REFLECTOR: Nose gear damper failure update.

[Laurence Coen via Reflector]

Bob,

This unit has the standard shear pin.  The purpose of the shear pin is to prevent internal damage to the damper.  If you make the external pin stout enough, you will simply move the failure from outside to inside.  The true problem is excessive damping forces when it gets cold.  Adjusting the base valve would be prudent no matter which shear pin is installed.  As a clarification to the settings I suggested in my original post, I backed off from the first setting, not from max damping, (warm=1/2 turn, cool=1 1/2 turns, cold 2 1/2 turns).  These are suggested starting points only and are not carved in stone.

Larry Coen
N136LC (builder)


From: Bob Huntingford via Reflector 
Sent: Tuesday, October 27, 2015 6:41 PM
To: Laurence Coen ; Velocity Aircraft Owners and Builders list 
Subject: Re: REFLECTOR: Nose gear damper failure update.


Larry
Was this the latest version of damper with the 1" shear pin that Andy Millin had machined?

Bob Huntingford
C-GAIT (builder)


On 10/27/2015 11:53 AM, 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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