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Baltic Birch for Dihedral Brace?

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Baltic Birch for Dihedral Brace?

Old 01-08-2006, 06:18 PM
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Rcpilot
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Default Baltic Birch for Dihedral Brace?

I need to fabricate a new dihedral brace for an ARF. The stock brace is made from 3 pieces of 3mm ply thats sandwiched and glued together. That turns out to be 5/16" thick.

I can't find 5/16" hard plywood.

Can I laminate 3 sheets of 3mm Baltic Birch together for a new brace?

I'll be scratch building a new wing center section about 8 or 10" wide and then gluing the ARF wing panels to that. I'll have to build the new center section in 2 pieces and then glue them together with a dihedral brace. The new dihedral brace will have to stick out about 3" past the end ribs, so that I got something to attach the wing panels.
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Old 01-08-2006, 06:48 PM
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kdheath
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Default RE: Baltic Birch for Dihedral Brace?

I don't see an obvious reason to build a 10" panel in two parts. Why?

What is the wing span, area, and projected weight of the model?

The birch, laminated with good slow set epoxy and plenty of clamping pressure, would probably work. However, this must be a larger? model. And my gut would be much happier with aircraft 5 ply plywood. You could laminate 3/16th and 1/8th inch, for example. to get your 5/16th.
Old 01-08-2006, 07:16 PM
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Default RE: Baltic Birch for Dihedral Brace?

ORIGINAL: kdheath

I don't see an obvious reason to build a 10" panel in two parts. Why?

What is the wing span, area, and projected weight of the model?

The birch, laminated with good slow set epoxy and plenty of clamping pressure, would probably work. However, this must be a larger? model. And my gut would be much happier with aircraft 5 ply plywood. You could laminate 3/16th and 1/8th inch, for example. to get your 5/16th.
This is a Hobbico Twinstar ARF.

I'm trying to add sq.in. to the wing as well as incorporate a place to mount mechanical retracts.

The new center section will add the wing area I desire. Approx 80--100sq.in. And it will house the retractable gear, flaps and servos.

The reason it needs to be built in 2 pieces is to accomadate the dihedral angle and brace.

I need to build the center sections in 2-pieces so that I can slip them onto the dihedral brace. After I slip the center pieces onto the dihedral brace and join them--I'll have 3" of dihedral brace sticking out on each side. Those 3" is what will plug into the factory ARF panels and join the panels to the center section.

Look at the pic in my first post:
If that center section is 10" wide and I need 3" of brace sticking out for each panel on the end--then I'll need a dihedral brace 16" long.

I do like your idea of using 3/16 and 1/8 in a laminated piece to get the required 5/16". That would be extremely strong.
Old 01-08-2006, 07:53 PM
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kdheath
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Default RE: Baltic Birch for Dihedral Brace?

Ok, thanks, that's clear. Just trying to get a picture of it in my head...
If that wing's gonna have a nacelle/motor assembly on each end, it does need a solid spar. Is three inches all that the original assembly calls for? Doesn't sound like any too much, but I may be thinking too much.
Old 01-08-2006, 08:35 PM
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Default RE: Baltic Birch for Dihedral Brace?

The 3" also sounds to me a bit short. But I bet that's the measurement from the original ARF before you stick your new center section between the original wing halves, right? You know, if that is how the original is done, then the original wing actually got some support from the wing saddle or the fuselage sides. So that original wing joint didn't have to rely entirely on that brace. If that's so, then I'd think about wrapping the two new joints with glass cloth for sure, and running it out each wing a bit.

If you're ever doubtful about plywood (especially self-made plywood) not being strong enough, just laminate some glass cloth in between the wood layers. Or some CF if you got it.
Old 01-08-2006, 08:36 PM
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da Rock
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Default RE: Baltic Birch for Dihedral Brace?

BTW, that's a clever idea to improve the model.
Old 01-08-2006, 08:38 PM
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Default RE: Baltic Birch for Dihedral Brace?

3" is the full length of the brace pocket on the ARF wings.

Stock brace is only 6" wide.[:-]

So, if I build my center sections 10" wide and then cut a 16" wide brace--I'll have 3" sticking out each side. Thats all I can stuff into the factory panels. Of course, if I'm gonna strip them down and build bigger nacelles anyway--might as well look at modifying the dihedral pocket. I'll have to get in there and see how it looks.

