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  #21  
Old 08-14-2017, 08:30 AM
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bret bret is online now
 
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Are the rod end bearing hinge lock nuts tight? any play here that could have excited flutter?
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  #22  
Old 08-14-2017, 09:27 AM
rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by bret View Post
Are the rod end bearing hinge lock nuts tight? any play here that could have excited flutter?
As already mentioned previously in this thread, if this damage had been caused by flutter, the pilot flying the airplane would have been well aware of it occurring (and I assume it would have been mentioned).
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  #23  
Old 08-14-2017, 09:37 AM
rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by AlexPeterson View Post
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!
I agree, but since I have not physically inspected it, it is still just one more guess.

What I do feel totally confident in saying is that this having been caused by a counter balance weight being 8.7 oz too heavy is highly unlikely.
I am not saying this to judge the person who owns the airplane.... but I do hope it doesn't start panic within the RV community.

Even at 8.7 oz over balanced, the vertical acceleration/gusts the airplane would have had to experience during the mentioned turbulence, to cause damage to the elevator, would likely have caused failure of something else way before the elevator would have.
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  #24  
Old 08-14-2017, 10:19 AM
BillL BillL is offline
 
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Quote:
Originally Posted by rvbuilder2002 View Post
I agree, but since I have not physically inspected it, it is still just one more guess.

What I do feel totally confident in saying is that this having been caused by a counter balance weight being 8.7 oz too heavy is highly unlikely.
I am not saying this to judge the person who owns the airplane.... but I do hope it doesn't start panic within the RV community.

Even at 8.7 oz over balanced, the vertical acceleration/gusts the airplane would have had to experience during the mentioned turbulence, to cause damage to the elevator, would likely have caused failure of something else way before the elevator would have.
I am glad you said this - +1. Something to be understood for sure.
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  #25  
Old 08-14-2017, 10:46 AM
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DanH DanH is offline
 
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No one has claimed an extra half pound of mass balance is The Cause. The question asked was how the half pound was measured, i.e. is it really over-balanced by that much?

Nobody has enough information, yet.
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  #26  
Old 08-14-2017, 11:10 AM
rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by DanH View Post
No one has claimed an extra half pound of mass balance is The Cause. The question asked was how the half pound was measured, i.e. is it really over-balanced by that much?

Nobody has enough information, yet.
Unless I interpreted it wrong, in his first post the OP was implying that the owner thought the damage was caused by the over balance of the elevator.
(note he wrote "unbalanced elevator", but I assume he meant not properly balanced)

Quote:
Originally Posted by Ron RV8 View Post
I am posting this event on behalf of a friend who does not wish to enter
the conversation directly. It is however a potentially catastrophic
safety issue which should be brought to the attention of the RV
community.

The aircraft, an RV-8 was not built by the current owner… Because he is
uncertain of the extent of possible hidden damage he is replacing the
entire empennage.

ISSUE:

* Torsional failure of an unbalanced elevator through flutter or metal
fatigue (undetermined).

RECOMMENDATIONS:

* If you did not personally balance your elevators (or trust the person
who did), it is suggested to check the balance.
* When doing so, it is suggested to disconnect the elevator pushrod to
check the balance of each elevator independently.

DETAILS:

* Aircraft - RV-8
* Total Time - 580hr
* Conditions - Moderate Turbulance
* Duration of incident flight - 32min
* Airspeed well under Vne due to turbulence.

THE INCIDENT:

My friend was returning from a fly-in under turbulent conditions. He
noticed the autopilot release following which the aircraft started to
climb. He reset the trim, reset the autopilot and continued to home
base, about a 20 minute flight. He did not notice any changes to the way the
aircraft handled.

While putting the aircraft away he noticed some cracks in the right
elevator which he did not notice during the pre-flight inspection. On
further investigation he discovered that the elevator had lost torsional
rigidity and was well on it’s way to complete failure.

He found the right elevator was overbalanced to the front by almost 9
oz. It had the old style weight installed which looks like a rudder weight.

It is suggested that it may be prudent to advise owners that have not
built their aircraft of this potential failure mode. Since the state of
balance of the other elevator can mask an imbalance situation it is
suggested that the elevators be disconnected from each other to check
the balance.

