BY TONY BINGELIS, 8509 GREENFLINT LANE, AUSTIN, TEXAS 78759

FOR A GOOD FITTING

. WINDSHIELD'S SIMPLE uncomplicated appearance belies the sometimes fussy preparations required to install it. By fussy, I mean the necessity to remove and reassemble that thing at least a half dozen times before you can really

lay claim to a completed installation. Almost every operation-trimming, filing, drilling, redrilling, deburring and finally painting - is followed by a disassembly or reassembly of the windshield. I just thought I would tip you off to this fact to reassure you that you are probably not overlooking any short cuts. Working with Plexiglas is fussy but not difficult. The Semantics Of It All

First a short aside. PLEXIGLAS is not really a word; it is a product trademark of the Rohm & Haas Company dating back to 1935. Other products like LUCITE, LEXAN, ACRYLITE, etc., are also used for windshields but we builders blissfully refer to our windshield or canopy as PLEXIGLAS when, in fact, that acrylic part might actually be some other brand. We use the trademark name PLEXIGLAS as a common word, which it ain't. However, I do the same so,

any reference I make to Plexiglas will apply also to the other common brands of transparent acrylics. Working With Acrylics Plan ahead and try to make your windshield and canopy work coincide with warm weather. The colder the temperature the more brittle and unforgiving Plexiglas becomes. Therefore, if a cold Plexiglas windshield is subjected to flexing, vibration and localized pressures while you are cutting or drilling it, there is a very real risk that it could

handling it. A Plexiglas windshield scratches easily so you must immediately see to it that it will be reasonably well protected against the hazards that crop up during its installation. If the Plexiglas is still masked, leave the masking paper on and remove only a narrow strip from the edges for your work. Most manufacturers of acrylics state that their plastics can be cut by almost any means at your disposal. The band saw apparently is the favored means although circular saws (bench saws), hole saws, routers, jig saws, saber saws, electric saws (Skil saws) and even hand saws are not ruled out. No mention is made, apparently, of abrasive discs (my favored method). If your windshield is already unmasked, cover it with paper (held in place with pieces of duct tape) to give it some degree of protection from scratches and drips. If you have to cut the Plexiglas on a bench saw or band saw, tape paper to the underside too, because the Plexiglas might get scratched as you slide it across the table and through the saw blade. If you must use a saber saw, be aware that this is the riskiest means for cutting plastic outside of using a chain saw . . . if you aren't extremely careful, that is. Be sure that the Plexiglas is solidly supported and that the reciprocating action of the saw blade does not cause the plastic to chatter. Before you make any cuts, lay a strip of masking tape over the line of cut and saw right through it. The masking tape will minimize the chipping damage that usually results when you use a saber saw. Use metal cutting blades and not the wood cutting blades oridinarily installed in your tools. This applies

equally to the band saw (use 18 teeth/inch), saber saw (use

crack or break. Since, on the average, a project will last a couple of years or more, you should be able to delay your

24 teeth/inch) and the bench saw (6 to 8 teeth/inch).

acrylics work until the warmer months. This is not to infer

for drilling plastics. A regular drill bit will almost certainly grab and pull itself into the hole abruptly. This increases the risk of chipping and cracking.

that you can't work with Plexiglas after the weather has turned cold; it's just that you must be more careful in

Finally, always use a drill bit that has been modified

SPORT AVIATION 57

The Windshield Frame (Bow) Small windshields do not require a frame at all and are often secured in place with simple metal clips. Larger

ESEKT INCORRECT WMD3HIELD FRAME OF BEVEL (l-l)

windshields, however, require a windshield bow that can

be made of laminated wood, fiberglass, steel tubing or aluminum tubing. The windshield frame determines the height and width of your windshield area. It does not limit or restrict the slope of the windshield. Of course, if you purchase a formed windshield designed especially for your aircraft type, this dimension will have been more or less locked in. A tubular windshield frame once bent and installed is ready for the Plexiglas installation. Not so with a wooden windshield frame. It must first be beveled to obtain a contact surface that will be parallel with that of the windshield. Gradually establish the bevel on a wood windshield frame with a sharp block plane, files and/or coarse sanding blocks. Check and bevel angle frequently with a stick smudged with colored chalk at one end. The chalk will smudge the windshield frame when the stick is rubbed against it indicating the high spots. Be sure the smudge stick is held at the correct windshield angle. Work these high spots down until the entire frame smudges uniformly across the bevel surface. See Figure 2.

