REFLECTOR: Runway Rash
Laurence Coen
lwcoen at hotmail.com
Wed May 17 09:06:58 CDT 2006
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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