Dico wrote:
Hello all,
I was looking through the TSIO-550 Power Settings Chart and
noticed that it said the min MP for pressurization was
31.5".... I'm just wondering if this is true for all IVPs?
I was up to FL170 the other day at 28.5" and the cabin was
showing perhaps 7800. Should I assume that if I was running
at 31.5" that the cabin might only have been showing perhaps
5000'? The reason I ask is because I'm thinking if we're at
FL170 and the cabin is pushing 8000, then at FL250 (or even
FL210) the cabin pressure would be well over 10,000'. I had
thought it might have been because I have a leaky cabin or
something.... but perhaps its because I'm being too cheap with
the fuel....
Dico,
The Lancair IV-P is designed
to operate at a maximum differential pressure (pressure
difference between the cabin interior and outside) of 5 PSI.
Your airplane has a Duke’s pressurization/control system
that allows you to set the cabin altitude to a specified
value. Most are calibrated to generate a cabin altitude of
1,000-1,500’ above the set altitude—for a couple of reasons.
In any case, the system will allow pressurized air from the
turbochargers to flow through the cabin until the inside
pressure altitude reaches about 1,000’ above what’s set on
the control. It then starts closing the outflow valve under
the rear seat to maintain this inside pressure altitude. As
long as there is a sufficient supply of pressurized air from
the
turbos, the controller will
automatically close/modulate as needed to maintain this
altitude-- until the inside/outside difference reaches 5
PSI. The cabin pressure then starts to decrease (higher
cabin altitude) as you continue to climb. I’ve attached a
chart that shows that with a 5 PSI differential system, you
should be able to maintain an 8,000’ cabin altitude to about
23,000’ PA and that the cabin should be about 10,000’ at FL
250. Because of leakage, inefficiencies and calibration
settings , the cabin altitude starts to go up as the
airplane climbs through FL 21-220---for most of us.
There are two big factors
that govern how well this works—supply pressure/volume and
cabin leakage. A high engine power setting generates a lot
of high-pressure air from the turbos.
This is moderated somewhat by the sonic nozzles in the hot
side feed (some airplanes were built with sonic nozzles in
both the hot and cold feed lines; some did not do this), but
you can definitely tell a difference in airflow through the
cabin between takeoff and cruise power settings.
If the cabin is leaky, even
max power (38.5” boost pressure) won’t generate enough high
pressure air to pressurize the cabin (e.g., you forgot to
activate the door seal). If it’s a tight cabin, 25” of MP is
about the minimum that will maintain the 5 PSI
difference…and since there are not anti-back flow valves in
this system, pulling the power below this will cause
pressurized cabin air to flow back through the engine—rather
rapidly.
To answer your specific
question, you should be able to maintain a cabin altitude of
about 4500’ IF your altitude controller was set below 3,000’
AND your cabin is tight. You can easily check for a
calibration or leakage problem by stabilizing at 17,500’ in
cruise with the altitude controller set to 0—or at least
below 3,000’--and check the cabin pressure. Then increase MP
by at least 2” of Hg and again check the cabin pressure. If
it increases (cabin altitude decreases), you can improve the
cabin pressure by sealing leaks; if it doesn’t change, you
can improve the cabin pressure by adjusting the pressure
controller.
If you can’t maintain cabin
pressure at 25-26” of MP, consider working on the cabin
sealing. Requiring 31” of MP to maintain cabin pressure
indicates leaks to me.
Bob