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I'm curious if there is ever a purpose to use full flaps like that? Was it a bit of an overkill?

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The MD-11 has one of the highest approach speeds, which is comparable to the 747-400 (Boeing), while having higher wing loading: 806 vs. 756 kg/m$^2$. In other words, the MD-11's wing is small, so flaps 50 is not an overkill.

The standard MD-11 landing flaps are either 35 or 50 depending on the weight, altitude, available stopping distance, etc.

Aircraft manufacturers don't usually go for overkill items. The bigger the flaps deflection the more loads and stresses there will be, and the stronger and heavier the flaps and wing support need to be made.

Further, the MD-11's flaps arrangement is covered in a NASA study, from which:

Because of high wing sweep, the slant angle on the inboard closure rib of the outboard flap is about 30°. When the flaps deploy, the slanted inboard rib exposes a forward-facing surface. The same out-of-streamline angle applies to the aft portion of the hinge fairings when the flap is deployed. (See fig. 1.24.) Both of these features degrade flap lift performance and cause drag. [emphasis added]


If McDonnell Douglas had went for an all-new wing for the MD-11, and not just adapt the DC-10's, it would have doubled the development cost, see: How much does it cost to design a jetliner's wing?

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