Among the range of mechanical, electronic, and aerodynamic machinery that make up the engineering creation that is an airplane, there´s one type known as high-lift device. In this blog we´ve already written about the concept of “lift”, one of the forces that keep an airplane in the air. And life has become the great enigma that flying machine designers have had to unravel since the earliest times.
Well, there are some phases of flight in which the aircraft needs extra lift. By increasing the coefficient of lift, the airplane will be able to fly at lower speeds than cruising speed, something essential, for example, during takeoff, initial climb, approach or landing. So how can lift be increased? Fairly easily with hight-lift devices added added to the aircraft which are activated during these phases but deactivated and sometimes retracted when the aircraft is in the cruise phase. Some devices are more common than others; here´s a rundown of the different types:
Passive High-Lift Devices
This is the type that increases the lift of the aircraft by modifying the geometry of the wings or blades. This can be done by varying their curvature or their surface. Passive high lift can also be achieved by creating gaps in the wing profile that allow the airflow to be controlled. There are mainly two of this type:
Flap
These are located at the trailing edge of the wing, and you may have seen them in action during takeoff or landing; they´re a kind of panel that opens like a fan, and is deployed or retracted according to the needs of the flight. Not only do they increase lift and resistance in takeoff or landing maneuvers but they are also very useful instruments when these maneuvers involve a shorter runway. Flaps increase the coefficient of lift by increasing the surface area of the wing or increasing the coefficient of lift of the profile and, although they are usually located on the trailing edge (that is, at the rear of the wing), they can also be installed on the leading edge. The flaps expand when pilots need to reduce the speed of the plane and they retract when cruising speed is reached.
Trailing edge flaps (the most common) usually have two or three sets on each side, and have several successive planes (three or four) that extend in a staircase-like fashion, downwards and symmetrically, and that allow air to pass between them through successive slots. How do they increase the lift and drag of the aircraft? This modification affects not only the curvature of the wing profile, but also its surface and angle. The four most common types of flaps are:
Simple: They are mainly used in light aircraft.
Split: The upper part of the wing separates from the lower part when the flaps are deployed.
Slotted: An evolution of the previous ones, which leaves a slot between the wing and the flap when opened; thus, the passage of air flow provides extra lift.
Fowler: The most commonly used in large commercial aircraft, they significantly increase both the curvature and the wing surface.
Slats
Like flaps, slats are elements that modify the profile of the wing to reduce the speed of the aircraft, and like flaps are deactivated when the maximum speed is reached. They are located in the front part of the wing (leading edge), and are more common in large aircraft than in lighter ones.
Active High-Lift Devices
These increase the coefficient of lift by introducing energy into the fluid in a certain way. Unlike their passive counterparts, active devices try to modify the boundary layer of a fluid (that is, the area in which the fluid is disturbed by the presence of a solid, in this case the aircraft). There are mainly four:
Blown flap
These flaps take in air injected from the compressor, which passes under pressure through ducts to the slot in the flaps. In this way the kinetic energy (that possessed by a moving body) increases and generates a favourable aerodynamic environment that prevents a detachment in the aforementioned boundary layer.
Leading-Edge Roller
The leading edge is the part of the wing closest to the aircraft at the front. And a cylinder can be installed there which rotates on its axis in a clockwise direction. What this hyper-lift device achieves is to artificially accelerate the air that flows over the top of the wing or extrados and slow down the speed of the air that passes over the bottom or extrados. What is achieved, in short, is to significantly increase the coefficient of lift of the wing.