Hi,
I have had a sig tristar kit for a while and have finally goten to the sheeting phase. I cut all the sheets and preped to glue them on. I was told to use the contact cement. The kit came with foam core wings and the container said it is used for foam. So I proceded to start gluing the sheets on, and brushed some of the contact cement on. As I put more on, I realized that the glue was eating the foam and wondered if it was the kind of foam used, or was it just the glue? The kit is pretty old so I was wondering if it could be old foam that caused the problem? Also would epoxy work on this?
-TJ

epoxy will work, but will be heavy. As per the contact cement, is it the water based stuff? I was told to only use water based.

I only put the cement on the balsa side. I wait until the cement turns clear before I applied the balsa to the wing.

Aaron D.

polyurethane glue works outstandingly!!

Epoxy will work fine, and need not be heavy at all. A PAIR of 78" bipe wings can be done with < 4 oz per wing, if you do it right. This method will still work for your 'eaten" cores, if the eaten areas aren't too extensive, and aren't on the leading or trailing edges.

You WILL absolutely need 3 (maybe 4) things for this to work..if you don't have them, stop reading now.

a) The wing shucks
b) A FLAT table. no bumps, waves, etc. Level is not necessary, but FLAT is.
c) Preferably a vacuum bagging system...if not, LOTS of weight (concrete blocks, car batteries, etc) is a suitable alternative. If you use heavy weights, you'll also need a large FLAT board...like a sheet of 3/4" MDF or something...that's at LEAST the size of a wing.


1) Get some poster board. Lay it in a shuck, put the wing core down on top of it, and put the other shuck on. Trace around the wing, leaving a good 1" or so overlap on all edges. You don't need to do 4 patterns...2 will do...a top and bottom. They can simply be flipped for the other wing.

2) Cut your 'templates" out of the poster board. Use these to plan the various holes, bays, etc you may need for servos, or retracts, or whatever. Cut them out.

3) Lay down sheets of balsa, and tape them together (not glue...tape) on what will be the top or exposed side of the sheet. Cut them to match your templates, including any of the holes for servos and such.

4) Lay in any spars/false ribs/servo bays/etc that you may need to add, prior to sheeting the wings.

5) Spray a bit of hair spray on the "foam side" of the sheets. yes. hair spray. plain ole hair spray. Don't soak them...just get them kinda tacky.

6) Now..prepare to sheet the wings by first laying a large sheet of plastic film/saran wrap/etc over your build table. This will keep drops of epoxy from getting on the table, making it "un-flat".

The next steps need to be done quickly and in order...so have everything prepared first.

7) Mix up a batch of 30-60 min epoxy. 60 min if you think you'll want the longer work time, but 30 will hold fine. For that bird, you'll probably NEED < 2 oz per TOTAL wing...for ease of working, mix up 4-5 oz per wing, and just accept the waste the first few times you do this, until you get a feel for how much a given wing will need.

8) Lay the sheet being epoxied down on the plastic film, tape side down. Pour some epoxy on there, then...using a credit card, or one of those "window tint" squeegees, or something like that, spread that stuff as THIN as you possibly can. Quite literally, if you're doing it right, the balsa won't even look wet. It'll BARELY even be sticky. I can ABSOLUTELY promise you that you will SWEAR there's not enough epoxy on there. There is. If you cover the whole sheet, there's enough.

9) Squeegee some more, because you didn't believe step 8. Seriously...the balsa will NOT look wet. It'll be just SLIGHTLY sticky. Really scrape that stuff off there, and spread it just as THIN as you POSSIBLY can.

10) Do the other side, if you wish to sheet both top and bottom of a wing at the same time. if not, that's fine. First couple i did, i did one side at a time, just to not be rushed with an unfamiliar process.

11) Do the other wing, if you're doing both at once. See comments in #10.

12) Make a "sandwich" that goes Bottom Shuck, Sheet (epoxy up), Wing core, Sheet (epoxy down), Top shuck. Be careful to line the wing core up very precisely in the shucks.

13) Put the wings into the vacuum bag, or set them on the flat table, set the large flat board on them, and weight them down...a lot. I normally put 40-60 pounds on each wing. the top board allows the weight to be spread out evenly, and push down uniformly on the entire wing...the same effect created by vacuum bags.

