Ground straps
Other than weight and cost, is there any reason to/not to run ground wires back to the ground bus under my panel?
No, other than weight and cost.
Dedicated grounds are a good idea for shorter runs behind the panel, but providing dedicated ground runs for each item out to the wing tip and tail is prohibitive weight wise. Cost is not a big deal for you, as it would in a production plane with labor cost. Bottom line it is just not needed on a metal RV, however there are exceptions.
High current items must/should have their own ground wire (of sufficient wire gauge)
Composite/wood planes run ground wires to the extremities of the plane because they have to, but most RV'ers take advantage of the fact we fly a metal plane and make local grounds to the airframe structure.
Avionics, especially AUDIO, usually use/need a dedicated ground, often also an isolated ground, at least if you don't want noise. Audio, mics and phones use very small voltages. If you ground the electronics in one place and the mic/phone or component in another place on the airframe, it creates small differences in resistance to the ground. The different resistance in the ground circuits adds a small voltage differential. Even small voltage differences in the audio signal often convert or are interpreted as noise.
Instruments like electric OIL or FUEL pressure transducers/senders often rely on airframe grounds. Make sure those grounds are good and the same as the instrument they connect to. Small difference in ground resistance results in poor instrument accuracy or flaky operations. Even if the sender does not have a separate ground terminal, case grounded, I run a ground to the senders attachment/mount.
NAV lights, landing lights don't not care where you ground it, just as long as voltage is sufficient. A poor ground creates extra resistance and voltage drop. You can check the resistance of you airframe grounds.
AC 43.13-1B, Chap 11 has everything you ever wanted to know.
SO HOW DO YOU KNOW YOUR AIRFRAME GROUND IS GOOD?
Usually the light turns on and it's bright. Good enough. The technical way is described in AC 43.13-1B, Chap 11. Below is my method, loosely based on AC 43.13-1B.
Ground resistance through the structure should not exceed 0.10 to 0.20 ohms.
You don't just use an ohm meter with probes touching at opposite ends of the plane. You can have low resistance and no current carrying capacity, so you measure voltage drop at max current, across the ground side of the circuit, from the Bat Neg terminal or main ground buss to the ground stud in question. The less resistance the better of course.
Example, with a 2 amp Nav light and 4 amp landing light in the wing tip, the return current (ground) is 6 amps. With the 6 amp current, a 1.2v drop would be measured across the ground at 0.20 ohms (max recommended). The ground stud in this example is a bolt near the wingtip in the spar, providing Gnds for both lights. With the lights on, using a voltmeter, put one probe on the Bat's NEG terminal, the other probe on the ground stud. At 6 amps, anything more than 1.2 volts drop is too much. To calculate the acceptable voltage drop, the equation is: (V=I x R) or (V = I x 0.20).
The 0.10 to 0.20 ohm ground resistance rule of thumb is general; some items may be voltage sensitive, requiring less voltage drop. If the voltage drop is too much, this might be a case to add a dedicated ground wire. Total voltage drop also includes the Pos side. Assuming the positive wire was sized properly, the deficiency would be on the Gnd side.
Another method, is measuring total voltage drop at the load, say the landing light for this example. Total voltage drop is the sum both Pos and Neg sides resistance, since they are in series. We can subtract the Pos side out of the circuit with some calculations. In this example voltage is measured across the powered landing light. The power supply is the plane's 12.6 volt battery (engine not running, alternator off-line). You measure 11.2 volts across the landing light, or a total 1.4 volt drop. You know the Pos wire gauge & length, 18 awg and 25 feet long. Look up and calculate the wires resistance and voltage drop. Using
standard data 18 awg wire has 6.395 ohms per 1000 ft. This gives 0.16 ohms for 25 feet and .64 volt drop at 4 amps. Since total drop is 1.4 volt, the ground side contributes a 0.76 volt drop. Ground resistance is, R = (V/I) = (0.76v/4amp) = 0.19 ohms. It’s under 0.20 ohms, so it’s OK.
This is just an example. Actual ground resistance in a RV should be much less. The bottom line, measure the voltage at the wing tip and tail devices. If the drop is less than 10%, its fine, but if it’s more (or you want less voltage drop for what ever the reason) than its time to rethink the ground and also possible the positive side as well.
Ground Straps?
The starter and alternator are big current items; both need good airframe grounds. It seems obvious you would have to use a good ground strap to tie the engine to the battery & airframe, however some miss this. One guy could not figure out why his engine turned over slow. He burned his throttle cable up because it was the only ground to the engine!
Another guy had alternator problems. The alternator had fancy anodized mounting hardware. Anodized parts are not conductive. The only ground was through a small safety wire on the tension bolt to where the wire contacted a small non-anodized area. The wire burned handling all the ground return current!
Lesson good grounds are a must. If you rely on structure make sure there's no surface paint, corrosion or anodized coating some where, to allow good conduction.
In an all aluminum plane it’s almost impossible to not have a grounded airframe, but from the two examples Mr. Murphy says stuff happens. Often the problem is local, right at the wire to structure attachment. Some times you are limited in contact area, so add more ground points might help, verses ganging a bunch onto one ground node. The lesson is don't assume every thing is properly grounded by just bolting it to the plane or engine.
Grounding straps, besides improving ground return resistance, they have a side benefit of improving lightning strike protection. When lightning hits one part of the plane, it exits another, creating huge voltages. With good grounds and conductive paths, it exits with little or no damage. With out a good path, it will arc and jump, burning holes where it jumps to get to conductive items. Composites have a little challenge with this; metal planes not so much. Also related but not as scary as lightning is static discharges. As static electricity builds, the voltage gets great enough to jump the resistance. Those mini spark discharges create radio noise. Often control surfaces have ground straps. Usually controls are grounded through their control linkage or hinge, which is the case with RV's. Flying through clouds and snow, especially at jet speeds causes lots of static build up.
Like anything you can over do grounding, "gold plate it", lots of effort for little gain, but a bad ground is a pain to trouble shoot.
