[skidmk]

HI all,,, RV8,,, ran the ground wire from batt to firewall (AWG2) to a "forest of tabs"....

my question is this. For the nav lights, strobes and landing etc... do I have to run separate grounds,,, or can I ground locally at the wing tips.

thanks

M

 

[hecilopter]

Grounding

I grounded all my avionics to the "forest of tabs", Lights are grounded to the airframe at the tips. Everything has worked fine with no interference.

 

[f1rocket]

I would (and did) ground the landing light and position lights to the spar at their mounting location. However, I would ground the stobe lights at the power pack and the shield at just the power pack end per the Whelen instructions.

 

[Rainier Lamers]

Generally it is advisable to run seperate grounds AND power cables to any power user that can cause interference (strobes, radios, some digital instruments, solenoids etc). Over longer distances however, with the cables running next to each other, the advantage is lost as electrical noise simply couples inductively and capacitively to other cables.

However, it is possible to create "virtual" ground points to minimise wiring. In this case you need at minimum a capacitor to create a virtual supply point to which you will join "star fashion" local power users.
This capacitor should have enough charge to supply short term current demands. The best way to achieve this is to use two or three capactors in parallel. A 4700uF (micro farad) will supply longer term current (but takes a little while to react), parallel to this, wire a 10uF. This little one reacts much faster, just long enough until the big one can take over. It is a good idea to add another 100nF (nano farad) which helps with high frequency stuff where the 10uF can't cope.

This is a general advice for standard capacitors which are easy to get. There are higher speed devices available, usualy at higher cost but with the above scheme they are hardly required.

Rainier
(CEO MGL Avionics)

HI all,,, RV8,,, ran the ground wire from batt to firewall (AWG2) to a "forest of tabs"....

my question is this. For the nav lights, strobes and landing etc... do I have to run separate grounds,,, or can I ground locally at the wing tips.

thanks

M

 

[Ironflight]

Another data point....

All of my "remote" loads (landing lights, pitot heat, Nav lights, etc....) are grounded to the structure locally. My avionics all ground together in the panel sub-asembly. The aft-mounted battery is grounded to the longeron back there, and the engine compartment all grounds to the firewall. The only noise I have anywhere in the audio system is when I have my 396 plugged in to both the audio panel and the 12volt outlet - it's a known "ground loop" problem with the 396.

Bottom line - local grounds have worked just great for me....

Paul

 

[skidmk]

Yeah, I was going to local ground it all with just a short neg to airframe in the back next to battery. I was convinced to do it like Randy and have a separate ground wire (2AWG) that runs up the right side of my ship (power from batt to starter is on the left). something about lots of amps running through the airframe (didn't quite understand, but lots of people know more than me and have done it).... so I'll locally ground the remote loads to the wing,,, everything else will be at the firewall or panel side. thanks again,,, this site is awesome.

 

[rv8bldr]

Yeah, I was going to local ground it all with just a short neg to airframe in the back next to battery. I was convinced to do it like Randy and have a separate ground wire (2AWG) that runs up the right side of my ship (power from batt to starter is on the left). something about lots of amps running through the airframe (didn't quite understand, but lots of people know more than me and have done it).... so I'll locally ground the remote loads to the wing,,, everything else will be at the firewall or panel side. thanks again,,, this site is awesome.

Hey Mike,

This is exactly what I did.

 

[Rainier Lamers]

Using the airframe as ground can be hazardous.
In particular if it is mostly aluminium. Older land rover models where quite comparable in construction to an aircraft in some ways with aluminium panels. Currents in the skin joints, rivets etc lead to galvanic corrosion. Bad corrosion. The very same is possible in an aluminium skinned aircraft if you are not careful. Watch out for tell-tale signals like fine white powder around joints, supports, rivets and fastners. It is preferable to use ground cables and ground the airframe in a single location only (usualy close to the battery negative). This way no electrical currents can flow in the skin - carefull also with the VHF antenna - it will likely be grounded at the base (and it must - you need a ground plane) you may form a ground loop with the aircraft skin. As long as the cable runs tightly along the skin that is no issue - if not, you may pickup electrical noise and transmit reflected antenna enery all over the place inside your aircraft whicj can make like difficult for other electronics and intercom systems.