14) Clean up your mess

15) Go away, and come back tomorrow.

16) Pull the wings out, trim the overhanging sheeting, and finish as desired.

17) Enjoy your INCREDIBLY light, AMAZINGLY strong wings.

==================

Couple of random comments here now.

This method was taught to me by a guy who, in his day, built wings for some awfully well known champions and pros. It's not some random made up way of doing things...it's a proven method, over several decades, on some of the highest profile airplanes in the hands of some of the most demanding competitors in the hobby.

While not necessary for wings the size you're dealing with, on larger (1/4 scale, 2m pattern planes, etc) wings, a layer of carbon fiber veil can be used in between sheets and core. In that case, epoxy the CF only, not the sheets. VERY strong sandwich.

Despite steps 8 and 9 above, you're going to use more epoxy than you need. That's fine..everyone who's ever tried this method did so the first few times. The key thing to understand is that there really only needs to be enough to contact wood and foam. Any more is simply wasted...it's dead weight. Truly excessive amounts can create bumps/ridges in the wing surface. As examples:

33% Pitts S2S with false ribs used 7.5 oz...both wings. 78" wingspan
25% Lanier Cap232 kit. We only MIXED 5oz total for both wings...and there was waste.
.40 sized Pitts S2S, fully sheeted wings. 4 oz was GOBS too much for both wings.

The most important "trick" is to make SURE the entire surface of the sheeting is covered...ESPECIALLY the edges. You get it all covered, they'll be strong enough to hit golf balls with (literally, I've seen this done).

=========

The process sounds more complicated than it is...just follow the steps, and it'll be simpler than you realize...and you'll end up with some very nice...and truly light....wings.

Ok,
Thanks guys, I am looking at the epoxy thing but it will be a bit heavy because the plane is a small, .15-.25 Sig Tristar and needs to be as light as possible. I was thinking of using something like the 3M 77 spray adhesive. This could help me keep the weight down and have a good bond. But I also wondered what should I use to plug the hole that the contact cement made? I was thinking of getting foam chunks and putting it in there with some hot glue or even regular glue and tape over it?

Thanks for the advise!
-TJ

GBOULTON,

GREAT step-by-step. Same method I use. It works GREAT.

Thanks for taking the time to type it all out for the newbies!! Good job man!! [sm=thumbs_up.gif]

Thanks for the compliment, Rcpilot.

You're right...it works fantastic, and time and time again folks are simply blown away by how light and strong a wing you can produce. I'll STILL put a quality sheeted foam wing, using the method above, against ANYONE'S built up wing...even with today's super-light ARFs. Add in the use of some quality Contest Grade sheeting, and I'll wager good money i can sheet a foam wing lighter than ANYTHING you can build.


Discount it as "a bit heavy" if you wish...I'd ask you though...

Upon what do you base your belief that the method above is incapable of keeping the weight where you need it to be?

Fill your Holes with light weight Spackle and go for it.

Gboulton-
You are right, that it would work very nicely and keeps the weight down, but for the task at hand, I am not building a contest grade plane, and it would be a lot simpler just to put some 3M 77 on it. I am a regular pilot, but I am starting to get into the kit building and this is my first one. Thanks for all your advise and Im sure I could use that technique on another plane in the future.

Live Wire-
My dad has something that works to fill the holes without adding too much weight I believe and hopefully it wont be too much structual damage to the wing itself.

Thanks for all your advise!
-TJ

Fair enough.

I used 3M 77(old can) for sheeting the wings on my Sig Kougar. A light mist on each surface then wait 45 seconds and be vewy, vewy careful when applying the sheeting to the foam cores. Once it's down...it's down. Now this is the old 77 version which is quite safe for foam. The new version seems to be a bit aggressive if over used.
BTW instead of gluing all those narrow 3" wide sheets together, I bought some 10" wide wing sheeting from BUSA. Saved me a lot of work!

Yeah, wider sheets takes some work out of it. It's nice if I have the cash at the time to purchase the wider sheets, but even 4" sheets aren't that bad to work with.

One thing I do is make sure EVERY long edge where the sheets will be glued together is sanded on a LOOOOOOOONG sanding box. If the edged of the sheets aren't true when you tape or glue them together, it makes warps in the finished product.

