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Old Oct 09, 2012, 04:13 AM
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Here's AG 12, 13, 14 and Clark Y plotted in AeroPlot. What is the aspect ratio (wingspan/chord) of your sailplane? Glider territory usually begins around 6.
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Old Oct 09, 2012, 01:35 PM
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So Drela, KFm, and Clark Y would be better than a NACA?
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Old Oct 09, 2012, 02:05 PM
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Originally Posted by sraden View Post
So Drela, KFm, and Clark Y would be better than a NACA?
For a 40" span sailplane.. The Drela would give genuinely good performance. The Clark Y isnt really very good for anything that small and slow flying and the KFm just isnt any good for sailplanes unless ease of building is your main driver.

Dont write of all NACA airfoils because there are thousands of variations on the NACA theme. Of the 4 digit type NACA a 3306 would be pretty good for a small sailplane, maybe not as good as the Drela but much better than the Clark-Y and in a different league performance wise from the KFm
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Old Oct 09, 2012, 02:12 PM
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Here are some polars for the drela AG13 vs. Clark-Y vs. NACA 3306 at Re=40k

Drela is good, the NACA is ok, the Clark-Y pretty awful. KFm airfoils are difficult to run on this software but performance would be worse than the Clark-Y
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Last edited by JetPlaneFlyer; Oct 09, 2012 at 02:18 PM.
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Old Oct 09, 2012, 05:12 PM
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Originally Posted by whitecrest View Post
Here's AG 12, 13, 14 and Clark Y plotted in AeroPlot. What is the aspect ratio (wingspan/chord) of your sailplane? Glider territory usually begins around 6.
Not quite sure yet...
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Old Oct 09, 2012, 10:03 PM
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Originally Posted by whitecrest View Post
Here's Clark Y and NACA 4412 superimposed in AeroPlot:
Interesting, you might also find this interesting. I went to UIUC database and found the Clark Y smoothed and I did a similar overlay. First, what I did was adjust the thickness and camber of the NACA 4412 so that it matched the "smoothed" Clark Y and then I overlayed the Clark Y. It's not just coincidence that the match gets closer.
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Old Oct 10, 2012, 12:14 AM
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Yeah, but a smoothed Clark-Y isn't really a Clark-Y anymore. For starters the (very strong) smoothing process caused the bottom surface to go slightly under-cambered. Also a NACA 4412 with thickness and camber parameters adjusted isn't a NACA 4412 any more

But I take your point.. The NACA 4412 is very close to a Clark-Y, but it's incorrect to say they are the same because there are small but significant differences.
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Old Oct 10, 2012, 01:26 AM
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What's an airfoil but a distribution of points around a mean line. That was the point of tweaking the 4412 to the same thickness and camber. Do that with a Selig or something(I should) and the visible differences would be more pronounced. It reinforces what historians have said about the NACA 4 digit and the Clark Y, they have the same blood lines.

As for the smoothed vs. non smoothed, I checked the difference between the smoothed and the non smoothed Clark Y and it was very small in thickness and camber, much less than what I did to the 4412 to normalize it. I even tried using CADD and PANE to smooth the standard Clark Y, but there are some strange extra ordinates that show up as squiggles.
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Old Oct 10, 2012, 01:50 AM
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Here, just for kicks. I normalized a Selig 4061 to the Clark Y and you can see the difference is in the back end, very visible. Eppler did a lot of tweaking on the back end of airfoils. That was where he figured he could control flow seperation, from what I've read.
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Old Oct 10, 2012, 07:28 AM
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Here's Selig 4061 and Eppler 193 superimposed on Clark Y. These are straight data plots. Both the Selig and Eppler airfoils seem to be tweaking Clark Y for less drag (thinner) and more lift (greater camber).
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Old Oct 11, 2012, 02:38 AM
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I was hoping one of you might be able to run a sequence on 4 different airfoils, RAF32, Clark Y, Selig 4061, NACA 4508. The version of Clark Y I have gives crazy results and Xfoil's pplot isnt as good as profili.

Here's what I did as a comparison, Re 50000, Mach .025.
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Old Oct 11, 2012, 07:04 AM
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Originally Posted by HELModels View Post
I was hoping one of you might be able to run a sequence on 4 different airfoils, RAF32, Clark Y, Selig 4061, NACA 4508. The version of Clark Y I have gives crazy results and Xfoil's pplot isnt as good as profili.

Here's what I did as a comparison, Re 50000, Mach .025.
I'll run them tonight
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Old Oct 13, 2012, 05:57 AM
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Wow, the more I play with the NACA 4 digits, the more I believe you can find a combination to do whatever you want.
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Old Oct 13, 2012, 02:36 PM
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Originally Posted by sraden View Post
Ok... Revised question
Best NACA and KFm (1, 2, 3, etc) for a sailplane with a roughly 40 in. wingspan. Also, can a Clark Y be used with this.
To answer the question we still need more info. What are you looking for - slope aerobatics, thermal, speed, turn performance? What approximate weight will the model be, and how will it be constructed (moulded, built up, foam etc).
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Old Oct 13, 2012, 07:12 PM
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To answer the question we still need more info. What are you looking for - slope aerobatics, thermal, speed, turn performance? What approximate weight will the model be, and how will it be constructed (moulded, built up, foam etc).
Full info:
-Roughly 4 foot wingspan
-constructed of foam (simple insulation board foam)
-not sure about weight, probably 250 grams or so
-mostly looking for smooth, steady, long flight with low stall speed

just looking for 3 different airfoils that i could compare
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Last edited by sraden; Oct 13, 2012 at 07:14 PM. Reason: forgot to add this
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