Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml@lancaironline.net
Date: Thu, 03 Jun 2004 22:01:32 -0400
Message-ID: <redirect-123097@logan.com>
From: bob mackey <nospam@pure-flight.com>
Subject: Pitot pressure in lb/sq in or inches WG
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> Somebody on the list once gave a formula or a conversion method to take
> airspeed and convert it into units of pressure.
>
> Due to my gear up landing I want to fit a pressure switch in the pitot line.
> I need to know how to convert airspeed into pressure units in order to buy
> the correct switch.  Electronics catalogs have adjustable pressure switches
> at under $20.

The "dynamic pressure" measured by a pitot tube, relative to the static port,
is:
   Q = 0.5 * rho * V^2

Where rho is the density of air, and V is the forward speed.
In reasonable units (approximating 200 kts at sea level),

   Q = 0.5 * 1.225 kg/m^3 * 100 m/sec = 6125 Pa

in screwy american units, that's 0.888 psi.


If you want the gear warning to come on below say 80 kias (~40 m/sec),
then Q = 980 Pa = 0.142 psi.


- bob mackey
  flying a 235
  building a 320
  103MD -at- pure-flight.com
  [use this email address - the one at the top is spam bait]