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  #131  
Old 09-28-2013, 05:12 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by krw5927 View Post
What is not shown in this sketch is what the actual load path "should" be. The jam nut should be transferring all of the rotational torque load into the spar itself, and then into the skin throug all the spar/skin rivets.
Sure, the locknut clamps the nutplate, doubler, and spar together. I can draw a locknut into the sketch, but it won't change the end result. The load path still passes through the circled spar section.

Quote:
Originally Posted by BillL View Post
I am assuming it is to scale.
Not precisely. No spar handy at my computer yesterday, so the sketch was scaled from photos and preview drawings. Somebody PM key dimensions (spar height, doubler height, rivet locations) and I'll tidy it up.

Quote:
I was loosely referring to a (yet unidentified) cyclic load overlaid on a static load, thus it failed only on one side where it is biased in favor of your sketch. (Imagine a sine wave where the "zero load" line is not centered between the peaks)
You're considering stress ratio, as am I.

Quote:
There has been a lot of discussion about the jamb nut, but it was tight and seems to have no relation to this failure mode.
Agree.
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  #132  
Old 09-28-2013, 05:27 AM
exp2ex exp2ex is offline
 
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Default Castle nut on Pitts

My pitts has a castle nut on the drilled bolt that connects the elevator horns to the push tube. There are 2 of them and a aluminum block spacer that sets the 2 elevator horns at the exact width for the push tub end bearing to fit between the horns with a washer on both sides of the bearing. The access is a clear inspection plate so you can visually inspect the bolts as well. I try to use tamper paint for all the nuts on everything I have now. I started using it when the U-bolts on my Citabria were loose. I have never seen the tamper paint cracked on any of my applications and it also forces you to go around your aircraft and check every accessible nut and mark it.

Quote:
Originally Posted by DaAV8R View Post
I don't currently have a dog in this fight as I'm a long way from flying, but.....

several here have reported loose nuts on the elevator pushrod bolt and mentioned adding the second bolt. While the second bolt may be a good idea, it doesn't address the loose Jesus nut. If the pushrod bolt comes out, your second bolt isn't going to help.

There have been discussions in the past regarding the nylon lock nuts. My testing indicates these nuts only provide 1 to 2 inch pounds of friction. Should there be any conversation about improved captivation of these nuts?
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  #133  
Old 09-28-2013, 06:13 AM
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apkp777 apkp777 is offline
 
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Since apparently the cracked spars in question didn't have a bearing mis-alignment problem (still my first guess), it becomes clear (to me at least) that the spar is not strong enough to carry the load and should be either beefed up at the hinge point or replaced altogether with a heaver gauge.

I don't see the "nut" being the problem, even though when loose the lower rivet gets loaded in tension, that's only because the spar is flexing, which again indicates it's too weak. Sorry Van's!
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  #134  
Old 09-28-2013, 07:27 AM
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AlexPeterson AlexPeterson is offline
 
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My apologies if this has been said already in this long thread...

Regarding misalignment of the bearings, be aware that they are not necessarily aligned just because all of the bolts freely slip in while the surfaces are in the neutral position, for example.

It only means that each rod end is matched with its corresponding attachment bracket. All bolts can freely go in, yet huge forces can be generated when the elevator (or other surface with greater than two bearings) is moved. This might be difficult to detect in moving the surfaces, as it may be a very subtle, and smooth, force feedback.

One way to check for this without complete dis-assembly is to remove one bolt at a time, and verify that the bolt goes in freely at both extremes of surface deflection, as well as at the neutral position.

The bearings need to be in a straight line.
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  #135  
Old 09-28-2013, 08:58 AM
alpinelakespilot2000 alpinelakespilot2000 is offline
 
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14 pages of thread for a issue that so far only seems to be affecting one aircraft out of a fleet of thousands? I think the op is great if it makes us check for cracks, alignment of the bearings, and jamb nut tightness, but is the part of the thread trying to engineer solutions to what may not even be a design problem helpful? I'm not trying to stifle discussion, I just hope new builders don't take from this discussion that they should think about mods for this area.
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Last edited by alpinelakespilot2000 : 09-28-2013 at 09:19 AM.
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  #136  
Old 09-28-2013, 09:46 AM
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Russ McCutcheon Russ McCutcheon is offline
 
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Thumbs up Mine are doing well

It?s time for my condition inspection, this year the tail group got a very close look, nothing loose or moving, no loose lock nuts, no cracked spars, elevators or rudder, no cracks of any kind anywhere.

