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  #1  
Old 12-16-2019, 02:02 PM
avrojockey's Avatar
avrojockey avrojockey is offline
 
Join Date: Aug 2019
Location: Appleton, WI
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Default Rudder Cable Chafing

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

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Last edited by avrojockey : 12-17-2019 at 10:13 AM.
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  #2  
Old 12-16-2019, 08:09 PM
pulsar pulsar is offline
 
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Location: NY
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Default No corrosion issue

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)
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  #3  
Old 12-17-2019, 10:54 AM
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avrojockey avrojockey is offline
 
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Quote:
Originally Posted by pulsar View Post
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.
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  #4  
Old 12-17-2019, 11:56 AM
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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.
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  #5  
Old 12-18-2019, 08:19 AM
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Greg Arehart Greg Arehart is offline
 
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Quote:
Originally Posted by fl-mike View Post
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).
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  #6  
Old 12-18-2019, 10:16 AM
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fl-mike fl-mike is offline
 
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Quote:
Originally Posted by Greg Arehart View Post
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
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  #7  
Old 12-19-2019, 06:49 AM
krw5927 krw5927 is offline
 
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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.)
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Last edited by krw5927 : 12-19-2019 at 07:01 AM.
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  #8  
Old 12-19-2019, 09:51 AM
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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...


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  #9  
Old 12-19-2019, 10:23 AM
MGRVAwesome MGRVAwesome is offline
 
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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.
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