Always thought that a thikker secion at the tip would avoid tip-stall. Way of my thinking was that the funflyers with the thick section can fly so slowly.
Today I read an article in a Britisch magazine RC Model World that states that for a symetrical tapered wing the tips should have a thinner section. So for example a 14% root section (NACA0014) should have a 12% tip section (NACA0011) to void tip stall at low speed landing. It had to do with a to low Reinolds number for the tip section if this is to thick at landing speed.
I'm in the process of choosing a wing for a Giles project (tapered wing with streight trailing edge). The root is Selig 8035 14%. What is the best thickness for the tip section?

Thanks, Hans

Well, I'm hoping that Ollie will jump in here and give you a more definitive answer, but I do know this..

In my hobby experience, I've found that most people preach a simple axiom that "thicker = more lift" and "thinner = less drag". Under certain conditions, this is true, and if you've stuck to similar planes for a while, you can believe that it's that simple. (Of course wing loading, aspect ratio, etc., all have an effect too.)

But there is a point, related to size (or "scale") where that breaks down... roughly when the wing chord goes below 5 or 6 inches, and at that point, a flat plate is nearly as efficient as a traditional airfoil. (You didn't mention the size of your Giles wing.) Of course, there are other things you can do to prevent tip stall too, like adding a bit of washout at the tips, which I'm sure you know, or changing to an airfoil with a higher critical angle of attack for the airspeeds in question.

As for the relationship between root and tip thicknesses... Help, Ollie!

Where a wing will stall first depends not only on the airfoils but the taper, twist and sweep. The sweep is determined by the 25% chord line rather than the leading or trailing edges. In this case the 25% chord line's sweep forward tends to cause the root to stall before the tip. The taper in the planform has the opposite effect. These effects cancel each other with the possibility that there may be a residual tip stall tendency. How much of a tip stall tendency there is remaining depends on the amount of sweep forward and taper ratio involved.

Tip stall margin and crisp snap rolls are conflicting objectives. You should deside where you want the compromise to fall between these conflicting objectives before you try to increase the tip stall margin by thinning the tip airfoil.

Thickness is not the only thing that controls the relative stalling angle of attack of various symmetrical airfoils. The generality that thinner symmetrical airfoils stall at higher angles of attack than thicker symmetrical airfoils is not true in all cases.

Because the stalling angle of attack of the S8035 and the unspecified tip airfoil have not been measured in the wind tunnel (to my knowledge) the only other tool to find the answers is a program like X-Foil which is available as a free down load for windows from the Charles River R/C web site. With this program of Mark Drela's you can calculate the polars of the specific candidate airfoils for the reynolds number ranges in question and see what the differences are in the stall angles of attack. Be warned that this is not a trivial effort but it will give you valid comparative results.

Mike and Ollie thanks for the help so far.

@Mike,
The sizes are: 180cm span, root 43 cm, tip 26cm

@Ollie
>>In this case the 25% chord line's sweep forward<<
No streight trailing edge, not leading edge. The 25% line sweeps backwards. See planform.
Both sections tip and root will be 8035, but if you have a different suggestion, please mention.
So if I conclude further, that is why it is said that a giles tends to tipstall because these effects add on here.

The sweep back of the 25% chord line and the tapered planform both promote tip stall. Unless you are prepared to investigate the stall angle of attack of various airfoils via X-Foil, I suggest that you fly the Giles with plenty of stall margin during the landing approach. Even if it involves comming in very hot. A better alternative might be to modify the Giles with inboard flaps for landing. Deployed inboard flaps will generally prevent tip stall on landing approach.

@Ollie
I understand that it is difficult to give a final conclusion but what would you advise me to do if I do not follow the route of investigation with X-foil or wind-tunnel-tests.

Thinner or thikker or equal thick tip section?

So you agree on the article that in general a thinner tip section is better?

I would apologize for the fact that I'm perhaps a bit persistant but I can not decide myself on this point and would like to leave the choise to you.

Thanks,
Hans

There are three kinds of airfoil stall.

1. Some airfoils stall very gradually. As the angle of attack increases the coefficient of lift levels off and there is little or no loss of lift for several degrees increase in angle of attack. Instead of an abrupt loss of lift the lift persists for as much as an additional 8 to 10 degrees angle of attack increase. During this soft stall the drag increases a lot and the plane seems to "mush" along.

2. Some airfoils stall very abruptly but recover quickly (no hysteresis).

3. Some airfoils stall very abruptly but as the angle of attack is decreased, lift does not resume until an angle of atttack much lower than that at which the stall originated (hysteresis). Spin recovery is difficult and altitude loss is more severe than in the first two cases.

The second and third case give much crisper snap rolls than the first case.

When you thin a particular airfoil you won't know what you are getting as far as the stall angle of attack is concerned unless you measure the thinned airfoil in a wind tunnel or simulate it with X-foil. I'm not sure if X-Foil will tell you if the airfoil falls into case one, two or three.

Instead of trying to solve the tip stall problem during landing approach by changing the tip airfoil, I strongly suggest that you install split flaps near the wing root. When the flaps are deployed they introduce a large washout effect that makes the wing tip stall proof and lowers the landing speed. This is a very simple modification that was detailed in a construction article in the July 2001 issue of Model Aviation on page 48. You can get a reprint of the article at a nominal charge from the AMA Headquarters Library in Muncie. The advantage of this approach is that it doesn't spoil the snap roll and the results are completely predictable. Fooling around with the tip airfoil thickness does not guarantee the results you are looking for unless you do the in-depth technical investigation via the windtunnel or X-Foil first. You may have to investigaete several thicknesses and even several families of airfoils to find what you are looking for.

In other words I don't know the answer to the airoil thickness modification question but, I have outlined a way for you to find the answer and I have given you a practical solution to the tip stall issue which avoids the need for an indepth technical investigation.

The question of airfoil thickness effects on stalling has received conflicting answers because people have generalized on the basis of special cases. The generalizations are not justified in my opinion.

Thanks, Hans

Thanks Ollie! I enjoyed reading all the info you shared, even though it wasn't my post. You're always informative!

Too bad X-Foil doesn't run on a Mac!

Actually, Iīm about to build a copy of a seduction freestyle wing, basing on pics of the plane (plain geometry) measurements and the data posted from some owners;
- Root: 13.5 % thick with max at 28% of the chord
- Tip: 16% thick with max at 20% of the chord;
So, Iīve searched matching airfoils. Now itīs time to cross my fingers and wait the results (everyone love this plane behavior), what do you think?

Hello Daniel,

Great! I'm very glad to see your posted specifications on the F/Style wing. I was trying to gather that data myself for a future project with a smaller engine.

What are the other dimensions such as span, root & tip chords, and the amount of L.E. sweep?

I've deluded myself into believing (dreaming) that I can build a scaled down clone of the F/Style for a .25 that will do some of the stuff that Ono does on the videos on the web.

If you need some help with the airfoils, let me know....I have CompuFoil and can do whatever thickness & highpoint location changes you need with a click of the mouse.

ok, the chord of the tip is 62% of the root and the root is 57 % of the half span, thatīs what i got...

Thanks Daniel!