MATCO Tail wheel shimmy

44 posts in this topic

Posted

Dear friends,
I am no more Avid/Fox breed owner as you already know, but my Bristell is tail dragger as well. Tail wheel used is MATCO light weight version of the dual fork eight inch tail wheel WHLT- 8LD installed as full swiveling version, mounted on dual steel spring. (see picture below)

During one year of operations (mostly on paved RWY) I am facing troubles with strong shimmy vibrations in the case of three-pointer landing (not often, rather on mains) or in the case I am putting tail to ground soon after landing.
I would like to know what could be o reason for such vibrations.
Thank you for any opinion

Tail_dragger TDO.jpg

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Posted

Can you tighten that bug nut a little bit. Or maybe grease it a bit more.

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Posted

I have a similar tail wheel (the single arm version) and had to cure the TW shimmy when first installed. The shimmy is more prevalent at higher speed, but can be cured by crushing the washers a bit more below the main nut, #3 in the photo. Try going one more flat on that castellated nut.

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Posted

Thank you friends,

I will try and let you know. For now, bad weather, rainy/snowing a little, freezing, low clouds ... no way to try now.

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Posted

If the tail spring is bent straighter then it should be, the tailwheel can shimmy because the caster angle of the tailwheel is wrong.  I'm half owner in an Aeronca  Chief that had the tailwheel spring flattened somewhat.  The Scott 3200 tailwheel started to shimmy, and we cured it with a new tail spring that had the right amount of bend in it.  JimChuk

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Posted

1avidflyer is quite right. One way to explain it is to be sure the axis of pivot of the TW is as near to vertical as possible. The spring often bends flatter with time, so getting it bent steeper, so the wheel stands more vertical, is a strong contributor to fixing shimmy.

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Posted

I think I understand, but ... maybe any hand drawing could be big help.

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Posted

Tailwheel.thumb.jpg.5cbcb554cef7702e26db

 

This photo has been around for a long time, but shows well the "Pivot Axis" everyone is talking about.

 

 

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Posted

This is a post I made on another topic about tailwheel shimmy. I was too lazy to try and remember exactly what I posted so here is a copy and past. I spent a lot of time chasing TW shimmy.

I have found tailwheel shimmy to come from two sources 1) larger tail wheels whose mass moves resonant frequency down to where it can be excited and 2) once in that situation, inadequate resistive damping in the back and forth movement of the wheel. Anything else you do whether it be changing catsor angle, tire pressure, asymmetrical springs, stiffer springs, looser springs, more tailwheel weight, less tailwheel weight....anything, only moves the resonant frequency around a bit and the system still can/will shimmy under the right conditions.

Adequate resistive damping is the only truly effective means to eliminate tailwheel shimmy from my experience...which along with $4.75 will buy you coffee at Starbucks!

 
 

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Posted

I do not know the construction of this particular tailwheel, but in order to increase resistive damping something has to be tightened. If it is possible to replace flat washers with belleville washers, that would be a great way to make the desired amount of resistive damping consistent over time.

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Posted

These are good points, Chris and I don't doubt your experience.  Damping and mass of the wheel itself are certainly factors to consider.

However, every single time I have encountered tail wheel shimmy on my plane and others, it was directly caused by (and solved by) adjusting the caster angle of the pivot axis.  My plane was shimmying badly during my trip out west.  The steel spring had sagged and was making for a negative caster angle.  On that trip I purchased a used Grove aluminum spring.  After installing the spring, all shimmy went away and as you can see, it has a positive caster angle.  During the spring installation I disassembled the entire assembly to check for wear and anything else I could find.  All was good and the swivel action is very free and loose.  The chains also have some slack in them.  No shimmy at all.

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Posted

EB27DAE2-541D-446C-94C0-8F0E53F38907.thu

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Posted

Caster is usually the number one culprit.Another useful item (which may not work on Jenki's setup) is to replace the tension springs with compression springs.Maybe the internal friction or the higher spring rate somehow dampens the shimmy?

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Posted

Well, thank you very much friends.

This is something pushing this forum forward - sharing ideas and experience.

My tail wheel is pretty COTS but I don't exactly know about tail spring and setup//geometry of this part. There is U-strap as well (No. 6 on my picture) where is free space (real clearance about 10 mm/0.4") in between fuselage and tail spring. It seems not correct for me, I would expect sort seal (sort of rubber? or elastic material ) there as spacer to damp vibrations. One more observation is that there is only one fix point of tail spring - bolt (pin) and U-strap without sort of spacer is leaving free motion up and down for spring.

I will check caster angle definitely anyway.

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Posted

Maybe you can recognize system how tail spring is fixed to fuselage.

