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Outside air temp. probe

N13BN

Well Known Member
If anyone has suggestions where to install an outside air temp sensor, I would be most interested. I'm trying not to re-invent the wheel.

Bill Newkirk
N283RV
 
Where ever you can reach the back of it, out of the sun, and out of the exhaust trail.

Mine is on the bottle of the wing, near the tank drain and I can reach the back through the inspection plate.
 
I mounted mine ON the inspection plate under the wing. Easy to remove and replace if needed.
 
Mine is also on the lower wing, and a lot more accurate than those that are stuck in NACA inlets on the side of these planes. The inlet mounted probes usually show higher temps.

L.Adamson --- RV6A
 
If you mount it anywhere on the fuselage it will not read accurately... heat from the engine will bleed out and effect it. I recommend in one of the wing bays on either side of an inspection cover, easily reached that way. You could put it in the inspection cover also but then you need to disconnect it to get the cover out of the way.
 
The NACA inlet doesn't work, I tried that and it reads high. I am in the process of trying a new place in the FAB intake. Maybe some one else has tried this but my first attempt is to put it just down stream of the Carb heat door inside the FAB. My theory is normally it will read inlet air temp and when I pull Carb heat I should be able to see a difference. The best choice perhaps will be two probes, one before the carb heat flap hinge and one down stream, that way you would always have an OAT. The compression of inlet air may raise the temp a tad but it has to be more accurate than the NACA inlet. Coments?
 
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Dynon Manual for OAT probe:
Mount Location
It is important that the OAT probe be mounted somewhere on the skin of the aircraft where it will not be affected by heat sources (sun, engine, aircraft interior, etc). It is acceptable to extend the length of the included wiring for the OAT. The ideal location would receive no heat from the aircraft engine or any other source in the aircraft body. While this may be impractical, it is a good idea to mount the probe as far away from heat sources as possible. On the RV series, common locations include the wingtip and under the horizontal stabilizer.
Depending on the "system" you install might affect the location e.g. on the Dynon it can be wired into the Remote Compass... so wherever you choose the remote compass to go might affect the OAT probe.

On our 8 we have the probe under the HS, and have no reason to believe it is anything other than accurate for it's needs - i.e. very occasionally look at it out of interest ;)
 
Interesting idea!

The NACA inlet doesn't work, I tried that and it reads high. I am in the process of trying a new place in the FAB intake. Maybe some one else has tried this but my first attempt is to put it just down stream of the Carb heat door inside the FAB. My theory is normally it will read inlet air temp and when I pull Carb heat I should be able to see a difference. The best choice perhaps will be two probes, one before the carb heat flap hinge and one down stream, that way you would always have an OAT. The compression of inlet air may raise the temp a tad but it has to be more accurate than the NACA inlet. Coments?
Roger on the NACA inlet not working, heard from dozens of builders on that. Locating it in the FAB is an interesting idea, never heard of it before. Might just work since it should be getting clean air before anything has a chance to warm it. I would think locating it as close to the inlet as possible would be the way to go just to reduce the chance of it picking up any heat. Regarding picking up heat due to the (slight) compression or speed of the, I'll leave that to the engineer types, but it should be considered.
 
Roger on the NACA inlet not working, heard from dozens of builders on that. Locating it in the FAB is an interesting idea, never heard of it before. Might just work since it should be getting clean air before anything has a chance to warm it. I would think locating it as close to the inlet as possible would be the way to go just to reduce the chance of it picking up any heat. Regarding picking up heat due to the (slight) compression or speed of the, I'll leave that to the engineer types, but it should be considered.
I'm not familiar with FABs, as I have fuel injection with the snorkel horizontal induction system. Would the back side of the probe be heated by air warmed by the engine? If so, it would almost certainly read too high, just as ones mounted in NACA scoops on fuselage sides read too high (they are warmed by cockpit air on the back side of the probe).

Any probe that sees the blast of moving air flow will read a bit high, no matter if it is in a FAB, on the bottom of a wing or in a NACA scoop, etc. But, the amount it reads too high should be fairly predictable, and you can take it into account when you look at the OAT. The probe will read too high by an amount:

ram temp rise = K * (TAS^2)/7592

where K = the recovery factor of the OAT probe - probably between 0.8 and 0.9.
TAS is true airspeed in knots
the temperature rise is in degrees C

So, at 180 kt TAS, the OAT indication would probably read about 3 deg C too high.

If the probe is mounted somewhere where it is not subjected to an air blast, it may not see a ram temp rise, but it probably is affected by a large delay when responding to changes in actual air temperature.
 
On or near an inspection plate on the wing is perfect, assuming you're either still building or can run the wires easily.
 
From Kevin's post "I'm not familiar with FABs, as I have fuel injection with the snorkel horizontal induction system. Would the back side of the probe be heated by air warmed by the engine? If so, it would almost certainly read too high, just as ones mounted in NACA scoops on fuselage sides read too high (they are warmed by cockpit air on the back side of the probe)."
I aways assumed the NACA read high due to blowby at the cowl attach point. If the temp of the base affects the reading, the the FAB may not work so good because the base will be in Warm engine heated air. Are you saying the probe tip will be at a temp affected by incoming air on one end and a hot base on the other and therefore the net has to be warmer than incoming air alone? Reasonable and interesting, I think I need to do some tests with my solder gun tip!!
 
