How to apply fiberglass cloth

I got a request on how I apply the fiberglass cloth on my center sections of the wings. After repling, I thought that maybe a post was in order since other people may be wondering the same thing. Most of what I got, I got through Windy's tapes with a little of my own ideas thrown in. Here is a copy of my reply:

Here is my recommendation. The first thing to do is prepare the surface correctly. There should be no "step" between the two halves of the wing. The joint should be smooth with no gaps either. If you have some small gaps fill them with epoxy or tick CA. The idea is not to have any air pockets under the fiberglass. If there is air pockets then the strength in that area is not as good. There should be a smooth transition between the two halves of the wing. The next step is to cut out some .5 oz glass cloth so that it is about three inches wider on each side of the fuselage in the center of the wing and say about an inch on the leading and trailing edges. The dimensions do not have to be exact. The second piece of cloth should be heavier say 1 to 2 oz stuff. Cut this so that the widest part is about 1 to two inches wider at the center and about a half inch or less at the front and the back. If you want you can add a third piece, just make this one about as wide at the fuselage and about half the width at the center sections. This third piece should be at the 1 to 2 oz size cloth. Now mix up some slow drying epoxy. I will use either 2 hour or finishing epoxy for this. It really does not matter as long as it is slow drying. You want the time. Now take some and put in on the center of the wing and spread it around with a business card. You do not have to worry about scraping it all off leaving a very small amount, it is actually best to leave some on. You will get the extra off a little later. Now put the half oz cloth on in position. Lay it out best you can, smoothing it out from the center outwards. Push it down so it becomes soaked with the glue you put on earlier. If you get dry spots, put some glue on the business card and smear it in the weave again from center out. You want the weave to become saturated with the glue but do not want so much glue that it fills the weave. That is too much. Now take an old hair dryer, not you wife's new one because you are likely to get glue on it, and set it on high to warm up the glue. Warm it up to the point of it is fairly warm to the touch but not so warm as you cannot touch it. This will thin the glue and cause the fiberglass cloth to settle down closer to the wood. With so much glue before the glass cloth can actually be suspended a little ways away from the wood with adds glue and weight. You will be able to tell when the glass cloth settles down because puddles of glue will start to appear. Now take a paper towel and blot the excess glue from the plane. This should give you the minimum amount of glue necessary to saturate the cloth and stick it down. If you are careful enough you can put the second thicker cloth on the first in the same manner before the first has a chance to dry. Waiting is much easier however but takes longer because of the drying time between coats. I like to use the really thin glass cloth on the wing first because it is the easiest to feather into the wing since it is so thin. The thicker center section then becomes a lot easier to feather with a harder under surface. Always use a sanding block when feathering in the cloth. Otherwise you will get a dip right where the hard cloth and the soft balsa meet. Hope this helps. Any other questions just ask.

I would add a couple of things to this. Fiberglass cloth has a tendency to frey at the edges. If this happens let it go and let it sit lightly on the wing. After the glue dries, it is easily sanded off. Also, I do not like to wrap the cloth around the trailing edge of the wing too much. Because of the large bend in the cloth, it has a tendency to pull up and bubble. I like to let it just hang over the edge and let the glue harden. After it dries, it is easy to sand the edge and "cut" the cloth forming a nice clean edge. Wrapping around the leading edge is usually not too much of a problem because the bend is not nearly as great.

That should about do it.

just a copule of comments... The freying along the edges is not necessarily a bad thing. I like to actually remove some of the threads that run parallel to the ribs to make the edge easier to blend into the surface. And, it is easier to cut along the trailing edge if you do it when the epoxy has cured to a "green" state. That is: not sticky but not yet hard.

