| Hi Paul,
The rate control on the cabin panel controller just changes the speed of the electric motor in the outflow valve. There is no vacuum connection to the cabin panel controller. The electric motor in the outflow valve has a very large gear reduction which slowly changes the set point altitude for the onset of cabin pressurization based on the panel controller setting.
The "vaccum source for differential control" is still in place in the form of a tube from the controller to the atmosphere located in the tub that holds the outflow valve. A second vacuum connection is provided by Dukes which provides enough force to fully open the outflow valve when rapid venting of the cabin is desired. Activating the dump valve solenoid in the outflow valve admits the vaccum to the outflow bellows fully opening the valve. Without the vacuum applied and the dump valve activated, the outflow valve only partially opens when below the set altitude.
Many of the Pressurized Lancairs weather new or upgraded no longer need vaccum for the instruments which raises the issue of providing a vacuum source to the controller. Since releasing the door seal is a very effective way to depressurize the cabin, most owners have elected to tolerate the minor pressure bump on take off with a rapidly advanced throttle caused by the partially opened valve. Per Dukes there is no other undesirable side effect from not providing the vacuum.
Hope this helps,
Jack Morgan On Oct 15, 2013, at 6:01 AM, Lancair Mailing List wrote: Subject: Re: [LML] Re: Removing vacuum pump? (LIVP)
Date: October 14, 2013 8:14:11 AM EDT
If the pressure controller on the panel has rate of climb and cabin altitude settings (like a Garrett), it likely uses the vacuum line to modulate the outflow valves (pull them open). That modulation is what provides the signal to allow the cabin to climb or descend. I don't know what controller is in place but the vacuum is for cabin differential control. The squat switch is for ground use or emergency and simple opens the outflow to a maximum. In a typical situation, the outflow closes with wheels up then the vacuum signal starts to open the valves to provide the differential required to either climb or hold the cabin as set by the controller.
If the vacuum is disconnected, I suspect you simply are running with a fully closed outflow valve and the cabin pressure is an open loop with whatever pressure is supplied from the firewall. Does that sound about right?
What I am missing is if there is no vacuum line then why install the controller? How does it control anything? Those controllers can't be cheap.
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