Re: 777 rejected takeoff test

Date:         01 Nov 97 18:55:53 
From:         "Udayan V. Bhapkar" <ubhapkar@erols.com>
Organization: Erol's Internet Services
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SHW wrote:
>
> wb8foz@netcom.com (David Lesher) wrote:
> >I watched the PBS series on the 777. Of special interest was the
> >full-load rejected takeoff test. I missed the figures for the
> >weight [and ISTM they were for load, vice emptyweight+load]
> >but they said it was 9.7 million ft-lbs of hot brakes.
>
> I also saw the special and had a question, not about the units but the
> order of magnitude.  The 707 has a limiting brake energy of 40 Million
> foot-lbs per brake.  What doesn't make sense here is that the Trip-7
> is roughly twice the weight of the 70.  With modern carbon-fiber brake
> rotors and an extra set of wheels per truck how could the failure
> limit on the 777 be so low ?

I noticed that also, and believe it's a typo.  For the 777, taking mass
= 288,000 kg and V2 = 210 mph = 94 m/s, the amount of kinetic energy
that must be dissipated is E = m/2 * v^2 = 1.27 billion Joules = 940
million foot-lbs. total energy.  The actual energy dissipated in the
brakes may be a bit less than this, due to assistance from aerodynamic
drag.  Compare to the 707, which would dissipate 320 million foot-lbs.
with two four-wheel main landing gear and brakes.

Udayan