Fred,

 

Righto.....

 

It took me a couple of years to get the most important OAT probe in the correct location.  The Rocky Mountain Instruments' Micro-encoder (RMI) probe is a thermocouple inside of a metal cylindrical cup sealed at one end by a cap that is slightly larger than the cylinder (cyl maybe .5" dia and .75" long.

 

My first try was thru the cockpit lower wall ahead of the right wing.  Sun and leaking cowl air could influence the reading.  Cockpit temperature could warm the cup and certainly heat conducted thru the wires from the cockpit to the probe.  Arrrrggghh.  I have a secondary OAT probe that extends thru the vertical filet face covered by the flap in cruise (a brass probe about 1/8" dia by .5" long).  There is some measurement variability from similar influences at this location even though I have tried to insulate it.

 

The RMI "button" probe is now mounted into the lower external wing skin just forward of the middle gear door with the wire traveling thru the cold wheel well.  This means a fast response to outside air temp changes with no extraordinary temperature influence on the probe or its wires.  The sun effect is eliminated, except on the ground where pavement radiant heat may cause higher readings (leading to higher density altitude displayed on the RMI, an added safety margin).

 

GPS runs have indicated that RMI's temperature/compression adjustments display a TAS within a knot of that determined thru the GPS (uh, even if one uses the GPS kph readouts for more accuracy).  It also displays the TAT.  Hmmmm, maybe its just compensating errors with my pitot mounted mid wing instead of on the stub wing where it might be influenced by prop wash/turbulence.

 

Also, I have dual static ports placed where Lancair said they should be in order to work properly.  I did spend some useless months trying to get a Piper "blade" and another gi-normous chromed pitot, both with built in static ports, to perform.  They never did.

 

I love experimental aircraft.............

 

BTW, I just can't seem to get the speeds or altitude where you and Brent operate.

 

Scott Krueger

 

As I noted elsewhere, at our lower speeds, compression heating at stagnation points and frictional heating elsewhere (and mixtures of both near stagnation points) makes the OAT error occur everywhere on the airframe.  The Piper curves published earlier give you a good estimate of the OAT error.  Going in the wheel wells only adds confusion if there is a fuel tank near by with a big thermal sink in the form of avgas that will be very slowly heating and cooling.   Additionally, the OAT response for a probe in the wheel well will be really slooooooooowwwwwwww. 

 

The thing to do is accept the error, and remove it.  Use the Piper charts to estimate the error in OAT.  To get an accurate TAS, you have to also correct for compressibility.