08.14.08

On Fiberglass…

Posted in Fiberglass Work at 1:56 pm by Administrator

I’m still cleaning up all of the old fiberglass pieces. I have the main body, the T-top and the two gull-wing doors left to work on. After that, I’ll start repairing all of the damaged areas, make any modifications that I’d like and scratch-build some additional parts.

Today, I’d like to continue on with the introductory discussion about Moldless Composite Construction, according to the School of Burt (Rutan, that is…). Last time, we talked about adhesives, today we’ll look at fabrics and next time we’ll explore core materials.

Composite fabrics include fiberglass, carbon fiber and Aramid fibers such as Kevlar. For this project, I’ll be using fiberglass exclusively. There are literally thousands of types of fiberglass with different weave patterns, weights, yarn sizes, sizings (coatings that make it easier to wet out the glass), etc. There are also materials called prepregs — fabrics that are pre-impregnated with an adhesive and stored at cold temperatures to keep the adhesive from curing. We’ll be using three or four types of fiberglass in our work, including bidirectional fiberglass (BID), unidirectional fiberglass (UND), deck cloth and perhaps unidirectional fiberglass tape. You can learn more about the properties of these materials here:

http://www.aircraftspruce.com/menus/cm/fiberglass.html

Bidirectional fiberglass cloth (known as BID or RA7725, where RA stands for Rutan Aircraft) has half of the fibers woven parallel to the selvage edge of the cloth and the other half at right angles to the selvage, giving the cloth the same strength in both directions. The cloth is almost always cut “on the bias,” meaning at 45-degrees to the selvage edge. To understand why this is done, imagine a crack in a part that you want to reinforce with fiberglass. If you apply the fiberglass such that one set of fibers runs parallel to the crack and one set runs across the crack, then you have reduced the strength of the repair by 50 percent. The fibers running parallel to the crack aren’t reinforcing the damaged area — they’re just holding the cloth together. By applying the cloth on the bias — so that both sets of fibers cross the crack at a 45-degree angle — you’re allowing all of the fibers to develop strength across the crack. As we start our repair work on the Bradley, you’ll see me do this over and over again. BID is good in tension and is great for torsional (twisting) loads. It also takes compound curves very easily but, because of this, it distorts easily. As we go, I’ll show you some special handling techniques that will help you place the cloth without distorting it.

Unidirectional fiberglass (UND or RA 7715) has 95% of the glass fibers woven parallel to the selvage, giving it exceptional strength in that direction and very little at right angles to it. The remaining 5% of the fibers are there just to hold the 95% together in the form of a cloth. UND is great in tension and good in compression. It is sometimes used to accomodate torsional loads but, in these cases, it is usually applied at 30 degrees across the load surface. We probably won’t have need to use UND in this way but the folks who build aircraft use UND across the faces of their wings’ shear webs in just this manner. UND will not bend around a compound curve. You can bend it parallel to the fibers or perpendicular to them, but not both at the same time. UND tends to ravel severely when it is cut. We’ll show you some tricks to minimize this problem when we start the repair work.

Deck Cloth (Aircraft Spruce P/N 1080-50) is not used for aircraft construction very often but it has a useful purpose in our work. The weave of deck cloth is very fine. As such, it can be used in place of BID where subsequent filling and contouring will be required. For example, repairs to the inside of a bumper can be done with BID but repairs to the outside should have one or two layers (plies) of deck cloth outermost. This will make filling, contouring and finishing go a bit easier.

Unidirectional fiberglass tape (Aircraft Spruce P/N 01-06800) is used where very high tensional or compressional loads are expected. The aircraft folks use it to construct the upper and lower spar caps of their wing spars. I’ll probably use it to reinforce the insides of the bumpers in high stress areas such as where the bumper mounts are attached to the bumpers.

The one type of fiberglass that you won’t see me using is glass mat. Most auto body folks use a lot of glass mat and, if you’re used to working with it, by all means do so — mostly as a replacement for BID. However, I prefer to use BID because it is much easier to wet out and control. Hopefully, by the time we’ve repaired all of the fiberglass parts on the Bradley, I will have convinced you of that!

When making a fiberglass repair, start by roughing up the area of to be repaired with either 36- or 40-grit sandpaper. You want to have a really rough surface so that your repair materials will develop a good mechanical bond with the underlying structure. Next, paint a coat of pure epoxy on the area where the fiberglass is to be applied. Be a bit generous with the epoxy. Then lay on the first ply of fiberglass. If you use excess epoxy under the fiberglass, it will wick up thru the glass, driving out any excess air. If the layup is too dry below the fiberglass, you have to force the epoxy down thru the fibers in the cloth by stippling the layup with your brush. This can be done, but it tends to drive a bit more air into the layup, which makes it weaker. Use enough epoxy to fully wet out the cloth, leaving no unwet white areas, but don’t use so much epoxy that it puddles on top of the cloth. As Burt says, “Not wet, not white.”

The number of plies of fiberglass that you need to use is something that you will learn by experience. The designer will specify the required number of plies, if you’re working from plans. However, here is a rule of thumb that you can apply. If a crack goes all the way thru a structure, build up fiberglass to half the thickness of the original structure on both sides of the structure. For a crack that is 4 inches long, place the first ply so that it covers the crack and an area 1″ around the crack. The next ply should extend beyound the first for 1 inch in all directions. The third ply should extend beyond the second for 1 inch in all directions. Continue this procedure until the area over the crack is half as thick as the original material. Then, repeat the layup on the other side of the structure.

If a crack fully penetrates a structure, you must repair both sides of the structure. If you don’t, the repair will be weak at best and will fail at worst. If you repair just the underside of a crack on a car part, the unrepaired outer side of the crack will telegraph thru any gel coat, body filler and paint job in short order.

Always use a hair drier or heat gun to warm the epoxy as you apply it. The kind of heat gun used by model airplane bulders to heat-shrink Monokote over their balsa-wood airplane frames works really well. Mine is made by Top Flight. The use of a heat gun lowers the viscosity of the epoxy, allowing it to wet out the cloth much better. Be careful not to apply too much heat or your epoxy may exotherm. I keep mine about 3-4 inches from the work. Keep the heat gun moving at all times. This is especially important when performing a layup over a core material such as a foam, because excess heat my melt the foam core.

By the way, you don’t need to use a new brush for each layup. When you’re done for the day, use a paper towel to remove as much epoxy from the brush as possible. Then, roll the brush in a zip-lock bag, squeezing out as much of the air as is possible, and store it in the freezer compartment of your refrigerator. The next time you want to use it, give it about 10 minutes to warm up and it will be ready to go. By doing this, you can re-use the brush for dozens of layups. I use 1″ and 2″ chip brushes, bought from Harbor Freight in boxes of 36. That’s the least expensive source of brushes that I’ve found. They tend to shed a bit, but a couple of bristles in a layup isn’t a problem.

You can use standard shears to cut dry fiberglass, but I like to use a rotary cutter (pizza cutter) that quilt-makers favor, along with a nice 4-foot metal drywall straightedge. For cutting wet fiberglass, nothing beats a pair of Dritz rechargeable electric scissors. You can cut thru up to 6 plies at once and easily get right up to the edge of the layup. Clean the jaws with a bit of MEK and a paper towel after use (before the epoxy cures). To protect them from getting all gummed up with epoxy, cover everything except the jaws with a baggie and duct tape.

Enough for now. Soon, you’ll get to see all of this in action. Next time, we’ll look at core materials (foams).