WMMHCLD FRAME

:SMEO WMOSMKLO ANOLE (...)

SMUDGE UNDERSIDE Of STICK iKITH COLORED CHALK IN THIS AREA

- NOTE SMUDGE STICK ALONG WINDSHIELD FRAME TO LOCATE HIGH SPOTS THAT MUST BE DRESSED DOWN

WMOSHCLO FRAME

CHALK SMUDGED AREAS

Windshield Hand Holds I don't know if a hand hold should be considered as part of the windshield. I do know, though, that it is a might handy aid to getting in and out of an airplane. If you intend to install one, you should do so before you secure the

FIGURE 2.

OBTAINING THE CORRECT BEVEL FOR WOOD WINDSHIELD FRAMES

WINDSHIELD

FRAME

(STEEL TUBING) WINDSHIELD FRAME

(WOOD BOW)

WINDSHIELD FRAME (STEEL TUBE)

HAND HOLD-

(V8" STEEL TUBE)

LAMINATED WOOD

^\

WINDSHIELD HAND HOLDS FIGURE I. 58 JANUARY 1984

Plexiglas to the frame. If your windshield frame is a steel tube, simply form a small tubular "U" of %" diameter steel tubing and weld it to the windshield bow. On the other hand, if your windshield frame is a laminated wood or fiberglass bow, the hand hold will have to be bolted to the frame. A hand hold for this type of installation may be fabricated from W thick aluminum plate . . . you might find a commercial handle that looks good and is sturdy enough to support your weight when you pull yourself up out of the seat (see Figure 1).

that has a slightly larger diameter than the metal fasteners that will be used. Smooth and chamfer the hole edges with a larger drill bit twirled lightly between your fingers. Chamfer both sides of the Plexiglas. While you are at it, finish the edges of the Plexiglas windshield by removing all saw and file marks. Scrape and smooth the edges with a sharp knife blade or single edge razor blade and/or sandpaper. Do not leave the slightest scratch or imperfection as it might, under localized pressure and vibration, become the starting place for a crack.

Installing the Windshield

That Bottom Retaining Strip You are now ready to make the retaining trim strip that holds the windshield in place against the fuselage. This trim strip can be a removable type as is typical with most aircraft, or an integral type that is faired into the fuselage (see Figure 4). First, prepare the Plexiglas by protecting it with overlapping strips of waxed paper taped along the bottom of the windshild. The waxed paper will allow you to pull out and remove the windshield after the fiberglass lay-up around the base of the windshield is completed. Reinstall the waxed paper protected windshield with screws. Always start at the top and work down both sides. Do not overtighten the screws . . . ever! A snug contact is good enough. Unless you are adept at hammering soft aluminum into nice shapes, I would suggest that making the bottom retaining strip of fiberglass would be easier and quicker. This would be true even if your retaining trim strip is intended for an all metal aircraft. A fiberglass trim strip can be riveted to the metal in the same manner as you would a formed metal strip . . . and it would be difficult to distinguish between the two types. Anyhow, the fit of a removable fiberglass trim strip built up in place can be far superior to most any hammered metal retainer. An even nicer installation than a well fitting removable fiberglass retaining trim strip would be one that is built into the fuselage so that no seam mars the smooth transition from the fuselage to the windshield. This integral restraining strip is a natural windshield treatment for composite or wood structures. If the bottom retaining strip is to be integral with the fuselage, scuff sand that area and make your fiberglass lay-up. If you want to make a removable trim strip, protect that area of the fuselage with waxed paper. Give the fiberglass lay-up at least 4 or 5 layers of regular (8 oz. is O.K.) fiberglass cloth set in epoxy resin. Note - When using this "in place lay-up" method, do NOT use polyester resin as it will have a bad effect on the windshield. It's fumes will cause the Plexiglas to craze . .. not immediately maybe, but soon enough. Use epoxy resins for this purpose only. Furthermore, the use of window glass cleaners, unless formulated for plastics, should be avoided. Naturally, acetone, lacquer thinner and stuff like that must be kept away from your Plexiglas even though no immediate effect can be detected. After the lay-up has cured, remove the windshield by working it around slowly (remove the masking tape strips first). The windshield will come out easily although most of the waxed paper may remain stuck to the fiberglass

