REFLECTOR: Runway Rash

Alex Balic alex157 at pwhome.com
Wed May 17 18:11:25 CDT 2006


That is exactly correct- the numbers for static vs sliding are closer with
the plastic -  a well lubed washer assy is going in the right direction, but
it is difficult to maintain the correct pressure relationship- dirt, grease
hardens, slings out ect.

 

  _____  

From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Chuck Jensen
Sent: Wednesday, May 17, 2006 3:27 PM
To: Velocity Aircraft Owners and Builders list
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 <mailto:ALVentures at cox.net>  

To: 'Velocity Aircraft Owners <mailto:reflector at tvbf.org>  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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