A perfect Vulcan Mind Meld!
Huh?
BTW; don’t be surprised if your
EM2 doesn’t read correctly. When I got mine it was off by about 2”.
I got a gauge and re-calibrated; but I have noticed it has drifted off
some over the last couple of years. But it doesn’t really matter. Think
of it as ‘relative’ MAP; the important thing is that it changes properly with change in
altitude.
Al
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Al Gietzen
Sent: Tuesday, June
23, 2009 3:48 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: progress
report #347c
Bill;
Yes;
at or near sea level the correction factor is 1.000. If your airport is
at 5000 ft, the correction factor is 0.832. So if the ATIS (or the
altimeter setting to give you field elevation) is 29.92, then your engine off
MAP should read 24.9”. And, yeah; you’re right – who
needs a chart, the correction is about 1” per 1K ft. My field
elevation is about 1400’, and the correction is 0.95, so my engine off
MAP is .95 x altimeter setting; if 29.92 then my MAP should read 28.4”.
Al
-----Original Message-----
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Bill Bradburry
Sent: Tuesday, June
23, 2009 10:00 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: progress
report #347c
Al,
Ok, Part of my misunderstanding lies in
the fact that my field elevation is 55 feet. So both will read close to
the same here. You are saying that his readings would have to be
corrected for his 5000 foot altitude which is about 1 inch per 1K feet?
Bill
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Al Gietzen
Sent: Tuesday, June
23, 2009 2:12 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: progress
report #347c
Al,
Hmmmm, Further explanation would be helpful
here. I thought that the
altimeter was actually reading the air column weight
or pressure at its
location. If you set the altimeter to the field
elevation, it seems it
should read the barometric pressure in the window
Gees;
I was hoping you wouldn’t ask – then I wouldn’t have to think
about itJ.
Well;
it goes something like this; your altimeter setting is the barometric pressure
of the station as if the station were at sea level. So the altimeter setting is
barometric pressure with an altitude correction applied. Your MAP is
absolute ambient pressure.
Absolute pressure is measurement relative to a perfect vacuum, as in outer
space. A zero reading occurs when the pressure is reduced to near perfect
vacuum conditions. The ambient atmospheric pressure at sea level is
approximately 29.92 inches of mercury, or 14.7 pounds per square inch, or 760
mm of mercury. As you go up the ambient atmospheric pressure decreases and your
Absolute pressure gauge (MAP) reading also decreases.
I
have a chart of the barometric vs ambient pressure, but I don’t know
where I got it. Anybody know? I’ll check the internet and see if I
can find something; or I could scan it and send it.
Al
G