What are Aircraft Flaps or High Lift Devices? Flap Types

Aircraft high lift devices are commonly known as aircraft flaps. Aircraft high lift devices are crucial components designed to enhance lift during takeoff and landing, allowing aircraft to operate safely at lower speeds and shorter runway lengths. These devices alter the aerodynamic characteristics of the wings, increasing lift and improving the aircraft’s ability to generate sufficient lift at low speeds.

High lift devices are designed to make aircraft wings generate more lift. They can be classified as either active or passive. Active high-lift devices need extra power, usually from the engine, and are mostly used in experimental settings. Passive high-lift devices, on the other hand, are commonly used. They are movable surfaces attached to the edges of the wings, either at the front or the back. When these devices are deployed, they change the shape of the wing, making it more curved and increasing its effective size. This helps the aircraft fly at slower speeds during takeoff and landing. Some high-lift devices also help control airflow over the wing’s surface, further improving performance.

Necessity of high lift devices

The necessity of high-lift devices in aircraft stems from the limitations imposed by the maximum coefficient of lift (CLmax). Here’s why they are crucial:

1. Stall Speed Consideration: In horizontal flight, the lift force must balance the weight of the aircraft. However, there’s a limit to how much lift can be generated, determined by CLmax. This implies that there’s a minimum speed, known as the stall speed (VS), below which the aircraft cannot fly safely. Increasing the wing area (Sw) and CLmax allows the aircraft to fly at lower airspeeds, reducing the stall speed.
2. Increased Drag: High-lift devices, such as flaps, also increase the aircraft’s drag coefficient. This is because they disrupt the airflow over the wing, leading to higher induced drag. Additionally, some devices increase the wing’s planform area, further contributing to parasitic drag. Despite increasing drag, high-lift devices play a crucial role in improving aircraft performance.
3. Shortened Takeoff and Landing Distances: By decreasing the operating speed and increasing drag, high-lift devices help reduce the distance required for takeoff and landing. They achieve this by lowering the velocity needed for the aircraft to generate enough lift to take off or land safely. This is essential for operations at airports with limited runway lengths or in challenging terrain.
4. Improved Climb Rate: During the initial phase of climb after takeoff, high-lift devices enhance the aircraft’s rate of climb. This allows the aircraft to ascend more rapidly, enabling it to clear obstacles and attain cruising altitude more efficiently.
5. Enhanced Landing Performance: When landing, high-lift devices help decrease the aircraft’s impact velocity and assist in braking. By reducing the speed at which the aircraft touches down, they contribute to smoother landings and improve overall safety during the landing phase.

In summary, high-lift devices are indispensable for optimizing aircraft performance during critical phases of flight, including takeoff, climb, and landing. Despite increasing drag, their ability to lower stall speeds and improve maneuverability makes them essential components of modern aircraft design.

High Lift Devices or Flaps Types

Types of high-lift devices, commonly known as flaps, utilize various principles to enhance aircraft performance during critical phases of flight. Here are some common types:

1. Plain Flaps: Simplest type of flap used in light aircraft. These flaps allow the trailing edge of the wing to rotate around an axis, increasing the camber of the airfoil and consequently, the lift coefficient.
2. Slotted Flaps: Similar to plain flaps but include a slot that allows communication between the upper and lower surfaces of the wing. This prevents the boundary layer from separating, enabling higher deflection angles without airflow disruption.
3. Fowler Flaps: Combines camber increase with chord extension, resulting in increased wet surface area and lift coefficient. This design enhances the slope of the lift curve, improving overall lift performance. Variations include double and triple slotted Fowler flaps, which further control the boundary layer.
4. Split Flaps: Also known as intrados flaps, these devices are positioned on the underside of the wing. They provide additional lift with increased drag compared to other flap types, but with less torque.

Leading Edge High-Lift Devices:

1. Slots: Slots are openings in the leading edge of the wing that facilitate airflow communication between the upper and lower surfaces. By preventing boundary layer separation, slots enhance lift generation at lower speeds.
2. Leading Edge Drop Flaps: Similar to plain flaps but positioned on the leading edge of the wing. These flaps increase camber, leading to improved lift performance during takeoff and landing.
3. Krueger Flaps: Krueger flaps modify the camber of the wing’s leading edge while also controlling the boundary layer. They are effective at lower speeds and contribute to increased lift during critical flight phases.

These high-lift devices are essential for optimizing aircraft performance during takeoff, landing, and other critical flight maneuvers. By increasing lift and controlling airflow, they enable safer and more efficient operations, particularly in challenging conditions or at airports with limited runway lengths.

Increase in C_Lmax

The increase in the maximum coefficient of lift (CLmax) of a wing can be determined based on the increase in the maximum coefficient of lift of an airfoil (∆clmax). This relationship is influenced by factors such as the type of flap used and the wing’s geometry, particularly its sweep angle (Λ1/4). Here are the typical values for this increase:

For slotted and Fowler flaps:

For plain flaps:

In these equations:

• ΔCLmax represents the increase in maximum coefficient of lift of the wing.
• Δclmax is the increase in maximum coefficient of lift of an airfoil.
• Sfw​ is the surface area of the wing between the two extremes of the flap.
• Sw​ is the total wing surface area.
• Λ1/4 is the sweep angle measured from the locus of the c/4 of all airfoils.

Below are typical values of CLmax and flap deflections in different configurations:

These values provide insights into the relationship between flap configurations and the resulting increase in maximum lift coefficient, which is crucial for optimizing aircraft performance during critical flight phases.