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What is the technique in balsa sheeting foam core wings. I have read the manual recomending to use the Zap Finishing Resin or the Hobbypoxy Smooth n Easy Finishing Resin. Is these glues just regular epoxy? I have read at other places here on RCU that one could mix 50/50 white glue and water and use a brush to put it on the foam core. Also if I would like to lighten the struckture is it wise to remove some of the foam by cutting out holes on the foam core? What other ways is there to lighten? Changing the balsa sheets for lighter ones?

majortom-RCU

My Feedback: (40)

As to choice of adhesive, there are a few ways to go, epoxy for one (look for laminating epoxy), aliphatic another, polyurethane (foams up as it cures) seems to be the new favorite. Here's one website with good photos & text: http://www.mackrc.net/patternwings2/index.htm

As to removing foam pieces from the cores before sheeting, it can be done, but is rather laborious. It also creates problems as to just where to remove and where to leave for structural integrity. It is a process best left to those willing to accept unsuccessful experiments. A lot of bother for not much weight saving, in my humble opinion.

As to weight of the finished wing, two areas to focus on are the amount of glue used and the weight of the balsa skins. Use the least amount of glue that will give good bonding from balsa to foam core. I would think aliphatic would be the lightest of the glue choices, but have yet to experiment with that myself. I like the polyurethane. Its foaming action creeps into voids, and its viscosity is thin enough that it can readily be spread out for a very thin coating. Also it has a very strong bond. Keep it off your skin (human skin). Not toxic (that I know of), but sticks good and doesn't wear off for days.

I weighed the balsa sheet pack that came with a quarter-scale kit a few days ago and calculated the wood is nearly 14 pounds per cubic foot. Contest balsa, if you can find any, would be less than half that. I figured if I used the wood in the kit, it would cost me close to two pounds in the finished weight as compared to contest balsa. That's a lot of heavy wood--more than a 10% weight penalty. Having been unsuccessful in finding any truly contest grade wood I decided to take a chance on a quantity of AAA grade balsa, which with a little luck might come out around 8-9 lbs/cu ft. We'll see when it gets here. I will sort the pieces out and use the very lightest for sheeting the tail parts, next lightest for sheeting fuselage parts aft of CG, then the rest for wings and forward fuselage.

Rate1

Great feedback. I have 2 days ago ordered a 1/4 scale myself, the Lanier Extra 300S 72".

Where can I find the polyurethane glue? Is it difficult to use or is it mearly using a brush spreading it out like regular glue? What is the price of contest balsa? If using contest should I go for the same thickness as stock or could one even try slightly thinner to save even more oz? How much sheeting are you replacing and what is it costing you? Where are you buying your balsa? is it worth buying contest balsa online or should one go to a store and inspect each sheet?

I had a chat with another guy here on RCU that said the fuz sides are of poor and very heavy and brittle ply. That there is a good idea to swap them for 1/8 lite ply instead to save several oz. He also meantioned the contest balsa as you did. Needless to say, I will try to lighten this fram and changing some wood for better one sounds not to difficult. In addition I will try to chose good equipment and hardware. Have for instance ordered CF wingtube and are looking into CG gear. Did order the fiberglass cowl and wheel pants option together with my kit.

....hmm, I can see that I came up with loads of questions on this on. However I have never sheeted foam core wings before and I want it to be done good as well as ligt weight.

Regards,

C.

majortom-RCU

My Feedback: (40)

Polyurethane glue is available in several different brands. Gorilla Glue is in our local hobby shop, ProBond is at the hardware store, pretty much the same stuff I think. Dribble out a small bead and spread it as thin as you can, roller works best for me.

http://www.lonestar-models.com/ for prices.

Thickness: I ordered 3/32" thick so I can sand without fear of sanding all the way through. If I were more careful I could use 1/16", but I'd rather work in my usual casual style and suffer the consequences. Master builder I am not. I think I ordered 100 sheets of 3" x 42" for considerably less than $100. That's more than I need, but I'll use it over the next few projects, then evaluate. If I could go to a store and weigh each sheet I would do that, but the local hobby shop sells the same stuff that comes in the kits.

It would be great if you could find a set or two of cheap foam cores and practice once or twice before doing your project. I learned that in art class years ago. (Our teacher would say, If you're not sure how it will work, and the whole class would join in: Make a model! She did not mean model airplane, but a throwaway before screwing up your masterpiece.)

CF substitutions for aluminum will help on wing tube and landing gear. On a plane this size I think titanium axles are worth the cost. On our grass fields I find the typical Dubro axles don't stand up well to even soft landings, and if you go to 1/4" for more strength they get very heavy, but the 3/16" titanium is light and stands up well. I believe they're a PSP product, available from Don's Hobby, Troy Built and others.

