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Re: Airspeed sensing switch: Pressurization
so how does the math work here?
If you land at sea level, you would have had to
tell your pressure controller to shoot for below sea level for it to be making
any cabin differential no matter what your engine setting is.
right?
So let's say you land at 9000 feet.
So I would have to be holding 22" or better on
final for that to be an issue, right?
Now let's say you are cruising at
25,000.
Ambient there is 11.12 in. Hg.
If you would like a 8000' cabin you need 22.23 in
the cabin.
and maybe you are running at 32" in.
MP. So there's about 20 inches differential available. and you
only need 10"
So a perfectly efficient system with no flow
through the cabin would still function at 22" MP.
So my real question: Is the reason you
need about 28" just that that's what it happens to take to get enough volume of
air moving to support the cabin flow rate?
----- Original Message -----
Sent: Sunday, January 24, 2010 6:07
PM
Subject: [LML] Re: Airspeed sensing
switch: Pressurization
I've never appreciated the value of this pressurization feature in our
LIVP planes. The recommendation is to wire the Dump Valve to the
airspeed switch according to the description below, but I did not, and see no
value in doing so. In fact, if airspeed is artifically reduced from
pitot ice or some failed pitot tubing at one of the couplings, you will still
want to maintain pressurization as the engine continues to hum along int he
flight levels.
I figure that the turbos provide pressurized air, and when the throttle
and MAP drop below pressurization levels (let's say 28", depending on
altitude) the cabin air will vent quickly either through porous sources or
back out through the mixing box and back into the upper deck.
Either way, when you reduce throttle to something below ambient pressure,
like when you're landing, the cabin will normalize with the outside air
pressure long before you arrive abeam the numbers on downwind. This
action does not require an airspeed switch to then open the Duke's Valve cabin
dump.
To restate my observation, in our planes, the way they are built, there
is never a time when you can or will hold pressurization on the ground after
you land, however you set up this auto cabin dump feature (in my case, totally
disabled). Other planes have one way pressurization valves, but we do
not. The mixing box input hole (bringing pressurized turbo air in) is
the biggest 1" hole in the cabin to vent air back into the upper deck when
cabin pressure exceeds mainfold pressure.
Jeff L
LIVP
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