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Working With Ready Made Fiberglass Parts, Part 1
By Tony Bingelis (originally published in EAA Sport Aviation, November 1991)
THOSE ready-made fiberglass components that come with most aircraft kits really look good. For the most part, they are beautifully shaped and have flawless silky smooth surfaces that you couldn't duplicate without hours and hours of tedious work.
In more complete kits the fiberglass components generally include cowling sections, wheel pants, wing tips, tail fairings and sometimes a fiberglass spinner complete with metal bulkheads.
These fiberglass parts were, undoubtedly, laminated with polyester resin and fiberglass cloth layers inside female molds.
Aircraft fiberglass fairings, cowlings and other components, as received, will typically have white shiny surfaces which may be attributed to the gel coat (a special colored resin which is sprayed inside the waxed female mold). After that, several layers of fiberglass saturated with polyester resin are laid in and allowed to harden. The completed part, when removed from the mold, will be white and shiny and virtually free of pin holes and minor blemishes, thanks in part to the gel coat.
Although a white gel coat treatment results in an extra nice finish, it does carry a slight weight penalty when compared to a bare fiberglass layup. However, in compensation for that, you have a smooth surface that is ready for painting after a light de-glossing with #400 wet/dry sandpaper . . . provided, of course, you don't have to modify it.
Modifying Gel Coated Components
Don't be surprised to find that one or more of your factory made fiberglass components doesn't fit your structure perfectly. This is true of some empennage and wing tip fairings.
How could that be? After all, you followed the plans carefully. Even so, slight variations do crop up from one builder to another.
For example, a slight difference in the incidence angle or in the alignment of the fin could affect the fit of that particular fiberglass fairing. Then, too, if the part was not stored properly it may have become warped.
At any rate, for a perfect fit you may have to add a bit of fiberglass here and grind some away there.
Sometimes it may even be necessary to split (cut) the leading edge of the fairing several inches to improve the fit. Of course, the resulting gap will then have to be filled with a scrap of wood, a piece of urethane foam, or a lightweight automotive body filler ("bondo") and glassed over.
More often than not, though, the fiberglass parts you receive will fit quite well with a bit of coaxing, careful trimming, and grinding away a bit of fiberglass here and there.
Ordinarily, most of the modifications you make to your fiberglass fairings will, more than likely, be to suit your particular requirements. For example, you might want to add faired-in wing tip lights, or perhaps, streamline the wheel pants into the lower landing gear legs.
As modest as these modifications may be, they will, nevertheless, require a considerable amount of work and whatever sculpting skill you can muster.
Fiberglass modifications are best made with a fiberglass layup over some sort of shaped form that is carved or molded in place.
1. For example, pieces of rigid foam can be lightly spot glued in place with a 5 minute epoxy, and carved to the desired shape for the subsequent fiberglass layup, or . . .
2. As do many builders, use modeling clay because it is easily shaped.
As for completing a fiberglass layup over a mold or form, here again you have two ways to go about it:
1. The underlying form (usually carved foam) over which the fiberglass layups are made is allowed to remain in place and becomes a permanent part of the component or aircraft structure, or. . .
2. The fiberglass shell is pried away from the heavily waxed mold and the mold is removed and discarded. It is often destroyed in the attempt to remove it, anyway.
The fiberglass shell (fairing) is then re-attached permanently with an adhesive, rivets, or screws. This makes a much lighter installation than other options where the mold becomes an integral part of the airplane. Fiberglass over the seams.
Here is something to keep in mind. Gel coat is a special resin (usually colored) that hardens when cured into a non-porous, shiny substance. Consequently it is a poor surface to which to add additional fiberglass layers because they may not adhere very well to it.
I, therefore, always remove the gel coat from the areas where further gluing or fiberglass work has to be done.
Removing the gel coat from a particular area is very difficult to do because that smooth shiny surface is so hard.
The quickest way to remove the gel coat from a particular area, I find, is by abrading it away with an electric drill fitted with a coarse sanding disc. Consider yourself warned, though! It is a very risky way to sand away gel coat because the sanding disc can gouge the surface where and when you least expect it.
