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Location of OAT sensors

keitht

Well Known Member
Any suggestion on locations for Outside Air Temp probes. Put mine on the bottom skin off to the left side back about mid fuselage but am getting quite an error due to exhaust and cooling air flowing out of the bottom cowling. Tried a couple of wedges taped on to deflect the air around the probes but will have to move the probes for a permanant solution.

KT
 
Wing

Out on the wing away from exhaust. On or near inspection cover would be easiest for flying aircraft.
 
I plan to put my OAT probe close to the pitot tube. My rationale is the air temp will be as close to the air entering the pitot, and that temperature is used to compute other relevant air data based on the pitot pressure. So if I keep the same air temperature for the OAT and the pitot, then all the corrections for the performance data will be minimal.
 
Carl,
Thanks for the reply. I was thinking I should have gone for under the tailplane location - existing wiring may be long enough, but not a problem to splice some in. Almost completed the phase one test flying with very few problems or issues. Now if only the weather would cooperate could finish off all the autopilot and coupled approach testing and finish up the 40 hrs.

KT
 
Stick it where the sun don't shine... Plus out of the exhaust plume.
That usually means under the wing so make it easy on yourself and put it on or near an access panel.
 
I tried all the OAT probe mounting places listed. The only one that did not have a significant error (as reading was 5-10 degrees higher than actual) was just inside the wing bell crank access plate.

Run a single shielded four conductor #22 cable to connect both OAT probes (for those running dual ADHARS). This shielded cable provides for a more robust (and boat load easier) wire run, the shielding here is for mechanical properties, not electrical.

I lived with this error until it bit me. IFR decent through a layer (really just the tops of a few build ups) OAT reading (high) at 36 degrees. In a few seconds I had a sheet of clear ice on the windshield and leading edges. Classic sub-cooled water plating out. This was my first experience with something like this and in hindsight should have taken a different path to get down. This and another rime ice incident shortly after created a new IFR rule “when flying at any altitude that can be below freezing where it is likely to be in the clouds, the surface to 3000’ (above ground) air temps must be predicted to be above freezing”. The reason is on this last ice issue I’m sitting there looking at a 100% obscured view as I’m continuing my descent. Luckily the OAT crept up above freezing, cleared the ice in short order and I was able to land.

I’m an amateur pilot and need such rules.

Carl
I learned the hard way on this (and a lot of other stuff)
 
“when flying at any altitude that can be below freezing where it is likely to be in the clouds, the surface to 3000’ (above ground) air temps must be predicted to be above freezing”.

Carl, please enlighten us. Your reasoning then is that the ice will melt and shed off before approach to landing?
 
“when flying at any altitude that can be below freezing where it is likely to be in the clouds, the surface to 3000’ (above ground) air temps must be predicted to be above freezing”.

Carl, please enlighten us. Your reasoning then is that the ice will melt and shed off before approach to landing?

Correct - as was my experience. Better is to avoid icing all together (and I do) but sometimes icing finds you anyway. I do mostly high IFR cross country of late (above the layer) so I’m very sensitive to this (as in I need not test my luck). An easier process to follow when flying south compared to north in the winter.

No defrost fan in the world will clear away ice when flying.

An accurate OAT is required. Don’t forget to compensate for Ram Rise. While not huge, at good RV speeds you could have actual OAT a degree or so below what you are reading on your OAT instrument.

Carl
 
I've the garmin probe, I will install in the plastic vent, because the Garmin probe hasn't a long cable
 
I second under the wing near the bell crank inspection cover plate.
My temps are always the same as what the AWOS at our field is reporting.
Wiring in the wing conduit and near the cover plate allows easy access if needed.;)
 
Add me to the outside prop diameter and in the shade group.


I tested several locations with thermocouples before settling on the inspection plate. Mine was inside the left wing root aft of the spar and when going from full rich to lean in the winter (900 to 1400F) the OAT tracked the EGT change (~8F difference). Odd things happen.
 
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I have mine on the NACA vent on the fuse, and I'm sure it reads a bit high.

Serious question - does it make any real difference if OAT is off by a few degrees?
 
I have mine on the NACA vent on the fuse, and I'm sure it reads a bit high.

Serious question - does it make any real difference if OAT is off by a few degrees?

The short answer, yes.

Off a few degrees high will:
- Have an erroneous high TAS reading (standard EFIS function).
- The high TAS reading will yield wind calculations that are off both in amplitude and direction (standard EFIS function).
- You will not be sure of your margin to icing.

Carl
 
Put mine on the bottom skin off to the left side back about mid fuselage but am getting quite an error due to exhaust and cooling air flowing out of the bottom cowling.

How are people verifying what is incorrect/correct? Where/how are you obtaining truth data?
 
How are people verifying what is incorrect/correct? Where/how are you obtaining truth data?

I took thermocouple wire and made bead ends, then placed two in several places (oven, water, ice) to compare readings and define a correction factor. One was strung to different places on the plane and the other was under the wing midway - thus both flight tested at the same time. The thermocouple reader was simply switched between location. I ran different power and EGT temps to compare . . . EGT yielded the largest effect if any. The under fuse (or any other fuse) location was not considered. Oil streaming, in prop blast, and exhaust contamination indicated it would not be a good test location, just me FWIW. I am not saying it is bad as it was not tested, it just did not seem viable.
 
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The short answer, yes.

Off a few degrees high will:
- Have an erroneous high TAS reading (standard EFIS function).
- The high TAS reading will yield wind calculations that are off both in amplitude and direction (standard EFIS function).
- You will not be sure of your margin to icing.

Carl

Thanks Carl. So for a slacker, daytime, VFR flyer like me, then essentially it doesn't make much difference, and slightly higher TAS reading actually gives me a bit higher Vne TAS margin.

If I use this calculator:

https://www.dauntless-soft.com/PRODUCTS/Freebies/TrueAirspeedCalculator/

and I change my OAT numbers by 5c, I get about 1-2 KTS of TAS difference depending on the IAS, altitude, and altimeter setting.

There is a much bigger numerical impact on density altitude, and like with TAS, it gives me a higher density altitude, which is on the more conservative side.

If I'm in the clouds, with my current level of skills and experience, my only concern will be to get out of them safely, immediately. If I upgrade my aircraft to become IFR capable, then I'll move my OAT indicator.

Thanks a lot for your feedback!
 
Just to pull up some old crude flight testing I did back in 2010 on OAT probe location here is link to old thread.

I did several plots of data in post 31 and post 53.

I have never came to a definitive conclusion on best truth data or if under RH H-tail is best position but that is where I have left it for last 10 years.
 
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