RE: DX8 telemetry RPM pickup for Gas engines - SOLVED!!
Too bad the Tach Output from the CDI boxes do not work. I personally am uncomfortable adding an extra connector in the ignition circuit on $3500+ planes. It introduces another potential failure mechanism very near the source of all vibration. The Y adapter is however good for initial testing to figure out what is needed for each type setup, though. That is an excellent idea. Maybe once it is all working on the bench, one should remove the Y and just solder in and re-insulate a tap on the needed wires? However, this too has it own disadvantages in that solder makes wires stiff and more suspetable to breaking.
I suspect that the different experiences seen with the need or not for a ground wire has to do with the various ground paths already on each different plane. For a signal circuit to be reliable, one has to have a complete circuit (i.e. a ground and a signal.) Also, multiple ground paths can also introduce its own set of problems. So, I suspect the differences that are being seen from plane to plane may just be grounding related. I needed the ground wire and a signal wire, probably because I have isolated the CDI circuits from the Receiver/Servo circuit. Anyone else done any significant testing on aircraft circuit grounding?
Last but not least; Usually, we take great pains (e.g. separate ignition batteries, fiber-optic kill switches, non-conductive throttle linkages, etc.) to ensure the CDI circuits and the Receiver circuits are physically isolated. This helps to preclude RF and other stray voltage from the CDI module impacting the Receiver and Servo circuits. RPM Telemetry seems to preclude this protective design feature. One can't help wondering if adding telemetry undermines the basic design of a well designed aircraft's isolated electrical systems. Maybe the manufacturers are buffering the telemetry? Maybe the need for electrically isolating the CDI from the Receiver is over-hyped by the various manufacturers? Maybe not! Guess we would have to look at the post-flight receiver data to be sure. Anyone else with telemetry and a isolated CDI system testing and/or monitoring this?