Effect of Flaps on B737

[Tomaski]

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Excerpt:

Leading-edge devices did not come into vogue, however, until the advent of swept-wing, turbojet airplanes. Such wings have a particularly nasty trait: the wingtip stall. Instead of the stall's occurring near the wing root and propagating outboard (as with straight wings), the stall of a swept wing begins near the wingtip. This can result in strong nose-up pitching moments, sharp and uncommanded roll rates, and reduced aileron authority. Aircraft designers obviously go to great length to preclude the possibility of such a stall.

One solution is to install slats or — as in the case of the Boeing 727 — a combination of slats and leading-edge flaps. It is natural to wonder why Boeing uses inboard flaps and outboard slats on the "three-holer." The answer is both simple and clever. Because the leading-edge flap is less effective in delaying a stall than a slat, the inboard section of the wing is forced to stall first, thus preventing the possibility of a tip stall.