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Re: [LML] Re: Airspeed sensing switch: Pressurization
Bob,
ok I get that.
I'm trying to understand abnormal operation in a
little more detail.
So I think concerning normal operation. two things
are true:
1) upper deck pressure must exceed
ambient
2) there's enough volume per second to overcome the
leaks and valve exhaust in the cabin.
You say you can't make any cabin pressure below 24"
@16000
My guess is the cabin pressure exhaust
valve never closes in normal operatoin because it has to meet the requirement
total air exchange every n seconds ( I forget the constant).
Now if you plug that thing, you would get pressure at some lower map,
right? I wonder what that is?
I also wonder how you would typically notice that
failure condition.
----- Original Message -----
Sent: Saturday, January 30, 2010 2:41
PM
Subject: [LML] Re: Airspeed sensing
switch: Pressurization
Colyn,
I’ve
not commented on this before, but for the Lancair IV-P using the
TSIO-550/turbo to pressurize the cabin (and this may not apply to other
configurations), it is not possible to maintain cabin pressure when the engine
is not producing power, or is off. The cabin is connected to the engine intake
system by a 1 ˝” duct/controller that can only be turned off by dumping it to
the ambient atmosphere. The result is that the cabin very quickly goes to
engine intake or ambient pressure when power is reduced sufficiently to spool
down the turbos. You can test this for yourself by pulling power to idle when
pressurized and timing how long it takes to “back-flow” the cabin air through
the intake system…there is not a one-way valve in this system. In my
experience, the time required to reach ambient outside pressure is a few
seconds; certainly less than a minute at the max pressure differential of 5.5
PSI.
Again,
other aircraft/setups may work differently, but for the IV-P with a TSIO-550
built to plans, I can’t see any way for the cabin to be pressurized after
landing. Other comments?
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