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The Science of OAT probe location?

Kooshball

Well Known Member
After searching and posting on the forum seeking recommended locations to mount an OAT probe for an EFIS I have come across many options and recommendations. Many of them make sense such as to keep the probe away from the exhaust, away from cowl exit air, out of the sun, etc..

The one recommendation that GRT makes that I can’t wrap my head around is to keep it out of the prop wash. How would prop wash air that isn’t blended with engine exhaust or cowl exit air give a different reading than a probe mounted mid-span on the lower wing skin? I believe that several certified planes have their OAT probes sticking out of the corner of the windscreen which is in the prop wash. This also goes against the recommendation to keep it out of the sun. Perhaps once there is airflow over it the impact of direct sunlight is negligible?

So what is it about the prop wash that will give a bad reading and how far off is it likely to be?
 
Good question. Maybe have to ask GRT. A standard location on many cert aircraft is poking out of the left side of the windshield as you said.

Impact air at SL at RV cruising speeds gives a pressure rise of about 4% and a temp rise of about 1.1% above ambient. In the slipstream, add ~20% or so for ~1.3% temperature rise. So, if the probe were immersed entirely in impact air, the temperature rise would be about 8 degrees F in the slipstream instead of 5 degrees F. outside. (very round numbers)

But a typical probe is a small cylinder and the impact condition is only present on a small part of the probe. So, I would guess that the measured impact rise would be much less and the slipstream difference in readings would be almost nil.

I am curious as well what reasons GRT gives for the probe location.
 
If it is out of the sun but in the prop wash, it will most likely get some diluted exhaust effect. There is a fair bit of mixing. There is also the higher velocity of the slip stream increasing the “ram rise” since there is a rise in stagnation pressure there is also a small rise in stagnation temperature.
 
Good question. Maybe have to ask GRT. A standard location on many cert aircraft is poking out of the left side of the windshield as you said.

Impact air at SL at RV cruising speeds gives a pressure rise of about 4% and a temp rise of about 1.1% above ambient. In the slipstream, add ~20% or so for ~1.3% temperature rise. So, if the probe were immersed entirely in impact air, the temperature rise would be about 8 degrees F in the slipstream instead of 5 degrees F. outside. (very round numbers)

But a typical probe is a small cylinder and the impact condition is only present on a small part of the probe. So, I would guess that the measured impact rise would be much less and the slipstream difference in readings would be almost nil.

I am curious as well what reasons GRT gives for the probe location.

In the case of the GRT probe it is not a cylinder but a blade shaped a bit like a football in cross-section so perhaps even less impact error?
 
SNIP

So what is it about the prop wash that will give a bad reading and how far off is it likely to be?

I just did this - nine degrees Fahrenheit. This equates to about a two knot high TAS reading. The far more impactful issue however is not having confidence when venturing into freezing conditions (ice).

I moved both probes to just outboard the wing bell crank area. Easy to reach, wires do not interfere with the bell crank, and outside the prop wash. Comparing readings with winds aloft data now show OAT readings to be very accurate.

Carl
 
I just did this - nine degrees Fahrenheit. This equates to about a two knot high TAS reading. The far more impactful issue however is not having confidence when venturing into freezing conditions (ice).

I moved both probes to just outboard the wing bell crank area. Easy to reach, wires do not interfere with the bell crank, and outside the prop wash. Comparing readings with winds aloft data now show OAT readings to be very accurate.

Carl

How did you deal with the amount of excess wire needed to remove or access that cover plate while keeping the wires clear of the bellcrank?
 
How did you deal with the amount of excess wire needed to remove or access that cover plate while keeping the wires clear of the bellcrank?

There is no wire in the bell crank area (other than the wing conduit that passes through the area). I mounted the probes in the bay outboard of the bell crank area. The same bay as the Pitot probe. The wires enter the conduit in that area. Another length of conduit continues out to the wingtip for lights and such.

The probes are easy to reach, and the wires are long enough for maintenance if ever needed.

Carl
 
The one recommendation that GRT makes that I can’t wrap my head around is to keep it out of the prop wash.

+1 for Kahuna's comment

My Rocket had a side skin air inlet for the front seat and an underwing air inlet, outside of the prop arc, for the rear seat.

On a 95 F day, the front seat received a measured 106 F air and the back seat received 95 F air.

I'll never build another plane with a cabin air inlet on the fuselage. Never.
 
I decided to put it just next to the bellcrank cover as the inboard cover was close to prop wash and right behind the wheel. The probe appears to be reading high during an ambient review in the hangar and there isn’t any trim available in the GRT efis. I’ll need to confirm with a calibrated thermometer but perhaps that location uses enough wire to impact the readings?
 
Outside the prop wash.

I placed thermocouples in a lot of different places once I found that my wing root location was affected by EGT. Yes, EGT tracked and changed by OAT. Plotting data showed an excellent correlation. Go figure.

I concluded that "outside the prop wash" ,and in the shade, description was an easy way to make a recommendation, but did not know it matched GRT.

I am not sure a 2-3 F variation is of any practical relevance anyway, but gathered data and made it accurate just because I wanted to and could. :D
 
My -8 has one OAT in the cabin air vent on the left fuselage side and another under the right wing on an access plate ob of the prop wash. Both read within 1* as long as I have the vent open with air flowing through it. One is GRT and the other is AFS. The fuselage vent is the standard one as on Vans plans.
 
I placed thermocouples in a lot of different places once I found that my wing root location was affected by EGT. Yes, EGT tracked and changed by OAT. Plotting data showed an excellent correlation. Go figure.

I concluded that "outside the prop wash" ,and in the shade, description was an easy way to make a recommendation, but did not know it matched GRT.

I am not sure a 2-3 F variation is of any practical relevance anyway, but gathered data and made it accurate just because I wanted to and could. :D

Good point. Even being off by 8F is only a 2-kt error in TAS calculations and it would be an error on the side of caution in this case.
 
Good point. Even being off by 8F is only a 2-kt error in TAS calculations and it would be an error on the side of caution in this case.

Again, TAS error is not the big issue. Decending through a layer with OAT reading 38 degrees you may be surprised at the ice you get on the plane - as actual OAT is 30 degrees. Here is where you make sure where ever you are going is warm enough to melt the ice before landing. No amount of defrost air can clear an ice covered windshield with OAT below freezing.

Just mount the probes out at the outboard inspection plate, or in the bay just beyond that and end the problem. The extra run of wire has no practical effect on the OAT probe reading.

Don’t forget to account for ram heating yielding a slightly higher than actual OAT indication:

28430-EA2-F143-48-F2-931-C-FC4-C1-E81-F246.jpg


Carl
 
I've recently mounted an OAT probe and connected to a newly installed Uavionix AV-30. The OAT probe gives accurate information and was installed into the centre of the first under wing inspection panel, about two feet out from the wing root, on a RV3b. The probe is out of the prop wash and hot exhaust zone.
The install was relatively easy and gives convenient access to the backside probe. Biggest time consumer was routing the wire through the wing conduit and securing and protecting the signal wire to the AV-30. A fellow at my drome with a gorgeous RV7a installed his OAT probe almost all the way aft on the underside of the tail cone. He has a consistent temperature error indicating approximately 10 degrees higher than actual and likely the result of being in the exhaust stream. Mounting the OAT to an inspection panel work well for me.
 
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