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Re: Airspeed sensing switch: Pressurization
(IV-P)
I have flown with the Duke's Valve completely occluded via duct
tape fully across the rear seat opening (for several month, in fact,
part fo an experiment). I was ready to puncture the tape if
needed. The cabin did not over pressurize, but it would have
above ~13,000' to ~15,000' during power climb. The initial slam
of higher pressure during power up and take off was noticible, but not
in excess of 3psi (it was just immediately noted). Over
pressurization at anytime was avoided with a controlled reduction of
the throttle (MAP) to demonstrate cabin pressure correlation to MAP.
Upon landing, with outflow still occluded, cabin was always at airport
ambient pressure before door seal was dropped. Again, this zero
differential is notable on short final, door seal tight, Duke's outlet
occluded, engine below 11.5".
During flight, outlet occluded, a small puncture through the
occluding tape demonstrated brisk exiting air through the puncture out
the Duke's Valve, but without a rapid cabin decompression (like one
might experience with door seal deflation). At any altitude,
door seal deflation results in immediate cabin decompression and cabin
climb to current flying altitude.
I also have a cabin alert above 10,000' and panel light
notification.
Jeff L
Of course I
wasn't suggesting manually blocking the outflow valve. I'm
saying you should consider all failure modes in the
analysis.
Based on the
evidence presented so far, I think I'm convinced that you can't get
the plane stopped and have pressure left in the cabin, even if the
outflow valve were blocked.
Based on the
evidence presented so far, I'm not convinced it's impossible to land
and have a little differential, given all failure
modes.
anyway,
thanks for the info. that's interesting about the mvp-50
pressure sensor.
----- Original Message -----
From: Robert
Pastusek
To: lml@lancaironline.net
Sent: Monday, February 01, 2010 7:18
PM
Subject: [LML] Re: Airspeed sensing switch:
Pressurization (IV-P)
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.
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