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Date: Mon, 4 Jun 2007 11:09:55 -0600
From: Bob White <rlwhite@comcast.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: EC-2/EM-2 manifold pressure
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On Mon, 4 Jun 2007 09:23:37 -0800
"Al Gietzen" <ALVentures@cox.net> wrote:

> Well Bill,
> 
> if the Kolsman window is where you set your altimeter setting (mb or inches
> mercury) you will never see field elevation (pressure) unless you are at
> sealevel.
> 
>  
> 
> E.g. if you have 29.92" at sealevel (and in the window) you still have 29.92
> in the window (you set it there!) when  you get to 6500'. Your actual
> ambient pressure is around 23.42".... (which you should be reading on your
> manifold pressure gauge - engine stopped... 
> 
>  
> 
> Unless, I got it wrong the last 15 years and 9500 hours - ohhh my...!!
> 
>  TJ
> 
> I think you've got it right.  It seems to me that if you want to find what
> your MAP out to be when parked, engine off; you set 29.92 in the altimeter
> window to get pressure altitude, then either use a formula or chart to get
> the expected air pressure at that altitude.  For example, if you set 29.92
> and it shows your pressure altitude to be 2000 ft, then your chart will tell
> you that the air pressure where you are is 27.8 in. HG for a standard day,
> then correct for temp.
> 
>  
> 
> So unless I'm mistaken, that is the same as calling the tower (or weather
> observation station, or get out your handy-dandy flight computer) and get
> the density altitude, then vary the altimeter setting until the altitude
> reads density altitude, and get the pressure from the Kollsman window.
> 
>  
> 
> Al
> 

Hi Al,

I think your first paragraph is correct, almost.  You don't need to do
any temperature corrections.  The pressure is whatever it is regardless
of temperature.

Just to review:
The pressure you read in the Kollsman window is the sea level
pressure.  Let's say it's a standard day.  The barometric pressure is
reported to be 29.92 and the temperature is standard for your
altitude.  Then actual altitude = pressure altitude = density altitude
and the reading in the Kollsman window is 29.92 no matter what altitude
you are at.  The barometric pressure doesn't take temperature into
account.  Set the Kollsman window to the reported pressure and the
altimeter will read actual altitude.  Set it to 29.92 and it reads
pressure altitude.  Take the pressure altitude and correct for
temperature and you get density altitude.


Bob W. (Assuming everything is calibrated perfectly.)


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