Resize of 2017_07_08 098.jpg

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Posted

I am trying to find detail ... but nothing better then this

Bristell_TDOdetail.jpg

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Posted

Hi Larry! There is no question changing castor angle can affect shimmy, and it is possible in some cases (maybe a lot of cases i don't know) to greatly reduce/eliminate shimmy by changing castor angle. However it didn't work for me when I tried a very large tailwheel tire. The particular tailwheel I was using did not incorporate any resistive damping by design. When I went to the large tire the shimmy began.

Trying to solve the problem, the designer built me various tailspring sockets to change castor angle. i was using a stinger type spring which made changing castor angle relatively easy by changing the spring socket angle.  I tried various castor angles from negative to neutral to positive. The effect on my tailwheel of changing castor angle was that with negative castor angle it would shimmy continuously. With positive castor angle it would shimmy violently but would then suddenly track without shimmy. But it still shimmied and was violent. The large tailwheel would ALWAYS shimmy under one condition or another until we applied resistive damping.

He finally modified the design of the tailwheel to add resistive damping and that eliminated the shimmy. And with adequate resistive damping, It did not matter what the castor angle was. it did not shimmy

I think that is why at least one newer manufacturer of really big tail wheels has eliminated steering springs altogether and uses tons of resistive damping in much the same form as Vans does using belleville washers to eliminate shimmy on their nose wheels. Vans specifies something like 10lb of force required to move their nose wheels back and forth. This resistance critically dampens any resonant frequency in the system. i think we have to look at tailwheels the same way.

 

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Posted

Sorry, can you explain more deeply "resistive damping" ?

I am not native so even "belleville" washers are something I don't know exactly ...

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Posted

Jenki,

Belleville washers are washers that are not flat, they are cupped, and if not made of spring steel, have some spring to them so they keep pressure on the bolt/nut when they are torqued down correctly.  The amount of pressure can be varied by changing how much torque is applied to the bolt.

Mark

 

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Posted (edited)

I fought the same issue with my MATCO single fork tail wheel.

I found that the shimmy was worse with a passenger in my MK 4 Avid. This is because the tail wheel spring would flatten more with the weight throwing off the geometry.

I found that the tail wheel had worn parts. Namely the triangle shaped part that the "wings" rest on when in the steering mode was worn so the tail wheel broke into swivel too early. Also MATCO sent me those washer that are bent...can't remeber the name...to put under the top nut. And finally I ran my rudder springs tighter.

I am seeking a shop that can rebend my tail wheel spring back to the original shape, but am not haveing an shimmy issues.

John M

Edited by skypics

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Posted

Anyone with a simple press can re-arch the spring.  I would only do that once and then replace when it loses its arch again. The spring will break if re-arched more than a couple of times.  I did it myself a couple of times before it broke causing a fair amount of damage to the rudder.  I suspect that the original MK IV spring is nothing more than cold rolled steel.  I am now using the long leaf, one leaf, of a Super Cub spring which is holding up great.

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Posted

I'm still waiting for someone to chop apart an old compound bow and make it into a tail spring. Lighter and it won't rust. Rocks might be hard on it though.

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Posted (edited)

Murphy aircraft and others have used fiberglass tailwheel springs.  I also made one for an Avid, but it cracked when I was taxing with a 275 lb guy in the other seat.  Picture is not of mine, but one I pulled off the web.  JImChuk

Image result for fiberglass tailwheel spring
 

Edited by 1avidflyer

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Posted (edited)

Guys, you are going to chase shimmy all day long re-arching springs to change castor angle in hopes of eliminating it. The only real solution is enough adding enough friction in the back and forth movement of the wheel (resistive damping) to critically dampen the resonance. Shimmy is a resonance. Just sayin.....

The trick is adding enough friction in the back and forth movement of the wheel to kill the shimmy but still be able to overpower the friction with steering springs..

That's why some of the really big tailwheels have gone away from steering springs altogether, because it takes so much friction that steering doesn't really work anymore and you have to steer with differential breaking.

 

Edited by Chris Bolkan

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Posted

Jenki,

Belleville washers are washers that are not flat, they are cupped, and if not made of spring steel, have some spring to them so they keep pressure on the bolt/nut when they are torqued down correctly.  The amount of pressure can be varied by changing how much torque is applied to the bolt.

Mark

 

Jenki,

Belleville washers are washers that are not flat, they are cupped, and if not made of spring steel, have some spring to them so they keep pressure on the bolt/nut when they are torqued down correctly.  The amount of pressure can be varied by changing how much torque is applied to the bolt.

Mark

 

Oh, thank you, we call this spring washer, split lock washer ... now I understand

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