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  #41  
Old 12-31-2019, 09:54 AM
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airguy airguy is offline
 
Join Date: Oct 2006
Location: Garden City, Tx
Posts: 4,951
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Quote:
Originally Posted by rocketman1988 View Post
You may want to look up Convective Cooling to get some idea of how heat transfer occurs without forced air...it appears, from your post, that you are not familiar with it...
That was.... kinda ... the point.

Yeah, nevermind...
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N16GN flying 600 hrs and counting! IO360, SDS, WWRV200, Dynon glass, 430W
Built an off-plan KC9A with too much fuel and too much HP. Should drop dead any minute now.
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  #42  
Old 12-31-2019, 09:57 AM
rocketman1988 rocketman1988 is offline
 
Join Date: Jun 2010
Location: Sunman, IN
Posts: 2,080
Default well...

Too much trouble to google it? Here is a brief overview:

https://en.wikipedia.org/wiki/Convective_heat_transfer

Happy New Year...
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Aerospace Engineer '88

RV-10
Structure - 90% Done
Cabin Top - Aaarrghhh...
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Wiring...

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  #43  
Old 12-31-2019, 05:52 PM
BillL BillL is offline
 
Join Date: Sep 2007
Location: Central IL
Posts: 5,327
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Since it seems to be a slow day to catch this much attention, here is a crappy vid on the 777 brake test. I have the 1 hour original recorded from a TV show, but this has the essentials of the validation test. Full load, rejected takeoff, 5 minute wait at the end, and pass criteria of having the ability to jack up the plane repair and return to service with only gear damage. Expensive test, required by the airline!

https://www.youtube.com/watch?v=f4LFErD-yls

For our RV technology level and specifics of mass, velocity and braking required, DanH nailed it, disc mass is a primary solution parameter. I imagine if we had glass wings, the tolerance for a wheel fire would be a lot less.
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  #44  
Old 12-31-2019, 09:42 PM
gasman gasman is offline
 
Join Date: Mar 2007
Location: Sonoma County
Posts: 3,729
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Quote:
Originally Posted by Square Feet View Post
The last time I was in the RV-6, it was on fire. *That was 12/26/2019 about seven hours and three beers ago.

I headed out west to the Lake Michigan coastal airport of Frankfort to fly the RV a couple weeks ago. *There was a small, dried puddle of something around the right wheel pant and the right brake pedal was mush. *The right line from the master was visibly white, and empty to the reservoir. *I removed the right pant and inspected the caliper and brakes in place. *Line and fitting were dry - the caliper had fresh 5606 drops dangling, the pads were wet and there was 5606 mud in the bottom of the wheel pant. *But no real culprit at this point. *I clipped the safety wire and plucked the caliper thinking the piston o-ring must be misbehaving.

Incidentally, the return from annual inspection was the last flight, and more than a month ago, where we had installed new tires, tubes, etc., and packed wheel bearings. *At the time, the left brake pads looked new, with the right showing slight wear, but definitely serviceable.

Back to the shop a week ago, an inspection of the cleaned brake assembly showed absolutely no signs of wear on the piston, nor the bore of the caliper. *The original o-ring had no visible defects, or stiffness, but it was likely 20 years old. *While I was at it with a new o-ring, just for good measure, I installed new brake pads at the shop then took the clean, fresh package out to install in today's 50+ degree balmy weather. *With the hangar door open, the bright sun lighted my workspace and warmed the recently-frigid wrenches and pliers to the bone. *The classical music station (keeping the mice out, because mice have no class) still played Holiday music; even the day after. *Bing, and Nat. *I was loving life.

We know that our phones will answer any question on the spot. *This would be a good time to take a micrometer to the brake disc, and compare to minimum thickness from Cleveland's specs. *The mic in the box was metric, but translated to .175-ish on a couple spot checks with min. spec for the 500x5 in the .160's. *Installed, the pad-to-disc clearances looked like a new installation.

I gas-ragged the 5606 mud (dirt and hydraulic fluid) from the belly of the right pant, pumped new brake fluid into the right bleeder, tested the right pedal until rock-firm while letting the volatile blue 100LL flash off before final installation of the pant. *Many brake cycles, no leaks, so on went the pant.

