Return-Path: <marv@lancaironline.net>
Received: from [65.33.85.255] (account <marv@lancaironline.net>)
  by logan.com (CommuniGate Pro WebUser 4.0.6)
  with HTTP id 2052257 for <lml@lancaironline.net>; Thu, 13 Mar 2003 23:50:31 -0500
From: "Marvin Kaye" <marv@lancaironline.net>
Subject: Re: Brake Lines
To: lml
X-Mailer: CommuniGate Pro Web Mailer v.4.0.6
Date: Thu, 13 Mar 2003 23:50:31 -0500
Message-ID: <web-2052257@logan.com>
In-Reply-To: <3E714A1E.3040907@regandesigns.com>
MIME-Version: 1.0
Content-Type: text/plain; charset="ISO-8859-1"; format="flowed"
Content-Transfer-Encoding: 8bit



Posted for Brent Regan <Brent@regandesigns.com>:

Gary asks:

<<

I've been told to reverse the
"orientation" of the plumbing in that it is better to run the reservoir to
the right side master cylinders, making the left side cylinders the ones
closer to the brakes.  This presumably reduces the sponginess of the left
(pilot) side pedals, which are the ones used 90% of the time.  Makes sense
to me, but there is another routing that might work.  If reservoir were
connected to the left pedal on the pilot's side and the right pedal on the
co-pilot's side the total run of high pressure tubing could be reduced.  The
plan would be to run the line for the left brake down the left side of the
fuselage and the right one down the right side.  Would this create any pedal
feel issues?  The pilot would apply the left brake directly, but he would be
pumping fluid through the co-pilot's master cylinder for the right brake.

>>

The "sponginess" or "springiness" of the brake pedal is due to the collective 
flexing and compressing of all the brake system components in accordance with 
Hooke's law. A little is due to the radial expansion of the aluminum tubing 
and brake cylinders. A little is due to the flexing of the pedal and linkage. 
A little is due to the flexing of the caliper and the compression of the brake 
pads. A lot is due to the radial expansion of the flexible brake line BUT MOST 
is due to the compression of the hydraulic fluid itself. Liquids are indeed 
compressible at brake system pressures. The compressibility is called the Bulk 
Modulus of  the fluid and can be one to two orders of magnitude greater effect 
than that due to the Young's Modulus of the other metallic system components. 
This effect becomes particularly egregious when there is gas dissolved in the 
fluid.

Nothing is rigid, everything flexes. The screen your are looking at right now 
changes shape with changes in barometric pressure.

Most brake fluids are designed to resist gas absorption and have high bulk 
modulus BUT we do not use brake fluid, we use hydraulic fluid. Brake fluid, in 
particular aviation brake fluid (aka Skydrol) would melt the seals and 
eventually many other parts in our aircraft and should NOT be used in Lancair 
brake systems.

In order to have a symmetrical "feeling" brake system where the left and right 
pedals deflect the same amount it is therefore important that both systems 
contain the same fluid volume. Therefore, the short answer to Gary's question 
is that it would not be a good idea to plumb the system as he described as one 
side would have an additional brake cylinder volume of fluid.

In order to maintain a symmetrical system, what I did was to run the brake 
lines down the center of the plane and then into the gearbox through  -4 
bulkheads where they split to the left and right side. This has proved 
reliable in over 1200 landings.

One other tip is to degas your brake fluid (correction, hydraulic fluid) 
before using. I use an old 10 pound freon can and a venturi vacuum pump. Once 
the fluid is degassed I put a couple of PSI in the tank, flip it over and 
VIOLA (pardon my using French anything) instant pressure brake bleeder.

Regards
Brent Regan


["VIOLA" isn't French... that's either the name of the little old Polish lady 
that owned the mom & pop grocery store next door to where I grew up, or the 
slightly bigger brother to the violin.  Now, VOILA... that's French <g>.   <M> 
    (sorry... I couldn't resist...)  ]