In article , "Highflyer"
wrote:

"Alan Baker" wrote in message
...
In article ,
Stealth Pilot wrote:

aircraft brakes were designed for use in holding the aircraft still
while the engine was started. after the taxy out and the engine has
warmed you do a run up check to make sure that the magneto circuits
are up to the bit of work that lies ahead for them. the brakes are
applied to hold the aircraft while the revs are bought up and each
maggy checked in turn.

from a design aspect that is the end of the use of a light aircraft's
brakes until after landing and we wish to hold the aircraft still for
shutdown and disembarkation.

of course brakes are brakes and people will use them like they were
driving cars. light aircraft brakes were never designed for slowing an
aircraft when landing.
I know that they get used for that by students of bad piloting
technique but the design intent is a fact borne out by their
diminutive size.

Stealth Pilot

Nonsense: complete and utter.

What aircraft brakes aren't designed for is stopping aircraft
*repeatedly*.

The chief advantage of putting larger brakes on any vehicle is that it
providess a greater heat sink to allow for more braking before the
brakes overheat.

Aircraft brakes need to be able to stop an aircraft *once* and then have
an essentially infinite amount of time to cool down again.

--
Alan Baker
Vancouver, British Columbia
http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg


Right. Almost. Maybe.....

My Stinson has 9 inch diameter drum brakes and has lots of
area on the brakes, but low pressure applied. It uses a largish
master cylinder to pump fluid into an "expander tube" under the
brake shoes inside the drum.

They work great for holding the airplane for runup and are essential
for ground handling and taxiing because the tailwheel is a swivel and in not
connected to anything that could allow it to be used for
steering. As a result, all steering is by differential braking.

You can apply the brakes on the landing roll and it will quickly slow the
airplane. However, if you do so, you may not be able to
leave the runway. Rubbing all that shoe area inside the drum makes
it quite hot inside the drum. My little infrared laser guided remote
reading thermometer gives temperatures in the 400 to 600 degree
range. It takes a while to dissipate that heat from inside the drum
and until it does the brake fluid inside the "expander tube" gets quite
warm. When it does it expands and the brakes tend to remain quite
"ON" until things cool down.

Wow. This is so wrong.

The expander tube expands because when you push on the brake pedals you
force fluid into it. If that fluid is heated, it will try to expand but
since the pressure in the system is determined by your feet on the
pedals, what will happen is that the pedals will push back on your feet.
If you don't push any harder, they will move back until the reduction in
pressure once again balances the system. If that moves the pedals all
the way back to the stops, then the orifice to the reservoir will open
and fluid will get pushed back into it.


Even a long and complicated taxi, like into a parking spot at Oshkosh, will
generally result in a noticeable loss of "differential"
in the braking activity and a substantial increase in the power
required to taxi. I have found that it is wise to NOT attempt a
takeoff if it requires over 1000 RPM to maintain a reasonable taxi
speed. :-)

Then I suggest that there is something wrong with your brakes...


Of course, this airplane weighs generally two tons and lands at 70 mph at
touchdown in a three point attitude. :-)

Ummm... What model Stinson weighs two tons? You're not claiming you fly
a Stinson Reliant, are you?


The general limiting factor in ALL aircraft brakes is heat dissipation. The
wheels are relatively small and the brakes are
in a small space. The more effective the brakes are, the more heat
they produce. All that energy they are dissipating when they slow
you down has to go somewhere. Thermodynamics tells us that most
wasted energy appears as heat! Randomized molecular activity. :-)
To stop the airplane you have to waste the energy. 1/2 M V^2.
You can't get around it.

And you don't have to. The kinetic energy of a 1,000kg light aircraft at
a landing speed of 27.8 m/s is 1/2 mv^2 = 386420 Joules

The specific heat of steel is 500 Joules per kg*K (degrees Kelvin).

So if you have two brakes weigh -- say -- 10kg each, then the rise in
temperature (dT) is

386420 = 20(dT)(500); dT = 386420/10000 = 38.64 K degrees. Even if the
brakes weigh a half of my estimate, the temperature rise is still only
77 K, or 139 F.


Fortunately, a taildragger with the flaps down and the tail on the
ground takes a LOT of energy to keep moving, so you can waste a
lot of the energy you have to get rid of by stirring up the air. Then
apply the brakes to turn off the runway after you have slowed down
without them.

Sorry, but you won't stop a Stinson in 290 feet on aerodynamic drag
alone.


FWIW Department. You scrape a lot more rubber off you tires by
landing and applying brakes vigorously while little weight is on
the wheels than you would in many many miles of taxiing around or
rolling out with the brakes off! :-)

You only scrape off much rubber if you brake close to the threshold of
adhesion.


Highflyer
Highflight Aviation Services
Pinckneyville Airport (PJY)

--
Alan Baker
Vancouver, British Columbia
http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg