Thread: On Fire
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Old 01-04-2020, 01:32 AM
agent4573 agent4573 is online now
Join Date: Aug 2019
Location: Mountain view
Posts: 169

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.

2020 dues paid.

Last edited by agent4573 : 01-04-2020 at 01:41 AM.
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