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Default RE: Airfoil selection

Never heard of an E-1213, I don't think he got that high in numbers.

The NACA 2415 would be my choice just to get the thicker wing, but you might want to look into an AG35 if it's a built up wing. This site might be of some interet to you.

http://www.charlesriverrc.org/articl...t-airfoils.htm
Old 02-11-2008, 12:59 AM
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combatpigg
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Default RE: Airfoil selection

Down in this size range, I can't see how you can get a floater type glider to cover any ground? I would shoot for low drag, reasonably light.......sounds like a job for some special order 1/4" thick balsa, shaped on the top only with no flat spots or ripples. I'm thinking of a plane capable of flying around enough to find some good air.
Old 02-11-2008, 09:18 AM
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Default RE: Airfoil selection


ORIGINAL: combatpigg

I would shoot for low drag, reasonably light.......sounds like a job for some special order 1/4" thick balsa, shaped on the top only with no flat spots or ripples. I'm thinking of a plane capable of flying around enough to find some good air.
CP
Makes sense to me, you thinking of something like this? I built one, flew pretty well. 39"

http://www.charlesriverrc.org/articl..._apogeehlg.htm
Old 02-11-2008, 10:17 AM
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DeviousDave
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Default RE: Airfoil selection

At the sizes we are talking about, the Reynolds numbers just don't work out too well-you can scale down the airplane, scale down the airfoil, but you can't scale down the air molecules. I have to say though, I am very partial to the stock thinckness RG14 for planes in the 1/2A ish size range. It's a glider airfoil, but has seen a lot of success in pylon. Hell, the winningest airfoil in F5D is the MH40, which is a thinned RG14..

As long as you have a clean semi-symmetrical wing, you won't notice a huge amount of difference between airfoils though. It only shows when you are really pushing it with the airframe and have everything dialed in that you can properly test between these engineered airfoils-they really are that close.

Dave
Old 02-11-2008, 10:39 AM
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MJD
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Default RE: Airfoil selection


ORIGINAL: DeviousDave

At the sizes we are talking about, the Reynolds numbers just don't work out too well-you can scale down the airplane, scale down the airfoil, but you can't scale down the air molecules. I have to say though, I am very partial to the stock thinckness RG14 for planes in the 1/2A ish size range. It's a glider airfoil, but has seen a lot of success in pylon. Hell, the winningest airfoil in F5D is the MH40, which is a thinned RG14..

As long as you have a clean semi-symmetrical wing, you won't notice a huge amount of difference between airfoils though. It only shows when you are really pushing it with the airframe and have everything dialed in that you can properly test between these engineered airfoils-they really are that close.

Dave
Yeah, but while the differences are not huge at these sizes, they are still there to some degree. What is there, is the difference between a decent choice of well engineered airfoil and shoe sole airfoils, though some disagree and that is fine. I've built many 36" ish span models and the leap in glide performance when I used a properly plotted airfoil profile versus the flat bottom french curve rounded LE airfoils, was very noticable. One powered glider I built used no more than the venerable old Clark Y (a real Clark Y not what people call Clark Y's sometimes) on a 5" x 36" wing, and the performance difference was big time - vey significant.

MJD

Old 02-11-2008, 12:12 PM
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Briannixon
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Default RE: Airfoil selection

What is there, is the difference between a decent choice of well engineered airfoil and shoe sole airfoils, though some disagree and that is fine.
To me, this is an area where many would benefit from some critical thinking before building smaller models from kits or plans. This may be particularly true for Guillow's builders. Look for a good justification of why a particular airfoil section was chosen (this is sometimes stated on the better plans or in build articles). If the section is a symmetrical size 10 medium, I would look for something better and known to work in the model's particular performance regime.

Accuracy in construction is also important, as well as issues such as washout, no washout, etc.

Brian
Old 02-11-2008, 12:18 PM
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Default RE: Airfoil selection


ORIGINAL: MJD


used no more than the venerable old Clark Y (a real Clark Y not what people call Clark Y's sometimes) on a 5" x 36" wing, and the performance difference was big time - vey significant.

