CWFMD,
I’d need more info to diagnose this completely, but write me directly and I’ll try to help.
rpastusek@htii.com
Some basics on the pressurization system (applies to the IV-P with Continental TSIO-550 engine ONLY--you didn’t specify
the aircraft/engine):
The engine is designed to “vent” part of the turbocharger output overboard during normal operation, through what are
called “sonic nozzles” that basically maintain a rather constant flow rate over a range of pressure differentials. The IV-P (and some other pressurized
Lancairs with big bore Continentals) uses this bleed air to pressurize the cockpit. The air is routed through a mixer/control box mounted near the top center of the firewall in the engine compartment. This gold-colored aluminum can
(about 2/3 the height of a quart oil can) has a cockpit-controlled shuttle valve that allows selection of a mix of hot air directly from the turbochargers and cooler air that has already passed through the intercoolers.
It also has a separate valve, with cockpit control, that shuts off airflow to the cabin and diverts it out the bottom of the engine compartment when cockpit pressurization (and associated heat) is not needed.
As to heat, at full throttle, the turbocharger output air temperature can be up to 300 degrees, and in the cabin heat
on mode, flows pretty directly into the cabin. With the heat control turned off, the turbocharger air passes through a pair of air-to-air intercoolers before heading to the cockpit for pressurization. The problem is that these intercoolers only lower the temperature;
they don’t deliver “cold” air out the backside. When the input is at 300, the output is warm, at best; hot in Texas in the summer. So, without air conditioning, there is not a good source of really cool air available to pressurize the cockpit.
As to the fixes: First check is to be sure the valves in the controller are functioning as intended.
Second check is to be sure you have sonic ports installed in your turbocharger output lines that feed to this controller. The variation you report in MP would tend to indicate there are no sonic ports installed, and that
you’re getting “full flow” through the system, rather than restricted flow as intended…but this is just a guess at this point… not enough data. Talk or write?
Bob
From: Lancair
Mailing List [mailto:lml@lancaironline.net] On Behalf Of cwfmd@yahoo.com
Sent: Tuesday, December 03, 2013 7:01 AM
To: lml@lancaironline.net
Subject: [LML] cabin pressure valve vs manifold pressure drop
We put a new canopy seal in the IV-P. I fiddled with the cabin pressure valve to try to reduce or locate a squeal. (My Grainger
smoke matches don't light, with reduced partial pressure of Oxygen at altitude) I noticed that pulling the cabin pressure control partially out, reduced the cabin inflow, as expected, but also dropped the manifold pressure by about 4 inches with a loss of
about 50 hp (estimated). This was at flight level 230. I had not previously noticed this large effect on manifold pressure, but it could demystify performance changes in previous data. Who knew the cabin pressure valve affected the "throttle" setting :) It
makes sense because the previous cabin pressure just dumps overboard at the firewall. Wondering why this is set up this way.... Why not just close the valve to the cabin, when not needed or wanted, and leave the manifold pressure in the manifold??
In general, I noticed others have similar issues with the cost of repair of the Dukes valve. I would rather redesign the system,
using the on-board instrumentation and computers that already know the static pressure and the cabin pressure. I hot climates I prefer to minimize the heat entering the cabin, and maybe just use 4-5 psi relief valves to distribute air flow. I think my system
where all the inter-cooled air routes thru the firewall to under the back seat was designed by Eskimos, who always need max heating.