Wing Design....

I'm currently trying to design a new wing for a Sterling Yak-9. I was thinking of using a NACA 0018 airfoil (symetrical airfoil which is 18% thick). On the Yak-9, both the leading and trailing edges are swept, meaning that the tip ribs are smaller than the root ribs. My question is...should I use the same airfoil shape at the tip of the wing as at the root? Using the same airfoil shape will cause the wing to get thinner as it approaches the tip, since the tip ribs are smaller than the root ribs. Keeping the wing the same thickness at the tip as it is at the root will cause the airfoil to get real fat at the tip. Anyone have any opinions on this subject?

Thanks,

Bob Balsie

Bob --

Classic design is to keep the % at the tip the same as, or a little thinner, than the root.

>Bob --
>
>Classic design is to keep the % at the tip the
>same as, or a little thinner, than the root.
>
That's for fullsize classics, maybe. Right or wrong, thicker at the tips than at the root is "traditional." Supposedly, it keeps any tendency to stall from developing at the tips and creating rolling behavior.

K.

That's "washout" or twist in the wing where the t.e. at the tip is higher than that at the center section, so that the tips stall first, before the main body of the wing does (where most of the lift is), so you don't get a flick over into a spin (not much of a problem in C/L). I don't think thickness to chord ratio has anything to do with it (I can always be wrong!).

Washout gives the tips a less angle of attack and will stall after the center, thus giving aileron control up to the spin or stall moment. Washin does the opposite. We use both on free flight models as a matter of course to give turn patterns with and without power to set up the glide pattern for the elusive hunt for thermals. The Cessna 172 I fly has washout at the tips to forstall stall at the ailerons.

On the other hand, the full-size Yak wing gets thinner towards the tip. Our models operate I think ,under a different set of aerodynamic rules than the "big" stuff does. Still, if it were my design, I'd thin the wing towards the tips, just in the interest of less drag and weight.

If it were mine, I'd use 18% at the root and 20% at the tip.

And I'd use the 0018 in both places. You might look up NACA Technical Report 586.

You sure don't want washout if you plan to do both inside and outside loops.

Speaking of B-17s and NACA 0018s, guess what the root airfoil of the real B-17 wing was.

I suspect everyone is right about this question in one way or another. I have seen many models with thicker airfoils at the tip than at the root. I recall that the Bridi Kaos series in RC has that arrangement. They have lovely low speed handling. The theory seems to be that the thicker foil stalls at a slower speed, thus resisting tip drop at the stall. Washout does the same thing, and is used a good deal in FF as part of the trim package. But I believe that the tethered nature of CL negates the tip stall isue for the most part. What we need is a perfectly straight wing that generates enough lift for our intended purpose.

Howard --

Don't tell me it was 0018! Don't leave us hanging. What is the rest of the story?

Yep, the B-17 root airfoil was an 0018.

Bob: I have two restored ones, and neither flies very well. Any change towards a thicker section would be helpful, I would think.

To be fair to Mr Kania and the model, both of these have Mickey Mouse (read: flexible) linkage from the flaps to the elevator, and this probably is part of the "problem". I must fix.

They look cool in the air, anyway. One has a gray and green camo paint job, the other is dark green. The top covering is dyed green on the latter, but some castor seeped underneath the OB wing covering, making it even a darker translucent green, even improving the looks!

The camo one is so good, you can't see it sitting in the grass. I copied a Russkie paint color scheme, so I guess Ivan knew what he was doing when the Huns came calling.

There are going to be so many different answers to this question. There are several reasons why each might be right as well.

First, unless you plan on enlarging the wing area, I wouldn't go with an 18% airfoil. The wing is small, the chord is relativly short. Why not consider something like a 14 or 15% airfoil. If it's good enough for the Nobler, it should be good enough for a profile Yak 9.

Make your root chord 14% with the high point in it's normal position. consider 15 or 16% at the tip with the high point moved forward a few percentage points. Also realise that even at the thicker percentage at the tips, the wing will still taper to the tips as the airfoil's chord is getting shorter on that tapered wing, so the percieved thckness is looking like it is getting less as you move to the tip. I'd consider enlarging the wing to ~ 500 sq's for better wing loading.

