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  #1  
Old 08-12-2017, 05:00 PM
Ron RV8's Avatar
Ron RV8 Ron RV8 is offline
 
Join Date: Nov 2010
Location: Okanagan Valley BC, Canada
Posts: 418
Default RV-8 Elevator Failure

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.

When the video was made, the left elevator did not appear to be cracked and already had the trim tab and motor removed and installed on the new one. As a result, the left elevator was not checked for an accurate balance but appeared to the owner to be slightly underweighted when removed from the aircraft.

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.

Scale shows 8.7 oz leading edge heavy



Pens point to cracks









VIDEO: https://www.youtube.com/watch?v=ec-eWnCr7cQ
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Okanagan Valley BC, Canada
RV-8 Completed Dec 2013

Membership renewed Nov 30, 2017
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  #2  
Old 08-12-2017, 07:19 PM
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JonJay JonJay is offline
 
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Location: Battleground
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Default

First, thanks for posting.
How do you know the imbalance caused this?
I hope they quit beating it up and zip the skin off to see what the structure looks like underneath. There may be construction errors or a material defect.
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  #3  
Old 08-12-2017, 07:23 PM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 7,059
Default

The video shows a hint of what may be a damaged (cracked/kinked) trailing edge in the area where the torsional flexing is occurring.
If the trailing edge of the elevator is compromised, it would have a negative impact on torsional rigidity.

Is he sure that it didn't get damaged in some way while at the fly-in?

If nothing similar to a flutter event was ever noticed (If it happened he likely would notice), I think it is unlikely this was related to improper balance of the elevators.

BTW, you wrote "failure of an unbalanced elevator". Did you mean it had no weights on it, or did you mean not properly balanced?
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Last edited by rvbuilder2002 : 08-13-2017 at 10:26 AM.
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  #4  
Old 08-12-2017, 08:43 PM
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Steve Melton Steve Melton is offline
 
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Location: Cincinnati, OH
Posts: 1,354
Default

seems like a significant finding. I doubt there is any test data for a 9 oz nose heavy elevator in turbulent conditions. thanks for posting.
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  #5  
Old 08-12-2017, 08:54 PM
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Ron RV8 Ron RV8 is offline
 
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Location: Okanagan Valley BC, Canada
Posts: 418
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The cause of the failure is unknown

There is no way to know if there was damage at the fly-in. I will ask the owner, (he lives in another city), to look at the crack at the trailing edge to see if there is obvious damage that might have led to the crack.

The elevator had a balance weight on it that was ~9 oz too heavy. It would seem that it might provide torsional flexing in rough air and is suggested as a possible factor. It is likely that some degree of metal fatigue may have occurred before the subject flight. We don''t know...

I think that if it had fluttered it might be gone, possibly along with part of the tail. The concern is that without rigidity it would seem more prone to flutter and he considers himself lucky to have landed in one piece..

The elevator is being preserved intact in case someone with more knowledge in evaluating structural failures wishes to have a look at it...
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RV-8 Completed Dec 2013

Membership renewed Nov 30, 2017
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  #6  
Old 08-12-2017, 09:10 PM
BillL BillL is offline
 
Join Date: Sep 2007
Location: Central IL
Posts: 4,006
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Those weights are about 820 grams each. Each side takes two on each elevator on the 7. 9 oz is 255 grams, so 16% off two of the weights. If the 8 is similar then, there is no way the weights are missing on the left side.

Is there a way to share the other "blanked" photos?
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  #7  
Old 08-12-2017, 09:41 PM
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RV6_flyer RV6_flyer is offline
 
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Location: NC25
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The crack in the trailing edge is a PROBLEM. Since it was not built by the pilot, I would be interested in knowing what the construction looks like inside.

Having built RV-6 elevators over 20-years ago and RV-8 elevators last year, the design is similar and share a lot of the same parts. This is the first report that I have seen of a problem. There may have been other problems but over 20-years of success says a lot about the design.

I can say that the new weights that are used on the RV-8 elevators will balance both elevators. IF like you say to balance them individually, the excess weight off the right elevator transferred to the left elevator with the trim servo will balance the two out as individual assemblies. That is what I did with the RV-8 elevators recently built and balanced.
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  #8  
Old 08-13-2017, 08:16 AM
judoka5051 judoka5051 is offline
 
Join Date: Aug 2010
Location: Stuart, FL
Posts: 192
Default Somebody sit on it?

From the the way it twisted so easily in the video makes me wonder it someone didn't sit on the outboard edge of the elevator while it was tied down. This comes to mind because that much damage from flutter would likely be felt through the stick.
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Stuart, FL
Plane at X58

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  #9  
Old 08-13-2017, 08:40 AM
Pat Stewart Pat Stewart is offline
 
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Location: Granbury Texas
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Lots of things running through my mind, the first one is that the autopilot disconnected and it started climbing, how aggressive was the climb and is it possible that the aircraft was significantly out of trim when the turbulence was entered. When on long trips in my 7 with the autopilot on I will turn it off about every 30 minutes just to make sure the EFIS is not lying to me when it seeks trim input. Cracks along the end of the internal stiffener is also a little curious, we have seen this in rudders. Flutter, not an expert at all but what I have read does not bring it to to top of my list. Balancing the Elevators, I don't remember what the plans say so if someone can provide that info to this thread it would be helpful.
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  #10  
Old 08-13-2017, 10:57 AM
Ron RV8's Avatar
Ron RV8 Ron RV8 is offline
 
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Posts: 418
Default Re-post of photos












https://www.youtube.com/watch?v=ec-eWnCr7cQ
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RV-8 Completed Dec 2013

Membership renewed Nov 30, 2017
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