Since the only problem discovered/mentioned was the unbalance of one elevator, to me it was being implied that this was the cause......
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  #27  
Old 08-14-2017, 11:23 AM
jrvssgl jrvssgl is offline
 
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As the owner and pilot of this aircraft I will answer some questions. I do know that the elevator was in good condition prior to flight. The turbulance experienced that day would have been uncomfortable with someone not used to flying but it wasn't more than I or my ship could handle. The airplane itself never experienced more than a couple g on the occurance flight and the speed was considerably less than cruise speed to penetrate the turbulence. I did feel something not right when the autopilot struggled to maintain pitch on the aircraft, and once it was disengaged the plane flew home just fine, however nothing would have made me even consider flutter or buzz at that time with the speed lower than typical cruise. The ONLY facts I do know is that I left with a straight undamaged and uncracked elevator, and returned with a badly damaged and fatigued elevator. Somewhere along the flight the elevator received lots of damage and i am entirely certain it never happened on the ground. All jam nuts and hinge points were still tight after the flight. The 8.7 ounces of extra weight on the elevator may have or may have not been a contributing factor but with the extra weight this contradicts Vans plans for having balanced elevators. The weight was taken with the elevator balancing on its hinge points. I will try to answer any questions either private or here but this is really all I know. I will try to post another video of the elevator before it was removed from the aircraft and let fall from neutral position once the bolt was removed connecting both elevators. Thank you.

https://youtu.be/y2C9qtbquUA

Last edited by jrvssgl : 08-14-2017 at 11:41 AM. Reason: Add YouTube link
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  #28  
Old 08-14-2017, 12:29 PM
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Ron RV8 Ron RV8 is offline
 
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Default Weighing method

Thanks to Jarvis (the owner) for entering the discussion as it is much better to have first hand answers to the questions this has raised. My apologies to the group for not responding more promptly to some questions but I currently have a house full of company and was hoping that Jarvis would join the fray...

The question has been raised wrt how the elevator was supported while weighing the degree of imbalance. The objective was to get the event out to the RV family quickly in order to potentially save a life so we did not build a jig to accurately measure the exact imbalance. We opted instead to use "fingers" on the bottom of the hinges under the pivots. This definitely introduced the potential for error so the person holding the elevator purposely rolled his fingers back and forth in order to observe the degree of error that might be introduced. It was found that the maximum "finger" error was in the order of a few tenths of an ounce, insignificant when compared to the total imbalance in excess of a half pound.



Please be aware that this is not intended as a criticism of Van's design, and it is suspected that building error is probably a contributor. We do however want to make people aware that failure of this nature is possible and that a check of elevators constructed by others may be prudent.

It would be nice if someone with the skills necessary to evaluate structural failures (perhaps Vans?) would do a post-mortem on the elevator and advise all of us what caused the event... If in fact it can be traced to hidden damage it would be valuable to know that. To the best of my knowledge the elevator is still intact and available for analysis...
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  #29  
Old 08-14-2017, 01:27 PM
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DanH DanH is offline
 
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Quote:
Originally Posted by rvbuilder2002 View Post
Since the only problem discovered/mentioned was the unbalance of one elevator, to me it was being implied that this was the cause......
Fair point.

Quote:
Originally Posted by jrvssgl View Post
I did feel something not right when the autopilot struggled to maintain pitch on the aircraft...
Which makes sense, given the elevator's demonstrated flexibility on the bench.

Quote:
The ONLY facts I do know is that I left with a straight undamaged and uncracked elevator, and returned with a badly damaged and fatigued elevator. Somewhere along the flight the elevator received lots of damage and i am entirely certain it never happened on the ground.
Jarvis, if you're absolutely sure the trailing edge was undamaged prior to boarding the aircraft, the cause must be something else. Note my use of "prior to boarding", as we should note the possibility of damage during ground operations. Such damage could be (for example only) a fair size rock kicked up by a main tire, or running over an object sticking up...a stake, or light, or a Yak 52 boarding ladder (don't laugh, there was a time when a Yak ladder was the most common FOD item seen at 08A). Any such possibility at the points of departure and landing?
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  #30  
Old 08-14-2017, 02:31 PM
jrvssgl jrvssgl is offline
 
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I will try to reiterate... With ABSOLUTE CERTAINTY there was no damage prior to flight. No one touched my airplane as I was there the entire time. I remember doing a walk around before leaving there and before leaving home base .5 hour prior and this was so evident that there is no possibility that this damage would not have been noticed. Also no dents or scratches that indicate foreign debris contacting elevators, HS or Wings which are more in line for FD than the elevators. And I never mowed over any runway lights either...
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