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Engine Grounding?

AviatorJ

Well Known Member
Quick breakdown on my RV-10 grounding. I have a ground from the Engine (close to the starter) to engine mount. Then another ground from the back of the engine to the firewall. This then goes to grounding tree behind the panel where ALL my other grounds are run to. Which then had a #2 line ran all the way to the tail terminating on the battery.

I did this to hopefully prevent grounding noise but that didn't work. I had alternator noise (Just alt 1) when it was doing the initial charging after start, but would die down once it leveled out. I also had landing light noise, but not taxi light noise, which use the same wire run. I decided I would deal with it later.

Today I went flying and noticed a complete lack of static noise. Even had to double check that I had my lights on a few times. When I landed I had to pull the top cowl to measure a few things. I noticed the ground I had between the firewall and the back of the engine broken at the terminal and was hanging there.

I'm going to replace it with some braided stuff from B&C and do better on my crimps. But is there any reason that just doing two grounds to the engine mount won't suffice? Has there ever been an accident or anything blow up because the RV only utilized engine to engine mount tab grounding? I'm more apt to do that, especially if it seems to solve my radio static concerns.
 
As far as I’m concerned it’s a lot of black magic. That said, you created a ground loop: engine to mount to firewall then back to engine. And that’s a general no-no. The alternator generates a magnetic field with some ac component. If that penetrates the plane of the ground loop, it will generate an ac current flow around the loop, and possibly some noise. Does (did) your noise vary with engine RPM?
You do want to be sure you have a good ground from the starter to the negative battery terminal. Otherwise, on engine start, a large return current will try to find a way (up mixture control wire sheaths, etc) possibly frying or welding stuff along the way.
 
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I want to say that the noise did vary with RPM. But that could have been another plane. It really was minor Andy didn?t bother me once flying.

The ground loop was by design. At some point I read a suggestion on essentially running your ground from the starter all the way back to the negative terminal. But if that increases the chance for noise then that?s no good.

I?ve read similar things as you mentioned, ie amps from the starter finding an alternate ground path through controls or other wiring. That?s sort of my question, has that ever happened? I?ve searched around and haven?t found anything where an issue was caused by ground through the engine mount.
 
Engine ground

I just put one ground from the firewall to the engine block at the mount ear. No issues with starting or alternator noise at all. I chose to keep it simple.
 
Not a big fan of engine mount grounds, as the current path includes additional joints, each a potential point of additional resistance. Run a braided ground strap between the engine case and the firewall ground block.
 
Ground

Also to add. I did not ground the powder coated engine mount as it is bolted to the airframe and I did not run a ground back to the tailcone. I grounded the airframe at the battery location.
 
If your biggest indicator of a problem is alternator whine in your headset, then your problem might have nothing to do with engine grounding, but rather with grounding at your headset jacks. Remember that with most audio systems we have in our lanes, you want headset jacks grounded at the audio panel (or radio) end ONLY. The jacks themselves should be isolated from the airframe - ther are special washers for this, and many people lose then along the way....

If nothing else, its something to check.
 
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Paul has a very good suggestion there. Here's how to think about induced noise: First, it is not due to magnetic field induction in ground loops. Alternators put out rectified, but not filtered AC. It is the resistive drop in the ground path that the alternator output current takes that can generate a noise voltage. So if you are looking for the method by which noise appears, you need to look at the path the charging ground current takes and where it might induce a series voltage that appears in the ground of the disturbed item. In the case of the headphones, that can easily be the shields of the headsets. If, like Paul suggested, they are grounded both at the jacks and the panel, then that ground path is in parallel with the airplane frame, which if conducting some of the charging current, then imposes the resistive drop AC voltage on that shield, which then is in effect in series with the audio signal. By grounding the audio stuff at the panel only, it is a single point ground which therefore cannot accumulate any series noise voltage. In fact, single point grounding is the key to noise control in any case.
 
I have a new braided cable on order. Will install it as I had my old one installed and see what noise I get. Noise isn?t overly annoying but if I can remove it all together I would like that.

In my case i doubt its the jacks. My front head sets are mounted in plastic and the rear carbon fiber, it did also use the washers since I had them anyhow.
 
Several thoughts; not intended as pokes at you; just observations.

If your original ground broke at the terminal, it was either fabrication error, or installation error (wire not supported a few inches away from the terminal). The support thing is often ignored, but is detailed in AC43. Any termination (solder or crimp) is a 'stress riser', so wire should be supported near the termination.

