Sunday, March 21, 2010

Big Baby Advanced Practical Composite Airframe

Now that we have finished with the basic calculations, we have determined our best shot at a low cost, practical airframe was to support the balsa with glass and later kevlar composites. Our plan is to use 4 layers of 2oz glass for the fuselage. That should give us more than enough structure for 3g in a 20# aircraft.

We are trying to make a new version of the rail system for the engines. That will make it easier to package and ship. Our goal is to be under 80oz for the airframe and under ten pounds for the aircraft without instrumentation. Which is great, to get more than five pounds of usable payload.

More Practical Composites

After several weeks of analysis, I came to the conclusion that using a glass or carbon fiber reinforced balsa composite is one of the best ways for us to get the weight out. Our other attempts were plagued with uneven resin densities. We have good luck matching allowables for 2-4 layer lay ups, beyond 5-6 layers we start to diverge pretty badly.

The best results that we have seen is with 60min epoxy and making sure that each layer is well wrung out before it is let cure at normal room temperature, 77F. As the humidity goes over 80% the pots react differently. The biggest issue for the divergence we think is the number of pots of epoxy used. Something happens with the weight and density between the pots.

Make sure that when your layups are done that you have at least a 2-3 layer pot. If you need a much thicker lay up, I would make sure that you make enough for everything. Also it is an issue with the humidity in the room. Do not add alcohol to the resin when you mix in the hardener. If the mixture is missed, it will just ruin the pot. At worst, it will react exothermically and create large amounts of bubbles both make a mess of the part.