Alongside stunning views, one of the big perks of the window seat is observing the aircraft’s flaps, slats and spoilers in action during different phases of flight. Flaps and slats are types of high lift devices – a collection of mechanisms or aerodynamic features installed on aircraft wings, but why do we need them? Let’s take a look.
Why do aircraft need flaps?
Aircraft need flaps to enhance their aerodynamic performance during critical phases of flight. Extending the flaps increases the curvature and surface area of the wing, therefore increasing the amount of lift the wing generates. This enhancement is critical during the slowest phases of flight – takeoff, approach, and landing.
- Takeoff – flaps give you a shorter takeoff roll, but a lower rate of climb.
- Approach – flaps allow the aircraft to fly slower on the approach
- Landing – flaps reduce the distance of the landing roll on the ground.
How are flaps deployed?
Flaps are deployed in stages using a lever to the right of the throttle. Whilst deploying the flaps generates lift, it also generates more drag. Pilots use procedures and experience to decide when to move the flaps and by how much. On small aircraft, flaps typically have three simple settings, such as ‘up’, ‘takeoff’ and ‘landing’. Flaps on larger aircraft are deployed in stages. In Boeing aircraft, these are referenced to the degrees of deployment, with flaps being set to 1°, 5°, 10°, 30° or 40°. The flap settings on Airbus aircraft are simply referred to as 1, 2, 3 and full. The flap handles on some aircraft are even designed in the shape of a flap, giving an extra tactile clue to the pilot in busy workflows.
As the high lift devices have such a fundamental effect on the aircraft, the actions of a flight crew when operating the flaps can be make or break in an emergency situation. After experiencing a dual engine failure on final approach to Heathrow Airport (LHR) in 2009, the crew of British Airways flight BA38’s decision to partially retract the flaps enabled the aircraft to safely reach the runway. On the flipside, the failure to set the flaps for takeoff at all led directly to the crash of Northwest Airlines flight 255 at Detroit Metropolitan Airport (DTW) in August 1987.
What are the different types of flap?
There are several different types of flaps used in aircraft design, each with its own unique characteristics and advantages. Let’s take a simplified look at the most common types.
Plain Flaps – these are simple hinged surfaces that extend downward from the wing when deployed. They increase the wing’s camber, enhancing lift generation at lower speeds. Plain flaps are easy to design and maintain, and they are common on light aircraft. They produce a considerable amount of drag when fully deployed.
Slotted Flaps – a modification made to plain flaps, essentially a more advanced design that creates a slot-like gap between the flap and the wing when deployed. This design allows high-pressure air from below the wing to flow over the top, delaying the stall and re-energising the airflow. Most complex aircraft flaps include slots to manipulate the airflow and increase lift.
Split Flaps – these are also hinged surfaces, but they move downward and backward away from the wing when deployed. They provide an increase in lift but introduce a substantial amount of drag, making them less efficient than other flap types. Split flaps were commonly used in older aircraft designs.
Fowler Flaps – a development of the slotted flap that not only moves downward but also slides backwards, significantly increasing the wing’s area. This flap design provides higher lift coefficients and increased lift at lower speeds compared to other types of flaps. Fowler flaps are found on larger commercial aircraft.
Krueger Flaps and Slats – these high lift devices are found on the leading edge (the front) of the wing. Krueger flaps are hinged and extend forward from the wing’s leading edge, while slats are movable surfaces that slide forward to improve lift at low speeds and high angles of attack. Leading-edge devices also improve lift performance during takeoff and landing.
Flaperons – In aviation, you can often smash two words together to form another. Flaps + Aelerons = Flaperons. Flaperons combine the functions of flaps and ailerons. They are located at the trailing edge of the wing and move up and down. Flaperons are usually found on large swept wing aircraft and provide additional roll control during flight.
The type of flap used depends entirely on the aircraft design and the desired performance characteristics for takeoff, landing, and other phases of flight. Modern aircraft often incorporate multiple flap types or even use innovative systems to improve overall aerodynamic performance and handling.
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Cover photo: Kevin Cargo, JetPhotos