Another couple of useful tricks. 1) Put masking tape down around the periphery of where you want to end the fiberglass. Then fiberglass over it. Sand the edge until you cut into the masking tape, peel off the tape, and you have a nice edge that's almost flush. 2) S.O.P. for FAI Free Flight is to thin the epoxy with alcohol (shellac thinner) until it flows about like 2% milk. Lay down the fiberglass, tape into place, and brush on the epoxy until saturated. Use a roll of TP, rolling across the surface and blotting up the excess. Remove soaked layers of TP and chuck. Result is no dry spots, no shiny spots, no heat gun, easy to do. The alcohol evaporation cools the epoxy, so there may be more working time, but it doesn't seem to delay cure time at all. Mix your epoxy and put into a shallow dish. Keeps it from kicking too fast due to self-generated heat. Steve

FWIW

My only comment relative to Matt's informative opener is that I think the amount of glass involved is a huge overkill. It's not a big deal but everything weighs something and, if you don't need it, why bother.

The only thing the glass is required for is to make sure the joint of the two wing halves is at least as strong as it would be if the sheeting went tip to tip right through the center section. It's only because we make foam wings in left and right halves that the subject even comes up.

Once that joint is as strong as the continuous wood sheeting would be it is only necessary to insure that the rest of the wing has uniform rigidity and is free from any stress risers ... most importantly, at or outside the fuse sides.

IMHO, a one inch strip of light cloth centered on the wing joint is plenty to insure the required strength of the joint. Follow the glass clothing with a tip to tip covering with either silkspan or, better in my opinion, 0.2 oz carbon matte. This covering provides the required "uniform rigidity" to preclude any stress risers and is tons lighter than the multiple applications of heavy glass and cloth. Especially when you consider at least a single layer of paper will usually be applied over the entire wing anyhow.

I think it is particularly important not to extend the very rigid glass cloth beyond the inside of the fuse. In fact, the edge of the cloth should be well inside the sides to avoid a compound stress riser at that point. The exposed edge of the cloth will produce an instant stress riser whereat the flexure rate is zero up to that edge and "whatever" degree of flex beyond that point. Under load the flex that would otherwise be uniform across the span will be concentrated at the riser and the opportunity for failure will actually be increased.

The ultimate choice to be avoided is to end the glass cloth right at the fuse side. The resulting stress riser coupled with the "cleaver" action of the fuse at that point will multiply the opportunity for failure.

My suggestions are particularly appropriate for a thick wing which has a lot of natural resistance to spanwise flex. If you are an advocate of thinner airfoils you might want to consider two or three layers of paper and/or CF matte with one layer going tip to tip, a second layer two thirds of the way out each wing and the third a third of the way out ... but again, beyond the fuse for sure.

Still overkill in my opinion unless you're talking a flat sheet balsa wing, but much more desireable from a flex standpoint.

I'm certainly no lightweight builder, though my newest effort is a bit better. I've never used any sort of glass cloth, carbon fiber or really any sort of re-enforcement at the center of the wing. I've just assumed that the spar doublers, LE and TE re-enforcement and such internal to the wing structure are plenty. Only “flexi-flyer” I’ve built was a foam wing job without much in the way of ANY re-enforcement. Just never saw the point, I guess.

Had an RC airplane that I forgot to glass. All was fine until I put a Honkin ST 45 on it up from an OS40fp. Made a sharp turn and half my wing didn't come along with me. I was going so fast and pointed at just the right angle so that the plane went from horizon to horizon before sliding in on it's side. 30 seconds later the other wing half fluttered to the ground. My buddies said, 'If only it would have caught fire, THEN it would be cool'.

Inspection showed it snapped right at the joint, also showed I forgot the glass. And the buddy who thought it would be cool if it caught fire, and had the same plane and same ST AND remembered to glass his which flew flawlessly until he didn't go OVER the trees, well, I still talk to him, now.

