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10-21-2013, 09:36 AM
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Join Date: Jan 2005
Location: Victoria, Canada
Posts: 2,251
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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:

__________________
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V e r n. ====
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RV-9A complete
Harmon Rocket complete
S-21 wings complete
Victoria, BC (Summer)
Chandler, Az (Winter)
Last edited by vlittle : 10-21-2013 at 09:51 AM.
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10-21-2013, 01:21 PM
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Join Date: Dec 2005
Location: Mahomet, Illinois
Posts: 2,195
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Thanx for the heads-up
... will be watching that area more closely.
__________________
Terry Ruprecht
RV-9A Tip-up; IO-320 D2A
S. James cowl/plenum
(Dues paid thru Nov '18)
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11-19-2013, 10:42 AM
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Join Date: Jan 2005
Location: Victoria, Canada
Posts: 2,251
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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!
__________________
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V e r n. ====
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RV-9A complete
Harmon Rocket complete
S-21 wings complete
Victoria, BC (Summer)
Chandler, Az (Winter)
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11-19-2013, 11:39 AM
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Join Date: Aug 2011
Location: Monterrey, Mexico
Posts: 121
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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
__________________
Miguel Vidal
EAA #1133587
RV-8 SB Wings in Monterrey, Mexico
N511MV Registered
MyKitLog
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06-21-2014, 08:14 AM
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Join Date: Sep 2006
Location: Defiance, MO
Posts: 1,674
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Quote:
Originally Posted by vlittle
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
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There were no crack in the skin?
Anybody know of cracks located in skin in this location?
__________________
Philip
RV-6A - 14+ years, 950+ hours
Based at 1H0 (Creve Coeur)
Paid dues yearly since 2007
Last edited by plehrke : 06-21-2014 at 02:52 PM.
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06-21-2014, 02:43 PM
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Join Date: Sep 2006
Location: Defiance, MO
Posts: 1,674
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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.
__________________
Philip
RV-6A - 14+ years, 950+ hours
Based at 1H0 (Creve Coeur)
Paid dues yearly since 2007
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06-22-2014, 04:40 AM
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Join Date: Apr 2011
Location: Cincinnati, OH
Posts: 2,791
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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.
__________________
Steve Melton
Cincinnati, OH
RV-9A, Tip-up, Superior O-320, roller lifters, 160HP, WW 200RV, dual impulse slick mags, oil pressure = 65 psi, EGT = 1300F, flight hours = 800+ for all
Simplicity is the art in design.
My Artwork is freely given and published and cannot be patented.
www.rvplasticparts.com
Last edited by Steve Melton : 06-22-2014 at 04:59 AM.
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06-23-2014, 10:53 AM
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Join Date: Mar 2014
Location: Laguna Hills, CA
Posts: 1,805
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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.
__________________
Doug
RV-9A "slider"
Flew to Osh in 2017, 2018 & 2019! 
Tail number N427DK
Donation made for 2020
You haven't seen a tree until you've seen its shadow from the sky -- Amelia Earhart
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