From Kevin's post "I'm not familiar with FABs, as I have fuel injection with the snorkel horizontal induction system. Would the back side of the probe be heated by air warmed by the engine? If so, it would almost certainly read too high, just as ones mounted in NACA scoops on fuselage sides read too high (they are warmed by cockpit air on the back side of the probe)."
I aways assumed the NACA read high due to blowby at the cowl attach point. If the temp of the base affects the reading, the the FAB may not work so good because the base will be in Warm engine heated air. Are you saying the probe tip will be at a temp affected by incoming air on one end and a hot base on the other and therefore the net has to be warmer than incoming air alone? Reasonable and interesting, I think I need to do some tests with my solder gun tip!!
I would hope that a high quality OAT sensor would be relatively insensitive to the temperature of the back side of the probe. But, many builders are quite interested in low price when purchasing engine monitors, OAT systems, etc. So it is possible that some vendors are purchasing lower cost OAT probes that might not be so well insulated on the back side, so they can quote lower prices.

I flew with an RV-6A builder many years ago to help him gather glide performance data. It was quite cool, so he was using cockpit heat. I was suspicious of the possible effect of the warm cockpit air on the OAT indication, as the probe was in the NACA probe on the fuselage side. We experimented, and the OAT indication would definitely increase if the cockpit heat was ON, and decrease if the cockpit heat was OFF. I don't remember how much the change in indicated OAT was, nor do I recall the manufacturer of the OAT system.

The OAT may also be affected by heat coming from the cowling as you mention. I don't know enough about heat transfer to have an informed opinion on this. Nor do I have any data to support or refute this hypothesis.
 
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Thanks for all the replies. My -3 is flying and running the wires from the wing will be a PITA, however, it seems to me that an under-wing inspection cover location might be the way to go. The wiring can be piggy-backed onto the auto-pilot wiring that I recently installed.
By the way, my new Trutrak is the cat's meow. I'm pretty dumb about wiring but I finally got it right. It follows a GPS heading very accurately and just this morning I was able to get it to intercept a GPS route. Just great. Now I'll be able to unfold and look at a chart. How novel.

Bill Newkirk
N283RV
 
We mounted our OAT sensor on the bottom of the wing, away from engine compartment heat and direct rays from the sun.
We drilled a hole in the bottom wing skin (just aft of the rear spar and in front of the flap brace and flap hinge) we located it as far outboard as we could get before reaching the aileron which will allow running the wires behind the flap attach brace.
The biggest problem we had was to remove and reinstall the flap which really is no big deal.
 
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Ok, I took an OAT probe and mounted it on a sheet of cardboard. With a solder gun I could raise the air temp beneath the tip of the temp probe easily and the the digital display responded accordingly. Then I went on the other side of the cardboard and heated the body of the temp probe far from the sensors tip and sure enough the digital reading went up easily. So this tells us in an application out on the wing where all parts of the probe will be generally at the same temp they should work well. However, on the NACA scoop or where I am going to put it, on the FAB, if the body will generally be hotter than the air being measured we will get a falsely high reading. The NACA scoop being plastic may be easy to fix. If we insulate the mounting hardware from cabin air temps it should read fine. Hey, maybe the NACA scoop will work after all if the inside mount is thermally insulated. Maybe the spray foam would be enough. I mounted my OAT on the Aluminum part of the FAB which will make it hard to insulate from warm engine air. I think I need to plug that hole (already there) and mount it on the fiberglas part of the FAB which conducts heat less well than AL. Then I should be able to insulate the probe mount from the engine's warmed cooling air. Maybe a foam plug fiberglassed over will do. Kevin, thanks for pointing us in this direction.
 
Try the aft bulkhead

I put mine here:

It reads high while on the ground but as soon as the plane start moving, enough air gets in there for it to be accurate.

BTW, this is an old topic, try doing a search and you will find even more options.
 
I like the idea of putting it in the FAB on the fiberglass side. On the -3 and -4 we usually don't have NACA vents or but one inspection plate under the wing. That plate is not easy to get wires to either.

I wonder if someone has done this and checked it against one located in a more conventional place?
 
OAT Probe Location

I also tried in the NACA duct and was not happy. I finally mounted mine in the side of the fuselage inside the wing root. It's just forward of the rear spar. It's out of the sun and exhaust stream and gets plenty of air from the flap opening in the back. You can get to the front by removing the aft wing fairing and the back by removing the FCP floor. I've measured it's reading both on the ground and in the air and it's easily within a degree. The best part is it's out of sight and no drag (I know, very little but it all counts).

Cheers,

Bill
RV-8 x 2
 
i put mine underneath the left cowl cheeck just forward of the wing bulkheads. Shortest run to instrument and very easy to check in the pre flight. I thought it might read a bit high but thats ok with me. Checks with ATIS are always within a degree or two.
Tom
RV3x2 2000+ in Rv3s
 
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