Whip Antenna - cracking around screw holes

chrispratt · #1 12-09-2006, 01:54 PM

While cleaning the belly of my RV-8 the other day I noticed some cracks have developed around one of the screw holes in the comm whip antenna (DM C63-2). I've marked the ends of the cracks for reference to see if they continue to propagate. I mounted the antenna according to spec (a gasket between the antenna and exterior belly skin and a doubler (supplied) on the interior side of the floor.

Anybody else experience this problem? Would a second doubler help?

The aircraft only has 135 hours. I'd hate to keep replacing this item at $175 a pop.

Chris

Please, no crack-the-whip jokes

gmcjetpilot · #2 12-09-2006, 02:57 PM

Take it off and really inspect it. Just to be sure they are in the antenna base and not the aluminum of the airframe. Is it two mount lugs or four? Take the antenna off and check. Remove the finish is you have to to make sure the crack is in the metal. After clean up the crack may go away. It could JUST BE cracks in the finish and not the metal.

You many want to leave the gasket OFF, many do. The soft silicon may make the antenna flex and put more load on the Antennas base mount lugs.
The gasket is a great place for mosture to collect and corrosion to develop, which is not good for the antenna's ground. All ground is thru the mount screws and nut plates which is marginal. If you primed and or painted the heck out of the plane inside and out, plus the coated nut plates and stainless steel screws (which are not as compatible or good as steel screws with cad plate) the ground can be compromised, aka poor to bad. Of course a RV princes pampered and hanger-ed is not likely to have an issue with corrosion at least for a a few decades.

Instead of the gasket, consider installing the antenna directly on the airframe, no gasket. Some will add a very thin layer of silicone or fillet of silicon around the edge of the base. Even if the airframe metal is bare under the antenna, if you seal just the edge mosture is less likely to get in and corrode. Having the direct metal antenna base to metal airframe is a better gound. Just a thought. I could be all washed up, but I have seen some horrible corrosion under antennas with silicon or cork gaskets.

It's not structural and can imagine the antenna leaving the plane but if you are worried call DM the manufacture and ask them. It's possible they had a bad batch of castings???? Who knows with out really looking.

chrispratt · #3 12-10-2006, 03:12 PM

Thanks gmc. I think I'll start with a call to Dorne and Margolin to see if this is a common problem.

Chris

swisseagle · #4 12-10-2014, 03:05 PM

I had the same on my plane, still not flown yet

It is very important to respect the torque that the supplyer recomend.

It is only 10"/lbs

I fist applyed to much torque and now there are cracks from the front two screwhole to the outside Diameter.

I now put cyano-glue into the crack, then immediatly loosen the screw, so the crack Closes.

After a day, I installed again with the recomended torque.

It is bad, but the design is "crack-prone", there sould really be more material in this area.