Jeff,

 

See if this is helpful :  http://www.csgnetwork.com/tasinfocalc.html

 

Also:  http://en.wikipedia.org/wiki/True_airspeed

 

You can try this for an estimate of TAS: Climb to smooth air, under AP heading control with altitude hold and constant engine settings record IAS and GPS GS and device TAS, turn bug 120 Degrees and when stabilized, record IAS, GS and TAS, again  turn bug 120 Degrees and when stabilized, record IAS, GS and TAS.

 

Indicated TAS should have been constant.

IAS should have been constant.

Average the 3 GS numbers and that is a good approximation of TAS unless the air mass movement is weird.

 

Grayhawk 

 

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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