Safety Alert Kolb 111 Extra Rudder and Tail boom flutter

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franc

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Feb 23, 2020
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2
Guys,
We had a serious incident at our club, involving a Kolb 111 Xtra.
The aircraft developed severe rudder and tail boom flutter. This after trying to alleviate very heavy aileron and rudder forces by increasing the lever arm on the aileron control, and by affixing a standard size trim tab on the rudder.

Please see the report below to the Kit Manufacturer, which as yet is unanswered.

I'm an Authorized RAAus aircraft maintainer and inspector and have been asked by the owner to research the cause of this incident
So my question is ; has anyone else suffered this problem?

And as I'm yet to find anything anything wrong with the rigging of the aircraft prior to the accident, I thought it prudent to alert all to this incident.

Franc

To: "customersupport@kolbaircraft.com" <customersupport@kolbaircraft.com>
Cc: Martin Ford <fordmartin89@gmail.com>, Martin Ford <av8rford@icloud.com>

Hi Brian,

This is just to let you know how things went with the Kolb.

We built and attached the new torque tube arm at the front of the torque tube this resulted in very manageable stick pressures and was a real improvement. I flew about 1.5 hours with this new configuration. I was able to trim the aircraft for straight and level and was becoming quite happy with the handling although the roll rate was slower than I would have liked.

On Saturday afternoon there was virtually no wind at all and it was an overcast and calm. I decided to practice some tight circuits to ascertain how it handled when pulling short base and shot finals in a series of touch and goes. The climb performance was always good but I was light on fuel and it really was exceptional. I climbed out at 80MPH to provide a safety margin incase of an engine failure. The take off from the sealed 05 airstrip went well and the subsequent touch and go on the sealed strip went well. The Base and Final legs were tight but very manageable with the new torque tube arm.

On the next touch and go I decided to use the shorter 05 grass strip that is parallel to the sealed strip again the handling on Base and final went well and the touch and go was a non event again climbing out at 80 MPH at a little over 125 feet the aircraft developed severe rudder flutter. (I will go into what witnesses observed later) I immediately reduced throttle and changed attitude the rudder pedals were oscillating rapidly and I could not suppress them. The stick exhibited no signs of flutter or movement. I took my feet off the pedals to see if it would improve, it continued to get worse. I could hear the cables slapping inside the boom. I pulled the throttle hard off and decided to try and put the aircraft back onto the field knowing there was insufficient field remaining. The buffeting in the aircraft continued even at the reduced speed and it really felt like it was going to break up the shaking in the cockpit was quite severe. I managed to get the wheels back on to the ground around 70 Mph due to the steep approach with little open field left in front of me (about 125 feet left before the boundary fence). 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. From then on I had no more control until I hit the corner posts of the boundary fence (not that I could have done much in the final 25 feet or so). I hit the boundary fence at about 90 degrees to the original direction of travel. Due to the inertia of the impact my hand inadvertently advanced the throttle I immediately pulled the throttle off again I then went through the shutdown procedures.

Witnesses on the ground said that the rudder was fluttering so severely that the boom was rotating in a circular fashion like a mixmaster and that the rudder and the tail plain was twisting with the rudder flapping.

I have attached photos of the result. It can beset that the impact was significant. The aircraft was severely damaged the engine and propellor did not contact the fence. There are signs of structural stress on the engine mounts (bending of the tubular structure).

Have there been any other reports of rudder flutter in these aircraft? I have been unable to determine what caused the flutter.

Aside from a bruising from the seatbelts and a bruised finger I am none the worse for wear… More than I can say for the aircraft.
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jedi

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Interesting to say the least and a good detailed report.

I do not see the added rudder trim clearly in the photos. Is it still attached firmly to the structure?

I am reminded of a similar incidence in a CGS Hawk that was traced to a bracket holding the elevator trim tab teleflex type cable housing to the tail structure.

The repeated operations prior to this incidence and the switch to operations on a possibly rougher surface are clews that something may have changed as it was the in the case mentioned above where the trim support gradually loosened over time.

How was the tab bonded to the rudder? Is it the darker rectangle just above the lower stripe in the last photo? If so, what is the structure under the fabric in that area?

I have no additional comments except that congratulations are in order on a successful conclusion of the flight.
 

cblink.007

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Jul 7, 2014
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206
Location
Texas, USA
Wow. Glad to hear the pilot is okay, but unfortunate for the aircraft.

I am unfamiliar with the Kolb; are the control surfaces mass balanced?
 

Victor Bravo

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Wow, I am very very happy nobody got hurt. Tailboom or control surface flutter is really really bad news.

What bothers me most about this story is that once excited, the flutter did not go away after the speed was reduced. Also, hearing that it fluttered at pattern speed (as opposed to aerobatics, full power cruise, descending out of a cloud, etc.) is very troubling.

