OAT probe question

lostpilot28 · #1 07-26-2009, 01:18 PM

I failed to do the appropriate research on mounting location for the OAT probe. I had heard of others mounting it in the NACA duct for cabin air and I thought that would be the end of it.

Well, after flying yesterday in the heat (it was about 103 F where I was at) I noticed that my "ambient temp" reading on my EFIS said it was 110 F! For a short while it bumped up to 112, but for most of my flight over the dessert area, it was around 110. After looking up a few threads on the subject, I found that NACA scoop mounting locations read significantly higher than actual temp.

Why is this? Was my temp probe really picking up heat from inside the cockpit? I find it hard to believe it was 110 in there. If anything, I thought the lower pressure from the NACA scoop would cause the temp to read lower.

Outside of relocating my probe, what can I do to correct this? Would insulating foam on the back of the probe help? Any tips/suggestions are greatly appreciated!

Ironflight · #2 07-26-2009, 01:37 PM

I have always figured that the heat there is coming from outside the back edges of the cowling Sonny - I know that was the case on my similarly configured Yankee, because when I put a big engine on that bird, the amount of heat coming in the vents increased considerably!

I don't know hoe you're ever going to get good temps in that location - better to bite the bullet and move the 'ducer somewhere in the shade. RV-8 guys are lucky - we have a NACA scoop under the wing that gives very accurate OAT's. On Louise's RV-6, I mounted the Dynon OAT probe on the inspection cover on the side of the fuselage under the horizontal tail - that has proven to be a good spot for that bird.

Paul

erich weaver · #3 07-26-2009, 01:41 PM

Heat coming off the cowl is being picked up by the probe at the NACA inlet. I dont think there is a good solution other than moving it. A better location is under the wing, near one of the access plates. I had the same issue. Under the wing mo betta bra

erich

roee · #4 07-26-2009, 03:02 PM

To get accurate readings, the probe should ideally be mounted:

1. Out of direct sunlight.

2. Well away from heat sources (engine, exhaust gas, cabin, landing light, etc.).

3. In clean airflow at the same airspeed as the aircraft / pitot tube (i.e. away from prop wash, and away from structures that would disrupt the airflow). The measured air temp at the probe rises with TAS due to adiabatic heating of the air compressing against the front of the probe, and friction. Knowing the airspeed and the aerodynamic properties of the probe, these effects can be compensated for numerically to arrive back at the true OAT. I'm not sure if all/most/any EFIS packages actually do this computation (they should), but if they do, then the OAT probe should be exposed to the same airspeed as the aircraft to arrive at the most accurate solution.

I'm about to mount my OAT probe under the wing, either on the outboard-most inspection panel or the middle one. This is a compromise between the above guidelines and other practical considerations, primarily accessibility for maintenance. I'm still well away from first flight, so I won't have results to report for a while. But at least in theory, I would expect good results with this location.

RV7ator · #5 07-26-2009, 03:13 PM

Hi,again, Sonny,

Ya gotta move it. All insulation does is slow down the response time.

My latest has the probe on the bottom center fuse skin that screws onto the inboard wing rib. Others have been nearby to the extent the factory pigtail allows. All worked.

Consider this. Instead of going through all the pain of MOVING the probe, abandon it and buy a new one to install where you like. How much is time, maybe ruination and frustration worth (and you've a hole to plug)?

If you mounted the remote compass abaft the 706 baggage bulkhead, figure a way to get the wires from there forward. Remove a wing root fairing. Drill the above mentioned skin for the probe (remember it's inside mounted) just aft of the aileron pushrod, then a hole through the fuse side skin, thence your chosen route to the compass, all within reach of the 10' wire, of course. A bit of groping with really long nose pliers should get the new one installed pretty easy.

You get your bird painted, yet?

John Siebold

RV7ator · #6 07-26-2009, 03:18 PM

Hi,again, Sonny,

Ya gotta move it. All insulation does is slow down the response time.

Consider this. Instead of going through all the pain of MOVING the probe, abandon it and buy a new one to install where you like. How much is time, maybe ruination and frustration worth (and you've a hole to plug)?

My latest has the probe on the bottom center fuse skin between its extension outboard of the fuse side skin and where it screws onto the inboard wing rib. There's room. Others have been nearby to the extent the factory pigtail allows. All worked.

If you mounted the remote compass abaft the 706 baggage bulkhead, figure a way to get the wires from there forward. Remove a wing root fairing and a seat pan. Drill the above mentioned skin for the probe (remember it's inside mounted) just aft of the aileron pushrod, then a hole through the fuse side skin, thence your chosen route to the compass, all within reach of the 10' wire, of course. A bit of groping with really long nose pliers should get the new probe installed pretty easy.

You get your bird painted, yet?

John Siebold

Webb · #7 07-26-2009, 03:28 PM

One of the best spots I've seen is the most inboard inspection panel cover. Put a connector on it and when you pull the cover, you can disconnect the wire. Wish I had put mine there. I'm on the bottom skin closer in.

roee · #8 07-26-2009, 04:20 PM

Quote:

Originally Posted by Webb

One of the best spots I've seen is the most inboard inspection panel cover. Put a connector on it and when you pull the cover, you can disconnect the wire. Wish I had put mine there. I'm on the bottom skin closer in.

Webb,

Why do you prefer the most inboard inspection panel rather than the more outboard ones? I was thinking generally the more outboard the better to get away from the prop wash and into clean air flow.

(I'm leaning toward the middle inspection panel rather than the outboard-most one only to get away from potential interference with the aileron bell cranks, A/P servo, heated pitot, tie down rings, etc).

-Roee

Rick6a · #9 07-26-2009, 04:57 PM

Like many builders, I originally installed the OAT probe in a NACA vent but quickly learned it produced highly inaccurate readings. After putting up with it for too long, I invested a few hours and relocated the OAT probe, centering it on an aft access cover that I fabricated for my RV-6A when cutting in an access on the starboard side was considered a builder option. The probe has a generous service loop so I can move the access cover well out of the way for routine inspection of the area. Although installing the probe out on a wingtip is probably the ideal location, I consider my OAT gauge to now produce reasonably reliable data.

Lars · #10 07-26-2009, 06:01 PM

Quote:

Originally Posted by erich weaver

Heat coming off the cowl is being picked up by the probe at the NACA inlet. I dont think there is a good solution other than moving it. A better location is under the wing, near one of the access plates. I had the same issue. Under the wing mo betta bra

erich

Erich, sent you a PM...