Canopy Installation Notes
by Steve Wilkinson
From "Construction Notes" in the March 1989 issue of the Falco Builders Letter.
It may already appear in The Wit and Wisdom of Chairman DeAngelo, since he was the one who put me onto it, but I've been using a "found-wax" method of forming all the small fiberglass fairings for the top and bottom of the wing and such things as the rudder-cable outlets. It requires only a few pounds of supermarket canning wax, several aluminum-foil "loaf tins" and a small implement called a melon baller, which is not something you do to the Orange Bowl queen but a small scooper with a different-size cup-like hemisphere at each end, used for making cute little balls of canteloupe for fruit salads.
Melt the wax, pour it into the loaf tin and let it harden. (It inevitably hardens with a declivity in the center, but no matter; you're going to be scooping out most of the wax anyway.) Sharpen your-preferably stainless steel-melon baller with a file and just go to work hollowing out, freehand, what you reckon the shape of the needed fairing will be, by sliding-not "scooping"-the sharpened mellon baller across the wax. As a finishing touch, put the scooped-out wax mold under the hot-water faucet and rub smooth the tooling marks with the heel of your hand or perhaps a sponge or balled-up wet paper towel. (Careful, since the wax remelts surprisingly quickly.) Then just cast your fairings in the hollowed-out mold, with the normal three plies of nine-ounce cloth and epoxy mix, and pop them out when done. (Obviously, they won't stick to the wax, no need for any mold-release agent.)
One advantage this has over the turned-spinner-shapes-cut-in-half technique suggested in the plans is that you can carve and try as you go: plop the hollowed-out mold down onto the wing where the fairing will go, and you can immediately see if you've hollowed out enough or if it's the correct shape. Also, since it's already a female mold, you get a finished exterior of your casting without having to convert a male shape to a female mold, as you would with the half-a-spinner method. And finally, for those who, like the neurosurgeon tallying his BMW's mpg, calculate the cost of glue and staples as they build their $70,000 airplane, the wax can be melted, recast and reused forever.
It's surprisingly easy to do the carving freehand with a sharpened melon baller, as long as you concentrate on shaving the wax rather than cutting it out, and you can make the mold more than adequately symmetrical with a little practice. Also, I worry less about the exact plan-specified dimensions of these fairings than I do about having identical twins on each wing. I have two loaf-tin molds at a time going-carving on one set of fairings while another is drying-and I'm just cranking these things out one after the other. Maybe I'll go into business, since writing is getting boring.
Alfred already described my happy-go-lucky procedure for fitting the acrylic bubble to the canopy frame, and it seems to have worked out just fine, other than a couple of misdrilled holes. However, it should be pointed out that fitting the canopy frame plus bubble to its tracks and rollers takes some shifting and adjusting even though you might have gotten the naked canopy framework to seemingly slide back and forth freely. I had to slightly rout out two small areas of fuselage skin at each rear lower edge of the canopy, for example-where the rear diagonal fuselage frame meets the cockpit side rails-to allow the acrylic to "nest" rather than being bowed slightly outward by the full width of the fuselage. I also had to play with the dorsal-fin "slider tube"-sand slightly deeper the groove within which it fits-in order to have the whole assembly slide neatly, and had to file a tiny bit off the bottom edge of the acrylic on each side so it wouldn't rub against the cockpit side rails. It's surprising what tiny, almost imperceptible hangups will affect the freedom of canopy movement.
Also, it should be pointed out that mounting the uppermost of the two black plastic rollers on each side of the canopy does not require drilling a hole through the acrylic in order to afford access for the capscrew and then for a hex wrench to to hold it steady; you simply slide the canopy aft about halfway, to its widest point, which allows space for the capscrew to slip into its roller, bushing and hole, and then use a hex wrench that you have shortened appropriately with a hacksaw.
Another thing: I'd suggest not going to a vast amount of trouble to laboriously cut and fit the spruce spacer that goes between the canopy framework and the acrylic on each side. Put blocks of wood in there-or whatever-to space the canopy the proper distance from the framework as you drill the necessary mounting holes. Then, when you get the entire canopy assembly mounted on its tracks, you can run it full forward and make the precise measurements that will allow you to cut and custom-fit the precise spacers for your airplane, so the curve of the acrylic conforms exactly to the flow of your airframe. Maybe every other airplane fits the plans precisely at that point, but I had to make new spruce strips on each side to account for variations on the order of one or two millimeters.
