REFLECTOR: Nose gear damper failure update.

Thu Oct 29 17:17:45 CDT 2015

I though I had made it VERY clear that it is not my damper and not my airplane.  When it was my airplane I maintained the friction at 14-16 lbs with no viscous damper and never had a shimmy.  This means that the viscous damper is redundant with friction set at 15#.  With a retract the gear comes into a guide so  precession displacement would be corrected during retract.

Larry

From: John Tvedte via Reflector 
Sent: Thursday, October 29, 2015 2:08 PM
To: reflector at tvbf.org 
Subject: Re: REFLECTOR: Nose gear damper failure update.

I am posting this discussion from Andy, as he is not on the Reflector.

The damper shear pin IS NOT NEEDED in our application.  PERIOD.  It is an unnecessary and creates a point of failure for us.  This was determined by working directly with Steve Zollinger at Scotts Performance.  This is why we have the square post damper that does not have a shear pin.  If Larry doesn’t want to have his damper upgraded to the square post, then I highly recommend he get the hardened shear pin.

FWIW, Steve is very confident in the strength of the damper.  He does not believe it can be damaged by our loads.  

The washers are not just belt and suspenders.  They are required.  They need to be set at 15 lbs of breakout force.  This provides two necessary functions.  It helps the aircraft maintain a straight line without dragging a brake.  It will also stop precession after takeoff.  Gotta have them and they gotta be tightened up.

The reaction of the damper does change according to temperature.  It gets stiffer as it gets cold.

The damper’s function is to catch a shimmy initiation event and stop it.  Steve had surmised it should be able to do this on the lowest reaction settings for both High and Low speed.

However, here was his last recommendation to me.

·         The low speed valve has the pointer on it and is not covered.  Turn it clockwise until it stops then back off one revolution (8 clicks).

·         The high speed is under the black cap.  There is no “click” on adjustment.  Turn it clockwise until it stops then back off 180 degrees.

These settings should create a very soft response to a jarring event.  Plenty to stop the shimmy.

Andy

From: Laurence Coen [mailto:lwcoen at hotmail.com] 
Sent: Wednesday, October 28, 2015 2:38 PM
To: John Tvedte; Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: Nose gear damper failure update.

John,

I am the builder but not the owner.  The new owner installed the damper and to the best of my knowledge there was no breakout set on the friction washers.

Larry Coen

N136LC (BUILDER)

From: John Tvedte via Reflector 

Sent: Wednesday, October 28, 2015 1:37 PM

To: Laurence Coen ; Velocity Aircraft Owners and Builders list 

Subject: Re: REFLECTOR: Nose gear damper failure update.

Hi Larry,

Did you also maintain the belleville friction washers – and how tight were they?

John Tvedte

From: Reflector [mailto:reflector-bounces at tvbf.org] On Behalf Of Laurence Coen via Reflector
Sent: Wednesday, October 28, 2015 11:23 AM
To: reflector
Subject: Re: REFLECTOR: Nose gear damper failure update.

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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