joining wing center section
 

A friend gave me a kit that had been started about 15 years ago. It is a trainer. This looks like a plane to try a couple of new techniques on. To make a better looking center section joining the wing panels with three layers of thin fiber glass, each cut wider than the one below it, may avoid the ridge that occurs using the typical 3" tape. Would laminating epoxy work in this application or should a slow say 45 or 60 min structural epoxy be used?

Jerry

RE: joining wing center section
 

If I have the ridge after sanding I use a little spackling compound to feather it out.

RE: joining wing center section
 

You want the widest layer first, not the narrower one. If you cover the layup with Saran Wrap (a type of kitchen plastic wrap) and smooth it out with your hand then let the epoxy cure overnight; when you remove the wrap you will have a perfectly smooth finish that will not require any sanding to smooth out any ridges.

RE: joining wing center section
 

Adding to Rodney's post... if you have any foam rubber (for RC gear) put that on the plastic wrap and weight it down with some magazines for a really smooth finish. Then do the other side.

RE: joining wing center section
 

Thin your epoxy a bit with some denatured alcohol. This will allow it to fill the weave of the glass better.

RE: joining wing center section
 

Doesn't the epoxy stick to the plastic wrap?

RE: joining wing center section
 

widest layer first. Not intuitive but this may be the strongest since the wide piece is in contact with the balsa. Now lets see. If the bottom of the wing is done first the top can be wraped around the leading edge hiding any irregularity since this is a high wing trainer. I will attempt to post an image of what we are working with. It is bad weather here in NC so the next couple of days are building days.

thanks for the ideas

Jerry

RE: joining wing center section
 

tryed to upload a picture of wing center showing problems to fix.

RE: joining wing center section
 

No need for the fiberglass just spackle like Gorisich mentioned above

RE: joining wing center section
 

Note the wedge shaped spacer between the two panels in the previous post. The first effort to join wing panels did not result in a straight wing. So sawed the wing into up to the main spar from leading edge and trailing edge. Re epoxyed with spacers to straighten and align. It is now straight but may not be as strong as it should be.

The next problem to work out is how to put hatch over the fuel tank. The original plan called for a simple straight piece of plywood screwed to the firewall and held at the back with a tongue under the plywood wind shield. With this tank set up more space is needed above the tank to accommodate the tubing. suggestions are welcome.

RE: joining wing center section
 

I would just build up the sides and firewall to get the hatch cover over the tank I guess.

RE: joining wing center section
 

Didn't Gorisich mean to feather with spackling to get rid of ridges after smoothing the epoxy with sand paper? The fiber glass is to give the center section added strength and with the errors so far this wing needs all the help it can get.

Jerry

RE: joining wing center section
 

That is what I meant. Don't omit the fiberglass.

RE: joining wing center section
 

I'd use waxed paper for effortless 'release'!

RE: joining wing center section
 

Curious...on a trainer why do you want to use 3 layers of glass?? These are lightly loaded and powered. One layer of 2-3 oz will be plenty even for a 60 size. Finishing epoxy works great for this.

If you do decide to layer it, it doesn't make a hill o' beans difference which width you lay down first. Final strength will be the same. So I suggest you go for the one that's easiest to finish

I'd use 2 layers tho. If you want to get a little more out of joint (I don't know why you would, but anyway...), lay down one of the layers on bias and the other orthogonally. That is, cut one layer of cloth at a 45 degree angle and the other at right angle. You get max torsional strength this way and you maximize tear strength in the center joint.

Again, for a trainer don't bother with any of that tho. How much strength do you think you really need for a 5-8 lb airplane with maybe 700-800 sq in wing

AS ALWAYS, YMMV...

RE: joining wing center section
 

MTK, thanks for your observation. I had wondered if anyone would hit on that. The purpose of this airplane is to work out the techniques that will be used on my next build which will be a much larger model with a 110 fourstroke. The other effort with this model is to work out the details in installing a three line fuel system. It is good to check out your methods before starting a big bucks airplane. Two years ago I spent over three weeks sanding all of the finish off of a model because the finishing epoxy never hardened, even after a month in a hot attic at over 130 degrees.

Just getting gun shy in my old age.

Jerry

RE: joining wing center section
 

You can get 36" rolls of that stuff from your local Hobby shop any weight and wide as you need.

RE: joining wing center section
 

Bad luck with todays effort. This is the reason to try this out on a not so imprortant model.
First problem lots of creases and wrinkles that would not go away no matter how much I stsreatched and pulled and brushed. I do not remembr this being a problem the first time I used this material. would it do any good to iron this stuff.

second problem. after putting the plastic wrap and foam and weights on the covered section the whole slimey mess slid toward the trailing edge of the wing abut 3 inches befor i saw what was happening. So pulled it up and tried to reapply it. wrinkles got worse.

the game plan now is to order new fiber glass and try again.

Jerry

RE: joining wing center section
 

Sounds like my first glassing attempts! Don't worry, you will get there. Very smart to add that reinforcement as the highest breaking forces (moments) are on that center section. I've seen models that just use the glass tape to hold the wing together but I like plywood dehderal braces on the spars plus the glass tape. Seeing a wing fold in flight is not a fun thing.

RE: joining wing center section
 

DIDyou REPLACE the spars you cut across the center section?? If not you may be in for trouble. Thoses main spars through the center section is what keeps the wing from folding up in flight. Without them and just putting glass in the middle may not do the trick. The glass is only attached to thin balsa sheeting. Just like 2 piece wings , those spars are replaced with carbon fiber tubes through the center. Just a thought

RE: joining wing center section
 

I can't recall the last time I glassed the center section?? The center section isn't the weak link on the wing. When most wings fold it is outboard about center of the wing. Plane I'm building right now is just two foam halves that glue together. I'm thinking in a case like this I will go ahead and do it? Then again I just may put in some wood to act as a spar to brace them? Glass isn't really a very strong support.

RE: joining wing center section
 

The cut was from the leading edge to the spar and from the trailing edge to the spar. The spar didn't even get a scratch. A wedge was cut to bring the two halves to proper alignment and epoxied in place.

The second attempt at fiber glassing, doing the top of the wing, came out much better. The first material that I used may not have been fiberglass. The lesson here is don't stay out of the shop for 6 months and forget what is where. If the camera does ok there will be some interesting pictures to post tomorrow. But then the weather man is promising 70 degrees and low winds for tomorrow afternoon.

Thanks for the help. It looks as though there is something to learn about how center sections are stressed.

Jerry

RE: joining wing center section
 

we've always used a heat gun when glassing the center of the wing. glass it and then apply some heat to get the epoxy to flow/smooth out nicely. no need for saranwrap etc.

It will cure faster if you heat it up, so keep that in mind. Have been doing it for 25 years myself and learned from my father... never had a wing fail here so I think it works pretty well....

RE: joining wing center section
 

The heat gun trick Matt mentioned is how I do it too when I bother with glassing the center section.I have also used CA glue but find it requires too much glue and doesn't fill any of the weave.

RE: joining wing center section
 

I do both, heat gun and then wrap with Saran wrap as that lets you feather out the edges much neater as you can use your fingers to smooth out any edges under the wrap and the result is much slicker (smoother) than if you do not use the Saran Wrap.