Rudder Cable Chafing

avrojockey · #1 12-16-2019, 01:02 PM

After watching Vic Syracuse's webinar on eaa.org the other day I did an extra detailed preflight to catch any of the common maintenance items talked about. Didn't find any loose nuts but I found the rudder cables chafing on fuse skin where they exit under the cable fairings at the tail. I didn't take a picture but I can tell you that it's been happening since the airplane was built 600 hours ago.

The cables look okay other than the coating of aluminum built up over time. The fuse skin has worn down to sharp edge where the cable is chafing. There's no mention of this issue in any of the conditional checklists in the aircraft records.

Questions:

Is there an issue with galvanic corrosion with the buildup of aluminum, assuming the cable is stainless or galvanized?
Should one worry about the stress point on fuselage skin? Repair or monitor?
What fixes are folks employing to prevent chafing? I've seen a couple solutions in the picts below...

pulsar · #2 12-16-2019, 07:09 PM

No issue with galvanic between Al and the steel wire. Aluminum is often used as a sacrificial anode meaning the steel won?t suffer corrosion from the dissimilar metals. (A side note... not relevant, but perhaps interesting... the ?zinc? people put on their boats for corrosion are often in fact made of aluminum)

avrojockey · #3 12-17-2019, 09:54 AM

Quote:

Originally Posted by pulsar

No issue with galvanic between Al and the steel wire. Aluminum is often used as a sacrificial anode meaning the steel won?t suffer corrosion from the dissimilar metals. (A side note... not relevant, but perhaps interesting... the ?zinc? people put on their boats for corrosion are often in fact made of aluminum)

OK...so my understanding is the aluminum can't hurt the steel especially if its already galvanized.

Now...my main concern is about any stress riser where the skin chafing. There are currently no cracks but not being an engineer I'm not sure how loads are transmitted to this area from the empennage.

fl-mike · #4 12-17-2019, 10:56 AM

You do have the poly tubes over the cable at the exit, correct? That is the standard design.
If not, they should be installed to keep the cable from chafing the exit.

Greg Arehart · #5 12-18-2019, 07:19 AM

Quote:

Originally Posted by fl-mike

You do have the poly tubes over the cable at the exit, correct? That is the standard design.
If not, they should be installed to keep the cable from chafing the exit.

FWIW, I encased my entire rudder cables in (split) poly tube from the exit point forward to avoid any potential damage due to rubbing where it passes through the bulkheads (and also have the bushing in the bulkheads per plans).

fl-mike · #6 12-18-2019, 09:16 AM

Quote:

Originally Posted by Greg Arehart

FWIW, I encased my entire rudder cables in (split) poly tube from the exit point forward to avoid any potential damage due to rubbing where it passes through the bulkheads (and also have the bushing in the bulkheads per plans).

Sailboat rigging cable covers are handy for that.
https://www.westmarine.com/buy/davis...E&gclsrc=aw.ds

krw5927 · #7 12-19-2019, 05:49 AM

Just like the photo you posted, in the standard Vans design the plastic (UHMW?) tube over the cable is retained in the fuselage by an adel clamp (held by the screw just forward and above the cable exit), and the cable slides through that.

You could replicate that setup rather easily, however the plastic would have to be split to get it on the cable. (If I recall correctly, the stock cables from Vans come with this plastic already installed since the end fittings are permanently swaged.)

avrojockey · #8 12-19-2019, 08:51 AM

Thanks for all the replies!

I didn't know this was standard installation as this is my first RV. Looking in the cable fairing I can see the adel clamp and poly tube but it's not extended through the fuselage...I'll fix this immediately.

My main concern now is the possibility of a stress riser where the skin is worn. Here's some pics...tell me what you think...

MGRVAwesome · #9 12-19-2019, 09:23 AM

Well if you haven't had any issues with stress risers up until now (600hrs), the chances are probably slim that it'll develop now. I see those fairings are fitted with blind rivets. I'd drill those out, remove the fairings for better access, deburr / round off / polish the sharp edges, move the poly tubes to where they should be and close up again. Remember to prime the bare aluminium before closing up.