REFLECTOR: Air pressure question...

Thu Jun 23 15:42:02 CDT 2005

Hiroo Umeno wrote:

>There are some great information that is coming out of this discussion.
>
>I am clear on the concept that temperature, DA, humidity, etc, all have
>impact on the pressure exerted on the pitot-static system.  However,
>what I am not too clear on is that how that is reflected on the
>instruments.
>
>As far as I know, the instruments (altimeter and ASI) are mechanical
>device that only show the mechanical force acting on them.  Meaning the
>end results of all the stuff that adds up.  So...
>
>The altimeter will always show the same altitude setting for the same
>pressure measured, assuming the setting is the same.  In other words, if
>the setting is 29.92, if the measured pressure is 25.84, it will show
>4000ft regardless of "actual" altitude is.
>  
>
Exactly.  Which is why they call it Pressure Altitude.

>Similarly, the ASI simply takes the mechanical differences between pitot
>and static pressures and translate that into airspeed regardless of the
>altitude.  So, using the previously discussed example, if the pressure
>differential is 1.45psi, it will indicate 250KIAS regardless of the
>altitude, humidity, etc.  Of course, at higher altitude, the air is less
>dense so it takes higher airspeed to actually create the same amount of
>pressure differential.  So while indicating 250KIAS, the TRUE airspeed
>is higher at FL200 than it is at 0MSL.
>  
>
That's true.

>Did I get it right?  Yes.
>
>So, if the above is correct, in order to calibrate the electronic
>pressure sensor to mimic what the mechanical gauges do, all you really
>need is a fairly linear formula for both.
>  
>
You probably don't want that.  Both "electronic" and "mechanical" start 
with some sort of mechanical transducer.  Granted, the conventional 
Altimeter / ASI is totally mechanical, with all of the attendant 
inaccuracies, is the "blunt instrument" of the transducers, but it all 
pretty much starts with something (however tiny) moving.  Calibrated 
Airspeed is not bandied about much any more, probably because ASIs have 
become accurate enough to where we can largely ignore calibration 
errors.  I suspect that even the "solid state" transducers used on 
encoders and air data sensors may have calibration tables (perhaps built 
in).  In any event, flight data displays will correct electronically for 
altimeter setting that you input (just like ATC).

>Did I turn the wrong way somewhere?
>
>Hiroo
>
>-----Original Message-----
>From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
>Behalf Of Jim Sower
>Sent: Thursday, June 23, 2005 8:31 AM
>To: Velocity Aircraft Owners and Builders list
>Subject: Re: REFLECTOR: Air pressure question...
>
>But NOT half your density (since it's a good bit colder). Not a dramatic
>
>difference, but significant..
>Jim S.
>
>J.P. Brooks wrote:
>
>  
>
>>You will loose half your pressure at 18,000 feet.... half the 
>>atmosphere better said
>>
>>
>>At 08:27 PM 6/16/2005, you wrote:
>>
>>    
>>
>>>Hiro,
>>>Following is data to 15K
>>>Alt in hg PSI
>>>SL 29.92 14.7
>>>1000 28.86 14.2
>>>2000 27.82 13.7
>>>3000 26.81 13.2
>>>4000 25.84 12.7
>>>5000 24.9 12.2
>>>6000 23.98 11.8
>>>7000 23.09 11.3
>>>8000 22.23 10.9
>>>9000 21.39 10.5
>>>10000 20.58 10.1
>>>11000 19.8 9.7
>>>12000 19.03 9.3
>>>13000 18.3 9.0
>>>14000 17.58 8.6
>>>15000 16.89 8.3
>>>
>>>    ----- Original Message -----
>>>    From: Hiroo Umeno <mailto:humeno at microsoft.com>
>>>    To: Velocity Aircraft Owners and Builders list
>>>    <mailto:reflector at tvbf.org>
>>>    Sent: Thursday, June 16, 2005 9:34 PM
>>>    Subject: REFLECTOR: Air pressure question...
>>>
>>>    I am not too up on my physics so I would like to pick the
>>>    community's collective brains on a little air pressure question.
>>>
>>>
>>>
>>>    The air pressure at sea level is about 14.5psi. What is the air
>>>    pressure at, say 20,000ft msl?
>>>
>>>
>>>
>>>    Also, what would be the pressure differential (in PSI) at 250KIAS
>>>    between Pitot and Static ports?
>>>
>>>
>>>
>>>    Thanks.
>>>
>>>
>>>
>>>    Hiroo
>>>
>>>
>>>      
>>>
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