I have never used Rohacell, but anything that has porous surface needs to be filled in this manner. I built a 22.5lbs 33% Extra 300SX out of Divinycell last summer, so I have gained a lot of experiance with this material.

After rereading your process, you should be able to use just the 2oz glass and your carbon reinforcements and balsa, you could use even lighter considering I am using 1.4oz glass on my 35% Extra wings.

Make sure you have the glass oriented the same (45°) on your laminate, or on a hot sunny day you will learn what thermal expansion is the hard way. You can add layers all you want, but put them in the same orientation.

Like this:

2oz@45° / 2oz@90° / balsa / 2oz@90° / 2oz@45°

These layers will heat and cool the same and will not twist the piece as the day goes on.

Darrin C

I don't mean to keep posting, but I teach a composites class and I am just trying to help by throwing out some different ideas.

We use Meguires Molds release wax which is very paint friendly and after waxing the mold about seven times, we directly paint the mold with Lustercoat, or some other laquer based paint. Your first couple of pieces might not be perfect, so I would wax the mold a few extra times, but the more you use the mold the better it will release.

We let the painted mold cure for at least two days, then do our layup by painting on a thin layer of epoxy then laying our cloth down, pushing the cloth gently into the epoxy with a small brush and getting out any air pockets. When the piece is cured and popped out of the mold, we have a perfectly painted piece with NO pinholes.

Just a thought,

Darrin C

I really appreciate your commens Darrin... It's nice to chat with people about "shop talk" on these things.

I have been trying to get away from PVA for awhile. Bought some wax to try on the Tsunami stab. That mold has been well waxed, so the part should pop out ok. However, the wax that I used did not like the primer. I had to mist it on or it would get dots where the paint pulled away from it (not sure what the term is), but basically it looked incompatible. We'll see how it goes when I pop it out of the mold.

Believe it or not, I only used the 4oz on the inside (I think it was 4, could be 3) because I ran out of 2oz. My layup was:

primer/2oz/balsa/4oz (plus carbon in some areas)

Do you think 1/32 would be too thin?

What do you use to close up the molds? what type of material?

What do you make your molds out of? I hear a lot about tooling resins.

Michael

The wax we us is Meguiars Mirror Glaze No.8

What direction are you laying up your glass? I would suspect you are running the weave at a 45° from the mold, this will give you the torional strength and keep the wing from twisting.

Well, the engineer I work with wants me, after I make some Nomex honeycomb wings, to use 1/16" balsa as the core material, but that sounds thin to me on a 35% wing that will be taking some massive amount of G's. But he has done the math and assures me that it will work. (It's not his $1000 engine if it fails)

I would stick with the 1/16" and would use 5-7lb contest balsa, because you might run into problems with not enough skin thickness for buckling strength if you use a thinner material.

Main thing I would do at this point is to figure out a way to seal the balsa, especially the endgrain balsa. Balsa absorbs in the direction of it's grain so a light resin layup will have the possibility of having the resin sucked up by the balsa and leaving dry spots in the glass.


What do you use to close up the molds? what type of material?

Do you mean once the halves are bolted? I run 1" glass tape saturated in a epoxy/microballon mix.

I have made my molds like you have, simply because I do not have any of the tooling resins, I would like to try them in the future.

One trick I have learned is after you lay down a couple of layers of glass, mix dry sand or sawdust, 8 parts to 1 part epoxy and spread out a 3/8" layer of this slurry, then put the same amount of glass layers on top. Just a cheap and easy way to build thickness. If it is a large part, lay cut pieces of conduit in the slurry then glass over them, this keeps the mold from twisting and adds strength.


If you look in the aerodynamics/scratch build section, Ollie and several others have made molds and can offer other ideas too. We are all learning so no one has all the answers.

Regards,

Darrin C

Actually, on the glass (blushing), I just ran it lengthwise.

Well, I meant on molds where it is basically impossible to put tape in there unless the ends are open (like my mold). What type of "splooge" - material mixed with the epoxy to give it strength do you use on the joint?

I'm going to do some thinking on the next wing. Definitely didn't need the 3 or 4 ounce on the inside. I'm going to have to get some more 1.4 or 2 oz as I'm out. I have a roll of 3/4oz that's lasted me a few years. I must have had 70 yards on it at once time, but it's got no strength for this application. Just good for finishing.

Michael

Photo of wing from today. landing gear cutout started and hinge line cut away on bottom of wing.

Shot of airplane from above.

Another perspective.

Well, I meant on molds where it is basically impossible to put tape in there unless the ends are open (like my mold). What type of "splooge" - material mixed with the epoxy to give it strength do you use on the joint?


