OAT probe position

[Chris Strube]

1965 M20E turbo.  I'm removing my "through the window" OAT probe, and have installed an AV-30.  Where is the best place for an electronic OAT probe?

[Igor_U]

Inboard inspection panel under (right) the wing.

My understanding is that's the factory setup.

[Ragsf15e]

36 minutes ago, Chris Strube said:

1965 M20E turbo.  I'm removing my "through the window" OAT probe, and have installed an AV-30.  Where is the best place for an electronic OAT probe?

Mine is in the left wing, but yeah, out under the wing so it’s away from the engine, propwash and out of the sun.

[Nippernaper]

Under the wing works well. I have seen some in the NACA inlet on the copilots side of the fuselage (J models). When I upgraded the panel on my J, avionics shop installed the OAT probe on the belly near the tail. It's hard to believe, but the exhaust raised the temperature of the probe by tens of degrees f. Had them relocate it to the factory position in the wing.


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[Hank]

On 2/27/2023 at 8:00 AM, Nippernaper said:

I have seen some in the NACA inlet on the copilots side of the fuselage (J models). 

That's where it is on my C.

[ArtVandelay]

Even my JPI probe mounted in pilot NACA duct runs about 10° warmer, the JPI allows adjustments but not 10°. My G3X probe replaced the factory one in the wing.

[flyboy0681]

Interesting subject. I have a GI 275 ADI and the OAT probe is mounted in the NACA scoop. Reading responses on the subject led me to believe that there would be a huge difference in temperature readings, which worried me because I love the wind speed and direction display that the -275 affords me. Now here's the rub, when landing I always compare the towers (or AWOS) wind speed and direction and the -275 agrees exactly with what is reported, even when the sun is shining on the probe. If the OAT was inaccurate, I would expect the display to be way off, but it isn't. Maybe OAT isn't a factor in the air data computer calculation?

[ArtVandelay]

Interesting subject. I have a GI 275 ADI and the OAT probe is mounted in the NACA scoop. Reading responses on the subject led me to believe that there would be a huge difference in temperature readings, which worried me because I love the wind speed and direction display that the -275 affords me. Now here's the rub, when landing I always compare the towers (or AWOS) wind speed and direction and the -275 agrees exactly with what is reported, even when the sun is shining on the probe. If the OAT was inaccurate, I would expect the display to be way off, but it isn't. Maybe OAT isn't a factor in the air data computer calculation?

When landing your engine exhaust output is minimal, try checking it in cruise and comparing to forecasted temperatures.

[flyboy0681]

1 minute ago, ArtVandelay said:

When landing your engine exhaust output is minimal, try checking it in cruise and comparing to forecasted temperatures.

I actually have. I feed the GTN 750 Winds/TAS utility with the required data (ie, CAS, Baro) using the temp from the trusty old factory OAT and the calculated wind/speed is very close to that of the GI 275. It's not always identical due to difficulty getting an accurate reading from the old pointer.

[N201MKTurbo]

If you are an IFR pilot, you can check the temperature when the ice forms. It should be zero C. It’s the only temperature that really matters.

[McMooney]

Mine's in the air inlet scoop on the copilots side of the plane, m20e

[Tom 4536]

1 hour ago, McMooney said:

Mine's in the air inlet scoop on the copilots side of the plane, m20e

On my 231 I have one AOT probe where yours is and one under the wing. On the ground they both read identical values; cruising they are within one degree of each other.

[flyboy0681]

1 hour ago, Tom 4536 said:

On my 231 I have one AOT probe where yours is and one under the wing. On the ground they both read identical values; cruising they are within one degree of each other.

Sounds about right.

[0TreeLemur]

My C had two: one in the passenger side NACA duct, and a second on the first inboard inspection panel outboard on the pilot's side.  The one in the NACA duct read a few degrees warmer than the one under the wing..

My J has it on the second inspection panel outboard on the pilot's side. 

[BDPetersen]

FWW I have probes in both NACA scoops, one for G5, one for JPI. Both read the same. Someday maybe do the underwing experiment, but I think it’s close enough for my purposes.

[M20F]

Here is a trick I have found to be very effective.  Put the OAT probe near the exhaust and bingo you never have to worry about icing.  I am always a comfortable 200 degrees and zero concerns about icing up.  

[NicoN]

@ArtVandelay We also have the factory probe under the right wing, while the JPI probe lives in this NACA inlet.

It is ~6° C higher inflight than the fctory gauge. In the hangar they read the same

[flyboy0681]

24 minutes ago, NicoN said:

@ArtVandelay We also have the factory probe under the right wing, while the JPI probe lives in this NACA inlet.

It is ~6° C higher inflight than the fctory gauge. In the hangar they read the same

Interestingly, my JPI probe and Garmin are two degrees off when I take a reading in a closed hangar with static air with no sunlight. When I quizzed others on MS about that, the response fro more than once was that the Garmin accounts for ram rise in its design. Even when I adjust the JPI display to match the Garmin, they eventually don’t agree in flight. 

[EricJ]

39 minutes ago, flyboy0681 said:

Interestingly, my JPI probe and Garmin are two degrees off when I take a reading in a closed hangar with static air with no sunlight. When I quizzed others on MS about that, the response fro more than once was that the Garmin accounts for ram rise in its design. Even when I adjust the JPI display to match the Garmin, they eventually don’t agree in flight. 

That might also be within measurement error tolerance of the probes.   Ram rise should be included in Total Air Temperature, but not OAT.   Ram rise is going to be negligible on our airplanes, anyway.

[PT20J]

I found a formula for ram air temperature rise in deg C = [TAS(knots) / 87]^2. I don't know if this is accurate or not. But, it predicts a temperature rise of 1 deg C at 100 KTAS, 3 deg at 150 KTAS and 5 deg at 200 KTAS which seems reasonable. Garmin corrects the measured ram temperature at the probe to OAT by some unpublished formula (maybe this one) for some instruments (I know it does it for the G3X -- I haven't checked others). 

[flyboy0681]

38 minutes ago, PT20J said:

I found a formula for ram air temperature rise in deg C = [TAS(knots) / 87]^2. I don't know if this is accurate or not. But, it predicts a temperature rise of 1 deg C at 100 KTAS, 3 deg at 150 KTAS and 5 deg at 200 KTAS which seems reasonable. Garmin corrects the measured ram temperature at the probe to OAT by some unpublished formula (maybe this one) for some instruments (I know it does it for the G3X -- I haven't checked others). 

3 degrees at 150kts is not insignificant.

[JeffMirs]

On 2/23/2023 at 1:38 PM, Igor_U said:

Inboard inspection panel under (right) the wing.

My understanding is that's the factory setup.

Left wing better as it won't be influenced by exhaust gases which are also on the right side. Agree on using the existing hole as guide in an inspection panel. 

[PT20J]

Here’s where the factory put it under the right wing on my M20J.

[Aerodon]

I have a bunch of 20' JPI thermocouple wires that are ideal for OAT probes - Type K so can be used for other brands too.  Send me a PM.

Aerodon