For DC9/MD-80, when aircraft take off, the leading slat is mid extent
(sealed),and flap usually 11 or 15 degrees; during landing, slat is
full extended(unsealed) and flap is set to 40 degrees.

My question is: why slat is sealed when taking off and unsealed duing
landing?

thanks!

During takeoff you want maximum lift, and minimum drag, as the
aircraft tends to be  quite heavy, and at high weight, MD80's and D9's
often didn't have especially good short field characteristics. Put as
much of the engine performance as you can into lift, and as little as
possible into drag. You want to maximize L/D. Keep the open spaces
between the control surface to an absolute minimum for minimum drag.

On landing, the aircraft is much lighter,  the unsealing causes a
signficiant increase in drag. Increasing the drag allows you to
increase the rate of descent, and to bleed off excess speed more
easily. The full extension of the leading slats provides more lift,
but with a hefty drag penalty, reducing L/D. On approach and after
touchdown, the hefty drag penalty tends to be a useful.

Boeing is in fact in the process of sealing many control surfaces and
altering the Kreuger flaps schedule on the 737-900ER to improve the
short field performance! Since CFM isn't building a bigger engine for
the -900ER, but MGTOW is going up about 13,000 pounds, there is a need
to improve the takeoff performance.

sleek,
thin, black hair was stuck eagerly forward, and all his motions
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quivering lip and dilated
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^^^^^^^^ -?
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