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-   -   Bending balsa help (https://www.wattflyer.com/forums/showthread.php?t=73825)

thepiper92 06-02-2014 12:52 AM

Bending balsa help
So I have to bend balsa. 1/8x1/4 to be exact. I left the balsa soaking in water and attempted to bend it around the template I made. I managed to make one tip, a few splits, but it is strong with 3 pieces built up. How can I make this easier, I went through a few pieces just to mange one tip working out?

Wrongway-Feldman 06-02-2014 12:56 AM

I have found windex to work better than water for bending. The ammonia in windex really makes a big difference.

fhhuber 06-02-2014 02:06 AM

Ammonia does help.

Hot soak in ammonia-water mix helps more

Using a steam box is how they make the tight bend in oak for a classic cane.

But for some bends its better to do a glue-lamination of 2 to 4 layers of thinner stock.

thepiper92 06-02-2014 03:04 AM

Well what I did was lamination, 3 layers of 1/4 inch. So hot water is a must. Also does windex even contain ammonia anymore? I also see there is different grades of balsa, clearly I need the more flexible type, but what I have at my LHS is what I have. The bend is essentially an arch. How much ammonia to water should there be?

CHELLIE 06-02-2014 03:42 AM

25% Ammonia with water should do the trick, boil the wood then bend it.

buzzltyr 06-02-2014 03:54 AM

I use ammonia with water as well. I have a 1" PVC pipe, about 4 feet long, capped at one end. I just put the balsa sticks in it, fill with the water/ammonia solution, and put a piece of plastic wrap over the open end, held on with a rubber band. For the thickness you are using, I'd probably let it soak for 48 hours, at least.


fhhuber 06-02-2014 03:57 AM

Use plenty of ventilation when working with ammonia in hot water...

And keep it away from chlorine.

thepiper92 06-02-2014 04:35 AM

So put ammonia in sink, pour in water from a kettle? How long does it need to sit? 48 hours seems to be for cool water. The site on the instructions state 30 minutes.

fhhuber 06-02-2014 05:20 AM

I generally limit my laminated bends to 1/8 inch per layer. Then I can do them with a 10 to 20 min hot soak in tapwater and I apply yellow glue for the lamination as I do the bend.

kyleservicetech 06-02-2014 05:25 AM


Originally Posted by fhhuber (Post 949535)
I generally limit my laminated bends to 1/8 inch per layer. Then I can do them with a 10 to 20 min hot soak in tapwater and I apply yellow glue for the lamination as I do the bend.

Just curious

Wouldn't several thin layers of balsa glued together be a lot stronger than one thick layer, bent to just before splitting?

thepiper92 06-02-2014 05:46 AM

I think it would be stronger with more thin layers. So I am getting opposing times here. 30 minute got soak in tap water, 48 hours in water and ammonia and water, not sure how long. By the way I am bending by the 1/8 thickness, not by 1/4. I have three pieces, so total dimension are 3/8x1/4.

fhhuber 06-02-2014 06:17 AM

It depends partly on thickness of the material.... partly on its hardness. Thicker harder wood needs to soak longer, especially if doing it with cold water.

Also it depends how tight the bend will be.

thepiper92 06-02-2014 06:27 AM

Hardness...it's just what my hobby shop has. It's either spruce or balsa. I can ask. Judging by it though, I would say medium in hardness, and I can feel areas of inconsistency where it gets harder. It's not the balsa that you can cut with your finger nail, but has give to it when dry. So can any balsa bend if it is soaked long enough? I may try adding boiling water in a bucket, when I get more wood that is, or maybe step up to ammonia and boiling water. The one wingtip is okay I feel; a split here and there, but the three pieces reinforce each other.

DHC Beaver 06-02-2014 09:15 AM

One thing i found with trying to make bends in balsa,is to do it gradually.What i mean is,put a slight bend in the piece,let dry under pressure,then soak and bend some more,etc.
It will take longer,but you waste a lot less wood.YMMV.
These days,if i need a rounded wing tip,i just cut pieces of 1/4 into a rough curve,glue them together ,then shape.Sometimes,i use thinner core,then add 1/16th top and bottom.That gives high strength for no weight increase.

thepiper92 06-02-2014 04:30 PM

So you don't bend the balsa? You cut into multiple little pieces to make a rough curve?

fhhuber 06-02-2014 05:47 PM

Piecing together a curve can work but its not anywhere near as strong a a bent curve of glue laminated strips.

