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Proper Tailwheel Geometry - Castor Angle and Shimmy

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Posted

This topic comes up every so often in the General Forums so I thought it was worthy of it's own post in the Technical Forum.

 

Read the EAA Counselor's comments and compare the attached pic to your own tailwheel. It may provide the solution to your problem.

 

Tail Wheel Shimmy.pdf

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Posted

I have spent more time than anyone should have researching tailwheel shimmy. I have actually had springs made for both positive and negative castor angle. That "report" is very interesting and sidetracked me from the reality of what was going on for a long time.

 

All you can really do by changing castor angle is change the characteristics of shimmy and when it happens. The bottom line is that the mass of the tailwheel, trail distance, "spring" in the tire, TW spring and fuselage all act together to create a tuned circuit that WILL resonate if excited. The ONLY way in my experience to stop shimmy is resistive damping (friction) to critically damp the resonance. If you don't have enough resistance to back and forth motion in the tailwheel it WILL shimmy sooner or later.

 

Not trying to sound like a know it all, but if you have a shimmy problem and it is not from worn parts like bushings or bearings or loose spring bolts, it is because there is not enough damping. Castor angle changes might seem to get rid of a shimmy problem, but it is not the real solution.

 

Just my 2 cents.

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Posted

I completely agree .I battled a tail wheel shimmy for 180 hrs on my plane .I wheel landed everywhere to avoid the shimmy .one quarter turn tighter the shimmy disapperaed

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Posted

Yeah. Tailwheel shimmy is like a balloon full of shit. Changing castor angle or tire pressure or bla or bla or bla, is like squeezing the balloon. It will go away where you squeeze it, but pop up somewhere else. Resistive damping is the ONLY way to get rid of it once and for all.

 

It was interesting to me that while I was struggling to figure this out on my own, that others were also having the problem. An interesting approach I have seen recently with really big tailwheels is the total elimination of steering cables and springs. The huge tailwheels require so much resistive damping that they won't easily steer, so the just eliminate the steering altogether. The tailwheel is steered by differential braking action exactly the same way the RV nosewheel is steered with differential brakes.

 

The RV nose wheel setup has large belleville washers that are compressed and tightened to produce the resistive damping to kill shimmy. Some of the big tailwheels are doing the same thing now. Very interesting. I never got a chance to experiment with this approach though.

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