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RV-8 fastback canopy question

scsmith

Well Known Member
I am thinking ahead to a repeat offense. Premeditation, if you will.

I have a question about the shape and fit of the canopy on a fastback.

When I look at my standard canopy and look at the cross section at the station where the fastback canopy frame/skirt would attach at the back seat bulkhead, the standard canopy has, well to use a naval architecture term, tumblehome. This means that the canopy contour is wider up away from the bottom edge of the skirt, and is getting narrower along the bottom edge. Said another way, the canopy bulges out beyond the skirt.

The Showplanes website says their kit uses a standard canopy, yet the shape of the bulkhead at the rear seat back does not fit this tumblehome shape. It is widest at the bottom, and has a normal oval shape.

So, it would seem that fitting a standard canopy to the fastback skirt and fuselage shape would require flexing the canopy to fit that shape. This concerns me because it puts a bunch of pre-stress into the canopy.

When I fit my standard canopy, I took care to shape the canopy frame so that it fits the canopy in its completely relaxed state - no built-in stresses. That plus Sikaflex and I think/hope it will never crack. I would be really worried that forcing the canopy into a different shape would be asking for trouble.

Thoughts? DanH? Scard? Scott Hersha? Others?
 
Thoughts? DanH? Scard? Scott Hersha? Others?

I am none of those people and I haven't done a show planes canopy installation, I am familiar with what the canopy does once it has been cut.

When the rear portion that will not be used is removed they canopy gets very flexible in that area and should easily adapt to the different shape without inducing any undo stress.

You may remember this with your current RV-8, when the fwd windscreen portion was removed from the main canopy.
 
There is no "pre-stress" at all in my canopy and it doesn't "bulge" anywhere. Mine is a Todd's canopy, but I suspect is quite similar in shape to the standard.
 
I'm using a standard Vans canopy, circa 2006. It has been glued into that glass frame 10 years or more, no issues.

As Stu suggested, I think a lot of the tumblehome gets cut off. That said, I cut mine tall.
 
As far as i remember, the canopy (standard Van?s) is thermoformed like a bath-tub and ends with a 90 degree angle to the edge (which is supposed to be cut of anyway).
Therefore, to create this (standard, according to plans -8 slider canopy) ?double bubble? / F-16 like / bulged canopy, the lower outer canopy edges are pushed inwards to meet the form of the canopy-frame. Thus creating a pre-stress on the canopy.
This doesn?t cause too many problems as over a thousand RV-8?s are out there flying and only once in a while you see posts about cracks.

I would figure that, if the showplanes/fastback canopy lower end is vertical (not bulged in), that there is no pre-stress on the canopy whatsoever.
 
Tumblehome reshape option

Steve,
Jeff at airplane plastic will stress relieve the plex to conform to your rear frame or bulkhead shape. Yon need a wooden form and a way to clamp the plex against it. He heats to about 180F and the plastic conforms when cool.
Just one option... Larry
 
So, is the bottom edge of the exposed canopy higher than the stock slider canopy?

Now that you ask, I don't think so.

Nothing handy to compare.

I assume the tumblehome observation comes from your existing slider? None of my photos show any at the rear of this canopy.
 
Steve,
When I fit my canopy on the Showplanes frame, I slid it forward to get the shape I wanted. I was looking for a little more slope at the front and that resulted in a little less slope at the back. As others have said, when you trim off the bottom edge and then trim the back portion where you want it, the canopy becomes quite flexible. If you do what I did, be mindful of canopy height. If you reposition it too far forward (before trimming) you will loose some headroom if you go too far. The standard Vans canopy is quite a bit longer than what you will need so there is room to adjust it where you want it. Mine was glued on in 2007 with a thickened structural epoxy from the Gougeon Brothers (West System) called ProSet. It has a fairly long pot life, giving you plenty of time to work with it when fitting before it sets up. It is compatible with all West System epoxies and will wet out cloth also if needed while doing the install. Spaceship 1 was glued together with this stuff...
 
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