|
I trimmed and drilled the main screen early in the build as part of the initial body fitting process but the edge preparation and reinforcing is a job that I’ve been delaying whilst I decide on the best way to tackle it. Of the few Rotorway screens I’ve seen close-up some are neat, tidy and well finished and some less so. As with all jobs on my project I try to aim for a safe, well-engineered and elegant solution although it sometimes takes a while for one to rise to the surface.
I’ve been cutting, shaping, drilling and moulding clear panels for many years. Polycarbonate, Acrylic, PVC, Polystyrene and Polythene all have their own quirks and characteristics but I’ve never been required to add a reinforcing border around the edge of any panel. It’s probably also fair to say that very few, if any, of my previous jobs have involved structural panels - where the panel itself constitutes part of the overall strength of the assembly. All other panels I’ve ever work on have been unstressed in their application.
The Rotorway screen is obviously structural, contributing considerably to the overall strength and rigidity of the body assembly. You only have to fix it in position with a few Clekos to see what a remarkable reinforcing effect it has on all the panels around it. Rotorway have specified that a border of reinforcing tape be bonded around the edge of the screen as additional reinforcement. Fair enough.
There has been some useful debate on the ROG about reinforcing the edge of the screen. It’s pretty clear (oops, sorry) that Polyester resin is a no-no because it doesn’t adhere well to Plexiglass and it will react with the surface causing tiny cracks. Epoxy Resin is certainly the way to go.
There may be better methods of tackling this job but here’s mine. Please remember, the following is not a set of instructions - it’s just the way I did it.
Laying up the reinforced border isn’t too challenging. I first applied a sacrificial border of 3/4 inch wide masking tape around the edge of the Plexi, where the reinforcing tape would eventually be bonded. I then applied a second line of tape inside and butting against the first. The first tape was removed leaving a perfectly spaced, masked 3/4 inch border. I then marked and cut some pieces of 2 inch masking tape in the curve of the corner radii.
 |
I abraded the surface up to the masking tape with 120 grit and laid on the Epoxy Resin and Tape, overlapping the corners. I cut some fillets of reinforcing tape to form the corner radii and overlapped the edge strips slightly. This gives a double thickness of reinforcing at some points but that’s not a problem. I allowed a few hours for the resin to begin curing - just enough that it doesn’t string when the tape is pulled then I removed the masking tape. The resin cures at different speeds depending on temperature so it’s an idea to make a test piece using some scrap material at the same time.
When the resin had fully cured (five or six days in winter) I carefully sanded off the highs and edges.
That was a few months ago. It’s a little warmer now and time to finish the job. The task at hand is to build up a layer of filler or adhesive on top of the reinforcing tape, sand it smooth and paint a border over it.
I first laid a 3/4 inch sacrificial tape border around the edge to form a straight guide. I laid a second line of tape spaced 3 mm (or 1/8 inch) inside the first, then removed the first.
I lifted the tape corners and slid a thin sheet of clear plastic under the tape and laid it down again. You can just make out the clear sheet in this picture. The purpose of this was to allow me to cut a matching curve on the masking tape with my modeling knife without cutting into the screen.
Removing the thin sheet and waste tape leaves a nice masking edge like this.
And this
I mixed some Epoxy Resin and Mica filler powder to a paste and used an old credit card to spread a thin layer over the reinforced edge up to the masking tape. Again, waiting for a few hours for it to partially cure before removing the masking tape. It’s important, when removing masking tape in these situations to always pull the tape off towards the adhesive to prevent stringing.
After three days I masked close to the resin with strong cloth ‘Gaffer’ tape to protect the Plexi from sanding scratches. The resin was rubbed down level with 120 grit.
Any small depressions that were left were filled with Acrylic stopper applied with an old credit card and rubbed down flat and level when dry.
I turned the screen over with the inside up and masked the inside exactly in line with the edge of the resin on the outside. I also drilled through with a 9/64 inch drill bit as clearance for the 6-32 screws.
When drilling Plastics it’s much safer to use a dull drill bit and ‘scrape your way through the hole. Removing the sharp cutting edge with an abrasive stone will prevent the drill snatching and cracking the material.
