REFLECTOR: Runway Rash

[Laurence Coen]

Chuck,

First, I don't advocate reducing the breakout force below the 12 to 15 lb 
speck.  At 14 lbs my aircraft is very easy to steer.  Secondly, yes a well 
lubricated Belleville washer set up should be real close.  Keeping it well 
lubed is a maintenance problem where as the nylon washer is the lubricant.

Larry Coen
  ----- Original Message ----- 
  From: Chuck Jensen
  To: Velocity Aircraft Owners and Builders list
  Sent: Wednesday, May 17, 2006 3:26 PM
  Subject: RE: REFLECTOR: Runway Rash


  Larry,



  Very good explanation of how static v. sliding friction affects our nose 
wheel steering, but I'm still having difficulty seeing how the plastic 
washer sharply changes the equation.  Are you saying that the plastic washer 
reduces the differential between static and sliding, hence the static (aka 
breakout force) can be set lower without compromising the necessary sliding 
friction which dampens the shimmy?



  If that's the theory, its comprehensible why it would reduce the breakout 
force necessary to still damp the shimmy, but is the change/improvement 
actually noticeable from a well lubbed Bellevile washer setup?



  Chuck Jensen

  Diversified Technologies

  2680 Westcott Blvd

  Knoxville, TN  37931

  Phn:     865-539-9000 x25

  Cell:     865-406-9001

  Fax:     865-539-9001

  cjensen at dts9000.com



  -----Original Message-----
  From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On 
Behalf Of Laurence Coen
  Sent: Wednesday, May 17, 2006 10:07 AM
  To: Velocity Aircraft Owners and Builders list
  Subject: Re: REFLECTOR: Runway Rash



  Al,



  The manual tells us to set the force to move the nose wheel at 12 to 15 
lbs. Mine is set to 14 lbs.  The basic problem comes from the fact that 
there are two types of friction involved.  The first is static friction 
which is what we measure during adjustment.  When the fork moves we have 
sliding friction which is less than the static friction.  With lack of 
lubrication, rust and/or dirt the difference between the two becomes much 
greater.  As a result the static or break out force is excessively high when 
you have enough sliding friction to prevent shimmy.  This gives a plane that 
is hard to steer with marginal shimmy control.  The idea of the nylon washer 
is to keep the value of the static and sliding friction as close together as 
possible.  Think of a rusty hinge that creaks.  What makes the noise is the 
static friction is so high the hinge starts to flex before it moves.  When 
it does move the sliding friction is low enough to allow it to spring back 
to its original shape. the rapid repetition of this process makes the 
creaking sound.  Lubrication is curative.



  My previous description was inaccurate.  When I replaced my original fork 
with the new thicker one I rearranged the washers.  The current set up is 
two Belleville washers with the cupped surfaces together above the fork and 
the nylon washer, flat steel washer and nut below. A word about materials. 
Phenolic is not self lubricating and absorbs water.  Teflon is self 
lubricating but suffers from cold flow which would require constant 
adjustment.



  Larry Coen



  ----- Original Message ----- 

    From: Al Gietzen

    To: 'Velocity Aircraft Owners and Builders list'

    Sent: Tuesday, May 16, 2006 11:13 PM

    Subject: RE: REFLECTOR: Runway Rash



    Let me describe my "Oreo" friction damper.  I have two bevel washers 
(the

    cracker part)  with a nylon washer making up the creamy filling.  I made 
the

    nylon washer from 1/8" flat stock using a couple of hole saws.   The 
science

    behind the idea is that nylon is self lubricating so there is no grease 
to

    squeeze out or attract dirt.  My experience to date with ground handling 
has

    gone slicker than snot on a brass doorknob.



    Larry Coen

    N136LC





    I think there is something missing here.  The 'drag' that makes it hard 
to steer IS the damping force that keeps it from shimmying.  If the wheel 
pivots easily, where does the damping force come from?



    Al


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