daven

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I'm hoping to be laying up some wings in a mold in the next couple of months and had some questions.

1. What are the advantages/disadvantages of using Rohacell vs. Balsa as the sandwich material for the wing?

2. Any good sources for Rohacell? I found one called Mr. Fiberglast and that looked pretty good, just curious if there were others.

3. I plan on laying up each half of the mold, and then bagging until dry. Clean up the edges, add the internal structure, and then join the two halves. This is the only way I can see keeping them light enough for my use. My question here, is that when I lay up each half, I assume that some glass will extend past the top of the mold. When I apply vacumm, where will this excess go, and how would I clean it up after it dries?

4. After the two halves are done, and I'm getting ready to put it all together, what mixture are you guys using. I assume some slow cure epoxy mixed with Cabosil?? Just curious as to the consistancy, or if I should be using somthing else.

If it matters, the wing will be 52" long with a 9 5/8" chord. The wing will hit speeds around 170 mph so strength is obviously an issue.

The wings will need to come in between 15-17 ounces at most.

This is all new to me, so I will kinda be learning as I go. I figure the first couple of wings will probably be on the heavy side, but the project will be worthless if I can't get them down to 16 ounces. There are several guys getting them there, so I know if can be done. I will be talking to them, but thought the experts here could help me start to get some info.

Thanks in advance.

sayno2glo

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Mother of all molding threads:
http://www.rcgroups.com/forums/showt...&pagenumber=10

I would go for contest grade balsa because it's easier to work with more robust and gives more strength. Make sure you have enough carbon in the spar and I would recommend to ask someone who has already built similar wings. End grain balsa is good for shear web. I talk about spar this much because without good one it's will snap.
Back to Rohacell- I would use it only if I could not build light enough from balsa. When a Rohacell gets hit it will brake much more than balsa, the difference is huge.
For joining halves I use mix of epoxy, micro-balloons, carbosil, little bit of chopped class or cotton, and sometimes foaming agent (not for joining spar caps to shear web) which will make about 50% foaming when curing, but it makes the mix much weaker.

/Antti

davidfee

Dave,
Here are some basic answers:

1) Rohacell is lighter than most balsa you will find. It can also have better compressive strength. Not better than end-grain balsa, but better than across a normal sheet.

2) Composite Structures Technology (CST) sells it. There are probably dozens of sources.

3) Yep, that's how it goes. Any excess material (you can cut the skins to ~exact size using templates) will be pressed flat on the mold flanges. You can trim it after curing with a sharp razor blade or chisel.

4) Epoxy and microballoons... lots of microballoons. Apply a thin bead around the perimeter and all spars/ribs using a large syringe.

Good luck,
-David

davidfee

Antti,
That wing mold looks great! That's a new one, right? Not your Avionik clone...

Balsa is also cheaper than Rohacell... another advantage.

-David

sayno2glo

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...Avionik Killer...[:@]
There it is, intentionally fuzzy picture of it
/Antti

davidfee

Awesome! I can't wait to see it in York!!

-David

Marco Vergara

Hi Dave

1.- Rohacell dents more easily and the price is higher than contest balsa wood. I use contest balsa on my wing skins q500 or q40 type.
3.- Trim the excess after the resin cure.
4.- Mix cabosil, lots of microballoms and resin. consistency should be like peanut butter.

To get a light wing you must be very careful with excess resin, select your wood, use peel ply on the entire skin surface for mold closing use very very little mixed resin.

To hold high G forces I use 3 plys of 3/4 " x 48" long carbon uni graphite stitched 4.5 oz upper and lower skin. The spar is 3/8 " cross grained balsa.

hope it helps

Marco

daven

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Antti,

Thanks for the link, that is very helpful. I will be very carefull with the spar, I was planning on 1/4" end grain balsa capped with .007 carbon fiber?? I'm suprised you mentioned balsa being easier to work with. I figured I could buy a 24" x 48" piece of rohacell and just cut it out with a razor blade. With Balsa, I'd have to go to the trouble of edge glueing the sheets and sanding. From what I saw, the Rohacell would be more money, but I found the 24" x 48" sheet of rohacell for about $40 U.S. Dollars which didn't seem to bad to me. As to durability, when these planes crash there typically isn't much left worth repairing anyways. You mentioned a foaming agent, what is this?

David,

Do you not use cabosil with your mixture to join the halves? My biggest worry is that the leading edge will open up, and the wing will explode.

