Wing section question

[IanN]

 I'm thinking of both sizes of Midget Mustang, and the Bootlace in particular, which have all been said to fly exceptionally well both in the build articles and by forum members. I know there are other designs aswell

 

This was probably mentioned in one of the articles somewhere, but try as I might I've been unable to unearth it

 

Peter, is there a common wing section you use for these semi scale racers, that contributes to said excellent flying charcateristics?

[Former Member]

[This posting has been removed]

[Peter Miller]

I use a flat bottomed section set at 0 degrees. This is usually NACA3412 these days.

 

I have used Clark Y  and NACA 3312 . They are all fairly similar but the Naca ones have more of a Philips entry and 3412 has a flatter bottom for easielr building

[kc]

I think Peter meant a flat bottom section set at a true 0 degrees at the chord line,  not that the flat bottom is at 0 degrees. There is a difference.  Mentioned in one of his articles ages ago I think.

Could you confirm Peter?

[Peter Miller]

Yes, always measure the incidence on the chord line which is a line drawn from the centre of curvature of the leading edge to the trailing edge.

 

Measuring from the bottom gives anything up to 4 degrees positive.

[John Peedle]

Peter,

 

What would you advise for a symmetrical airfoil (for a true aerobatic beastie)?

[Bob Cotsford]

more or less on track for this thread, I recently came into a PM design, American Nightmare.  This is a small twin, with Peter's usual flat bottomed wing section.  What surprises me is just how sensitive to CofG it is.  With the CofG a few mm behind the plan position it was almost unfyable, whereas most semi or fully symetrical sections seem to treat the CofG as a rough guide, not really biting unless extremelt tail heavy.

So, is the sensitivity down to the section, or some other design/buiding trait?  It all looks square and aligned.  The weight is around 1.5Kg ready to fly with electric power, call it 52 oz, against the designer's result of 59oz for an ic version.

[Peter Miller]

For a fully symmetrical section I would go for NACA 0012, 13 or 14. You could even go upt to 0016. For a true Fun Fly model NACA 0020 is OK.

 

American Nightmare uses Clark Y, a pretty docile airfoil really, CG is shown at 25%.

 

Since then I have moved to NACA 3412 which is still 12% but has a better Phillips entry and this makes it more like a symmetrical section.

 

I have found...to my cost...that 25% is fine but going back to 30% makes models very unstable at best.

[John Peedle]

Thanks, Peter. So useful to be able to lean on your expertise.

[IanN]

Looks like NACA 3412 will be an excellent starting point then. Thanks, all.