If I can add something here based on the TSIO-520 experience, the turbos are controlled to provide a positive differential in the upper deck and that is higher than MAP and higher than ambient and generally references absolute pressure.  I know the TSIO-550 uses the slope controller but the differentials I think are basically maintained.   So, the upper deck is always higher than the MAP and always shoving large amounts of air into the cabin through the sonic venturi (I think the 4P has similar plumbing) and the pressure differential in the cabin is controlled only by the leaks, either the outflow valves and/or cabin leakage.  The reason I twigged to this thread was my personal experience in other aircraft and reading about Jeff's disconnect on the airspeed switch.  When you dump the pressure it can take more than a minute to return to zero differential (Twin and TBM as examples).  If you land with a positive differential and someone opens the door you can take an arm off or kill someone on the outside.  My previous twin Cessna was wired to dump on the squat switch and it triggered a few times in my life from being behind the airplane.   It would have been disastrous if a passenger or line guy tried to open the door (and both of those have happened before I could say something) with a positive differential.  I always confirm zero differential on the pressure controller before opening doors.  I would suggest that unless the controller has dumped everything overboard that you would be maintaining some differential in the cabin regardless of throttle setting and that those safety triggers are designed possible to dump everything before doors are opened (squat switch or low airspeed).  That upper deck is there to provide fast MAP increase on throttle application and to maintain cabin differential at low throttle settings.  Setting the controller to 500 feet above sea level gets the differential to zero by the time landing has occurred.  If something isn't set perfectly you can have differential in the cabin if a dump switch is not wired in.   Does all this sound correct for the 4P and would that be why the airspeed switch is wired for this situation?

best,

Paul Miller TBM 700, L2K

On 2010-01-25, at 9:42 PM, Colyn Case at earthlink wrote:

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.

Now ambient pressue is according to this table http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html 21.39 inchesHg.

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 -----

From: Jeffrey Liegner, MD

To: lml@lancaironline.net

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