REFLECTOR:Nose Gear Shimmy

[John Dibble]

I also doubt that wheel balance is a shimmy issue even though when I spin the nose
wheel, it does seem to be out of balance.  But I never get shimmy on the take off
roll, only when I brake during landing.  I'm comfortable with the nose wheel at 16
psig, in fact I think that it might squeek by at 0 psig, but I don't plan to try that.

John

Chuck Jensen wrote:

> At this point, about 12 hours and as many landings with nary a hint of
> shimmy.  Lightly loaded and gross.  No difference.
>
> And, the times when the nose drops on a little harder (rare, I assure you),
> I'm nearly positive that the "thump" is muted by the softer tire, yet I
> don't really see any "squishiness" in the front tire when loaded....and this
> at only 16 psi.  And, as John observed, the nose is lightly loaded compared
> to the mains, so the low pressure may not result in much tire deformation.
>
> Its fair to say there is some interesting "stuff" associated with this nose
> wheel thing.  I grew up on a farm (which probably is a surprise to no one)
> and the hay rake had a castering wheel.  Sometimes, for reasons unknown,
> it'd shimmy like it had gotten into some bad home brew.  Then other times,
> it'd track like a dog.  Who knows.
>
> The wheel balancing is certainly a good idea as that little wheel spins like
> the dickens at 80 mph, but I have some doubts that the out of balance issue
> is a causative factor in the shimmy problem.  I think the shimmy is likely
> initiated when we land and, with a little cross wind, the nose wheel touches
> slightly out of alignment with the runway, though in perfect alignment with
> the plane.  Upon contacting the pavement, the wheel is kicked in the
> opposite direction, which then kicks it back and so on and so forth til it
> self destructs and is sliced-an-diced in the prop.
>
> Clearly, the stiff resistance of the tightened washers on the forks kills
> this oscillation.  Perhaps, the additional friction of the soft(er) tire
> also helps reduce the oscillation vector.  To date the stiff washers
> definitely work (even though its a little bit of a cuss to steer) and the
> jury is out on the reduced air pressure in the front tire.
>
> I'm just ignorant enough to try the continued loosening of the washers,
> and/or, increasing the air pressure to see if I can induce the shimmy, yet
> get to a grass field to land and rectify.  Wouldn't it be a kick if all we
> had to do was reduce the front tire pressure to 16 psi, then we could loosen
> the front forks so a light tap on the brakes would turn the nose ever so
> subtly?  There, but for the grace of god, goes a stearable nose wheel.
>
> And, besides, if the test is a failure, MT can always use the extra money!
>
> Chuck