Position the untrimmed Plexiglas windshield on the airplane to see how it will look. Clamp it to the windshield frame with spring clamps and check the overall fit. The secret to a good windshield fit is in obtaining the correct shape for its bottom edge. This contour must make contact with the fuselage along its entire length. If the purchased windshield was intended for the type airplane you are building, the initial fit should be pretty accurate without having to force or restrain the bottom edges to any degree. After the Plexiglas windshield is trimmed and fits to your satisfaction, you are ready to drill the screw holes for the fasteners that will be used to secure it to the windshield frame. Clamp the Plexiglas in place on the frame with padded spring clamps. Begin at the top center of the frame and work equally down both sides using as many clamps as necessary to insure that the Plexiglas is tight against the entire windshield frame. Drill the first hole about l/z" away from the centerline of the windshield frame. Use a small drill (about V«") and drill through the Plexiglas and into and through both sides of the windshield tubing. Fabrication of your trim strips will later be simplified if you drill the screw holes completely through the tubing. Do this whether you will be using machine screws and nuts or simply sheet metal screws that need only to penetrate one wall of the tubing (see Figure 3). Space your holes uniformly 3" apart, more or less, as may be needed. You will find it easy to drill through the transparent Plexiglas and hit the tubular frame dead center . . . but be sure to hold the drill perpendicular to the surface of the Plexiglas. Next, remove the clamps and the Plexiglas from the windshield frame and enlarge the screw holes with a drill HOLES IN PLEXIGLAS VK*

MOLES MOST BE DRILLED PERPENDICULAR TO

PLEXIGLAS

PLEXIGLAS

NOTEHOLES IN PLEXIGLAS MUST BE OVERSIZED FOR FASTENERS USED

FIGURE 3 SCREW HOLES IN WINDSHIELDS (TYPICAL)

lay-up. But that too will pull out easily. Trim the fiberglass retaining strip to a uniform height but don't get carried away with the task and trim it too low. Allow sufficient height above the coaming so that if you install a rubber molding inside the windshield it will not show from the outside . . . that would look blah! A judicious amount of filing and sanding with a sanding block will nicely fair the lay-up into the fuselage (if you are making the integral type). The removable trim type, too, is easier to finish while it is still in place after the lay-up. Work at it. There is nothing more unattractive SPORT AVIATION 59

than a cowling, fairing or a trim strip that has pock marks, dimples, pin holes and other imperfections. After the final trimming and sanding are completed, give the lay-up a finish coat of resin. When that cures, finish it off with a fine grit oxide sandpaper wrapped around a small foam sanding block . . . always use a sanding block to obtain

surfaces you like to see.

METAL CLIPS ALUMINUM .060" (INSTALL INSIDE OR OUTSIDE) NO FRAME REQUIRED

RUBBER CHANNEL TRIM (WIND SEAL) EPOXYED FIBERGLASS LAY-UP (5 LAYERS) REMOVABLE TYPE RETAINER

COAMIN6

PROTECT LOWER PORTION OF WINDSHIELD WITH WAXED PAPER

WINDSHIELD FRAME

EPOXY/FIBERGLASS LAY-UP

PLEXIGLAS

(SLAYERS)

FIGURE 5.

SMALL WINDSHIELD INSTALLATIONS

Why Use Trim Strips? Some European windshield installations have the fasteners (screws) bearing directly on the Plexiglas. These fasteners usually are of the button head or round head type backed by a thin washer under the head. That does make a lighter and cheaper installation as no trim strip of any kind is used.' However, the practice in this country is to install a metal or fiberglass trim strip between the screw head and the Plexiglas. In some cases a fiberglass trim may be glued to the Plexiglas and the Plexiglas to the windshield frame. At any rate, a trim strip provides a clamping effect over a wider area and lends a nicer overall appearance. Furthermore, any dust that may become entrapped between the Plexiglas and the tubular frame will not show because the trim strip will hide it. The final appearance of your windshield installation, however, does hinge on the quality of your trim strips. They need to lay snugly against the windshield Plexiglas.

