2nd OAT Probe

[Cth6]

I am adding a second OAT probe. This one is for my engine monitor. I am planning on placing it 2” behind the existing probe on the inboard wing access panel. I realize it will be in the downstream turbulence zone from the leading probe, but not sure that will really translate into any measurable temp delta. I did some searching, but could not find issues with an install like this. Does anyone have experience with an install like this or issues I should consider?

 

[RV8JD]

Here is what the Dynon SkyView manual says on OAT location. They do not specify next to, or behind one another.

"Sensor Location
The SV-OAT-340 is an outside air temperature sensor. In order for it work properly, it must be able to measure air temperature accurately. Avoid exposing the sensor to sources of heat that would interfere with outside air temperature readings such as:

• Direct sunlight
• Engine heat and exhaust
• Aircraft interior (back side of sensor)
• Heated air from the cabin exiting from an open window or cabin air exhaust port
• The installation area should have space for a nut and wires on the back side of the sensor. It is acceptable to extend or reduce the wire length if necessary. Reduce the wire length by cutting out the desired length from the middle of the wires and splicing together the remaining ends.
• If there is a backup ADAHRS in the SkyView system, it is acceptable to install a backup OAT sensor a few inches away from the primary sensor. Consider running primary and backup OAT sensor wiring together. Tape sensor wire pairs together to avoid confusion later."

 

[Auburntsts]

I'm not understanding why you are installing a 2nd probe.

 

[rvmills]

I also added a second probe when installing a JPI EDM-350 engine monitor. That OAT provides the needed data for %HP. Mounted it adjacent to a wing access panel as well, though the Dynon OAT is back in the aft fuselage...so I cant tell you if there is an airflow interference issue. From the Dynon manual quote above, it appears not to be an issue.

Multi-RV and Rocket builder Tom Martin told me about his OAT placement experience, in which he placed an OAT probe on a wing-bottom access panel, but had ram-rise issues, where his OAT went up as speed increased (it was a really fast Rocket). I actually have measured the same on my aft-fuse mounted OAT (100 knot increase in IAS...70-170...resulted in a 4-5°F rise in temp...slowing back to 70 brought it back down the same amount). Tom's solution was to build a bracket, and mount the OAT probe inside the access panel, with the tip just over the hole where it was previously mounted. He said it alleviated the ram rise issue, and was, to him, a very accurate OAT probe from then on. Might be an interesting experiment to see if your new probe, placed inside the access panel (over a hole in it), reads any different from the existing probe, and if ram rise affects the two probes differently. Dan H would have fun with that data!

Cheers,
Bob

 

[Jpm757]

I have 2 probes installed on left and right inboard inspection covers. They are wired to my dual ADAHARS units.

 

[Bavafa]

I had installed a second OAT probe in my RV7A which was inside the wing root area and it read within 1F different than the other one which was in the airstream.

 

[Toobuilder]

Multi-RV and Rocket builder Tom Martin told me about his OAT placement experience, in which he placed an OAT probe on a wing-bottom access panel, but had ram-rise issues…

I also fought inaccurate OAT’s until I ran across Tom’s account several years ago. As a result, I moved my OAT probe directly behind the pitot mast so that the latter would act as a “wind break”. Temps are far more accurate now. Ram rise is a real thing WRT OAT probes, but in the end, not sure how scientifically accurate we need some of this stuff to be.

 

[Cth6]

I'm not understanding why you are installing a 2nd probe.

One probe is used by the AV-30s for TAS calcs and what I look at normally during flying. The second is for my engine monitor. Really more useful when reviewing performance information after the fact as well as used to compute horsepower. I am upgrading the Engine Monitor and time for a new OAT. The original was installed in a sub-optimal spot, so I am going to use this opportunity to address it.

 

[Cth6]

I had installed a second OAT probe in my RV7A which was inside the wing root area and it read within 1F different than the other one which was in the airstream.

The original probe is currently in the wing root and it shows 4-5 C hotter. I temporarily pulled it in the wing root and while it was great the first 30 mins, it kept climbing 2-3 C hotter than the wing probe. I am guessing the exhaust really is heating that area up.

 

[rvmills]

I also fought inaccurate OAT’s until I ran across Tom’s account several years ago. As a result, I moved my OAT probe directly behind the pitot mast so that the latter would act as a “wind break”. Temps are far more accurate now. Ram rise is a real thing WRT OAT probes, but in the end, not sure how scientifically accurate we need some of this stuff to be.

Michael...did you actually just say that last sentence out loud? Fell outta my chair!

Cheers,
Bob

 

[Toobuilder]

Michael...did you actually just say that last sentence out loud? Fell outta my chair!

Cheers,
Bob

Moment of weakness Bob…. Moment of weakness.

That said, in context of the OPs stated need (horsepower readout), no matter how perfectly calibrated the sensor on an airframe mounted EFIS component, the calculated HP is just a wild approximation anyway.

 

[rvmills]

Moment of weakness Bob…. Moment of weakness.

That said, in context of the OPs stated need (horsepower readout), no matter how perfectly calibrated the sensor on an airframe mounted EFIS component, the calculated HP is just a wild approximation anyway.

Michael, I wouldn't disagree with that last sentence at all!

Cheers,
Bob