>FWIW
>
>My only comment relative to Matt's informative opener is
>that I think the amount of glass involved is a huge
>overkill. It's not a big deal but everything weighs
>something and, if you don't need it, why bother.
>
>The only thing the glass is required for is to make sure the
>joint of the two wing halves is at least as strong as it
>would be if the sheeting went tip to tip right through the
>center section. It's only because we make foam wings in
>left and right halves that the subject even comes up.
>
>Once that joint is as strong as the continuous wood sheeting
>would be it is only necessary to insure that the rest of the
>wing has uniform rigidity and is free from any stress risers
>... most importantly, at or outside the fuse sides.
>
>IMHO, a one inch strip of light cloth centered on the wing
>joint is plenty to insure the required strength of the joint. Follow the glass clothing with a tip to tip covering with either silkspan or, better in my opinion, 0.2 oz carbon matte. (snip)
>
>Ted

I should probably post the picture of Matt's plane here. I think this was in reference to His Rabe Mustang that I posted the picture to a while back (see one of the posts that Al started with his Mustang). What is being addressed here are two different problems but with overlapping themes. Matt's plane used a built up wing that was sheet covered. Al builds his in two separate sections (or even three) and then joins them together. As I said, it was a built-up wing. But some of the problems being addressed are shared with a foam wing as well.

First off, if you are merely joining a foam wing, then the one inch strip in the center is enough to insure the required strength of the joint at the center section. If the foam wing has a plywood spar and other structure that may be adequate. But...we have a foam Das Ugly Stunter that had its wing break three times in the air. And the place where it broke was right at the fuselage. It never sheared, but cracked. It was repaired and cracked again.

And once, in a crash (line snagged on take off and plane did a loop into the ground) the fuselage broke apart and the wing then broke inside the fuselage along that one inch fiberglass joining strip. Oops, a stress riser inside the fuselage.

This is why I would now recommend using a three layer approach that Matt was mentioning. Instead of a narrow strip of two ounce cloth inside the fuselage, we would use a wider strip of half ounce cloth extending well outside the fuselage. However, as Ted suggests, do not cut it square. A single or several football shaped pieces accomplish the job without placing a square shear point across the wing. Then a second piece can be added over the first, but narrower and still extending outside the fuselage. To this a third is added that just makes it outside the fuselage. Oh, we have added tremendous strength and no shear points inside the fuselage or at the wing to fuselage joint. And you have three layers of half ounce cloth holding the center section together.

Since Matt had the wing fold on his white Stuka at an inopportune time (is there ever an opportune time?) he has added this cloth routine to the center section of his succeeding planes--even those with built up wings. By placing these football shaped pieces over the center section of the wing prior to gluing it into the fuselage, you remove the stress riser from the fuselage joint. And by using the oval shaped patch, you do not have a sharp, square edge anywhere for this to end and to cause its own problem.

Perhaps it is overkill, but so are his leadout wires. Although you want to go for as light a weight as you can, there are some areas you simply cannot afford to allow to let go. Leadout wires and wings are pretty much at the top of the list. Well, they are at the top of Matt's list at least. He has lost a plane in competition to a failure of each.

Although there are many different ways to travel, this is Matt's present method of insuring that he does not suffer the loss of another plane due to wing failure at the wing to fuselage joint. It is very effective and it doesn't add very much weight at all. Very little indeed.

A few pointers for glass work as applied to constructing the real thing; a full size Lancair 360.

situation

Reinforcing a joint at a right angle such as model aircraft fuselage to wing or rib to lower wing skin in the real thing or bulkhead to fuselage side or floor.

We used 2 inch wide bid tapes, two ply cut on a bias. That is the weave is cross hatched at a 45 degree across the 2 inch wide tape. This allows all glass fibers to cross the joint providing ultimate strength. To do otherwise limits ultimate strength of the tape and the parallel fibers are adding weight, taking up space and do not add to the joint strength. This also provides torsional strength as well.

