X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Mon, 25 Jan 2010 21:42:39 -0500 Message-ID: X-Original-Return-Path: Received: from elasmtp-curtail.atl.sa.earthlink.net ([209.86.89.64] verified) by logan.com (CommuniGate Pro SMTP 5.3.1) with ESMTP id 4094433 for lml@lancaironline.net; Sun, 24 Jan 2010 20:55:18 -0500 Received-SPF: none receiver=logan.com; client-ip=209.86.89.64; envelope-from=colyncase@earthlink.net DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=dk20050327; d=earthlink.net; b=USlYxUxWIU7WghIb12OB/UxKAagNQYdnlEXTad4HT2kU3GECt1/evulc2pCmZbNG; h=Received:Message-ID:From:To:References:Subject:Date:MIME-Version:Content-Type:X-Priority:X-MSMail-Priority:X-Mailer:X-MimeOLE:X-ELNK-Trace:X-Originating-IP; Received: from [216.57.118.190] (helo=ccaselt3) by elasmtp-curtail.atl.sa.earthlink.net with esmtpa (Exim 4.67) (envelope-from ) id 1NZEA6-00012o-Dw for lml@lancaironline.net; Sun, 24 Jan 2010 20:54:42 -0500 X-Original-Message-ID: From: "Colyn Case at earthlink" X-Original-To: "Lancair Mailing List" References: Subject: Re: [LML] Re: Airspeed sensing switch: Pressurization X-Original-Date: Sun, 24 Jan 2010 20:54:41 -0500 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_004B_01CA9D37.733A8EA0" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.5843 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 X-ELNK-Trace: 63d5d3452847f8b1d6dd28457998182d7e972de0d01da940cec2c8e8bbf974310972849400af8a54350badd9bab72f9c350badd9bab72f9c350badd9bab72f9c X-Originating-IP: 216.57.118.190 This is a multi-part message in MIME format. ------=_NextPart_000_004B_01CA9D37.733A8EA0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Re: Airspeed sensing switch: Pressurizationso 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? 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. =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. 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 -----=20 From: Jeffrey Liegner, MD=20 To: lml@lancaironline.net=20 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 ------=_NextPart_000_004B_01CA9D37.733A8EA0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Re: Airspeed sensing switch: Pressurization
so how does the math work = here?  =20
If you land at sea level, you would = have had to=20 tell your pressure controller to shoot for below sea level for it to be = making=20 any cabin differential no matter what your engine setting is. =20 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=20 inchesHg.
So I would have to be holding 22" or = better on=20 final for that to be an issue, right?
 
Now let's say you are cruising at=20 25,000.  
Ambient there is 11.12 in. = Hg.
If you would like a 8000' cabin you = need 22.23 in=20 the cabin.
and maybe you are running at 32" in.=20 MP.    So there's about 20 inches differential available. = and you=20 only need 10"
So a perfectly efficient system with no = flow=20 through the cabin would still function at 22" MP.
 
So my real question:   Is the = reason you=20 need about 28" just that that's what it happens to take to get enough = volume of=20 air moving to support the cabin flow rate?
----- Original Message -----
From:=20 Jeffrey=20 Liegner, MD
Sent: Sunday, January 24, 2010 = 6:07=20 PM
Subject: [LML] Re: Airspeed = sensing=20 switch: Pressurization

I've never appreciated the value of this pressurization feature = in our=20 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=20 value in doing so.  In fact, if airspeed is artifically reduced = from=20 pitot ice or some failed pitot tubing at one of the couplings, you = will still=20 want to maintain pressurization as the engine continues to hum along = int he=20 flight levels.

I figure that the turbos provide pressurized air, and when the = throttle=20 and MAP drop below pressurization levels (let's say 28", depending on=20 altitude) the cabin air will vent quickly either through porous = sources or=20 back out through the mixing box and back into the upper deck.

Either way, when you reduce throttle to something below ambient = pressure,=20 like when you're landing, the cabin will normalize with the outside = air=20 pressure long before you arrive abeam the numbers on downwind.  = This=20 action does not require an airspeed switch to then open the Duke's = Valve cabin=20 dump.

To restate my observation, in our planes, the way they are built, = there=20 is never a time when you can or will hold pressurization on the ground = after=20 you land, however you set up this auto cabin dump feature (in my case, = totally=20 disabled).  Other planes have one way pressurization valves, but = we do=20 not.  The mixing box input hole (bringing pressurized turbo air = in) is=20 the biggest 1" hole in the cabin to vent air back into the upper deck = when=20 cabin pressure exceeds mainfold pressure.

Jeff L
LIVP

 
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