X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Thu, 28 Jan 2010 12:47:36 -0500 Message-ID: X-Original-Return-Path: Received: from betsy.gendns5.com ([65.254.38.234] verified) by logan.com (CommuniGate Pro SMTP 5.3.2) with ESMTPS id 4102156 for lml@lancaironline.net; Thu, 28 Jan 2010 12:32:28 -0500 Received-SPF: none receiver=logan.com; client-ip=65.254.38.234; envelope-from=paul@tbm700.com Received: from 0200-142-27-72-dynamic-dsl.cwjamaica.com ([72.27.142.200]:64590 helo=[192.168.1.206]) by betsy.gendns5.com with esmtpsa (TLSv1:AES128-SHA:128) (Exim 4.69) (envelope-from ) id 1NaYDZ-0003if-Pg for lml@lancaironline.net; Thu, 28 Jan 2010 12:31:46 -0500 From: paul miller Mime-Version: 1.0 (Apple Message framework v1077) Content-Type: multipart/alternative; boundary=Apple-Mail-1-777378777 Subject: Re: [LML] Re: Airspeed sensing switch: Pressurization X-Original-Date: Thu, 28 Jan 2010 12:31:43 -0500 In-Reply-To: X-Original-To: "Lancair Mailing List" References: X-Original-Message-Id: X-Mailer: Apple Mail (2.1077) X-AntiAbuse: This header was added to track abuse, please include it with any abuse report X-AntiAbuse: Primary Hostname - betsy.gendns5.com X-AntiAbuse: Original Domain - lancaironline.net X-AntiAbuse: Originator/Caller UID/GID - [47 12] / [47 12] X-AntiAbuse: Sender Address Domain - tbm700.com --Apple-Mail-1-777378777 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=us-ascii 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? =20 > 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? > =20 > 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? > =20 > Now let's say you are cruising at 25,000. =20 > 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. > =20 > 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 >=20 > 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. >=20 > 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. >=20 > 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. >=20 > 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. >=20 > Jeff L > LIVP >=20 > =20 --Apple-Mail-1-777378777 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=us-ascii 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&n= bsp;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 -----
 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

 

= --Apple-Mail-1-777378777--