BFoote

Balsa insert is an ok option for increasing section modulus. Unless you can make sure that it is end grain it shears too easily. You can buy 1" end grain balsa, but it will be heavy density balsa. The shear modulus in this condition is much better, but it will still shear long before the composite laminate breaks. This is assuming a thin laminate not a thick... No precise terms here, but close apporximations will work. Neoprene has better bonding characteristics than balsa because depending on the epoxy, the epoxy will meld with the neoprene creating an intmingling effect especially a high temp epoxy, softens up the neoprene. Honeycomb substrates also have the same problem of delaminating from the top laminate, but this is often offset by adding extra epoxy creating a gusseting condition in the cells of the honeycomb. Thus, your shear modulus will be that of the epoxy. I have found that when using balsa, one wants to "stitch" the balsa, creating a larger area for the top and bottom laminates to "work" on. By stitching I mean cutting a thread on one end of the laminate, poke a hole through the balsa and run the epoxied CF thread through this pin hole. Or, throw some Thin CA on it and get your fingers off Immedietly, since it will burn you in a jiffy. For every type of lamination method, there are always a ton more.

I will have to make some AL powder!!!
What size of AL powder do you use? Have lathe, will use =)

Brian

davidfee

By Al powder, I literally mean powder. I don't know if you can make it small enough using a lathe. An example would be the aluminum powder #420 from WEST Systems, which you can get lots of places, including CST: https://www.cstsales.com/Epoxy_&_Mol..._additives.htm

Be careful with the stuff because it is quite reactive and is considered explosive around static discharges.

-David

Darrinc

My Feedback: (5)

Make sure that you use a respirator with the Al powder.

davidfee

...or with the graphite, or with colloidal silica... or when cutting, grinding or sanding the raw fabric or cured composites. Wear safety goggles, as well as tripple-duty gloves.

Ok, just taking it to the extreme. Maybe it's not funny.

Darrin, what do you see about aluminum powder which makes it warrant a respirator?

-David

Darrinc

My Feedback: (5)

Sorry, no technical data, just seems like a good idea. Everytime I try to mix in anything I get a cloud, no matter how heavy the powder.



Ahhhh, I must finally be a Californian!!!!!!!!!!!!!!

Must be safe, must be safe, must be safe!!!!! (grin)

davidfee

And whatever you do, don't mount a propeller to a shaft which might spin it round and round... thus voiding all warranties.



Yes, be safe... it's always a good idea.

-David

rcairplanenut

Hi guys

What temp is generated by a black glad bag in the sun post cure?
Is it good or bad?

davidfee

Hot enough to warp your mold if you're not careful. A friend warped his wing molds doing that. I think it may have been mostly epoxy shrinkage in his case, though.

Stick a digital thermometer in the bag so you can read the inside temp. If it's a free-standing part made with room-temp epoxy, you might want to keep the temp to 120F or so. If it's supported by a mold (or a foam plug), you can go a little higher.

-David

Magne

You can do the cowl the same as the hatch, or layup the parts seprately in the molds, trim them and release them and put them back in the molds and then do the layup over them. (don't forget the cabsil/epoxy around the edges)

This will give you a all most seamless look to your fuse. (post No. 35)

OK, this is interesting.
Assuming you have a fuselage mold in two halves, where you want to make a canopy (on the top, obviously) and an engine cover/belly pan on the underside, all parts with Nomex core. When do you actually join the respective parts together (i.e left and right hand side) Is this done right at the end, after you have manufactured your canopy and bellypan halves, trimmed them, sealed the edges, released them, placed them back in the mold halves, and layd up the "main" fuselage halves with a 1/4" overlap??
Next question:
When you have "popped" a part out of the mold, and later put it back, then obviously the surface of the part is no longer "stuck" to the mold surface. When joining the two halves (using epoxy/micro balloons, cabosil or whatever) Is there not a big risk of epoxy creeping in between the part and the mold?
Finally:
Quotedon't forget the cabsil/epoxy around the edges)
Do you ever fill up the honeycomb where you are going to trim, during the lay-up process? Would save doing it later on, as you trim/cut a "solid" material. If done carefully it should not add much extra weight.

Thank you Darrin, you have given me many good ideas.
Magne

Darrinc

My Feedback: (5)

Yes, my method has me join the halves at the end of the process.


Yes, the epoxy will creep, so be aware. You can put the trimmed and released part in place with 3M77 and vacuum bag it into place, this seems to help.

I usually bevel the core where it ends, typically 3~4 to 1. This helps tranfer the load.

Thanks, you will probably come up with even better ways to do this. Keep experimenting