Since we used these bid tapes often; we usually cut two 14 inch strips of glass cloth on the bias, put down a 3 mil plastic sheet underneath, layed the two ply on top, mixed up a batch of epoxy, poured it over the glass, used a squeegee to spread out the epoxy and penetrate the fibers as per Leonard's post. We also used the one inch throw away bristle brush, cut off over half the bristles so the brush is stiff and use it to stipple the glass cloth and soak up the epoxy. After the cloth is sufficiently wet ie no white spots, place another 3 mil plastic sheet on top of the whole mess. Smooth it out and use a 4 inch wide roller. Working from the center and radiating outward to squeeze out the excess glue. Leonard's idea of the heat gun may be appropriate here as well if the room temp is below say 70 degrees. The objective is to have a 50/50 weight ratio of glue to glass. Once you are satisfied, take a roller cutter used by the ladies for cutting cloth for sewing, a straight edge and cut two inch strips. These strips can then be applied to a wetted joint after removing one of the vinyl coverings. This eliminates a significant mess and keeps the cloth fibers straight which is necessary for stress forces to be properly transferred from one part to the other. Remove all air bubbles and smoothly apply the bids to the joint with your hands and any other method that works. Them remove the last vinyl covering. Now use the stipple brush and stipple out the air bubbles per Leonard's recommendation. After you are satisfied that all surfaces are wetted and sufficiently bonded take paper towels and blot out the excess glue. The ideal is to see the fiberglass weave and no shiny glue on top but there is glue between the fibers of the upper ply. The glass should look transparent, not white but otherwise dry of epoxy. This provides the lightest and strongest joint. Glue above the top fiber layers is excess weight and brittle subject to cracking and causes failure to rest of the joint. There are other lighter fillers you apply later for that smooth finish. Oh yes, Do not sand into the glass fibers to achieve that smooth finish. You are destroying the strength of the cloth you already applied; thus losing the benefit and carrying useless dead weight.

We used two inch bids in many locations with one inch on each side of the joint.

Other joints should have built up bid joints with staggered plies as stated by Ted.

In this case you would lay down a 2 inch, a 2.5 inch, 3 inch or a one inch increment such as 2,4, 6 inch bids. You can see that this approach will reduce and distribute stresses in a gradual manner.

Another concept.

Glass and epoxy is good in shear but poor in tension. That is if you glue glass or two pieces of wood together in an overlap fashion and pull on each piece from each end such that the glue joint is parallel to the force applied; Fiberglass is very strong when bonded to a surface. If however, you pull perpendicular to a glue joint; the bond will fail miserably. Ie lifting the fiberglass away from the bonded surface.

When you apply glass to a right angle corner you actually create a bonded joint however, at the corner the joint is weakest for the above reason. It is therefore necesary to distribute the forces in a gentle curve around the joint by applying a fillet to make the turn. The larger the better for load distribution but micro balloons and epoxy is weak therefore you should use a small radius fillet so that a smooth turn of the glass is made and most of the surface is bonded to the parts, not the micro. Large fillets then are heavier, and weaker than small filets.

How is this applied to models? Apply fillets first before glass. I suppose you could use two ply glass with stepped plys, no more than two for our purpose, small fillets, apply glass such that all fibers cross the joint on a 45 degree bias, apply or heat the epoxy and blot out the excess but still have wetted glass, (ie no white spots).

Ater the glass has cured, inspect for air bubbles. If any, you have a weak joint at that point. You make the determination in model world if you should do anything about it. In the real world, you solve the problem. The fix? Drill two small holes on each end of the bubble. Inject heated epoxy with a srynge in one hole and fill it up until it flows out the other hole. This adds weight but strength integrity is maintained. This works well for small voids but larger voids require cutting out the affected part and doing it over.

The vinyl sandwich method may be appropriate in model building where say you must directly apply glass in a difficult location. You can remove one plastic side, roll the piece up,insert it in a tight location, using tools unroll the piece and affix it to the location, stipple it in place with tools, then remove the plastic sheet and finish the work. The best thing about this method is that it keeps the glass integrity while working it in place.

Bob Smiley
A Lancair 360 builder/pilot trying to be a modeler