wing comparison- MiniMax and Kolb

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13brv3

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I'm thinking of a one-off project, and I'm considering different two different wings. One would be the MiniMax type wing, and the other is a typical Kolb wing. They're both used in similar speed and weight aircraft, but I wonder if the airfoil is significantly better on one or the other. I do MUCH prefer the all aluminum Kolb wing construction.

Is anyone familiar enough with the airfoil of both of these wings to give some pros and cons of each, in layman's terms? Is there any significant difference in performance between the two up to maybe 80 mph? I have to imagine they're very similar at the low speeds they're made for.

Thanks,
Rusty
 

Dana

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Structure and airfoil shape are two different things, any airfoil can be made in any construction type (within reason). That said, the Kolb has a somewhat sharper leading edge than a Minimax because the Kolb's is shaped by the aluminum tube leading edge, whereas the Minimax is a plywood skinned D-tube. Both do just fine.
 

Victor Bravo

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The Kolb wing is built with no twist or "washout", and has a sharper leading edge as Dana mentioned. This creates a phenomenon called "The Kolb Quit", which is Ultralight-Speak for a sharp stall with no buffet or warning. I have experienced it myself, and I wrecked a friend's Kolb Mark 3, simply because I was not expecting such a light, slow, "beginner friendly" airplane to behave like that.

I am guessing that the Kolb is built with no twist because it is easier to just lay everything on a flat table instead of twisting the table (or raising the wing off the table), and because the people who knew him well said that Homer Kolb would never have given up any performance or lift for any reason. Apparently that's why it also has just about no dihedral.

When I got my Kolb Firestar flying, I noticed it had no significant stall buffet or other warning. But it wasn't quite as sharp because I put VG's on the wing but left the two inboard rib bays without VG's, to force the stall to begin at the root. That worked a little but it is not a full subtitute for twist IMHO.

For whatever my opinion is worth, I like the Kolb wing a lot from a build time point of view, but I would make the leading edge a little larger diameter if I were starting one from scratch

I would also really like to put twist into it, but that creates a domino effect of other changes you would have to make. If you twist the wing then the Kolb's aileron hinging system goes from bad to worse. The ailerons already are the one bad thing about the Kolb (heavy aileron forces), but trying to make them work when the trailing edge tube has a kink in it is going to be a mess. Essentially Kolb probably tried this at some point and realized that their wing building system was really good but it prevented you from having a viable way to put twist in. So they just left it alone. You can do a google search for "the kolb quit" and read all about it.

There is a lot of good things about the Kolb wing. But one very bad feature is that they use a couple of feet worth of piano hinges for the ailerons. That works just fine if you have a stiff structure on a Cessna or Cherokee. But for a very light structure like this, the normal wing flex in flight, even under 1G, binds those hinges up and doubles or triples the stick force required. Definitely change over to "shear bolt" style hinges, it is very well worth the effort.
 

Dillpickle

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About 20 years ago, we built a wing spar for a single place, strut braced, one off design using aluminum angle for spar caps and sheet for the webs. It was simple to build and fast. Drill a few holes, and line up on a C frame riveter, and done. Spars were two guys, two days work. The biggest obstacle to building in wood is finding good wood at a decent price--shipped! The Minimax and Legal Eagle wings are works of art in their lightness, simplicity, and strength, but...getting the wood can be problematic and expensive. 6061 angle is ubiquitous in the US--every major metropolis has a supplier. So is the sheet. I spent a few hours cruising Cub and Cub clone sites reading debates on aluminum vs wood wings. The Conclusion is build what you have or are comfortable with, not weight or longevity. I DO wonder, that almost every time someone mentions an aluminum alternative to Minimax/Eagle/Ultralight wings, the convo turns to TUBE spars, and very rarely, extruded I beam spars.
My BIG question would be this:
Why so little conversation about built up spars?
 

13brv3

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I have about 100 hours on Kolb Slingshots, and have never noticed any heavy stick force compared. Those wings are shorter and stiffer than the typical Kolb though. The sharp stall is real, but I've flown RVs that behaved the same. I don't believe the MiniMax wings have any twist either, and certainly RVs don't, so Kolb is probably in good company there.

I much prefer aluminum, and I'm not opposed to more conventional spars with my RV background. I would certainly want to stick with a known wing though, and not get too creative, thus the Kolb option.

I've never flown a minimax, or anything with that wing. The main goal of the thread is to determine differences like this. If the minimax wing has more stall warning, or more gentle stall characteristics, that's good to know. If either has significantly more or less drag at cruise speed 80-90 mph, that would be good to know as well. I already know I'd much rather build a Kolb structure than an all wood wing.
 