MJD

The true Clack Y is a great airfoil!
Old 02-11-2008, 12:22 PM
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Default RE: Airfoil selection

Sorry guys, but while I was half asleep when I was writing this thread, it looks like I omitted the part about this being a small 4-channel electric powered aerobatic plane that was a take-off based the LLS, not a glider. I know that will make a wee bit of difference. I'm going to fix the existing fuselage and build a new wing.

Hogflyer
Old 02-11-2008, 12:35 PM
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Default RE: Airfoil selection


[quote]ORIGINAL: hogflyer

I’m in the process of rebuilding a model designed/built by one of the illustrious members whom frequents the 1/2A forum. He shall remain anonymous (unless he wishes to make him self known), lets just say he likes very small gliders .

Hey I like the sound of that member already. Good luck with the wing.

Hec
Old 02-12-2008, 07:00 AM
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Default RE: Airfoil selection


ORIGINAL: MJD


ORIGINAL: DeviousDave

At the sizes we are talking about, the Reynolds numbers just don't work out too well-you can scale down the airplane, scale down the airfoil, but you can't scale down the air molecules. I have to say though, I am very partial to the stock thinckness RG14 for planes in the 1/2A ish size range. It's a glider airfoil, but has seen a lot of success in pylon. Hell, the winningest airfoil in F5D is the MH40, which is a thinned RG14..

As long as you have a clean semi-symmetrical wing, you won't notice a huge amount of difference between airfoils though. It only shows when you are really pushing it with the airframe and have everything dialed in that you can properly test between these engineered airfoils-they really are that close.

Dave
Yeah, but while the differences are not huge at these sizes, they are still there to some degree. What is there, is the difference between a decent choice of well engineered airfoil and shoe sole airfoils, though some disagree and that is fine. I've built many 36" ish span models and the leap in glide performance when I used a properly plotted airfoil profile versus the flat bottom french curve rounded LE airfoils, was very noticable. One powered glider I built used no more than the venerable old Clark Y (a real Clark Y not what people call Clark Y's sometimes) on a 5" x 36" wing, and the performance difference was big time - vey significant.

MJD


I was assuming that he was starting with a modern airfoil (RG&MH series etc.)-coming from a Clark Y or generic 8-10% thick semi or full symmetrical wing you'd notice the difference. I'm used to talking about airfoils with pylon weenies and glider nerds, it can border on religion when talking about airfoils with these guys. I have found that in the 1/2A size range, there is very little perceptable flying difference between the modern airfoils at the weights we fly. When the wingloadings get hairy, some are better than others but on a 7-20oz model you'd never see it.

Haven't had a plane with a clark y or french curve airfoil in ten years or so...
Old 02-12-2008, 09:26 AM
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Default RE: Airfoil selection

Ol Hogflyer has taken on the challenge of Re-V-Amping my old Re-V-Amp. I partially re-kitted the plane while flying inverted about 3 foot off the deck. That'll learn me! Doh! He has the plane and is going to build a new wing for it and repair the fuse. The fuse isn't bad but the wing! OH Nelly! Here are some pics of what he is working on.



Hec

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Old 02-12-2008, 10:30 AM
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Default RE: Airfoil selection

Kind of a SpaceWalker color scheme going on there, with power from the Darkside, looks cool. I have a SR250 looks very similar.
Old 02-12-2008, 05:22 PM
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Default RE: Airfoil selection

This is an interesting thread. I tend to come from the french curve school of airfoil design, but I'd like to try the more scientific approach, so here's a question:

What airfoil should I use for an own design old-timer cabin style model, say 45", powered by a small ic (049 or 06 diesel, or 07 glow), girder style/lightly built, weight under 30oz, ruddder/elevator/throttle control. I like them to fly like old-timers - floaty, slow-medium speed. Can anyone advise pls?
Old 02-12-2008, 10:24 PM
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Default RE: Airfoil selection

I've flown AMA fast combat long enough to have seen the battle between ultra precisely executed airfoils VS your basic french curve designs. These planes are RTF at 18-20 ozs with .36 engines [that contribute 8 ozs to the RTF weight]. These planes don't quite have the speed of Q-40s, but will "out turn" anything. There is really no difference in performance between the 2 schools of thought as long as the wing loading is kept within limits. As we speak, the clod-hopper airfoiled planes rule the roost and most of the guys who used to show up with the carefully plotted wings have switched to your basic Russian ARF types that have about 8 inches between each rib..

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