RAFTSMAN'S CORNER By Ben Owen. EAA Technical Counselor Administrator
MISCELLANEOUS TIPS ON HOMEBUILT AIRCRAFT Adapted from information supplied by Terry McClain, EAA Chapter 168, Dallas, Texas. Turtleback Manufacturing Process For KR-2 (easily adapted to other aircraft)
The turtleback skin of fiberglass is first laid up in the flat on a waxed or greased surface (on a smooth sheet of metal or equivalent). The layup is
squeegeed and allowed to cure for a day. It is then stripped off the greased surface and you have a super smooth and flat surface, needing only degreas-
Windshield Bow For The KR-2 (also adaptable to other aircraft) ches wide with the grain perpendicular We decided to use a flat wrap to the face. The strips were soaked in windshield on the KR-2. The first step water, then loaded on the form and left was to mockup the windshield and window door bows to check for headroom to dry. Dryness was checked with an Ohmmeter on the highest Ohms scale. and looks. Templates were made to the When the meter wouldn't move, they outside mold line. Two forms were were DRY. The probes were approximade, one for clamping and one for rubmately 1/4" apart and in full contact (see ber band clamping. The rubber band form worked very well. It is easy to load, sketch). The rubber bands used were cheap — and fast — to build. The sold for fishermen's troutlines. They are clamp-type was too much work and pro1/2" x 1/32" x 50 and we bought about vided poor clamping. $5.00 worth. We protected the form and With the rubber band tool, the inside • the rubber bands with plastic wrap. (Some T-18 builders have made a mold line was determined and a 3/4" plywood form was cut. Braces were windshield bow/roll bar in much the then cut and nails driven in for rubber same way, using thin strips of aluminum bands to hook to. Total tool building that are epoxied and riveted together instead of using metal tubing. Such a time — about 2 hours (see sketch). The wood is Douglas fir ripped aproll bar can be made oversized and then proximately .080 inches thick by .75 insanded to the shape desired.
ing before paint. It is quite flexible at this stage, and it is laid into the rough female form for the turtleback with the smooth side next to the form (see sketch). Stiffeners of wood or foam are then laid in and epoxied to the skin. The foam is used as the inner core and the inner skin of fiberglass should be added and bonded to the foam core. The inner skin can also be laid up on the sheet of metal like the outer one, allowing to cure 24 hours before being put into the mold. NOTE: This process is basically the way Formica is made, and is the only way that hours, days and weeks of laborious sanding can be avoided. Take note that this same method can easily be adapted to make fuselage sides and bottoms as well. A little experimentation with small pieces of scrap material is advised before doing the real thing. If you use scored foam for easier bending, you may have to score it more deeply to make it conform to the form near the tail where the radius of curvature is tighter. On the Dragonfly we used a Mototool with a triangular shaped bit to rout out deeper and wider "trenches" in the foam.
Rubber Band Canopy Bow Form
TEST FOR DRYNESS SPORT AVIATION 67
How We Formed A Windshield After reading many horror stories on how Plexiglas is formed, I read this method in the KR-1 Newsletter. It seemed simple and low in cost. The method would be too slow for factory use, but I only wanted to form one set and so an extra 4 to 5 hours wouldn't matter all that much. Step One is to mock up the windshield (or build the aircraft up to this point). As I wanted to be sure I could build up a replacement if required and also make sure the windshield would fit the ship, I chose to build a mock up. The area that would be taken up by the fuel tank, the windshield bow and side attachments were built and installed. The forward deck about the gas tank was mocked up and installed. Step Two — Using poster board, a pattern of the windshield was made. This pattern was then transferred to a tin sheet. I now had a tin windshield (.025" 2024 aluminum would also be O. K.). My tin windshield was cut out on the forward edge. The tin windshield was then installed on the ship. A few clamps, some small nails and a little wire held it in place. Over the tin I draped thermo-liner. (Thermo-liner was used as flannel cost too much.)
Step Three — I borrowed a 100,000 BTU space heater (kerosene type) with a blower. We set the heater on two trash cans and a box about six feet from the Plexiglas. I tied a candy thermometer to a stick to check the temperature from the discharge to the Plexiglas. I wanted to keep the Plexiglas below 200 degrees F., so we moved the heat up three inches at a time until we ended up with it three to four feet from the Plexiglas. Soon the Plexiglas began to droop. When it began to take light pressure to cause the Plexiglas to form, clamps were applied one at each end of a Woodstrip on the lower edge. The heater was moved to blow on the formed area. The clamps were then tightened and the Plexiglas was heated for another 10 minutes. The heat was then turned off and the Plexiglas allowed to return to room temperature. When the clamps were removed, it only took a few
ounces of pressure to hold the Plexiglas in place. So far, so good. One side now done. The other side was done in the same manner. The first side took 2-1/2 hours, the second took 45 minutes. The optics looked great! By the way, the Plexiglas we used is 1/8" thick, bought locally from Handy Dan Hardware. They carried clear and heavy smoked in acrylic and styrene. Styrene is very brittle and doesn't form well — not recommended. The cost of the 36" x 72" sheet of acrylic was $37.00 or about $1.94 a square foot. It is not well protected, so check it well for scratches. It sure beats $5.00 a square foot! 68 SEPTEMBER 1985
HOMEMADE GRINDING DISC
Circle When Dry
— Glass Cloth
Open 1 Gallon Can
Homemade and Handy Abrasive/Cut-Off Wheel
I needed a thin abrasive cut-off wheel to cut some Plexiglas in the past and used to use one called a "Zippity-do", but over the years this wheel has changed. It is thicker and courser and not as flexible as it used to be. I had some course grit left over from a rock tumbling project, so I stretched some nine ounce glass fabric over a one gallon round can with both ends cut out. A small amount of epoxy was mixed up and applied to the glass on both sides. Then the grit was sprinkled on the wet resin on both sides and the resin left to cure thoroughly. Then it was only a matter of cutting out a disk, drilling a hole in the exact center and installing a proper mandrel so that it could be used. The resulting disk has worked very well on both Plexiglas and wood. While we are on the subject of cutting Plexiglas and acrylic, it can be a real tear shedder if you are cutting a windshield or canopy and it suddenly cracks because a saw tooth grabs. The only safe way to cut Plexiglas or acrylics is to use an abrasive flexible wheel and
cut a series of shallow straight line cuts. Once around the part with the shallow cuts and then you can go back and deepen them on the next pass. When you have cut through completely, you can sand and polish the edges to eliminate any rough spots where cracks could start. Some even flame polish the edges with a torch. All these items should be practiced on scrap first, naturally. Also never drill Plexiglas with a standard drill bit... as the flutes come through there is a 90% chance one of the flutes will grab and crack it. The bit must be modified. I have used a Miller Falls wood countersink with good success. On the T-18 canopy installation drawing, a 1/2" hole is called for first. A rubber bushing with a small metal bushing in that is inserted. Then when the mounting bolt is installed with large area washers on both sides, the bolt can be snugged up against the metal bushing with no direct pressure on the Plexiglas to get it in a bind and crack. Each fastener floats in rubber in an oversize hole.
T-18 CANOPY PLEXIGLAS BUSHING