[Loman]

I am doing a full-Sikaflex tip-up and I am fitting the aft window. Just behind the roll-bar on the right side I have a length of about 8 inches where the edge of the top skin does not lie flat against the plexi. The apex of the bulge is about 1/8 off the plexi. Elsewhere along this interface, there is perfect contact

I happen to have an edge shrinker that I have never used but the skin is already riveted on (other than along the longerons forward of the baggage bulkhead where you see the clecos) so it would have to be used 'freehand' and I doubt if there is any way enough force could be applied.

I have also considered relaxing the skin by deliberately elongating the holes in the lower skin and longeron and then using oops rivets but that feels like desperation and anyway, the two countersunk screws into the roll-bar would still be off.

Has anybody else seen this and how did you tackle it?

 

[Loman]

Bump

No replies yet, and the post is dropping rapidly out the bottom of the 'Today's Posts' listing. Over 70 post views but nobody knows how to fix it. I'm getting worried now.

 

[BSwayze]

Loman, obviously I haven't crossed this bridge yet but I'm thinking there must be some way to push the plexi from the inside to close this gap. ???

 

[Lionclaw]

I went with a sika'd slider, so my experience is a bit different. That said, were you planning on using any sort of gap for the sika? I put ~1/8" thick spacers in between my windscreen/canopy and the mating surfaces to allow room for the sika.

If you add spacers, maybe the plexi will sit differently?

 

[agirard7a]

tip up

Just glued my 9a tip up main canopy two days ago. (100% sika all around) Have only dry fit the rear section. I agree with Bruce, maybe a wood brace cut to length one side to the other? I saw also that the top skin is a bit loose in that area. I am also glueing the back glass to the roll bar but plan to install screws on the skin. I thought for any unkown future reason the roll bar and back glass needs to be removed it can be with the glass attached to the roll bar.

 

[Loman]

That's more like it. Good Thoughts

Bruce,

Good thinking, but the angle of the plexi is different from the skin angle along this bulge. Here's what is going on (vertical section down through the centre of the bulge, looking back along the right hand side - plexi angle slightly exaggerated):

The plexi is in contact with the skin already - just not at the skin edge. I will be using flexible wooden slips with cushioned ends to brace the plexi out against the skin. The bottoms of these will rest on a Board laid across the longerons

Lionclaw, I have a good 1/8 or more of Sikaflex pretty much everywhere else but I was not planning that kind of depth of fillet here - mainly due to a concern to keep the join smooth and slippy. I think it is the leading edge joint that is most important for strength and a thin Sikaflex joint of very large area should be plenty around the aft edge. After all, the plexi is trapped under the skin here and can't depart the airframe unless it breaks into pieces or the leading edge lets go. Only an inward (downward) blow or a shear force related to thermal expansion will break that joint on the aft edge.

The aft window is less than 2 ft in its fore-aft dimension but is 5 ft or more span-wise so thermal expansion should only really be significant across the pane, driving it's sides down into the baggage hold.

That being said, your basic idea is well worth trying. I will use some 1/16th popsicle stick spacers this evening to see if I can spread out the bulge.

I am still attracted to using the shrinker. I think I can rig up a bracing arrangement so that I can put enough force on the handle

As regards the removal point raised by Agirard7a, I have always thought that a cheese wire, possibly electrically heated, is the way to go, if that ever arises. That's what the automotive windscreen guys do, I think.

Thanks to everyone who has replied so far. The 'long arms' of VAF have been wrapped around me again.

 

[bruceh]

While dry fitting in my rear window, I have the same issue. The window is flush with the skin all around, except for about 6 inches back from the roll bar on each side. I don't think that bending the aluminum will help much -- and will probably make things worse. Pushing the window outward on the aluminum is also just going to set up stresses on the plexi. My amount of overlap with the plexi to the skin is larger than specified (plans call out 5/8", I have just over an inch). I will probably cut down my overlap in this area to 5/8" and hopefully that will reduce the gap. My plan will be to just let the sikaflex fill that small gap. I am not drilling any holes in the window.

Another potentially risky fix would be to apply some heat to that area and see if the plexiglass can be shaped.

 

[agirard7a]

Sand edge

I have used a 120 grit belt sander on the plexi
To fair out slight inconsistencies of the die cut.
Maybe try sanding in a slight angle on the glass.
At least take off the edge. Being that the sika is so ****
Strong, a slightly thinner glass edge should not be a structural
Issue. My 2 cents.

 

[Loman]

I dunno

I'm not sure we are on the right track here.

The plexiglas is perfectly positioned. It is just that 8" of skin that is out of position. If I alter the plexiglas by reducing the overlap or grinding the edge to a matching slant, it will have to be locally pushed out into that bulge (perhaps using braces as Bruce Swayze suggested. Now the Plexiglas is also bulged (if that is even possible - it is fairly rigid).

No, I am convinced that the solution has to focus on the skin material. I either need to shrink the skin locally or try again to spread the bulge out using spacers. My first attempt at that lsecond option ast night did not work at all but maybe I need to try harder. I think I can find a way of using the Shrinker on the skin 'in situ'. I tried it on a scrap last night, putting in a maximum amount of curvature. It needed substantial sanding to get the marks and bumps out but I would not be working the actual skin material anywhere near as much.

