How to bend balsa
Quite frequently in building with balsawood we need to bend balsa into a curved surface.
For curves with fairly large radiuses 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 sections
- One sided Moisture / heat
- Long 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
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 radiuses 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.
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 task the potential risk, consider using the long-soak method.
If using chemicals such as ammonia or vinegar isn't your idea of a plesant 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 accellerate 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 forwarned 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 airframe.