Message-ID: <390C3A87.FD372B79@regandesigns.com>
Date: Sun, 30 Apr 2000 08:52:07 -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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John, thanks for the kind words. To attach the air supply to the cabin, I
made a plastic 1.25" OD plastic plug with a 3/8" female pipe thread. I
clamped the plug onto the hose that would normally go the the sonic venturi
and threaded in an air fitting. I also brought out the door seal pressure
line so that I could control the door seal. It took about a day and a half
to find all the leaks (which were mostly around the rear seat bottom panel).

I was one of the first IVs to fly pressurized (#4 I think) but I didn't want
to wait till after the 40 hour fly off to go to Redmond to do the
pressurization test and get my cabin controller. That is why I did the test
in my workshop, video taped it and sent the tape to Lancair so that they
would send me the cabin controller.

Regarding the cabin differential, I may have confused you with my terms. I
pressure tested the cabin to 7.5 PSI differential or 22.2 PSI absolute
(referenced to a vacuum). The ambient pressure at FL300 is 4.37 PSI. Add a 5
PSI differential pressure to this and you get 9.37 PSI cabin pressure or a
cabin altitude of about 12,000 feet. The safety pressure valve limits the
cabin differential pressure to 5.5 PSI (aka 5.5 PSID).

On the topic of pressurization, there is a misconception that the IV
requires a dump valve to bleed off any residual cabin pressurization upon
landing. This is not the case as the cabin pressure is always already at
ambient pressure when you go to open the hatch. There are two mechanisms
already in place that ensure this. First, the sonic venturie is basically a
0.55" diameter hole between the cabin and the "deck" (the manifold between
the turbo and the throttle butterfly). There is no reverse flow check valve.
When the engine stops the deck pressure and therefore the cabin pressure
quickly equalize to the ambient pressure. Second, due to mechanical loads,
you can't actuate the latch until the door seal is deflated. Deflating the
door deal will depressurize the cabin from 5 PSID in less than 3 seconds.

The one thing that the dump valve function on the Dukes controller does give
you is the ability to fully open the outflow regulator during takeoff
(assuming you have a vacuum source connected to it), reducing discomfort
caused by rapid cabin pressure fluctuations.

Regards
Brent Regan

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