Is there any compendium of airfoils out there, and why they work?  I know that the stunt world seems to have gone toward fairly thick (up to 20%) airfoils with blunt leading edges and really thin flaps trailing off the back.  What I'd like to find is an article that discusses what each design choice is, and what the tradeoffs may be.

Tim-

There aren't many (any?) authoritative articles on CL Stunt airfoil trade-offs. However, using this forum's search function on "airfoil" gives these among the earliest of the resulting 26 pages:


http://stunthanger.com/smf/index.php?topic=14655.0

http://stunthanger.com/smf/index.php?topic=11753.0

http://stunthanger.com/smf/index.php?topic=15467.0

http://stunthanger.com/smf/index.php?topic=13931.0

http://stunthanger.com/smf/index.php?topic=2811.0

Of course I'm partial to my own posts among these (ha!). Anyway, looking elsewhere you should probably consult NACA TR 586 (cited and excerpted in these posts) and look on Igor Burger's site:

http://www.netax.sk/hexoft/stunt/index.htm

Igor discusses both wing and tail airfoil choices. You can see some of the evidence for his findings in figure #28 of TR 586, which is a free download on the NASA Technical Report Server site and has been posted by me in one of the search results above. The most vast repository of this kind of information is the SSW Forum archives. A search there will turn up more.

In short:

At our wing Reynolds Numbers, 18% and thicker sections have greater maximum lift coefficients than thinner sections. These work well with modern power, since weight is not so critical, and these thicker sections allow more weight to be carried without stall. As Igor suggests, we should stay in the linear section of the lift curve though. For tails, the RN is lower, and there is some disagreement on how to treat these narrower-chord surfaces. Sometimes turbulators are needed for flat stabs with rounded leading edges. Sometimes, if you can make them stiff enough, a 3% section is best. Otherwise, the thick sections have more advocates also specifying an "airfoiled" section at those (ca. 1/2" thick) thicknesses.

Regarding flaps. there is one NACA or NASA report (changed names in 1958) that demonstrates that the thin, flat flaps generate more lift than flaps that just continue the airfoil shape. Al Rabe concluded the same thing from his experiments, and XFOIL (FWIW) showed similar results on my airfoil. This is especially true for stationary flaps. Some of this is on the threads posted above. Martin Simons' book has some theory, but I believe he stops short of these particular CL questions. Such things are much more important to us than the RC guys, since we place heavier loads on our wings in our small circles.

Essentially you have asked very valid questions about something that has engeandered a lot of discussion on both of the top forums. This material is tough to come by; so first see whether the material already posted is sufficient. Then, if not, I'd suggest some more specific questions in the context of available data and opinions.

There are now some airfoil compendiums that include symmetrical sections, but most are best improved using profili or proven airfoils already in use. Most collections are for FF and soaring.

That is, UNLESS someone actually finds such an article outside our forums. I don't see that happening - at least not to my satisfaction. People whose opinions I recommend include, but are not limited to, Igor Burger, Al Rabe, John Miller, Phil Cartier, Ted Fancher, and Bob Kruger, since they have done some work on this question. They don't all agree on everything, but I think that they would agree on most of the basics, ruling out semantics.

Easiest thing to do would be to throw together a wing with the thicker NACA 00xx sections, use a movable or stationary flat flap and go fly. We each have posted what we think are improvements, but we agree (I think) that ours are just "a bit" better. Igor's are designed to have a continuous lift curve with flap deflection, and Al's may accomplish that too. I like mine - on paper - but have no valid flight data. They're all in the material cited above. Have at it!

SK

Tim

George Aldrich once told me that, in his opinion, the leading edge of the 4-digit NACA sections was too blunt. Despite that, plenty of people use them—me included in my early days.

If you have access to an airfoil program like Profili, Compufoil or Winfoil, I suggest using NACA 63A with the max thickness point moved forward.

I suggest the thickness be about 18% of the chord including the flap and the max thickness point about 27% of chord including the flap.

Cheers, Geoff

If you have access to an airfoil program like Profili, Compufoil or Winfoil, I suggest using NACA 63A with the max thickness point moved forward.

I suggest the thickness be about 18% of the chord including the flap and the max thickness point about 27% of chord including the flap.

Cheers, Geoff

Above I posted that I had no flight data. This is literally true, but a bit misleading. I like the airfoil I posted in a thread or two in my listed search results. It fits Geoff's description (18% 63Axxx with and without stationary flap), except that the maximum thickness is located at .24c or .25c. That wing flew quite well and seemed to do what I wanted in wind and glide. However, it was a high-aspect-ratio wing, and in the few flights before loss due to engine failure overhead, it was impossible to separate out A/R and airfoil effects. XFOIL results illustrated in my posts indicated a higher Cl and smoother, more continuous stall, if you trust any CFD program at our Reynolds Numbers. I still suggest reading what has been said on these forums.

Edit: PS - XFOIL also liked the flat stationary flap as an improvement over continuous airfoil.

SK

The NA63XX modified airfoil Serge and Geoff are talking about is almost identical to several of the best airfoils being used today. Laying one over the other, and there's often no, or very little difference. We're used to calling one a Patternmaster, Randy Smith, Impact or some such airfoil, but they are very very close to the NA63XX modified as described. 

Interesting.  So the Flite Streak had it right.

George Aldrich once told me that, in his opinion, the leading edge of the 4-digit NACA sections was too blunt.

  George was probably wrong, particularly after God gave us AAC engines,  4" of pitch,  and cheap nitro. No one has ever had a fatal problem from making the LE too blunt, but a whole lot of them have had a fatal problem making it too sharp.

    Of the examples on this page, I can pretty well guarantee that no one is going to be able to tell the difference between them - differences in the details not given and the state of construction, trim, and power will swamp any microscopic performance differences from the airfoil selection. Not that the topic isn't interesting, of course, but from a practical standpoint only the gross characteristics (i.e. percent thickness and wing loading) will be detectable.

     Brett