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Doug,
This may sound stupid, but did you leave your
alternate air switch on?
Anyway, it sounds like the static input to
your AS came loose, or has a sizeable leak or something.
Keep in mind your airspeed indicator is essentially
a differential pressure gauge, and if it's reading low, it could be due to low
ram pressure OR high static pressure. Now, on the ground, static
pressure is, well, pretty static. With your engine off and plane not
moving, the pressure inside your fuselage will pretty much equal the pressure
outside the fuselage, so even if your static line is leaking, a test using
pressure on the pitot tube (but relying on ambient pressure for the static port)
will not indicate any problems. Now, with the plane moving, the pressure
inside the fuselage and the pressure outside the fuselage can vary
somewhat. Normally, the airflow over the various openings in the fuselage
will cause the pressure inside the fuselage to be less than that read at a
correctly-positioned static port, leading to higher than normal AS
readings, and this is what had me stumped for a while. The key here
is that your plane is pressurized, and from what I gather from reading other
posts on this list, you L-IVP guys "puff up" your planes at low altitudes.
The pressurization system will cause the pressure inside the fuselage to be
higher than it would be externally, leading to lower airspeed readings if the AS
is getting it's static input from inside the fuselage vice the external static
port.
Keep in mind I don't have a IV-P, probably never
will, and am 10 years into building an LNC2, so take this
FWIW.
Gary Fitzgerald
LNC2 extra-slow build
~70%
engine: TBD
St. Charles, MO
Subject: [LML] Airspeed Indicator
Consult
In true medical
fashion, I would like to request of you all 2nd and 3rd opinions about the
cause of an Airspeed Indicator (AS) problem I developed last
week.
So, we have here a
situation in which the AS tested out fine on the ground, and bad in the air.
Any idea why?
Doug
Johnson
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