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Tail Spring Fatigue Failure

rv8or

Well Known Member
Landing back at my home strip I had landed and was on roll out and as I rolled over some very small bumps the tailwheel spring broke. Probably doing about 30 kts at the time, the chains held onto the tail wheel which bounced up and damaged the rudder.

The spring broke at the tail wheel end where it comes out of the tailwheel swivel boss.

If anyone wants to see a picture of it please send me a PM with your email address and I can send a picture or if someone has the ability to put it in the thread.

It is a classic fatigue failure

Rob
 
@RV8or

As you probably already know, hours and landings are only part of the cycles equation. Fatigue failures are pretty easy to identify in the field. An ultimate strength rupture where you’re describing is almost unimaginable. That said, we have a world class materials lab where I’m employed with some truly talented NDE engineers and techs. I can see if they’ll have a look for an expert verification if you wish. Let me know.
 
If anyone wants to see a picture of it please send me a PM with your email address and I can send a picture or if someone has the ability to put it in the thread.

It is a classic fatigue failure
Rob

Pictures posted as requested.
DSC_0282.jpeg DSC_0276.jpeg
 

Looks like it broke right where the tapered spring transitions to the straight section that goes into the fork. There should be a gradual transition right there. I’d say that it broke in a good place, rather than trying to pull the assembly and weldment out of the aluminum tailcone where it’s attached. A new tail spring is $95 I think (Vans or FlyBoy). You should probably replace the fork too. The Bell, Screaming Eagle, or JD Air forks would probably be an improvement. If you want to stick with the standard issue Vans fork, let me know. I have a couple and you can have one of them if you want.
 
Fatigue failure

Carlos has kindly posted the pictures so here is the background.

The tail wheel spring was bent at the end as per original design with a non castering tail wheel.

A previous owner cut the bent end off to fit the the standard full swivel tail wheel.

The diameter of the original RV3 spring is smaller than the standard to fit of 0.625" in the full swivel tailwheel so the spring was machined to fit a parallel sleeve to make up the difference in diameters.

A sharp edge was left where the spring was machined, then it was only a mater of time before it failed due to the high stress point and also the reduction of cross sectional area.

This crack would almost impossible to see as the sleeve / spring joint covered it.

The picture clearly show the sleeve around the broken spring, and the nature of the fatigue break.

Rob
G-BVDC not flying at present
RV8 slow slow build
 
...

A previous owner cut the bent end off to fit the the standard full swivel tail wheel.

The diameter of the original RV3 spring is smaller than the standard to fit of 0.625" in the full swivel tailwheel so the spring was machined to fit a parallel sleeve to make up the difference in diameters.

A sharp edge was left where the spring was machined, then it was only a mater of time before it failed due to the high stress point and also the reduction of cross sectional area.
...
Thanks for the update, Rob. I breathed a sigh of relief when I realized that this was a non-standard part. From time to time, my landings are "firm". :eek:
 
The diameter of the original RV3 spring is smaller than the standard to fit of 0.625" in the full swivel tailwheel so the spring was machined to fit a parallel sleeve to make up the difference in diameters.

So did this happen on an RV3 or an RV8? :confused:
 
So did this happen on an RV3 or an RV8? :confused:

RV-3 spring, the -8 never had a non-swivel tailwheel spring. This failure was an outlier due to the way it was modified in the past and doesn't create any concern for similar failure of the full-swivel spring that has been in all tailwheel RV kits since the mid-'90's.
 
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Tail Spring

The reason I highlighted this was in case anyone else had a similar mod done.

As a retired engineer I consider this failure falls into the lessons learned category, making changes can have an adverse effect if not thought out and executed properly.

I always discuss with my inspector to get a second opinion / guidance for any work that I carry out on my aircraft to reduce the chance of introducing a future failure.

Rob
RV3 Repairing
RV8 Slow building
 
I did that mod to my RV6 (current), and made sure to machine a gradual taper in the transition from the .625 straight section for the TW fork on the tail spring. Hopefully I won’t suffer the same consequence.
 
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