Rainier

CEO MGL Avionics

Yeah, I was going to local ground it all with just a short neg to airframe in the back next to battery. I was convinced to do it like Randy and have a separate ground wire (2AWG) that runs up the right side of my ship (power from batt to starter is on the left). something about lots of amps running through the airframe (didn't quite understand, but lots of people know more than me and have done it).... so I'll locally ground the remote loads to the wing,,, everything else will be at the firewall or panel side. thanks again,,, this site is awesome.

 

[skidmk]

hmmm,,, okay... I'm confused. Rainier, you're saying that running a single wire from batt to "forest of tabs" (its a 2awg about 11 feet long) at the firewall is not a good idea? The only "local" grounding will occur at the wing tips, and where the antenna's will be (floor behind front seat) Is it the local grounding that will cause the "galvanic corrosion"? running a couple of ground wires from the wing tips to the forest of tabs is not a big deal. ... doing that would keep all the grounding (except antenna ground plane) at the firewall via the cable to batt neg. ???

 

[skidmk]

Hey Mark,,, yeah we had this discussion at Steve's.... btw ... did you comm antenna in the wingtip? I was figuring nav in wingtip, comm and xpndr on the belly.

another btw.... if your flying this weekend,,, I'm available Friday after 18:30, Saturday after 15:00 and Sunday after 0730 (finishing a midnight)... dieing to buy you some gas

 

[Yukon]

Using the airframe as ground can be hazardous.
In particular if it is mostly aluminium. Older land rover models where quite comparable in construction to an aircraft in some ways with aluminium panels. Currents in the skin joints, rivets etc lead to galvanic corrosion. Bad corrosion. The very same is possible in an aluminium skinned aircraft if you are not careful. Watch out for tell-tale signals like fine white powder around joints, supports, rivets and fastners. It is preferable to use ground cables and ground the airframe in a single location only (usualy close to the battery negative). This way no electrical currents can flow in the skin - carefull also with the VHF antenna - it will likely be grounded at the base (and it must - you need a ground plane) you may form a ground loop with the aircraft skin. As long as the cable runs tightly along the skin that is no issue - if not, you may pickup electrical noise and transmit reflected antenna enery all over the place inside your aircraft whicj can make like difficult for other electronics and intercom systems.



Rainier,

As compelling as your Land Rover example is, I think a more pertinent illustration would be the tens of thousands of production general aviation aircraft which have utilized local grounding for the last 70 years.

Galvanic corrosion, by definition, requires dissimilar metals, as in your Rover with aluminum fenders bolted to steel chassis components. http://en.wikipedia.org/wiki/Galvanic_corrosion . Aircraft structure, being aluminum, seem to function well as a path to ground, and history has shown little propensity for corrosion.

Interesting to note that the alternator doesn't even have grounding terminals, but rather depends on local ground through dissimalar metals to conduct our entire electrical load. Same goes for the starter, it's path to ground being the bi-metalic conduction through the crankcase of the engine.

 

[airguy]

One more note about galvanic corrosion that should be remembered - an electrolyte is REQUIRED. Even moisture from humid days can be enough of an electrolyte, but an electrolyte MUST be present for true galvanic corrosion. There is a dry corrosion phenomenon known as "fretting" which is different (and rare), but we are not generally worried about that.

If you have dissimilar metals, and you SEAL them to keep them dry, you won't have any galvanic corrosion.

 

[Rainier Lamers]

Yes, the comments are quite correct and most certainly applicable to RV aircraft which are excellently designed - no question.
As an AP I get to inspect and sign out all sorts of interesting aircraft. Aluminium on wood, steel parts connecting to aluminium, T61 aluminium joined to T65, etc etc.
Do not forget that an aluminium aircraft is seldom made up from IDENTICAL aluminium. You get all sorts depending on the metallurgical composition. That is all you need to *potentially*, in a corrosive atmosphere (we like to fly low level over the beach) cause galvanic corrosion. Just needs the right combination of stuff and a little bit of DC current.
What cetainly helps the majority of cases is very low hours operating and lots of hours spent in a nice hanger. But take a badly built aircraft (from viewpoint of galvanic issues) that operates a lot (for example training) in a coastal town...
It happens !