Since you are redoing the wing, and you didn't ask, I'll ask you to consider using a straight flap hinge line in liu of the original tapered forward one. You can still have the right shape, by keeping the original trailing edge line. I think you'll like the results better. Since you'll have more flap area by doing this, consider using 2/3 flaps with the last third fixed solidly to the trailing edge. You could easily encorporate an adjustable trim tab on the outboard wing tip this way as well.

Another item you might want to consider, since you have really made a nice wing, is to redo the fuselage, Give yourself enough nose moment to mount your engine and tank with perhaps a half inch to spare at the leading edge. Use a full balsa tripler to stiffen up the nose, inset your tank to the inside of the inboard plywood doubler. ( you'll get smoother runs this way, sice the output of the tank will be closer to inline with the spraybar.) Use a 14 to 15 inch tail moment hingeline to hingeline, and enlarge the tail feathers to at least 24% of the wing area. consider a 60/40 split between the stab and the elevator. You could also set the gear up as wing gear and it would look more scale like.

Since you would be using good wood you picked out your self, the plane, though larger in size, it will probably come out lighter than most kitbuilt planes. It'll fly circles around a stock one as well.

18% though great for todays powerful engines, Are a bit to draggy at that size and loading. Normally losing speed so fast after the engine quits that they really are difficult to make a decent landing with.

I realize I'm getting verbose and going in directions that you were'nt asking about, and for that I apologise, but it seems to me that if you are going to take the time to design and make a new thicker wing, with a little more effort you could fix more of the short comings of the Sterling design and have a much better plane in the end. If you choose to simply build a new wing for the kit, be sure and check the alignments between the thrustline, wing Center Line, and the stab. The last kit Yak I biult was badly misaligned with an extreme positive angle on the wing. It would do great insides, but the outsides were less than acceptable. Aparently the die alignment moved and they never caught it on some of the later kits.

Best of luck in your endeavor, whichever way you choose to go.

I built one Sterling Yak-9 that flew very well with McCoy redhead 35 and fixed flaps. I've seen a couple recently, with flaps, that flew excellent. An 18% airfoil should do fine. There are arguments all directions for making it thicker, same, or thinner at the tips, so do what's easiest. If I were doing flaps, I'd go with the straight hinge line idea.

I hate to disagree, John, but I would definitely not, under any circumstances (have I made that positive enough) even consider using a thinner airfoil with a coupled flapped wing. The Nobler's 17% (or 18%) wing was based on the fixed flapped designs of the time. If you make the airfoil thicker on the Nobler you will dramatically improve the design. A case in point is the profile Nobler and Forerunner designs formerly kitted by Custom Models. The profile Nobler used a Nobler wing and airfoil. The Forerunner used a much thicker percentage. And there is a dramatic difference in the turnability of those planes.

Al Rabe, when he was "going for broke" on his Sea Fury (and several other) designs was trying to keep as close to scale proportions as he could, and thus chose the thickest airfoil he could get away with (for looks) in order to accomplish this. But he also tested these airfoils in his "wind tunnel" to prove his theories before building.

We aren't trying to get an endurance glide after the engine quits. We are trying to get a plane to turn on a minimal (if you stick with the Yak proportions) wing. Going to a thinner wing will just hurt it all the more. If you were going to change the airfoil and keep the proportions, I would take it to 25% or better. And, yes, straightening the trailing edge and increasing the wing area in the process and lengthening the tail and nose moments would also improve the model. But if you stray too far away, you might as well start with a new design. Find a plane you like. Copy it. And make it look like a Yak. Matthew did that with his early and latest Stuka designs. His 97 Stuka was based on our own "Das Stunter" design (using a Forerunner airfoil, by the way. and also kitted by Custom Models) and his latest is based on a Randy Smith design (but you can't tell it by looking. I dare you to trace its lineage without asking. Flies beautifuly, by the way.)

Hi Leonard.

I understand where you are coming from regarding your thoughts on foil percentages.

