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  #21  
Old 10-17-2019, 08:42 AM
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rocketbob rocketbob is offline
 
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3K 5.8oz "tape", 50% overlap for two layers. Doing it over I would get a 6" wide roll.
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N9187P PA-24-260B Comanche, flying
N678X F1 Rocket, under const.
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  #22  
Old 10-17-2019, 08:51 AM
David Paule David Paule is offline
 
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Location: Boulder, CO
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Default Different Potential Ways To Eliminate Shimmy

It's interesting that there are several approaches to eliminating shimmy -

1. Increase the stiffness of the gear leg, like the OP did.

2. Change the moment of inertia in one axis to decouple the fore/aft and lateral modes of vibration. Since the wheel and brake are off the axis of the leg, the motion might be a combination of those modes. This is what I think the wood additions do. I think Larry took this approach in a different way.

3. Actually add dampening, which I haven't seen yet.

4. Reduce or eliminate the forcing vibration that starts it, like AntiSplatAero does with their wheel balancing/bearing mod.

5. Adding balance weight at the forward end of the wheel pant. I'm not entirely clear on why this would work but several people have said it does.

Dave

Last edited by David Paule : 10-17-2019 at 04:34 PM. Reason: Added #5.
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  #23  
Old 10-17-2019, 06:16 PM
VIN2020 VIN2020 is offline
 
Join Date: Feb 2015
Location: Fort Worth
Posts: 26
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Quote:
Originally Posted by rocketbob View Post
About 5 years ago a friend of mine installed 380 tires on his RV7. I fly this airplane regularly. With the 380 tires a shake would develop right around 19 KTS GS which required firm braking to slow thru that speed fairly rapidly to get the shake to stop. Lived with it but it was pretty annoying. Things tried to fix the shake: carefully balancing the tires, new tires instead of recaps, new wooden gear leg stiffeners, various tire pressures, tried one gearleg with a stiffener without the other having one thinking a resonance was developing between the two. Nothing really worked but there were varying degrees of effect on dampening the oscillation. That is, until the new idea we came up with and implemented a few weeks ago.

Wrap the gear legs with 3K 5.8oz carbon fiber. No stiffeners.

Using a 48" roll of carbon fiber, squeegee wetted a 4ft or so square, 8" wide strips were cut using a rotary knife which were then rolled up and wrapped around clean gearlegs bottom to top in one pass overlapping 50% which amounts to two layers of carbon.


No peel ply or vacuum bagging was used as this was considered an experiment.

The layup was done with mains on the floor.

After about a dozen or so hours still no shimmy or shake at all and it seems to be holding up fine.

The gear feels absolutely perfect now with no tendency to rebound or skip. Tires are 35psi. In no way does the gear feel stiffer, but feels well dampened. For this reason I don't think any additional stresses are placed on the engine mount.
Any chance you could post some pictures (worth a thousand words)?

My RV7 has the same problem. Tried running lower tire pressure, which significantly help the vibration, although had started having problems with my tires going flat way to often.

Thanks!
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  #24  
Old 10-17-2019, 06:51 PM
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rocketbob rocketbob is offline
 
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Quote:
Originally Posted by VIN2020 View Post
Any chance you could post some pictures (worth a thousand words)?

Thanks!
Didn't take any. To be honest it got slapped on and didn't look very nice.
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N9187P PA-24-260B Comanche, flying
N678X F1 Rocket, under const.
N244BJ RV-6 "victim of SNF tornado" 1200+ hrs, rebuilding
N8155F C150 flying
N7925P PA-24-250 Comanche, restoring
Not a thing I own is stock.
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  #25  
Old 10-17-2019, 07:40 PM
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AlexPeterson AlexPeterson is offline
 
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Quote:
Originally Posted by DanH View Post
Indeed. Anyone care to speculate about how it works?
If there is damping, it could be from carbon fiber moving slightly within the epoxy. Just a guess.
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  #26  
Old 10-17-2019, 08:01 PM
PilotjohnS PilotjohnS is offline
 
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Location: Southwest
Posts: 1,119
Default My take

I think this is a great idea. Seems it makes the gear behave more like the rv8 flat gear legs where they are stiff(er) fore and aft, except the RV7 gear is now stiffer in both directions, I aint picky.
Anybody want to wrap mine for a small fee?

