On Fri, 7 Nov 2008 23:55:23 -0600, "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.

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. :-)

At about 2300 or so RPM the Deb will happily accelerate with the
brakes locked and the tires skidding. Once it starts to skid it feels
just like it hit ice as the rubber begins to melt/vaporize.


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

Man, but that's fast!. :-))
The old Deb comes down final at 80 MPH minus 1 MPH for each 100 under
gross. With me and half fuel, final is about 73 MPH. touchdown?
Gotta be in the 40 MPH range. Those big flaps slow it quickly.
Course it only weighs a ton and a half so you're hauling the
equivalent of the entire useful load of the Deb on top of the Deb's
gross.:-))

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.

Those big Cleveland's will fade if they get a lot of use, but it's
rare to ever use them that much. Even after a short field, with
maximum effort they still work. You can lock them up, but after that
the taxi gets kinda bumpy.

Steep final into the round out, firm touch down on the mains,
immediately let the nose down, get on the brakes, and go full up
elevator if the brakes will keep the nose gear down. The aerodynamic
braking with that big wing and tail is very effective giving it the
ability to stop shorter than many 172s (book figures)

I'm certainly not that proficient at present.



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.

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! :-)

Highflyer
Highflight Aviation Services
Pinckneyville Airport (PJY)

Roger (K8RI) ARRL Life Member
N833R (World's oldest Debonair)
www.rogerhalstead.com