High-Lift Devices

8.1 Purpose of High-Lift Devices 176176

• High-lift devices are fitted to aircraft to reduce takeoff and landing distances by increasing
• the wing's maximum lift coefficient (Cʟₘₐₓ). 177177177177This allows the aircraft to fly at lower
• speeds, which is essential for safe takeoffs and landings on reasonably short runways.

8.2 Trailing Edge Flaps

• Flaps are hinged portions of the wing's trailing edge that deflect downwards to increase the
• wing's effective camber and, in some cases, its area.
•  Plain Flap: A simple hinged section of the trailing edge. It provides a good increase in Cʟₘₐₓ.
•  Split Flap: Hinged from the lower surface only, leaving the upper surface contour
• unaffected. It produces about the same lift increase as a plain flap but with more drag.
•  Slotted Flap: When lowered, a slot opens between the wing and the flap. This slot directs
• high-pressure air from the lower surface over the flap, re-energizing the boundary layer and
• delaying flow separation. This provides a greater increase in Cʟₘₐₓ with less drag than a plain
• r split flap.
•  Fowler Flap: This flap moves rearwards on tracks to increase the wing area before deflecting
• downwards to increase camber. It gives the greatest increase in lift and is often multi-slotted
• n large transport aircraft.

8.3 Effects of Flaps

•  Cʟₘₐₓ and Stall Angle: Lowering flaps increases Cʟₘₐₓ but reduces the stalling angle of
• attack.
•  Drag: Flaps significantly increase drag, especially at large deflection angles. Split flaps
• produce the most drag, while Fowler flaps produce the least for a given lift increase. This
• high drag is beneficial for landing as it allows for a steeper approach path. 186186186186
•  Lift/Drag Ratio: Since flaps increase drag more than they increase lift proportionally, they
• reduce the maximum L/D ratio. This reduces the aircraft's climb angle and glide range.
•  Pitching Moment: Lowering flaps typically moves the center of pressure rearwards, creating
• a nose-down pitching moment. However, the increased downwash over the tailplane creates a
• nose-up pitching moment. The overall pitch change depends on which effect is dominant for
• a particular aircraft design. 188188188188

8.4 Leading Edge High-Lift Devices

• These devices are used on wings with sharp leading edges (common on high-speed aircraft)
• to delay flow separation at high angles of attack.
•  Leading Edge Flaps: These are simple hinged sections of the leading edge that droop
• downwards to increase the leading edge camber.
• Krueger Flaps: A type of leading edge flap that hinges forward from the lower surface of the
• wing. They are often used on the inboard section of swept wings to promote a root stall.
•  Slats and Slots:
• A slot is a fixed gap in the leading edge that allows high-pressure air from below the wing to
• flow to the upper surface, re-energizing the boundary layer.
• A slat is a movable aerofoil section that forms part of the leading edge. When deployed, it
• moves forward and down, opening a slot. This is the most effective type of leading edge
• device, significantly increasing the stalling angle and Cʟₘₐₓ.