Potential elevator skin crack

I apologize for not having a photo (yet), but here's the scenario.

I noticed a smoking rivet at the outboard forward corner of the skin where the trim tab is cut out of the elevator. At this point, the rear stub spar of the elevator intersects an elevator rib. The elevator skin overhangs and the trim tab hinge is riveted to this overhang.

The outboard portion of the hinge has a lot of stress from the trim tab flexing the skin. This is where I noticed the smoking rivet. I think that there is a high probability of a crack if this is not addressed. The trim tab on the RV-9 is much larger than most of the other RVs, so I expect the strain on this junction to be greater.

My solution was to make a reinforcing bracket that connects the elevator rib to the hinge. I will post a photo later with the details, so this is just a heads up to inspect this weak point. Aircraft flight time is close to 500 hrs.

EDIT: this is not my elevator, I just grabbed the pic from the interweb for illustration:

Thanx for the heads-up

... will be watching that area more closely.

OK, here are the pics of my modification.

First, you can see the damaged paint around the smoking rivet in the trouble area. This and one other rivet were drilled out to allowing attaching a stiffener bracket.



Here's an underside view of the bracket.



This mod ties the spar and rib together and eliminates the working of the elevator skin which led to the smoking rivet.

There are other ways to do this, for example with a bit more complex bending, a web can be added to the flange that lies against the skin. However, the result of what I have done is quite strong. Best of all, it has the minimum impact on the paint!

mind the gap

Nice fix.

Did you ensure to maintain proper distance clearance between the Trim tab and the new bracket?

For the -8 this is a tight spot that won't allow for such modification.

My two cents

vlittle said:

First, you can see the damaged paint around the smoking rivet in the trouble area. This and one other rivet were drilled out to allowing attaching a stiffener bracket

Click to expand...

There were no crack in the skin?
Anybody know of cracks located in skin in this location?

Found During Pre-Flight this Morning

Today on my RV-6A (early Quickbuild with .016" tail skins, 600 hours, no aerobatics) I saw thin little cracks across the paint over the rivets on the aft spar of the LH elevator where the trim tab hinge attaches. There is one end rivet that looks OK. The picture shows the next most outboard 3 rivets. The one with the paint chip is after I picked at the crack over that rivet and popped off the chip.



Removed the paint (PPG Concept) over the rivet using prolonged contact with MEK on a q-tip. This picture below does not show it well, even if you zoom in tight, but it looks like there is a micro crack right around the rivet forward side of the rivet (RH side in this picture). Very small crack starting to grow away from the rivet at the 12 o'clock position. I suspect I will find similar after cleaning paint off the other two rivets.



My thoughts on this cracking is that I noticed the skin in the panel just forward of the rivet line can oil can. The panels to either side of this panel (between the next ribs over) do not oil can. I think maybe the constant flexing in and out of that skin panel may have caused the cracks.

I have done several searches and not found any other mention of elevator skin cracking in this spot. Only found this thread of any mention of problems in this location.

Next action will be to drill out this rivet and really look at the hole for the crack. If there, as I suspect, I will probably put an external doubler on the skin across to pick up these rivets on the aft spar hinge attach.

trim tab rivets

it seems the trim tab steady force loading should be higher at the actuator end and slightly less at the free end because of slight flex in the trim tab when loaded during flying but the smoking rivets seem to be closer to the free end. is this correct? there is a possibility of slightly higher vibration at the trim tab free end causing a fatigue crack. Vern's stiffener seems to be a reasonable solution for a 9.

both affected aircraft are 500-600 hrs flying time.

Great fix, Vern. And a timely post, as I'm at this exact stage of construction. It does seem odd that the outboard trim tab spar is "floating" (anchored just by the skins) while its inboard attachment to the root rib is beefed up with a gusset and AN-4 rivets.