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Stabilator Counter Weight Failure

Tacco

Well Known Member
Van’s analyzing. Expect something soon.
 

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Originally posted on FaceBook RV-12 Builder’s page. Not much info there except Van’s posted they were aware of it happing on one other aircraft and that they would be providing their assessment soon.
 
Just as a point of information, under FAR Part 25 (transport catagory) mass balance/counterweight systems are to be designed for limit load of 100 g's in primary motion direction and (IIRC) 30 g's in transverse direction. Under FAR Part 23, balance/counterweight systems are to be designed for limit load of 25 g's in primary motion direction and (IIRC) 10 g's in transverse direction.

Note these are limit loads, subject to a Factor of Safety of 1.5 for failure.

This just gives some context for the level of robustness that counterweight attachments usually have.

BTW, is that blood I see next to that nylon zip-tie holding an antenna cable to a longeron/stiffener?
 
Looks like transverse failure to me, loose weights acting as hammer load with every turn? No hours on airframe provided. Is it an SLSA IFR trainer with the VOR antenna installed? Or an EAB/ELSA flown in IMC that got rough? Be fun to see what the engineers come up with. Clearly not a weld failure. Imagine someone would like to look at that counter balance arm under a microscope.
 
I wonder how the airplane flies when the weight falls off in-flight? Apparently controllable... as witnessed by photo above where airplane appears intact.

I bet a SB to be issued soonest....
 
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Also mentioned by Van's in the FB groups post is that they are aware of another case with similar or same failure.


Looks like transverse failure to me, loose weights acting as hammer load with every turn? No hours on airframe provided. Is it an SLSA IFR trainer with the VOR antenna installed? Or an EAB/ELSA flown in IMC that got rough? Be fun to see what the engineers come up with. Clearly not a weld failure. Imagine someone would like to look at that counter balance arm under a microscope.
 
I wonder how the airplane flies when the weight falls off in-flight? Apparently controllable... as witnessed by photo above where airplane appears intact.

I bet a SB to be issued soonest....

God help me if I have to crawl into the back of that tailcone again....
 
didn't everybody put a hands on that weight and wiggle/shake it when you were back there with those ''clips''? i hope the sb just says wiggle/shake.
 
BTW, is that blood I see next to that nylon zip-tie holding an antenna cable to a longeron/stiffener?
It looks like there are pieces of tubing or something RTV'd to the edge of the stringer, probably to protect the cable where it's cable tied in place. Dried blood would be darker. :p
 
One of the mods I did was to install a 4” diameter handhole on the right side of the fuselage near the counter weight. That’s looking more and more like a great decision. I always thought Vans should have added more inspection holes to facilitate stabilator removal and elevator cable wear block replacement. I also replaced those blind rivets on the wear block with machine screws to facilitate replacement.
 
Engine Prop balance and or Blade pitch not the same

The break appears to be broke in the horizonal direction. (small neutral access)
Engine vibration and the length of the tube transmit it to an inertia stable counterweight.

A rubber isolator may solve this problem. (Harmonic balancer)


My view
 
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My guess is that the plane was parked outside without a control lock on the stabilator and it banged up and down in the wind until the weight attach bracket fractured.
 
The Perfect Storm

This problem has been on my mind since my pervious my post.

I believe this failure is caused from a frequency-based metal fatigue from harmonic vibration. (Over the maximum number of cycles allowed)

Also, the kip frequency caused by the engine balance and the prop balance when in alinement? (caused by the gear box ratio)

The engine Ballance and the prop balance and the gear box move the force in changing degrees from a horizonal plane.

The engine is the on the front end of a teeter totter (so to speak) inertia stable.

The center mass is at the spar, balance point (so to speak), The spar the fuel tank and the occupants. Note full fuel tanks in the wings would help dampen the vibration.

The counterbalance is on the longer arm of the teeter totter (so to speak)

Note my background is in engineering (I am not a wordsmith)


A good prop balance and pitch match would mitigate this problem.

My view



From my pervious post

The break appears to be broke in the horizonal direction. (small neutral access)
Engine vibration and the length of the tube transmit it to an inertia stable counterweight.

A rubber isolator may solve this problem. (Harmonic balancer)


My view
 
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So, if the stabilator counterbalance falls off during flight... what will happen? I think that is the BIG question here, probably followed by... what will it take to prevent failure?

Will stabilator flutter wildly? Will airplane become uncontrollable? How heavy will pitch forces be? Etc., etc....
 
The comment on the RV-12 Facebook page was "Counter weight broke off the stab rod. Pitch got a little heavy."

