I have a Airtronics radio system (vg400) the servos have 3 wires, center red with black and blue I can plug the servos into the receiver in both directions what is correct black facing outside or blue?

Plug it in one way or the other. If it works-you were correct. If it doesn't work-no harm done. Just flip it. It will NOT damage anything to have it reversed.

Not the preferred way I know but functional.

Airtronics receivers are keyed so that the servos will only go in one way. The plug looks like the one in the sketch.

THESE ARE THE NEW "Z" conectors and go either way

Airtronics Z, Hitec S and Jr are keyed by those tiny bevels however they can be easily plugged in backwards. Another simple method is to just remember all the majors which use a topside plug orient the ground (Black lead) toward the outside of the case. Never have to stare at those tiny bevels agine.

John