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I mean why using Krueger flaps in some aircraft instead of using slats all over the leading edge?

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There are different types of Krueger flaps used, with increased efficiency coming at the cost of increased complexity.

  • The simple ones are basically flat panels that folds out from the wing lower surface into a very steep, deployed position and seals against the wing leading edge; though these are extremely simple, they don't offer much in the way of stall protection.

  • The next type has large folding 'bull' nose that offers increased performance at the cost of slightly increased complexity. This type of flaps is used in B727, B737, and inboard of B747, as shown in the figure below:

b747

By Arpingstone - Photographed by Adrian Pingstone in August 2002 and released to the public domain., Public Domain, https://commons.wikimedia.org/w/index.php?curid=118310

  • The most complex type of Krueger flaps are the variable camber type, which has a flexible panel (which forms a slot at the trailing edge) and a bull nose. This provides a better $C_{L_{max}}$ performance compared to a slat, but gives poorer takeoff L/D performance compared to the slat. However, the main problem is that the mechanism is exceptionally complex, as can be seen in the figure below. As a result not many aircraft use these.

Krueger flap

Image from aviation.stackexchange

This means that simpler Krueger flaps are used in the inboard with slats in the outboard region, so that the flow remains attached in the outboard regions at higher angles of attack compared to the inboard region.

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Many large airliners, particularly Boeings, use Krueger flaps closer in to the fuselage and slats further out on the wing. This is primarily because slats resist a stall more effectively, so by placing Krueger flaps on the forward parts on a swept wing and slats on the rearward sections, it encourages a more balanced, controlled landing stall. Another reason to use Krueger flaps is that they are lighter, both with the equipment itself and with its actuators, and can be used on thinner wings that are too thin for slats.

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