COMPLETED LAY-UP- -.-

BOTTOM WINDSHIELD RETAINER AND TRIM (INTEGRAL METHOD)

TRIM OFF EXCESS

FINISHED CONTOUR

• REMOVE MASKING TAPE AND PULL WINDSHIELD OUT. • PULL OUT WAXED PAPER REMNANTS. • FAIR FIBERGLASS LAY-UP INTO FUSELAGE. • TRIM FIBERGLASS TO CONTOUR.

FIGURE 4.

FIBERGLASS LAY-UPS FOR WINDSHIELD RETAINERS - BUILT IN TYPE

60 JANUARY 1984

The Trim Strip Dilemma With the holes already drilled through the Plexiglas and the windshield frame tubing, how in the world can you tell where to drill the holes in a bent blank trim metal strip. Insert a screw from the outside. Drill the next hole and so on until all are completed. Remove the screws and the trim strip. Deburr it and reduce its width and overall dimensions as needed. The hole alignment will be perfect only because you were able to back-drill them all while the entire windshield assembly was in place. If you had originally chosen not to drill all the way through the tubing, or if your windshield is a bulky wood bow, you will now have to resort to a less accurate hole locating technique. Clamp a strip of stiff paper around the outer edge of the windshield and trace its outline for a pattern. Then,

without removing the pattern, rub your fingernail or a rounded dowel over each screw hole to fix its location accurately on the paper pattern. Transfer this paper pattern to your metal trim strip blank and punch mark the screw hole locations carefully. I warn you though, a hole or two might not be exactly where it should be when you try to install the trim strip - this in spite of the care you exercised. The cure? Elongate the offending hole(s) with a small round file. If the elongation is slight, it might disappear when you open the hole to its correct size. At any rate, the screw head will cover it completely. If that is repugnent to your artistic senses (or if you have nothing better to do), you can use that first trim strip and make another more accurate one. Be sure, though, that you clamp the new blank while both strips lay flat. Drill through the corrected old trim strip into the new one. Don't attempt to transfer the holes with punch marks . . the results may not be any better than your original attempt. Windshield Security Your windshield can be secured with screws (stainless), machine screws and nuts, rivets, epoxy and simply be clamped in between channels. The method you use will be influenced by the type of windshield frame installed. A tubular windshield frame, for instance, does not lend itself to the "glued in windshield" technique but a fiberglass or wood frame does. In my estimation, countersunk screws should not be used except, perhaps, with thicker strips (.050") and even then, with smaller diameter screws. In the same vein, riveting through holes drilled in Plexiglas windshields is asking for trouble because of the localized pressures rivets impose on the Plexiglas. If it doesn't crack on installation it probably will some time later. Rivets can be used successfully if properly employed with framed strips and channels. I would suggest that your initial installation be made with self-tapping stainless screws. Later, if a hole becomes stripped you could switch to a larger size screw or change to machine screws and nuts. If self-tapping screws are used you could plug the unused drill holes on the backside of the tubing with a lacquer putty (Dupont's "Spot 'N Glaze", for example) to conceal them from the cockpit side. Epoxying a windshield in makes it a very permanent installation and future replacement, if mandated for any reason, could require a partial demolition of the windshield structure and a major rebuild of the coaming and windshield frame elements. This prospect, however, is not too great a concern as, with care, most windshields will last for years and years . . . especially when the airplane is regularly hangared.

As you can see here, small windshields need no frame. These windshields are typically secured to the fuselage with a trim strip or with small metal clips.

A nice framed vintage effect is achieved here without resorting to the use of a heavy structural frame. (Earl Davit photo)

A nice light low drag windshield installation, however, builders must assure that the screw holes in the windshield Plexiglas are a loose fit and that the screws are uniformly snugged down . . . just barely.

A fairly typical Volksplane windshield with a built-in bottom retaining trim strip of fiberglass. Note that no trim strips are installed around the windshield frame and that the Plexiglas tends to bulge out between fasteners.

A good example of a windshield that is virtually molded into the fuselage. The integral bottom lay-up technique described in the text will produce similar results. SPORT AVIATION 61