Any chance this will trap moisture between wrap and leg? Would solid carbon rod be better?
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Last edited by PilotjohnS : 10-17-2019 at 08:04 PM.
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  #27  
Old 10-18-2019, 04:15 AM
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Steve Melton Steve Melton is offline
 
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some material expansion numbers:

spring steel 6150 coefficient of thermal expansion = 6.8

woven fiberglass / epoxy coefficient of thermal expansion = 10

unidirectional fiberglass / epoxy coefficient of thermal expansion =7

woven carbon fiber / epoxy coefficient of thermal expansion = 3.4

unidirectional carbon fiber / epoxy coefficient of thermal expansion = -1

wooden gear leg stiffeners wrapped with carbon or glass would also provide stiffening in the fore / aft direction. the build up would be a stiffening beam.
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Last edited by Steve Melton : 10-18-2019 at 04:26 AM.
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  #28  
Old 10-18-2019, 04:21 AM
jask jask is offline
 
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Location: Ramona, CA
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Default gear leg vibration

Just a thought on vibrations. It would appear that the gear leg is acting as a mechanical amplifier to the wheel vibration. So, there are two solutions. First, eliminate the source of the vibration by balancing the tires (best method) or limit the ability of the gear leg to act as a mechanical amplifier. Whatever you put on the legs to change the resonate frequency needs to be firmly attached. I would think that you could add weight in selected spots that would change the resonate frequency or harmonics of the rod.
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  #29  
Old 10-18-2019, 05:43 AM
BillL BillL is offline
 
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Location: Central IL
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Quote:
Originally Posted by jask View Post
Just a thought on vibrations. It would appear that the gear leg is acting as a mechanical amplifier to the wheel vibration. So, there are two solutions. First, eliminate the source of the vibration by balancing the tires (best method) or limit the ability of the gear leg to act as a mechanical amplifier. Whatever you put on the legs to change the resonate frequency needs to be firmly attached. I would think that you could add weight in selected spots that would change the resonate frequency or harmonics of the rod.
If I was back in grad school this would present quite an interesting engineering mechanics derivation challenge. Natural frequency calculation of a tapered rod with a spinning mass at the end. Combining both bending and torsion. Now (42yrs later) I am puzzled and mentally challenged in a different way.

It is curious, though, that some planes have the issue and some do not. It would seem that the tires are the primary damper, and the pants are an additional spring mass part of the system. It can't be just the forced vibration element, as all planes would eventually have the issue, even if transient.

Dave: couldn't the mass center of the pants be off axis thereby converting a linear vibration into a torsional element?

With all the fresh brain power on VAF it is amazing no one has ever identified (posted) the precise root cause(s).
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Last edited by BillL : 10-18-2019 at 06:26 AM.
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  #30  
Old 10-18-2019, 06:36 AM
506DC 506DC is offline
 
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Location: Fresno, CA
Posts: 38
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Quote:
Originally Posted by David Paule View Post
It's interesting that there are several approaches to eliminating shimmy -

1. Increase the stiffness of the gear leg, like the OP did.

2. Change the moment of inertia in one axis to decouple the fore/aft and lateral modes of vibration. Since the wheel and brake are off the axis of the leg, the motion might be a combination of those modes. This is what I think the wood additions do. I think Larry took this approach in a different way.

3. Actually add dampening, which I haven't seen yet.

4. Reduce or eliminate the forcing vibration that starts it, like AntiSplatAero does with their wheel balancing/bearing mod.

5. Adding balance weight at the forward end of the wheel pant. I'm not entirely clear on why this would work but several people have said it does.

Dave
I thought number 5 prevents flutter like counter balancing a control
surface. It would also not magnify shimmy.
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