REFLECTOR: Phenolic Pad Options

Chuck Jensen cjensen at dts9000.com
Fri Oct 6 08:14:14 CDT 2006


Kent,
 
I was ready to reply to your email about a SS divider instead of
phenolic board but thought I would read the other posts first.  Mmmmm,
nobody seems to be on the same path that I was ready to stumble down.
The reason for the phenolic board is not to protect the leg from radiant
heat, only conductive.  Even though the air gap between disc and leg is
small, one would have to have a very hot heat source at that distance
for an extended period of time to start affecting the gear leg.  Before
that radiant heat became a problem, there would have been so much heat
conducted through the bolts and surface attachment points that a little
radiant heat would be the least of our problems.
 
Looking at the overal design, it seems quite obvious that the phenolic
board is intended to be a thermal break to prevent conductive transfer
of heat.  Yes, some will still make it through the bolts, but the
surface area of the bolts for heat transfer is small compared to the
area of the mating surfaces.  Regarding the use of SS, yes, it is a poor
conductor compared to copper, but still quite efficient compared to
phenolic board.  So the SS substituted is still a thermal break, but one
of modest value.
 
So, why do the SS plates work?  Probably because there's not much of a
problem to begin with.  With the older style, small Matco brakes in the
big XLRG, and the original brake-rudder tie-in, brakes were worked a lot
harder and got a lot hotter....a LOT hotter.  With the bigger Clevelands
and their mass and ability to absorb energy, the whole brake assembly
rarely gets hotter than 'warm to the touch', so the thermal break, be it
phenolic or SS, is a solution to a problem that barely exists anymore.
 
That's my theory and I'm stickin' to it.
 
Chuck Jensen 
 
 -----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Kent Murley
Sent: Thursday, October 05, 2006 11:05 PM
To: Velocity Aircraft Owners and Builders list
Subject: REFLECTOR: Phenolic Pad Options



	Anybody use something other than phenolic pads to protect their
main struts from brake disc heat?
	
	My situation:
	
	XLRG
	
	I changed to Cleveland brakes a dozen hours ago (and toe pedals
- love 'em) and installed new phenolic pads per the usual method: the
top of each phenolic pad runs vertically up each strut to a height
nearly a centimeter above each brake disc. The only attach points
between the phenolic pads and struts are down by the axle to minimize
transfer of heat.
	
	The shape of my well used struts made the tops of the phenolic
pads warp away from the struts a couple centimeters and nearly touch the
brake disc. After the first flight one pad actually did touch its disc -
wearing an arc into the phenolic pad less than a millimeter deep. I
jambed a one centimeter spacer between the pads and the discs to sit
overnight. Neither pad has touched its disc since but they are close -
within 5 millimeters - and it varies after each flight. It may be the
strut or the phenolic pad or both that is changing shape with each set
of heat cycles.
	
	For other reasons I'm now replacing the left main strut and,
after taking the wheel off, the phenolic pad has cracked and is
unusable, perhaps from my manipulation. Should I use a new phenolic pad
or can I fashion an aluminum (or other material) plate that would not be
subject to such large (3mm) flexure in heat cycles but still protect my
carbon fiber wrapped new strut from the hot disc?
	
	Kent
	
	 
	

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