Airfoils

FAT and BLUNT is my favorite.

Of course, that means more thrust is needed because FAT is DRAG.

I think a safe rule is “Fox 35-light airplane and thin wing, schnuerle engine over killer and overweight FAT WING”. This is not hard and fast, but not a bad guide.

AS a node in that domain I prefer "fluffy" to fat....

>However, if anybody knows of
>any wind tunnel or CFD
>data on symmetrical airfoils, particularly
>with flaps, I'd be interested
>in seeing it.

I think I have seen and been on a NASA web site where you can enter an airfoil and then watch its performance interactively. If not, I think I have seen some shareware that will do essentially the same. The "blunt" airfoils (I'm using the old NACA0024 on my new plane) do perform well at relatively high AOA's and when the do stall, tend to do so gently.

I'll see what I can find in the archive of sites.

FWIW,

>Just curious to hear some discussion
>regarding thick airfoil with blunt
>leading edge vs a somewhat
>thinner airfoil with a sharper
>leading edge. Any thoughts....especially
>with regards to flying where
>windy conditions prevail?

Boy, lots of variables here.

I first break it down to flapped and unflapped ships. Unflapped ships seem to perform better with a blunt airfoil, especially in the squares. In windy conditions, it seemed (this is purely subjective) that the blunter airfoils helped reduce "windup" that you get when flying in high winds and no flaps.

For flapped ships, I have seen and experimented with blunt LEs, sharper LE's, and various thicknesses. With the engines we have today, constant speed, even in the wind, seems to be more easily attained than back in the days when the Fox 35 and the OS Max-S 35 reigned supreme. Anyone who has flown a Nobler with a Fox 35 when a good breeze was blowing know that by the third loop, they better hang on.... On windier days, I have had the same experience with a NACA 0022 airfoil as I have with thick but differently shaped airfoil that comes out of the Cardinal. A thick airfoil helped (regardless of the LE shape), but even more important (again purely subjective) was the engine/prop/RPM combination in keeping the speed constant.

I read a comment once by George Aldrich on how impressed he was with the Brits in their ability to fly in high winds, and do a good job of it. Considering the climate, they are probably more practiced in windy weather flying (along with rain, cold, dampness, and fog) than most of us here in the US. Note - this is by no means a slight to our Pacific Northwest flyers who have to deal with much of the same. Perhaps some of the UK readers could comment as to how they manage windy weather flying.

I am sure there is a bias here. I fly regularly with the designer of the wing I fly and he has been flying it for 35+years so there is the real trim secret. He now has a high aspect ratio wing with a more blunt thicker airfoil. He says the same thing about trim work. Once it is in though it is strong flying machine.

I gues what I am saying is you can accomplish good flights with either type of wing. The real trick is building it as straight as possible and light as possible.

DMoon

I still like the compromise approach, fairly thick airfoil at root about 22-23 percent, 24-25 percent at the tip, with the leading edge somewhat blunted but not highly blunted with the high point at about 25% this give a compromise between high manuverability and easier tracking. Like you said in another post everything is a compromise and find the one that fits what you like. If you say well that sounds pretty much like an SV type airfoil, that's probably a close assessment.

An additional advantage of a seriously fat airfoil is the fact that it can be built extremely light and still be strong and flex free.

Mark

Doug,

I have better luck with blunt and high aspect in the wind. But that's probably just me. The plane I flew this year is very high aspect and a very thick airfoil with blunt LE. Was great in the wind. It had more problems on calm, hot days. Of course, this plane also has "pocketed" flaps and elevator (inset into TE of flying surfaces) and a very, very, very long tail moment along with very large stab (elevators are pretty small).

And just ot show my consistency, The new plane is very low aspect with a very Juneau like airfoil.

As for airfoils, there was a good article in Stunt News about May of 2000, and what I got out of it was that too blunt and the airfoil would not track well. The week after it hit the mailbox, I noticed at Stunt-a-thon 2000 a new Brodak Profile Cardinal (OS .46LA) of Terry Mitchell flew very well with excellent tracking. Looking at the airfoil (Patternmaster, I expect), it was thick at the high point and yet was not terribly blunt. I don't recall the wind speed, but the Clover Park site is almost always turbulent, over the adjacent lumberyard warehouse/yard. You almost hope for rain, when it is less likely to be windy. Steve

The Sea Fury, the plane described in that article, is my all time favorite Nats PA Winner. Bob's right, the article is a must read for a treatise on the design of a great airplane as well as the trimming process for same. A lesson in study and persistence.

Thanks.

OS

...How do we reconcile what happened to Tod Lee? When his airfoil on his Magnum/Mustang collapsed at the Nats to darn near a pollywog or flat airfoil right behind the highpoint? I seem to remember reading he said it made surprisingly little difference in the airplanes flying ability. (see cover shot of new Stunt News, it shows real well!)