I do not know off hand since mine have always been open on the ends, but I will makes some phone calls tommorow and find you a answer. I would imagine that you will be adding some milled fiberglass to the epoxy to keep the seam from being brittle and cracking and some Colloidal Silica to make it a gel so that it does not run.

Let me know if you need any lightweight panels for your project. I make really light weight firewalls, landing gear plates, servo trays, ect...


The pictures are great and you should really be proud of the work you've done!!!!!!!!!!!!!!!!!!!!!!!!!!

Regard's,

Darrin C

I used milled fiber and cabosil as my joint. Worked good as far as I can tell...

I would be interested in light weight stuff to use for ribs, etc. Like landing gear uprights.

Thanks for all the nice comments. I just take things a step at a time and move it to the next level. This wing weighed (gulp) like 2 lbs, 6 oz with the heavy lg mounts, etc in it. My balsa/foam equivalent was 1 lb 10oz. My target for the next wing will be 1 lb 10 or lighter. It shouldn't be hard to do. This wing was a feasibilty "study" so to speak and my brother got a free wing out of it for himself. Not a bad deal. I think it will be easy to cut the weight down now.

Michael

Great information on this thread, and the photos make everything much more obvious. Keep us informed!

Mluvara, first off that’s for replying to my thread on glassing over foam wings and pointing me to this thread. I am sure many will get a better understanding from your posts. I run a Model Shop for Industrial Design Product Development and I just finished 3 molds for my 25% Edge 540 and I have some questions for you. Why do you prefer to add primer to the mold before laying up parts? It’s the same amount of work as spraying the body? Are you spraying this over the wax parting or over PVA? Can you describe your splitter plate please? When I CNC machine my plugs I cut them in halves (very easy with CNC) and when you lay-up the first mold half you have a flat parting line? I have never tried the glass stabilizer before but I like it. Do you add any ribs or foam to the inside? I usually vacuum-bag over foam for this. Thanks again.

Hi Av8tor,

I'm somewhat an amateur at molding. Started making parts a few years ago and progressed over time to what you see here.

I like to use a "gel coat", primer, or something to reduce pinholes when molding. I find that I get better results and it's just been a habit. A lot of places paint or put a gel coat of some sort on in the mold. I've just followed suit.

I have been using PVA as my main mold release and are trying some other waxes now as PVA tends to give a lot of imperfections. After I apply the mold release (whatever it is), I spray on the primer, let it dry, and then do my layup.

Man, I'd love to CNC all my molds! Mine are all hand-made.

For a splitter plate, it tends to be what I have around the garage. I have generally used wood shelving with a smooth surface on it, or it can be plastic, or metal. It is hard to make sure that they are perfectly flat and I do get some good parting lines, but not perfect. Works for my type of usage I guess. Basically, I make a cutout of the profile of the wing or fuselage, drop it into the splitter plate, center it where I want the parting line, and then use clay to fill in the voids.

On the composite stab, I put a main spar (just endgrain balsa) and then maybe a 1/8" balsa center rib and some stringers or wood near the hinge line. What I put near the hingeline depends on whether or not it is skin-hinged or going to be conventionally hinged. Other than that, it's hollow.

I'm going to finish the stab shown in the pictures tomorrow and possibly lay up another wing or fuse this weekend. More photos to come...

Michael

My compliments on sharing your learning experiences. Sounds like your molds are in great shape, but you need some experience with layup techniques.

I suggest reading how the RC glider guys build composite planes. Particularly F3B models, which often see loads greater than 18g's over 3-meter wingspans!

Wing Weight
I suspect all of the weight of your wings is contained in unnecessary epoxy. I have vacuum bagged quite a few wings with either balsa, carbon, or fiberglass skins.

The main task is to:

1) completely wet out the cloth

2) Immediately get all the epoxy you can off of it. Use an old credit card, or squeegy. You have to work very gently with light weight cloth or you will stretch it. The strength of any epoxy (FG or carbon) matrix is in the combination of the two. Extra epoxy adds nothing to the strength, only weight. You should target a 50% epoxy by weight. So if you are laying down 3 oz / yd^2 and 2 yards of cloth (for 6 oz cloth weight), you should plan on using no more than 6 oz of epoxy.

Yes, balsa will soak up the epoxy. You can thicken it with cab-o-sil (or similar) a little to help the absorpsion.

You want to get all the epoxy you can out of the layup (after completely wetting out the cloth). The surface of the cloth should look almost dry after using the card to get as much epoxy out as you can. Certainly not shiny, just kind of damp.

