grounding question

skidmk · #1 05-15-2007, 10:22 AM

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 · #2 05-15-2007, 10:27 AM

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 · #3 05-15-2007, 11:46 AM

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 · #4 05-16-2007, 05:42 AM

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)

Quote:

Originally Posted by 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

Ironflight · #5 05-16-2007, 06:10 AM

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 · #6 05-16-2007, 07:25 AM

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 · #7 05-16-2007, 07:59 AM

Quote:

Originally Posted by 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.

Hey Mike,

This is exactly what I did.

Rainier Lamers · #8 05-16-2007, 08:19 AM

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

Quote:

Originally Posted by 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.

skidmk · #9 05-16-2007, 08:28 AM

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 · #10 05-16-2007, 08:33 AM

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