Again it depends on what you are doing.

Bending forms I made when doing Proctor/VK Sopwith 1/6 scale.

thepiper92 06-02-2014 06:21 PM

The angle of bend is roughly what your rudder is. The wing tip I made was from letting the wood sit for an hour. I had work the next day, and pipeband practice the next, so the other pieces were sitting for 2 days, even with that, the wood still snapped. Would this mean that I need ammonia? I also tried doing little bends, giving a slight curve, then going back to the soaking.

fhhuber 06-02-2014 06:48 PM

you need ammonia and/or thinner stock. 1/8 not making it.. go to 1/16

The bends for the Sopwith were done using 1/16 spruce, soaked in ammonia water for 24 hours. But look at that tight bend at the front of the fin...

thepiper92 06-02-2014 06:58 PM

Is that 1/16 squared, or does not matter, other than 1/16 being the thickness that is experiencing the bend? Also was that curved all at once, or for incredible bend of the fin, was it bent little bits by little bits? Is spruce better to bend?

fhhuber 06-02-2014 07:36 PM

The pieces were 1/16 thick, 3/16 wide. Width doesn't really matter.

It does matter to have a good bending form if you want to be able to duplicate the bent part.

carpetbagger 06-02-2014 08:08 PM

HEAT - Hot, near boiling water, or steam works. You need to get the wood core around 200 deg F. Forget Windex or ammonia, stinky and not needed, you need HEAT.

Lamination. If you bend a hot wet wood pierce around a form and let it dry it will "springback" a bit when released. However say a lamination of four thin strips to equal single strip thickness glued up on a form will only "springback" a tiny tiny bit.If you search springback you can find a formula for % of springback Re lam #.

Yes, A grain balsa will wrap around a small mandel, but C grain (the kind with sort a freckled appearance) will not - C is great for flat sheet surfaces because it hates to bend/warp.

thepiper92 06-02-2014 08:53 PM

I don't know what grain I have, it is just what my shop has, and there aren't any other shops really. I can't say it looks freckled, more grain, although both sides are not smooth, guessing the pattern from cutting. Looking at pics, I'd say A to B. So boiling a ton of water, tossing it in a bucket and putting the lid on should do it? I did have spring back on the piece I made, but when I made two more pieces and laminated them, the piece had no springback at all really, seems strong, even with the slight splits in the wood. Would spruce work better though, as I can get spruce in the same sizes? Just wondering if the spruce bends better with moisture, as I am finding myself to be liking spruce for stringers, and I don't mind using it in the wingtips.

fhhuber 06-02-2014 09:57 PM

Spruce was used in my airplane because of the strength needed. You can bend almost any wood to a radius of appx 6X thickness with use of steam and ammonia. Some you can go down to a radius of 2X thickness.

The grain can make it harder to get the bend without splitting. Straight along the length of the stick bends best.

The whiter the balsa looks the lighter it is and the easier it bends.

Spruce tends to all be very straight grained. Its heavier than balsa and may not be appropriate for some purposes. For other purposes its better than balsa.

Without knowing the exact bend being done.... I can't say what to do. I can only point at the general techniques that work in most cases.

Thinner bends better.
Wet bends better than dry.
Ammonia softens a substance in wood that "glues" the fibres together allowing tighter bends.
Heat (steam or hot water) allows tighter bends than just wet or using ammonia.....
steaming ammonia lets you do the tightest bend (and is the most dangerous to your health)

Glue laminated bends can be more than 50% stronger than a single stick bend of the same final dimensions.

thepiper92 06-02-2014 10:53 PM

My plane is coming out pretty strong, it's a 60 inch wingspan Hurricane, the stabilizers are built up of multiple layers of cross beams. The fuse itself is built up of spruce stringers, resulting in a very stiff fuse. For the wings I'll be using spruce too for stringers...so maybe just going to spruce wing tips will be the best. I'll stick to 1/16x 1/4, in boiling water.

DHC Beaver 06-02-2014 11:05 PM

Piper,I did mean several smaller curved pieces.But this will only work on thick(ish) wingtips.I can see this would be a problem on your Hurry-up.
Laminating it is then.

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