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Photo © Alexander Y.A. Kueh
Slants as you can see from the pic above, are basically inverted flaps. Instead of being at the trailing end of the wings, its on the front. Their jobs are the same as flaps; to expand the surface area of the wing so that air has a larger area to act on, thus increasing lift. On landing, they can be used, like flaps, to increase drag, so that the plane slows down without having to decrease engine power or applying speed brakes. Flaps and slants have a max operating speed. Deployment above that speed (indicated air speed) can cause structual damage.
Depending on the model of aircraft, slants can be controlled independent of flaps. Onc control for flaps and one for slants, both located next to each other. On newer planes, slants are incooperated with flaps. Im not sure but I think slants are 50% of flaps or somthing like that. (ex 40% flaps= 20% slants) Moving the flap lever also deploys slants. There might be an override but Im not sure. Smaller aircraft, older aircraft usually dont have slants, just flaps. There are also many types of flaps. From slimple single ones that just drop down to complex
tri-slot ones used on large jetliners.