Stabilizing thin wing sheeting - SSE
 

I have a question for the gurus. I have already sheeted the wings on my Somethin Extra, and I realize that the sheeting is VERY fragile. I am afraid that I will grab ahold of the wing one day and crack the sheeting. If I had to do it over again, I would just add a small strip of balsa between the first and second rib to stiffen it up, however it is too late for that. I am really only concerned with the root bay (first bay) as that is where it will most likely be handeled.

I have 2 ideas, hear me out

One is to cover the first wing bay ONLY with very light fiberglass (leaving the outer bays like Sig intended). I have used fiberglass before to do bodywork on cars, so I have an idea of what is involved. I would be able to blend the extra thickness out over the rest of the wing with filler or resin.

Second is to drill a small hole on the root rib and squirt some Great Stuff or other expanding foam in to the wing therefore filling the void. It would end up similar to a foam core wing for ONLY the first bay.

My main concern is to get the Monokote/Ultrakote to stick. Will it stick to the fiberglass if I scuff the resin? I plan to use regualar polyester resin for easier sanding and probably better flow.

My second concern is adding too much weight. Does anyone know how heavy Great Stuff is per volume? I know the Fiberglass will add a little weight, but if I apply the resin using a squeegee, that will minimize excess resin.

RE: Stabilizing thin wing sheeting - SSE
 

Plastic films do not stick very well to epoxy or polyester resin. A coat of Balsarite over the resin really helps the fim to stick. Even with a glass covering if the sheet underneath is fragile it will still fracture under pressure. Cut some slots in the sheeting and fit some sub spars between the ribs, sand flush and then glass.

Be very careful with expanding foam. It is more likely that as the foam expands it will blow the wing apart. The area where the foam is to be placed has to be clamped to prevent this. I would advise against it.

Either way however the amount of weight that will be added will be negligible.

Ed S

RE: Stabilizing thin wing sheeting - SSE
 

I try to be careful when I handle my wings but I see what you're saying. I feel that fragile every time I pick them up that way.

RE: Stabilizing thin wing sheeting - SSE
 

Good point, cutting slots to install the subspars would be easy and there would be no noticeable strength hit.

I have a question about the Balsarite. I know some people swear that it causes problems. However, when you rean the product discription, it seems like it does a lot of good things including fuel proofing and strengthening the wood itself. Is this a good product to use over most of the model to promote better adhesion, or should I only use it when absolutley neccesary (like over fiberglass). I can see where it would make it harder to disperse bubbles, but I have applied decals and paint protectand films to cars before and I wonder if all their problems are due to trapping air by sealing the perimeter of the film first rather than starting near the middle of the sheet.

RE: Stabilizing thin wing sheeting - SSE
 

Glassing a sheeted wing is a very common finish. Usually, the entire plane is glassed. The glass that is used for a plane is very light, weighing .5 to .75 oz per square yard, an only one layer of glass is used. The important thing is to use as little resin as possible. Resin is added to the center of the area you are working on, then squeegeed with a business card size piece of plastic till the entire glass area is whetted. Then, toilet paper is used to remove more resin to the point where fresh paper will not pick up any more resin. When cured, the surface will have the texture of the weave. This is covered with light coats of primer and sanded down between coats till just the weave is filled. Any deeper depressions can be filled with a lightweight filler. You don't need to worry about feathering the edges where fiber overlaps because the glass is so thin, the primer is enough for feathering. The plane is then painted. The result will not be a bullet proof surface, but it will help prevent your fingers from poking holes. Do a search and you should be able to find detailed instructions for glassing your plane. The main reason sheeted planes are glassed is not for strength, but for producing a smooth, even surface for painting.

Scott

RE: Stabilizing thin wing sheeting - SSE
 

If you're poking hole in the wing sheeting, it's usually not a too-thin sheeting, but the fact that fingertips are pressing into the wood. As you pick up the model, the model's weight is being concentrated onto small areas, your fingertips, which puts a very large amount of localized pressure and will 'pop' the sheeting.

Even 1/16" sheeting on a wing is plenty thick. Some wings use 3/32" sheeting. The key is to not use fingertip pressure when picking up the model. Use the flat of the hand. Yes, you'll need two hands to pick it up.

Better yet, pick it up just under the fuselage below the leading edge. On a low-wing model, your thumb can stick up at the leading edge while your palm and fingers are splayed across the bottom in the center. On a high-wing model, the landing gear is a good place to stabilize the model. Reach under the model with the flat of the hand, use a thumb to stabilize it and then lift. Don't allow any part of the wing to be squeezed between the fingers and thumb. The other hand should be on a part of the fuselage to help stabilize the model. You don't need a viselike-grip to hold onto your model.

I learned this technique long ago because many of my models were covered in Silkspan and tissue, and you's easily poke through if you weren't careful.

RE: Stabilizing thin wing sheeting - SSE
 

Plastic film covering usually sticks to bare balsa with no problems. I use balsarite only on glassed areas. Do not get the iron too hot it will bubble the resin.

Ed S

RE: Stabilizing thin wing sheeting - SSE
 

why not add a layer of sheet balsa, a couple bays out on top of the existing skin. fair it in and no known will know the difference, except you. we won't tell. dick

RE: Stabilizing thin wing sheeting - SSE
 

Ignore..

RE: Stabilizing thin wing sheeting - SSE
 

By the time you do a good fiberglass finish and/or add foam between the ribs you could've simply removed the sheeting from the bottom of the wing, reinforced the upper sheeting and fitted a new re-enforced sheeting back to the bottom. At least then you'd have a surface to finish with no issues of smoothness or whether or not the covering would stick.

RE: Stabilizing thin wing sheeting - SSE
 

Many good suggestions.

As for bubbles, they're not caused by sealing the perimeter first, they're caused by the adhesive giving off gas as you heat it to shrink or adhere it. Going over wood doesn''t seem to be as much of an issue, but covering over covering, or covering over fiberglass will result in bubbles. [:@]
Dennis-

RE: Stabilizing thin wing sheeting - SSE
 

I have had the same problem. I have used ultra lite drywall spackle and fiberglass drywall tape. Take the tape and place it down over the weak sheeting, it is self adhesive, then spread the spackle on and build up the weak area, let it dry and sand it down. It will give considerable extra strength. I also use the technique to build up areas that I have over sanded when I shapped them.

The Fiberglass tape gives the spackle something to stick to,and when the spackle drys on it, it will be more solid with the tape.

Give it a try.

RE: Stabilizing thin wing sheeting - SSE
 

Don't even worry about it, 1/16 sheeting is thin but is still plenty strong enough. Whether your wing has any sheeting on it or not the vast majority of the stress is on the main spar. Look at 4* wing it has no leading edge sheeting at all. Reinforcing the sheeting (unless its already damaged) will just add extra weight. Your wing will also become stronger after it has been covered.
I always caried my SSE wing halves around by sticking a finger in the wing jointer tube and a thumb on the outside, and the assembled plane buy the landing gear never had any problems

RE: Stabilizing thin wing sheeting - SSE
 

I agree with last post.
You are doing a composite structure that will be very strong once finished. This is very strong airplane and very good engineering have been done by SIG.