DaveK

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Other than antidotes there isn’t a way to compare the two objectively. The two airframes are totally different and the fuselage, engine placement, gear, etc all contribute to the overall drag and cruise speed. You’ve flown RV typ aircraft, many of which have a NACA 23012 airfoil, same as a Taylorcraft. Totally different performance envelopes due to differences in the overall configuration.
And as pointed out the construction style is generally independent of the airfoil choice.

I have about 100 hours on Kolb Slingshots, and have never noticed any heavy stick force compared. Those wings are shorter and stiffer than the typical Kolb though. The sharp stall is real, but I've flown RVs that behaved the same. I don't believe the MiniMax wings have any twist either, and certainly RVs don't, so Kolb is probably in good company there.

I much prefer aluminum, and I'm not opposed to more conventional spars with my RV background. I would certainly want to stick with a known wing though, and not get too creative, thus the Kolb option.

I've never flown a minimax, or anything with that wing. The main goal of the thread is to determine differences like this. If the minimax wing has more stall warning, or more gentle stall characteristics, that's good to know. If either has significantly more or less drag at cruise speed 80-90 mph, that would be good to know as well. I already know I'd much rather build a Kolb structure than an all wood wing.
 

13brv3

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Other than antidotes there isn’t a way to compare the two objectively. The two airframes are totally different and the fuselage, engine placement, gear, etc all contribute to the overall drag and cruise speed. You’ve flown RV typ aircraft, many of which have a NACA 23012 airfoil, same as a Taylorcraft. Totally different performance envelopes due to differences in the overall configuration.
And as pointed out the construction style is generally independent of the airfoil choice.
Good point about the use of the same airfoil on such different planes. I guess I never would have imagined that. The MiniMax wing is said to be just drawn by hand, and not really based on any existing airfoil. The only thing I can find on the Kolb is a statement saying the FireFly uses NACA 4412, but clearly it's modified from any drawing I can find of a 4412. They seem to have flattened quite a lot of it to make it easier to construct. Thanks, Rusty
 

TFF

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Not calling it Clark Y is semantics. I think both are designed to spar height and type. I think it depends on the range you want to capture. Slow or fast. Ease of making in the materials. I doubt there is more than a couple of percent difference.
 

Dana

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The Kolb airfoil is a TLAR section ("That Looks About Right"). The story is that Homer Kolb bent the wing ribs around a tire, and when people asked what airfoil was, he gave them the tire size. The leading edge radius is defined by the 2" leading edge tube, the overall thickness is enough to accommodate the 5" irrigation pipe he used as the main spar, and the thickness in the back is set by the rear spar tube size. If I was starting an ultralight design from scratch I'd likely pick a Clark Y but yeah, almost anything will work on aircraft in that speed range. The NACA 4412 is very similar to the Clark Y but the Clark's flat bottom makes it easy to accurately build a wing on a flat bench.
 

Dana

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When NACA (predecessor to NASA) developed the 4 digit airfoil series, they started by looking at existing airfoils that performed well. Up until then most started as "TLAR" sections, and some worked better than others. NACA found that the best performing airfoils, including the Clark Y and very similar USA35B (used on the Cub) had a very similar thickness distribution when the camber was removed. They reduced that thickness distribution to an equation, then experimented with a range of cambers, varying the amount and position of max camber, and a range of thicknesses, all with the same thickness distribution. The 4412 is very similar to the Clark Y, but without the flat bottom that makes for easy construction.

People [mistakenly] call any flat bottom airfoil of that approximate shape a Clark Y, but a true Clark Y is a very specific airfoil.
 

13brv3

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Thanks for the great info! I really have to believe that most of these light aircraft started with some form of TLAR design, maybe changing it a bit here or there. Once they found something that worked pretty well, they just stuck with it. Next, other companies borrowed it. The end result is that most popular wing types probably started out on the back of a napkin.

The Kolb wing is easy to build, and I restored a 20+ year old SlingShot that has absolutely no corrosion on the 6061 structure. Of course it wasn't stored outside, but I tend to trust aluminum to outlive me with any reasonable care. My preference is also to avoid anything that depends on glue, which leaves out fiberglass and wood. I really don't have any good reason for my glue-phobia, but I don't see my opinion changing anytime soon.

The latest Kolb FireFly seems to be using an undercambered wing, which may be optional. I took a picture a year ago at the Kolb facility in KY that shows it. The 5" spar tube makes this possible, vs the 6" tube that is used in the SlingShot, Kolbra, and Mk3. Clearly some modification of the rib tubes could change the shape to be a bit less flat if desired. The biggest issue building a Kolb style wing from scratch is sourcing the spar tube. That size tube just doesn't seem to exist from any of the usual sources, so it would likely have to come from Kolb. Fortunately, I'm close enough to Kolb to make an easy drive to get the material if needed.
 

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