lostpilot28

lostpilot28

Well Known Member
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!
 
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
 
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
 
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.
 
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
 
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
 
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 said:
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.
Click to expand...

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
 
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.

2v30t47.jpg
 
erich weaver said:
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
Click to expand...

Erich, sent you a PM...
 
Well, I kind of figured that's what you all would tell me! :D

Thanks, Paul...I was thinking of the engine heat escaping the aft cowl, but didn't know if it would affect the readings that much.

I think the rest of you guys are right about putting it under the wing. I was thinking of the inner most panel, but moving the wires through a couple more bulkheads to get away from exhaust might be a better idea. The only problem is that I have 2 MGL Voyagers and each has it's own probe...so I need to do this twice! Maybe I can mount both of them in the same place...

The thing about the MGL probes is that they're very small, and not very sturdy looking. They don't really have a mounting flange or anything, so I think I just need to drill a 1/4 - inch hole and epoxy the probe so that the tip is exposed to the slip stream.

One other thing to note about the MGL probe is that they only give you about 6 feet of wire length (I think...maybe less)...anyone care to guess whether these leads need to be the "special wire" variety, like the CHT & EGT probes?

P.S. - John, I swung by your hangar the other day, but you weren't there. Just wanted to see how things were going. I haven't painted mine yet...still working up the courage to do it, and waiting for the temps to drop below 85 or so! I'm thinking it'll happen in a couple months. Drop on over anytime...I'm out there often!
 
I put my probe in the left wing, at the location where Van's has you drill the hole for their very simple pitot tube, just a piece of bent stainless tubing. I originally thought I would do that, and drilled the hole in the wing. Later I decided to go with a Dynon heated AoA pitot tube and Gretz mast, which is mounted differently - so now I had a hole to plug. The OAT was going to be installed somewhere on the airplane anyway, so it was an easy decision.
 
On two RVs, I put the OAT probe in the fuselage skin, sticking into the wing root area. It is shaded and gets plenty of air flow for accurate readings. Also, it allows you to use a shorter wire run and you do not have to transition into the wing. I ran the wire under the floor for a behind the spar location but you could also put it in front of the spar.
Worked for me. Temp readings seemed to be very reliable.
 
"accuracy"

sf3543 said:
...Temp readings seemed to be very reliable...
Click to expand...

This brings up an interesting question: How do you know? I mean, it's pretty easy to verify OAT accuracy on the ground, but what about in flight, which is what we really care about? Those of you how say your OAT readings are "reliable", or "accurate", how exactly did you arrive at that conclusion? How accurate is accurate? And what reference did you compare your readings to? Fly side-by-side with another aircraft with a known accurate OAT instrument? I think it's an interesting question because to really talk about accuracy we need to 1) define accuracy quantitatively (how accurate does it need to be?) and 2) be able to actually measure to verify it under true operating conditions, i.e. in flight.
 
You must log in or register to reply here.