How to bend Balsa

[Papa]

came across this article and thought I would share it with you.


How to bend balsa
Quite frequently in building with balsa wood we need to bend balsa into a curved surface. For curves with fairly large radii this can be done without any problem. When it comes to convincing balsa to bend around complex, varying and tight curves (such as tail planes or wing tips) balsa has to be assisted into making these curves without crimping or snapping.
The reason why we choose to bend balsa around such curves is for a couple of reasons;Strength - Balsa is strongest when the grain runs the length of the wood.Finish - Sanding with the grain produces a smoother surface.Economy - It's cheaper to make a wingtip out of a strip of balsa than to use up a much larger sheet of balsa and having to discard the bulk of it.
The available methods of getting balsa to bend more can be broken down into the following sectionsLaminatingOne sided Moisture / heatChemicalsLong soak
With all bending operations it's suggested that you start out with the most flexible piece of balsa that you can obtain, typically this is referred to as A-grain balsa. Do not attempt to use C/quarter-grain balsa as it'll tend to split very quickly.
Stage 1 - Getting the wood flexible
Laminating
The process of using laminating to make balsa curve around corners is based on the principle that a thinner sheet of balsa can be curved at a tighter radius. The radius of curvature limit varies between materials but essentially it represents a percentage of compression (or tension), caused by the difference in curve radii between the inner and outer limits of the balsa. Thinner balsa will be able to be bent tighter before the same critical difference of curvature occurs.
Using the laminating process can be a fairly tedious one but it does produce an appealing (to some) visual appearance. Laminating produces the strongest but also heaviest resulting form.
One sided Moisture / Heat
If you take a sheet or strip of balsa and dampen one side you'll see that in a few seconds that the balsa starts to curve away from the dampened size. Conversely, if you apply a hot iron to the sheet of balsa, the balsa will curve towards the heated side. The reason why this occurs in both cases is due to a difference in moisture content in the balsa wood cells. The more moisture in the cell the more it expands.
In the damp application the damp side of the balsa expands causing the sheet to curve away. With the iron application the moisture is being driven out of the balsa cells on that side to contract and causing the balsa to curl in.
Chemicals
Sometimes you really need to get a piece of balsa around and things are already too thin for laminating practically - the solution can sometimes be to chemically adjust balsa to bend. Clouded Ammonia (water with ammonia in it) or Windex will make balsa especially flexible. The action by which this occurs is the breaking down of the balsa cell walls. Interestingly some people have reported that using vinegar also works, the key appears to be to soak the material in a non-neutral pH substance.
For clouded ammonia, use a 50/50 mix with water. CAUTION - use this mix in a well ventilated area. Ammonia can suffocate you. If you would rather not take the potential risk, consider using the long-soak method.
Long soak
If using chemicals such as ammonia or vinegar isn't your idea of a pleasant experience, you can soak the balsa in hot/warm warm water for an hour or more (depending on the thickness). The heat is useful to accelerate the absorption of the water into the cell structure.
Stage 2 - Setting the shape
Once you've made your balsa flexible you can commence to shape it to your needs. For simple curves, such as cylinders, cones and such, you can simply apply the wood to the formers or suitable shape holder (having a good selection of tins, tubes and rods helps here) and tape / hold the balsa to the required shape and allow to dry. Even if you're using the framework itself to form the curve, do not attempt to glue the balsa at this stage. Wet balsa and glue do not work together. Wait until the balsa is completely dry. Be forewarned that this sometimes can take a day or two in cold weather. When you remove the balsa from its former or shape holder, you'll notice that it tends to spring back a little bit, that is okay, it's normal. You can now glue your balsa to the air frame.

[sihinch]

You don't seem to mention licking it!?

[Papa]

Try "non-neutral Ph substance"

That should cover saliva!

Jack.

[Michael]

In case anybody's wondering how to break balsa, well, all you have to do is follow the same instructions for bending, but continue bending the balsa until you hear it break.


If one is hard of hearing, then concentrate on how the wood feels in your hands as you continue to bend it, and then wait until you feel the wood snap.


Both methods work well.


********


Is anybody interested in how to glue your fingers together?

[Skyking]

Simon,


If you were intended to lick balsa it would be coated with sugar or catsup or something.
Another thing, what happens if you are dehydrated? No bending!
And you gotta watch out for tongue slivers.


I just have your best interests at heart. If I want you to get hurt I'll do it myself.


Ken

[Wingnutz]

In case anybody's wondering how to break balsa, well, all you have to do is follow the same instructions for bending, but continue bending the balsa until you hear it break.


If one is hard of hearing, then concentrate on how the wood feels in your hands as you continue to bend it, and then wait until you feel the wood snap.


Both methods work well.


********


"Is anybody interested in how to glue your fingers

I think this belongs in the "Sticky Business" aka glue thread.

[Adrian Page]

You can also steam bend balsa. I use the technique to bend thick balsa for the nose of my Gee Bees. Just put the balsa into a large pot with a steam insert in the bottom. Put about an inch of water in the bottom of the pot, cover and bring it to a boil. Once the water comes to a boil you can turn the burner down to a lower setting so the water does not evaporate as quickly. Steam bending the wood is fast and allows you to use the wood as soon as you are finished bending it. It can also bend thick balsa... the balsa in the photo is 3/8"

When the wood has been steamed for about 5 minutes it will be ready to bend. Time the 5 minutes from when you see steam coming out from around the pot lid. You will have to wear
rubber gloves to handle the wood and be very careful when you are taking the parts out of the pot. Steam will give you a very nasty burn instantly. Take a piece of wood out and quickly bend to the radius required. I bend it in my hands as shown in the photo. I blow on the part to cool it and after about 45 seconds it will hold the new shape. If the bend does not come out right you can put it back in the pot and try again in 5 minutes.

Adrian

[wollins]

Just put the balsa into a large pot with a steam insert in the bottom.

Hey Adrian,

For those of us who are culinarily (is that even a word? lol!) challenged, what the heck is a steam insert?  (seriously)

Colin

[battlestu]

guessing something like this

http://www.amazon.ca/Amco-10957-Collapsible-Steamer-Stainless/dp/B000Q4N2LO

[wollins]

guessing something like this

http://www.amazon.ca/Amco-10957-Collapsible-Steamer-Stainless/dp/B000Q4N2LO

You guys are good! Thanks Greg, methinks the wife has one of those. (it might smell a little woodish going forward. lol!)

Colin