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Date: Mon, 13 Aug 2012 11:39:34 -0400
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From: "Lynn Farnsworth" <farnsworth@charter.net>
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Subject: RE: [LML] Question about TAS Error (and Winds Calculation) based on OAT (in a LIVP)
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Jeff,

I have a Legacy so I can't say how my experience would relate to a IV. That
being said, my OAT was originally installed in the right cabin air duct
which is on the fuselage aft of the engine. I was looking at recorded (data
from a Chelton) Reno Air information. I happened to notice that the OAT
showed a marked increase from engine start to race completion. I think it
was about 25 to 30 degrees. We decided most of the rise was from engine heat
flowing back along the fuselage. We moved the sensor out to the right wing
on a inspection panel just forward of the right main landing gear. There is
still compression effect from flying at IAS of 300Kts plus though.

Lynn 

-----Original Message-----
From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of
jeffrey liegner
Sent: Monday, August 13, 2012 9:00 AM
To: lml@lancaironline.net
Subject: [LML] Question about TAS Error (and Winds Calculation) based on OAT
(in a LIVP)

Recently, I've been doing (and sharing with you) some engine setting tests
with WOT climbs and LOP cruise settings at different RPM/MAP settings.  This
has been partly due to new GAMI injectors, a recent top end overhaul (400
hours), and renewed curiosity (not the Martian lander).  The experiment is
not over yet.

My LIVP has the OAT installed at the standard location on the pilot side
fuselage, near the cabin door.  I have calibrated my Cheltons for the
compression effect on the OAT reading at high/low speeds.  But I have seen
ice form at indicated OAT 36*F, and my Malibu friends have shared their
POH's "Temperature Rise Due to Ram Recovery" graph.  I shared this with the
LML on Aug 9, 2009.

And recently, someone on the LML wondered why he always sees a headwind on
his EFIS.  During my weekend flight from Oregon to New Jersey (duration 8
hours), I expected some decent tail winds based on forecasts.  But my
Chelton EFIS often showed a crosswind, sometimes a headwind, and rarely a
very mild tail wind; not what was expected.  This was at FL230 and FL250.  

So I was thinking that if my OAT was reporting an erroneous reading, then
the TAS calculation provided would be wrong.  Then the calculation of winds
based on Magnetic Heading compared to Ground Track mixed with the GPS
derived ground speed compared with my TAS would be in error.  This makes the
OAT a critical value to proper reporting and judging.

Everything ties back to accurate OAT reading.

First, what suggestions do you have regarding this OAT question?  Do you
have some keen examples showing an OAT error of X produces a TAS error of a
certain magnitude Y?

Then, based on prior experience (not installer intuition), what have you
discovered regarding your OAT readings and then sensor locations in the
LIVP?

Maybe this is leading me to rewire the OAT, but I'm not wild about the idea
of this without some solid experience by others.

Thanks.

Jeff L
N334P

Footnote: On the ground, the plane's OAT matches the measured temperature on
the cars, the bank and on the radio.


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