Rainier
CEO MGL Avionics

One more note about galvanic corrosion that should be remembered - an electrolyte is REQUIRED. Even moisture from humid days can be enough of an electrolyte, but an electrolyte MUST be present for true galvanic corrosion. There is a dry corrosion phenomenon known as "fretting" which is different (and rare), but we are not generally worried about that.

If you have dissimilar metals, and you SEAL them to keep them dry, you won't have any galvanic corrosion.

 

[Rainier Lamers]

No, that is not going to be much of an issue.
The cable is short. That helps.
If you really want to do things "the electronic way" then you will find a capacitor (or combination of several) between the positive and negative power bus.
This creates a correct virtual power supply with all users that may cause electrical interference each having their own cable runs to this central supply point (with the caps).

If it's my aircraft, I will elect not to run any currents through the airframe itself (I know it is tempting).

The supply scheme I am talking about is commonly refered to as a "star supply" (in some cases star ground - if only done with the gound). It is the safest from viewpoint of avoiding issues due to interference between electrical users.

But there is no cut and dried scheme - you would have to look at every electrical user in your aircraft closely and decide which can safely share power, which units should share power (avoidance of ground loops in connected comm equipment and which units definitely should not share power.

Rainier
CEO MGL Avionics

hmmm,,, okay... I'm confused. Rainier, you're saying that running a single wire from batt to "forest of tabs" (its a 2awg about 11 feet long) at the firewall is not a good idea? The only "local" grounding will occur at the wing tips, and where the antenna's will be (floor behind front seat) Is it the local grounding that will cause the "galvanic corrosion"? running a couple of ground wires from the wing tips to the forest of tabs is not a big deal. ... doing that would keep all the grounding (except antenna ground plane) at the firewall via the cable to batt neg. ???

 

[gmcjetpilot]

Local ground cool for remote stuff, but..

Corrosion and good practices.

Cadmium seems key. They don't mention stainless steel, for a reason.
For more info refer AC 43-13-1B, Chap 11, section 15. There is a ton of stuff on grounding.

 

[mgomez]

I used separate ground wires

I ran separate wires from the wingtips to the fuselage, all ending up in a "forest of tabs" on the firewall. It's a pain to route all that wire through the spar (and I'm not done yet) but I'm anal retentive about ground loops, noise, etc.

I'm sure that using the airframe as ground would work (as it does in most production airplanes) but it wasn't THAT hard, or that heavy, to run 3 ground wires to the tips (nav, strobe, landing lights.)

 

[ptrotter]

I run seperate ground wires for everything. Every device has a hot and ground wire that will originate at the panel. The hot will go to the appropriate switch or breaker and the grounds will go to 1 of 3 ground buses, avionincs, lighting or engine. Since I have a rear mounted battery, I will use the longeron to carry the ground forward, but I will pull a connection off the longeron at the firewall to the ground buses, therefore using the longeron only as a piece of wire. I do not want to rely on connections between metal parts of the airframe to carry the ground through since everything is primed. The seperate ground buses are only for convenience as to where they are located. Avionics on the panel itself, lighting behind the panel and engine on the firewall. All will be connected together to ensure that they are at the same potential. The important point is that all similar equipment will ground to the same ground location so there can be no ground loop potential between devices, e.g., the audio panel and radios will have the same ground.

Overkill I'm sure but I want to avoid problems as much as possible.

 

[N941WR]

I would (and did) ground the landing light and position lights to the spar at their mounting location. However, I would ground the strobe lights at the power pack and the shield at just the power pack end per the Whelen instructions.

Ditto for me. You could hear the strobes through the headsets when grounded with the rest of the instruments and this resolved the issue. I also clipped the strobe?s shielding at the wing tip and continued all the way to and grounded it at the strobe power supply.

Using the airframe as ground can be hazardous.
In particular if it is mostly aluminium. Older land rover models where quite comparable in construction to an aircraft in some ways with aluminium panels. Currents in the skin joints, rivets etc lead to galvanic corrosion. Bad corrosion. The very same is possible in an aluminium skinned aircraft if you are not careful...

Rainier is correct on this point and it is something I took a long hard look at. However, like a lot of other decisions I had to make, I thought, "There are a lot of other airplanes flying for many years with X".


Good luck.