Re: Wing "flaps" that raise on landing

From: (Robert Dorsett)
Organization: Capital Area Central Texas UNIX Society, Austin, Tx
Date:         01 Feb 93 14:28:48 PST
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In article <airliners.1993.118@ohare.Chicago.COM> you write:
>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?

Braking is accomplished by two factors, aided by a third:
	1.  Wheel brakes.
	2.  Thrust reversers.
	3.  Spoilers.

Brakes provide the certification standard for determining landing distance
necessary to stop the airplane.  Brakes incorporate anti-skid, to help
alleviate hydro-planing.  Most modern types have automatic braking systems,
with "intensity" scales, such that the brakes can start working without
pilot interaction (but the pilot can increase braking pressure if necessary).

Thrust reversers are gravy: they deflect engine exhaust at some forward angle,
thus applying "reverse thrust."  There are two types of thrust reversers:
cascade and clamshell.  The type used depends on the geometry of the engine 
and its nacelle: smaller engines use clamshells; large fans tend to prefer 
cascade reversers.  There are exceptions in both directions.

Spoilers "spoil" lift: these are small panels that raise on the top of the
wing.  There are two types of spoilers: ground spoilers and flight spoilers.
Flight spoilers are used for roll augmentation in-flight; they are normally
closer to the fuselage than ground spoilers.  They may also be used as in-
air "speed-brakes."  Ground spoilers are the flight spoilers + panels on the
outer sufraces.  By spoiling lift, they increase the weight of the airplane
on the runway, thus improving braking effectiveness via the wheel brakes.  

Flaps are *not* normally raised during the landing roll.  It takes over
20 seconds to go from Flaps 15 to flaps-up in a 727: by then, it's all over.
It is a valid contention that IF the flaps were raised, it could increase
braking effectiveness, but it isn't necessary--the ground spoilers do it all
for you.  Additional considerations are the sudden demand on the hydraulic
system, plus the additional workload in the cockpit--landings are very much
"heads up" procedures, and workload has to be minimized.

Robert Dorsett!!rdd