Some US planes(P40) use the split flap, where the lower half of the wing drops down increasing wing camber and thereby increasing lift.

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Most use the conventional flap(51), where the wing trailing edge surface deflects down incrasing wing camber and lift.

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P38 uses fowler flaps, where the flaps dont just move down but OUT and DOWN, increasing both wing camber and wing area.

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The difference is that for every pound of lift generated they produce a lot less drag than the conventional flaps. The split flaps are the worst in this respect btw.
This has allowed me at time to pull off landing like this one........

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