Mike has asked me to provide a tutorial on framing, glazing and installation of clear plastic canopies for scale sailplanes. I am sure there is more than one way to accomplish the task, however the following is the procedure that I generally follow. I will provide some links at the end to content allowing readers to explore methods employed by modelers with more experience than my own.
BEFORE YOU BEGIN
Canopy construction cannot proceed until you have framed up the fuselage. Its important to have completed any requisite sanding and have sheeted or fabric covered at least around the area of the canopy side rails and forward and aft bulkheads. Without these steps, there will not be accurate referencing surfaces to ensure a good fit of the completed canopy. If provisions for radio gear and its installation within the fuselage are not complete, or if you plan on adding interior details which would be more easily fit prior to sheeting or covering, do so before proceeding to the the canopy. In addition, make sure you have a clean working area with all tools and materials laid out before beginning.
You will need the following for most installations:
Masking tape, 1/2" to 1" wide (12-25mm) both high and low tack
Two part hobby epoxy, 10-15 minute preferred
Filler, either wood-based or microballoons
Hobby knife with No. 11 and razor saw blades
Waxed paper or clear food wrap
Sanding block with coarse and medium papers
Fine tip felt marker
Scissors
Glass cloth (small scraps will do)
DRY FIT THE CANOPY FRAME
First dry fit the parts in the frame and check fit and
mark with a fine point pencil where trimming or beveling is required.
In dry-fitting of the canopy frame components, make note of the relative fit of parts, one to the other, and make any necessary marks to indicate trimming required. Note in the photo above that the side rails have been slid inboard to allow approximately .032" of clearance (.75mm) between frame and fuselage side so that the canopy outer surface will land flush with the finished fuselage sides. I've not made any adjustments to the forward or aft bulkheads as they will need to have their edges beveled to accept the rising angle of the vacuum formed clear part and this will be more easily and accurately accomplished once the frame is fully assembled.
The goal is to have all frame parts initially cemented so that when placed into the canopy opening for final fitment, all parts are at least net proud of their final dimensions so they may be sanded or trimmed later to allow best fit of the canopies outer surface to the finished surface of the fuselage leaving no gaps where cement will be applied to make the clear part to frame bonds.
CEMENTING THE FRAME
When you are satisfied with the frame position and fit in the cockpit opening of the fuselage, place reference marks at the junctures of each set of parts if needed, and remove the parts. Chop a small amount (about a teaspoonful) of glass cloth into about 1/4" bits and set aside. Next mix a small batch of epoxy and with the side frames taped into position, apply cement to both the ends of the bows and the point on the sides where they will meet, and position them together in the fuselage, clamping them into place to cure with masking tape.
LAYING IN THE FILLETS
Immediately blend the chopped glass fibers with more of the epoxy mix and carefully push a small pinch directly into each corner joint in the wood frame. Press the slurry down to form a matt that fills approximately 1/4" on either side of the joint. Using a small scrap of waxed paper or clear wrap, press down with thumb or forefinger to compress them out and into the joint, creating a short radiused fillet.
After the epoxy has has cured for about 30 minutes, prepare another mix of epoxy, this time using powder filler such as microballoons to reach the consistency of peanut butter. Spread it carefully with a small flat stick over the chopped glass fillets and again press it out to form a smooth radius across the same area which feathers out to meet the wood about 3/8" or more from the center of the joint on each side. Using your thumb or forefinger its quite easy to make a fairly smooth surface but don't be too concerned about excess squeezing out to the sides. This will trim away easily with the sanding block so its more important to focus on the radius which is will require more work if not properly smoothed out during application.
TRIMMING THE FILLETS
Finally, after fully curing for several hours, trim off the excess chopped glass and filler that extends inboard and outboard of the joint. Use coarse sandpaper or ideally, a bench-top disk/belt sander will make short work of this if one is careful and takes their time.
It is not necessary to clean up the joints much further than shown above as in most cases the canopy framing is designed and cut to be slightly smaller than the finished outer trim when scaled to the full size aircraft. This will allow final paint or trim to cover and hide the frame joints. More on this later.
PREPARING THE FRAME TO FINAL FIT
The next step is to mark and bevel the bows to match the canopy angles. First, center the canopy frame back in the opening and tape it in place securely. Mark the bows by tracing around the fuselage with the pen held at an angle that will mark approximately 1/32" in from the fuselage surface (about the thickness of your canopy material).
Then with a sanding block or again carefully with the disk scander, bevel the parts upper surface back to visually get the angle to match that of the canopy when held behind and sighted over the fuselage. Note that most of the material will be removed at the top and fair away to nothing being removed as you reach the flats of the fuselage sides.
This process has puzzled me for a long time. It's good to see a thorough treatment. I'm looking forward to the next episode!
Posted by: greg smith | January 21, 2010 at 09:38 PM