About the shear web I have already made what I intended to use for this spar but it is essential and shows better what composite processing is all about than my rather special and improvised method of making sparcaps, so I thought I'd make a material plate that would be suitable for a shearweb. The shearweb needs to encounter two forces; compression forces and shear forces - to prevent the spar from bending. Vertical grain balsa is hard to beat for it's light weight and compression strength compared to just about anything, and it is relatively easy to get hold of and not too expensive so that is what I'll be using. I take one sheet (100x10cm) and I cut it in 5 parts to make a material plate of 50x20cm, I only cut it and see that the edges fit reasonably together - no gluing, no sanding, no nothing else.
The direction of the wood grain should be vertical between the sparcaps to make the most of the balsas exceptional qualities, the spar will eventually be wrapped in fiberglass at 45/45* so this takes care of the shear forces and at the same time prevents the sparcaps from lifting from the shearweb at high loads. I am applying a light weight layer of fiberglass on each side of the balsa that will hold it together when cutting it in to narrow pieces, from a spar structure point of view it would be beneficial to the spar construction if the fiberglass is at 45/45*, however from a practical point of view a layer of 0/90* would make sure the narrow lengths of vertical grain balsa remains nice and straight after it is cut and it might make it easier to handle.
I am using one layer of 55g fiberglass at 45/45* and one layer of 49g at 0/90* on each side of the balsa, I hope some of the readers would perhaps be so kind to help me translate these fabric weights approximately in to imperial measurements - that becomes just a little too complicated for my simple mind to grasp at the moment.
When I have saturated the first side of the balsa and fiberglass with epoxy I have a perforated self releasing plastic film on top of it, then I turn it around and lay it on to the absorber/glass plate. The absorber/breather could very well have been a couple of layers of paper towels, I am using some suitable polishing material/cloth that have the correct width to my material plate and it makes a nice and even layer. What this does is to compress the fiberglass fabric on to the balsa and at the same time squeezes and pulls out any air bubbles and excessive epoxy, the result is a very light weight and very stiff, straight and strong material plate. It is difficult to feel the difference in weight with your hands between the original balsa sheet and the laminated one, but the strength and stiffness is something else indeed.
