Types of Aircraft Flaps: Plain, Split, Slotted, Fowler Explained

Aircraft flaps are essential components of an airplane’s wing system, playing a crucial role in improving lift and controlling drag during different phases of flight. There are various types of flaps, each designed to serve specific purposes, such as enhancing lift at slower speeds or increasing drag during the landing phase. In this article, we’ll explore four primary types of aircraft flaps: Plain, Split, Slotted, and Fowler.

Plain flaps are the simplest and most basic design, hinged to the back of the wing and pivoting down when extended. While they do generate some additional lift, they are relatively limited in their capability. Split flaps, on the other hand, consist of two sections with the upper portion fixed and the lower part moving downward in a pivoting motion. This design provides additional drag, helping to slow the aircraft down during landings.

Slotted flaps and Fowler flaps are more complex in design, offering better performance in terms of lift enhancement. Slotted flaps are designed to allow smoother airflow over the wings, reducing the chance of separation and increasing lift. Fowler flaps, meanwhile, not only change angle but also extend outwards, generating both increased lift and higher drag. These advanced flap types are commonly found in modern aircraft, ensuring safe and efficient flight operations.

Fundamentals of Aircraft Flaps

Lift and Drag

Aircraft flaps are essential components of the wings that help produce lift, reduce speed, and control descent during landing. The primary purpose of flaps in aviation is to increase both the lift and drag of the aircraft. This is crucial during takeoff and landing when lower speeds and increased lift are required. By adjusting the angle of the flaps, the pilot can modify the lift and drag coefficient, which helps maintain the desired speed and angle of attack.

Wing Flaps

There are four main types of wing flaps, each with its unique characteristics and purposes. These include:

1. Plain Flaps: The most straightforward and basic flap type that pivots down when extended. They are somewhat limited in the amount of lift they can generate, but still useful for light aircraft. An example of an airplane with plain flaps is the Cessna 185 (source).
2. Split Flaps: These flaps consist of two sections, with the fixed upper part being an extension of the top trailing edge of the wing, while the lower split flap is movable and pivots down to generate drag (source).
3. Slotted Flaps: With slotted flaps, the air from the lower part of the wing flows upward, energizing the boundary layer and reducing the severity of flow separation (source).
4. Fowler Flaps: A more complex type of flap that helps increase the wing’s surface area and its chord, resulting in higher maximum lift. Fowler flaps are ideal for STOL (Short Takeoff and Landing) aircraft.

Camber and Curvature

The camber of the wing refers to its curvature, which affects the airflow around the wing and, consequently, the lift and drag produced. When flaps are deployed, they increase the wing’s camber, creating a more pronounced curvature and augmenting the lift generated.

By altering the wing’s curvature, flaps also affect the stall characteristics of the aircraft. Deploying flaps tends to lower the stall speed and angle of attack, providing the pilot with greater control over the aircraft during critical phases of flight, such as takeoff and landing.

Using various flap settings also helps optimize aerodynamics for different flight conditions, allowing for more efficient and versatile aircraft performance. Flaps play a crucial role in aviation by enabling greater control and maneuverability tailored to specific flight situations.

Different Types of Aircraft Flaps

Plain Flaps

Plain flaps are the most basic type of aircraft flaps. They are hinged to the wing’s trailing edge and pivot down when extended. These flaps help change the wing’s curvature when extended, increasing flow separation at the wing’s trailing edge and creating a larger wake and drag source. However, plain flaps create a fairly limited amount of lift due to the air losing energy as it moves over the wing source.

Split Flaps

Split flaps, unlike plain flaps, extend from the lower surface of the wing-trailing edge, creating a gap between the wing and the flap. This design leads to a more significant increase in lift, but also greater induced drag. Split flaps are commonly used for their simplicity and effectiveness in increasing lift source.

Slotted Flaps

Slotted flaps are designed to create a slot between the trailing edge of the parent aerofoil and the flap, allowing higher pressure air from the lower surface to move through the slot and reenergize the airflow over the upper flap. This design results in less airflow separation, making slotted flaps more efficient than plain or split flaps in generating lift source.

There are a variety of slotted flap designs, such as the single-slotted flap, which has one slot, allowing air to flow from below the wing to the flap’s upper surface. Other types of slotted flaps include SAP flaps, Junkers flaps, and Krueger flaps.

Fowler Flaps

Fowler flaps are one of the major types of aircraft flaps and are particularly versatile. When extended, Fowler flaps not only pivot downward but also slide backward, increasing the wing area and camber. This results in a significant increase in lift, with less drag compared to other flap types source.

Fowler flaps can be found in various configurations, such as double-slotted or triple-slotted flaps, which have multiple slots for improved efficiency. These flaps are commonly used in high-lift systems for aircraft that require short takeoff and landing distances.

Historical Context and Notable Aircraft

Northrop P-61 Black Widow

The Northrop P-61 Black Widow was a twin-engine night fighter used during World War II. It featured a trailing edge flap system that improved its performance at lower airspeeds. These flaps were essential for maintaining stable flight control during challenging nighttime missions.

Douglas DC-1

The Douglas DC-1 was an important commercial aircraft prototype in the early 1930s. It was designed with a mechanical simplicity in mind, which included the use of plain flaps on the trailing edge of the aircraft. These flaps were chosen primarily for their simplicity and effectiveness in controlling the aircraft at lower airspeeds.

