S

szicree

Well Known Member
I'm mounting my Com antenna on the belly and using a rectangular doubler. The antenna base will ground to the doubler via screws into 4 riveted nutplates, and the doubler will, in turn, be pulled against the inside of the belly skin. I've removed the primer from the doubler and belly surfaces to achieve good grounding, but I worry about corrosion. Is there something I can smear on the surface that will allow good electrical contact, but prevent corrosion?
 
What does the antenna maker say?

szicree said:
I'm mounting my Com antenna on the belly and using a rectangular doubler. The antenna base will ground to the doubler via screws into 4 riveted nutplates, and the doubler will, in turn, be pulled against the inside of the belly skin. I've removed the primer from the doubler and belly surfaces to achieve good grounding, but I worry about corrosion. Is there something I can smear on the surface that will allow good electrical contact, but prevent corrosion?
Click to expand...

DC 4 is an insulator - not really suitable for the job....

Doesn't the comm. antenna come with a rubber or cork gasket for the outer surface?
If this is the case, the only primer you need to remove is on the inner surface under the mounting screw nuts.
A sealer (RTV) is usually recommended around the antenna base edges to stop moisture getting under the gasket.

The best thing is to follow the antenna manufacturer's instructions (surprise... :) ...)

A typical set is here....

http://www.comant.com/htmls/guide2.html

If in doubt, follow the instructions.... :D ... don't do a Tim Allen..

EDIT - UPDATE

Sorry ... the Comant link is for GPS ... this one is for Comm.

http://www.comant.com/htmls/guide1.html

They do talk about direct metal to metal contact... so the DC 4 comment still applies.

Not all antennae are identical... the one I mounted on my Tiger belly had a cork gasket, so removal of external paint would have had no effect. You need to check the specific instructions for your antenna...

SECOND UPDATE

This antenna from EDO (D & M) does use a 0.080 external gasket...

http://www.edocorp.com/documentation/antennas_c70.pdf

gil in Tucson
 
Last edited:
Gil,

I have instructions just like the ones you posted. They suggest two methods; I'm using the one that grounds through the backing plate.
 
Second try

szicree said:
Gil,

I have instructions just like the ones you posted. They suggest two methods; I'm using the one that grounds through the backing plate.
Click to expand...

Steve, I think I'm getting it now.... :)

You are only talking about the inner surfaces? ...correct?

If that's the case, just use some RTV at the seams (edges) after the antenna is bolted down to the backing plate. If you stop moisture getting between the plates, you'll be OK.

DC 4 is an insulator, and is not the best for that application where you want good conductivity...

Note that one set of the instructions called for alodine before installation.... this would really help on the long term corrosion prevention, and not upset the conductivity...

.. hope I got your installation correct... gil A
 
We're on the same page now, Gil. I saw the part about alodine and will use it. I like the idea about using rtv around the seam. Thanks.
 
Here's a picture from the Aero Electric Connection. Per Bob Nuckolls, step 1: discard the cork gasket.

Put a bead of RTV around the perimeter of the antenna once installed. The doubler and skin only need to be cleaned around the screw holes in a 1/4 inch radius. In other words, primer is OK except within 1/4 inch of the screw holes. The torque will create an air tight seal around the screws where you stripped to the bare metal.

http://www.aeroelectric.com/Pictures/Antenna/Antenna_Installation.gif

My comments are based on those from Bob Nuckolls on the Aero Electric list.

Jekyll
 
As some have already stated, alodine provides good protection of bare metal surfaces of the skin and doubler while maintaining good electical bonding between the surfaces. An RTV bead around the perimeter of the antenna base should keep out moisture, but don't apply RTV between the antenna or doubler and the skin - it is an insulator.

An alodine "pen", such as the Touch-N-Prep 1132 sold at Aircraft Spruce, Stein Air and elsewhere, while somewhat expensive works great for this without the need for rinsing after application. If you use Alodine 1201 be sure to rinse the surfaces after application.

I am using doubler plates riveted to the skin, the Touch-N-Prep alodine pen applied to bare aluminum between the doubler and the skin - and between the skin and the antenna base, and RTV sealant (only around the perimeter of the antenna base) for all external antennas.