The really big question is whether the flutter happened because of the addition of a trim tab tht was likely an ounce of aluminum , or whether some structural part broke and allowed flutter to start, but that flutter would not have started if X had not broken.

I truly hope Bryan has real-world useful information for you, he's a good guy. BUT, regardless of anything you get from the factory, PLEASE get someone from the Australian EAA, or Australian Civil Aviation engineering branch involved. A real engineer needs to look at this and determine whether it was caused by damage, by a design flaw in the Mark 3 Kolb, or whether the design is good but adding even an nounce of weight at the rudder can cause this.

Although completely unsolicited, some of the things I would look into are... how much does the rudder weigh? Is there 15 coats of heavy paint on it? Has it been modified form the original Kolb design (4130 steel tubes instead of aluminum and rivets)? Is the vertical in front of the rudder secure,a nd are the tail wires taut enough? Can the fin or rudder be twisted by hand? Is there any previous damage or cracking on the tail boom which would have allowed it to sway left and right?
 
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Victor Bravo

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I am unfamiliar with the Kolb; are the control surfaces mass balanced?
The ailerons on most of them have a mass balance at the outboard end. Not particularly robust, but far better than nothing. There are no balances on the rudder or elevators that I have seen, but I only have one flight in the Mark 3.

DEFINITELY report and discuss this incident on the Matronics Kolb list. That list represents the "fleet experience" for the Kolb series, and there are several very very highly experienced Kolb owners there. The primary guru there, John Hauck, has extensive experience with the Mark 3, having flown it in rather extreme conditions, all across North America, Canada, and Alaska. If there is a known cause for rudder flutter, those guys will be able to get to the bottom of it.
 

franc

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Feb 23, 2020
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Guys,

Thanks for your input. To answer some of the questions
I saw the rudder trim tab only from a distance, as at that stage I was not involved with the aircraft. But it appeared "Standard size"
I will be talking with the owner tomorrow and will confirm the size of the tab.
The Tab was secured by "100mph" tape, as it was a trial.
The Tab had been in place for 1.5 hours flight testing prior to the incident.
The Tab was dislodged by the flutter early in the incident
None of the control surfaces are mass balanced.
From what I can see the aircraft was correctly rigged, and had only one coat of paint on all surfaces.
 

billyvray

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Been around Kolbs a while, and never heard of such an experience in one. My first thought is was something not rigged right, or maybe not final-fastened that allowed some movement, or anything done different from stock. They are robust designs for the envelope.
The only slight apparent difference I see from the pictures might be the top of the rudder appears to be taller than standard, though that might be a perception mistake from the pictures. I don't know if that would cause flutter but it would be in the problem area...maybe...
 

Victor Bravo

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But if I am reading the original post correctly, he was climbing out and at light weight when the flutter happened. So I would assume that this means somewhere between 50-65 MPH ?? That is far below the likely "design flutter speed' on the Kolb if it was a stock plans built airplane. Any normal size sheet metal trim tab just doesn't seem like it could excite flutter at that speed, no matter if it was banging back and forth on a piece of duct tape.

My gut feel is that there's gotta be some other smoking gun problem... one side rudder cable or nico sleeve broken, rudder control horm weldment busted loose, one of the rudder hinge pins wiggled all the way out ... something big like that.
 

don january

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The way I see it something terrible went wrong. I hope the entire owners of said model of plane gets parked and a solution is found to solve the problem before some one is killed. franc you got lucky
 

Victor Bravo

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Been around Kolbs a while, and never heard of such an experience in one. My first thought is was something not rigged right, or maybe not final-fastened that allowed some movement, or anything done different from stock. They are robust designs for the envelope.
The only slight apparent difference I see from the pictures might be the top of the rudder appears to be taller than standard, though that might be a perception mistake from the pictures. I don't know if that would cause flutter but it would be in the problem area...maybe...

Actually that is the stock rudder, and it doesn't really stick up that bad. The vertical fin is shaped at an angle that meets that top radius on a tangent line of sorts.

Dig my high-end professional graphics skills....

KolbTailProfile.jpg
 

Kyle Boatright

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Marietta, GA
Actually that is the stock rudder, and it doesn't really stick up that bad. The vertical fin is shaped at an angle that meets that top radius on a tangent line of sorts.

Dig my high-end professional graphics skills....

View attachment 98711
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.
 

tdfsks

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Aug 29, 2005
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Franc,

I heard about this incident through another source (one of the witnesses) and have some other photos sent to me the day it happened. At the time I remarked that this sounded serious and that the pilot was very lucky (I have some background with flutter from an engineering perspective).

