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
|