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High-Lift Airfoils and Flap Systems in Aircraft

Key Takeaways

  • High-lift airfoils play a key role in creating the high lift-to-drag ratios essential for high-speed cruises.

  • Usually, in high-lift airfoils, the wings are equipped with a flap system to produce the maximum possible lift, reducing landing speed, producing high lift-to-drag ratio, and reducing weight and mechanical complexity. 

  • The required lift coefficient is achieved with the help of flaps and slots in wings without compromising aerodynamic efficiency or performance. 

Airfoils

 High-lift airfoils play a key role in creating the high lift-to-drag ratios essential for high-speed cruises

Aircraft is one of the most productive transportation forms humans have created so far. In aircraft, there are so many parts that are essential for safe flight. How far and fast an airplane can fly depends on these aircraft components.

The performance, control, and stability of an aerial vehicle are influenced by airfoil components. For example, airfoils are key to creating lift. The angle of attack, speed, and other flight conditions must also be considered when determining whether an airfoil performs as a high-lift airfoil or a low-lift airfoil. In this article, we will focus our discussion on high-lift airfoils.

High-Lift Airfoils

In aircraft manufacturing, a major goal is to reduce aircraft costs. One way of achieving this goal is to build simple, low-cost high-lift airfoils. As we all know, the four basic forces acting on an airplane are lift, drag, thrust, and weight. High-lift airfoils play an ineffable part in creating the high lift-to-drag ratio essential for high-speed cruises. The lifting coefficient is a metric used to relate the lift generated to the density of the fluid around the body, reference area, and fluid velocity. It is a dimensionless parameter. The maximum lift coefficient required at takeoff is around 2.6, which is much more than an efficient high-speed airfoil can supply. Designs require high-lift airfoils that enhance the lifting force at low speeds without affecting the high-speed aerodynamic efficiency.

The objective of a high-lift airfoil is to create maximum lifting force. High-lift airfoils are responsible for creating high-lifting force. The lifting force created is dependent not only on the shape of the airfoil, but also influenced by the angle of the airfoil to the flow, its Reynolds number, and Mach number. 

Benefits of High-Lift Airfoils

High-lift airfoil aerodynamics not only save money but also:

  • Increase payloads
  • Shorten landing and takeoff distances
  • Lower stall speeds 
  • Reduce aircraft noise 
  • Reduce fuel consumption during takeoff and climb

High-Lifting Principles

High-lift airfoils and their associated components always pose the challenge of maintaining maximum lift with minimum flow separation. High-lift devices or airfoils work by following some of the principles given below:

  1. Increasing the wing area
  2. Increasing the airfoil camber
  3. Controlling the boundary layer by:
  • Removing the old boundary layer
  • Improving the pressure distribution
  • Feeding high-energy airflow to the boundary layer

The Flap System in High-Lift Airfoils

Usually, in high-lift airfoils, the wings are equipped with a flap system to produce the maximum possible lift, reducing landing speed, producing a high lift-to-drag ratio, and reducing weight and mechanical complexity. 

Trailing-Edge High-Lift System

  1. Plain flap - Plain flaps are formed as the pivoted rear section of an airfoil. The principle used in a plain flap is the increase in airfoil camber. Flap length equals around 30% of the chord. 

  2. Split flap - Split flaps increase the lift as same as plain flaps. However, they are not used, as they produce drag. 

  3. Slotted flap - Slotted flaps consist of a gap between the wing and the flap. The air flowing from the bottom to the top of the airfoil through the slot produces a new boundary layer on the flap top surface. The flap angle goes up to 40° without flow separation and gives maximum lift and reduced drag.

  4. Fowler flap - Fowler flaps are just like slotted flaps. The first extension of a fowler flap to the rear uses the principle of increasing wing area and enhancing lift. In the further extension, the fowler flap increases the airfoil camber, thereby bringing improved lift and drag.

Leading Edge High-Lift System

  1. Leading edge slot - In a leading edge high-lift system, a leading edge slot exists between the leading edge and the wind. 

  2. Leading edge flap - The leading edge flap increases the coefficient of lift by increasing the curvature of the top of the airfoil.

  3. Slotted leading edge flap - The principle of increasing wing area and increasing camber are employed in slotted leading edge flaps, and this influences the flow.

  4. Krueger flap - Krueger flaps are high-lift devices that are fitted to all or parts of the leading edge of the wings. 

High-lift airfoils are complemented by trailing and leading edge flap systems. The required lift coefficient is achieved with the help of these flaps and slots in the wings, without compromising aerodynamic efficiency or performance.

The complete set of CFD simulation software from Cadence can help designers with high-lift airfoil design. Cadence’s CFD simulation tools offer customizable workflows for high-fidelity designs, which are important in the designing of aircraft.

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