High Lift Devices And Their Purpose

High Lift devices - AviatorsBuzz
(Source: aircraftcompare.com)

High lift devices are movable surfaces or, in some cases, stationary components that are designed to increase lift during some phases or conditions of the flight.

The most common high lift devices are flaps, slats and Krueger flaps, but the category also includes less common installations such as lead-edge root extensions.

Main Purpose of using High Lift devices

  • To allow a steeper approach without increasing the speed of the aircraft
  • Reduces the distance required to take-off and land
  • Reduces the speed at which the aircraft will stall
  • They increase the pilot’s visibility of the runway

There are two types of high lift devices classified on the basis of their position on the wing:

Trailing Edge Devices Leading Edge Devices
Plain flaps Krueger flaps
Split flaps Slats
Slotted flaps
Fowler flaps


Flap - AviatorsBuzz
(Source: Flight –mechanic .com)
Krueger Flap - AviatorsBuzz
(Source: wikipedia)

Effects of using Flaps

  • Relation with Centre of pressure:
    • Flap movement, up or down, will cause a change of pitching moment. This is due to Centre of Pressure (CP) movement.
    • Centre of pressure is a point where all the lift of the aircraft is considered to be concentrated.
  • Relation with lift and drag:
    • Lowering flap increases both lift and drag, but not in the same proportion. Although the lift is a larger force, and proportional increase in the drag is greater, so the maximum lift/ drag ratio decreases.
What is a SLAT?

Slats are extendable, high lift devices on the leading edge of the wings of some fixed wing aircrafts. Their purpose is to increase lift during low speed operations such as takeoff, initial climb, approach and landing. They accomplish this by increasing both the surface area and the camber of the wing by deploying outwards and forming a slot which allows passage of air from the high pressure region below the wing to the low pressure region above it.

Fixed & Automatic Slot - AviatorsBuzz
What should be the sequence of operations of these High Lift devices?

Leading edge devices re-energise the boundary layer and delay the aircraft from stalling. Whereas, deploying flaps would increase drag in very high amounts.

Hence, it is advisable to always deploy slats and then flaps and do the opposite while retraction, so that the boundary layer remains energised for a longer period of time and there are low chances of stalling.  

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