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Re: Airspeed sensing switch: Pressurization
(IV-P)
Specific to the question of puncturing the occluding tape over
the Duke outflow opening:
As expected, upon puncture in level flight, air could be felt
exiting through the puncture opening for a short period ot ftime.
This supported the idea that the controller was in a relatively open
position BEFORE the pucture, trying to vent the cabin more than what
was allowed, as commanded by the controller. Once pressurization
behavior was restored and the controller commanded a tighter Duke's
bellow, the outflow decreased through the pucture.
The cabin was at a higher presssure differential before the
pucture, the pucture resulted in a drop in cabin pressure (and ear
pop), but not a complete loss of pressurization, and then normal
pressurization behavoir was demonstrated.
I beleive that when the door seal is dropped and cabin pressure
differential goes to zero, the Duke's valve is fully closed as it
tries to retain cabin pressure, not aware what the door seal is doing
for the problem. This is different from the Cabin Dump switch,
which forces the Dukes valve full open.
Jeff
Thanks for
the info Jeff. Did you happen to note what MAP it takes to get
ANY differential?
Can you
say more about the "puncture tape during flight"
situation? Does this mean you could not hold AS MUCH cabin
differential with the tape punctured?
Does it
also mean the dukes valve never fully closes?
b.t.w. - I
rechecked the pressure chart which shows that even at an extremely
high airport, ambient pressure is well over 20" Hg.
so I'll take back that you can actually get it on the ground with any
differential.
----- Original Message -----
From: Jeffrey Liegner,
MD
To: lml@lancaironline.net
Sent: Wednesday, February 03, 2010 7:42
AM
Subject: [LML] 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
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