CNC help required for final year Uni project

[ChristoT]

The pre-preg I hope to source hardly weighs anything, because the amount of resin mixed into the fibres is the bare minimum for structural strength. A single CF spar down the middle, with ribs in, say 4 positions made up of 3 ply pre-preg, with a single carbon skin over the lot will weigh mere grams. It's an expensive method, true, but one I am familiar with. Not to mention the fact that the University has an autoclave on site we plan to use!

Truth be told, I am not as confident working with foam: all my experience at e-Go was either working with moulds, or applying patches in wet layups for small areas. It's also worth mentioning that as each wing will be 1m long, and has fins and rudders mounted at each end, there's going to be a certain dynamic load on the wing in multiple directions (certainly a torsional element from the rudder input), one which I'm not sure the foam can take.

Wet layup over a pocketed foam core certainly is a process worthy of consideration; in fact, it was the initial plan for the wing. However, due to the loading, I figured that the spar, rib and single skin method would work out substantially lighter. If no CNC sponsor steps forward, that may well be the process we have to go for, out of necessity rather than choice.

Anyone here able to hot wire cut foam?

[charlieuk]

if you are worried about twisting ext from other aspects then when you do your layup you are going to have to orientate your fibres in more than 2 directions to stop this which will mean multiple layers in places. i think you will find a carbon skin for the hole wing will be over kill and just add wait. try to only use carbon were you need the strength ie in the spar and in the dbox possibly. you can purchase carbon T section that will provide you with more then enuff strength for your needs and only way a few grams and then its just a matter of crating the wing shape as light as posable so using a lighter weight material like mylar were you are just need to cover the structure. There was a program by guy martin not long ago about building a human powered aircraft and they used the same foam mylar technique

Hot wire cutting foam can be as easy as a length of stainless steel wire and a 12v battery and a bow of some kind and then just two templates at ether end of the shape you want you also can get a range of densities of eps ranging from just under 1lb cubic foot to 3 or 5 so lots to choose from.

[EddyCurrent]

@ChristoT, did you look to see if there was a FabLab near you, that might be another way to do them yourself.

http://www.fabfoundation.org/fab-labs/

[ChristoT]

Didn't know about them, but now I see there's one in London - thanks! Now to see if any near me have a CNC machine big enough.

[ChristoT]

Annnnnnnnd... No luck there. Their machines are too small for our purpose. The University has a mid-size Denford machine (I believe it has a a 500*250mm deck - something in that order of magnitude anyway), but we're trying to avoid mucking around with 6 different sections - as it is, the fuselage will probably take between four and 8 sections of tooling board to make!

[Jonathan]

On a similar note to Eddy, have you spotted Hackspace? e.g.
https://london.hackspace.org.uk/

Have you got a drawing / image you could post showing what you want machining, or just something similar to show the general idea? My machine is big enough...

[ChristoT]

Yep, come across Hackspace, but I don't think their machine is up to size.

The wings will be in two moulds (top and bottom), and be around 1m long each. Not sure what the chord is yet, that depends on the wing designer. The shape of the mould will be the negative of the wing, so a effectively a long curve. A quick google throws this up :
http://static.rcgroups.net/forums/at...g?d=1261911797

The only difference I can see is that in that example, material is removed around the shape of the wing - we'd be looking for the shape of the wing to be removed from the billet instead so that we can layup directly in it.