Arvid,

I think you are talking about slotted and plain flaps...

Slotted flaps (which have a "gap") produce more lift than plain flaps (which don't).

You also get Fowler flaps which are like slotted flaps except they bodily slide back on guides and generally have less angle of deflection than slotted flaps.

check this out

http://www.centennialofflight.gov/es...es/Tech6G3.htm

http://www.centennialofflight.gov/es...ices/Tech6.htm

Flaps inrease lift co-efficient, increase drag, and reduce stalling angle of attack they effectively increase alpha by inreasing the camber of a wing.

Without them 747's would need massively more powerful engines, longer runways, and no cargo or passengers to take off!.