REFLECTOR: Phenolic Pad Options

Sat Nov 4 06:54:54 CST 2006

Kent,

Looks like a nice fix. The most important part of the fix is...you're
back in the air!!!  Good job and good flying.

Chuck Jensen 
Do Not Archive

-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Kent Murley
Sent: Saturday, November 04, 2006 2:17 AM
To: Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: Phenolic Pad Options

Thank you Chuck Harbert, Jim Agnew, and Chuck Jensen for your responses
to my phenolic board replacement questions. I'm finally flying again and
using a thin stainless steel plate to protect my new left strut from
brake disc heat (picture attached). I'm still using a phenolic pad on my
right strut. Both are working well.

Not too evident in this picture: the leading edge of the SS plate is
curved inboard around a 6mm radius to provide some stability in the
relative wind but I doubt it is necessary. The plate still only contacts
the strut down by the axle. The orange RTV sealant may be providing some
rigidity but I would bet those two little dobs will dry out over the
years and fall out. I wonder if I can paint them.

To summarize: I have an XLRG with Cleveland brakes and toe pedal
actuators, a carbon fiber wrapped left strut (replaced this month after
some surface cracks revealed the original strut was damaged in an
off-airport landing 5 years ago by the previous owner) with stainless
steel heat barrier, and a standard wrapped right strut with a phenolic
board heat barrier.

Email me directly for more pictures of either strut, if you like. 

Kent Murley
Tacoma Narrows WA

Chuck Jensen wrote: 

	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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