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  #91  
Old 09-26-2013, 08:06 AM
David-aviator David-aviator is offline
 
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Mark,

The vertical crease could have been formed if the the AN4 rivets were set with excessive pressure on the rivets.

It is difficult to image sufficient side load force on the elevator to cause that bending crease without damage to the elevator skin, trailing edge of the HS itself.
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  #92  
Old 09-26-2013, 08:13 AM
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Mark Burns Mark Burns is offline
 
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Default Some answers for Dan

Quote:
Originally Posted by DanH View Post
Mark, Bill,

Best to confirm facts before moving further. Could you each address a few basic items about your installations?


Dan, see by answers below
(1) The five hinge points on the stabilizer are in perfect alignment?
It's been almost five years since the build. I'll remove the left elevator and re-check.
(2) The three hinge points on this elevator were in perfect alignment before dis-assembly? I can say that the bolts just slid right out with little effort. And just the other day I did look down through the rod ends and saw the hole in the horn in what looked like perfect alignment.
(3) Elevator balance was neutral, under-balanced, or over-balanced?
Again, it's been to long to remember.
(4) Elevators balanced separately or together?
Don't remember. At the airport last night, left elevator very free and slighty tail heavy.
(5) Bill, which elevator, right or left?
I do plan to replace left elevator spar also. If it was caused by wind or tail impact the left side was there too

One thing I just thought of, the left elevator counterweight is trimmed to offset the weight of the electric trim if I remember correctly.
So there would be a slight difference in force applied during a storm or tail impact.

Mark
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  #93  
Old 09-26-2013, 08:16 AM
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Mark Burns Mark Burns is offline
 
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Quote:
Originally Posted by Walt View Post
Mark,
Do you by chance use gust locks on the tips of the elevators?
Walt,
Never used any gust locks on the elevators. I just used the seat belt around the stick to hold them back.

I was thinking that a gust lock on the tips would be better. But now you have me thinking otherwise.

Mark
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  #94  
Old 09-26-2013, 08:51 AM
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az_gila az_gila is offline
 
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Exclamation

Quote:
Originally Posted by David-aviator View Post
Mark,

The vertical crease could have been formed if the the AN4 rivets were set with excessive pressure on the rivets.

It is difficult to image sufficient side load force on the elevator to cause that bending crease without damage to the elevator skin, trailing edge of the HS itself.
No, I could see it happening if the side load was applied directly along the line of the spar. The other hinge points would assist in this happening.

It would be interesting to see a detailed view of the other hinge points.
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  #95  
Old 09-26-2013, 09:23 AM
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Toobuilder Toobuilder is offline
 
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Default

Quote:
Originally Posted by Walt View Post
Perhaps this would be a good time for folks to review Van's article on VNE as well.

http://www.vansaircraft.com/pdf/hp_limts.pdf
Good advice, but we also have to keep in mind that there are plenty of Rockets flying with completely stock RV-8 tails. Normal cruise speed for many of these Rockets is at or in excess of the published VNE for the RV-8.

Based upon this fact and the lack of other RV-s falling out of the sky, I think it is more likely that we have an issue with basic craftsmanship or external damage than a basic design flaw.

I think it is prudent to do a careful inspection, but I'm hearing lots of "beef up" talk in this thread. let's make sure we are addressing the real problem before we try "fix" a design issue that might not exist.
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  #96  
Old 09-26-2013, 09:29 AM
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Mike S Mike S is offline
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Quote:
Originally Posted by Toobuilder View Post
I think it is prudent to do a careful inspection, but I'm hearing lots of "beef up" talk in this thread. let's make sure we are addressing the real problem before we try "fix" a design issue that might not exist.
Yes, exactly correct.
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  #97  
Old 09-26-2013, 09:33 AM
BillFear BillFear is offline
 
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Default Vans input

I have spoken with the factory twice on this so they are aware and following the thread. I have not suggested any design flaws, just wanted their advice and input. I appears the parts in the 7a elevator (spar, doubler and nutplate) are that same that were used in the RV6 tails.



Quote:
Originally Posted by Toobuilder View Post
Good advice, but we also have to keep in mind that there are plenty of Rockets flying with completely stock RV-8 tails. Normal cruise speed for many of these Rockets is at or in excess of the published VNE for the RV-8.

Based upon this fact and the lack of other RV-s falling out of the sky, I think it is more likely that we have an issue with basic craftsmanship or external damage than a basic design flaw.

I think it is prudent to do a careful inspection, but I'm hearing lots of "beef up" talk in this thread. let's make sure we are addressing the real problem before we try "fix" a design issue that might not exist.
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  #98  
Old 09-26-2013, 09:45 AM
Clarkie Clarkie is offline
 
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Quote:
Originally Posted by BillFear View Post
I have spoken with the factory twice on this so they are aware and following the thread. I have not suggested any design flaws, just wanted their advice and input. I appears the parts in the 7a elevator (spar, doubler and nutplate) are that same that were used in the RV6 tails.

How can that be when the -6 tail is shorter than the -7's?
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  #99  
Old 09-26-2013, 10:06 AM
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N941WR N941WR is offline
 
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Quote:
Originally Posted by Toobuilder View Post
...I think it is prudent to do a careful inspection, but I'm hearing lots of "beef up" talk in this thread. let's make sure we are addressing the real problem before we try "fix" a design issue that might not exist.
Good point.

Many times, when a part is "beefed up", it simply shifts the load to a different part of the structure which can can cause cracks / failures in other areas.

I would suggest you wait until Van's has a chance to evaluate the problem and engineer a solution before "beefing" anything up.

As for those who have cracks that need to be addressed, follow Van's advice. You will be good for another 900 hours and can address it then, if the cracks reappear.

Just my $.02 worth and remember, you got what you paid for.
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  #100  
Old 09-26-2013, 01:38 PM
BillL BillL is offline
 
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This is very interesting from a design standpoint. The crack appears to have formed on the thinner piece, but still at the sharp edge presented by the plate nut. Additionally, the crack appears to be within the area covered by the doubler. This means that 1. the doubler could also be cracked, 2. the stiffness of the flat doubler appears to be less than the spar when the rod eye is loaded vertically and 3. the edge of the doubler did not cause a stress concentration that led to the fracture. I hope Vans will be doing an FEA of this area. A simple channel shape of the doubler would spread that load better laterally, but the designer would have to evaluate that option as well.

This may address why it happened there, through no apparent failure of the builder or maintenance, but not what developed a loading condition sufficient to fatigue this area. Warm days on the gulf coastal areas do present a lot of bumps in the warm months (many of them) at relatively low altitudes, but what other reasons could this bird be seeing more loading cycles than another? And, is it only on the lower edge and not showing much loading on the upper edge because this is the typical load direction for keeping the nose up and it is not precipitated by some overwhelming aerodynamic cyclic load?

I hope some answers will come from Vans. Meanwhile, the good news is that it can be inspected, it can be found, and seems to be a pretty robust design, this crack notwithstanding.
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