Pmerems

Pmerems

Well Known Member
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Gents,

I am into my test program for my RV-7A. I have noticed in the last few flights that when I push back the plane into the hanger I hear the left brake pads rubbing on the disc/rotor. It is cyclical sound by that I mean it is not a constant rubbing (more like the pad is slightly touching the disc/rotor) and can be heard during half the tire rotation.

I inspected the brake pads and they were free to move.

I noticed this sound the previous flight but when I pulled the RV out of the hanger today there wasn't any sound at all. However once I landed today and pushed the plane back into the hanger the sound was back.

This leads me to believe that when the disc/rotor heats up it warps a fraction and or the pads retract a bit more when they are cold, but the rotor is still warped.

I have extra external brake cylinder return springs so I know the pedals are returning to neutral position.

I also notice early on in the that one disk/rotor has some plating on it that is flaking off and the other disk/rotor looks like it never had the plating. I had an experience A&P look at the disks/rotors and said it was normal to see this condition.

80% of the hours flown on my RV were by a pilot who doesn't use his brakes unless he has to. I tend to use them more. I can't believe they have been overheated this early in the test program.

Any thoughts or experiences would be appreciated.

Thanks,
 
Perfectly normal!

That's how disk brakes work. There are no return springs on calipers. Jack up the front of your car and rotate the tire. Same thing!
 
While we're on that subject...

....don't sit still and hold the brakes after a hard stop with a lot of hard pedal usage.

I used to race in the SCCA and guys quickly warped their discs after very hard braking, IF THEY STAYED ON THE PEDAL, because the rest of the disc cools, while the area between the pads doesn't, resulting in a warped disc.....same thing applies to your car. If you have to stop fast (unexpected red light), put it in park and get off the brake pedal.

Best,
 
keep in mind--

The rotors are pretty thin, so they will heat pretty quick. Not sure about the rotors you are using, but in automotive, some "imported" rotors weren't the same quality of materials, and were prone to warping, even under "normal" usage. Also, make sure your master cylinders fully retract, and dont leave some residual pressure on the lines-can cause the caliper pistons to extend slightly and keep the pads touching the rotors.
My 2 cents
Tom
 
Automotive disc brakes are more sophisticated...

...than "aircraft quality" brakes.

As a former Sr. Engineer for a Tier 1 OEM brake supplier (Kelsey-Hayes), I have to chime in. I am certain that this is more information than most (all) of you want to know.

The short answer:

1. Normal for aircraft brakes
2. Improvement not likely to occur because it requires increased manufacturing expense and "permission" from the FAA.

The too long answer:

Mel is correct that aircraft brake linings drag slightly on the rotors. During various endeavors to improve car fuel economy, the issue of brake linings dragging on rotors was investigated by brake manufacturers (well, K-H anyway. Bendix - not so much).

Auto disc brake calipers use an o-ring with a square (rather than round) cross-section. The groove in the piston bore is cut with a form tool rather than a single point cutting tool. The form of the tool results in a groove that has a slightly smaller diameter on the disc pad side than the caliper side. In the section view it looks like a "ramp." This naturally causes the square section o-ring seal to "roll-back" upon brake release, which greatly diminishes brake drag. In actual service, the lateral run-out of the brake rotor (.002 - .003") is generally enough to knock the pads back so they don't rub.

Because the last operation before jacking a car up is typically low speed maneuvering followed by braking, the "knock-back" that occurs driving down the road doesn't happen and there can be slight brake drag. If it is more than slight, one should investigate the cause, because it is costing $$$ for gas!

LarryT
 
Photos of disk

Gents,

Take a look at the photo. You can clearly see the flaking. I am going to call Parker to see what their technical support has to say.

Any thoughts?

Brake disk.jpg
 
Bumping an old thread on warped brakes

Gents,

Take a look at the photo. You can clearly see the flaking. I am going to call Parker to see what their technical support has to say.

Any thoughts?
Click to expand...

My brakes were sticking before I loosened the nuts on the bottom of the brake pedal. As a result, they were heated up and I am seeing "Flaking" identical to the photo in the post above. I also have a bit of warping that I can hear when I pull the plane out of the hangar or push it back in.

Should I worry about this?

Should I replace the wheels to get new rotors?

The brakes no longer stick, the stopping power is fine and they hold me still during run up. I just look at the flaking and hear the little scrape as part of the rotor hits the pad and worry. I do not notice anything different when slowing down on landing. They track straight and true. I have been pulsing the brake to slow, coast, slow, coast and using more runway, as I was a bit worried after seeing the heat up and flaking. I don't really think that is required, as I am pretty sure the original problem was caused by the nut being to tight to allow the master cylinder to release. So the original heat up was from a stuck brake pad. I found I couldn't push the plane on the taxi way, until I went back in the plane and pulled the brake pedal back the last 1/16". That no longer happens since I made the nut finger tight on the bottom of the brake pedal (finger tight and then cotter pinned in place).
 
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