Still, I may be wrong, and it wouldn't be the first time, but I believe the Noblers foil is closer to 14% when the flaps are figured in. I suppose I could dig out the plans and measure the thickness, but I'm just too lazy right now to do that.

I beieve the foil at even 14% would be considerably thicker than the stock wing in any case.

My experience with thick wing sections at high loadings, is similar to a semi controlled crash into terrain. No way Icould make a stunt landing with it.

Yes, many planes are really another design in new "clothing"
It's the evolution of the design thing isn't it? Most designs are so close to the same numbers that the differences are marginal. But the looks and appeal can really be enhanced with some careful thought to esthetics. Matt has shown that he can do this better than most.

Actually I went way further in details than I should have, seeing that he just wanted a better sport flier.

I forget sometimes that not everyoe wants a good stunter.

;_) john

John, you may be right about the Nobler being as thin as 14% with the flaps. I guess now that you have the "juices" flowing, I will have to measure it. As I recall it, the Nobler used essentially a Barnstormer airfoil which worked effectively with fixed flaps. Moveable flaps at the time of the Nobler were new, and nobody had much scientific data on their airfoils then yet. And being truthful, very few airfoils on any of our stunters--even today--are truly scientifically derived.

When Bob Gieseke wanted a thicker airfoil than the "standard" airfoil of the Nobler, he drew around it with a pencil and increased it by that thickness. That obviously increased the thickness percentage more than the length, so he got what he wanted. I don't know what the thickness is, however. Nor did he.

Al Rabe did determine, however, that increasing the thickness with moveable flaps will proportionally increase the lift, and increasing the thickness towards the rear of the airfoil is more effective in producing greater lift with the flaps deflected. Since this is where we want the lift (with flaps deflected) it would seem to be the way to go. Yet I find very few people following up on these findings. Still, Al was going for the smallest wing possible to get the desired amount of lift and ended up with 28% thickness in some cases.

The only detrimental effect I see to all this would be the drag, as you mentioned. Yet the profile plane is so draggy already with the engine and tank sticking out in the breeze, that a little extra thickness to the airfoil shouldn't make that much difference. Just keep the nose down when the engine quits to keep up the speed, and then flare it over the deck.

The Piper Tri Pacer in real life is a good example of this. We used to joke that we could land the thing down a chimney. Short, thick wing, overweight, but it flew well if you knew how to handle it. Just don't expect to glide it forever if the engine quit.

Why not just build a Brodak LAAG Stunter?

Guys,

Thanks to all of you for the plethora of ideas. I will consider all of them in putting my wing design together. By the way, I should have mentioned that I'm building the Yak for fun flying, not for contest work. I like it's looks, but I've read so many negative comments about it's handling characteristics that I wanted to improve upon it somewhat to make it more maneuverable and easier to fly. Thanks again for all of the info.

Regards,

I think Currell mentioned the key thing--sloppy controls. If you use good hardware and "bulletproof" those controls, and then keep the weight relatively low for the size of the plane, it will turn just fine and dandy. If the controls are sloppy, then so will be the flight characteristics.

RCJ, you figured it out didn't you?

Hello,
I have an idea for you that you may find that helps - substitute a foam wing. I've got some gotcha 460 wing cores from Phil Cartier and I am using them with a Ringmaster - I cut the foam wings down to a 42" span and put the controls on the outside. The airfoil is thinnner than most, but thicker than the original airfoil with the ringmaster. The wing is also tapered, for and aft and just needs a set of spruce spars to tie it all together. You can even add wing tips if you want a more true to scale outline.

Just a thought

Joe m.

Good thread. After reading everything, I would go ahead and increase the thickness, and maybe even sneak in a couple of inches of added span. But I wouldn't mess with moments, tailfeather area, eliminating the tapered LE and TE, etc. You might as well start from scratch, otherwise.

Since the Yak is challenged, stunt wise, by its early 50s DNA (not a knock on Matt Kania, so no flames, please), you need to do all of the usual smart things to make the plane fly better. Like keeping the wt. down (choose really light wheels, don't forget), seal the hinge areas, extend the mounts, etc.

And solid pushrods. Currell