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Date: Tue, 05 Jun 2007 01:24:46 -0400
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Subject: Re: [LML] trailing static / pitot bomb ?
From: "bob mackey" <n103md@yahoo.com>
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Grayhawk wrote...
> Borrow a decent GPS (Garmin ?96, ?30, etc.) and use any one of the many
> methods (2,3 or 4 legs) to calibrate the true airspeed.  ...
> Thus, you can avoid all the, uh, limitations of old methods.


Thanks for the suggestion Grayhawk. If you can convince everyone else
to switch over to GPS-derived altitudes, then I will use the GPS to
calibrate my static port.

Until then, I'll use the same old-fashioned, limited method as everyone
else so that my alitimeter indicates the same as theirs. Speaking of
which... does anyone have a static bomb they can loan?

For anyone who is not familiar, the standard method of experimentally
verifying the correct static port location is to trail a calibrated
static "bomb" several wingspans behind the aircraft. The pressure at
the static port under test is compared to the pressure seen at the bomb.
The aircraft static port location is then adjusted until it sees the same
pressure as the bomb over the desired airspeed range.

My aircraft is fitted with a heated Piper blade pitot+static probe
under the left wing. The pressure at the port decreases with airspeed,
causing an altitude error that varies with speed. As others have
noted, this is worth fixing. Most of us have had no problem with the
standard static port locations below and behind the aft window, but then
I don't know anyone who has actually tested their system to see if it
is accurate. Even the CAFE tests did not calibrate the static system,
but rather the pitot-static difference.
( http://cafefoundation.org/v2/pdf_apr/Lancair%20320%20APR.pdf )