I wanted to build my retracts into the scratched center section, but my wheels are going to have to be farther apart than 10". On the Twinstar--the wheels have got to be about 20" or so apart. Thats for stability.

I'll have to strip the wings and mount my retracts in the factory panels.

I guess, I should start ripping the skin off this puppy.
Old 01-08-2006, 08:45 PM
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Default RE: Baltic Birch for Dihedral Brace?

Adding the area is good but I don't see why you need to build the center section in two peices. Just build the center around a one piece dihedral brace.

If the old brace was strong enough then this new one will likely also be strong enough. But consider that plywood only has just over 50% of the grain going in the correct direction. The other is going up and down and does little to add strength. If you were to use 3/16 solid maple it would have 100% of the grain going the right way and be just as strong or stronger than the plywood.

Finally you'll be better off if you use two braces. One on the front face and one on the back. That way there's double the glue joint area to transfer the loads into twice the wood grain and limit the ultimate loading of each of the wood grain fibers next to the glue joints.

The increase in span isn't much but technically you're placing the original wings out further from the fuselage where they will have more leverage to load up the spar in the center section. The bending loads in the spar at the center line will therefore be higher than before. And adding to this will be the extra weight of the center section and retractable gear.
Old 01-08-2006, 11:16 PM
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kdheath
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Default RE: Baltic Birch for Dihedral Brace?

ORIGINAL: BMatthews
Finally you'll be better off if you use two braces. One on the front face and one on the back. That way there's double the glue joint area to transfer the loads into twice the wood grain and limit the ultimate loading of each of the wood grain fibers next to the glue joints.
He can't do that easily without modifying the spars. The idea was to use the original ARF spar doubler pocket. I dunno about maple for a spar. Good plywood is still probably stronger in shear and bending than the maple. The more I think about it, though, the more i think that the ultimate way to go would be to scratch a new wing and build a fair size box spar for the center section. Maybe spruce top and bottom with plywood faces.

It could be that what you gain in wing loading by adding the center section will be lost and then some by the additional weight of structure and retracts.

Maybe the question we should have asked first was, "What are the goals of this project?" That knowledge makes the decision making easier.
Old 01-09-2006, 01:00 AM
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Default RE: Baltic Birch for Dihedral Brace?

I've got one wing stripped of covering and gutted some of the center section sheeting on the bottom to expose the LG block and ribs.

My goal is just to turn this Twinstar in a plane with retracts and use .46 engines for power.

I want to increase the wing span and gain some square inches. I want to try and compensate for the added weight of the retracts, bigger engines, bigger fuel tanks, and additional servos.

Not sure if adding an additional 100sq.in. is worth it or not. More drag and weight. It's an 18% increase over the stock wing--but at what cost?

This throws all the other ratios out the window too. By increasing the sq.in. of the wing by 18%--now my tail surfaces are undersized. Will most likely end up scratching new tail surfaces. I don't want to build new tail surfaces. Scratching a whole new vertical and horizontal stab is going to put a dent in my balsa reserves. I'm gettin' a bit low. I would be tickled if I could get away with just adding larger control surfaces.

I'm committed now--I got the right wing torn apart. I got the fuse stripped and the covering torn off the left wing. Can't put it back in the box and return it.[:-]

I may be able to knock out part of the next rib and lengthen the dihedral brace pocket in the factory panels. That would give 5.5" of bracing into each panel. Rather than just 3"
Old 01-09-2006, 03:19 AM
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Default RE: Baltic Birch for Dihedral Brace?

I didn't think of having to use the existing pocket.

As for the maple vs plywood I stand by my claim. Long grain for a dihedral brace is going to have a higher failure load under bending than plywood. And the thickness of any sort of decent brace will easily handle the shear loads. Maple and the birch used in our aircraft plywood are very close in strength to each other. But a similar sized maple joiner has almost double the cross section of long grain wood compared to plywood. This means it's going to be both stronger and stiffer as well.
Old 01-09-2006, 07:33 AM
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da Rock
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Default RE: Baltic Birch for Dihedral Brace?

I'd say that everything you're doing sure sounds "right".... or at least close to right considering what you've got to work with.

Heck, it sounds like fun. Wish I could see it when it flies.

I agree with BMatthews about the wood strengths, but have to throw in a "but".... but, I've never seen a plywood brace that was well built break. And especially one with a layer of cloth in it. I'd bet what you're doing with the brace is going to work better than necessary.