This is not the first case of flutter with Kolb designs. There have been cases of aileron flutter and I believe that is where the aileron mass balance referred to above originated.

Flutter can occur at low speeds such as this (for example the wing / aileron flutter that killed Gerry Ritz many years ago) but the frequency will generally be lower and amplitude higher.

The initial indications are that this appears to be a case of binary flutter with rudder rotation coupling with tail boom torsion. However this needs more investigation as there are some things that don't make sense to me.

A few questions ....
1. How was the tab taped in place ? You don't have a picture by any chance do you ?
2. Is that the tab that I can see in the last of the photo's you posted or did it definitely fall off ?
3. I assume the damage to the inboard end of the elevators is as a result of the rudder hitting them ?
4. Can the pilot provide an estimate of the flutter frequency and rudder pedal amplitude ?
5. Is there any permanent damage to the tail boom ? What is the diameter and wall thickness of the tail boom ?
6. Was covering / paint on the tail surfaces standard ?
7. Is the rudder size and shape standard ?
8. Were there any mods to the horizontal tail surfaces that might have increased their weight / mass moment of inertia about the tail boom axis ?
9. Can you provide any info on the rudder control circuit (pages from the manual or description) ?
10. Is the fin / rudder spar bent ? If so, where ? (Fairly important)
11. I note that the leading edge of the LH stab is bent. Did this hit the fence or did this damage occur in flight ?
12. Do you have any details of the old and new rudder horns ? Material, dimensions, fasteners etc

Happy to talk about this offline if you would like ... might be easier. My time zone is 2 hours different to you.
 
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jedi

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But if I am reading the original post correctly, he was climbing out and at light weight when the flutter happened. So I would assume that this means somewhere between 50-65 MPH ?? That is far below the likely "design flutter speed' on the Kolb if it was a stock plans built airplane. Any normal size sheet metal trim tab just doesn't seem like it could excite flutter at that speed, no matter if it was banging back and forth on a piece of duct tape.

My gut feel is that there's gotta be some other smoking gun problem... one side rudder cable or nico sleeve broken, rudder control horm weldment busted loose, one of the rudder hinge pins wiggled all the way out ... something big like that.
VB: As noted in Post #2 above a loose aerodynamic trim will excite the structure upstream. The trim excites the aerodynamic control it is attached to which in turn excites the stabilizer surface it is attached to and that in turn drives the primary structure. This can occur at a low speed on climb out and once it has started the flutter will persist to an even lower speed. In the case noted in Post #2 the flutter started on climb out with a frequency of about 2 or 3 Hz. A transition from controls fixed to controls free will likely exaggerate the flutter as noted in post #1.

If the trim tab breaks loose and departs from the aircraft the flutter would likely stop provided that structural damage is absent which could cause the flutter to continue. In the case of the Reno P 51 crash the strong out of trim condition that resulted after the trim departed the aircraft created additional control issues.
 
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av8rford

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Jun 30, 2020
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Hi All
Franc's post relates to my aircraft. Also note that I was not the test pilot or the builder.
The Kolb was purchased as a fairly complete project and to my knowledge, never flown.
My friend, Don agreed to test fly the aircraft due to his experience in pushers.
His comments initially when flying it, were related to it being way out of trim. We were able to sort the aileron trim easily but took a while to get the rudder trim to be neutral in cruise. Just prior to the last flight I fitted a larger tab which was temporarily secured with duct tape.
This method of temporary attachment has worked well even on higher performance aircraft such as the RV6.
Don commented after the event that the rudder trim seemed ok prior to the flutter incident.
My theory is that while there was a constant need for right rudder input, the flutter was held at bay. Also the slightly heavier tab added momentum to the subsequent rudder swing. Another thing, I think the Rotax 912ULS is too powerful for the airframe!.
I was observing the event and I feel that immediately after the touch and go the tailfeathers started shaking fairly violently. 100horses on climb! Even when doing a runup the tail on the Kolb seemed to shake quite severely.
A few seconds after the touch and go the whole tailboom seemed to whip around in a spiral motion, with the rudder smacking back and forth.
The damage to the rudder and right elevator was purely from the flutter. The left horizontal and elevator were mainly damaged from the impact with the fence. The trimtab was not loose after the impact.
From what I can see of the build everything appeared to be very much to plans.
Even though I have no plans to rebuild the Kolb, I hope that others may benefit from our experiences.
Thanks for all your input....Martin
 

Wild Bill

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Dec 11, 2013
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Vidalia, GA
I don’t know if it would make that much of a difference, but the rudder does look like it was built wrong.
Most Kolb rudders that I’m aware of have the top tip bow starting more or less tangent with the leading edge tube. This one appears to extend straight up several inches before making the bow.
Would help to see all of it as the picture is cut off.
 
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