Hi All,

I am in the process of designing a Pitts Model 14. The documentation lists the airfoil as a NACA 23012 but searching around the net on this airfoil has me a little scared of using it on a 1/4 scale project (~65" wingspan). I've only ever designed mono wing aerobats using NACA 00** airfoils in the past. Should I stick to those or go with the 23012?

Also, the airplane's original documentation shows the bottow wing with dihedral. However, the actual build thread photos on steenaero's website show the wing without dihedral. Which way should I go with that?

Thanks!

John

The more powerful Pitts are closer to model airplanes - the 12 is a BIG model airplane
using a symmetrical airfoil and and setting the panels at 0-0 will work just fine - On our Buckers we did just that -for best results
here is the caveat - if your mode l is relatively heavy or underpowered -it will be closer to an underpowered full scale bipe
all this busines of special airfoils and placing one wing at a different angle - is just compromised settings to compensate for low power and higher weight.
If yours is to be mildly aerobatic - use the dihedral in lower wing - it looks better but for full ot aerobatics it won't matter - you wil have to allow for rudder couple tho--

Thanks for the response Dick!

I think I'll go with all four wing panels flat. Makes life easier and will match the current prototype anyhow. So any objections to going with a NACA 0013 for the wings?

I'd like to get this bird as light and aerobatic as possible. Shooting for 15.5lbs to 16lbs and a DA50 for power. I like to hover and other 3D stuff

So if I read your post correctly, no negative incidence is required on the top wing even though it is offset in front of the bottom (the three view shows about 2 deg of neg incidence by my estimation)? Also, thoughts on the elevator incidence (plan shows about 2.5 deg positive)?

Cheers,

John

kept light -- all zero/zero/zero-- add some right thrust. I designd my Buckers such that stabs were adjustable but feature never needed

Thanks again Dick,

In my past designs, I never added any firewall right thrust...rather I found it simpler to take advantage of my computer radio's mixing ability. Is the torque effect especially pronounced on a bipe?

Cheers,

John

Unless your desire is to make it 'scale' for that reason alone, there is never a good reason IMO to mimic a full size airfoil simply because your model is likely operating at a significantly different Reynolds number. So right from the getgo, the polars which were *presumably* optimized for the full size aircraft will not be translated to your model & you may be passing up many significantly better airfoils optimized at the model's scale.

Anyway, just for discussion purposes, here is how the Cl/Cd polars look between the 2 airfoils you are curious about, the (semi-sym) naca 23012 & (sym) 0012. I chose a token Re, Im not sure how relevent it is without doing the math. The first thing that stands out is the non-symetery of the Cl/Cd profile at pos/neg AOA. I assume there is a logical reason the full size designer chose this feature which yields an extended Cl profile at positive AOA vs negative. Usually you would prefer this airfoil type on something like that spends more of its life making inside G-turns like a pylon model or HP glider. A classic aerobatic/pattern model should ideally have identical charactersitics (or as close as possible) in upright & inverted (as shown by the naca 0012). Otherwise you have differnt control inputs & a very differnt feel = extra pilot work. Is there some biplane inter-wing effect they are taking into account? I dunno, Im pretty much aerobatic biplane illeterate. Maybe someone more qualified can answer. The second observation is the 23012 has some humps & bumps in the profile vs something noticeably smoother in the 0012. There could be reasons for this relating to the vintage of coordinates, but all things being equal, bumps are less desirable than smooth, again from a pilot control stanpoint when traversing this range in normal aerobatics. I didnt look at pitching moment, but its generally accepted that sym airfoils operating at low AOA have lower pitch coefficients vs semi-sym. That means less tailplane work & variation through changing AOA, ie again sym is better IMO.

In (mono-wing) F3A models I designed, I think the classic 10% naca airfoils were pretty good, or at least a close starting point in the 2m span, 10-poundish range. Area & planform shape can often overide airfoil selection if you go too far off the ideal. The naca A series were a bit easier to build with minimal compromise in perfrmance, but these days you can cnc hotwire foam to look like anything so who cares. Certain (similar) Eppler foils are also good choices too IMO.

If this were my project, I would database as many succesful TOC/IMAC 'RC' bipes as I could get my hands on. This would be a much more meaningful & 'contest proven' approach simply because these are flown by the best aerobatic pilots in the world. Bad designs just dont survive organic evolution & it should be no surprise they start to look similar after a decade or so. Im guessing you will find something like naca0010 (sym) airfoils, maybe even thinner max thicknesses.

Good luck, love the cad rendering!

I used the E168 12,45% on my M12 26%. Check this RCU thread : [link=http://www.rcuniverse.com/forum/m_2645862/tm.htm]Pitts M12 'Spirit of Pitts'[/link]