Steven Shelikoff wrote:

aircraft brakes are in many cases under engineered since they
depend so much on engine braking to slow down.

Incorrect. The brakes on transport category aircraft are certified to
stop the aircraft on the runway remaining after a rejected takeoff at
the highest speed it would still be on the ground (V1) without using
thrust reversers. Thrust reversers provide little braking at high speeds
anyway.

Rick

On Mon, 03 Nov 2003 17:11:20 GMT, Rick wrote:

Steven Shelikoff wrote:

aircraft brakes are in many cases under engineered since they
depend so much on engine braking to slow down.

Incorrect. The brakes on transport category aircraft are certified to
stop the aircraft on the runway remaining after a rejected takeoff at
the highest speed it would still be on the ground (V1) without using
thrust reversers. Thrust reversers provide little braking at high speeds
anyway.

Yeah, right. But not over and over and if that does happen, i.e.,
stopping the plane with the remaining runway after an aborted takeoff,
you're almost guaranteed a brake fire. No matter how the brakes are
certified, if a heavy gets up to takeoff speed on most runways, aborts
and only has the brakes to stop it, chances are it's gonna go off the
end of the runway.

And I'm not sure where you get the idea that thrust reversers provide
little braking at high speeds. They way they work, they really *only*
provide braking at high speed and very little at low speed. They are
the vast majority of braking at landing speed.

While a jet thrust reverser can be used to back up the plane, very
little thrust is actually "reversed". Mostly, it's just diverted into
an unuseful direction, like up and down or outward, and very slightly
forward for backing up. They slow the plane mostly by engine drag, not
by reversing the thrust forward. And engine drag is greater at higher
speeds. In fact, most of the accidents involving thrust reversers occur
when they are inadvertantly or uncommanded deployed in flight, causing
massive drag on the deployed side and throwing the plane out of control.

Steve

Steven Shelikoff wrote:

Yeah, right. But not over and over

Well, duh ...

and if that does happen, i.e.,
stopping the plane with the remaining runway after an aborted takeoff,
you're almost guaranteed a brake fire.

Not true. But that was the point of my original statement, that the
tires are more likely to be heated by the wheels and brakes than cooled
by them.

... if a heavy gets up to takeoff speed on most runways, aborts
and only has the brakes to stop it, chances are it's gonna go off the
end of the runway.

Not really apples to apples. RTO's at V1 are rare in any event and when
they do occur it is likely because of a tire, or multiple tire failures
so there is little braking available in any event.

And I'm not sure where you get the idea that thrust reversers provide
little braking at high speeds. They way they work, they really *only*
provide braking at high speed and very little at low speed. They are
the vast majority of braking at landing speed.


They are aerodynamically most efficient at high speeds but they do not
provide the majority of braking nor are they required to be used or even
desired at all times. They cannot be used until the engine is at idle,
there is weight on the wheels, they buckets have cycled open, and the
engine spooled up again. By this time the autobraking has slowed the
aircraft considerably. They must not be used below around 60 knots to
prevent compressor stalls and sucking up garbage. They are useful only
in a very narrow range, not at the highest speed where brakes are needed
most or at the rollout when autobraking is off and manual braking is
used. They are hard on engines and the modern design trend is toward no
reversers, depending instead on carbon brakes.

Rick

On Tue, 04 Nov 2003 17:00:12 GMT, Rick wrote:

Steven Shelikoff wrote:

Yeah, right. But not over and over

Well, duh ...

and if that does happen, i.e.,
stopping the plane with the remaining runway after an aborted takeoff,
you're almost guaranteed a brake fire.

Not true. But that was the point of my original statement, that the
tires are more likely to be heated by the wheels and brakes than cooled
by them.

... if a heavy gets up to takeoff speed on most runways, aborts
and only has the brakes to stop it, chances are it's gonna go off the
end of the runway.

Not really apples to apples. RTO's at V1 are rare in any event and when
they do occur it is likely because of a tire, or multiple tire failures
so there is little braking available in any event.

And I'm not sure where you get the idea that thrust reversers provide
little braking at high speeds. They way they work, they really *only*
provide braking at high speed and very little at low speed. They are
the vast majority of braking at landing speed.


They are aerodynamically most efficient at high speeds but they do not
provide the majority of braking nor are they required to be used or even

They really only work well at high speeds, not low speeds.

desired at all times. They cannot be used until the engine is at idle,

They may not be used or desired at all times, only about 99% of the
time.

there is weight on the wheels, they buckets have cycled open, and the
engine spooled up again. By this time the autobraking has slowed the
aircraft considerably. They must not be used below around 60 knots to
prevent compressor stalls and sucking up garbage. They are useful only
in a very narrow range, not at the highest speed where brakes are needed
most or at the rollout when autobraking is off and manual braking is
used. They are hard on engines and the modern design trend is toward no
reversers, depending instead on carbon brakes.

