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Windshield Trim Work
By Tony Bingelis (originally published in EAA Sport Aviation, May 1992)
Not much of a subject, you say? Well, as the saying goes, there is more to it than meets the eye.
I don't know about you but the "simple jobs" give me the heebie-jeebies and the most trouble. Finishing off a windshield installation falls into that category.
How can so simple a job turn out to be so fraught with frustration . . . and end with less than perfect results? Obviously, it is a job that deserves your best effort.
After all, windshield and canopy areas are highly visible and nobody can look at an airplane without noticing how well, or how poorly, the installation was made.
Most noticeable, perhaps, will be the fit of the windshield trim strips, and especially the fit and finish of the retaining strip at the base of the windshield.
Plexiglass windshields, bubble canopies and windows are typically installed with mechanical fasteners or are bonded in place.
Sometimes a combination of the two methods is used. It depends upon the type of structure and the builder's preference.
For example, builders of wood, metal or tube and fabric aircraft have, for many years, relied almost solely on mechanical retention methods for their windshield and canopy installations.
These mechanical retainers include various types of small metal clips, metal channels, screw fasteners, rivets, and even small nuts and bolts.
To finish off plexiglass installations, metal or fiberglass trim strips are typically installed between the fasteners and the plexiglass to better distribute the clamping stresses.
These trim strips also serve to dress up the appearance of the windshield or canopy, and protect the plexiglass edges.
As for composite structures, screw fasteners and separate trim strips are seldom used for the installation of windshields and windows. A windshield installation in a composite kit project is comparatively easy to complete because, for the most part, the preformed plexiglass windshield simply has to be lightly trimmed and then bonded (epoxied) into pre-molded recesses.
Basic Plexiglass Bonding Tips
As already mentioned, windshields, canopies and windows are generally bonded to composite structures with an epoxy adhesive.
NEVER ever use a polyester resin around plexiglass because it, like lacquer thinner, MEK, acetone and similar volatile substances, can ruin the plexiglass. In time, the plexiglass, once exposed to any of these substances will almost certainly begin to craze and become discolored and brittle.
Before attempting to bond fiberglass to plexiglass, it is a good idea to scuff-sand the surfaces to be glued with sandpaper (wet/dry 180 or 240 grit is O.K.).
However, don't start sanding before you take the precaution of laying down a strip of black vinyl electrical tape to serve as a protective barrier along that portion of the plexiglass you don't want to get accidentally scratch-sanded during the sanding operation.
The vinyl electrical tape barrier will allow you to safely sand right up to the edge of the tape.
Don't use masking tape . . . if you do you'll have a mess to remove afterwards.
Protect your plexiglass before you proceed with any work on the windshield. Be absolutely sure that you have all nearby areas of the plexiglass windshield and/or canopy carefully masked off with paper or a polyethylene film. Carefully tape it down around the edges.
Composite builders already know this, but if you have never used epoxy resins or adhesives before, take note of the following:
Epoxies are temperature sensitive. Some require a minimum working temperature of 60 degrees F., preferably more. Do not attempt to use epoxy that is cold . . . it will not mix thoroughly. It might even fail to cure and may remain sticky . . . then you really will have a problem.
Composite builders keep their epoxy resin warm by storing the resin near a light bulb (that's right, Wilbur, it has to be turned on).
At a temperature of around 80 degrees F. the resin will be easy to pour and will mix readily. It will also cure sooner.
Accept this as a fact. Epoxy, soon after it is applied, thins noticeably, and it will sag and run . . . boy, does it ever run!
Those runs, as well as accidental drips on the plexiglass, will be difficult to remove. Consequently, your expensive windshield will ultimately suffer damage during your attempts to scrape away the epoxy mess. These areas, then, will have to be refinished mechanically and polished with Micro Mesh . . . and that spells extra work.
Now that I have your attention, here is something else. No matter how careful you try to be, epoxy will also get on your hands and everything else . . . so work slowly and carefully.
