Return-Path: Received: from relay03.roc.ny.frontiernet.net ([66.133.131.36] verified) by logan.com (CommuniGate Pro SMTP 4.1.3) with ESMTP id 2575964 for flyrotary@lancaironline.net; Thu, 11 Sep 2003 10:57:22 -0400 Received: (qmail 17975 invoked from network); 11 Sep 2003 14:57:21 -0000 Received: from unknown (HELO frontiernet.net) ([65.73.142.80]) (envelope-sender ) by relay03.roc.ny.frontiernet.net (FrontierMTA 2.3.6) with SMTP for ; 11 Sep 2003 14:57:21 -0000 Message-ID: <3F607FAF.3CF4F986@frontiernet.net> Date: Thu, 11 Sep 2003 09:59:11 -0400 From: Jim Sower X-Mailer: Mozilla 4.77 [en] (Win98; U) X-Accept-Language: en MIME-Version: 1.0 To: Rotary motors in aircraft Subject: Re: [FlyRotary] Re: turbo wastegate requirements References: Content-Type: multipart/alternative; boundary="------------D96706A81C998630E6F4FE69" --------------D96706A81C998630E6F4FE69 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit John, For conversion purposes, one atmosphere is 30" Hg and 14.7 psi. Estimating 1 psi = 2" Hg is close enough. Aviation convention is Manifold Absolute Pressure. When you're on the ground before you start the engine, you have 30" on the gauge. Low power that would indicate 17" of vacuum will be read as 13" MAP. The auto industry dumbed this all down for Joe Schitts the rag man and, since altitude is not a real factor, depicts boost as overpressure in psi. Overpressure from what is unclear, since 6 psi overpressure at partial throttle (say, 9 psi absolute) is 15 psi absolute and is a lot different from the 20 psi absolute you would get from the same 6 psi boost at WOT. This convention is confusing since it requires that you know the local atmospheric pressure and add that to boost in order to know what the engine is seeing. For the first 10k' or so, you can assume that ambient pressure decays at about 1"/1000' (the 500 mb level, or 15" level or ~ 7.4 psi level occurs at about 18k' (the 250mb level is more like 35k', so the accuracy of linear assumptions stops below 15k', roughly at 10k or 12k). For my own part, I discuss and think in terms of MAP so that I don't have to do all the interpolation of ambient pressure and guessing what the throttle position represents in unboosted MAP so I can add things up. The engine sees MAP. Period. Why not fix it so I see the same pressure as the engine? I truly think PSI boost is a really REALLY dumb convention since there is so much important information left out that you have to figure out for yourself (largely without benefit of instrumentation) The T-28B/C I did my basic training in had a 2-stage blower with a limit of 58"-60" for TO an 36" (at lower RPM) for climb. You weren't supposed to go "over square" (RPM < MAP) in cruise. My (as usual, unbiased) $.02 .... Jim S. John Slade wrote: > Rusty,You seem to know a lot about turbos. Could you give > some of us (eg me) a bit of basic education?I'm confused by > the mixed units. There's talk about x inches of Manifold Air > Pressure, then about so many PSI of boost. Are we talking > about the same thing using different units? Regards,John Slade > (all leather, no gas) -- Jim Sower Crossville, TN; Chapter 5 Long-EZ N83RT, Velocity N4095T --------------D96706A81C998630E6F4FE69 Content-Type: text/html; charset=us-ascii Content-Transfer-Encoding: 7bit John,
For conversion purposes, one atmosphere is 30" Hg and 14.7 psi. Estimating 1 psi = 2" Hg is close enough. Aviation convention is Manifold Absolute Pressure. When you're on the ground before you start the engine, you have 30" on the gauge. Low power that would indicate 17" of vacuum will be read as 13" MAP. The auto industry dumbed this all down for Joe Schitts the rag man and, since altitude is not a real factor, depicts boost as overpressure in psi. Overpressure from what is unclear, since 6 psi overpressure at partial throttle (say, 9 psi absolute) is 15 psi absolute and is a lot different from the 20 psi absolute you would get from the same 6 psi boost at WOT.

This convention is confusing since it requires that you know the local atmospheric pressure and add that to boost in order to know what the engine is seeing. For the first 10k' or so, you can assume that ambient pressure decays at about 1"/1000' (the 500 mb level, or 15" level or ~ 7.4 psi level occurs at about 18k' (the 250mb level is more like 35k', so the accuracy of linear assumptions stops below 15k', roughly at 10k or 12k). For my own part, I discuss and think in terms of MAP so that I don't have to do all the interpolation of ambient pressure and guessing what the throttle position represents in unboosted MAP so I can add things up. The engine sees MAP. Period. Why not fix it so I see the same pressure as the engine? I truly think PSI boost is a really REALLY dumb convention since there is so much important information left out that you have to figure out for yourself (largely without benefit of instrumentation)

The T-28B/C I did my basic training in had a 2-stage blower with a limit of 58"-60" for TO an 36" (at lower RPM) for climb. You weren't supposed to go "over square" (RPM < MAP) in cruise.

My (as usual, unbiased) $.02 .... Jim S.

John Slade wrote:

Rusty,You seem to know a lot about turbos. Could you give some of us (eg me) a bit of basic education?I'm confused by the mixed units. There's talk about x inches of Manifold Air Pressure, then about so many PSI of boost. Are we talking about the same thing using different units? Regards,John Slade (all leather, no gas)

--
Jim Sower
Crossville, TN; Chapter 5
Long-EZ N83RT, Velocity N4095T
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