RV-8 Elevator Failure

[scsmith]

When you say the scale shows the leading edge is 8.7 oz heavy, how is the elevator supported? Do you have it supported at the hinge bearings? How was that done? So it is free to pivot, and then you put a scale under the counterweight, as shown in the picture?

It is hard for me to imagine there is enough room in the countereweight area to get enough extra lead to overbalance that much. The proper counterweight pretty much fills up the available space.

[BillL]

Originally Posted by scsmith

When you say the scale shows the leading edge is 8.7 oz heavy, how is the elevator supported? Do you have it supported at the hinge bearings? How was that done? So it is free to pivot, and then you put a scale under the counterweight, as shown in the picture?

It is hard for me to imagine there is enough room in the countereweight area to get enough extra lead to overbalance that much. The proper counterweight pretty much fills up the available space.

Yes, lead would have to be added, there is room in the tip. And, it is at max radius so would be the best location.

[rockwoodrv9]

From the video, I am trying to figure out what failed to allow it to rack so much. Did the connection to the spar tear so the ribs are not connected to the spar? Maybe with a mirror or camera, a weak spot could be found. I checked my elevators and they have zero rack. I can only imagine the force it took to damage it that bad. From flutter videos I have seen, there is plenty there to do the damage. I HOPE that if someone put that much force on the elevator at the show that they would let someone know.

I would love to see pictures with the skin off of the spar and ribs.

[sblack]

I think, based on a friend's experience with aileron flutter, that if there was flutter there would be absolutely no doubt about it. One doesn't forget that.

[rvbuilder2002]

Originally Posted by rockwoodrv9

From the video, I am trying to figure out what failed to allow it to rack so much.

The failure of the trailing edge.
That is why I asked if it had gotten damaged before the flight home.

[Mel]

Originally Posted by sblack

I think, based on a friend's experience with aileron flutter, that if there was flutter there would be absolutely no doubt about it. One doesn't forget that.

I experienced aileron flutter on a Moni Motorglider that ripped up wing skins and actually broke the rear wing spar in-two at the fuselage.
I promise you, when you experience flutter, there is no question that it happened. Witnesses on the ground estimated the wing tip was moving at least 2' vertically.
I was VERY fortunate to get the aircraft slowed down enough to stop the flutter and got it safely on the ground.

[Ron RV8]

"There was no damage prior to the flight. I was stopped for ten minutes
total at Unity where I never left the airplane. The elevator trailing
edge was flawless up until this point. The auto pilot does have an auto
trim so it wasn't an out of trim issue either when the plane pitched up.
The autopilot and the autotrim has worked spot on up until this incident
meaning that when the autopilot is disengaged it is in perfect trim. It
certainly is an unexplained cause of destruction to the elevator that I
can not explain and it just happened now at this point in the airplane
history. According to plans the elevators are supposed to be
individually balanced and these ones clearly were not. In my opinion this
is builders error and the smoking gun for this incident and something that
should be not taken for granted. I to agree that the design is good but
certainly rattled at how fast good can go bad."

Posted by Ron Townson who is not the owner...

[PaigeHoffart]

What were the Max/Min G-Meter readings? Any EFIS data recorded? Autopilot servo torque limit? Reported to NTSB? (or Canadian equivalent)

Thanks,
Paige

[DanH]

Originally Posted by scsmith

When you say the scale shows the leading edge is 8.7 oz heavy, how is the elevator supported? Do you have it supported at the hinge bearings? How was that done? So it is free to pivot, and then you put a scale under the counterweight, as shown in the picture?

There's the winning question. How about it Ron...was the above photo taken with the elevator pivoting on its hinge bearings?

[AlexPeterson]

The pictures and text would indicate that we are looking at the under side of the elevator, generally out of typical, easy visual inspection.

The two cracks look like classic prop pulse induced fatigue cracking, certainly not from a single or low cycle damage. (Classic meaning it was not uncommon on .016" elevator and rudder skins of 6's and 4's.)

The kink seems to have been caused by a single overload, such as would happen if someone leaned on the one end of the elevator while it was locked at the other end via control stick or other means. Or, outboard locks holding the elevators, but someone hauling forward or back on the stick. It is very hard to believe this kink happened in flight. If it had, whatever started it would have likely not stopped there...

The two failure modes may be unrelated.

It might be good to check the integrity of the seam between the top and bottom skins, where they curl together. This seam is pop-riveted, but it would seem to be an important contributor to torsional rigidity.

Regardless, we're glad you caught this!