Braided ground in the same spot may be a 'workaround' for the support issue, but won't change electrical characteristics. If the problem goes away, something else changed.

Van's uses the motor mount as a ground path on the new -14 kits (just helped a friend hang an engine), but a lot of people consider it bad form (including me).

If the battery is in the back of the plane, grounding the engine & avionics at the firewall and grounding the battery in the back of the plane does open the door for 'ground loops' to cause noise issues.

Draw a 'road map' of the physical paths all your ground returns follow. If a 'noise maker' (alternator, DC brush type motor, strobe power supply, even magnetos, etc) has its ground 'driving on the same road' back to the battery negative post as small signal stuff like your audio system, then there is the *potential* (pardon the pun) for you to hear the noise maker in your headphones.

Charlie
(electronics tech, location sound engineer in a couple of previous lives)
 
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Don't worry I don't get offended by forum posts. I would agree my original termination was weak and the stiff cable moving around probably didn't help.

My intent on starting this thread was to see if anyone could find a case where utilizing only the engine mount as a grounding source resulting in an accident or substantial damage. I spent time combing through NTSB and other forums and couldn't dig anything up. I get there are a number of people adamantly against it, I'm fairly indifferent because when I did conductivity testing I had the same resistance through the mount tab as I did from the bus bolt punched through the firewall.

I'll keep my dual setup, engine ground near the starter to engine mount tab, and a second ground from the back of engine to the bus bolt on the firewall (using braided cable). That bus bolt leads to a grounding block which then has a #2 to the battery negative. Nothing is ground local and all runs back to the block of tabs behind the panel.
 
As I mentioned, Van's uses the engine mount as ground path on the -14. The plane won't spin out of the air if you use the motor mount for a ground path. But...

DC welding on motor mounts has been documented to magnetize the steel in the mount. You're looking at ~60-80 amps (at most) to weld that size 4130. Starter current is no less than 150A; with inrush current *much* higher. Probably not much of an issue for the magnetometer in the tail, but can be a different story for a cockpit mounted compass.

As to resistance measurements: A typical ohm meter is inadequate for the task of measuring extremely low resistances, as you (should) find in a starting circuit. But the bigger issue, as others have mentioned, is the radically increased number of failure/increased resistance points *over time*, you add when you insert the mount in the ground path.

Again, it's not that it's completely wrong; it's just not 'best practice'.

One noise producer I haven't seen mentioned is failing diodes in the alternator. Alternators are not usually big noise makers, but if you have a diode failure, noise levels will often go up quite a bit. Unfortunately, it's kinda hard to diagnose without either an oscilloscope or substitution of a known-good part.

Charlie
 
.

One noise producer I haven't seen mentioned is failing diodes in the alternator. Alternators are not usually big noise makers, but if you have a diode failure, noise levels will often go up quite a bit. Unfortunately, it's kinda hard to diagnose without either an oscilloscope or substitution of a known-good part.

Charlie

Actually, it's easy. Just pull the alternator field breaker. If the "whine" goes away, it's a bad alternator bridge diode. Or in my case, a broken stator wire.
 
Not a big fan of engine mount grounds, as the current path includes additional joints, each a potential point of additional resistance. Run a braided ground strap between the engine case and the firewall ground block.

I agree with Dan- go straight to the engine case. As an option to the braided straps, B&C offers super-flexible insulated cables with a very high strand-count used in welding applications(only #4, but #2 can be found on Amazon). I’ve had great luck with these and find them to be a bit easier on the eye than the braided stuff in addition to having built-in anti-chaffing properties. Because they are so flexible, much lighter vibrational loads are imposed upon connections and battery lugs, and they are far less likely to work loose. I also use di-electric grease to coat all components used to secure the cable at both ends(not inside the crimp connector, just make sure that is fresh, clean, tight, and protected by adhesive shrink tubing to prevent moisture access). The grease itself is not conductive and it does not improve electron flow in a new joint, but it does inhibit corrosion caused by dissimilar-metals within the joint that can lead to arcing and resistance within the connection over time. Proper torquing and regular torque checking are also important, and the use of star washers(brass ones are available) and self-locking nuts is a good idea. I’ve seen many sluggish starter issues cured by proper attention to these connections.- Otis
 
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DC welding on motor mounts has been documented to magnetize the steel in the mount. You're looking at ~60-80 amps (at most) to weld that size 4130. Starter current is no less than 150A; with inrush current *much* higher. Probably not much of an issue for the magnetometer in the tail, but can be a different story for a cockpit mounted compass.