Junkers Ju 52

Junkers Ju 52 was a German trimotor transport and passenger aircraft used in the 1930s and throughout World War II. It utilized Gouge flaps, which were an innovative high-lift device that increased the lift generated by the wings, allowing for better performance at slower airspeeds.

Junkers Ju 87 Stuka

The Junkers Ju 87 Stuka was a dive bomber used by the German Luftwaffe during World War II. Its distinctive feature was its slotted flaps located on the trailing edge of the aircraft. These flaps improved the handling characteristics at low speeds and during steep dives, making precision bombing more attainable.

Denney Kitfox

The Denney Kitfox is a series of light, homebuilt aircraft designed with mechanical simplicity in mind. These aircraft typically use plain flaps to provide effective flight control at lower airspeeds. The straight forward design of the Kitfox has contributed to its continued popularity among homebuilt aircraft enthusiasts.

Boeing 727

The Boeing 727 was a mid-size, narrow-body commercial airplane introduced in the 1960s. As one of the first swept-wing airliners, it used a combination of double-slotted and triple-slotted Fowler flaps on its wings’ trailing edges. These flaps were essential for allowing the 727 to achieve the required lift at slower speeds for takeoff and landing. The aircraft’s chief designer, Edward Zaparka, oversaw the engineering of these high-lift devices, which became a standard feature on many large jets that followed.

Flap Systems and Technologies

Tracks and Rails

Aircraft flaps are designed to adjust the wing’s shape and surface area, helping the airplane during takeoff, landing, and maneuvering. Aircraft flap systems mainly consist of plain, split, slotted, and Fowler flaps. These flaps are often guided and stabilized by tracks or rails, essential components that provide a smooth, controlled movement of the flaps during deployment and retraction.

Boundary Layer and Wake

The boundary layer is a thin layer of air that flows over an aircraft wing’s surface. It interacts with the wake, the turbulent flow of air behind the wing, and directly affects the wing’s aerodynamic properties. Aircraft flaps help in altering the boundary layer and wake, improving overall lift and control, especially during low-speed flight conditions like takeoff and landing.

Flow Separation and Stalling Speed

Flow separation occurs when the airflow detaches from the wing’s surface, resulting in a loss of lift and increased drag. Flaps help mitigate flow separation by changing the wing’s geometry, enhancing the lift and delaying the stalling speed of the aircraft. Notably, flaps like the slotted and Fowler type are highly effective in managing flow separation due to their advanced design, which allows better control of the air and pressure distribution over the wing.

High-Pressure Air

High-pressure air beneath the wing is crucial in creating lift. Effective flap systems help manage the high-pressure air, manipulating the pressure distribution to optimize lift and maintain an ideal aircraft performance. Fowler flaps, for example, extend rearwards and downwards, directing high-pressure air to improve both lift and drag for low-speed flight conditions.

Extension Settings

Each type of flap system allows for various extension settings, which are primarily based on the aircraft’s configurations during takeoff, landing, or other in-flight adjustments. These settings differ for plain, split, slotted, and Fowler flaps, with each design offering specific performance advantages. For instance, slotted Fowler flaps are known for their effectiveness in enhancing lift while maintaining low drag, making them a popular choice among modern aircraft.

Practical Applications and Scenarios

In this section, we will discuss the practical applications and scenarios for the different types of aircraft flaps, including Plain, Split, Slotted, and Fowler flaps. We will cover their effects on take-off and landing distances, climb rate and descent angle, crosswind landing and thunderstorm situations, and overall aircraft performance and efficiency.

Take-Off and Landing Distances

During take-off and landing, flaps play a crucial role in shortening ground roll and reducing stall speed. By increasing the lift coefficient, flaps allow the aircraft to take off or land at lower airspeeds. Plain flaps, while simple in design, are limited in the amount of extra lift they can provide. However, they are still useful for aircraft like the Cessna 185.

Split flaps, on the other hand, provide more significant drag and lift, making them more effective for reducing take-off and landing distances. Slotted and Fowler flaps, being more advanced types of high-lift devices, offer even greater lift, which translates to shorter take-off and landing distances.

Climb Rate and Descent Angle

The use of flaps also affects the climb rate and descent angle of the aircraft. When flaps are extended, the aircraft’s climb rate is decreased since the extra lift comes with increased drag. Conversely, during descent, the extended flaps can be used to increase the descent angle without increasing airspeed, allowing for a steeper approach.

Notably, Slotted Fowler flaps are effective in controlling the lift distribution over the wing, thereby optimizing aerodynamic performance.

Crosswind Landing and Thunderstorm

Aircraft flaps also play a role in handling crosswind landings and navigating through thunderstorms. In strong crosswinds, pilots may choose to use lower flap angle settings to minimize the side force on the aircraft, thus enabling them to maintain better directional control during landing.

During thunderstorms, aircraft flaps help in stabilizing the aircraft by increasing the lift and drag forces, making it less susceptible to turbulence. Appropriate flap use allows the pilot to manage the aircraft’s ground speed and descent rate more effectively in such challenging situations.

Aircraft Performance and Efficiency

The choice of flap type and configuration has a direct impact on the aircraft’s performance and efficiency. For instance, Slotted Fowler flaps are known to provide higher lift coefficients, allowing for better short field performance, particularly in airports where runway length is limited.

Incorporating advanced technologies such as RNAV and GPS in modern aircraft allows for more precise control of flap settings, enabling pilots to optimize aerodynamic performance based on the current flying conditions.