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Stiffner sealant?

pjoshyjosh

Well Known Member
I was discussing build with someone who had built an RV4 years ago and the person mentioned a 'must' of using sealant on the stiffners between stiffner/skin. Pretty sure this isn't part of instruction - just prime. Does anyone do this?
 
Some early builds suffered from rudder and elevator skin cracks. Applying a sealant as an adhesive was commonly done as a preventative. It was never proven to work.
There where many theories for the cracks, the most plausible is poor construction technique.
Search for ?skin cracks? and you can find a lot of discussion.

I have never heard of the newer kits having an issue. Build to the plans.
 
Actually IIRC the sealant was plain old RTV and was applied as a blob near the trailing edge where the stiffeners from both side almost meet.
 
Sealed stiffeners

I used a very thin layer between stiffeners and skin on rudder and elevators. Also a blob at each intersection.
 
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Some early builds suffered from rudder and elevator skin cracks. Applying a sealant as an adhesive was commonly done as a preventative. It was never proven to work.
There where many theories for the cracks, the most plausible is poor construction technique.
Search for ?skin cracks? and you can find a lot of discussion.

I have never heard of the newer kits having an issue. Build to the plans.

To add to this......
The stiffener is not a rib, so it is only attached to the skin. The attaching rivets at the ends are not at the very end for edge distance, etc. as the skin flexes, the "end" of the stiffener can move away from the skin, causing the skin to crack at the last rivet. This situation is magnified on thinner skins and higher HP prop blasts/P factor. So using a blob of RTV or completely gluing the stiffener to the skin stops the movement of the two pieces to stop the cracking at the rivet. As Jon said, this is only one theory of the cracking, but it is the theory related to the blob or glue on the stiffeners.
To add to the thread, the poor construction techniques can cause cracks as well and the main one is misaligned VS Spars....if the spar is not straight and the rudder rod end bearings are adjusted "to fit" then the rudder structure bends or flexes with every use of the rudder and the skin has to yeild somewhere and it shows up in the rivets on the end of the stiffeners.
So if this situation is not as prevelent in newer kits than most likely the skins are thicker and the plans are better about parts alignment. Just my 2 cents.
 
Obsolete Tribal knowledge?

Was this adopted for the early 0.016" aero surface skins? Would be a good bet, at least
 
Was this adopted for the early 0.016" aero surface skins? Would be a good bet, at least

No. The “Blob” that ties the stiffeners together on the end has been in the plans for a long time. Gluing the stiffeners to the skin was never in the plans.
The biggest contributor to skin cracks in my opinion is poor construction. Many did not properly bend the skins to lay flat on the Spar before riveting. This preloads the skin and that force gets magnified at the end of the stiffeners where most cracks occurred although cracks can happen anywhere along the stiffeners.
Anyone building an older kit with .016 skins should search the threads and become familiar with the discussions. Many of us are flying .016 skins with thousands or hours, higher horsepower engines, and no cracking. Built to plans. Care must be taken in construction but gluing down stiffeners has never been proven to be the “fix”.
These concerns are not for the newer kits.
 
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