The incredible shrinking/expanding canopy skirt

I recently trimmed, glassed and made fit against the fuselage skin (pretty nicely) my RV-8 canopy skirt. As is typically the case, the "make it fit" effort was focused mostly along the sides toward the rear of the canopy. I got a nice, clean and relatively tight fit. That was in about the September/October time frame.

Then, a week or two ago I went to slide the canopy closed on the fuse in a cold-ish hangar, in which the propane heater had been on and running for about 25 minutes. When it closed, I saw there was about a 1/8 to 3/16 gap along the sides, basically from the rear seat-back area back a couple feet, in the area where the skirt curves up toward the rear. The right was slightly prouder than the left side. I opened and closed it a few times, thinking maybe it just wasn't seating right. I cussed and stamped my feet and whatnot for a bit. Then I just took a deep breath and started pushing on the skirt sides, and determined that if the "crossbow" portion of the canopy where the slider block attached had just a slightly smaller radius, it would almost certainly fit just right.

The hangar heater's on a timer. It puts out a lot of heat when its running. I kept it running (it was cold out and I'm a cold wimp) and kept messing with the skirt gap, trying to decide if I should grab a pipe bender and tweak that rear bow every so slightly so as to reduce the radius and pull the sides in about 1/8 to 3/16.

Eventually I'd had enough of that and went on to do some other work. The heater shuts off because I let the timer run out and I needed to run up to the restroom (yeah, TMI). When I come back to the hangar, it's a cooled off a bit. I start to walk toward the heater timer and happen to glance at the closed canopy as I walk by. WAIT, WHAT?? It's shrunk back in, narrower gap. Apparently cold means shrinkage. Go figure. I waited a bit, wanting to be sure it wasn't my mind playing hopeful tricks on me, and did the metaphorical opposite of watching water boil: As the temperature dropped, the gap closed up. Once it gets cold enough, it once again fits pretty much perfectly.

So, now I am wondering what (if anything) I should do to tighten things up. If I tighten up the radius represented by that canopy frame bow I can bring the sides in slightly and make it fit tighter. If it's warm, okay fit and if it's cold it would be a bit of an interference fit (fit even tighter) and get sprung out a little. In theory. My canopy is sika glued, and so is the skirt.

Anyone else solve this problem specifically? I searched VAF and found a variety of example posts related to closing up gaps with various materials, and I already have a plan for stopping any errant air. But I do want to get the fit on this expansion/contraction things as close as possible before employing any sort of gap filling materials. Anything else I am missing?

Thanks!

Sorry I don't have anything to add, but I am very interested in any responses you get, so this post gives me an opportunity to subscribe to the thread.

I would love to bend the front bow on my 8 about 1/4", but I assumed given the springiness of the frame, that you could not bend the frame with the canopy and skirt on far enough for the bend to take, and have the plexi survive.

Thermal propertys

It has been a long time now that I went through this but the metal in the airframe and the plastics used in the construction have a wide difference in their rate of shrinkage do to temperature chances. We all see the aft edge of the canopy lift up some in winter. Most use some rubber seal around the base of the skirt to keep the cold air off the back of our necks. I would have to go back to some book to see how much that change is, but bottom line you really don't want to get it to tight. It need to breath like an exspantion joint in a highway bridge. This is why you oversize the screw holes in the plastic if you use screws or rivets to attach the canopy to the frame. Hope this helps, Yours, R.E.A. III #80888

Yeah, thinking about this more, I am sure that not only do they expand and shrink in different amounts, but also at different rates. Maybe a cold (say 40 degrees F) aluminum fuselage compared to a warmed up canopy is a different fit compared to the same fuselage and canopy when the outside temp is 70 degrees, for example. Hmm...

Any ideas anyone has about best overall fit year-round would be interesting to hear! I don't plan to do much about it yet. Will probably watch it and see how it behaves in different temps. Even with the gap at its worst point the other day it's still tighter than some skirts I have seen. I just really prefer the tight, clean fit.

Greg,

As stated in this thread, the expansion/contraction of the canopy is common.
I encountered this in very similar conditions on my first build. while fitting during winter months. I had a near perfect fit at the end of the day. Hangar was warmed up, even had a space heater in the fuselage to keep the plastic warm while working it. Shut the heat down and went home feeling really good about my workmanship.

Next day (temp in the teens) came back to a cold hangar and the skirt was 3/8 inch off the turtle deck. While still in disbelief turned the heater back on. Went to get some hot coffee, returned and the fit was tight again.

It amazed me how much expansion/contraction the plastic exhibits with temperature change. It occurs in every -8 I know. There are about 6 at HAO and at least a dozen in the near surrounding area. Everyone has a larger gap at the duck tail in winter.

Some carry a bath towel the pax can tuck in to cut the draft. Others try thick soft foam. I heard of someone crafting an inflatable seal.

As far as best temp to fit. The plastic needs to be warm while working. The brittle nature while cold invites cracking while cutting, drilling, and riveting. So there is not much room to change that. In my mind it reinforces the need for oversized rivet holes, per Vans recommendations. Keeping the plastic in the 70 - 80 degree range seems to work well.

Good luck!
Keep Riveting!

Useful feedback and info - thanks. When I fit and assembled the canopy bubble/rail/skirt it was late summer - much warmer weather and consistently so. Sounds like I should be patient and let things be for now and see what it's like in the spring. My goal is to get it flying sometime next year, so not like I have an active draft bugging back-seaters just yet.

Adendum

Just a tip for those that are still building. You may want to fit in the med point of you areas temp. range. Are as we did close in a small area and control the temp. so everything is normalized to the med range calibrated to the center of the curve so to say. Just what we did, Yours, R.E.A. III #80888

All temperature gap seal

I am struggling with the same thing on my -3 with Todds canopy. Fits perfect in my 68F shop and leaks badly in the rear in cold weather. I have had some success with applying a light weight vinyl cloth to the inside if the skirt so that air pressure trying to get inside then seals the flexible fabric against the fuse skin. This avoids serious bending moments on the canopy structure from foam compression when the clearance is small. Just something to add to your options.

Its been quite a while since I did this, but I recall taking advantage of the slight flex in the fiberglass tighten up the fit. Basically you can build in a bit of extra curvature that snugs up against the fuselage. I spent a ridiculous number of hours making small adjustments but the end result was that there is little or no gap along the sides in either warm or cold weather. The gap at the back end is another matter; I don't think there is a cure for that other than artful weatherstripping (mine lifts up 1/4" or so on cold winter flights).

You're looking at the effect of four different thermal expansion rates. The coefficient of thermal expansion (CTE) for plexiglass is about 6-1/2 times more than the steel frame. The CTE for aluminum is about double that of steel. The epoxy/glass CTE is fuzzy, but pretty close to that of steel.

I made the same experience when fitting the canopy skirts this year during summer. While the temperature change was not that much I realized different gaps at different times. I just filled up the skirts on the inside after the fit was not bad at all. Now - when flying the canopy lifted at the rear may be 1/4 inch. I glued some soft rubber on the inside of the skirts and meanwhile the soft rubber goes forward where there is no visible gap protecting the paint on the fuse. I am very satisfied with the fit after 40 hours. When opening the canopy after flying high in the cold the rollers have a little pressure towards the inside obviously because the canopy contracted.
I think even after the first flights the skirts can be trimmed if necessary and also repainted off the airplane.