For anybody approaching this stage, I would strongly suggest not riveting that top skin on until dry-fitting the aft window so that you know if you have this problem or not. Your options will be much reduced once the skin is riveted on.

 

[RViter]

I'm not sure we are on the right track here.

The plexiglas is perfectly positioned. It is just that 8" of skin that is out of position. If I alter the plexiglas by reducing the overlap or grinding the edge to a matching slant, it will have to be locally pushed out into that bulge (perhaps using braces as Bruce Swayze suggested. Now the Plexiglas is also bulged (if that is even possible - it is fairly rigid).

No, I am convinced that the solution has to focus on the skin material. I either need to shrink the skin locally or try again to spread the bulge out using spacers.

This thread is a bit 'dated' - now about 2+ months old, and no follow-up resolution was posted. I'm putting you in 'my shoes' here and wondering if, like me, you reached an impasse and moved on to greener fields, maybe hoping for some inspiration to distill upon the gray matter. (Been there and after sufficient procrastination - sometimes it works.)

One observation is from the photo - you are clecoed with silver, (#40 holes), and that leaves a little wiggle room for incrementally shifting the two surfaces, as you progressively work to large holes in the match drilling process. I've used it a couple of times with success. The idea is to shift material A to the left and material B to the right, like tectonic plates under the earth's crust. Sometimes the 'gap' appears HUGE, but it doesn't take much misalignment to produce that 1/8" gap if it's incrementally squeezed to the center, for example. A little bit of shifting in the match drilling can relieve a surprising amount when spread over 12-18" either side of the bulge. Without specifically knowing YOUR dimensions, it's just an idea to consider. [I used this once to correct the error when I drilled material A under B on a curved surface, when it should have overlapped the opposite way. Realized the mistake in judgment when I positioned them properly for match-drilling #30 holes. A few outside holes remained a little 'tight' but better than the alternatives.]

Because of the curvature of the plexi INSIDE the skin, any layer of Sika or other sealant between them would seem to benefit by the condition that you face. That and progressively match drilling might make for a near perfect fit.

Just another thought.

I'm currently evaluating Bruce Swayze's Sika solution for the front canopy but might do the rear window, too. Appreciated his documentation on the process he used.

 

[bruceh]

I just completed the bonding of my canopy with Sikaflex, and am about to start work on the rear window. I think in my case I might try to taper the plexiglass just a bit where there is a bulge in order to get it to lay a bit more flush with the skin.

Time to get out the electric sander and some heavy duty grit sandpaper!

 

[Loman]

Good results with edge-shrinker

Sorry guys, as the original poster, I should have given a final report on this issue. It was only because I don't have a photo showing how good the final result is that I hesitated. However, better late than never.

In the end, I followed my instincts (and available tools) and went with the edge-shrinker.

Remember, my top skin was already riveted on, otherwise I would have done this on the bench. I started by turning my fuse on its side (engine not yet fitted), which placed the site of the problem more than three feet above any support structure that I could use to push against while working the lever of the shrinker. To overcome that, I made a 'leg' and secured the shrinker on top with steel banding from a Vans crate.

The thick blue Sharpie marks indicate the beginning and end of where the bulge was. I went as gently as I could, beginning with the full depth of the jaws. I was thinking that I could go back later and put in another row of 'bites' at half depth but that turned out not to be necessary. The first fitting showed that the problem had disappeared and there was now a very slight compound curve beginning at the bottom of the shrinker marks and curving gently toward a perfect contact with the plexi at the skin edge as well as around the window opening - Result!!!. This curve looks as if it was meant to be there, so much so that I can't get a photo that shows it well enough.

The marks left in the skin were quite heavy and I had to use a fine file to dress them out, followed by progressively finer grades of sandpaper and ending up with a scotchbrite pad. Any loss of skin thickness as a result of the dressing is compensated for by the extensive area of the Sika-plexi joint and the strength of the plexi itself. Also, the shrinking action bunches up the edge and therefore thickens it slightly.

If you leave out 'fretting time', this took well short of two hours including making the leg for the shrinker and dressing the skin. The shrinking itself was the work of minutes.

Here's a wider view of that side of the aircraft with the canopy more or less complete. The windscreen fairing has been done since then (worked out great). The board across the longerons in the baggage area is what I used to brace the inside of the aft window against when bonding it to the underside of the skin.

I will try to take a detailed picture showing the final result where the bulge used to be and post it here asap.

RViter, the clecoes are into the longeron, not through the skin and plexi. As I have said, unfortunately the rest of the rivets from the bulkhead back were already set, so the skin was effectively permanently installed

Bruceh, I really feel that altering the plexi is not the solution, unless you are prepared to remove material everywhere except were the bulge is. Even then, I would worry about your proposed action. After all, a new skin is cheaper than a new canopy bubble.

The shrinker is borrowed from our homebuilders society tool library. Every chapter should have one.

 

[cnpeters]

Would not another option be a clamp that reaches around the roll bar and presses some slightly curved forms on either side? The clamp can be made with some wood arms, an end connecting block so you create a u shape device, with a bar or other type clamp applied in the middle of the arms of the U to give force. After all, the skin will be pulled snug in the drilled and screwed method, and doesn't fit perfectly snug til clecoed. The clamp would take minutes to build, plus a few more for the shaped clamping blocks that are on the inside plexi and outer skin.
But kudos to your excellent work with the shrinker.