REFLECTOR: Nose Gear Shimmy

Mel Bina pusherpilot1 at gmail.com
Fri Mar 23 18:48:55 CDT 2012


Hi Don,
Are you on the Reflector now?  And, how are things in general, that is, are
you flying?

Mel

On Mon, Mar 19, 2012 at 4:36 PM, Donald Bates <aeroopt at yahoo.com> wrote:

> 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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