[kirkbauer]

I'm working up to starting on my electrical design for my RV-10 build and I had a few basic preliminary questions. I have read the AeroElectric book and have a pretty strong electrical background.

1) It feels like there should be a clear answer on how to handle grounding in an aluminum aircraft, yet I can't find that clear answer. For example:

• In the FlyEFII installation manual (something I'm considering), it definitively says that there should be a single ground bus hardwired to the battery
• But the RV-14 kit comes with wiring harnesses that utilize the chassis as the ground

Is there a clear "best" solution for grounding?

2) I thought about putting in an external/ground power connector to (1) jumpstart the plane if the battery is dead and (2) to provide power for setting up and configuring the avionics. I don't love the idea of adding a custom external connector, and I'm not clear on what plug standard I would use for this. So, alternatively, I'm thinking about putting external battery terminals in the cargo area with a cover, connected directly to the battery. I could connect any battery charger or jumper cables to these to accomplish the same thing.

Question: I have seen some electrical diagrams with a ground power port, and every one of them has a contactor for the ground power. Why is that? Is it just to prevent the terminals from being energized when not in use? Do I need a contactor if I do this? I'm thinking something like this in the cargo area:

 

[Carl Froehlich]

While it is a good idea to have external power for Hangar Flying, this need only handle ~20amps or so. I suggest that having a ground power connection robust enough to jump start the engine is not a good design goal - as in jump starting and engine then flying off into the ether with a dead battery may not be a great idea. Use the external power to charge your battery then go fly. If the battery will not take a charge, replace it.

A simple pigtail tucked under the panel when not in use is the simple solution for external power. Use a real regulated power supply, not a battery charger. Such 30 amp supplies are available for $100 or so.

Carl

 

[Mich48041]

Bob Nuckols' Z-31 shows ground power jacks. A contactor serves 2 purposes:
1. Prevents reverse polarity from damaging your expensive avionics and equipment.
2. Prevents overvoltage from damaging your expensive avionics and equipment.
A combination breaker-switch is needed to deenergize the ground power contactor.
Without that, the contactor will remain energized (back-fed from battery) and run the battery down.
Installing a flashing light will remind that the ground power contactor is energized.

 

[DerekS]

Is there a clear "best" solution for grounding?

No real best answer. Airplanes are electrically noisy, so we do our best at generating as little noise as we can and protecting other wires from having noise induced into them. I like forest of tabs but I'll also ground locally to avoid having to run a fat wire all the way back. Generally the idea of a central grounding point is to use the same zero reference so all the noise comes in on the signal side. There are little dynamic voltage differences around the airframe, and these get bigger if you are flowing large currents through the airframe.

I think you'll find universal alignment that anything having low voltage signals should be centrally grounded. e.g. all the avionics though you could argue 12v RS232 isn't really all that low voltage in this context. This is to avoid having differences in voltage at the ground add noise to the signal and create either noise you hear or spurious signals. Certainly anything related to audio is in this category. Things like fuel pumps, flap motors, lights, etc don't really care about the noise, but do send out a nice magnetic wave looking for a wire (or even better a loop) to induce current into when you turn them on and off. You hear that as a tick or squeak, which isn't bad if you only switch them once, but more frequently can be annoying. So wig-wag landing lights, led strobes etc are better to have the offsetting return current right next to (or twisted with) the supply since it cancels out the magnetic field and noise. (This is why you will see twisted wires / shields specified for these)

For the 10, with the battery in the back, unless you want to run a #2 all the way to the front or a zillion wires all the way to the back you will be using the fuselage as a ground. The key is that for anything noise sensitive, it has a common reference which is what I use the forest of tabs on my firewall for.

2) I thought about putting in an external/ground power connector to (1) jumpstart the plane if the battery is dead and (2) to provide power for setting up and configuring the avionics. I don't love the idea of adding a custom external connector, and I'm not clear on what plug standard I would use for this. So, alternatively, I'm thinking about putting external battery terminals in the cargo area with a cover, connected directly to the battery. I could connect any battery charger or jumper cables to these to accomplish the same thing.

A lot of chargers use the SAE type connectors, and there are nice bulkhead versions of them. I put them on the back baggage wall, one for each battery. They won't take the full load of trying to start the plane but do just fine for quick charging, and powering avionics. If I run down my battery I want to have a good feeling that it will take a charge, and isn't planning to melt down before I launch, so I'm happy to wait the hour or so for the charger to do its thing.

Question: I have seen some electrical diagrams with a ground power port, and every one of them has a contactor for the ground power. Why is that? Is it just to prevent the terminals from being energized when not in use? Do I need a contactor if I do this? I'm thinking something like this in the cargo area:

Yep - and reversed polarity.

