when the flap is lowered, a duct forms between the flap well  in the wing and the leading edge of the flap. When the slotted  flap is lowered, high energy air from the lower surface is  ducted to the flap’s upper surface. The high energy air from  the slot accelerates the upper surface boundary layer and  delays airflow separation, providing a higher C$_{L}$. Thus, the  slotted flap produces much greater increases in maximum  coefficient of lift (C$_{L-MAX}$) than the plain or split flap. While  there are many types of slotted flaps, large aircraft often  have double- and even triple-slotted flaps. These allow the  maximum increase in drag without the airflow over the flaps  separating and destroying the lift they produce.  Fowler flaps are a type of slotted flap. This flap design not  only changes the camber of the wing, it also increases the  wing area. Instead of rotating down on a hinge, it slides  backwards on tracks. In the first portion of its extension, it  increases the drag very little, but increases the lift a great  deal as it increases both the area and camber. Pilots should  be aware that flap extension may cause a nose-up or down  pitching moment, depending on the type of aircraft, which  the pilot will need to compensate for, usually with a trim  adjustment. As the extension continues, the flap deflects  downward. During the last portion of its travel, the flap  increases the drag with little additional increase in lift.  High-lift devices also can be applied to the leading edge of  the airfoil. The most common types are fixed slots, movable  slats, leading edge flaps, and cuffs.  [Figure 6-18]  Fixed slots direct airflow to the upper wing surface and delay  airflow separation at higher angles of attack. The slot does not  Leading Edge Devices  Figure 6-17.  Five common types of flaps.  Figure 6-18.  Leading edge high lift devices.  6-9  Plain flap  Basic section  Split flap  Slotted flap  Fowler flap  Slotted Fowler flap  Leading edge flap  Movable slot  Fixed slot  Leading edge cuff 