REFLECTOR: Runway Rash

Wed May 17 15:26:30 CDT 2006

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 and Builders list'
<mailto:reflector at tvbf.org>  

	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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