I also suggest you stick with 1/16 balsa and not go with 1/32. You need some thickness in the skin to give it strength (resistance) to deformation.

I suggest a fairly light weight FG cloth for the surface layer only. This will give you a smooth finish for painting, etc. and many fewer pinholes.

Spar Comments
You mentioned using vertical grained balsa for the shear web. This is fine. What about spar caps? You need something to take the compression/tension loads. I suggest either pre-preg carbon from ACP or CST. For formulas to calculate the strength you might need, see the following link:

I'm not sure how your spar is built, but if the spar takes all the bending loads, then the skin only needs to be able to take up the torsional loads (twisting). This is best accomplished, as others have mentioned, by using all glass on a 45 degree bias to the spar. This 45 bias cloth layup will also help keep your ailerons from warping.

Trimming the edges
I suggest completely trimming the extra cloth. Before joining the wing halves, completely trim all surfaces so that they are perfectly flush with the surface of the flange of the mold. This is easiest done while the epoxy is partially cured. Easy to cut with a single edged razor blade, but stiff enough to hold it's shape. Then you require a relatively small amount of spooge (epoxy & cabosil) to join the wing halves.

My best wishes to you and your project. I am hoping we can all learn from each other.



Here's a few links

Glider Spar Design

Example of advanced molded F3B glider construction

ScrollSander page - molded fuses, seamless joining of fuse halves

Soarhead,
Thanks for the info and links. I found them interesting.

I am very familiar with a lot of the gliders and construction used therein. I fly giant scale racers which are laid up mostly by RnR products. We turn very tight radiuses at close to 200mph with them. MY Gr-7 weighs 27lbs and has a 131" winspan composite wing that RnR built.

I knew this wing was going to be heavy. It's like you said - technique. The wing sheeting was heavy (I didn't order AAA - should have), the landing gear area was overkill (and heavy), plus too much cloth on the inside, along with having too heavy a weave on the inside. I did use peel ply and soaked up whatever excess there was, but the balsa still acts like a sponge.

The spar is endgrain balsa and I consider the cap to be the carbon tow that was laid on each side of the wing in the mold. The spar is then laid right over it when the wing is closed up.

My next wing is going to use AAA 1/6" balsa, 1.4 or 2oz glass to sandwich the cores, and a little more care on my part when I lay the wing up. This was my first wing, so we'll see how the next one goes. I'm positive I can easily knock 10 oz out of the wing.

Michael

Thanks for the post. The RnR guys are nice guys. I've heard they are (rightfully) somewhat secretive about their building techniques. If you can learn from that, then you have a good source.

I missed the part about your tow on the inside of the wings. So you're doing your spars right. Thanks for everything. Again, hopefully we can all learn from each other.

I'm learning a lot of stuff from this thread.

One question about the quote above. How can you trim the extra cloth while the epoxy is partially cured if you are bagging the wing? Do you have to stop the vacuum and pull everything out at just the right time, or what?

Actually, I have not learned any of RnR's "secrets", unless I have figured them out myself. I have just looked at a lot of crashed wings from racing and from other manufacturers and come up with my own method. RnR's local to me, but I see a lot of their models around - from gliders to giant scale. They are very popular. I know some of the guys at the shop and they are all nice people.

Michael

Good point. I was thinking about trimming the glass when laying up a fuse, as described in the ScrollSander link.

I don't know how to do that when bagging a wing in a mold.

Sorry if I mislead you.

Ok guys -

Closed the stabilizer mold up tonight. Snapped two photos. This stab was layed up on top of a wax mold release rather than PVA. Let's see what it looks like tomorrow morning.

Michael

Stab with "splooge" before mold is closed.

Another angle before closing the mold. This is the inside top of the stab we are looking at. The hingeline has the balsa removed.

Michael

Pulled the stab out of the mold this morning. Came out pretty nice. I used just wax for release on this mold and it came out good. Only complaint is that when painting or primering the mold, you have to gently mist the paint on, let it flash, and then come back and shoot an even coat. Otherwise it will act like there are spots of moisture on the mold. Could have been my gun, but the PVA did not seem to do this. However, PVA is water soluble. I need to put a moisture trap on this compressor.

Total stab weight was only 4.6 oz!

If you notice in the above photos that I put carbon near the ends of the hinge line. This reinforcement is too much and makes the hinge hard to move.

Michael

Well, did some more work today. Chris sanded and filled, fiberglassed his firewall, and is close to mounting his stab.


Stab cutout and fit. Awaiting wing incidence check.

Fuselage layup prior to peel ply being added and then placed in the vacuum bag.

Fuselage in bag and all set for the evening.