Hope this helps.
 
Jekyll said:
Here's a picture from the Aero Electric Connection. Per Bob Nuckolls, step 1: discard the cork gasket.

Put a bead of RTV around the perimeter of the antenna once installed. The doubler and skin only need to be cleaned around the screw holes in a 1/4 inch radius. In other words, primer is OK except within 1/4 inch of the screw holes. The torque will create an air tight seal around the screws where you stripped to the bare metal.

http://www.aeroelectric.com/Pictures/Antenna/Antenna_Installation.gif

My comments are based on those from Bob Nuckolls on the Aero Electric list.

Jekyll
Click to expand...

Just to update this thread for future visitors, the above link has now changed to:

http://www.aeroelectric.com/Pictures/Antenna/Comm_Antenna_Installation.gif

.
 
I've heard of at tool called a burnishing brush, or at least that's what I think it's called that will make a nice primer free area around a bolt hole and is the perfect tool for this situation.

However, I have been looking a few times and not found where to purchase this tool. Anyone know where to get one?

TIA,

-Jim
 
All of the conductors at power sub-stations are made of aluminum. To avoid corrosion, the two surfaces to be joined are coated with a grease called NoOxid or petroleum jelly, the two surfaces abraided with something similar to crocus cloth, and then the two surfaces joined without wiping off the grease. The tightening of the fasteners should squeeze out excess grease which can then be wiped off.The reason for this procedure is that aluminium immediately begins to oxidize at soon as bare metal is exposed to the air. This procedure prevents this.
In addition, if the area of the doubler is sufficiently large, there should be sufficient capacitive coupling between the doubler and the skin to provide a very low reactive coupling. For instance, a doubler 4" square, separated from the skin by 0.001" with a grease with a dielectric of 2.0 would have an effective capacitance of 0.017 uF with a reactance of less than 0.0001 ohm.
 
Dumb question: stainless steel filaments on the wire bonding brush okay to use on aluminum? I've been religious about not using anything steel to abrade anything aluminum, to avoid corrosion from bits of steel embedding in the Al, but in an overabundance of caution, didn't really think about if it's okay to use stainless for this type of application...
 
Ellipse, I've always wondered about putting dielectric grease between conductors and trying to understand how it works. To re-phrase what you are saying in your post, if the layer of grease is very thin and the mating surfaces are large, then the overall resistance is low and you have a good electrical joint?

With regard to the bonding brush, Bob Nuckolls is adamant that one should clean the metal surfaces to be joined, but not roughen or abraid the surface in any way.
 
Last edited:
aturner said:
Ellipse, I've always wondered about putting dielectric grease between conductors and trying to understand how it works. To re-phrase what you are saying in your post, if the layer of grease is very thin and the mating surfaces are large, then the overall resistance is low and you have a good electrical joint?

With regard to the bonding brush, Bob Nuckolls is adamant that one should clean the metal surfaces to be joined, but not roughen or abraid the surface in any way.
Click to expand...

The information I presented is based upon information supplied by Alcoa for use in joining electrical conductors. As I wrote, as soon as you scratch the surface of aluminum it immediately begins to oxidize. By coating the surfaces with a grease to prohibit air getting to a recently cleaned surface, and by abrading the surface through the grease, you have a clean conductive surface available. The abrasion causes minute hills and valleys to be formed on the surfaces, which, when clamped together at 1000psi-1500psi will squeeze out the grease and give very low ohmic connection.
We used this method on our grounding grid below our Atlas guidance computer at Vandenberg AFB to produce a very well bonded ground grid. When I first read of this I immediately was horrified at the thought of grease on conductors until I read more about the prcess! You'll see pretty much the same thing on connectors on automobiles where they are filled with grease to keep out moisture. The other thing I was pointing out is that even if you don't have good electrical contact between the doubler and the aircraft skin, at least there will be a capacititive coupling of very low impedance. The other thing that aids preserving the low contact resistance is to use Belleville spring washers to maintain the clamping force when you have a difference in the coefficient of linear expansion between the steel fastener and the aluminium skin ('luv the Brit spelling!).
 
You must log in or register to reply here.