REFLECTOR: Nose Gear Shimmy

[Donald Bates]

Velocity Nose Wheel Shimmy
Explained
 
Recent comments have focused
on wheel balance. A well balanced wheel can prevent varying side loads while
rolling and turning, which may initiate or contribute to shimmy. Here is what
else I learned while fixing my shimmy problem.
 
Poor shimmy axis alignment can
also initiate or contribute to shimmy. The shimmy axis must be truly vertical
in the sidewise plane. With sidewise lean the weight of the airplane will force
the wheel to turn out of the vertical fore-and-aft longitudinal plane, while
the caster arm will tend to center the wheel thus initiating a shimmy.
 
If the shimmy axis is raked
forward at the bottom in the fore-and-aft vertical plane, the tire-ground
contact point will be aft and under the shimmy axis extension. Again the weight
of the airplane will tend to force the wheel to turn outward while caster tends
to center the wheel which can initiate a shimmy. 
 
If the shimmy axis is raked
aft, the tire contact point will be aft but above the axis extension and the
weight will now tend to force the wheel to center giving us two centering
forces. This is a stable or convergent geometry rather than divergent. How much
aft rake is needed? On my SERG the shock in the retractable nose gear actuator linkage
allows about a 4-5 degree forward rake when compressed. The Diamond trainer
aircraft has a similar nose gear setup and I measured about 8 degrees on their
shimmy axis so anything in the 5-8 degree range should work well. 
 
Getting the nose gear leg
adjusted for enough aft-rake may require more room at the top of the gear leg.
I had to cut a two inch diameter hole in my gear-canard bulkhead and pivot
channel to allow space for the clevis at the top of the gear leg. However, even
with a balanced wheel and good shimmy axis alignment, a shimmy damper is still
needed in case the wheel isn’t centered at touch-down.
 
The viscous shimmy damper is
preferable. The friction dampers have a problem generally called sticktion or
static coefficient of friction. It is quite a bit higher than the sliding
coefficient of friction. If the wheel is turned at touch-down the initial resistance
to centering is high. As soon as it begins to slide to the center the friction
drops suddenly and there is then an excess over-force that gives the wheel a
kick and accelerates it across to the other side, and this kick repeats every
time the shimmy reverses direction. This can result in a very hard shimmy especially
in a touch-down hard enough to force the wheel to turn outward, if there is
forward rake, before it starts to center. It can be very destructive. Don’t ask
how I know.
 
The viscous damper eliminates
the sticktion factor of a friction type damper and can be designed with a
higher shimmy resistance to help stop the shimmy quicker. I designed and built
my own viscous damper before I knew of the new Velocity viscous damper which is
a bargain compared to my machine shop costs. It may be all you need and you
will find it takes less braking to steer the airplane. However, if your damper
leaks, having a balanced wheel and good shimmy axis alignment provides an extra
margin of safety. 
 
Don Bates
N14B
SERG
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