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Old Oct 06, 2012, 09:28 PM
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Common airfoils

Hello,
I am conducting a test with different airfoils to see what their flight characteristics are. In your opinions, what are the most common NACA and KFm airfoils, and is the Clark Y commonly used?
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Old Oct 06, 2012, 09:56 PM
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There is a thread in modeling science about the origins of the Clark Y. It came from Goettingen and was tweaked into the Calrk Y, then NACA came along and generalized what the Clark Y had and came up with the 4 digit NACA series. The beauty of the NACA 4 digit is that you can dial in whatever thickness and camber you want and be pretty sure it will work nicely on a model because of the Clark Y roots. Xfoil makes it even easier to do that with the command "naca ####" and bammmm there it is.
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Old Oct 07, 2012, 05:05 AM
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I've used the Clark Y on several light balsa, scratch-built ships. In my experience, it seems to fly very nicely with a flat glide angle and good penetration. It can be built flat on the building board though it is not a completely flat-bottomed airfoil. It is also thick enough to install a sturdy wing spar. I matched the wing saddle to the airfoil to ensure the correct angle of attack.
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Old Oct 07, 2012, 08:01 AM
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What is the most commonly used NACA and KFm though?
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Old Oct 07, 2012, 10:06 AM
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Probably kfm2 or kfm4...not sure about naca
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Old Oct 07, 2012, 11:22 AM
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There are no coordinates determined for any KFm airfoil, so basically anything with a step in qualifies, that's one of the problems in testing them. You only test your versions which will be different from any others because the precise geometry is undefined. KFm airfoils have a very loyal following among the scratch built foamy fans, but suffice to say that their main advantages are simplicity and ease of construction. In terms of absolute performance they wont compete with a well designed conventional airfoil.

When you say 'common airfoils'.. Common for what types of plane? Different types of plane use very different airfoils, are you thinking of general 'sport' models, aerobatics, racers, sail-plains, slow-flyers??? The airfoils on each are very different and wont perform very well if used outside of their intended flight envelope.
  • For aerobatics something like the NACA 0012 would be a typical symmetrical airfoil.
  • For trainers and mildly aerobatic sport models the Clark-Y is typical.
  • For faster more aerobatic sport models maybe something like a NACA 2412
  • Pylon races might use something like an MH23
  • For sailplanes Drela airfoils are popular, for larger gliders the AG2x series (AG24 -AG27) are common.

Obviously there are thousands of airfoils out there, so these are just examples of the types.
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Old Oct 07, 2012, 06:40 PM
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Parkzone Radian uses a Gottingen 398 airfoil.
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Old Oct 07, 2012, 07:37 PM
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I'll let you guess whether this is Clark or NACA.
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Old Oct 07, 2012, 11:44 PM
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It isn't a Clark-Y because the underside has slight undercamber
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Old Oct 08, 2012, 12:53 AM
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Hmmmmmmmm, I think the illusion of undercamber is attributable to the Xfoil graphics. It's actually a NACA 4412 which is the Clark Y. Any difference is tiny.
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Old Oct 08, 2012, 02:21 AM
CPA 30, SPA 523
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In reproducing a couple of the SPA pattern planes I found the wing root was very close to the NACA 8020. It fit the Dirty Birdy and it's very close to the Phoenix 6 and Cutlas airfoils.
I use compufoil and it has the NACA airfoils in it. You can download the program for free but you can't print them unless you pay for it.
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Old Oct 08, 2012, 05:53 AM
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Here's Clark Y and NACA 4412 superimposed in AeroPlot:
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Old Oct 08, 2012, 06:01 AM
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Quote:
Originally Posted by whitecrest View Post
Here's Clark Y and NACA 4412 superimposed in AeroPlot:
Yep the NACA does have a tiny bit of undercamber. But you would be hard pushed to detect any difference if both were built by traditional methods.
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Old Oct 08, 2012, 08:18 PM
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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.
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Old Oct 09, 2012, 12:17 AM
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Quote:
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.
Ahhh.. well that does narrow it down considerably

The Airfoils designed by Mark Drela for small hand launch gliders would be the most popular and some of the best performing airfoils for a sailplane of that size. Take a look at the AG12, AG13 and AG14. Each is optimised for different Reynolds numbers which depends on wing chord. For a tapered wing the 12 would be used on the root, the 13 mid span and the 14 at the tip.

No KFm airfoil is going to get anywhere near a properly made AG airfoil. I did some testing a while back of a Drela AG wing against a KFm3: http://www.rcgroups.com/forums/showt...&highlight=elf
Average flight times were (averaged over many flights back to back):
  • Drela wing: 61 sec avg
  • KFm wing: 37 sec avg.
The difference in flight performance was very obvious to the naked eye, you will see it in the videos. Although the KFm flew fine it had much higher sink rate and would not maintain speed during the launch phase nearly as well as the Drela wing.

You could use a Clark-Y. It would fly but it's far from optimum for a small sailplane. It would give nowhere near the performance of the Drela airfoils which are optimised for low Reynolds number performance.
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