None of that changes the fact that aircraft braking requirements and
capabilites and tire heating have nothing to do with race car or boat
trailer braking or tire heating. While it may be true for aircraft
braking that the tire is more likely to be heated by the brakes then by
the heat from tire friction, that's not true for most types of racing
and especially NASCAR restrictor plate racing, when the brakes aren't
even used but the tires still get very hot and might benefit from
cooling through the wheel.

Steve

Steven Shelikoff wrote:


None of that changes the fact that aircraft braking requirements and
capabilites and tire heating have nothing to do with race car or boat
trailer braking or tire heating.

Never said it did, I just made the statement that tires are more likely
to be heated than cooled by the wheels and brakes and used aircraft
tires as a spectacular example.


While it may be true for aircraft
braking that the tire is more likely to be heated by the brakes then by
the heat from tire friction, that's not true for most types of racing
and especially NASCAR restrictor plate racing, when the brakes aren't
even used but the tires still get very hot and might benefit from
cooling through the wheel.

I have absolutely no idea what "restrictor plate racing" is, what do you
do, run with them? 8-)

If the brakes are never used then the brakes won't add heat. Unless the
area of the wheel exposed to the filling gas is a fair proportion of the
area of the sidewalls then I can't see much heat going out the wheels
regardless of the gas used. Are you sure there is a large area of wheel
surface exposed anyway?

I haven't seen a racing tire up close and personal but if they are like
most other tires the bead/s run pretty close from side to side and it
doesn't appear that there is much metal not covered by rubber in most
wheels.

Anyway, I don't buy the "runs cooler" argument for nitrogen any more
than anyone should buy the "nitrogen expands less" nonsense.

Rick

snip

If the brakes are never used then the brakes won't add heat. Unless the
area of the wheel exposed to the filling gas is a fair proportion of the
area of the sidewalls then I can't see much heat going out the wheels
regardless of the gas used. Are you sure there is a large area of wheel
surface exposed anyway?

I haven't seen a racing tire up close and personal but if they are like
most other tires the bead/s run pretty close from side to side and it
doesn't appear that there is much metal not covered by rubber in most
wheels.

I have done a fair amount of measurements of tire temperature and heating
using IR sensors in the wheel wells under racing conditions. The data was
recorded with a data aquisition unit during racing, and downloaded to a
laptop between races. We did 10 sample per second, with 1 degree resolution.
I can state with confidence that a large portion of the tire cooling is due
to airblast on the tire carcass. The percentage of cooling by the metal
wheel is a very small fraction of the total heat dissipation.

Going a step further, with certain high end racers, the inner safety liner
completely insulates the metal wheel from the fill gas and tire face. This
does not seem to affect the heat balance in any measureable way.

Anyway, I don't buy the "runs cooler" argument for nitrogen any more
than anyone should buy the "nitrogen expands less" nonsense.

I am not sure what you have taken from this thread. Conventional wisdom is
that the measured pressure increase is due to liquid water flashing to steam
above the boiling point of water. It has nothing to do with the fraction of
oxygen or nitrogen in the fill gas. In the turns NASCAR and F1 cars run peak
tire temperatures between 225 and 250 degrees F. I leave it to you to offer
an alternate explanation of the measured 4 to 16 PSI jump (nominal 30 PSI)
under racing conditions. This increase is enough to completely scuttle
chassis tuning. While you are at it, explain how switching from running
"air" to dry nitrogen combined with a few forced purge-fill cycles
eliminates the effect - the tires pressure changes pretty much as predicted
by PV/T = PV/T.

This stuff is not conjecture - it is measured data. If it does not match
your expectations - perhaps it is time to reexamine your expectations.

Mark Browne

Mark Browne wrote:

I am not sure what you have taken from this thread. Conventional wisdom is
that the measured pressure increase is due to liquid water flashing to steam
above the boiling point of water. It has nothing to do with the fraction of
oxygen or nitrogen in the fill gas.

I don't place much weight in "conventional wisdom" when it comes to
physical phenomenon that follow well defined laws of physics.

Water will not" flash to steam" at the pressures and temperatures you
describe.

In the turns NASCAR and F1 cars run peak
tire temperatures between 225 and 250 degrees F. I leave it to you to offer
an alternate explanation of the measured 4 to 16 PSI jump (nominal 30 PSI)

If there was liquid water in the tire at the start of the race, at say
80 degrees F, all but the tiny amount required to saturate the filling
gas would still be liquid. The filling gas will follow the gas laws.