Now, back to the phone for a reminder of the Cleveland brake lining conditioning sequence... Oh yes, taxi 1500 feet with engine at 1700 rpm and brake(s) applied. *Stop. *Let cool 3 minutes, then test the brake-hold at a full power run-up. *If it holds, go fly. *If not, repeat process.

So that I did. *Sort of. *The frisky wind on the 15-33 runway was 200-240 at 12G22, so I thought maybe I'd just break-in the brake today instead of fly. *AWOS was showing an enticing downward trend of wind however, and I was being tempted to fly as I taxied in the stiff-ish left crosswind. *Dragging the right brake for the prescribed 1500 feet I couldn't hold 1700 rpm. *That's nearly take-off power in an RV-6 with a 160hp Lycoming 320. *Sub-1400 rpm *was getting me there just fine. *And largely because I was only breaking one brake in, (the right) as the left pads were fine, remember? *Everything was splendid. *The right brake was holding my taxi straight in the stiff left crosswind. *And braking was firm and effective. *I probably *ran an extra 500 feet to the run-up area with the break in procedure to generate glazing heat because of the cold.

When I finally slowed, then stopped at the runway end I could, not surprisingly, smell a little heat from the right brake side. *Then maybe a glimpse of smoke? *Or was that my imagination? *Nah, couldn't be that hot. *Then, at this stopped position the right brake pedal suddenly went limp. *#*!$$@. *Now what's going on? *And I'm 3000 feet down the runway from the barn. *Well, the now-right crosswind was a blessing having no right brake, I was able to idle very slowly, cautiously and carefully back to the hangar-ish. *All left turns, and all very slowly, and deliberately, because once the tailwheel unlocks, any asymmetrical braking situation, as in this case, makes this plane spin about aimlessly like a dazed housefly. *I was actually wondering about my insurance coverage as I straddled this delicate situation.

I did seem to get an occasional whiff of heat on the way back, but certainly no additional heat could be generated by the now-dead right brake.

I taxied to within 50 feet of the hangar before I ran out of directional wind benefit, and it's maneuvering luck. *I stopped, and now saw another puff of smoke. *Or not? *Could it be? Fuel off! mixture lean! electrical off! *key off! I hopped out and rounded the beak only to see the right wheel pant of my favorite airplane burning like yesterday's Christmas hardwood in great-nephew Kenny's home-heating fireplace. *I mean really burning, and right under the wing tank. *Yes, very momentarily, but there was again denial on my part. *Then an immediate *sprint to the hangar looking for something wet - yet not frozen. *Nothing in the wash bucket... Windex won't be adequate... WD40? *McGuiars won't ever put a shine on this again... the 5 gallon can of 100LL avgas sure ain't gonna help this situation!

"You idiot. *You have a fire extinguisher in here somewhere." And there it was., by the door, where it's supposed to be. *Dated, but there. *I grabbed the extinguisher, pulling the pin as I sprinted to the burning airplane. *You know, an extinguisher about 4" in diameter and 18" tall isn't quite adequate to put out a wheel pant, and now, tire-fire. *In retrospect, I wish I had choreographed this moment ahead of time to include time for a quick photo, or video, or selfie of this persistent, olympic torch. So a sprint back to the hangar for the bucket hoping the frost-faucet at the terminal building is working. Then a sprint back to the rekindled campfire with five gallons of water. *Boom! went the new $350.00 tire and tube combo with the five-gallon surgical douse of cold water. *And it wasn't out yet. *So, another sprint to the terminal building and another wet five, which got things under control. *Then one more, walking this time in disbelief of what just happened, and what nearly happened.

Well, with a little neighborly help, Richard, the Airport Manager, and another neighbor were able to help get the right gear on a dolly and back into my hangar before dark. *I still have a plane, but also a boatload of redundant, needless work.

The way that fire-retardant 5606 hydraulic fluid and fiberglass combo burned has me re-thinking the aluminum vs. composite construction world. *My wheel pant was very difficult to extinguish, and turned into a literal dishrag that I simply ripped from the mounts before dollying.

