Hi All,I am planning on building the Project Extra designed by Mike Hurley, published in MA about 5 years ago. It has a span of 106" and is powered by a 100 cc twin. I am wondering about the wing airfoils. It has a 14% wing root and a 12% wing tip airfoil. It seems to me that this would encourage tip stalling. I would think the tip should be thicker than the root. What do you guys think?Jim

The airfoil thicknes is fine for this application - run em even thinner  with lighter wing loadinds  have done so for many years</p>

The optimum thickness for models is 12%.  Thicker airfoils don't show improvement until high Reynolds number ( full size ).  Looks like Mike Hurley knows what he's doing.

The thinner airfoils on the tip are done by design.  These help the plane to start and stop rolls and snaps more crisply.  Other models (extras, edges, yaks, etc.) have the same.

A 12% symetrical airfoil is the thinnest you can run without a loss of angle of attack/lift. There is really no advantage to going thicker for aerodynamics sake; butfor structural reasons, it can allow for a stronger wing with no weight penalty. In an Extra, you want to be able to stall the wing quickly and completely, so thats part of what your buying. This is assuming we are talking about NACA 4 digit airfoils.</p>

We used  thinner foilsyears back at  1990's TOC - worked fine -on the bipes  some foils were  9%</p>

 If weight is  low enough the foil can be  flat  dead flat  </p>

 no problemexcept structural.</p>

 In real world of  1000sq in n up stuff   sticking close to 12% allows for  a strong shape  woith good  stall /recovery  character.</p>

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Wow, thats a pretty broad statement. Are you refering to all R/C? I fly a pattern plane with 18% and its one of the most popularwings ever designed for R/C. The same wing is found on the Kwik-Fly, Kaos, and UltraSport's.

18% was common years back but present competition stuff is more in the 12-9% range.
My super light larger electric stuff (like the ERATIX is a thick wing - mainly for structural reasons
it is far easier to make a light wing if yo make it thicker.

That design was meant primarily for 3D and did pretty well at it. It actually resists snapping to the point that it is a bit of a liability in sequence flying.</p>

When you're looking at a plan for an aerobatic model that has a proven track record you're wise to accept the design as it comes.  As Dick has pointed out the details inherent in the design were not put into the plan on a whim.  Rather they are products of evolution from a series of models that led to what you see in front of you.  If the wing is set up with an airfoil transition that seems to favour tip stalling it was almost 99.999% sure to have been done because it was found to be needed to fly the model the way it is made to fly for the events it has to fly in.

It's like the stabilizer anhedral in one of the mid 70's pattern ships.  Everyone thought it looked uberkewl but when you read the details in the article about the design evolution it was done because the original stabilizer was too high and it didn't knife edge or something the way the designer intended.  So it was cut up the fuselage to move the stabilizer or somehow lower the stabilizer.  Since that model had a fiberglass fuselage cutting the fuselage would have been messy at best.  So he did the easy thing and put anhedral in the stabilizer.  It worked, he won a few big contests and others followed and put anhedral in their stabilizers wether it was needed or not because it was thought that the anhedral had magical powers.....

Now if this was for a scale model plan where only one was ever built and the designer basically said it was an evil demon to fly then you'd be right to be asking about fixes.  But on models that come with a proven flight performance you shouldn't bother trying to second guess until you fly the model.  If it doesn't suit you then it becomes more a case of you needing to adapt to the model and learn to fly it within the bounds it presents so that it can do the things it is intended to do.

I'm not the one interested in building this model, but I am saying to the guy who wants to build it that he doesn't need to worry about the tip stall stuff. The design, as-is, is difficult to stall. As a result, it doesn't snap particularly well, but does the 3D stuff about as well as anyone might expect. That's primarily what Mike designed it to do, and ofcourse wanted it to do sequence work well, but as with all compromises it does not excel in all areas.