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From: "pete@leapfrogventures.com" <pete@leapfrogventures.com>
Subject: RE: Figuring out the pressurization system IV-P
Thread-Topic: Figuring out the pressurization system IV-P
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I guess I should have used different words.  In speaking to an engineer at =
Lancair, I was told the plane was tested to 7.5 psi, and that this person s=
uggested a 5.5 psi max differential in operation was perfectly fine.

As for the pop-off valve, I discovered that mine needed to be adjusted.  I =
built a plenum out of an old candy container and pressurized it to calibrat=
e the pop off valve to start releasing at about 5.8 psi.  During my initial=
 testing, the Dukes wasn't operating correctly and was letting the pressuri=
zation climb infinitely.  I also discovered that the pop off valve, even wh=
en calibrated, did not let enough air bleed out to keep the pressurization =
from far exceeding 5.5 psi (I think it got as high as 6.5 psi before I manu=
ally released it with the mixer).  As a result of this, I added a 6 psi ove=
rpressure switch that lights an annunciator on my G900 (which already exist=
ed and was called Cabin Pressure) to warn me if this condition happens agai=
n.  I calibrated the pressure switch using the same plenum I used for the p=
op off valve.  Luckily, since getting my Dukes properly calibrated, I have =
not had an over-pressurization incident.

As for Brent's calculation, apparently the Dukes is designed to allow a spe=
cific volume of air to pass through the cabin at all pressurization setting=
s so as to provide fresh air to the occupants.  To determine the minimum MA=
P needed, this flow rate (essentially a calibrated leakage) needs to be add=
ed to his numbers.  I forget how many CFM it was, but it is a significant n=
umber.

Pete

-----Original Message-----

Pete's statement is mostly true but indicates some misconceptions about pre=
ssurization.

FIrst, the design operating point is 5.0 PSI differential (Inside Pressure =
- Outside Pressure). 5.5 PSI is when the over Pressure Relief valve should =
start to open. The design flow through the cabin is specified by the FAA at=
 10 SCFM per passenger, 40 SCFM for a IV-P.

Second, planes leak air. The pressurization system utilizes a Sonic Venturi=
 to provide air from the Deck (after the turbo but before the
throttle) to the cabin. A Sonic Venturi is a little like a Black Hole in th=
at what happens downstream doesn't much effect what is happening upstream s=
o releasing the door seal does not significantly change the Deck Pressure. =
Deck Pressure is above Manifold Pressure by an amount determined by the Was=
te Gate Controller, but usually runs around 2 InHg.  Regulation of cabin pr=
essure is done by the Outflow Valve which controls how much air leaves the =
cabin. Air can also leave the cabin via air leaks. All planes leak and the =
amount of the leakage varies from one aircraft to another. A perfectly seal=
ed cabin could be pressurized with a bicycle pump, but this would require t=
he smallest pinhole be plugged.=20
In practice, a well sealed IV-P can be pressurised on the ground with a 10H=
p air compressor. I performed a static ground proof test on my plane to 7.5=
 PSI (150% of operating pressure). It took me two days of chasing leaks to =
achieve that pressure.

Third, Cabin Altitude is the absolute pressure altitude of the cabin while =
Cabin Pressure is the differential between inside pressure and outside pres=
sure.

 From all this we can deduce that the manifold pressure needed to maintain =
a specified Cabin Pressure is a function of Altitude and Leakage. At FL280 =
the ambient pressure is 9.75 InHg. 5 PSI is equal to
10.15 InHg so if we assume the condition where the deck pressure is 2 InHg =
over manifold and the Sonic Venture pressure drop is also 2 InHg, the minim=
um needed manifold pressure is about 20 InHg, assuming low leakage. As the =
leakage increases the outflow valve must restrict its flow until at some le=
akage rate the outflow valve is completely closed and can no longer control=
 the cabin pressure. As the leakage rate increases above this point the man=
ifold pressure

In conclusion, every aircraft's leakage rate is different, so the relations=
hip between Cabin Pressure, Manifold Pressure and Altitude will be differen=
t for every aircraft.

Get to know your aircraft with flight testing. You will gain valuable knowl=
edge, skills and become a better pilot.

Regards
Brent Regan