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Safety Alert Kolb 111 Extra Rudder and Tail boom flutter

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av8rford

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Jun 30, 2020
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In pictures 2 and 5, the rudder is clearly sticking up past the fin, in pictures 3 and 4 it is flush with the fin. Why is this?
The horizontal stabs and elevators are covering the rudder when folded up
 

wktaylor

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In an incident like this... with properly rigged controls... I would expect the control stick and rudder pedals to pulse/snap/bang to a highly uncomfortable degree... yet there does not appear to be any mention of this. Perhaps control cable under-rigging [pre-tension]... or loss-of-rigging... due broken bracket(s) or pulley(s)… may yet to be discovered... then its a question of 'chicken or egg first'.

On the other-hand any aerodynamic-item or mass added to a flight control surface that was non-standard could be a threat… especially if 'soft/flimsy'.
 

Victor Bravo

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You sure about that? Take another look at the 5th picture in the initial post. The rudder looks substantially taller than what your beautiful drawing shows.
You're right, that one looks significantly different than what I've seen, but again I have only a little experience with this type.

Here's what I have seen as "stock" on the Kolbs:

Kolb Mk3.jpg
 

BoKu

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The Kolb design appears to feature a rather large overhang beyond the upper hinge; this one looks to have increased it by 25-30%. That plus the increase in horsepower is where I'd begin looking. That rudder is right in the very turbulent propwash. It's possible that there was a harmonic resonance that coupled the bending and torsional deflections of the rudder.
Screen Shot 2020-06-30 at 8.12.08 AM.png
Screen Shot 2020-06-30 at 8.08.22 AM.png
 

12notes

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My guess would be related to this:
"Standing on both toe brakes as hard a I could I attempted to ground loop to lessen the impact. About 20-25 feet before the fence the applied pressure broke the left hand rudder pedal off at the lower wield. "

Looking at a picture I found of a Kolb mark III rudder pedals (below), I'm not sure how the toe brakes were done, but if the pilot were in the left seat and the weld cracked but hadn't broken off or were otherwise twisted at the bottom of the left rudder pedal, would/could there be slack in the left rudder cable? I'm not sure what is the function of the part the passenger right pedal springs are hooked into at the front.

Does anyone have a photo of the toe brake setup?


1593536850526.png
 

Victor Bravo

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that coupled the bending and torsional deflections of the rudder.
The height of this overhang above the tailboom tube would sure give it a lot of torque on the tailboom. So if this upper portion of the rudder was starting to swing back and forth, it would sure as heck put a cyclical or reciprocating torsional vibration into the tailboom tube... I think Bob's on the right track here.
 

rv7charlie

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I'd strongly suggest joining the Matronics Kolb email list, and post your questions there. That's where you'll find the most knowledgeable Kolb guys, including, if he's still monitoring the list, John Hauck. He probably has more hours in a 100HP Mk3 than the next 10 owners, combined. Literally thousands of hours in a Mk3. I suspect you'll find that quite a few Mk3s are flying successfully with ULS engines. There's even one flying with a reduction drive VW engine, so the design is pretty 'stout'.
Matronics Email Lists :: View Forum - Kolb-List

IIRC, Hauck has talked on the Kolb list about mass balancing his elevator, but you need to do your own research and ask questions over on that list yourself. I think I've seen some pics of a mass balanced rudder on some model of Kolb, but don't recall which one.

I did see a few recommendations that don't make sense to me. Grounding the fleet because one example that already had flying quality issues (build issues??) and significant mechanical and aerodynamic modifications, had an issue, seems a bit...extreme.

Taped-on trim tabs do work on RV-x's, but don't forget that they are applied to a metal surface, which has underlying structure to stiffen it. Taping a trim tab to a fabric control surface *could* be the equivalent of adding one to a flag. Even if the tab was over a longitudinal stiffener, it could still pull the fabric loose from the stiffener which would allow the tab to flutter constrained in only one direction by the stiffener. I know that it's an 'after' pic, but if I'm seeing the image of the added tab correctly, it looks as if the tab doesn't extend forward past the trailing edge tube of the rudder at all. If that was the case, the tab would have almost nothing to keep it from moving. RV trim tabs typically have at least as much 'meat' forward of the bend line and attached to the control surface, as the area hanging aft.

Glad you survived the experience, hope you can get some answers. and I hope the a/c is repairable.