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Hmm titanium wheel axels sounds expensive and very nice. I ordered the NMP Light Weight Aluminum wheel axels from Central Hobbies, together with their 3/16 carbonrods with 3 mm titanium fittings and Dave Brown Treaded Lite Flite Wheels. I also ordered a Tru Turn Light backplate Spinner, eventhough I know it is not as light weight as a plastic one. I would like to have a CF spinner but are unsure if I can use a starter on it?

http://www.centralhobbies.com/landing_gear/Axles.html

As soon as the kit arrives I will check out the wood and see what balsa and ply I need to replace. Not meantioning where to find high quality ply and balsa here in Sweden. Eventhough I know that pretty much every hobby store is keeping AAA balsa in stock I am unsure about the contest. I will only buy the quantity needed for this kit so I suppose it will not be too expensive.

Another way of reducing weight, as I have been told, is to replace all the fuzdoubler with contest balsa. Claiming that contest is strong dispite its weight and in a sandwich construction as double it will be sufficent. Also to replace some ply formers in the fuz with contest balsa... Sounds a bit flimsy to me but then I don't have much experience. The only build I have done before was the GP Extra 300S .60 kit. However I manage to complete that one to me very satisfying.

As said, I am planing to replace the included ABS turtle deck and conopy hatch by foam/balsa construction. However I am a bit conserned the balsa structure to be too fragile. I would like it to withstand some handling without getting brused with nicks too soon.

What quarter scale kit are you working with?

C.

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A bit prematurely I know, but I am leaning towards this colour scheme.

majortom-RCU

My Feedback: (40)

My project is the Lanier CAP 232 81" wingspan. We must have bought from the same clearance sale. I have yet to be greatly impressed with any Lanier product, but I can't resist deep discounts, so I figured this would be a good exercise in rehabilitating a cheap kit.

I have seen suggestions along the line of replacing fuselage ply doublers with balsa (but not contest balsa!). Since I will most likely be powering with gas, I will wait until I actually start gluing up before I decide about wood for the forward fuselage. I am not looking to make a high-performance model here, just trying to avoid producing a flying pig.

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Or worse yet, a screaming pig running at full throttle to stay aloft. I actually are hoping my OS 120 FS will power this thing allright for traditional aerobatics. lets see how this stunt will turn out. Still, it is fun and motivating to set goals before a build and if this means that I actually shaved off 1,5 lbs when it is done then I suppose it all was worth it. Dispite needing to buy a YS 140 for it.
Do you have other weight reduction tricks in that bag of yours?

Is contest balsa as strong but lighter weight than regular AAA balsa?

robotronski

I built a kit way back when from the SIG models company called a "Kommnander" It was a sheeted foam core wing. Sig sold (and hopefully still sells for your sake) a glue that was called something like CORE-Bond??? Anyhow, it would not eat the PS foam and was easy to apply. My advice is spend extra time pre-building the skins as an entire sheet the entire chord of the wing. I used a razor plane to shave the edge of each 3"x36" plank square. I then butted individual planks together on top of waxpaper on a flat worksurface. I used wood glue to edge join the sheets until I had a single composite piece big enough to cover, lets say, the top left half of the wing. I would then apply the entire sheet as one application, being careful to roll the piece on from leading edge to trailing edge and not just plop it down (avoid risk of a warp or bubble) ONCE THE GLUE TOUCHES, ITS A KEEPER!!! So line things up, get you a good helper. Also the seams where the individual sheets a butt spliced requires only enough glue to hold them while installing. The real strength of the wing is in the foam/balsa sandwaich. I say this because, you need to use the wood glue sparingly to prevent ugly glue edge lines from peeking thru that gorgeous monokote film you are going to cover the sheeting with. If you use CA, the hardened CA will not sand out and your sheeting will look crudy under the monokote. By the way, I crashed my plane 5 yrs ago from a stupid move at full throttle and the wing only had a ding where it caved in the fuselage. They no longer make that kit, but I did keep that wing as a memento of some sort. Best of luck with the build, you should have a gorgeous wing that can take all sorts of G-loads without any problems whatsover.

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robotronski,

Thank you for sharing this. As you gather, I have never sheeted a foam wing before so I need all help I can get. I was actually planing in using CA for joining the sheets but remember now, after you meantioned it, that it is difficult to sand over a seam where it has been glued together. I agree that a better alternative is to use wood glue.

Newc

Perhaps the following thread will answer your initial question. By the way, ACP Composites has a great system for $100 that allows the vacuum bagging technique.

http://www.rcuniverse.com/forum/m_38...tm.htm#3818590

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It does. Boy is that serious or what... Way out of my league. []

camdyson

Latex-based contact adhesive works well, if not after the high-strength of epoxy resin. This is probably what Robo' was referring to. DEFINITELY prepare the sheets into skins big enough to cover whole top or bottom surface, and PRE-SAND the wood so you don't have to do it on the wing - causes flat spots. Aliphatic resin, used sparingly, is a good sandable glue for joining the sheets.