Marco,

This mold will be for a ama 428 Q500 wing, so you understand what I'm doing. Your pictures were great, and I am sure I will be contacting you with questions in the future. What thickness of carbon are you using around the wing hold down bolts?

Thanks again, you've all been very helpfull.

Goggles

My Feedback: (4)

#2 - I've been looking into ALL kinds of sandwich material to keep my stuff light and I'll be trying a few different materials soon to see what I can come up with. I'll let you know.

#3 - What I have found is when it's still a little tacky (green) is the best time to trim the edge. When it's dry it very hard and when it's green it a little easier to cut.

#4 - [link=http://www.contacteast.com/product/catlist.cfm?cat=DB47D0FA-0C85-11D4-A60C009027AFC798]Contact East[/link], check out their list of adhesives.....Very strong. I seen a few all composite planes that were joined with some of this stuff and talk about strong!! I have started using it and with a 3000-5000 shear strength I'm sure my stuff won't fall apart.

Take a close look at the picture, you'll see the adhesive used. (no, it's not mine)

daven

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Wow Goggles, you sure that thing has enough aileron throw

That is one beautifull looking structure.

Goggles

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No kidding, but like I said it isn't mine, it's JohnVH's new kit. I seen it in person and it's HUGE!! The new CA Yak 3.3M, all composite and I took a really close look at how it was made.....amazing is all I can say. Very interesting how it's all put together, the sandwich material used, and whole structure is something to behold.

Marco Vergara

Daven,
It is the same that I use for the spar Uni-graphite stitched 4.5 oz , I buy it at Aerospace Composite Products (ACP) if you have more questions just ask it will be a pleasure to help.

Marco

davidfee

Dave,
No, I have not used cabosil in the joining mixture. My efforts have focused on keeping things light.

If you are trying to make it strong, then you should use some chopped glass or something. Cabosil won't really add any strength. It's just a thixotropic agent which keeps the mix from flowing down into the wing.

-David

sayno2glo

My Feedback: (2)

I assumed you are building Q40 or similar but with Q500 the spar is not so critical. I don't know what you mean by .007 carbon fiber because I'm used to think it as number of 12k tows. I would put about about 30 12k tows in the middle and reduce it to about 10 in the middle of the wing half and to zero at the tip. Don't take these as a fact you better ask someone who has built q500 wing.
That foaming agent is EPS 50 and it's from emc-vega.de from Germany. It's not necessary and I don't advise to using it if strenght is a consern.
/Antti

daven

My Feedback: (1)

I've been collecting info on this layup, and this is where I think I am going to start.

Please let me know if this sounds ok for my layup schedule and if you have any comments or tips.

I will paint the inside of the mold with a simple scheme using KlassKote Epoxy paint
1.4oz fiberglass outside
2mm Rohacell
.7 oz fiberglass inside
3/4" wide Carbon Laminate Where the spar will go .007 thick. 1 full span layer, a 2nd layer 32" long, and a 3rd layer 16" long in the center.
2 oz carbon cloth approx 1 1/2"" wide top and bottom of wing at TE from outside of ailerons inboard.
2 oz carbon cloth top and bottom, roughly 2" x 4" where front wing hold downs will go.

At this point I will bag each half of the mold seperately and wait till dry. After it is dry, I will start the sequence below.

1/4" end grain balsa full length spar
Cap the LE of the aileron with 3/16" wide end grain balsa
add fitted blocks for front wing hold downs
install torque rods into blocks in ailerons. Have torque rods supported at a couple places with wood blocks and/or Pro-Set / Colladial Sillica
When I join the two halves I will use slow cure Pro-Set mixed with Colladial Silica roughly the consistency of Ketchup.

I have a question in regards to the torque rods. Many are using 1/8" rod with simple wheel collars which they use to mount the threaded portion after it comes out of the mole. I would rather use prebent 1/8" wire, but this would mean that I would need to drill holes into the mold so the rods could exit on the bottom of the wing. Is this an acceptable method, or should I use the wheel collars like everyone else?

TIA

daven

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Any responses?

Just curious if I am on the right path........................

davidfee

The layup sounds ok, except, where on earth are you going to get 2oz carbon? The lightest I've ever seen was 2.4oz. Are your carbon spar caps going to be inset into the balsa sandwich, or lay on the inside? You will also need a shear web ahead of the aileron hinge to close out the opening in the fixed portion of the wing.

