REFLECTOR: Nose Gear Shimmy

[Mel Bina]

Hi Don,
I'm really sorry to hear about your incident.  I hope that you will get the
airplane repaired start to enjoy the flying.
Mel

On Sat, Mar 24, 2012 at 12:23 PM, Donald Bates <aeroopt at yahoo.com> wrote:

> Hi Mel,
>
> Yes, I joined the Reflector last year. I guess my article was a little too
> technical for most as no one has commented on it.
>
> We are in Texas now and the airplane is in Placerville. Won't be back
> until June. Still need to repair the main gear doors and fix the radio
> problem with the headsets and mic.
>
> Last year February on my way to Texas I landed at Casa Grande to refuel.
> and got yanked off the runway by bad brakes and hit a ditch braking the
> nose gear and mangling the mains. The local mechanic pulled me out and
> rented me hangar space for four months. I spent a  couple weeks there
> ordering parts and getting started on the repairs, then went to Texas for a
> month and came back to finish the job. Took two and a half months to get
> the plane back to basic flying condition to get to Placerville. Steve M.
> had just signed it off before the trip but was too lazy to pull the wheels
> and told me to do it and signed it off and left. I thought the brake pads
> looked thick enough and so went to fly to TX. The mechanic pulled the
> wheels and found the left one with grooves well into the disk and the right
> rivet heads riding on the disks (the left had locked and the right did
> nothing so I couldn't bring it back on the runway).
>
> Was a big job. A friend from L.A. came there for a couple of days to help
> get the mains remounted and aligned. The mechanic there didn't know
> anything about composites and didn't have much time to help anyway. He was
> busy keeping other planes flying.
>
> So, I still have only about 15 or so flying hours and maybe 30 landings. I
> was just getting comfortable with it now that the shimmy problem is gone.
> Hope to be getting some good flying time this summer.
>
> Don & Dianne
>
>
>   ------------------------------
> *From:* Mel Bina <pusherpilot1 at gmail.com>
> *To:* Donald Bates <aeroopt at yahoo.com>; Velocity Aircraft Owners and
> Builders list <reflector at tvbf.org>
> *Sent:* Friday, March 23, 2012 4:48 PM
> *Subject:* Re: REFLECTOR: Nose Gear Shimmy
>
> 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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