Design, Manufacture and Machining of Carbon composites for RC planes

[RKD89]

Hi Guys

Recently I have started a new hobby into RC planes. What I have noticed is extensive use of Carbon composites weight yet sturdy structures.

Now the problem is that these structures seem to be cut (milling/drilling/laser cutting/water jet cutting) from a sheet (or laminates) of carbon composites. Here are the problems:

1. Don't carbon composites have very bad wear properties. If nuts are too tightened , they not only tend to scratch the material but the laminate also often cracks. I have seen lots of immature hobbyists due that.
2. There is often not enough material between the holes and the edge, ie critical holes for attachment are too close to the edge of the component. There does not seem to be enough fibre to support in-plane loads around the hole
3. There are often slots in the components to reduce weight (topology optimization?). They seem to be machined later into the sheet rather being included in the original manufacturing. I am assuming they either buy off the shelf prepeg sheets and cut them as and when required. Shouldn't a better practice be to incorporate in the holes/slots directly in the original manufacturing method (say making a proper mold for hand layup or vacuum bagging instead and avoid machining altogether).

I understand, hobbies don't require such detailed engineering and need to be cost effective, but I want to know how a professional would do it? Here are some of opinions on the same, kindly correct me where ever I am off the track.

1. Composite ideally should not be joined by bolts. In cases bolts seem the most economical option, at least washers should be used to redistribute the loading over a larger area?
2. Holes near edges is poor design. There is should be enough material ( in terms of number effective load carrying fibres ) around a hole/slot. Is there any empirical relation for that (both for woven and single direction fibres). Or do we just use appropriate stress concentration factors as we do with most metals?
3. Any tips on incorporating the slots/holes in the original composite manufacturing process, rather than machining them later (irrespective of how complicate the mold might become).

Looking forward to suggestions from you guys.

 

[British Menac]

Hi RKD89...

Interesting post and thread here.
To start, I am not an Aeroplane guy. I build and race RC cars but the subject of your post I do have some experience with.
I hand laminate parts for my cars and also machine them, Either my own design or improvements on existing designs.

While I believe you are right on with part strength being compromised by machining, I personally use machining slots and part size reduction for the prototype part only. Production parts being formed via Vacuum Bagging technique incorporating the improvement's (ie: the machining work) once proven.
Hole strength can also be an issue as you have pointed out. Washers is a good idea where space is restricted. Greater thickness of material used can help a lot.
I do think the stresses place on the composite part has to be analyzed before contemplating the viability of CF (composite) use for the part involved.

How are you getting on in your hobby?

Regards
BM

 

[RKD89]

Thank you BM for your reply.

How do you in corporate slots in your final design (to be vacuum bagged directly)? Most of the DIY/online videos are usually for sheets/curved plate types only. Can you share your work? I am looking to make typical truss like shapes (think a rectangle with only one diagonal/ the triangular slots to be incorporated directly in the primary manufacturing process and not machining).

Do you try to incorporate fibers in orientations other than 0/90?

Never knew RC cars needed composites. I am a newbie in this hobby but hope to build my own frame one day (preferably of composites).

Thanks for the reply

 

[British Menac]

Once final in design the mold incorporates the slots directly. I usually machine bosses to be used in the mold.
I also like the first layer of fabric to be slightly too big in the slot area... the boss so as it curves into the recess or radius of the boss/ mold. Subsequent layers lying within this first, radius'd layer. If you get what I mean.
Its a lot easier to do this work with pre-preg' fabric
As far as orientation of fibre's / twill I usually stay with 90 degree's unless multiple layers (Less then say 4.) Are used but high strength needed then I will orient at 33 degrees each time to achieve max strength.

Regards
BM