Back over to the outside and it was time to countersink the holes for my Tinnerman washers.
This is the back of a Tinnerman washer. It’s similar in principle to the countersunk screwcups I used on the body panels but has a larger contact surface to spread the load on more fragile surfaces like Plexiglass. It’s difficult to see in the picture but what looks like the flat part of the washer is actually slightly domed. This has the added benefit of acting somewhat like a spring washer, maintaining an even pressure on the joint. It is very important to use a sharp countersink bit - new if possible and make accurately sized countersinks for the Tinnermans. If you place a washer in the countersink you should just be able to slide it around a few thou. with the tip of your finger.
I re-masked the front, directly opposite the masking layer underneath and painted a thin coat of aerosol Etch Primer inside and out.
And here it is. The bottom corner of the screen and the front tub with Tinnermans and screwcups fitted. They’ll blend in a little more when the screen border and the tub are painted but I’m happy that this fixing method achieves what I planned - A strong, positive, accurate location for the panels with stainless steel fixings that will not wear-out or enlarge the holes in the GRP with repeated assembly and disassembly. A less disruptive path for the airflow around the ship, no painting or chipping of the screws and an improvement to the overall look and professional feel of the assembly.
EYEBROW WINDOWS
I had a dilemma with these little Plexi panels. The manual shows them bolted into their reveals with a dozen or so nuts and bolts around the edge. Hmmm. They’re not a structural item and they’re moulded quite nicely to fit the curvature of the upper panels so why the overkill of nuts and bolts? I’d already been toying with the idea of dumping the moulded roof liners because I have plans to flock the inside of the cockpit panels. Besides, the curvature and shape of the liners doesn’t really match the inside of the roof panels as well as it could and as far as I could see, their only purpose is to cover the exposed nuts on the inside of the Plexi panels. I decided to bond the eyebrows in place.
We routinely bond Plexi to GRP on our cars using a professional Polyurethane windscreen adhesive and primer system but I thought I’d test our method on scraps of the Rotorway materials to check the bond strength.
I bonded four strips of screen off cut Plexi to an off cut of GRP from the body labeling them with differing degrees of surface preparation. All adhered OK but No 3 - the one I prepared ‘by-the-book’ proved to be the strongest bond, adhering well to both surfaces.
I cut a piece of timber to wedge between the upper body panels. You’ll see why soon.
The plexi panels were a little oversize so I dressed them down by hand with a large sanding block and 60 grit abrasive.
I chamfered and radiused the inside edges to match the moulded radii in the eyebrow reveals.
Holding the panel firmly in the reveal, I marked the window around the inside edge of the reveal with a chinagraph pencil. I then masked up to the line and keyed the contact area with 240 grit. The outside face of the Plexi panels was masked right up to the edge.
I primed the bonding surfaces of the window and reveal using a soft cloth.
I masked around the outside edge of the radius on the reveal then applied a bead of adhesive around the reveal and smeared a layer on the inside edge of the plexi. The panels were placed in position and secured with a couple of pieces of tape while I positioned a ratchet strap through the cockpit and over the roof.
I made a pair of ‘dumbells’ from wood and Gaffer tape to transfer the pressure of the strap on to each side of the plexi. The spanner here is doing the same job with a piece of rubber under one end and a wedge of wood under the other.
When the panels were secured in the correct position I smeared the adhesive into a neat fillet and removed any excess with the best tool available for the job.
Smearing done, I removed the outer masking and cleaned up any excess adhesive with a rag dampened with thinners. I repeated the smear tactic on the inside. The job was then left to cure for 24 hours.
Unmasked, securely bonded and weatherproof. I’ll tidy up the fillet around the edge, inside and out. The adhesive has created a neat black border on the inside of the panel but I’ll eventually mask and paint over it in the ship colour.
UPDATE Jan 05
The CAA aren’t happy about the eyebrow windows being bonded in. It would take a ‘Major Modification’ and all that entails to gain acceptance. Probably a few months of procedure, a stack of paperwork and not far short of a thousand pounds in costs.
It’s a shame but not the end of the world. I’ve used countersunk screws and screwcups.
|