At 34 psig the gas temperature would have to reach approximately 280
degrees F to evaporate any liquid water in the tire.

At 46 psig the gas temperature would have to reach approximately 290
degrees F to evaporate any liquid water in the tire.

I have no idea what the tire volume is but if you do you can calculate
the weight of water present in the filling gas as a saturated vapor at
atmospheric pressure and temperature and if you know there is liquid
water flying around in the tire you can calculate what temperature and
pressure it takes for that liquid to change state.

under racing conditions. This increase is enough to completely scuttle
chassis tuning. While you are at it, explain how switching from running
"air" to dry nitrogen combined with a few forced purge-fill cycles
eliminates the effect - the tires pressure changes pretty much as predicted
by PV/T = PV/T.

IT looks like you are ignoring the vapor pressure of water and you
probably do not calculate the partial pressure of the water vapor in the
air filled tire. You are using the wrong gas law to begin with and when
you get a dry tire with a dry gas the tire acts as predicted.

This stuff is not conjecture - it is measured data. If it does not
match your expectations - perhaps it is time to reexamine your
expectations.

I am only a simple mechanic, it is my place to follow the laws, not to
change them.

The gas laws are not predicated on anyone's "expectations" they are
physical phenomena that scientists and engineers have used for a couple
of hundred years with great reliability and repeatability. It appears
that the only place they are held in abeyance is the race track.

If I am missing something here I would really like to know what it is.
It is an interesting subject.


Rick

"Mark Browne" wrote in message
news:Q_Zpb.109411$Fm2.94923@attbi_s04...



Conventional wisdom is
that the measured pressure increase is due to liquid water flashing to
steam
above the boiling point of water.

There are a few simple things you could do to eliminate the liquid water in
the tires:

1) Install a water separator on the air line between the compressor and your
inflation nozzel. Standard equipment on most air systems.

2) Don't mount the tires outside in a driving rain!

In the turns NASCAR and F1 cars run peak
tire temperatures between 225 and 250 degrees F. I leave it to you to
offer
an alternate explanation of the measured 4 to 16 PSI jump (nominal 30 PSI)
under racing conditions.

Are you saying that as the tire temperature changes from 225 and 250
degrees F (685 to 710 degrees R) the tire pressure changes from 30psig to
46psig ( 44.7 psi to 60.7psi at sea level) ??

pv/t = PV/T . IF the volume stays constant (not a good assumption) so
44.7V/685 should equal 60.7V/710
0.065 does not equal 0.085. Nope, something else going on here.

Hmmm, sure doesn't follow the steam tables. According to the steam tables
publised by the American Society of Mechanical Engineers (the accepted
standard) the internal temperature would have to be up to around 294 degrees
F to have boiling water create that kind of pressure. Nope, isn't steam.

And you say this doesn't happen when you use Nitrogen?

You said that this happens in the turns, so I can explain why you would get
a sharp increase, but I can't explain why you didn't see the same increase
using nitrogen. In the turns, the tire is going to be subjected to a
significant amount of lateral force. This force is going to distort the
shape of the tire, hence its volume will decrease. As you decrease the
volume, the pressure will increase. You are changing a second variable in
the PV/T equation.


This stuff is not conjecture - it is measured data. If it does not match
your expectations - perhaps it is time to reexamine your expectations.

What other gases did you try this with? Did you try dry air? Did you try
carbon dioxide?
The water theory would have been trival to eliminate by simply eliminating
the water, did you do that?
Did you try adding a little water to nitrogen and seeing if it behaved just
like air?

Numbers are wimps, if you torture them enough they will confess to anything.
To throw out the ideal gas laws because your measurements didn't agree, and
then say "Conventional wisdom is that the measured pressure increase is due
to liquid water flashing to steam" is absolutly conjecture.

Maybe there is something else going on. Too bad you didn't follow the
scientific method properly and try to figure out what it was. Change one
variable at a time and you have a much better chance of establishing what
the cause and effect relationships are.

Nitrogen is a funny gas. At sea level pressures and room temperatures it is
generally inert and safe to use for many applications. If you increase the
pressure, that is no longer true. SCUBA divers all know that under a few
additional atmosphers of pressure Nitrogen does BAD things to the human
body. Oxyen is even worse, double the pressure of oxygen and it suddenly
becomes toxic in its pure state. Perhaps what is really going on is that at
those pressures and temperatures the oxygen in the air is reacting with the
rubber compound of the tires, makng them more pliable.

