Message-ID: <3908380F.61A4A2A4@regandesigns.com>
Date: Thu, 27 Apr 2000 07:52:31 -0500
From: Brent Regan <brent@regandesigns.com>
To: Lancair List <lancair.list@olsusa.com>
Subject: Re: LNC IV-P window blow-out
Mime-Version: 1.0

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I have a little advice for IV-P builders/owners out there regarding the
cabin door window. My comments are based on my personal experience and
common sense. Unfortunately, I am an idiot so you must discount everything I
say and figure things out for yourself.

Here we go:
The windows in the IV-P are like the skin of a balloon, tough and flexible,
but if you apply the right stress at the right place they will rupture. Any
surface imperfection in a high stress area will concentrate the stresses and
provide the starting point for a crack.

The forward latch has three kinds of stress applied to it. Tension along the
axis of the latch, shear between the latch base and the door and torsion
about the forward edge of the latch base. I know this because I was able to
see it when I ground pressurized both planes (Derek Hine's and mine). IMHO
tension and torsion stresses are acting on small cracks that were formed
when the window was drilled for the latch bolts. The stress causes the
cracks to grow. SB040-1197 addresses this.

Some advice that that is worth repeating:

1) Acrylic is extremely sensitive to some solvents.  The accelerant used for
instant glue will cause cracks to spontaneously form on a cut edge. Keep all
solvents away.

2) Drilling acrylic is a tricky matter. Prior to using a drill bit on
acrylic "dull" it by grinding a negative rake (30-45 degrees by 0.03+- wide)
into the cutting edge. Practice on some scrap pieces. Examine them closely
for cracks on the far side of the hole. Don't drill the window until you get
it right.

3) I used carbon cloth on the door instead of glass and flox. I used carbon
around the window frame (same bid schedule) and used carbon to build up
under the latches. Flox was limited to 1/8 inch or less.

4) Ground pressure test to 7.5 PSI. I know the factory recommends 6.5 psi
but I also know of two lower pressure bulkheads that have failed (due to a
manufacturing defect) at just over 6.5 psi. I have talked to a couple of
builders who are flying pressurized without having first ground testing
their airframe or have attempted to test it and failed to get to the test
pressure. To any IV-P pilot out there who has done this I have one question.
Are you freaking nuts?

FAR 23.1428 calls for proof pressure testing to 1.5 times the normal
operating pressure. This is not a requirement for the builder but I think it
is a good idea. Perform the pressure test prior to painting the airframe.

If your airframe isn't sealed well enough to perform a ground pressure test
then it isn't sealed well enough to allow the cabin controller to work
properly. Spend the time and do it right. I used a 7.5 Hp compressor on a
200 gallon tank. I was able to hold 7.5 psi for about a minute before I ran
out of stored air in the tank.

I do not believe that the door design is defective. The prototype did
survive cycle and burst testing. I do believe that the door is particularly
sensitive to the skill of the builder and that a seemingly inconsequential
defect can lead to a catastrophic failure.

Regards
Brent Regan

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