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Old Jan 13, 2011, 12:28 PM
The earth sucks! Thankfully.
United States, OK, Tulsa
Joined Jun 2003
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Alternative to the Classic Clark Y Airfoil or Profile Sought

Howdy folks:

The Clark Y airfoil has a camber of 3.6% and a thickness of 12% depending on whose data you’re looking at. The newer Eppler E205 is 3.01% and 10.5% and the Selig S3021 is 2.96% and 9.47% respectively. I am looking for a flat bottomed airfoil near 12% thickness with the advantages of newer design, any suggestions?

Thank you.
Jim

Clark Y


Eppler 205


S3021-095-84


Jim
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Old Jan 13, 2011, 12:50 PM
York Electronics
Gary Warner's Avatar
Dallas Tx USA
Joined Apr 1999
2,505 Posts
Starting with the S3021, thicken it to 12.9% and camber to 3.5%, or 3%... I can't remember. I did this on two airplanes that were designed to be huge, slow and a very soft stall. It's the best modified airfoil I've ever flown.

My numbers are only from memory. I'd have to check when I get home on one of the templates. The point is, it results in a flat airfoil section from the spar to the trailing edge, making flat-board building and covering easier.

Here's one of the two airplanes that use this airfoil:

http://www.rcgroups.com/forums/showthread.php?t=157130
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Old Jan 13, 2011, 01:47 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
11,166 Posts
If you look at the lift/drag chart for the Clark Y you'll find that it really doesn't need any "upgrading". It's a great airfoil as it sits.

The point behind the Eppler and Selig airfoils was to get a lower camber thinner airfoil that works for modern sailplane flying. The Selig version being inspired by the Eppler version but that takes advantage of newer pressure gradient coding. Both of these airfoils shift the lift drag best performance "bucket" down to a lower drag value at the same time that they shift the best lift range down so that there's less drag at the very low coefficients of lift near to 0. The idea being that it optimizes these airfoils more for faster flying but that can still slow down quite well. Meanwhile the Clark Y and any similar airfoil with a similar amount of camber is intended to fly best at a higher range of Cl's and at slower speeds.

The question becomes what do you want to use the airfoil for? We know you want 12% but what other things did you want to change or enhance? Lower drag at high speeds and low lift coefficients? Higher lift coefficient ability for slower speed flying? Something else? Is this going to go onto a power model for sport flying or a sailplane for thermal flying efficiently over a wide speed range?

All of these questions and more need to be answered before we can suggest anything for you as there really isn't a blanket "better" option that is readily available that I know of.

The other option would be to run the Clark Y coordinates through Xfoil with a sharp eye paid to the pressure distribution looking for bobbles that would encourage an early flow separation. Some minor shifting of the shape may offer some advantages in that area.

But any minor improvements would require that you reproduce the airfoil in your wing to a high degree of accuracy. For example open frame ribs and spars that are then covered would totally ruin any small improvements that could be made. To realize the improvements you would need to do an all sheeted wing or foam and skin wing so that the airfoil is reproduced accurately to within a very small percentage of error. Bascially the thickness of a pen line in the airfoil accuracy over an 8 inch chord would make the difference between seeing some improvement or not.
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Old Jan 13, 2011, 05:29 PM
The earth sucks! Thankfully.
United States, OK, Tulsa
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Give me a day or so and I'll post the things Bruce mentions in post #3.

Jim
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