CNC help required for final year Uni project

[charlieuk]

Hi Jazz,

Looking again, I reckon I might be able to make the fuselage and fin moulds in house, meaning only the 4 1m long wing moulds need outsourcing.

That would probably make more sense, as my current project plan is looking at a fortnight for the pattern manufacture, and another week for mould manufacture. If someone can machine each wing moulds in one hit, that's three weeks for the 4.

Charlie, I am very familiar with the technique you describe, as that is how e-Go Aeroplanes (where I was on placement) lay up their wings. They use controlled polystyrene cores, with large sectional cutouts. They have to wet layup, though, which isn't ideal due to weight considerations. Whilst you can cure pre-preg outside an autoclave, the stuff we're going for will need autoclave treatment due to the resin/fibres ratio. Obviously, polystyrene can't take autoclave temperatures! We are looking at a wing loading of about 3.57kg/m2 - which is nothing. Makes more sense to get the weight down early, especially as the electric motor we're provided with is, well, pathetic.

During the design phase, we'll be running a simulator alongside the design, tweaking as required. Another reason why we need the moulds to be spot on - as the BwB has no horizontal stabiliser, it is nigh-on impossible to trim. Unless the centre of gravity and centre of lift line up, you have an unstable aircraft. So you have to add ballast weight to trim, and your advantage of a high-lift, low weight aircraft start slipping away...

We are trying to scrape every single gram off - the contest is highly competitive. Last year, the teams were using the more powerful glow ignition engines (about 2x as powerful), and the winning entry weighed under 1kg - nearly half the weight of my University's design. If we want to win, we need to be one of, if not the, lightest plane on the flight line.

if you are carefully doing a wet layup on a layup table and then vacuum bagging with all various extras and not just mylar a long with then removing a majority of it through pocketing you can achieve results incredibly close to a moulded or autoclaved product at a fraction of the cost and you can still post cure with xps or eps to get every last bit of strength.

For what you are doing carbon for the entire build will not only be expensive but also over built and heavy for the purpose. we used to do builds comps using micro cars for all the components which were like doing small scale payload challenge as the were heavy brushed motors and copper coil actuators, the size you can cut everything down to get the strength to fly if you relay try is crazy we were using something like .5 mm carbon caps on wing ribs were still ott a lot of the time

While moulding is great and you can get some fantastic results if you have a large budget and a proven design if you are doing a one off or prototyping it is a expensive way of doing it to get the same result as perhaps a little more time but significantly cheaper and faster method that allows you to be flexible with your design.

[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...