Posted for Gary Casey <glcasey@adelphia.net>:

 Colyn, Craig, Rob,
 
 I guess I shouldn't have sounded so positive in my posting (people  actually
read these things!) - yes there are a number of combinations  that work for
race cars.  My experience was only with TransAm cars  which are relatively
heavy (2700 pounds, 750 hp) and have to stop  for  a couple of hours.  The hot
setup (literally) back in the 80's  was to use massive rotors and organic
pads.  The primary failure mode  was from boiling fluid and the organic pad
provided better  insulation.  We had to add ballast anyway, so the heavy
rotors didn't  increase vehicle weight.  They also had a very high and
consistent  coefficient as long as the rotor stayed reasonably cool.  The wear
 rate was very fast, usually completely using up a set of pads in a  race, and
these pads were about twice the thickness of street car  pads.  The slots or
holes were there to sweep away the pad debris,  liquid binder and gases from
the face of the pad.  Rotor temperatures  stayed fairly cool because of the
massive rotors and good cooling and  the high coefficient allowed the system
to be run without a booster.   Another combination that could have been run is
a high metallic- content pad, but we couldn't figure out a way to effectively
insulate  the fluid, even with ceramic insulators and ceramic pistons - and a
 booster was required.  HIgh metallic content, depending on the binder
 chosen, might be expected to slough off less, reducing the advantage  of
drilling the rotors.  And, yes, drilling the rotors reduces the  mass of the
rotor, a bad thing.
 
 We (Bendix) found that for a single stop a good assumption to use is  that
ALL the energy goes into heating the rotor.  Some goes into  heating the pad,
but not nearly as much.  Some goes into the air, but  in the time it takes to
make a stop virtually none has been  dissipated.  Most large-aircraft brakes
appear to be designed (and  they are) with no air cooling capability at all.
 The heat is  absorbed into the brake and then dissipated while the next batch
of  passengers is being loaded.  Carbon brakes are interesting in that  the
materials themselves are relatively poor at absorbing heat (the  figure of
merit - I forgot what it is called - is the density times  the heat transfer
coefficient and that sort of times the melting  point).  Copper is the "best"
material and aluminum would be except  for the low density and low melting
point.  The advantage of carbon  is that it essentially has no melting point
and its strength is  retained or even increases as the temperature goes up.
 Red-hot iron  brakes are pretty hot, but hard-working white-hot carbon brakes
could  light up the sky.  Carbon burns, so why doesn't the brake just burn
 up?  It does, although slowly enough to last a race.  Aircraft brakes  are
intentionally designed to keep air out to increase the life of  the brakes.
 That being said, I don't see any advantage of using  carbon pads with steel
rotors, as the rotors can't be run any hotter  with the "carbon" pads than
without.  I suspect that people are  selling "carbon" pads that are simply
conventional pads with enough  carbon added to turn them black.  Real "carbon"
brakes are designed  to use a carbon/carbon friction pair.
 
 Another incidental thing we found was that very little heat was  transferred
to the air within the slots of a ventilated rotor - the  big advantage of the
"ventilated" design is that the rotor is  structurally more rigid and tends to
warp less.  Iron that is very  far away from the friction surface isn't useful
because the low  thermal transfer coefficient of iron prevents heat from
traveling  that far, so ventilating the center portion reduces weight and
 increases rigidity without degrading the thermal capacity.  Almost  all the
heat is dissipated directly from the friction surface as the  rotor turns -
not usually recommended to come to a complete stop with  the rotors very hot
(the rotor inside the caliper cools at a  different rate than the rest of the
rotor), but that's not usually a  problem with aircraft as we usually taxi at
low speed before coming  to a complete stop.
 
 What's all this have to do with our aircraft?  I'm not sure, but the  "best"
compromise might be to use a ceramet (metallic with ceramic  binder) pad with
lots of mechanical advantage to overcome the low  coefficient and then expect
to change the rotors often.  A  disadvantage of this combination is that the
coefficient is very low  when cold - expect erratic braking when first
touching the brakes  after landing - that would make me very nervous.  We
don't have power  brakes like the airliners, and increasing the pedal ratio
isn't all  that practical, although I'm thinking about drilling new holes in
my  pedals.  A problem with that is that any rotor warpage could push  back
the piston enough so that the brakes couldn't be actuated with  available
pedal stroke.  Too many questions, not enough answers...
 
 Gary Casey