One would tend to think with all the attention we give airfoils, that his ships flight would have changed drastically. Is this a red herring?

Curtis Comers, Skydancer was designed that way back in the early 60's. The plane flys very well. So it would seem that the shape of the aifoil rearward of the high point makes very little difference in overall performance of the airfoil.

Hi Curtis, I know you read these and don't comment though!

DMoon

This whole thread and your brief note reminded me of an old discussion you and I had many years ago. Unfortunately, I can't remember squat about it. I am picturing a graph where the X-axis is Reynolds numbers and the Y-axis is C/L or perhaps just L.

What I do remember quite accurately is that Reynolds numbers play a big role, and the graphs showed a huge leap as we crossed some magic Reynolds number threshold. That is, the graphs showed a flat linear relationship up to some point, and then a big vertical jump, and then a straight linear after the jump. Combat planes were near the edge of the jump.

Gerry Ritz designed strictly low RN airfoils for gliders, and used a log curve for undercamber, and didn't care much what happened on the upper surface, as I recall.

Anyway, in the 60s 18% was pretty radical, and as we learned more of how seperation occurs behind the high point, we got thicker. I don't know that NACA data covers the RN range we fly in.

Just to take this point a little further, help me get a good grasp on this... Could take apart the airfoil for our use and purposes (not real world full size planes) and break them up as these, with the following effects:

The leading edge shape -- wind penetration, tracking, stall?

The leading edge distance to the highpoint -- generate lift, some effect on stall?

Overall thickness at highpoint -- sets the amount of lift possible?

Behind the highpoint -- maintains the slipstream, keeps lift?

Trailing edge thickness -- tracking(amount of neutral around flap?)

Trailing edge shape --- ????????????

Great discussion!

I have noticed that the gliderboys often use turbulator strips on
the topside of the wing, in front of the wing thick point.

It is the idea to generate turbulence, so that the laminar flow
over the wing is interrupted. The danger with this laminar flow is the creating of a laminar bubble that might initiate stalling of the wing at low speeds (glider) or at higher angle of attack
(Stunt) by separating the airflow from the wing behind the thick
point. This is real possibility with the shiny surfaces used in PA.

When using a build up wing, the LE plancking usually extends until the thickpoint. The covering will have a small dip behind this planking, creating this turbulator effect, without us realising. For fully plancked wings however, this effect is not there and the use turbulator strips might help in preventing high speeds stalls.

But in general I suppose wings that have a tendency to stall might benefit from turbulators.

Because the problem corners are usually inside corners (Triangle/ hourglass) I suppose having these strips on the topside of the wing might be sufficient.

Anybody has experience using turbulators for stunt?

Somewhere in the deep, dark, recesses of my brain is the memory of either Brett Buck or Ted Fancher mentioning that Ted put a turbolator on a balky ship with marked improvement. Perhaps he can weigh in on this one.

There was a fairly recent thread on this. Some of the locals have used turbulators to good effect on several airplanes that were marginal wing-loading wise and had a tendency to stall. They were simple "tape a wire to the high point of the wing" gadgets. Ted's cream original Trivial Pursuit benefitted from this in the rain, and Jim Aron's "Frankenstunt" also had decent results. I think the mechanism is exactly as in the glider case.

There' has been a lot of taping wires to various places on the stab as well. This was to overcome some issues with thick flat stabs. Some airplanes clearly benefitted in level flight tracking and corner exits, others did not. The exact mechanism isn't entirely clear to me. I'm not sure I entirely buy the "stagnation point" argument. I also think that either moderately pointy airfoiled stabs are probably ideal. Flats stabs probably should be thin but it's difficult to build them stiff enough. And sometimes thick flat stabs are just fine (like Pauls, and mine).

Stab alignment makes a lot more difference than the stab airfoil in any case.

That might have been NACA Technical Report 586, which led me to pick a pretty thick airfoil for combat planes in those bygone days. NACA did some tests in a pressurized wind tunnel on NACA four-digit airfoils from 9% to 18% thick over a Reynolds number range from about 100,000 to about 10,000,000. In our Reynolds number range the 0018 had the highest maximum lift coefficient. Clmax of the 0018 was about 1.1 at a Reynolds number of 700,000. The 0015's was higher above 1,100,000. The 0012 had a jump in Clmax, from .9 at 1,000,000 to 1.4 at 3,000,000.

Modern 15% airfoils are good for a Clmax of 1.4 or so. Mind you, this is not a valid comparison, because the data come from wind tunnels with different levels of turbulence, and there may have been other differences in test method.