I don't know about the torque rods, except to say that having holes in the mold where you poke the wires would be a big source of problems. The holes would constantly will with epoxy. You would probably need to have dummy wires to plug the holes while you lay up the wing skins. Remove the plugs and install the torque rods before joining the upper and lower skins. Then you have the problem of demolding the wing with the torque rods sticking into the mold. Obviously this needs more thought. The thought of using wheel collars scares the begeebers out of me. Sorry for my rambling... it's late.

-David

daven

My Feedback: (1)

You are correct, I meant 2.4 oz Carbon.

The spar caps will be layed up inside of the sandwich.

Someone else recommended the shear web ahead of the aileron hinge, thanks for pointing that out.

I spoke to someone in the "know" yesterday, and he said that the holes would be fine, but I'd probably have to drill them out every time I used them. I was thinking about drilling and tapping the holes in 1/4 x 20 thread and just screwing in a waxed up bolt so that it was flush with the bottom of the mold??

You'd be suprised how many people are using the wheel collar method on these. Scares me also, thats why I want the holes

Thanks for your help.

Ed Smith

I use wheel collars on my racers without any problem. Grind a flat on the end of the torque rod. Slip on a 1/8 collar. Screw the nylon horn onto an allen screw and then tighten the screw into the collar to lock it. Using an allen key to tighten it will allow the screw to be realy tightened down. Use loctite on both.

HOWEVER!!

On skinned hinged wings there is a problem. The torque rod has to sit against the underside of the top wing skin. On a high wing quicky wing the wheel collar would protrude through the skin. On a Q40 wing the hole is covered by the fuselage.

The holes in the mold method is the best solution. Just prior to laying up fill the holes with modelling clay (Plasticene?). When installing the torque rods just push out the clay.

Ed S

daven

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great idea on the clay Ed. Its amazing sometimes how the best solution is the easiest?

As to the torque rods, the prong will be glued on the underside of the top of the wing (hope that makes sense) with a mixture of epoxy, glass, and collidal silica. I was planning on putting a small shim under the torque rod sleeve so that the rod would not sit flat on the underside of the wing, but be slightly (1/16") up off the glass.

Ed Smith

Some things to consider with skinned hinges.

The true pivot point of the control surface is a point that is on the top of the skin, (You may think I am splitting hairs here but read on) The centreline of the torque rod should line up with this pivot point. This would put half of the rod on top of the skin and half beneath it. OK so far?

So we put the rod beneath the skin. This means that the rod now swings in an arc beneath the skin. The further away the rod is from the skin pivot point the bigger the arc it has to swing. But how can it if it is glued in place? A point is reached where the the whole setup will not work, it is jammed. Problems will be encountered if the torque rod is fixed in place right up to the endcut of the aileron. Flexibility is needed. to allow the rod to swing.

I have skin hinges on my foam wing quickies. When I install the torque rods I cut away the foam and the top balsa skin. The rods sit right under the glass skin. they can be seen from the top of the wing. The only reason we get away with this type of setup is because we use very little control surface movement. The alternative is to use two wing servos and no torque rods.

Be warned!!

Ed s

davidfee

I like the idea of the modeling clay to plug the holes. Make the holes oversize (say, the size you want the final opening) to allow you to demold the finished wing without the torque rods binding up.

The way I have done the torque rods on F5D models is to bond them at the centerline of the aileron thickness. This gives the torque rod enough room to make its arc. I leave the rod unsupported outside the aileron, but then have a bearing block just at the 90deg bend where the torque rod exits the wing. It gives plenty of travel in the places I've used it.

Separate aileron servos with just straight pushrods would eliminate this problem, as Ed already mentioned.

good luck,
-David

daven

My Feedback: (1)

I understand the arc, and how the rods work, but it just haven't beeen a problem on the skinned hinge quickees I've been building, so I'm not going to worry about it.

I completely embed my torque rods in glue from the edge of the aileron to where the torque rod pops out the bottom of the wing. It just doesn't seem to be a problem when your moving 1/4" throw at max.

MarkVZ

What sort of surface coating are you guys using on your raw MDF (chipboard) wing molds? In the past I've had problems with pulling chunks out of the molds when removing the cured part.

Thanks in advance

Ed Smith

Not quite sure what you mean. The chipboard would be use to create the flange around the mold at the mold building stage. After that the mold is all glass. Or is it?

Ed S