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Jim,
My shimmy symptoms sound identical to
yours - same onset speed, same results from releasing and smoothly re-applying
brakes. To me, my butt says the shaking is coming from the nose, but I
know from my first experience when I had a witness that the origin may be from
one or both of the mains, which seems to translate forward.
Like Mike Easley, I'll hold aft
stick as I roll out to keep as much weight as possible off the mains during
rollout. I also retract the flaps as soon as I'm on the ground to put more
weight on the mains. I think that also takes a bit of weight off the
nose. I haven't really taken notice of the effect of a more aft CG on the
shaking, but I'll pay more attention to that in the future.
If I have a short runway (like the
2000' one we use for our Young Eagles events), I land on-speed and do heavier
braking as soon as I'm on the ground, which gives me a lot of deceleration
before the shimmy onset. That results in a much shorter landing
rollout.
I don't have a taxi speed that I
avoid, though I can get a shaking if I try to brake lightly at low taxi
speeds. I don't get any shimmy at all when taxiing with the brakes
off.
The surprising thing to me is what a
well known issue this is and the lack of a solution for it. You'd think
that between some of the engineering types who are flying ES's and the factory
guys, someone would have put their finger on exactly what causes this and
come up with a fix. I think Lancair has chosen to bury their heads in the
sand on this other than to equip the new kits with a beefier nose strut.
But there are a lot of old struts flying out there and at least three of them
that I know of have had catastrophic failure, two resulting in collapsed
nose gears, broken mounts and $50K+ in damage. In the other case, it was a
miracle that the strut didn't fold, as the post flight tear down of the strut
revealed a total failure of one of the damping O rings.
Maybe your video tests will point us
in the right direction. I sure hope so!
Skip
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