Fred,

 

Right on!  On a flying 360/320 with a pretty good seal betwixt the engine compartment and the nose wheel well, there is still an upper section of the strut being bathed in the hot air from engine cooling.  If the gear is dropped at 45 degrees from the numbers on the downwind, there is not enough time to cool the strut down.  Think about most instrument approaches where the gear comes down at the FAF, yielding 5 or 6 miles of flight before touchdown.  I use the gear to help slow down upon entering the airport area (about 4 miles out) and still the strut has been heated enough to expand the nitrogen enough to have added an effective 3/4" to the strut length.

 

I once was advised to use heavier motorcycle fork oil and found I could barely get the nose wheel to turn when taxiing on a cold winter's day.

 

Ah, the yin and yang of it...............

 

Grayhawk

 

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