The importance of flaps in an aircraft – why does the wing change shape?

🕗 6 minutes | August 19, 2024 | Text Mateusz Łapuć

If you’ve ever observed an aircraft’s wing before takeoff or landing, you may have noticed its shape changing—the trailing edge extends and retracts. These are flaps, located on the edges of the aircraft’s wings, and they serve a crucial function. What exactly do they do? Read on to find out!

Aircraft flaps are a key element of wing mechanics, allowing the wing’s shape to be adjusted to optimize flight performance. Depending on the phase of flight, they can either increase lift or increase drag to slow the aircraft down.

Flaps are especially important during takeoff and landing, when an aircraft needs additional lift at lower speeds. Their deployment significantly affects the aerodynamic properties of the wing, ensuring safe and efficient flight operations.

When flaps are extended, they change the shape of the wing profile and increase its surface area. This modification raises the angle of attack, causing the air above the wing to move faster than the air below, creating a greater pressure difference.

This increased pressure difference generates more lift, which is essential for keeping the aircraft in the air at lower speeds, such as during takeoff or landing. In simple terms: when flaps are deployed, the wing becomes more curved, leading to a higher lift coefficient.

Flaps play a critical role in ensuring safe and efficient flights. They allow aircraft to fly at lower speeds, reducing the minimum speed required for takeoff and landing.

Flap deflection is gradual and is measured in degrees:

• Takeoff – typically around 15 degrees
• Landing – typically around 30 degrees

A higher flap deflection angle increases aerodynamic drag, which helps reduce landing speeds but also affects the aircraft’s ability to accelerate.

Modern aviation technology has led to the development of several types of flaps, each designed for specific aircraft and purposes. Different flap designs enhance lift, reduce speed, or improve airflow efficiency. 

The simplest type of flap, hinged at the trailing edge of the wing. When extended, it increases the wing’s angle of attack and curvature, generating more lift. However, it also significantly increases drag, limiting its efficiency. Commonly used in small aircraft.

Similar to the plain flap, but the lower surface of the wing moves downward while the upper surface remains unchanged. This type of flap generates high drag and can act as an air brake, reducing speed without sacrificing lift.

A more advanced type of flap that creates a gap (slot) between the flap and the wing when extended. This allows air to flow through, preventing airflow separation and maintaining lift at higher angles of attack.

Three types of slotted flaps:

• Single-slot flap
• Double-slot flap
• Triple-slot flap (used in commercial airliners)

The most effective type of flap. Instead of just tilting downward, it extends rearward, significantly increasing the wing’s surface area. This results in a major lift boost while minimizing drag. Commonly used in large commercial aircraft.

Other specialized flaps include:

• Krueger flap – mounted on the leading edge of the wing to enhance lift.
• Junkers flap – an external, pivoting flap commonly found on older aircraft designs. 

Flap movement is precisely controlled using electro-hydraulic or electrical systems. These systems allow smooth operation and adjustment to the desired flap position.

Pilots use dedicated cockpit levers to deploy or retract the flaps at different stages of flight.

Another important wing modification system is leading-edge slats, also known as slots. These are located along the leading edge of the wing and, like flaps, improve the aerodynamic properties of the aircraft.

Slats delay airflow separation, allowing the aircraft to fly at lower speeds without stalling. In combination with flaps, they provide a significant boost in lift, reducing both takeoff and landing distances.