Why do we still drill holes in Plexiglas? (Chalkie Stobbart)
SPORT AVIATION, February 2003 page 86 refers.
Nuts & Bolts / Building Basics.
The article ‘Clear Objective, five steps to installing your aircraft’s windows’ by Greg Laslo, caught my attention and my first thought was why do we still drill holes in Plexiglas?
December 2000, time to fit the canopy to my RV-6 (Vans build #23680) Horror, shock… time to call the professionals. John Mc Kerchar, a friend of many years has been working with Plexiglas for at least 20 years so he is the natural candidate to advise me. Amongst other useful bits of advice he gave me, one stuck in my brain. ‘If you apply a load to Plexiglas, it will crack. The only question is when?’ My mind goes back a year or so; the first RV-6 to fly in this country was built by Noel Drew in Durban. One problem he had was with the fiberglass fairing from the bottom of the windscreen to the fuselage, moving and letting in air which inflated the material covering the top of the instrument panel. This was resolved by injecting a sealant into the air gap between the fairing and the fuselage structure. Another RV builder in the Cape, having heard of Noel’s problem, decided to use Sika Flex 255FC to seal the gap at the bottom of his windscreen (before fitting the fiberglass fairing) to prevent this from happening. Unfortunately, this builder turned his aircraft upside-down in a landing accident. When the time came to remove the windscreen he found that even though he had not used any sort of primer on the metal or the Plexiglas, he still had to use a wire to cut the windscreen from the airframe. This made me start thinking, this stuff must be a really good adhesive, why not glue the canopy and windscreen down? So I ran the idea past John and he was most enthusiastic.
The company SIKA make all sorts of industrial adhesives, you name the two materials you want to join and they will have a product. A relatively common adhesive is Sika Flex 252, which is a bit thinner than 255, which is used to glue the windscreen into automobiles making the windscreen an integral structural part of the car. This product is a one-part polyurethane polymer, which I understand means it takes moisture from the air to polymerise. I understand that the panels of refrigerated trucks are glued together the same way and the aluminium sheets do not move and crack as they did when riveted.
Having located the agents, I went to see them, needless to say they had never had a similar request, but they did accommodate me and advise me. I selected the black 252 adhesive, as my roll bar was painted black; these adhesives are also available in other colours. To ensure a proper contact between the three materials, polyurethane / Plexiglas / aluminium, two primers are needed. Sika Primer-210T for the aluminium and Sika Primer 209 for the Plexiglas.
I do not pretend to be a master at the art of fitting the RV canopy having only done one, but I do believe that gluing it down is far superior to attaching it with pop-rivets. This is how I attached mine…
Start off by trimming the canopy to fit the frame, the roll bar and the fuselage as per Van’s plans and / or any other more detailed information, which is available on the Internet. Mark the canopy and tape off the areas where the primer is to be applied to the mating surfaces. When applying the adhesive the trick is to warm it up to allow it to flow better. There are other adhesives that are thinner than the one I used and which have better UV properties (see later in this article) but in Africa you use what is in stock at the time. At the autoglass shop the mechanics place the tube of adhesive on the hot engine when you drive in, then remove the old windscreen. By the time they come to use the adhesive it is nice and warm! When ready to fit the canopy, I did so with it still in one piece.
Apply Sika Primer to the aluminium and the canopy. I had already painted my canopy frame and roll bar and figured that as I had used a polyurethane paint, the polyurethane adhesive would bond to it. So all I did to the painted surfaces was to scuff it with a coarse abrasive paper and then wipe it down with thinners to soften the (cured) paint. I applied the adhesive in a triangular bead as directed, with a thinner bead on the thin pipes of the canopy frame. Do not go overboard as you can always come back the following day (or week) and apply a neat fillet in the corners. Carefully lower the canopy onto the frame and clamp the lower edges into the position they will be when the side skirts are fitted. Then apply the adhesive to the front of the canopy and smooth to desired shape.
