REFLECTOR: Take Off Shimmy

[Vance Atkinson]

tstockmn at aol.com wrote:
> Velocitites
>
> For those of you flying fixed gear birds, I've got a question.   I 
> recently replaced the rubber shock mount on my front gear strut with 
> the "new" rubber mount due to suffering three failures of the factory 
> mount.     So far no shimmy on landing or on the take-off roll.    
> However . . . . . . I've notice that as a I climb out the first 500 
> feet or so, I experience a "shimmy" that starts about five to ten 
> seconds into flight and then dissipates after 5-10 seconds.  You feel 
> it in the fuselage, but no in the stick or rudder pedals.    It only 
> happens on takeoff and I can't reproduce it in flight while simulating 
> takeoff or anything else.    Nothing else has been done to the plane 
> (600+ hrs TTAF) recently other than the gear mod.  I checked all the 
> flying surfaces and not indication of flutter.   Prop tight, motor 
> mounts good.   I even put the brakes to see if it wa! s the main 
> wheels turning causing the shutter.    My rule of thumb is that the 
> most likely cause of a problem is normally the last thing you 
> "fixed."    I suspect it is either due to the front wheel spinning 
> down and being slightly off balance or some other transitory vibration 
> coupled with the spinning wheel.     My guess is that the previous 
> nose gear installation was loose enough that it didn't transfer any 
> shimmy or vibration once the wheel left the ground.    Now with the 
> new rubber shock mount, the nose gear is solidly pressed against the 
> fuselage (though the bumper and capture plate) and it amplifies any 
> transient vibration.   
>
> Any suggestions or anyone have a similar problem.   

The jets I fly have this problem occasionally and its the nose wheel 
spin down.  You can feel it in the rudder pedals and airframe.  Some 
jets have "snubber" that stops the spin as its going into the wheel 
well.   One of three things cause it.

1.  Tire out of balance.
2. Tire out of round, due to flat spot on tire from landing hard.
3.  Bolt axle torque incorrect.

Vance Atkinson
EAA Tech and Flt Advisor
COZY N43CZ
VEZ N3LV