Charlie

I would argue that the magnetism is created by the changing polarization of the steel molecules due to the heating / cooling cycle. Various characteristics of steel can be manipulated by achieve different temps and by accelerating or slowing the cooling speed, as well as the gas it is exposed to during that cycle. Think hardening. I know it is common for some SS alloys to become magnetized by some welding operations. However, only the weld area is magnetized, though the magnetic energy will carry through the rest of the steel, just like putting a metal object on a magnet. The attached steel will now act magentic.

I do not believe that you can permanently magnetize steel by running 150 amps through it. If this were the case, most automotive frames would be magnetized. A simple test on your car will prove/disprove your theory. Yes, a magnetic force will be generated while 150 amps is running through it.

Larry
 
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Secondary (high current) ground path needed ?

I have an RV6 with an O-320.

It has a heavy gauge ground wire running from a tab under one of the starter mount bolts to a common ground firewall penetration, the inside of which attaches to the battery (nearby in the stock location).

Is it necessary or advisable to have a second ground strap from another location on the engine to the firewall (presumably using the same penetration) ?

If I have read other comments correctly, a failure of the primary ground can cause high currents (while cranking) to find undesirable ground paths, possibly damaging things along the way.

Thanks in advance.

Peter
 
The voltage drop between battery negative and the engine will be reduced if
another ground wire is installed. And there will be a backup in case one cable fails.
The disadvantages of a second wire are that it will cost time and money and add weight.
If everything on the plane had a backup, then the plane would weigh twice as much.
The existing ground wire could be carefully inspected to be sure that it is unlikely to fail.
So the answer is: It is up to you.
 
Case!

Run a 2AWG from the lug on the engine case by the lower left Dynafocal cup to an AN-4 bolt through the firewall and into a K1000-4 nut plate on the diagonal angle. Use a little acetone, MEK, xylene - whatever, to strip the paint off the case at the lug to ensure a good conductive surface.

RV-7 DWGs OP-30 and OP-31 show this detail.

No worries about insufficiently sized wire, funky return paths, etc.
 
Run a 2AWG from the lug on the engine case by the lower left Dynafocal cup to an AN-4 bolt through the firewall and into a K1000-4 nut plate on the diagonal angle. .

I'm in the middle on doing this exact task. Can you explain by what you mean when you say "K1000-4 nut plate on the diagonal angle"? What diagonal angle are you referring to?
 
A lot of discussion on this, but perhaps not enough on specific steps and issues to consider on engine grounding.

My thoughts:
- Establish a common ground point on the firewall. ACS and Stein have nice ?Forest of Tabs? do to this. I like this one but I cut it in half, mounting the part with the bolt on the cabin side of the firewall. The other half (if I use it at all) gets mounted on the bulkhead behind the panel to add ?convenience? ground tabs. It connects to the same big brass ground post with a #10 wire. https://www.steinair.com/product/40tab-grounding-buss/
Everything in the plane gets grounded here - no airframe grounds.
- Your battery(s) are grounded on this same post.
- On the firewall side I run a #2 welding wire to the ground lug on the stater. For you snorkel builders this is not convenient so the alternative is use the mounding bolt on the alternator to alternator bracket. The logic is the starter is what draws the current, not the engine - this minimizes voltage drop across the various engine joints as compared to just running a ground strap under some greasy engine cast bolt.
- Be very careful of building in other engine ground paths (like instrumentation, mag switches and such). In other words verify that if your engine ground failed (or you forgot to put it back on after maintenance) that your starter would not find some other path back to the battery ground. I hear horror stories of such incidents frying wires and panels.
- One engine ground only.
- I too am not a fan of the RV-14 engine mount ground arrangement.

Side note - I use welding wire for all battery, alternator and engine ground runs as I find it superior to branded straps (for grounds), more flexible and durable for all the other connections (as compared to standard aircraft large gauge wire). I do not use brass or copper buss bar for anything other than the ground Forest of Tabs. Specifically:
- #4 from each battery positive terminal to its master solenoid. Output of the combined solenoids gets a #2 to the starter solenoid and then from there to the starter.
- #4 for each battery ground
- #6 for the alternator output

Carl
 
Braided ground straps are better at attenuating electrical fields than cables. The woven strap presents a larger surface area making it better from a noise perspective. That said, I use an AWG#2 wire from a engine case connection near the firewall, to a single point, large area ground (3/8 through stud)on the firewall. The battery ground, as does my equivalent to the forest tabs, goes directly to this point on the cockpit side. No noise. If I had engine related noise, I would change to a braid.
 
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