Sorry to hear of another nose
wheel shimmy problem. We have been over this ground many times. I
have a further thought to add to the comments already contributed.
The nose wheel shimmy is
controlled by internal damping using the oleo hydraulic fluid bleeding through
an orifice as the strut rotates from side to side. Too little fluid or
too much clearance, too little damping, and destructive oscillations set in.
Consider: The viscosity of the
strut oil varies dramatically with temperature. When retracted, the nose
gear gets heat soaked in the hot air under the engine which is roughly 150F
above ambient in normal cruise conditions. The strut and oil get hot,
and the oil viscosity drops – a LOT.
Then you drop the gear and get a
nice (comparatively) cold blast across the strut that cools it and the oil
inside. The oil is in the annular space between piston and cylinder, and
probably cools fairly rapidly as the external surface is exposed to the air
blast. Without doing the heat transfer calculations for flow
around the strut, my guess is that the time to cool the oil in a 100 knot air
blast is a few minutes.
So here is the thought: if the
nose strut is truly heat soaked, and the gear are extended only 1-2 minutes
prior to touchdown, the oil may still be warm to hot, and the ability to damp
shimmy is therefore substantially reduced compared to a cold damping test in
the hangar.
So here is the proposition: shimmy
may well correlate with time between gear extension and touchdown. If in
doubt, lower the gear early, and extend downwind.
This is pure supposition.
Other thoughts?
Fred
Moreno