I waited for the adhesive to cure, then cut the canopy from the windscreen, smoothed the cut edges then filled the void between the windscreen and the roll with adhesive. This was smoothed then a squeegee modified to cut a semi-circular groove in the adhesive next to the Plexiglas edge and into this groove I placed a 5mm (3/16 in) rubber hose so that half of the hose stood proud of the flat surface. The rubber hose is the common synthetic rubber, UV stabilised rubber hose used in garden irrigation to the sprinkler head. The windscreen to body fairing, which I had made out of commercial aluminium, was then glued into place using the adhesive with a few pop-rivets to hold it to the fuselage. The lower sides of the canopy were then primed plus the two aluminium sheets that capture the lower edge of the Plexiglas. Adhesive was applied sparingly and the sheets fitted then pop-riveted to the lower 4130 pipe of the slider canopy frame. Finally after this had set, the rear skirts were glued to the top of the canopy, here I made the mistake of not pre-heating the adhesive and had to fit the pop-rivets to pull it down. I was hoping to be able to get away with using Cleco’s and then filling the holes with adhesive.
To finish off the seal between the canopy and the windscreen, I built up the leading edge of the canopy frame, by filling the void between the frame and the Plexiglas with the adhesive. This took a few layers, with each layer being ground down with a high-speed grinder and an abrasive wheel, till the canopy and the windscreen had a perfect, watertight seal, which is provided by the rubber hose. Incidentally, the gap between the sliding canopy side skirts and the canopy rail can be sealed by gluing on a length of 30mm shrink sleeving, then splitting it at the upper edge. Contact adhesive sticks better to shrink sleeving than rubber, such as a bicycle tube. To protect the Sika from UV rays, I then masked the adhesive lines, painted the area with Sika Primer 209 and then painted the areas with silver polyurethane paint, followed by the colour of the aircraft. Hopefully these layers of paint will offer the adhesive some protection from the sun’s UV rays.
Having attached my canopy in rather a different way, than recommended in the plans, I then spoke to Van at Sun ‘n Fun and passed on the literature from Sika regarding their adhesives. This was followed up, on my return to South Africa, with an email to Vans giving a description of my method and experience gained. Whether this has been passed on to other RV builders, I do not know. Here at home I passed on my experience to another RV builder. George Morphis was completing his RV-6A and had reached the canopy fitting stage when I suggested my method. He investigated the Sika options further and settled on Sika 295UV as this has better UV qualities than the 252 I used, but he did take the system further. The only hole he has in the canopy of his RV-6A is the hole where the handle protrudes through. George glued down the canopy as I had, but as the adhesive he used is of a thinner consistency, he was able to fit the rear canopy fairings without using the pop-rivets, he simply taped them down in position and waited for the adhesive to cure. At the base of the windscreen he did away with the fairing by shaping the adhesive to the shape required. No pop-rivets, no fiberglass fairings, no stress.
According to the agents George bought his Sika Flex from, the Sika Primer 210T is not really necessary for the aluminium structure as the adhesive will adhere equally well to an epoxy primer and the bond between the epoxy primer and the metal is as good as the bond between the Sika Primer and the metal. Because of the smooth surface of the Plexiglas, a specialist primer such as Sika Primer 209 is required.
To me the benefits are obvious…
1. The canopy rests where it wants to be till the adhesive cures, reducing stress to the Plexiglas.
2. There are no point loads applied to the Plexiglas such as pop-rivets and screws.
3. The adhesive remains flexible, allowing the canopy some movement.
4. There are no unsightly fiberglass strips between the canopy and the windscreen. The joint is neat, clean, smooth and watertight.
5. The lower edge of the canopy does not have the holes and all the screws plus the nylon strip and it looks neater.
6. All the stress of drilling holes in your canopy is reduced to the single hole needed.
I hope this is of interest or perhaps use to other folks fitting Plexiglas canopies. Perhaps this method can be refined to the benefit of all aircraft homebuilders.
After George's success with Sika he convinced Noel Drew to replace the windscreen of his RV-6. Here is noel's story...
Your RV needs a new windscreen! Words to strike despondency into anyone who knows what that entails. My windscreen had a crack, no two cracks to be strictly accurate and although a glued repair had been made and stop-drilled holes provided, the fact remained that the ship was less than perfect and to top it all, a replacement windscreen had been acquired. Motivation was provided by George Morphis who offered to help with the new technique of urethane gluing using the know-how he had gathered from Chalkie and developed in the successful construction of his RV6A.