What a day. *Now, I need to think about *the root cause of this event. The quick answer may be that there are times "to go the extra mile" but riding the brakes on break-in, or ever, ain't one of 'em. *Funny, it could even happen in a fiberglass sailplane with no fuel aboard. *Some have disc brakes, and of course, that flame-retardant 5606. Dang, I always felt safer with no gas aboard. Does that even make sense?

Parachutes, Baby!
Happy New Year.

Miketitan
Check the extinguisher that you used. ABC, the most common powder type are very corrosive to aluminum and electrical connections.
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  #45  
Old 01-04-2020, 01:32 AM
agent4573 agent4573 is offline
 
Join Date: Aug 2019
Location: Mountain view
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Quote:
Originally Posted by Snowflake View Post
Your brakes will heat up during your landing roll to their maximum temp, so you're probably only concerned about the last half of that roll where the highest temps will be. That's when you're moving from about 40mph down to about 10mph. At those speeds, you won't be getting enough air through any ducts that would put out a fire... You'll get enough air to stoke one though, just like blowing on the embers of a campfire.

As for cooling, i'd expect the effect to be minimal. The temperature is maximizing as your ability to blow air into the space is diminishing rapidly (as you slow down). You'll be hottest just before you stop, and will have almost no airflow.

Clearly, instrumented testing is needed.
Maximum brake fluid temperature doesn't occur on rollout, it occurs after you stop completely. This is why the airlines require a 5 minute sit after the emergency stop test. The small area under the pad will heat quickly, it takes time for that heat to spread through the rotor and get to anything flammable. Mild steel can be run at over 1000 degrees before losing a significant amount of strength. It will melt and deform long before it spontaneously ignites. Things that will ignite are wheel bearing grease and brake fluid. It takes a while for the heat under the pad to make its way to the axle and wheel grease. If your pads retract correctly and dont drag it takes longer to reach the brake fluid. Most caliper pistons are stainless to minimize heat transfer through the Piston, and if the pad isnt touching the rotor while taxiing, the only transfer is radiative and minimal convective, no conductive.

Flashpoints of fluids only matter if there is already an open flame to ignite the vapor. When looking at fluid specs, autoignition temp and boiling temp are the primary concerns. At the boiling point you'll lose the pedal and braking ability. It's difficult to get beyond the boiling point once you can't brake anymore. This happens during sustained operation of the brakes above the boiling point but below the ignition temp. If you do get the rotor over the autoignition point, you then have to have a leak develope that hits the rotor and raises the fluid temp over the autoignition temp. It does happen, but if the system is sealed correctly, the chances are low. What normally happens is you exceed the maximum energy of the rotor, then while sitting still the heat makes its way to the o ring, which eventually breaks down and leaks onto the overheated rotor and ignites. If the rotor is large enough you don't exceed the maximum energy, you have very little chance of a brake fire.

Brake cooling ducts only help with repeated brake applications, that's why Ferrari and other racecars run them. There is a steady state point on a car that the rotor operates at once it's fully heated after multiple laps. The amount of airflow changes the steady state operating temperature. It does very little to lower the peak temp seen during an emergency stop and sit scenario. Cooling flow is also ineffective if blown over the rotor in the same plane of the rotor. The airflow really has to hit the rotor at 90 degrees to have a major impact to cooling.

Main takeaway: if you want a lower chance of a fire, put in bigger rotors, higher temp seals and a brake fluid with the highest auto ignition to you can. If you want to use your airplane brakes for repeated applications with no cool down period, add some cooling ducts.

Source: designed and built the brakes for the SAE racecar in college. Did landing gear and brake testing as a flight test engineer in the Air Force.
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Last edited by agent4573 : 01-04-2020 at 01:41 AM.
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  #46  
Old 02-04-2020, 05:53 PM
Sue Sue is offline
 
Join Date: Nov 2012
Location: Ok
Posts: 94
Default Brake pads

I have never “broke in” brake pads?
Never recommended it and never had
a issues or gotten negative feedback
from any of my customers over telling
them to just operate it normally.
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