IMPORTANT NOTE: I'm sure most every EAAer knows by now -epoxy is very toxic. Many builders, in time, suffer skin rash and other physical ailments from its use. Take extra care to have adequate ventilation - and protection for your hands.
I became allergic to epoxy several years ago and had to stop using it. However, I have since found I can protect my hands by coating them with "Invisible Gloves", a hypoallergenic pomade . . . a most remarkable product. It really works. Anyway, it allowed me to work with epoxy again and complete my windshield/canopy trim work without developing itchy and inflamed sensitive skin areas.
Fasteners In Plexiglass
When the plexiglass is to be attached to aluminum frames, tubular structures, or to any other type of structure with metal fasteners of any sort, first drill the holes through the plexiglass and structure with the correct size drill bit. Start at the top of the windshield and work down both sides inserting a Cleco fastener in each hole as you go.
After all the holes are drilled, remove the Clecos and redrill the holes in the plexiglass only with a larger drill.
Screw fasteners must not fit tightly in the drilled holes because of the plexiglass' propensity for expanding and contracting considerably with temperature changes.
A tight fitting hole will, therefore, often result in localized cracking in the plexiglass . . . especially when rivets are used.
Incidentally, don't use high strength structural rivets (like Monel MD series) as they impose too great a stress against the `glass.
For example, USM Corporation's lower strength AK series (270 lbs. tensile) commercial grade rivets are less risky to use in plexiglass.
Machine screws are safer to use than rivets when securing plexiglass to windshield bows and to heavier gauge tubular frames as the drilled holes can be tapped to accept the screw threads.
Holes for machine screws may also be tapped into laminated wood windshield and canopy bows to secure the plexiglass in place.
However, it is most important that you assure yourself that the screw holes in the plexiglass are always a bit larger than the screw diameters used. In other words, none of the screw threads should engage the sides of drilled plexiglass holes.
When installing a screw type fastener be careful not to snub it down too hard. The safe way to do it is to torque the screw until it is snug against the plexiglass - then back it off at least a half turn. Don't worry, the plexiglass will not fall out.
Making That Bottom Retaining Strip
The retaining strip along the bottom of the windshield provides a means for securing the windshield to the fuselage.
This bottom-retaining strip can be a removable type or it can be an integral fiberglass type bonded to the fuselage.
A separate windshield base trim strip, as in common in many commercially produced aircraft, may be made of metal or fiberglass and riveted in place. However, it would not look as nice as a well-made fiberglass-retaining strip smoothly bonded to both the windshield and fuselage skin.
Such a permanently bonded fiberglass base strip must be carefully prepared or it will end up with a dimpled irregular surface that no amount of paint can hide.
Here's an abbreviated procedure for installing one:
Step 1 - About one inch above the fuselage skin, lay down a strip of black vinyl electrical tape directly on the plexiglass. This will establish the top edge of the windshield-retaining strip. Epoxy resin will not stick to the electrical tape. In addition, the electrical tape is easy to remove and will not leave a residue on the windshield as ordinary masking tape will do.
Step 2 - If the windshield is not already masked for protection against inadvertently smeared epoxy, do it now.
Step 3 - Scuff-sand about two inches of the fuselage skin and the bottom one inch of the windshield (up to the bottom edge of the vinyl electrical tape barrier) with wet/dry 180 grit sandpaper or coarse emery cloth.
Step 4 - For a better bond, make the first layer using a light weight fiberglass mat. If you have no mat, proceed with the fiberglass layup. Lay up 4 or 5 layers of ordinary 6 ounce fiberglass cloth at one time . . . that's right, with epoxy resin.
You can use the selvedge of your precut fiberglass strips to obtain a nice straight edge by laying the first layer of cloth flush against the bottom edge of the vinyl tape line (see Figure 1). Wet the edge thoroughly with resin.