Charlie
 

tdfsks

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Taped-on trim tabs do work on RV-x's, but don't forget that they are applied to a metal surface, which has underlying structure to stiffen it. Taping a trim tab to a fabric control surface *could* be the equivalent of adding one to a flag. Even if the tab was over a longitudinal stiffener, it could still pull the fabric loose from the stiffener which would allow the tab to flutter constrained in only one direction by the stiffener. I know that it's an 'after' pic, but if I'm seeing the image of the added tab correctly, it looks as if the tab doesn't extend forward past the trailing edge tube of the rudder at all. If that was the case, the tab would have almost nothing to keep it from moving.
This was my initial thought as well but I was initially reluctant to speculate about this because they said that the tab fell off and yet the flutter continued. However, since they have now confirmed that the tab did not fall off, I think this would be the likely cause. However, there may be other contributing factors. The redesign of the control horn may have resulted in a horn that was less stiff and a reduction in the torsional frequency of the rudder. Also changes in the rudder shape may have had an effect on frequency.

If the tab is still attached to the rudder, pluck it like a guitar string and see if you can get a feel for the frequency although it is probably fairly highly damped .... that would be interesting. Same with the tail boom. Lift the tailwheel off the ground and give the stabilizer a slap and see if you can get an idea of the torsional frequency. My guess is that it is low (for a fuselage anyway). Is this a 4" or 5" tube ? Wall thickness ?

Control surfaces that are unbalanced do, in some cases, depend on control system stiffness to help prevent flutter. If the cables slacken off, then this can cause flutter as occurred in the Zenair CH-601XL. The unbalanced ailerons fluttered because the rib that the bellcrank attached to would bend allowing the cables to go slack. That is why FAA AC 23-629 on flutter requires the ground vibration test to be conducted for all cable tensions from max recommended to zero. Having said that, many aircraft with fabric covered control surfaces are flutter free within their approved flight envelope with no mass balance. The same cannot be said for aircraft with control surfaces skinned in metal, wood or composite. All will generally require mass balance (at least partial balancing) to meet FAR 23 requirements ... if you see a homebuilt with these types of control surfaces without mass balance be very suspicious ......

The thing that concerns me is how violent this was at such low speed and the inability of the pilot to stop it. Clearly the critical speed of this flutter mode (i.e. the speed at which damping becomes negative) was at some lower speed than that at which the flutter initiated. The violence of the flutter suggests it initiated at a speed higher than the critical speed where the damping was substantially negative and the fact that the pilot could not stop it also points to that. The question is why was it encountered after a reasonable amount of flying in this configuration. One possibility is that the damping is only negative over a limited speed range and becomes positive again at higher speed allowing safe flight at cruise. Another possibility is that torsional vibrations were induced in the tail boom due to the landing on the grass runway (as opposed to the sealed runway which it had previously been operating on) and this initiated the flutter but that is just speculation because I don't know enough about how the flutter initiated.

In this particular case, I doubt the use of a 100 hp engine had much if anything to do with this.

As to the question of wider implications for the fleet ? There is no immediate case to ground the fleet given that the most likely cause was a modification. However, the speed at which this occurred does suggest to me some wider concerns about the stiffness of the tail boom combined with unbalanced controls, particularly on higher powered aircraft that are capable of higher speeds. The problem with flutter is that it is a complex phenomena and only detailed analysis and proper testing can uncover the complete picture and ensure that you are safe. This is beyond homebuilders and most kit companies. So, with most homebuilts, you never really know .....
 

Dana

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The Kolb tail boom is 6" diameter, .065" wall I believe.

The flutter may have started when something, possibly the trim tab, came loose or shifted, but that's just a guess. Or perhaps something happened during the touch and go as the tailwheel bounced over the grass. Especially if parts were modified, like the taller rudder, that doesn't necessarily point to a fleet wide problem as many examples of this model have been flying safely for many years, though of course it should be looked at.
 

Kyle Boatright

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Or perhaps something happened during the touch and go as the tailwheel bounced over the grass. Especially if parts were modified, like the taller rudder, that doesn't necessarily point to a fleet wide problem as many examples of this model have been flying safely for many years, though of course it should be looked at.
These are my thoughts as well. An odd bounce on the grass started an oscillation that would have damped itself out on a stock airframe, but the taller rudder and other modifications created a situation where the initial oscillation didn't damp, but instead amplified. That taller rudder has a lot more ability to twist the boom than the stock rudder, and that might have been the real underlying issue. Just a guess, though.
 

BBerson

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Taping a piece of metal to any control surface is a major alteration. All tabs should be rigid and irreversible.
The weight of the tab changes the aeroelastic properties. But the lack of rigidity (as Jedi mentioned in the Reno crash) is the major concern.
 
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Peer Ebbighausen

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Robertl, in pictures 3 and 4, I think you're seeing the horizontal stabilizer, which is longer than the rudder and seems to cover it when folded up.
 

rv7charlie

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In pictures 2 and 5, the rudder is clearly sticking up past the fin, in pictures 3 and 4 it is flush with the fin. Why is this?
In 3&4, the horizontals are folded up, completely hiding the fin and rudder. The blue stripes appear to be on the *bottom* of the horizontal stabilizer; not the side of the rudder.
As mentioned, the horizontal half-span is longer than rudder or fin height.
 
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