REFLECTOR: Nose gear damper failure update.

Tue Oct 27 14:34:12 CDT 2015

Great info.... Thanks for letting us all know. I got away from the 
dampener after mine failed. But this is really good info to
get out there.

Ron
On 10/27/2015 12:53 PM, Laurence Coen via Reflector wrote:
> What failed on N136LC was the shear pin in the Scotts damper.  This in 
> turn allowed the nose wheel to shimmy totally un-damped which led to 
> major damage to the nose gear.  The difference between this landing 
> and all previous successful landings was the temperature.  It was 50 
> degrees F when the shear pin let go.  All previous flights had been at 
> 70F or above.
> The damper is filled with oil and like all oil, it gets thicker when 
> the temperature drops.  I set up an experiment to see what the 
> difference would be in damping between a 90F summer day and a cool day 
> of 50F.  The damper was mounted on its side with a 12lb weight 
> suspended from it's damper arm.  The adjustment valve was set to one 
> half turn backed off from full damping.  This was where it was set 
> during the incident.  I hot soaked the damper in a 90F oven for an 
> hour and measured the time it took for the arm to travel through it's 
> arc being pulled by the 12lb weight.  I repeated this test after cold 
> soaking the unit at 50F.  At 90F it took only 4 seconds. At 50F it 
> took 18.3 seconds.  That's 4.6 times the damping with a 40F drop in 
> temperature.  Backing off the adjustment one full turn set the damping 
> back to 4 seconds.  The change in viscosity due to temperature is not 
> linear and becomes exponential with falling temperature.  I would 
> recommend backing off a full turn for Spring and Fall and two for 
> places that experience Winter.  Your mileage may vary but it is 
> obvious that changes in temperature have a large effect on how much 
> damping you get.
> Larry Coen
> N136LC (builder)
>
>
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