Re: Braking

Date:         17 May 97 15:15:47 
From:         Pete Mellor <pm@csr.city.ac.uk>
References:   1 2
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On Fri May  9 13:37:31 1997, ctillier@leland.Stanford.EDU
(Clemens Emanuel Tillier) wrote:-

> Correct me if I'm wrong, but isn't the whole point of spoilers ("air
> brakes") to dump the lift and get the weight on the wheels as quickly as
> possible, such that the wheels can brake effectively?

Yes. (At least that is my understanding from the various sources
that I looked at when boning up on the background to the Warsaw
A320 crash.) The spoiler flaps on the top of the wings are referred
to as "ground spoilers" when used in this way. About 45 degrees of
extension is used to dump lift. In flight, the same surfaces *are*
used to increase drag and reduce air-speed, and are then referred
to as "air brakes". For this purpose they are not extended so far.
(I think not beyond 15 degrees, if that.)

The above applies to most civil airliners. On some models of
aircraft (I think mainly military) different control surfaces are
used as air brakes and ground spoilers.

> As I understand it
> the extra drag from spoilers wouldn't slow the aircraft much, compared to
> wheel brakes.  Could someone confirm/deny?

The extra drag would have a miniscule effect in reducing
the ground speed.

> (And while we're at it, what fraction of the braking (energy-wise) can be
> done by thrust reversers?)

Again, from the information I gathered in relation to the
Warsaw crash, about 20%.

Disclaimer: I am not an aeronautical engineer, and stand to be
corrected. Apologies to anyone who was one of the sources of
my information (Clive? Karl?) whom I have not specifically
acknowledged.

Peter Mellor, Centre for Software Reliability, City University, Northampton
Square, London EC1V 0HB, UK. Tel: +44 (171) 477-8422, Fax: +44 (171) 477-8585
E-mail: p.mellor@csr.city.ac.uk
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