The windshield: Obviously, the canopy has to be completely fitted, rolling freely, and the leading edge of the acrylic sanded so that it conforms exactly to the windshield bow before you start work on the windshield. Then latch the complete canopy in place. Draw the outline of the canopy bubble on the aft face of the windshield bow. And draw another line 3/8-inch inside that line, to account for the thickness of the windshield. That latter line will be the one down to which you sand the bow.
I used a yard-long float-sander to do the finish sanding of the windshield bow at the correct angle all the way around. First you put the windshield in place temporarily, draw a pencil line at the arc where it hits the fuselage skin, and then you can float-sand freehand keeping the forward end of the float-sander "pointing at" that line all the way around. It comes out close enough for gummint work if you keep checking your angle with a straightedge. (Tony Bingelis also details a slightly more complex procedure using a stick and blue marking chalk in his excellent newest book.)
One problem that might come to light as soon as you try to fasten the canopy to the windshield bow using the hook-and-handle arrangement supplied with the canopy kit is that overcentering the U-shaped "hook" with the handle to snap it fully shut severely stresses the bow. You either need to file oval the holes in that hook (which I did, and I might even have to end up welding a little more metal to them) or redrill the hole in the handle around which the hook pivots. Either way, you need to make the hook in effect longer, so it isn't trying to torque the windshield bow so severely-which I imagine could crack the windshield eventually.
The seemingly obvious way to mount the windshield is to fasten it first to the windshield bow starting at the top center and working out to the left and the right, and then mount it where it abuts the fuselage skin at its forward bottom edge. If you do this, however, you'll find that the slight weight of the windshield, and perhaps the pressure of repetitive drilling and screwing, may have almost unnoticeably bent the windshield bow slightly aft. You'll end up with the windshield bow and the canopy-framework hoop no longer parallel, and when the canopy is tightly fastened at the top, there will be quarter-inch gaps at each side, at the bottom, between the wooden bow and metal hoop.
So mount the windshield at the top of the canopy with, say, six or seven screws. Then hook the canopy to the bow and use it to shove the windshield forward as far as it needs to go in order to keep canopy hoop and windshield bow parallel all the way around. Draw a pencil line at the lower leading edge of the windshield, about six inches out from the center on each side, where it then meets the fuselage skin. This is where you will establish your windshield-to-fuselage mounting wedge.
The plans show that "wedge" to be a continuous strip between windshield and fuselage skin, all the way around the lower edge of the windshield, assumedly of varying thickness and angle all the way around. I found it much easier to use 30mm-wide, wedge-shaped blocks, one at each screw location. This way, each wedge-shaped block could easily be tapered and shaped to fit exactly in its proper location. I made a total of 13 of them, 80mm apart-one on the centerline and six progressing down and aft on each side. (There's room for two more on each side, but the angle between windshield and fuselage skin at those points is so slight that it doesn't seem worth bothering with. Maybe I'll add them later simply as spacers, since at those points the windshield-mounting screws go straight into the fuselage frames and make no use of the external wood for mounting strength.)
The entire canopy and windshield procedure is a matter of mounting and fitting, removing and adjusting, remounting and fitting. So it goes with the windshield. You glue in place the single wedge on the centerline, then drill the windshield and the block to fit it. With the windshield properly fastened to the top of the windshield bow, pull it forward-if you have to-so you can snug it down to that one centerline wedge. Then check your windshield-to-fuselage line and redraw it if necessary. Then dismount the windshield and glue in place the next two or three wedges, say, on each side, in exact alignment with the pencil line you've drawn. Drill and remount the windshield and continue in this fashion both along the windshield bow and the windshield forward edge.
The series of wedges doesn't look nearly as elegant as a continuous strip would, but they'll ultimately be out of sight anyway, hidden from the outside by the metal "framing strip" around the windshield and from the inside by... jeez, I dunno: Glareshield carpeting? Vinyl? Flox? Numerous old Jepps and yellowed E6Bs?
The secret, I think, it not to work too fast-not to drill too many holes at any one time. As you work from the aircraft centerline out-and this is true both for the canopy and for the windshield-and fasten the acrylic snugly in place screw by screw, it seats and sets and works its way into its permanent shape, and that shape can have a considerable effect on where the bottom edges of each piece of acrylic ultimately establish themselves.
And when you finally establish the true, ultimate perimeter of the bottom-most edge of the windshield, you can mark it, remove the windshield one more time and rout out a ramp-like area of fuselage skin (and under-skin blocking) so the windshield melds with the airframe at that edge. But I haven't done that yet, so what do I know.