When you do it, sheet top and bottom in one go, and weigh the lot down to dry in the foam cores' "beds" they were cut from.

It's actually pretty easy - practise on foam tailplanes first if possible!

Cam

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Camdyson, good point to practice on the tail plane first. Will do that.

Well I swinged by the boatstore and picked up epoxy resin and very fine cell rollers for this job. On the instructions for the resin it says that it will be dry to handle in 8 hrs and to fully cure in 24 hrs. I plan to use the smallest amount of this stuff to keep the weight down. Lets see how it will turn out.

Newc

Before you go any further, take a look at the video (free) at this site.. http://www.rcgroups.com/links/index.php?id=4901 ..Jim shows how much - virtually all - of the epoxy can be removed if you are vacuum bagging, even with his low tech system. This is a major benefit of any sort of vacuum bagging - it allows the use of the minimum amount of resin, glue, etc. so that the resulting structure is strong and yet as light as possible.

By the way, Jim is a friend of mine and he may be using Gorilla Glue for the vacuum bagging now instead of epoxy, since I did the experiments and then my wings, tail feathers, etc. with Gorilla Glue and the ACP system.

You will also notice in the video that with the vacuum bagging system we don't need to worry about what glue to use to edge glue the balsa sheeting (and how easily is will sand smooth) since we don't edge glue at all.

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I cant find a vacum system and really cant justify such a purchase for just 1 set of wings either. I guess I will just go don the traditional route. As I understood, the vacum system is simulating a very large weight on the construction. That I can do too by putting a table upside down over the foam and loads of weight on top of that.

Newc

Actually, to simulate the vacuum's 'weight' you would need to have a large car (not just a SAAB) on top of the parts being joined, and have to ensure that all sides of the parts (shucks) were being held together by the exact same weight - as a vacuum produces. Didn't you look at the video? Jim uses - very successfully - a $50 food saver vacuum system.

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I did watch the video and it looks like a great technique producing a awesome result, but I dont even know where to buy such a vaccum machine. Then it is the aspect that I am just doing one set of wings and after that not in need of the machine anymore. I think I will just follow the techinqes discribed to me initially and keep it low tech. I have however found the compound for the job, epoxy resin 24 hrs cure time, at my local boat store (the one hobby store I asked for it directed my to the boat place).

huggins

If you have an air compressor, a vacuum pump can be made fairly cheap. Check out:

http://www.joewoodworker.com/veneering/welcome.htm

Mike

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Good tip but I don't have a compressor either. Justy basic tools, glue, table, and a desire to build something nice.

Newc

Seems like you are in good shape, especially by getting the proper type of resin - for lamination, not for adhesive uses. Remember to use only enough resin to do the job - any more and you are just adding unnecessary weight.

Hey - lots of wings were successfully sheeted before folks started vacuum bagging! Good Luck.

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Newc,

I will have that in mind when going thru this building step. Thank you.
However, I will keep my eyes open for a vaccum machine. If unsucessfull finding one I will do it the traditional way.

majortom-RCU

My Feedback: (40)

Just a follow-up note on my AAA balsa sheets, which came in today, 100 sheets averaging 10.24 lbs/cu.ft. This was described on their website as '8 lbs and up'. So that is better than the almost 14 lbs/cu.ft. sheeting that came in the kit, but a long way from contest balsa. Other than being a bit heavier than I was hoping for, it's nice wood, clear, regular grain. I'm sure it will all find a good use somewhere. Now I have to find a little gram scale somewhere to weigh and sort the individual sheets.

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majortom,

Myself took the extra expense of ordering contest balsa for all the sheeting and AAA for all the leading edges. Lite ply to replace the fuz sides. I will have to dig up a digital scale myself to see if it was worth the extra cost and to play the matching game.

What was the materials weight that you are going to swap, and what is the all up weight of the new material. Hence what is the weight gain going to be on you flier?

majortom-RCU

My Feedback: (40)

Those are sensible questions, to which I don't have much in the way of real answers at the moment. I have already forgotten the cost of the new wood, but let's say somewhere around $.70 US per sheet. Let's say I use 30 sheets to do my model, so cost is $21. Let's say I use the 30 lightest sheets, which will all be less than the average weight in the package. Let's say they weigh 9 lbs/cu.ft, compared to the 13.8 lbs/cu.ft. that came with the kit. So I'm saving 4.8 lbs/cu.ft., with a net usage after trimming and sanding of maybe .15 cu.ft. that will actually end up flying on the plane. So .15 x 4.8 = .72 lbs or about 12 ounces. That's nothing to sneeze at on a plane that might finish out around 15 pounds. I consider that to be money well spent.

Of course the wood I will not have used, that will now be available for a future model or two, will be heavier... so I will be going backwards in my career, which has been the story of my life for some time now. It's tough getting old.