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Author Topic: Undercamber and incidence  (Read 581 times)
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GeoffinIN
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« on: September 04, 2021, 06:54:15 PM »

I'm slowly building a 23 7/8" span Bellanca Viking from factory drawings I got from the Smithsonian Institution some years ago.  I note that a center line on the wing rib drawings show the trailing edge above the line, whereas the leading edge is in the middle.  The wing has slight undercamber.  Looking at full-size Bellancas it appears that the wing root is indeed at negative incidence.  Are any of you guys aeronautical engineers, and can explain this?  I don't want to build the wing mounts in the fuselage at such an angle that if it flies at all, the nose wil be so low I'll look like a coon hound sniffing out critters!

Of course, I don't have to use the scale airfoil, but if I do...?
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TimWescott
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« Reply #1 on: September 04, 2021, 10:45:20 PM »

I'm not sure that using a factory airfoil is going to mean much at that small a size of airplane, especially if you're going with rubber power or rubber-power-ish flight speeds.  The Reynolds number is just so different that the airfoil will think it's flying through molasses.

That said -- if the original didn't fly nose down (or up, more likely), then yours probably won't either.
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lincoln
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« Reply #2 on: September 04, 2021, 11:20:10 PM »

Strongly suggest that you use a relatively thin airfoil. Maybe 7 percent and a bit thinner at the tip, with some washout. If you want to use the original airfoils, you should build a very large model.

It would help if we could see relevant parts of the drawing.
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GeoffinIN
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« Reply #3 on: September 05, 2021, 09:51:13 AM »

Thanks, you two.  Yeah, Tim, it WOULD be nose up.  I've sniffed too much Sigment lately.  I guess I'd better switch to Titebond.

As for showing the drawing of the airfoil, wellll...  the drawing is about six feet long. (Full scale)  And I'm convinced; I'll scrap the full scale airfoil.
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dosco
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« Reply #4 on: September 08, 2021, 11:59:03 AM »

I was thinking about responding to your initial post and decided against it.

Unless that line on the drawing is labelled (and I'm assuming it's not), there's no way to tell what the hell it is.

I'd like to guess it's the zero lift line, but that's a total WAG and probably wrong. And, as you're probably aware, aircraft drawing methods trace their history to boat drawing methods, and there are all sorts of "reference lines" that could be used. If not labelled, there's no point in guessing.

So ... don't worry about it. Especially if you're using a modern, not-to-scale airfoil. And it seems like you've reached that conclusion.

Best-
Dave

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GeoffinIN
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« Reply #5 on: September 08, 2021, 02:59:36 PM »

Thanks, Dave.  My original query was just to display my idiocy! Embarrassed  Gotta say, though, that the Bellanca airfoil shape sure is beautiful!

I did build a stick and tissue scale model with undercamber recently, and it seems to work well.  The Aeronca Sedan has undercamber, as does
the plan from which I built it - an old Berkeley kit.  The Aeronca has a good deal higher aspect ratio than the Bellanca, and the model's span is
50% greater.

Geoff

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lincoln
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« Reply #6 on: December 16, 2021, 03:50:30 PM »

Wondering about the status of this project.

If you have a smart phone, it should be easy to take a photo of the drawing and post it here. That would clear things up.

I just saw a drawing purporting to show, roughly, the airfoil shapes on a Cruisair.  The root airfoil was quite thick, with no "undercamber". The tip was much thinner and was a bit concave on the bottom. If appearance is paramount, you might be able to make it work if you can figure out an unobtrusive turbulator at, say, 25 percent of chord. One example would be a piece of thread doped on, though it would have to be at least a certain thickness. More than that, of course, would add drag. I think most airfoils with Reynolds numbers this low would benefit from a turbulator in slow flight. At some point, the Reynolds number gets so low that a turbulator wouldn't work, but my guess is that it's lower than on your model.
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GeoffinIN
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« Reply #7 on: December 16, 2021, 04:47:03 PM »

I decided it was all a tempest in a teapot and built the model with as close as I could come to the Bellanca mid and end-span airfoil drawings that I have.  It glides well and flies with just a few turns on the rubber, but the weather's turned too llousy to give it a proper flight.

If I could figure out how to post photos on heer I'd post one.  There's one on the FAC group on Facebook, but good luck finding it there!
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Indoorflyer
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« Reply #8 on: December 16, 2021, 06:54:16 PM »

Guess  the CG / Balance thing worked out OK-- I recall you were concerned about nose heaviness...
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Indoorflyer
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« Reply #9 on: December 17, 2021, 01:03:10 AM »

BTW, this is a good video on performing a Bellanca Viking wing inspection.  You can get a good idea of the wing's complexity and subtle contours.   

Dave Pasquale is a Bellanca and Bonanza specialist.

https://www.youtube.com/watch?v=kdf4qMaqBGQ
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GeoffinIN
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« Reply #10 on: December 17, 2021, 07:10:34 AM »

I recall you were concerned about nose heaviness...
It's still nose heavy, so some tail weight's been added.  If I build another one I'll build the nose structure with sticks going clear to the nose instead of having lots of carved balsa up front.  It's too heavy for any decent performance as it is, but it does make for a neat looking model.
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lincoln
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« Reply #11 on: December 17, 2021, 02:20:50 PM »

Unless you've already done it, what you need is MORE carved balsa up front. Specifically, the prop. It's really not very hard to do.
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GeoffinIN
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« Reply #12 on: December 17, 2021, 03:53:46 PM »

That makes perfect sense!  I just happen to have some 2" square X36" balsa, so I'll give it a go.

Verrry interesting video on Bellnca wing inspection!
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Indoorflyer
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« Reply #13 on: December 17, 2021, 05:06:47 PM »

He forgot to mention if something taps back, you may have a real problem!  Cheesy
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GeoffinIN
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« Reply #14 on: December 18, 2021, 08:25:59 AM »

It's a normal part of a preflight to check for bird nests in the engine, but woodpeckers in the wings is a new one on me!
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