That method is not for the timid builder. Still, it is the fastest way I know . . . and, boy, does it make that dust fly. Of course, I use a dust mask. A dust mask helps keep my lungs clear of that white stuff and airborne fiberglass residue . . . but not my hands and clothing.
Another good tool to use for working gel coat and fiberglass is a large new coarse half round file. It will cut fairly well once the surface glaze is broken.
But next to the disc sander in effectiveness nothing is equal to a skillfully manipulated handheld hacksaw blade.
Hold the blade vertically so you can scrape its teeth (32 per inch is fine) across the glassed surface. This is also a quick way to remove lumps in fiberglass surfaces.
Once you develop the right technique you can even easily scrape away the gel coat from areas having compound curvatures. Sometimes nothing else will work as well. Anyway, use whatever abrading tools work best for you . . . don't overlook the use of sanding blocks and dowels fitted with coarse sandpaper.
Don't skimp or omit these surface preparations or you may fail to obtain a reliable bond for your fiberglass layups.
At the very least, you should scuff-sand the gel coat surface, where needed, with coarse sandpaper before doing any kind of layup.
Installing Wing Tip Fairings and Lights
There is no valid reason to make your fiberglass wing tips removable unless you perhaps have to mount individual strobe light power packs inside. Internally located tip antennas may also influence you to make the tips removable.
To do so will cost you. It will cost you a lot of extra work and the price of a large handful of anchor nuts (about $18 per 100).
If you will be installing only an external nav/strobe light, why make the tips removable. Simply allow about 12 inches slack in the wires and fit them with quick disconnect terminals.
Pop riveting the fiberglass tips on often yields a much nicer job. If a wing tip ever has to be removed, I dare say, drilling out the pop rivets will be just as easy as trying to remove a lot of screws.
How you mount your navigation and/ or strobe lights on the wing tips will depend on their shape. Naturally, wing tip lights will provide the most effective light pattern when they are mounted to project beyond the wing tips.
Unfortunately, a lot of builders think the submerged type tip mounted lights look better even though the lights can only project a severely limited light pattern (see Figure 1).
If your navigation lights include strobes, the units should be externally wing tip mounted on the wing tips to provide the greatest azimuthal coverage possible.
A well equipped aircraft will have three strobe lights, so positioned that at least one of the strobe flashes will be visible from any direction.
An externally mounted nav/strobe light unit is usually mounted on a built-up fairing molded into the wing tip.
The usual procedure is to glue and carve a piece of foam or balsa wood, into a streamlined shape to which the light unit can be fastened. The built-up base is then overlaid with a layer or two of light weight fiberglass cloth and sanded smooth.
Installing Empennage Tip Fairings
Here's how you can do it:
1. Install the fairings and don't worry about the lap joint being visible. This method is most used for installing tip fairings on metal aircraft.
2. The fiberglass tips are usually pop riveted in place on the tail surfaces of metal aircraft, and glued to wood and composite surfaces.
3. There is no reason for making the vertical stabilizer tip removable unless you intend to install a navigation antenna there and will need access to the wiring.
4. You can install your empennage tip fairings so that no joint is visible. This is the common practice in composite aircraft and in plywood covered wood aircraft.
Sometimes, builders of metal aircraft also try to make the joint between the fiberglass fairing and the metal structure invisible. They fill the gap (seam) with Bondo or micro balloons and proudly display their work to visitors . . . indeed, the joint is no longer visible. However, these poor souls don't know it, but such joints will crack and become very obvious (make that read ugly) someday in the not too distant future.
About the only way you can guarantee that a joint will not crack and become visible is by overlaying the seam with a narrow strip or two of fiberglass.
Always roughen the metal surfaces with coarse sandpaper before applying any fiberglass treatment. When the resin cures, feather the edges and you will be pleased with the result.
Fitting Wheel Pants
Preparing a new set of wheel pants for installation can be a pretty tricky operation. If you goof, you will have to do a lot more fiberglass work than you intended.
Here are a few suggestions:
- When you drill the axle attachment hole be sure it is located low enough that the tire will not rub against the inside top of the wheel pants, and that the tire is centered in the cut out opening for the wheel.
- Before you start drilling the wheel pants support bracket attachment holes, be sure to set both wheel pants at the same angle. Do this by slipping identical wood blocks under the aft end of each of the wheel pants.
