snsmith

My Feedback: (5)

I'm getting started on designing my first plane from scratch...I've finally collected enough information on the Technoavia SP-55M to be able to design a scale model. While I'm a mechanical engineer, I've spent the last 16 years designing plastic and metal components, not wings, so I'm at a bit of a loss for selecting an airfoil. I have Andy Lennon's book on R/C model design, but it really doesn't go into the practical "whys" an airfoil should be selected for a specific type of model.

The full-size airfoil for the SP-55M is very thick and blunt and (I'm guessing) probably doesn't scale down to R/C size very well. What's a good airfoil for a 22%-25% (72"-82") aerobatic airplane? What are the most important properties for an aerobatic wing? Should the wing have a consistent airfoil or would there be a benefit (to aerobatics) for different airfoils from root to tip?

I'm still debating the size of the model and may very well wind up building both sizes, the smaller first as a test. Also, I'm planning to use airfoiled tail feathers, so would a fairly simple symmetric airfoil like NACA 0012 or Eppler 168 be OK or is there something "better"?

Thanks in advance for any insight you can provide!

Mike James

I'm familiar with, and have used the particular airfoils you mentioned. In fact, I started using them shortly after reading Andy Lennon's articles about 10 years ago.

Most modern pattern-type airplanes have airfoils in the 12 to 13% thickness range. I personally have designed and flown several successful models that used the Eppler 168 for both the main and tail surfaces, and have been completely pleased with the performance. (And the Eppler 197 does a great job for wings too, although not fully symmetrical...Great performance with slotted flaps, per Andy's articles.)

You can't lose with the E168 or the NACA 0012. My newest "pattern-ish" design uses it too. (image attached) See http://www.nextcraft.com/j45.html

Happy flying!

snsmith

My Feedback: (5)

Mike, thanks for the feedback!

I've been doing a little more research, and I'm starting to think more in the vein of a 14% thickness for my project. NACA 0014, Eppler 169 and Selig 8035 are all roughly 14%, but with different shapes in the trailing geometry. Eppler is thin and actually somewhat concave near the trailing edge, while Selig is fatter and NACA somewhere in the middle. I ran the coordinates through CalcFoil and couldn't really discern any significant difference between them at the Reynolds numbers I used (200K, 400K, 600K and 800K), at least nothing I could immediately intuit as being "better". Any ideas on the relative benefits and/or performance of the three airfoil shapes?

Also, if I were to go to a 14% airfoil for the wing, should I use the same for the tail surfaces or stick with a 12% shape for the tail?

Thanks again!

Mike James

Well, the short answer is "I don't know."

I've done a lot of experimenting with airfoils, including the NACA 0014. Much as I'd like to say "Aha!" and point out some significant difference, the fact is that at the Reynolds numbers most of us fly at, very little, if any difference can be detected. I would only say the obvious, which is that generally the thinner airfoils will have less drag.

In my own experience, I use the Eppler 168 almost exclusively for the tail surface airfoils, regardless of the main wing airfoils, and again, can detect no difference.

When you get into semi-symmetrical airfoils, the differences are somewhat more pronounced, but with symmetrical airfoils in the 12% to 15% thickness range, I haven't seen measurable performance differences. (Most of my models have have wing chords in the 12" to 20" chord range, and flown with propeller propulsion.)