Upside Down Battery Contactor

[Raz]

Hey, y'all.
Any proplem mounting the battery master contactor upside down? I'm building an aft-battery RV-8, and the 2awg cable fits much more cleanly in that configuration. Thanks in advance.
-raz

[Ron Lee]

You would have to install a hemispheric polarity rectifier.

[Phil]

I think the answer depends a little on your mission.

There's some logic in how they are mounted, so G forces won't open a weak contactor. If they're right side up an you pull positive G's, they are pulled closed. If it is upside down and you pull positive G's, they're trying to pull open.

I haven't seen it happen, but I do know that's the logic for mounting them that way.

You can twist them 90 degrees and protect against both, if that's an option for you?

Phil

[Phil]

By the way, this is the reason the starter contactor is mounted the opposite direction.

If you pull positive G's, it is being pulled open so it won't close and attempt a mid-air restart with the engine already running just fine at 2300 RPM.

[Flying Scotsman]

Originally Posted by Phil

By the way, this is the reason the starter contactor is mounted the opposite direction.

If you pull positive G's, it is being pulled open so it won't close and attempt a mid-air restart with the engine already running just fine at 2300 RPM.

I have to ask...is this *really* a problem? I know that's why they're mounted that way, and did it myself, but honestly...is this preventing a problem which doesn't exist? And what about the negative gs? If I pull -3 gs, is that okay, but +6 would cause the thing to engage?

Heck, maybe it would, I dunno...I'm just asking if this is a) a real problem, and b) the "fix" truly fixes it or just makes us feel better

[erich weaver]

I know that bob knuckolls has said on his AeroElectric list that the g force is insignificant relative to the electromagnetic force generated by the contractor and so it is basically not worth worrying about. One of those myths that never seem to die apparently.

Erich

[aerhed]

If a starter contactor is upside down the only thing keeping the points apart is a little coil spring. If it fails the starter will crank. If it gets overheated and loses temper it may not be able to rebreak the points after a start. Can't hurt to have gravity on your side. Same for master relay only not as critical and its backwards because you're trying to keep the points closed.

[Mel]

Originally Posted by erich weaver

One of those myths that never seem to die apparently.
Erich

May be a myth, but a few of "old" mechanics have seen it happen; especially when the solenoids are old and the springs get weak. Granted, not often, but sometimes.