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Here are my experiences on my IV-P with 560TT and about 1000 landings and some
thoughts.
Shimmy has been noted when one or more of the following are present:
High speeds (>90kts)
Goodyear FCII (Flight Custom Two) nose gear tire
Low tire pressure (<40 PSI)
Low strut pressure
Shimmy has not been observed when the following are present:
McCreary nose tire
>50 PSI tire pressure
Proper strut pressure (85% strut extension)
Hold the nose off on landings until <70kts
I believe that the rounder profile of the McCreary tires is less prone to shimmy.
I also believe that shimmy is due to the excitation of the nose gear assembly at
it's resonate natural frequency. Since the natural frequency of a system is a
function of it's mass and spring rate, changing either will effect resonance. Mass
is hard to change without effecting structural integrity, particularly in the hub
assemble. I would not attempt a hub modification without careful analysis first.
The two big springs in the system are the strut and the tire. The strut is also a
rotational damper (in the yaw axis).
Since the strut pressure must be maintained within a narrow range to preserve
stroke, the tire is left as the most obvious candidate for experimentation with
system spring rate. Changing tire pressure up or down may effect the propensity
for shimmy, explaining the experiences already posted. Since the nose tire has the
vertical strut to cushion the ride, my preference is to increase tire pressure.
I also believe that tire pressure will have a much greater effect on shimmy than
bearing play or wheel balance. However, a nose wheel tire with an asymmetrical
defect (perhaps to a hard, crosswind landing) would likely be more prone to
shimmy. One thing that may be tried is to check the tire run out by suspending the
nose wheel and rotating it and observing the profile, perhaps with a dial
indicator.
Just trying to do my part in man's quest to defeat entropy and chaos.
Brent Regan
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LML homepage: http://www.olsusa.com/Users/Mkaye/maillist.html
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