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<< Lancair Builders' Mail List >>
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Bob Young wrote:
<<..the nyaflow lines worked fine for me. <snip> Incidentally,
the Matco parking brake valve works well mounted on the fire wall just up
and
slightly to the right of the copilot right rudder pedal.>>
Brake fluid ran cold in my lines when I read this.
Using a Matco parking brake valve in combination with Nylaflow tubing is a
REALLY bad idea. Here is why. Nylon will cold flow, that is it will slowly
yield at room temperature when a strain is applied. The rate of strain is a
function of time and temperature. Matco parking brake valves work by
trapping pressure between the Matco valve and the brake caliper. In
operation, you land the airplane, set the parking brake and go home. The
brake rotors, still hot from the landing are being squeezed by the calipers.
Heat from the rotors is migrating through the pads and pistons into the
brake fluid (actually hydraulic fluid). As the fluid heats, it expands at a
rate greater than the increased volume expansion of the caliper. The
resulting pressure spike, on top of the existing pressure applied for days
or weeks or more, cold flows the Nylaflow in the system. The Nylaflow gets
slightly larger in diameter and it's walls get slightly thinner. This will
happen every time the parking brake is set so it is just a matter of time
before the Nylaflow ruptures. When, not if, this happens, it will likely
happen near the end of a hot landing into a short field. The resulting
ground loop will be unpleasant and expensive.
IMHO Matco valves are a bad idea on aircraft. The reasons include the heat
soak problem, logistic problems at FBOs, the propensity for metalized pads
to stick to rotors and the natural tendency to not tie down the aircraft if
the "brake" is set. If you still want one, it is imperative that you use
hose with a strength member to prevent failure.
Regards
Brent Regan
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LML website: http://www.olsusa.com/Users/Mkaye/maillist.html
LML Builders' Bookstore: http://www.buildersbooks.com/lancair
Please send your photos and drawings to marvkaye@olsusa.com.
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