Are you SURE you're an Electrical Engineer?
When I say "common-mode" the static potential is present on the main batt. positive, also.
The majority of helis on the market today use carbon fiber frames, a conductive material. There isn't going to be much in the way of static charge buildup that will cause any operational problems.
Helis with plastic tail cases and/or painted/hard anodized tail booms, AND BELT DRIVEN tails MAY
suffer from static discharge -- and its attendant spark.
With a plastic tail case or a painted/anodized tail boom, the tail pulley and shaft are insulated from the frame and the front drive pulley. The front end of the boom is also usually well insulated through the use of a plastic tail block and through the paint or anodizing.
As the belt contacts and leaves the pulleys, they generate a high potential static charge (triboelectric charging by separation). As the charge builds up to a high enough voltage, that potential can create a spark, as the dielectric strength of the surrounding air is exceeded. You get a conductive path to an adjacent, lower potential surface.
There are a few videos out there showing the discharge occurring at the front end of the tail boom, near where it enters the frames, with the spark terminating on the case of a tail rotor gyro, or perhaps a receiver.
Plastic and painted/hard anodized aluminum are non-conductive. The idea behind grounding the tail rotor shaft to the frame is that you provide a low impedance path from the tail rotor shaft and its pulley, to the frame, so that no excess static charge can build up, and therefore, no spark occurs.
I don't believe I have ever heard anyone talk about "common mode static electricity" till now. And static charge present on the positive terminal of the battery, too? Last time I checked, the impedance of a battery (voltage source) is extremely low, as in a few milliohms. That would be between the positive and negative terminals. You're NOT going to have any static charge voltage differential between + and -.
Only a FEW helicopter designs have been prone to static discharge events, and the problem ONLY occurs in helis with belt-driven tail rotors. It's a rare event. Dry air is a contributing factor, and we see a lot of that in the winter.
Grounding all sorts of stuff in the airframe to your battery is not a useful or productive effort for all helis. It may be required for those few belt-driven tails prone to static build-up and discharge.
72 MHz (as well as the the 27 and 35 MHz) radios of days gone by were quite prone to interference from the electrical noise generated by loose metal to metal contact. This was not static electricity, by the way. Loose mechanical connections or dissimilar or corroded metal connections form non-linear electrical junctions, which act as unintended "diodes" or mixers. Add some nasty oil with bits and pieces of fine metal particles, you have a fun dielectric in the mix. When these devices are excited by sufficient level of one or more signals they generate intermodulation products.
I used to demonstrate this quite easily by simply dragging a small screwdriver tip along the cooling fins of my motors on planks. I learned real early in this hobby, as a young lad, NEVER to use a metal clevis on a throttle connection.
I also fought the metal to metal gremlin in a Schluter Heliboy that took down every radio I threw at it...AM, FM, and even PCM, Airtonics, Futaba, and JR. The culprit turned out to be a sharp burr on a stamped metal pitch change mechanism at the base of the rotor shaft. Found the burr, filed it off, and voila, the Heliboy was cured.
Static discharge is a force to be reckoned with on electronic equipment. As an avionics hardware designer, one of the common required qualification tests all avionics is required to pass is an "ESD" test. The unit being tested is powered up and and put in its operational mode. The unit is then zapped multiple times, in multiple places, with an ESD gun -- at 15 kilovolts. The unit has to play through the discharges without interruption or unexpected behavior.
But back to the problem at hand. As this helicopter uses a SHAFT drive, it's NOT going to have a static electricity build-up/discharge problem. The behavior that usually follows an ESD hit on our electronics is not just an annoying glitch on a single control function. It is usually a catastrophic result, as the discharge disrupts the processors in the RX and/or gyro, causing a full system reset in the air. That ain't pretty.
2.4 GHz radios ARE quite susceptible to ESD created by static discharge.
If the OP isn't using a ferrite toroid on the ESC to RX connection, this would be the time to install one. Get as many turns on the toroid (donut) as you can...the more you get, the better this will work. ESCs output quite a bit of electrical noise that if they find their way to your power and/or ground signals, can wreak havoc with today's processor based systems.
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