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Date: Tue, 04 May 2010 16:23:19 -0400
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From: Brent Regan <brent@regandesigns.com>
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Subject: Re: Cabin Door Window-IV-P
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Replacing the windows from the outside is the common practice and much 
easier than attempting a replacement from the inside. This is especially 
true for the door due to the stiffener and latch geometry and 
placement.  I too had a bit of trouble accepting the concept but after 
some thought and analysis it became clear that replacing the window from 
the outside was the preferred option.

The reason for original installation of the windows from the inside is 
less a function of strength and more a function of labor and cosmetics. 
Imagine the sanding work needed to fair in all the windows and the high 
chance of slipping with some 60 grit. Ouch!

Remember that the entire fuselage is a series of layers that are glued 
together so it becomes a question of if you can get reasonably close to 
the prepreg bond strength with a wet layup. A quick analysis shows that 
the window area is about 400 square inches. If we assume a 10 PSI burst 
pressure we get 4,000 lbs force. The window bond perimeter is about 100 
inches so the stress is about 40 pounds per linear inch of bond, which 
is about the peel strength. But the loading is cantilever, not pure 
peel, and the tensile strength for Jeffco (now Rhino) 1307LV is better 
than 2 KSI. Bottom line, no problem.

The trick is to grind the door down about 0.060" around the window and 
taper it out about 4" so that when the 6 bid carbon is applied the 
original thickness is preserved. I made patterns to cut the wet layups 
to shape to minimize the work around the window edge. Once the 6 bid is 
on you can taper it back. The bid layers leave lines like a topo map so 
it is easy to get an even taper BUT, this is NOT a repair for an epoxy 
newbie.

The BIG advantage of replacing the door window from the outside is that 
you don't mess with the door shape or latches. When my friend blew a 
window climbing over Colorado he landed in a remote town. He removed the 
door and shipped it to me. I was able to replace the window,  paint the 
outside of the door in primer, reassemble and adjust the latches, pack 
and ship it to Colorado in less than a week. He was able to install the 
door in about an hour and flew the plane home.....pressurized! Once home 
he removed the door and took it to the paint shop. Now you can't tell 
any work was done. You have to love composites.

Regards
Brent Regan