What I did note is that my rod end bearings are all adjusted such that the elevators and the rudder are all as close to the stabilizers as possible, barley more than enough room for the lock nut between the spar and the end of the threads on the rod end bearing, about one thread left showing, this should make these assemblies as strong as they possibly can be, most ideal condition. There is also no binding of any kind, elevators and rudder move very freely.

Early 80s RV-4 with 2708 hours.
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  #137  
Old 09-28-2013, 09:53 AM
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skylor skylor is offline
 
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Quote:
Originally Posted by apkp777 View Post
Since apparently the cracked spars in question didn't have a bearing mis-alignment problem (still my first guess), it becomes clear (to me at least) that the spar is not strong enough to carry the load and should be either beefed up at the hinge point or replaced altogether with a heaver gauge.

I don't see the "nut" being the problem, even though when loose the lower rivet gets loaded in tension, that's only because the spar is flexing, which again indicates it's too weak. Sorry Van's!
Be careful with a statement like this. In Mr. Burn's case, there is very clear evidence that the assembly was exposed to a significant overload at some point, as evidenced by the crease on the spar at the end of the doubler.

Simply beefing up components is not necessarily a good way to handle failure of aircraft parts when the cause of the failure is unknown.
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  #138  
Old 09-28-2013, 10:27 AM
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apkp777 apkp777 is offline
 
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Quote:
Originally Posted by skylor View Post
Be careful with a statement like this. In Mr. Burn's case, there is very clear evidence that the assembly was exposed to a significant overload at some point, as evidenced by the crease on the spar at the end of the doubler.

Simply beefing up components is not necessarily a good way to handle failure of aircraft parts when the cause of the failure is unknown.
Keep in mind that we are talking about a failure of a Primary Structural Element. Be it a wing spar or a elevator spar, cracks, deformation of any sort is a MAJOR failure. The fact that there are 2 aircraft with very similar cracks is definitely cause for speculation and concern. We do know the cause of the failure...the deflection of the spar common to the rod-end. What caused that deflection is unknown and may remain unknown. Whether it be assemble error, excessive aerodynamic force or under engineering of unanticipated normal loads...you can be sure, if I find mine cracked I am not going to put a new stock spar back in it's place. The stock spar material is not sufficiently handling the structural load. It is NOT uncommon for a manufacturer to "beef" up parts of the aircraft that time has determined that the original design was insufficient.
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Last edited by apkp777 : 09-28-2013 at 02:21 PM.
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  #139  
Old 09-28-2013, 10:29 AM
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apkp777 apkp777 is offline
 
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Quote:
Originally Posted by alpinelakespilot2000 View Post
...seems to be affecting one aircraft out of a fleet of thousands...
I thought there are 2 aircraft? I am not sure that "a fleet of thousands" is correct. How many RV7's are flying with 900 hours? I would guess not that many.
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  #140  
Old 09-28-2013, 11:01 AM
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rvbuilder2002 rvbuilder2002 is offline
 
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Default Food for thought....

With regard to all of the "not strong enough" and "needs to be beefed up" comments....

Keep in mind that pretty much this exact same design was used on all RV's.

There is now over 8400 known to have flown. Thousands of those have thinner skins than an RV-7 elevator (.016 vs .020)

If there was a general issue with the design, it would have become evident many years ago. At the very least, on RV-8's. They use the exact same (prepunched) elevator.

I said it once, but I guess many don't believe... this type of a crack does not occur from one (or even several) overload/bend events localized at the crack. It is many (probably many thousands, possibly even many ten thousands) load cycles. The damage from a single overload in a location like this would be a visible crease or bend. Not an original flat surface with a (fatigue) crack.

Deductive reasoning says that something has been causing a localized load cycle on this (maybe these two) airplane, that does not occur on thousands of others that have much more flight hours on them.

The challenge is discovering what. Once we know that, a solution is right behind.

My opinion is that we haven't yet heard all the details. I am not saying anyone is withholding info... It is often something that is missed, and not considered significant because of lacking experience in evaluating this type of problem.

It is entirely possible that it is a combination of factors, that on these two airplanes, they just happened to have them all, but the majority of the fleet does not.

I still am of the opinion that it is some type of alignment issue. The photo evidence of the lateral edge of the doubler working on the face of the spar web could be an indicator that there is a lateral alignment issue at the hinge point.

If I was the investigator in charge would want to see photos of all three hinge points showing a thread pulled through them, and dead center (or nearly so) in the hinge bracket holes, and the same done with the hinge points on the elevator (if there is misalignment, it could be on either one, or both); and a video showing the damaged elevators, two rod end hinge points slipping into the stab. hinge brackets without any prying or unusual force required. ( impossible to do now, since the elevator has been disassembled)
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Last edited by rvbuilder2002 : 09-28-2013 at 11:15 AM.
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