From: "Marvin Kaye" <marv@lancaironline.net>
Subject: Re: [LML] kinetic energy
To: lml
Date: Fri, 14 Nov 2003 20:10:26 -0500
Message-ID: <web-2732231@logan.com>
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Posted for "Mark Ravinski" <mjrav@comcast.net>:

Impressive engineering.
But, losses in speed due to aerodynamic drag should not be overlooked.
I normally let my 360 roll out the full length of the home field 3500 ft.
runway without braking until the last 1000 or so.  I'd guess my speed to be
only half of that 80 kts you used when I start braking.(depending on winds)

Mark Ravinski


> All that doesn't mean anything much, so let's compute the temperature of
the
> brake rotor after a maximum-energy stop.  For a 3400 lbm aircraft stopping
> from 80 kts the kinetic energy to be absorbed by the brakes is almost
> 474,000 ft-lb, or 609 BTU's (some, but not a lot of energy is absorbed by
> aerodynamic drag and tire losses, while the residual thrust is working the
> other way).  I don't know what a brake rotor weighs, but I'll guess 5
> pounds.  One thing I couldn't find is the specific heat of cast iron, but
> most steel alloys have a specific heat of between 0.1 and 0.14, so I'll
pick
> 0.12.  The temperature rise during a braking event is then
BTU/(sp.ht.Xlbm),
> or about 1,000 degrees F.  The brake temperature at the beginning could be
> 100, but if you drag the brakes a lot during taxi, heating them up to
maybe
> 500 and then do a rejected takeoff, you could see 1500 degrees.  The brake
> pad material will probably continue to work at 1,000 degrees, but probably
> not at 1,500.  You might think that some of the heat gets absorbed in the
> caliper and wheel, but it is not so - almost all the heat goes directly
into
> the rotor and very little is rejected into the air during that short time.
> Metallic linings will keep working at higher temperatures than
conventional
> organics so that would help.