airfoil selection site

[KenSt]

I am trying to select an airfoil for a small Waco cg-4a glider. I found this site.
http://worldofkrauss.com/ I understand the numbers such as L/d ratio and the lift coefficient but when choosing my airfoil, I don't know which one is more important than the other. What data on this site is most telling, or is it an all encompassing kind of thing?

[lincoln]

I just looked at what they had for the Clark Y (which might be a good choice for something like this, depending). It only goes up to 100,000. If you're doing a big scale job, this won't be accurate.

Check out the "Incomplete Guide to Airfoil Usage" at:
http://www.public.iastate.edu/~akmitra/aero361/design_web/airfoil_usage.htm#conventional
According to it, the scale airfoil is a NACA 43012. That might be a problem, because according to Profili, at 100,000, it's lousy. However, it's a lot better if you put a turbulator at 10 percent of chord. That way it gets a lot more lift, which means you can land slower, plus there's a wider practical speed range. If you didn't mind the airfoil looking a little different, the Clark Y would be pretty good for Reynolds numbers in the 100,000 to 200,000 range, which is where a reasonable sized RC model would be. (Ok, if the span is 60 inches, maybe not.) The Clark Y has a wide speed range and higher lift in this Reynolds number range than the 43012. On the other hand, the 43012 has a smaller pitching moment, though there are better choices available if that's important. If the Waco's stabilizer is large, this is not important. (The Schweizer 2-32 uses the 23012 airfoil, and it has what looks like a very small stabilizer. Handles fine, though.)

At the full scale Reynolds number, the 23012 has a little bit more drag, but significantly more lift. Enough that you could make the wing chord shorter, get the same lift as the Clark, and have comparable drag.


Why don't you tell us what kind of performance you want, what the aspect ratio is, tail moment, etc. and we'd be able to tell you something more meaningful.

[KenSt]

A/R 8.2:1
span 45 inches
chord 5-1/2 inches
fully sheeted wing
Tm= 15-1/2
wing area 240 sq. inches
stab. area (elev. included) 38 sq. in. (this can be enlarged to get the stability I need, I have Andy Lennon's book on R/C design :cool:)

I'm looking for slow gentle thermal gliding on a typical summer evening with little to no wind, hi start launch, over weak lift from a parking lot maybe some weak ridge lift from a tree and building line.

Reynolds, 45000 to maybe 60000 (10 to 20 mph)

It's not going to be a record setter :rolleyes: I like the scale part more than all out performance. I expect lots of landings per flying session. :p

Your right, the original 43012 looks unsuitable.

[lincoln]

I don't think you're going to see 10mph unless your wing loading is under 4 oz. and you use a fairly high lift airfoil. I'm going to guess that if you're careful you can get to a 6 oz. wing loading. An easy rule of thumb is that flight speed in feet per second is 7.3 times the square root of the wing loading in ounces per square foot. That's at a lift coefficient of 1. More realistically, I think you're going to be looking at something more like 0.7 or less. Flight speed at Cl of 0.7 is like that if you increased the wing loading by the inverse of the Cl, or in this case 6/0.7 gives a "virtual" wing loading of 8.6 oz/ft^2. So that gives us 21.4 feet per second or 14.6mph, and Re of 62k or so at that CL.

I've just been playing around with Profili. Looks like, if you want an airfoil of plausible thickness, a Clark Y wouldn't be bad, with a turbulator at 20 percent. If you don't care about the thickness, one of those fancy, thin free flight airfoils might be good. Say, an A18. Also with the turbulator, and a little less drag. But I think if it was me, I might pick the Clark Y. On the other hand, if you want something that looks more like the 43012, if you turbulate it, it's only a LITTLE worse than the Clark Y.

You don't have to worry all that much about pitching moment, since the tail volume is adequate.

Not sure how you could hide a turbulator, but it looks like in this range it makes a big difference.

[KenSt]

I looked at the Clark "Y". With a chord of only 5-1/2 inches, I didn't know if I would benefit from something thinner like the NACA 4310. I don't see anything wrong with your predictions but is the Clark going to work at that Re and will the drag penalty be too high? I'm not too concerned about absolute scale so even a mild undercamber would be OK. like a NACA 6410. My concern here would be the limited speed range. Hiding a tubulator is not a concern. If it's there and it helps, so be it. What are your thoughts on the 4310? BTW, Thanks for the reply, as you can see I haven't had much discussion :(