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G3X OAT probe and shielding

G

GTWreck

Active Member
I'm wiring up the OAT probe for my G3X, and it comes with a 3 conductor shielded cable. The manual calls for the shield to be connected to the backshell at the GSU73. I had planned for a circular connector (CPC) at each wing root. This would require that I cut the OAT cable to connect it through the CPC.

How important is it that the shield have connectivity all the way back to the AHARS (GSU73)? I figure I have 3 choices:
1. Connect the shield to a pin in the CPC
2. Ground the shield at the wing root
3. Don't connect the shield to anything.

Thoughts or suggestions?

Thanks,
Aaron
 
Hi Aaron,

My plan is the same as your's - a CPC connector at eh wing room. I am planning on carrying the shield through a pin - I am just guessing that the shield on the OAT is probably not THAT significant, and if I am wrong, it will not make the airplane fall out of the sky - I'll just have to go in and fix that detail. But I strongly suspect that the shielded OAT wiring is overkill....

Paul
 
Since this same OAT probe is used on everything from LSAs to the Citation Ten, I believe the fact that they went overboard with shielding is due to lightning protection requirements for the big iron. On my airplane, I plan to carry the shield through the connector via a pair of pins, just as Paul described.

mcb
 
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I can report from experience that we have several flying with the OAT shield carrying through connectors - both CPC and D Sub and like Matt said, there is not going to be a problem.
 
Hi all.

An additional question on this - since the probes are resistive, do you see a wrong reading from the resistance added by the connectors? (I'd imagine not or just varying the wire lengths would also have some effect)

Thanks!
 
With more than four years of experience, I can now say that the the CPC connector has been a pain - not for the shield, but becasue of some sort of corrosion that changes the resistance and biases the OAT readings high. I've had to go in and clean this up a couple of times now - and on an RV-3, that is a HUGE pain becasue there is just no room to easily get a hand in there to disconnect the CPC. It has taken me more than an hour of fiddling each time. Likewise, getting at it from the inside is hard becasue it is under the riveted seat pan.

Next time, I'd realize that the only way I am ever going to pull the wings off is if the airplane is in a field somewhere, so I woudl just run extra wires out the root and make the connections inside the fuse. Until you mount the wings, you just don't realize how little room is availabel between the root and the fusealge (on a -3).

Paul
 
Maybe a thin coat of grease on the connections will help. If the connection is tight enough, the grease will be forced out from between the metal to metal contact point. The grease surrounding the contact point will help prevent corrosion.
 
Data point

So far, I'm just under 1.5 years and 130+ hours past my first flight. Thus far, I have not had any corrosion issues with connections in the CPC. However, the number of nights that the plane has had to live outside a hanger is small. On the other hand, I do live in one of the most humid areas of the country.

No problems either on temperature probes reading the correct values.
 
I have used terminal strips in the fuselage just inside the side skin. Slip the wires through a grommet in the skin with a little service length. Eyelet terminals on each wire go to the strip that is glued to the floor. Label each connection by writing on the skin or label maker. When and if a wing has to come off just unscrew each wire. Could have as many as 12 connections that are relatively easy to get to with very solid connections.
Food for thought.

Rt
 
Ironflight said:
With more than four years of experience, I can now say that the the CPC connector has been a pain - not for the shield, but becasue of some sort of corrosion that changes the resistance and biases the OAT readings high. I've had to go in and clean this up a couple of times now - and on an RV-3, that is a HUGE pain becasue there is just no room to easily get a hand in there to disconnect the CPC. It has taken me more than an hour of fiddling each time. Likewise, getting at it from the inside is hard becasue it is under the riveted seat pan.

Next time, I'd realize that the only way I am ever going to pull the wings off is if the airplane is in a field somewhere, so I woudl just run extra wires out the root and make the connections inside the fuse. Until you mount the wings, you just don't realize how little room is availabel between the root and the fusealge (on a -3).

Paul
Click to expand...

Are you using sealed connectors? Those would make corrosion much less likely since the pins are much less exposed (they're more expensive and you get higher temperature increase with high current though).
 
rdamazio said:
Are you using sealed connectors? Those would make corrosion much less likely since the pins are much less exposed (they're more expensive and you get higher temperature increase with high current though).
Click to expand...

No to the sealed connectors - becasue of the seals, they would take even more force to make or break - they'd be impossible to use on an RV-3 at the wing root to fuselage point becasue the space is so small.

I was always convinced that I wanted wing root disconnects in my airplane's, but the truth is, I have no need to pull the wings off once they are on, and if I do have that need, it would be easy to cut the wires and deal with the few-ring on re-assembly. Now I am of the belief that the fewer connections the better. Live and learn.
 
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