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Old Jun 19, 2011, 01:03 AM
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Eppler 197 vs Clark Y

any thoughts on the 2 foils? I'm curious if anyone has tried them on similar air frames. I have a plane currently using a Clark Y and had read that the Eppler 197 is a good choice because it's high lift, gentle stall. Not being a numbers person, I'm thinkinig a 15" chord & 80" w/s on a 6 lbs model. I'm NOT after a screaming fast plane. What I need is slow flight with as little energy expended as possible. (speed around the 8-10mph range)
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Old Jun 19, 2011, 02:21 AM
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You're asking a awful lot of a wing that size at that speed regardless of the airfoil. Do you want to cruise that slowly or just land. At that speed the Eppler 197 is not nearly as good as the Clarck-Y. You could use a higher camber airfoil like MEG 59 to get more lift but even then you're not likely to get more than 3 pounds of lift from 80"x15". You either need to double you're surface area or increase the target speed to 15 mph.

--Norm
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Old Jun 19, 2011, 04:32 AM
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To get a stall speed of 8-10mph then your going to have to aim for a MUCH lighter flying weight.. About 2lb would have to be the target given your stated wing area. As norm said.. that weight and wing area will give you a minimum flying speed of 15mph or a little faster.

To get a rough idea of minimum flying speed in mph take the square root of the wing loading (in oz/Sq ft) and multiply by 5.25

Steve
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Old Jun 19, 2011, 04:48 AM
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I'm guessing that you have misunderstood just how slow even a slow model flies.

The wing area and weight you specified is actually a big wing and a light wing loading. The model will fly in a very graceful manner and be grand to watch float by overhead. The actual speed may not be in the 8 to 10 mph range but it won't be much more than that.

I'd suggest you stay with the Clark Y as it's a proven performer and works well on that size of wing. But if you really must try something out instead I'd suggest you go with the slightly higher camber Eppler 201. I've got a 2 meter electric glider and I can vouch for the good manners of this airfoil at this size. On a model the size you're considering it would be that much better. But with the Eppler comes the need to deal with the undercamber on the lower rear portion of the airfoil It'll also require a fairly good arrangement to ensure adequite support for the covering on the upper surface in order to maintain a decently true airfoil. In my case this meant fairly closely spaced ribs and lots of "turbulator" support spars for the covering. On the 8 inch wide chord there's two such spars ahead of the main spar and another supportive tubulator spar behind the main spar.
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Old Jun 19, 2011, 08:45 AM
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Under camber isn't a really an issue, the wing is un-sheeted foam construction, which at very best will get some covering on it but not likely.
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Old Jun 19, 2011, 03:41 PM
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In that case the Eppler 201 would be a good choice. It's thicker than the Clark Y so the added thickness will tend to stiffen up the foam of the wing. And if you use top and bottom spars notched into the foam the greater distance between the two caps will make the wing that much stiffer and resistant to bending loads.

Flying the 201 for some years now showed it to be an airfoil that stalls and behaves a lot like the Clark Y. I suspect if you try it you'll find that it's a nice option.
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Old Jun 20, 2011, 12:11 AM
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If you want to fly slowly on low power what you need is for the minimum drag the occur at the highest possible coefficient of lift. I had some time to play with the airfoils mentioned in this thread. At the Reynolds number of the wing you described the MEG 59 is the best of the bunch. The Epplers would benefit a lot from turbulators but so would the MEG. The performance gap is smaller with turbs but it's still there.

--Norm
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Old Jun 20, 2011, 01:36 AM
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With un-skinned / un-sheeted bare foam wings (which the OP says is his build method) then airfoils with razor thin trailing edges like the MEG-59 will be impossible or at least impractical.

If using foam without any skin very much limits your airfoil choice. The Clark-Y is probably going to be hard to beat once your build method is factored in.

Steve
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Old Jun 20, 2011, 02:04 PM
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Another airfoil which would be OK for bare foam is the Selig 4233. Again a thick Eppler 201 like shape with a little bit of a reversed cusp on the lower side near the trailing edge.

Depending on your foam cutting skills you could try the Eppler or Selig if you think you can produce clean and decently accurate shapes. If the cutting skills are a bit rough then JPF's suggestion to stick to a ClarkY (the PROPER ClarkY and not just any old flat bottom round top shape) is sound.

Note that the leading edge on any airfoil counts for a lot. They can deliver or deny on their performance promises based on what the first 10 to 15% of the wing chord looks like. So be a bit more fussy on your leading edge trueness for any airfoil.
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