mede wrote: ↑
Mon Mar 07, 2022 8:25 am
I would like to sand my mast and foils, in order to avoid ventilation and increase speed.
Did my online research, but did not find consensus on what is the best grit to use at what places on the foil and mast.
Older posts seem to favor 1000 - 2000 p, newer ones rather 400 - 800 p.
Recommendations? I'm also interested in the reasoning behind those.
Many thanks in advance,
Not sure about the race side but I've been making my own foils this last year and a half culminating with 3 new front wing designs and several new stabiliser designs, all axis black fuselage compatible and cast in carbon fibre this year. And with a model still to mould and another mould yet to cast carbon in. For me, I currently do a twill skin, minimum 2 layers unidirectional sandwiching a layer biaxial each side, with some odds and ends going into additional reinforcing around the mounting holes and middle section. Sanding with an orbital sander on 400 grit will deform small areas such as the leading edge, tips and trailing edge after not too long but on the flat areas it's going to take you an appreciable time to deliberately damage the surface significanly if you are sanding flat. With 800 grit and above, you could be sanding away for a very long time to cut through a layer. On my PLA plastic models I clean up the surface with 80 grit and higher, working my way up but the important thing is if not hand sanding, use an orbital sander. Start and stop it on the surface, DON'T let it build up speed spinning away from the surface, it will cut too fast then. Unless you are making your own foil fron scratch, NEVER use a belt sander or disk sander, they cut much too fast and need a lot of experience to control, especially on an already finished foil. Here in NZ, I can't get orbital sanding disks above 280 grit from hardware shops but auto shops have the finer grits available. I have nothing for my machine between 280 grit and 800 grit and the slowness of 800 grit is astonishing. I use a battery orbital sander and mostly wet sand, you do NOT want to breath carbon fibre dust, it's a nasty health hazard.
For my type of construction, the unidirectional carries most of the loads, compression at the bolts and over the top span and tension in the bottom skin. Double bias thickens the skins and provides warp resistance to the tips, the twill skin is to stop runs on the unidirectional, provide a pleasing apearance and keep everything underneath from splitting. You will find that the carbon fibre tow in the surface cloth is likely some 15 thicknesses + of carbon so it takes some time to sand through, but I do just this, tiding the "flash" left at the edges from my moulding process to complete the correct trailing edge shapes of my foils. You can get an idea of the amount of carbon in your construction(if there's no glass) by getting the volume from it's displacement and comparing that with it's weight. Googles give a density for prepreg carbon laminate of 1.5 x more dense than water so for example if your wing displaces 1 litre but only weighs 1kg then it's likely that a third of your wing is light weight foam core.
If you do sand through the epoxy into carbon(if there's a gel coat) then the appearance will change, look side on(even if you've polished it afterwards) and you will be able to see where the carbon is at the surface.