# P-38 Lightning Nose Wheel Shimmy Damper Kinematic Problem



## JayW (Feb 24, 2014)

I am modelling a large scale (1/18 ) P-38 and will be adding a considerable amount of detail to it, including the landing gear. The noses gear will be displayed turned maybe 15-20 deg from center. To do this the shimmy damper pistons (one on each side of the gear strut) will be retracted inside the cylinder a certain amount on one side and further extended on the other side. As I study pictures of the shimmy damper from my sources, I clearly see that the cylinders are firmly attached to the lower rotating portion of the gear (basically clamped to rings on the torque link fitting), and the piston rod ends are attached, and pivot about, joints common to the upper strut drag link fitting which is stationary. But kinematically this doesn't make any sense. The rod ends want to shift outboard or inboard of their attachment points as the nose gear turns one way or another. I realize of course the piston rods extend and retract, but without the cylinders themselves having a rotational degree of freedom, the rod ends cannot stay on the attach points.

That is unless there is some feature of the system I am unaware of, whic is clearly the case. Do the pistons wiggle inside the cylinders? Is there some rotating eccentric bearing at the rod ends? 

Is there a P-38 expert out there who knows how the shimmy dampers move?


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## JayW (Feb 24, 2014)

Hey, why the funny face on the scale? That is one eighteenth scale, to eliminate the confusion.


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## FLYBOYJ (Feb 24, 2014)

JayW said:


> Hey, *why the funny face on the scale*? That is one eighteenth scale, to eliminate the confusion.



That was corrected - it happens when you place an "8" next to a ")."


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## FLYBOYJ (Feb 24, 2014)

JayW said:


> I am modelling a large scale (1/18 ) P-38 and will be adding a considerable amount of detail to it, including the landing gear. The noses gear will be displayed turned maybe 15-20 deg from center. To do this the shimmy damper pistons (one on each side of the gear strut) will be retracted inside the cylinder a certain amount on one side and further extended on the other side. As I study pictures of the shimmy damper from my sources, I clearly see that the cylinders are firmly attached to the lower rotating portion of the gear (basically clamped to rings on the torque link fitting), and the piston rod ends are attached, and pivot about, joints common to the upper strut drag link fitting which is stationary. But kinematically this doesn't make any sense. The rod ends want to shift outboard or inboard of their attachment points as the nose gear turns one way or another. I realize of course the piston rods extend and retract, but without the cylinders themselves having a rotational degree of freedom, the rod ends cannot stay on the attach points.
> 
> That is unless there is some feature of the system I am unaware of, whic is clearly the case. Do the pistons wiggle inside the cylinders? Is there some rotating eccentric bearing at the rod ends?
> 
> Is there a P-38 expert out there who knows how the shimmy dampers move?



Here's a pic of a P-38 NLG. I believe the shimmy damper will take up loads from the torque links (just as they do in many GA aircraft today). The area where the top of the torque links attaches moves, it is free castering and NLG steering is done by differential braking.



http://www.largescaleplanes.com/reviews/Revell/P-38J/P38lsfm2.jpg


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## JayW (Feb 24, 2014)

FlyboyJ - thanks for the response. I agree with everything you stated. Yes the torque links caster with the wheel freely, and yes the steering is accomplished with differential braking or rudder. The dampers are passive devices that are merely loaded up with pressure; they do not steer. But look at the picture - the damper cylinders move with the castering upper torque link; they have to as they are firmly attached to the same fitting on the strut. But the piston rod parts of the dampers have their rod ends attached to lugs that hang down from an upper strut fitting that is fixed i.e. it doesn't caster. That means as the wheel casters to and fro, the pistons of the dampers must plunge in and out of the damper cylinders, providing the damping. But the damper cylinders are firmly attached to the upper torque link strut fitting and are not allowed to wiggle or rotate or whatever you want to call that secondary motion. They just move with the whole lower part of the landing gear. If the piston rods are tight fitting in the cylinders and can only slide in and out, and if the cylinders are firmly attached to the lower strut, and if the piston rod ends each have a simple single pin attachment to the fixed upper strut lug which allows only simple rotational motion about the bolt centerline, then there is a kinematic problem with the device. 

The only thing I can think of is that the forward (piston head) end of the rod, which slides on the inside surface of the cylinder, and which is a larger diameter than the rod itself just like an engine piston, is shaped such that the rod can wiggle side to side as well as in and out, and there is clearance at the aft end of the cylinder for teh rod to do so. But that is a guess on my part. I saw a set of shimmy damper cylinders on E-bay of all places, and the photos seem to show clearance there, whereas on a classic hydarulic linear actuator there would be no clearance at all; just a tight seal against the rod surface. 

By the way, the way this is solved in a reciprocating piston engine is that the piston heads have a rotational joint which attaches them to the connecting links. I don't think the shimmy damper has such a thing inside that little cylinder.


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## FLYBOYJ (Feb 24, 2014)

JayW said:


> FlyboyJ - thanks for the response. I agree with everything you stated. Yes the torque links caster with the wheel freely, and yes the steering is accomplished with differential braking or rudder. The dampers are passive devices that are merely loaded up with pressure; they do not steer. But look at the picture - the damper cylinders move with the castering upper torque link; they have to as they are firmly attached to the same fitting on the strut. But the piston rod parts of the dampers have their rod ends attached to lugs that hang down from an upper strut fitting that is fixed i.e. it doesn't caster. That means as the wheel casters to and fro, the pistons of the dampers must plunge in and out of the damper cylinders, providing the damping. But the damper cylinders are firmly attached to the upper torque link strut fitting and are not allowed to wiggle or rotate or whatever you want to call that secondary motion. They just move with the whole lower part of the landing gear. If the piston rods are tight fitting in the cylinders and can only slide in and out, and if the cylinders are firmly attached to the lower strut, and if the piston rod ends each have a simple single pin attachment to the fixed upper strut lug which allows only simple rotational motion about the bolt centerline, then there is a kinematic problem with the device.
> 
> *The only thing I can think of is that the forward (piston head) end of the rod, which slides on the inside surface of the cylinder, and which is a larger diameter than the rod itself just like an engine piston, is shaped such that the rod can wiggle side to side as well as in and out, and there is clearance at the aft end of the cylinder for teh rod to do so.* But that is a guess on my part. I saw a set of shimmy damper cylinders on E-bay of all places, and the photos seem to show clearance there, whereas on a classic hydarulic linear actuator there would be no clearance at all; just a tight seal against the rod surface.
> 
> By the way, the way this is solved in a reciprocating piston engine is that the piston heads have a rotational joint which attaches them to the connecting links. I don't think the shimmy damper has such a thing inside that little cylinder.



I believe that's the way it operates.


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## Deuelly (Feb 28, 2014)

The piston rods don't have rod ends. They are flat on the end and rest on bearings on each side. I'm doing some work on our P-38 right now and will try to get pictures if I have time.


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## JayW (Mar 8, 2014)

Deuelly - you are absolutely right, as I have found out independently from a shop that was "annualing" a P-38. It seems such a crude setup, for a flat rod end to be contacting a round bearing, like an afterthought. But whatever works - I guess shimmy damping doesn't necessarily involve alot of force or alot of perfect mechanical motion. I also understand that as the lower strut turns, at about 10 or 15 degrees of rotation, one of the shimmy damper rods loses contact with its bearing and begins to hang out in the breeze fully extended, while the other one compresses and makes that unseemly contact with the roller misaligning more and more the more the strut turns. Anyway, it solved my "kinematic" problem which was vexing me. Thanks.

Flyboy - we stand corrected!

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