Canopy frame to plexi "best fit"

[Airhead]

I have not found a single discussion on this subject so mine must be a unique situation. I just finished making the first trim of the canopy (7A slider) removing the mold flanges. I set the frame upside down into the plexi looking for the "best fit" position according to instructions, so I can drill the latch hole. There is no best fit ony "less lousy" fit positions.

Here's what I find; the front and rear bows are not a determining factor, as the spacing doesn't change much in the range where a fit is possible.

Therefore, the only factor seems to be the shape of the top center support relative to the plexi. About 2/3 of the way back from the front bow where the center support reverses curvature and has a slight "sag" (concave upward), the plexi has no significant sag to it. The side view drawings in the plans all show this curve, so I don't think my frame is badly off. The farther aft I place the frame, the gap (between frame and plexi) in the sag gets less, but never less than 1/4" and at that point the forward part has high centered on the plexi (rocks back and forth) so no joy here. If I push it toward the forward limit, the front part aligns pretty well but the spacing in the sag part increases to almost 3/8". Anybody delt with this situation before?

[Rick_A]

Bob, I think you will find that fitting the plexi to frame is a moving target... so the "least lousy" position is a good starting point.

My experience was that the plexi seemed to have a mind of it's own. Work gradually to get to the final size. Once you make the big cut, the shape will change again. It seemed to me that the plexi & frame never fit together exactly the same twice in a row!

I just did my best to not pre-load stress on the plexi when I fastened it down. I tended to drill the holes oversize to give the plexi a little room to move to allow for temperature changes.

I also did a fiberglass rear skirt.

[Airhead]

Thanks for the reply Rick. At the encourgement of another builder, I am thinking of going with Sikaflex and a FG one piece skirt all around, even though I have no experience with FG layups. To that end, I went out this morning and got some plastic tubing, made spacers, and inserted them at various points between the frame and plexi. That changed the fit between the top center tube and plexi but still a little too much space at the previously mentioned aft "sag" point. My hope is that when I lay the plexi on top of the frame, gravity will help out.

The thing I am most worried about is positioning the frame too far forward, making the big cut and not having enough fior the windscreen portion. I wish Van's would give a little more dimensional guidance here even if it was a plus/minus dimension.