- Don't be unduly concerned with trying to get the wheel pants to be perfectly streamlined with the line of flight. Whether your RV-6A tri-gear axles have the specified toe-in (or your taildragger has a bit of toe-out) the longitudinal axis of your wheel pants will naturally be perpendicular to the axle axis unless you force them to do otherwise. Anyway, this natural alignment with the axle is no problem as the design of the wheel pants makes it difficult to detect.
Before you can do any work on the wheel pants you will have to slip them on over the wheels, right? Well, that means you will have to cut away enough of the inboard side of the wheel pants that they can clear the brake housing, if necessary, and/or the gear legs.
Incidentally, some kit manufacturers thoughtfully emboss location dents and trim lines in their fiberglass components like the cowlings and wheel pants, for example.
Such embossed location markings are very helpful but double check to see that they are correct for your airplane. Then, too, they may not be visible on your fiberglass parts anyway, so just go ahead and make your own measurements.
Just how much to cut, and where, is difficult to determine so start with a small cutout and gradually enlarge it.
The opening should be large enough to allow you to slip the wheel pants on easily.
A saber saw is ideally suited for making such cut-outs. Tin snips, too, will cut fiberglass easily enough but will tend to crush the fiberglass along the cut edges. Always leave a little margin for a second trimming and for filing the edges smooth.
Next, determine where to drill the axle attachment hole in the outboard side of the wheel pants. Look the wheel pants over closely. There should be a small depression marking the location for the drilled hole. Unfortunately, sometimes this mark is non-existent and the builder is on his own.
If you have to determine where to drill the axle attachment hole, measure your wheel radius from the axle to the tire surface and add 1/2" to this dimension to allow for tire clearance with the top inside of the wheel pants.
(Note: The diameter of a 500 x 5 wheel and tire is approximately 14 inches, so, the distance from the center of the axle to the inside upper surface of the pants should be about 7-1/2" to ensure that the tire won't rub.)
Make a mark on the outside of the wheel pants to establish the location of the axle using this dimension.
Center the tire in the wheel pants cutout and you will have the other dimension.
Drill the bolt hole for the axle attachment where your two marks cross.
Figuring where to drill the wheel pants attachment holes on the inboard side of the wheel pants is a bit more involved but can be very accurately done using either of the following methods (see Figure 2):
- The Carpenter Square Method -This simple method for locating the screw holes does not require making a template. Chock the wheels before you begin.
- With the wheel pants removed. Draw a horizontal line on the installed attachment plate (bracket) where you want the attachment screw holes to be located.
- Set a large square on the floor against the wheel assembly and make a mark on the vertical blade of the square to match the height of the screw hole line drawn on the pants attachment bracket.
- Turn the square 90 degrees and make a mark across the screw hole line on the attachment plate and on the floor where you want the first screw hole to be.
- Do the same thing for each of the other screw locations.
- Slip the wheel pants on again and tighten the axle nut attachment bolt. With the square positioned on the first floor mark, make a mark on the wheel pants at the height mark drawn on the vertical blade of the square. This is the correct hole location.
- Move the square over and repeal the steps for the remaining holes.
- Block the aft end of the wheel pants at the correct angle and drill your attachment holes through the pants and through the underlying attachment plate.
Cardboard Template Method:
- Remove the wheel pants so you can cut out an opening in a stiff cardboard template large enough td allow it to lay flat against the wheel pants attachment plate (remove the brake housing for easier access). The bottom of the cardboard template must rest firmly on the floor (see Figure 2).
- Trace around the attachment plate and cut the upper part of the cardboard template to that shape.
- Now, mark the template where you want each attachment hole to go.
- At the same time, without moving the template make an alignment reference mark at the bottom of the template and on the floor (this will assure that neither the aircraft nor the template has moved).
- Remove the template and reinstall the wheel pants by tightening the hub bolt and blocking the aft end of the pants at the angle you want.
- Duct tape the template against the inboard side of the pants assuring yourself that the alignment mark on the floor coincides with the template alignment mark.
- It is now a simple matter to drill the holes where marked on the template. If you did it right the holes will be properly located in the attachment plate after you remove the pants to check the results. Oh, yes, be careful that you don't drill into the tire!