 

[wjb]

1) It feels like there should be a clear answer on how to handle grounding in an aluminum aircraft, yet I can't find that clear answer. For example:

• In the FlyEFII installation manual (something I'm considering), it definitively says that there should be a single ground bus hardwired to the battery
• But the RV-14 kit comes with wiring harnesses that utilize the chassis as the ground

The ground return is just as important as the power feed; treat it the same. Design in an explicit return for all electronics. Bringing each back to a single point at/near the battery is now a no-brainer. Lots of planes use the fuse as a "free ground return" but the fuselage is not designed for this (ie, explicitly electrically-bonded joints/seams) and it is just asking for problems in the future. YMMV, but my plane has explicit ground returns to a single point "forest of tabs" on the firewall battery negative terminal passthrough.

 

[kirkbauer]

While it is a good idea to have external power for Hangar Flying, this need only handle ~20amps or so. I suggest that having a ground power connection robust enough to jump start the engine is not a good design goal - as in jump starting and engine then flying off into the ether with a dead battery may not be a great idea. Use the external power to charge your battery then go fly. If the battery will not take a charge, replace it.

A simple pigtail tucked under the panel when not in use is the simple solution for external power. Use a real regulated power supply, not a battery charger. Such 30 amp supplies are available for $100 or so.

Carl

Thank you! I should explain the origins of my jump-start requirement. I once stopped to pick up some items and a few hours later I got back to my aircraft only to find out that I had accidentally left the master switch on. The battery was dead and I was sitting on the ramp at a random airport. The airport had a jumpstart pack and I had to remove the rear cargo wall in the RV-10 to connect it to the battery. My partner has also left on the dome light in the plane before (over a longer period of time) and had the same issue.

In my case, with a backup battery (that can run the essential bus but not the starter) and two alternators, in VFR conditions, I don't think it was unsafe to jump-start the plane and depart.

So, my thought was -- why not have a ground power connector that can server multiple purposes: trickle-charging the battery, "ground flying", and/or jump-starting.

 

[vlittle]

A jump start may be a one-way trip for your alternator. After startup, your alternator will be at maximum output for an extended time to charge your battery. During taxi-out and runup, there is minimal cooling airflow. Bad combination. This may damage the alternator's (internal) regulator or melt a rectifier diode.

As an aside… if your maximum operating load is say 30A (with a typical 15A), having a 60A alternator is overkill. In the dead battery scenario, it may self destruct (see above). A 40A alternator would be a fine choice.

I went through all of this with a famous brand alternator… destroying both a regulator and a diode pack on two different low battery events. One could argue that the alternator was at fault… and the manufacturer agreed… they advertised that no additional cooling was required… but that may only be true in flight, not on the ground.

I have considered developing a device that will automatically limit alternator output during ground ops, but the simplest solution seems to be just using a smaller alternator.

V

 

[wcalvert]

Not being an electrical engineer, I can only tell you what things have been helpful in my builds:

Most builders use their previous experience from auto/truck/trailer/RV's to color their aircraft system designs. Most autos type systems do not have radios, intercoms, strobe lights etc... so noise is not an issue (until you allow Car Toys to install your new stereo with wire twist connectors and power it thru the dash lights) So the advice here: everything (except closely related systems) gets a separate dedicated ground that traces back to the battery buss. I do not use the airframe for any grounding duties. That's there to hold you off the ground.

Like was said earlier, having an easy way to charge your batteries without removing panels is a good thing. Easy to do. Keep in mind that the terminals at the plug/connector are connected directly to your batteries. Clearly a shorting hazard that must be protected and clearly identified for polarity.

As also said earlier, old cars are ok with being "jump started" .. I'm thinking generator vintage. Anything auto later than that has all the risks of reverse polarity, spiking voltage, fried alternator diodes, smoking display screens etc... just like an aircraft. Dead battery means dead plane until the cause is found and corrected. Jump and go only works when you can pull over and call AAA.

IMHO, powering avionics for hangar flying should be done on battery power alone, or with the batteries disconnected using a dedicated and properly monitored power supply. Some will disagree, and I'm not an AE, but think through the plus/minus.

Minor aside: a contactor on an external power port would be used on a LARGE system (think 767) to protect the aircraft and the poor dude pulling power without removing the internal power source first (APU/Engine generator). Probably not applicable to an RV type system.

Keep at your research and asking questions. Only drawback is like old saying: "Opinions are like ...holes, everyone has one and they mostly stink" Good luck!

 

[Jpm757]

My EarthX battery is designed for a max alternator output of 60 amps. I would think that jump starting from a car or truck with a large 100 amp + alternator would either damage the battery or shut it down via the Battery Monitoring System (BMS). Not sure how a PC-680 would handle same.