Right, here's a practical and theoretical update.
Just vac-bagged a t-foil on a budget, with great success.
Shopping list:
- Fridge compressor (this one was borrowed from will, they easily obtainable from broken fridges as it's usually the starter relay that packs up rather than the compressor itself)
- Polythene dust sheet
- Plastic-based loft insulation for breather (its high density polyethylene i believe, not the fibreglass stuff though i guess that would work too)
- 5mm poly tubing (to extend the pipe on the pump)
- peel ply
The method:
- cut an oversize bit of dust sheet and lay it out flat. too big is better than too small
- cut a piece of breather just larger than your layup and lay it on top of the polysheet
- cut a piece of peel ply bigger than your breather, but smaller than your polysheet
- do your layup on a separate bench making sure all your layers are pulled as tight as possible
- lay it on the peel ply and fold the peel ply over smoothing it down around the front edges and making it tidy as possible
- place the sucky bit of polytube in on top of the peel ply but not on the laminate (it might dent it, i don't know)
- fold the breather over, followed by the polysheet, be patient if you have a good pump within a minute it'll seal itself and flatten down nicely. taping and folding is not necessary
Thoughts:
- we used 1" thick breather which was quite squishy, it worked well but could maybe have been even thicker
- even a 2:3 resin to cloth ratio the vac pulled a lot of epoxy out of the mix so you really can go very dry and get good results
- after speaking to a friend who is a refridgeration engineer he advises that a domestic fridge compressor will possibly burn out fairly quickly as it relies on a lubricant in the refrigerant gas it usually pumps to oil it
- be meticulous in preparing your mold/core - even tiny imperfections will show through with a vac bagged laminate!
Pictures to come later...
A note on the core:
As a bit of a trial and exercise in laziness more than anything I decided to use a balsa core and bought some 3/8" leading edge, block, and trailing edge from a local model shop; stuck the three bits together and gave it a quick fair. As a result I decided to use a fair amount of laminate to strengthen the crushable core. This could be avoided by gluing in carbon stringers, but again i was feeling lazy.
I ended up with a core just over 90cm long, 12.5cm chord in the middle tapering to 9.5cm at the edges, and around 1cm thick. The plain balsa core weighed in at 92 grams.
Laminate was two layers of 200g glass weave all round, with 300g carbon uni-d sandwiched between. On the top there was one 3/4 length layer and one full length layer. On the bottom there was one 1/2 length layer, one 3/4 length layer and one full length layer.
It's ended up at 354g roughly trimmed. Still needs a little fairing and attaching to the bottom of the rudder blade..
Time will tell if it's strong enough..
