REFLECTOR: Air pressure question...

[Hiroo Umeno]

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.

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.

Did I get it right?

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.

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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