What I would do for sure, is to get a look at the single engine version, at it's wing panel joints and brace. If they're trying to do a twin with the same design and components they use in the single....... And it's an ARF right? So the "designers" aren't really designers, they're toy manufacturers who've seen the possibility of expanding their product base with little or no effort. But that rant raved, the extra length you're putting into the brace sounds like it'd work fine.

BTW, a twin does put stress on a wing that basically turns the area of the wing, from one engine to the other engine, into more of a fuselage than a wing. So if you got the time, you might look into how strong that between-the-nacelles area is. (although, I'd bet you already have)
Old 01-09-2006, 09:53 AM
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Default RE: Baltic Birch for Dihedral Brace?

I was able to cut away the rib material at the end of the pocket and make the brace pocket longer by 1 more rib bay.

You know how you build a wing-then when your done framing, you just cut the rib material out for your dihedral brace pocket. Well, I had 3" to start with, and I knocked out 1 more rib to make the pocket deeper. Now, my joiner will penetrate 5.5" into the wing panel. Thats got to be enough. This is going to work out great.

I am going to cut my dihedral brace from hard ply. 1 piece of 3/16" and laminate it to a piece of 1/8". I'm no wood grain expert, but I've been building plywood braces from hard ply for years. Haven't killed a plane yet.

With a 10" center section, I'll need a 10" brace. Then add 5.5" to each side and the total length will need to be 21". I'm gonna have to cut my brace from a piece of 24" ply.

After knocking out that next rib and effectively lengthening my brace pocket out to 5.5"--I'm feeling a lot more confident about this.

I've got a bashing thread started in the Twin forum. Will post pics over there.

Gotta run to the doctor this morning--will put up pics this afternoon or early evening.

Thanks for the help.
Old 01-09-2006, 04:43 PM
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Default RE: Baltic Birch for Dihedral Brace?

You're right, Bruce. I'm deleting most of this because of severe recto-cranial inversion at the time.

But I am still interested in the question of how much weight you estimate it will gain and the effect on wing loading. If we start with a stock TwinStar with a pair of BB 25s for power, add 16 oz by going to 46AXs, add 12 ounces for three retracts, servo, and mounts, add 6 ounces for flaps and servo, and add 6 ounces for the new center section and big spar carry-through, we add 40 ounces by that SWAG estimate. That runs the wing loading from the 24 oz. sq ft (that Tower gives) to over 27 ounces.

One thing we didn't think about is that if you leave the engine nacelles in theit original spots and extend the wing, the engines will each move five inches out from the original location. What effect will that have on engine out and spin behavior?

Interesting set of problems. I'm trying to eyeball how the modified plane will behave in the air.
Old 01-09-2006, 11:25 PM
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Default RE: Baltic Birch for Dihedral Brace?

Kd, that's true but what about the sections of the original wing that came out of hiding? That should bring it back up to a gain of 100 sq inches.
Old 01-10-2006, 09:05 PM
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Default RE: Baltic Birch for Dihedral Brace?

I agree with Bruce that long grain is what you want. Also most of the strength needs to be in the top and bottom chords of the 'beam' you are making. The middle stuff is irrelevant.

But I disagree fundamentally about using maple or even plywood, which is way too heavy. Use spruce, or if you really want strength, Douglas Fir. There is a beautiful Norsman at our local museum. While they were restoring it, they had the wing all uncovered. The main spar was a single piece of Sitka Spruce, hollowed like an I-beam. It's incredibly strong in tension.

Plywood is not that strong in this use. I speak from experience!

I build my dihedral braces from Douglas fir. I cut a 1/8 or 3/16" wafer from a 2x4, and then I drill lightening holes down the middle. Really strong and light. Then I sandwich the spars. Works great.

My C$.25.
Old 01-10-2006, 11:18 PM
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Default RE: Baltic Birch for Dihedral Brace?

If anyone tries the Douglas Fir idea be sure to shop for stuff that shows lots of growth rings per inch. At least 10 to 12 or more. Some of the second growth stuff can be as little as 4 growth rings per inch and it's not as stable or strong. The fast growth means the fibers are not as compacted throughout the wood.
Old 01-10-2006, 11:31 PM
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Default RE: Baltic Birch for Dihedral Brace?

I keep a supply of old growth scraps around. Do not use Hemlock fir, which is different stuff.

The best is to find an old 2X4 - very dry, light, and as strong as hardwood.

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