Another image - taken from the Facebook post
 

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What happens if it falls off in flight?

This topic has shown up on several forums recently.

The counterweight is dynamic - meant to reduce control surface deflection in case of gust loads.

In a separate, related context, it improves flutter risk by lessening load and avoiding development of "aeroelastic" conditions which is a definition of flutter when you add "positive divergence." (i.e. displacement of the control surface increases - to failure)

Just slow her down soft turns, etc., and hope it isn't turbulent - till you get to the ground.
 
Little doubt in my mind

My guess is that the plane was parked outside without a control lock on the stabilator and it banged up and down in the wind until the weight attach bracket fractured.

I always cringe when I see an Rv-12 parked downwind. The sound it makes when that stab slams against the up-stop is deafening. Even while taxiing, a strong tailwind gust can rip the stick right out of your hand. This is probably why the stab spar was cracking and the corrective SB was published.
My guess is that these "tail slam" events probably load the counterweight shaft beyond the limit loads. Frankly, I expected we'd be seeing them failing at the root, where it's attached to the spar itself, but I can believe this failure mode as well.

I "fixed" mine by installing a rubber cushion on the shank of the up-stop bolt. I simply slit a piece of 3/8" fuel hose length-wise and snapped it onto that bolt. It does limit my up-travel a bit, but it turns the "tail slam" into a silent, soft landing. I'd guess the shock loads are reduced fifty-fold at least.
 
Random Vibration Fatigue Analysis

I am still waiting for van's analysis of this problem and solution.

This video shows what I think is the cause of the metal fatigue.

This helps explain what random vibration fatigue is.

Fast froward to 8:50 in the video if you are not interested in all the engineering.

Rember the counterweight inertia (a body at rest tens to stay at rest)

https://www.youtube.com/watch?v=Jt-9KOBbIdY

From my previous posts

This problem has been on my mind since my pervious my post.

I believe this failure is caused from a frequency-based metal fatigue from harmonic vibration. (Over the maximum number of cycles allowed)

Also, the kip frequency caused by the engine balance and the prop balance when in alinement? (caused by the gear box ratio)

The engine Ballance and the prop balance and the gear box move the force in changing degrees from a horizonal plane.

The engine is the on the front end of a teeter totter (so to speak) inertia stable.

The center mass is at the spar, balance point (so to speak), The spar the fuel tank and the occupants. Note full fuel tanks in the wings would help dampen the vibration.

The counterbalance is on the longer arm of the teeter totter (so to speak)

Note my background is in engineering (I am not a wordsmith)


A good prop balance and pitch match would mitigate this problem.

My view



From my pervious post
 
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I think it’s red RTV, used to hold the abrasion protection in place. Or a lot of blood!

I'm sure I'm misremembering things but I very vaguely recall a warning not to use Red RTV on aluminum. I think it was when I was building my RV-7A rudder or elevators and the instructions called for a dab of RV inside the stiffeners at the trailing edge. Now I don't remember if it was in the instructions where the caution was made or if it was in an issue of the RVator (which is how we got our RV news from the factory way back when).

Am I dreaming this? Because I've made sure to always use blue RTV where it's called for on the airframe since.
 
I'm sure I'm misremembering things but I very vaguely recall a warning not to use Red RTV on aluminum. I think it was when I was building my RV-7A rudder or elevators and the instructions called for a dab of RV inside the stiffeners at the trailing edge. Now I don't remember if it was in the instructions where the caution was made or if it was in an issue of the RVator (which is how we got our RV news from the factory way back when).

Am I dreaming this? Because I've made sure to always use blue RTV where it's called for on the airframe since.
I'll jump in on this one even though it's serious thread drift...
That was a long-running debate way back in the day that was eventually debunked with some testing. I'm going by memory but as I recall the results were published in the RVator and showed no increased corrosion from RTV versus other materials. The theory is that the acetic acid (which is there to etch and help make it stick) outgasses off leaving no ongoing corrosive effect.
That said, RTV also breaks down over time and so shouldn't be used anywhere that could cause a problem, like between the carb and sump for example. For that and other reasons, I prefer proseal for most things other than baffle seals etc.
 
counterweight failure

I took a close look at mine - maybe found the failure signature? A square notch in the flat plate welded to the CW arm was cut to fit the CW arm and then welded. However, the corner radius of that notch - at least on mine- was not radiused properly and creates a stress riser in the corners. if the speculation voiced here about resonant horizontal vibration is correct, then I wouldn't be surprised to see the failure mode we're seeing here.
Just my impression, but I'll wait for the VANS engineering team to opine!
 
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