Step 5 - Remove the vinyl tape divider after the epoxy has gelled slightly, but before it hardens completely, in order to obtain a smoother blended edge between the fiberglass and the plexiglass.
Step 6 - After the epoxy has hardened, replace the vinyl tape with a fresh protective strip and begin the smoothing and finishing process. Use a large half round file, medium coarse sandpaper, and a sharp scraper (your block plane blade will do) in any manner you can work out to smooth the surface.
Windshield and Canopy Trim Strips
How about no trim strips at all?
Just for the record I must admit that adding trim strips around the edges of a windshield or canopy may not always be necessary . . . or for that matter beneficial.
Many European builders seem to prefer attaching their windshields and canopies mechanically with washer-backed screws . . . and without the "benefit" of underlying trim strips.
Although this may appear to be a marginally safe way to install plexiglass - it is not. These installations appear to be just as long lived as those fitted with trim strips.
One thing is certain. Such a smooth windshield/canopy installation (without a protruding trim strip) produces less drag than one where the add-on trim strips disrupt the smooth airflow.
Metal Trim Strips
Metal trim strips do an excellent job of protecting the edges of the plexiglass from damage. However, they fit best when there is no compound curvature in windshield or canopy surfaces where they will be attached. Otherwise, the metal would have to be stretch formed to fit . . . and this is not easy because hand forming aluminum has become a lost art).
You would normally make your metal trim strips of .032" or .040" 2024 T3 aluminum.
Because of the shape necessary to accommodate the slope and curvature of atypical windshield, a rather long wide piece of aluminum sheet would be required if you intend to cut out a windshield trim strip in one piece.
You will waste less aluminum if you first develop the shape and length for the trim strip with the help of a stiff paper pattern. Then you will be able to trace the shape directly onto your aluminum blank.
Cut out the trim strip roughly on a bandsaw if you have one. Leave the final trimming and smoothing of the edges until after you have drilled all the holes for the fasteners.
Since the plexiglass windshield (or canopy) will already be screwed to its windshield bow, securing the metal trim strips poses a dilemma. You have the option of drilling additional screw holes or removing the existing fasteners and drilling matching holes in the trim strip with the aid of a hole duplicator tool.
Here's where the vinyl electrical tape comes in handy. Lay down the tape to establish the bottom edge alignment for the metal trim strip.
Clamp the trim strip in place, aligning it with the vinyl tape.
If extra new holes are to be drilled for the trim strip's attachment, drill them spaced between the underlying existing fasteners . . . remember to enlarge each hole slightly (in the plexiglass only).
The usual installation practice is to tap the drilled holes in the windshield tube (frame) and secure the trim strip with #832 countersunk machine screws. However, this won't work with thin walled tubing. In that case, you might consider using pop rivets, or drilling the attachment holes completely through the tubing and installing #632 screws and nuts.
Fiberglass Trim Strips
Many builders prefer to make their windshield and canopy trim strips of fiberglass.
If you want to lay up a fiberglass trim strip for your windshield, use about 4 layers of regular fiberglass cloth.
First, tape polyethylene or wax paper to the windshield so the fiberglass layup won't stick to the windshield and you will be able to remove it for trimming after the epoxy resin has cured.
Lay up 4 or 5 layers of 2 or 3 inch wide fiberglass with epoxy resin. After the cure, remove the fiberglass layup and cut, trim and sand it to the width you want. This will provide the trim strip with a nice even edge.
If you intend to bond the fiberglass trim strip to the windshield, temporarily clamp it in position. Next, lay down an alignment strip of vinyl electrical tape about 1-1/2" from the edge of the windshield (more if you prefer). This will keep the epoxy from forming an irregular edge and will help prevent runs.
You may prefer to fasten the trim strip solely with machine screws or rivets. In that case, proceed as you would for fastening a metal strip to the windshield.
Yes, Wilbur, you can remove all the polyethylene masking after you finish so you can see through the windshield better.