lml@lancaironline.net Mailing List Archive

Re: [LML] Re: Airspeed sensing switch: Pressurization

My note was about on-the-ground retained pressurization after landing (which is not possible), and the (absent) need for an automatic Cabin Dump command to the Dukes Valve when airspeed drops below a measured value, given the potential problem with cabin pressure dump in flight when the IAS system misreads the pitot signal (eg, icing, tubing leak).

If you're planning to land at an airport elevation of 9000' and you have the panel controller set for sea level, I can guarentee your cabin will be 9000' plus your height above threshold when you round the downwind-to-base leg at 11.5" MAP, gear out, half flaps. It may have been at 4000' cabin altitude (under good Duke's control) as you entered the Terminal Area still carrying 23"-31" MAP, but as soon as you pull back on the throttle and drop your MAP below ambient, the cabin will begin to "climb" to the local altitude. As you approach the pattern at 18" MAP and descending 200 KIAS ('cuz you hate to use the speed brakes if you can avoid it), your cabin will climb to 9000' promptly, since the Duke's Valve is no longer the only thing in charge of cabin venting, with air flowing back through the sonic ports into the upper deck.

One can determine how porous the cabin by checking at what MAP is required to hold full 5 psi differential pressurization. Some are 28" MAP, some are 26" MAP, and maybe someone is lower still. My cabin begins to reduce from 5 psi differential below 28" MAP. This test must be done at sufficient altitude to have/hold 5 psi differential (probably >16K').

Others on the ListServ may be able to expand on how our controllers work, what their minimum MAP is to maintain 5 psi cabin differential, and tricks of the system. Clarifications on what I've stated here are warmly welcomed. Great topic that took time for me to understand.

Jeff L

LIVP

5 psi is approx 10" MAP

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? If there's no MAP to support cabin pressure above ambient, then the cabin will be at ambient.

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? 22" on final, indicating MAP behind the throttle, is not the upper deck pressurem which is the supplier of cabin pressure. landing 9000' altitude on final with 22" MAP will give you (probably) a 8000'-9000' cabin altitude and very little pressure differential.

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. I do not believe the 5 psi differential (~10" pressure) limit permits a 8000' cabin at 25000' altitude. This cabin is more like 9200'-10000' in our planes. To maintain 5psi differential at 32" MAP indicated, one needs >42" upper deck pressure produced by the turbos, siphoned off to the cabin, including cabin leakage, and a totally closed Duke's Valve.

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