Rod

On Wed, 05 Nov 2003 02:20:32 GMT, Rick wrote:

Steven Shelikoff wrote:


None of that changes the fact that aircraft braking requirements and
capabilites and tire heating have nothing to do with race car or boat
trailer braking or tire heating.

Never said it did, I just made the statement that tires are more likely
to be heated than cooled by the wheels and brakes and used aircraft
tires as a spectacular example.

And I'm just saying that while in racing, the brake rotors themselves
can get extremely hot during braking, if there is so little heat taken
away that the rotors alone are causing the wheels to be heated up to
over 250 degrees then something's wrong with the setup. The same
statement might not be true for an airplane, where the brakes are
applied hard for only a short time and then they have hours to cool
down.

While it may be true for aircraft
braking that the tire is more likely to be heated by the brakes then by
the heat from tire friction, that's not true for most types of racing
and especially NASCAR restrictor plate racing, when the brakes aren't
even used but the tires still get very hot and might benefit from
cooling through the wheel.

I have absolutely no idea what "restrictor plate racing" is, what do you
do, run with them? 8-)

Uh, yeah. You run with them. They limit the horsepower available on
the superspeedways to around half of what it normally available. That
way, the cars never get going fast enough to have to use the brakes.

If the brakes are never used then the brakes won't add heat. Unless the
area of the wheel exposed to the filling gas is a fair proportion of the
area of the sidewalls then I can't see much heat going out the wheels
regardless of the gas used. Are you sure there is a large area of wheel
surface exposed anyway?

If there's not an inner liner, then yes, a large area is exposed. An
inner liner is used for some races and not for others.

As a quick and dirty example, Nascar wheels are 15" dia x 9.5" wide.
The tires are 27.5" dia with a width of not more then 13.2". To make
things easier, assume flat sidewalls, which will make the area
calculation below come out on the low side. The sidewall area is around
2*(27.5-15)*pi = 78 sq in. Also ssume the wheel is a cylinder, which
will also make the area calculation come out on the low side so it sorta
cancels out. Also, assume that the bead takes up around 1/2" of the
wheel width on each side even though it's a little less, so the area
calculation of the wheel area will be a bit low. So the surface area of
the metal inside the tire is around 15*8.5*pi = 400 sq in, or about 5
times the sidewall area.

I haven't seen a racing tire up close and personal but if they are like
most other tires the bead/s run pretty close from side to side and it
doesn't appear that there is much metal not covered by rubber in most
wheels.

If you're using passenger cars as your example, you need to look at
today's larger and wider wheels mounted with very low profile tires.
They're closer to most racing wheel profiles. The area of the wheel
inside the tire is significant.

Anyway, I don't buy the "runs cooler" argument for nitrogen any more
than anyone should buy the "nitrogen expands less" nonsense.

I don't buy the "runs cooler" argument either. But I do buy the
argument that you can control the amount of moisture in the gas easier
if you're filling it with nitrogen then when plain compressed air.
There's no reason I can see that extremely dry compressed air shouldn't
work as good as nitrogen. But it may be cheaper and easier for the
teams to buy a tank of compressed nitrogen then to dry compressed air to
the same level of water content.

Steve

On Wed, 05 Nov 2003 06:25:44 GMT, (Steven Shelikoff)
wrote:

As a quick and dirty example, Nascar wheels are 15" dia x 9.5" wide.
The tires are 27.5" dia with a width of not more then 13.2". To make
things easier, assume flat sidewalls, which will make the area
calculation below come out on the low side. The sidewall area is around
2*(27.5-15)*pi = 78 sq in. Also ssume the wheel is a cylinder, which
will also make the area calculation come out on the low side so it sorta
cancels out. Also, assume that the bead takes up around 1/2" of the
wheel width on each side even though it's a little less, so the area
calculation of the wheel area will be a bit low. So the surface area of
the metal inside the tire is around 15*8.5*pi = 400 sq in, or about 5
times the sidewall area.

Holy cow, major brain fart. It's too late for deep thought. The area
of the sidewalls is 2*((13.75*13.75) - (7.5*7.5))*pi = 834 sq in. So
that's around twice the wheel surface area. However, those tires work
out to be a profile of around 50. Other types of racing, like F1, have
much lower profile tires so they have a much greater wheel area vs.
sidewall area.

In terms of profile, if you have a profile of 50 you'll have twice the
sidewall area as wheel area. A profile of 33 will give you about the
same sidewall area as wheel area inside the tire.

The funny thing is, that's the way I thought about it first but when the
math didn't work out, I didn't post it. The reason the math didn't work
out is that I was using the formula for circumference of a circle
instead of area of a circle to figure the sidewall area. Sheesh!

Steve