Brake system design


Marc J. Zeitlin
 

Folks:

I have to believe that we've discussed this in the past, but it bears repeating. A customer's plane has been beating the crap out of me for the past 5 hours of paid work time (and I HATE charging customers for time spent due to design or implementation issues), due to a brake system that is extremely difficult to bleed. Although this is an OSH award winning COZY MIV, which in almost all other ways is the epitome of what to do on COZY MKIVs, the brakes have always been marginal from a torque application (meaning pressure developed) standpoint.

This is not the plans system, or even a laydown MC system, but a "Velocity style" hanging pedal system, with the MC's up high, and the reservoir only a bit higher (the lines from the reservoir to the MC's are mostly BELOW the MC's). On a Velocity, due to the much larger depth of the fuselage, it's easily possible to have everything arranged well as described below, but on a COZY, it's a lot more difficult due to the small depth of the fuselage, particularly in the nose area. 

In this case, the feed lines from the MC's to the LG bulkhead route up and over the pedal torque tube and are pretty much guaranteed to be air traps at the high point. Given the lack of access in the nose due to the brake mounting system, bleeding the lines (pressure bleeding from the caliper to the reservoir, to try to push all the air out) has been completely unsuccessful on one side, even after manipulating the MC's to try to position them so that the lines are running uphill all the way to the reservoir. We may resort to replacing the tubing inside the fuselage; rerouting it and using smaller tubing.

At any rate, when designing your brake system, ensure the following:
  1. From the bleeder valve on the caliper to the top of the reservoir, ensure that EVERYWHERE upstream (closer to the reservoir) is higher than any place downstream. IOW, for those of you with math backgrounds, the uphill slope from the caliper to the reservoir MUST BE MONOTONIC - anything else will be an air trap at a high point.

  2. Sometimes it's not possible to perfectly do #1 - in these cases, use the smallest possible tubing, which would be AN-2 or even 1/8" SS with a 0.020" wall. If you can't use that, use nothing larger than 3/16" tubing.

  3. Unless you can GUARANTEE #1, do NOT use Matco 1/4" tubing, or 1/4" Matco fittings, as shown here:

    https://www.aircraftspruce.com/catalog/lgpages/matcotubefitting.php

    These are air traps at the point where the tubing goes into the fitting. The 1/8" tubing and fittings is probably acceptable, but why not just use metal hard lines where nothing moves, and SS/Teflon braided line where things move.

  4. Position the calipers so that there will be a minimum air trap in any attached fittings (and the smaller the brake line and fittings, the smaller the air trap).

  5. Position the calipers so that the fluid flow is uphill from the outlet to the inlet from the reservoir, and the moreso, the better.
You will be far happier with the performance of your brakes, and make things a lot easier when you need to bleed them (and you will), if you stick to these rules of thumb. The closer you can be to having all 5 things be the case, the lower the chance of issues. Sometimes you can get things to work even if you violate all 5, but your margin for success is a lot slimmer.

--
Marc J. Zeitlin                      marc_zeitlin@...
                                            http://www.cozybuilders.org/
Copyright © 2022                     Burnside Aerospace


cozygirrrl
 

Marc,

I know this is a really dumb question but I'm going to stick my neck out anyway and ask, if using a pressure bleeding system where no pumping or actuation of brake apparatus is involved, if it were possible or the brake system were laid out in such a manner, whats to stop you from unbolting the components and stringing them up in the air to get as much a vertical lineup as possible and then doing the pressure bleed?

...Chrissi

On 6/10/2022 7:17 PM, Marc J. Zeitlin wrote:
Folks:

I have to believe that we've discussed this in the past, but it bears repeating. A customer's plane has been beating the crap out of me for the past 5 hours of paid work time (and I HATE charging customers for time spent due to design or implementation issues), due to a brake system that is extremely difficult to bleed. Although this is an OSH award winning COZY MIV, which in almost all other ways is the epitome of what to do on COZY MKIVs, the brakes have always been marginal from a torque application (meaning pressure developed) standpoint.

This is not the plans system, or even a laydown MC system, but a "Velocity style" hanging pedal system, with the MC's up high, and the reservoir only a bit higher (the lines from the reservoir to the MC's are mostly BELOW the MC's). On a Velocity, due to the much larger depth of the fuselage, it's easily possible to have everything arranged well as described below, but on a COZY, it's a lot more difficult due to the small depth of the fuselage, particularly in the nose area. 

In this case, the feed lines from the MC's to the LG bulkhead route up and over the pedal torque tube and are pretty much guaranteed to be air traps at the high point. Given the lack of access in the nose due to the brake mounting system, bleeding the lines (pressure bleeding from the caliper to the reservoir, to try to push all the air out) has been completely unsuccessful on one side, even after manipulating the MC's to try to position them so that the lines are running uphill all the way to the reservoir. We may resort to replacing the tubing inside the fuselage; rerouting it and using smaller tubing.

At any rate, when designing your brake system, ensure the following:
  1. From the bleeder valve on the caliper to the top of the reservoir, ensure that EVERYWHERE upstream (closer to the reservoir) is higher than any place downstream. IOW, for those of you with math backgrounds, the uphill slope from the caliper to the reservoir MUST BE MONOTONIC - anything else will be an air trap at a high point.

  2. Sometimes it's not possible to perfectly do #1 - in these cases, use the smallest possible tubing, which would be AN-2 or even 1/8" SS with a 0.020" wall. If you can't use that, use nothing larger than 3/16" tubing.

  3. Unless you can GUARANTEE #1, do NOT use Matco 1/4" tubing, or 1/4" Matco fittings, as shown here:

    https://www.aircraftspruce.com/catalog/lgpages/matcotubefitting.php

    These are air traps at the point where the tubing goes into the fitting. The 1/8" tubing and fittings is probably acceptable, but why not just use metal hard lines where nothing moves, and SS/Teflon braided line where things move.

  4. Position the calipers so that there will be a minimum air trap in any attached fittings (and the smaller the brake line and fittings, the smaller the air trap).

  5. Position the calipers so that the fluid flow is uphill from the outlet to the inlet from the reservoir, and the moreso, the better.
You will be far happier with the performance of your brakes, and make things a lot easier when you need to bleed them (and you will), if you stick to these rules of thumb. The closer you can be to having all 5 things be the case, the lower the chance of issues. Sometimes you can get things to work even if you violate all 5, but your margin for success is a lot slimmer.

--
Marc J. Zeitlin                      marc_zeitlin@...
                                            http://www.cozybuilders.org/
Copyright © 2022                     Burnside Aerospace


David A Froble
 

On 6/11/2022 11:49 PM, cozygirrrl via groups.io wrote:
Marc,

I know this is a really dumb question but I'm going to stick my neck out anyway
and ask, if using a pressure bleeding system where no pumping or actuation of
brake apparatus is involved, if it were possible or the brake system were laid
out in such a manner, whats to stop you from unbolting the components and
stringing them up in the air to get as much a vertical lineup as possible and
then doing the pressure bleed?

...Chrissi
Not dumb ...

Since the master cylinders are in the nose, why not just hoist up the front of the aircraft until brake lines have no high points to trap air?

Some of Marc's suggestions seem to be good advice.

--
David Froble Tel: 724-529-0450
Dave Froble Enterprises, Inc. E-Mail: davef@...
DFE Ultralights, Inc.
170 Grimplin Road
Vanderbilt, PA 15486