Spoilers - Was Re: Wing "flaps" that raise on landing

From:         megazone@obsidian.WPI.EDU (MegaZone)
Organization: WPI Discordian Society, Undocumented Cabal
Date:         01 Feb 93 14:28:45 PST
References:   1
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In article <airliners.1993.118@ohare.Chicago.COM> pshuang@athena.mit.edu (Ping Huang) writes:
>When a jetliner lands, usually flaps on the wings are raised. Is the
>primary braking effect from the additional air resistance, or from the
>fact that the airplane is also pushed downward and therefore the brakes
>on the landing wheels exert more friction against the runway?

Those 'flaps' are really called spoilers. Flaps are the large trailing
edge devices that extend prior to takeoff and landing to increase wing
area/camber...

Anyway, the spoilers extend up into the airflow 'spoiling' the lift effect
of the wing by stalling it, hence the name. This cuts any of the lift the
wings were still producing and therefore transfers all the weight to the
landing gear. Since braking power depends on the Normal (perpendicular)
force * the coefficient of friction this increases the max braking available.
(Normal can be considered to be the Force of gravity minus any lift for
simplicity's sake)

Some braking is gained from air resistance, ala air brakes, but this isn't
the main purpose of the spoilers.

Spoilers are also used on some aircraft (B-52 for example, esp. late models)
as part of, or all or, the roll control system. By spoiling a bit of the 
lift on one side you induce roll into that direction.

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