Pics:
1st sheets will glue together along the seams and lie down flat on your wing.

I exaggerated the curved lines on the edges of the 2nd pic. These sheets might glue/tape together somewhat flat on the bench. But they will warp and bubble up on the foam cores.

Edge true all the sheets, with a long metal straight edge, or a long sanding block (4 or 5 feet long) before assembly. It makes a better finished product.

Alright, Thanks for that tip. One wing is only like a foot and a half long so Ill use a sanding block accordingly. I have yet to get some of that expanding foam to fill the hole in the wing from the contact cement, once I do, Ill finnish up the wings and the plane will go along pretty quick.
Thanks!
-TJ

TJ

Status on your Tri Star build? I have an old one (crashed) and a NIB kit ready. Looking to follow RocketMan's build thread. Wil use a Medusa Research 1700 kv 28 mm motor (~400 watts).

Scott

you said this plane has foam wing cores which is goodas they are stronger than a wood wing.but I'm not clear what you meant when you prepped the sheets with contact cement.did you edge glue the sheets together? the best product is southern sorgum from dave brown products.just take a brush and brush the stuff on the foam and the the balsa sheet and carefefully lay the leading edge lined up on the edge and roll it down over the sheet.do it right cause even with 3m77 which I have also used in the past you get one chance .once the two glued sides touch its permanent.somone mentioned a sig kougar.I used the dave brown product for that and my foam core wing on my pattern ship.

go to dynamic balsa they have instructions to show you the correct methods.

gB,
Thanks for the step by step. I'm sure that it will be helpful to many. What you have described is pretty much what I have been doing and you're right. it takes a few wings for you to realize how little epoxy you really need. I also think that the use of hairspray on the cores keeps the amount of epoxy [weight] penetration to a minimum. The only thing that I do differently is that I still glue the sheets together using Ambroid glue [love the stuff] as it sands so well.
One question though......Do you have any experiemce with fenestrating [cutting holes/sections out] in the core before you sheets them ? I had a couple of pattern planes from RC City that were built this way and the wings were incredibly LIGHT.
Thanks in advance for any info,
paul

Glad this thread has survived, and glad the info will be useful.

I agree...Ambroid glue is good stuff. i WOULD submit, however, that it DOES have some weight...and there's no glue in the world that sands as well as no glue. Give the tape method a try some time...you'll find that enough epoxy wicks into the edge joints to hold just fine. When you're obsessive about weight, you'll find any excuse to lose even an ounce or two.

I've not personally done it, but I've watched it done (by the guy who taught me the method above) and seen the results. Again, it gets back to that obsession with weight thing.

The principle is pretty simple. What most of us don't realize is that the vertical structure (ribs in the case of built up wings, foam in the case of cores) of a wing is almost completely unnecessary.

Think about the forces on a wing...

Is there any 'crushing" force from top or bottom? Hopefully not! There's a lifting force from one side or the other, but the wing is free to move with that force (lift the airplane)...so there's no resistance.

What about bending forces? Well, let's remember...a rib in a built up wing is going to do NOTHING to stop a bending force. Pull up on a wingtip, and the rib is parallel to the bending line, right? It can't do a thing to prevent that bend, and indeed might even encourage a break right at its attachment point, depending on installation/engineering.

There's SOME merit to the idea that a "twisting" force is handled, at least in part, by wing ribs or the foam core...but considerably more is handled by the leading and trailing edges, and the spar. The critical element to this is the fact that they're all tied together, making a "frame". The ribs server this purpose in a built up wing.

In a sheeted wing, however, the SKIN serves exactly the same purpose! So long as the LE, TE, and spar(s) form a "wire frame" if you will, they'll create a box that's very difficult to twist or break. Nothing says the elements tying them together have to be inside the wing.

So...again...it's down to obsessively saving every ounce you can. Honeycombing (or as you said, fenestrating) the cores simply removes un-necessary weight. It's a pretty low "bang for your buck" process, since it takes considerable effort to save just a few grams, but there's certainly no reason not to do it. Give me enough foam to hold everything in place while the epoxy cures, and I'll built a strong wing.