After much hesitation the decision was made and the destruction of my original work of many hours was accomplished in less than an hour. The fact that the old windscreen almost fell out after removing the screws on the roll bar was a lesson in itself. The epoxy and fiberglass that had been molded around the front of the Plexiglas had no significant integrity after 9 years of flying and the recommended tabs of aluminum pop riveted to the forward skin were doing all the work in front.
The new screen was easily fitted over the roll bar and trimmed to meet the old rear section on the slider. There was a snag in the alignment of the profile of the two sections as the rear was not able to be adjusted for height and there is a slight mismatch in profile in places that is as much as 1/8 inch. I might fair this in later with a wedge of fiberglass or urethane but this priority is rather low when put against flying.
The original construction that had the Plexiglas captured on the sides by the glareshield was altered to the simpler option of gluing it alongside. To get it tucked in tightly, a wooden support was wedged off a temporary angle screwed onto the outboard tank nut plates. When all was looking ready for gluing we marked the edges of the windscreen on the fuselage and used these marks as a guide for masking off the black primer that is so necessary for a good bond. An area outside the edge of the Plexiglas was also primed to accept a fairing of urethane in the form of a wedge to finish the job. The roll bar had been painted black to make it easier to hide any errors with the black gluing urethane and this was rubbed down with Scotchbrite, masked and primed. The Plexiglas itself was marked for gluing, abraded with Scotchbrite, masked and primed. The position of the priming of the Plexiglas is also important for the final appearance as the primer will provide a line inside the Plexiglas that should hide any irregularities left by the gluing.
It is recommended that a spacer of about 1/8 inch be used to hold the Plexiglas off the surface to provide a more flexible joint. These spacers were removed after the first round of urethane had set and fresh material was injected into these holes. I could not spare any extra packing on the roll bar because of the match needed with the rear canopy, but, considering the fact that a fair wedge of urethane was going into the tangent between the Plexiglas and the roll bar, I felt the situation provided enough strength and flexibility.
The urethane is easily removed from the polished Plexiglas after setting but careful masking is recommended especially on the painted or epoxy primed surfaces.
The urethane glue will set in a matter of hours with the cure moving in from the surface. The setting rate was sufficient to allow us to remove the spacers and inject in more glue within two hours and then follow up with an application of glue for the fairing wedge.
Take care with the application of the wedge using a custom made scraper to help minimize sanding or grinding that may be necessary to finish the job.
The joint under the Plexiglas must be protected from the UV radiation and I painted a light grey frame on top of the Plexiglas to cover the urethane glue.
My repair appears to be many times stronger than it ever has been and I will no longer be warning passengers about leaning on the roll bar. In addition the 9 year old RV6 is looking quite rejuvenated.
Thanks Chalkie, thanks George.
Durban, South Africa
At about this time the article was published in Sport Aviation and the emails started rolling in. Now many people in the world are refining the system and learning the benefits of not drilling holes in your Plexi.
If you MUST drill a hole, than take the drill and drill for a second or two into a brick to blunten the cutting edges! Better still take your high speed grinder with a small cutting disk and square off the sharp cutting edge / leading edge of both flutes (blunten them) of the drill so that you have a flat surface about 0.5 mm wide. The cutting edge MUST be at 90 degrees / vertical to the Plexi / parallel to the centre axis of the drill. This way it will bore (not CUT) through the Plexi (drill slow so you do not heat / melt the Plexi as this will also induce stress) and with the 'blunt' / square (as opposed to sharp) cutting edge of the drill exiting the Plexi it will not DIG, TWIST, TEAR as it exits, which always results in a crack.
http://www.rv8.ch/ In this site enter Sika in the search window and it will bring up a bunch of articles, here is a quote from one of the Sika Agents...
"Sorry, we do not support your application", right after that, " now that I have said that, how can I really help you............We talked for a while, Steve mentioned that his phone has been ringing off the hook ever since the Sport Aviation article came out. He kinda jokingly grumbled that he has had more response to an article Sika did not write, for a use Sika does not support in a magazine Sika did not know about, than what his total advertisement budget generated last year."
Makes you think....... Doesn't it?