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Old 06-07-2012, 03:41 PM   #1
CACN
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Default How can this happen?

During flight today, all of a sudden I lost elevator control, I hit throttle hold and auto down immediately without damage. My first thought is that the problem must be electronically related. To my surprise I found the elevator rod broken. Anyone has this happened before? How can this break in the air? I'm thinking metal fatigue. This is my first incident in 230 flights.

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Old 06-07-2012, 04:25 PM   #2
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WOW...You're very lucky to have got it down in one piece!!!!
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Old 06-07-2012, 04:53 PM   #3
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Precisely at the point were the sleeve ends, interesting.
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Old 06-07-2012, 05:21 PM   #4
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The sleeve does not butt up against the links? IF I remember correctly mine did...


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Old 06-07-2012, 05:27 PM   #5
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Mine doesnt. Great save!
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Old 06-07-2012, 08:52 PM   #6
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Man; you got lucky on that one. It could of been a lot worst. Great save.
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Old 06-07-2012, 08:56 PM   #7
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Now I remember, trimmed down one of the longer links to replace the 15mm link that is supposed to go there. I had mine setup so the links were tight against the sleeve. It was a much stronger setup.

You did one heck a job with that machine, very nice save
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Old 06-08-2012, 12:38 AM   #8
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Superb job recovering with that handicap!
Most probably metal fatigue, constant pressure against the swash rotation so depends how free the swash bearing is to how much force there is, shouldnt be very much but constant flexing on the pushrod is the killer.
I do have a gap on mine, may just cut a longer link to get rid of it, thus reducing the ability to flex at the end point, very good point that, great that you posted it

Post edit: Just modified mine, cut a link 2mm longer and filed carefully for a snug fit with no gap, weekend nearly here, cant wait, thanks again.
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Old 06-08-2012, 02:09 AM   #9
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I noticed this on my TDR build and didn't like it one bit ... so I used a spindle washer from a 450 Pro to take up the slop. Considerably tighter than having that tube sit there loose.
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Old 06-08-2012, 02:35 AM   #10
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NOt sure whether a gap a good thing - on flybar helis with collars on, leaving a gap between the collar and the flybar holder/cage was essential to stop the FB from snapping. I think is bad luck - obviously snaps where the thread ends as a focal point for stress. I do prefer the more ruggede links found on the Kasama Srimoks and the Minicopters - also worry about the swash to grip link which seems pretty fragile to me for such a high speed 700.
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Old 06-08-2012, 08:41 AM   #11
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Thanks guys for the suggestions! In hindsight, it is really so fortunate that my TDR came down in one piece. If it would have crashed, I would NEVER think that it was caused by the elevator rod failure. I would be wasting my time looking for the cause of it.
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Old 06-08-2012, 10:13 AM   #12
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Time for Titanium links

I think I got some titanium turnbucles a while back. I'm sure that there is something available that might work.
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Old 06-08-2012, 11:47 AM   #13
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Having the link butt-up to sleeve-end is a must. Look at underneath-side of guide-plate and make sure that you don't have any burring to the slotted-opening (for the sleeve-link).

This was very-common on the MiniCopter Jokers, if there was a gap (exposed threaded-rod at the end) there was a good chance that it would eventually "catch" on the edge of the slotted-opennig (mostly on the bottom-edge).
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Old 06-08-2012, 12:53 PM   #14
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This looks like a common fatigue failure. The root of threads creates a stress concentration. A very small crack forms at the surface of the thread root, and then slowly propgates under the cyclic loading forces.

You are very lucky. Take the rest of the day off, have a drink, and ponder your good fortune.
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Old 06-08-2012, 01:18 PM   #15
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Quote:
Originally Posted by NRPY View Post
This looks like a common fatigue failure. The root of threads creates a stress concentration. A very small crack forms at the surface of the thread root, and then slowly propgates under the cyclic loading forces.

You are very lucky. Take the rest of the day off, have a drink, and ponder your good fortune.
You're right. I stopped flying for the rest of the day yesterday. I don't want to push my luck. I wrote Jan see if he has heard of this happened before. He replied in just a couple of hours.

this is a extremely rare failure but generally everything can break at a model helicopter.
Nothing is aviation proved like on real planes.
Reason can be vibrations or a damage from a crash or hard landing before. Also a material failure can be the reason.
It is difficult to say.
Anyway you are lucky that you got the helicopter save down what is like a miracle without elevator control !
According to the theory of probability you won´t have this problem again for the next 100 years ;-)

Best Regards
Jan
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Old 06-09-2012, 01:22 AM   #16
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It's just a piece of thin metal... Everything breaks eventually if it's being used and moved... I'll just make a mental note to replace critical parts when I get to some two hundred flights...
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Old 06-09-2012, 07:33 PM   #17
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It's a good idea to put tension on the elevator rod for the following reasons:

1. Compression on the outer plastic sheet makes the assembly stiffer, and less swash phasing error will occur at extreme throws.

2. After the metal rod is pretensioned, there is reduced risk of fatigue fracturing. Fatigue is worst when the material cycles from tension to compression. Once the rod is under strong tension, it will only fluctuate from more tension to less tension, thus reducing stress fractures from propagating. Under tension, you also avoid a stress concentrator at the junction between the outer sheath and the metal rod, where the op's rod failed.

From the appearance of the op's photo, the rod and sheath were not compressed together, leading to flexing at the junction and what looks like fatigue fracture (roughness on the failure surface without apparent deformation, and along a known stress concentrator - ie the thread groove).
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Old 06-11-2012, 02:58 PM   #18
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Another factor: friction reducing the move-ability of the rod within the guide-plate?

I tried reducing slop by putting a shrink tube between outer tube and rod.
(Leaving the outer tube at the top)
When i was doing a pitch pump test on the table
(http://www.helifreak.com/showthread.php?t=398142)
with partly folded blades, i immediately broke the elevator rod. With less load
i bended the rod a couple of times. Removing the shrink tube - reinstating the slop and move-ability seems to help. No more bend rods so far.
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Old 06-12-2012, 12:40 AM   #19
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Quote:
Originally Posted by mkovalcson View Post
Time for Titanium links

I think I got some titanium turnbucles a while back. I'm sure that there is something available that might work.
Erm, steel is better than Ti for the same size part, especially with regards to fatigue (steel is better than pretty much everything for fatigue). The Ti advantage is specific strength (strength/weight) which is a non-issue for a part this small. There are some very funky Ti alloys that are as strong as steel but they tend to be brittle (and they cost a fortune and need to be carefully heat-treated).

Ti link rods are bling (like SS screws).
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