Colyn,
Yes, to pressurize the cockpit, upper deck pressure must exceed
ambient. I THINK (but have not verified) that the pressurization controller
will close completely to achieve the commanded cabin altitude. If anyone knows
about this, please advise. I know there are some small leaks in my cabin that
provide some air exchange, but they are not very large. I say this because I had
a piece of masking tape over a ¼” shoulder harness attachment hole that
dumped into the baggage compartment (don’t ask!). The tape blew through
the hole one day, and in addition to the whistle (Judy said SHRIEK), the cabin
differential pressure dropped to less than 1 PSI…because of the leak.
Power was at low cruise (60% or so) so I might have done better with more
power, but I was surprised that a ¼” hole effectively kept the cabin from
pressurizing…
I don’t know about the results of plugging the outflow
valve, but it may be academic? At least theoretically, a pressure differential
can be generated from any ambient (outside) pressure, but not sure how, or why
one would try plugging the Dukes controler outlet to do this? I actually
covered and sealed the whole pressure “bucket” in the rear seat when
I was looking for leaks, but my input was from the vacuum cleaner. As this
effectively blocks the emergency dump valve as well, I’d not consider
doing this with another pressure source, and particularly not with the engine,
nor in-flight…
On the “failure condition” you ask about: I have a
cabin pressure sensor hooked to my MVP-50 engine monitoring system. It shows
cabin altitude in feet, and is alarmed to annunciate a cabin altitude below
-500’ (overpressure on the ground—but only at sea level) and a
cabin altitude above 10,000’. I’ve only seen it alarm during
intentional tests, but any discrepancy between the cabin altitude displayed on the
MVP-50 and what’s set on the Dukes controller would indicate a
failure/problem, noting that this will occur when the differential gets to 5.5
and the aircraft continues to climb. BTW, I don’t have a separate cabin
altitude gauge…the MVP works well for this, AND can be alarmed.
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 ½” 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?