When an aircraft is flying normally at high altitude, its wings appear to be a single piece. In fact, movable panels of different lengths and widths are installed on both the leading and trailing edges of the wings. Some can deflect downward, some extend forward, and some slide backward, in a variety of designs. As these panels are attachments to the wings and can be deflected, much like the lower hem of clothing fluttering in the wind, scientists gave these panels a vivid name—flaps. When the aircraft is parked on the ground or flying at high altitude, flaps are retracted against the leading or trailing edges of the wings. Once the aircraft enters the takeoff or landing phase, their true form is revealed. Why do aircraft need flaps? Please read the following.
The function of a wing is to generate sufficient lift to enable the aircraft to fly. If the wing is a single piece, with a fixed area, airfoil shape and aspect ratio, its maximum lift will also be fixed. If the total weight of an aircraft is 50 tons, the wing must generate more than 490 kilonewtons of lift to get airborne. As we know, a larger wing area produces more lift, and higher speed also produces more lift. In other words, with a required lift value, a larger wing area allows a lower takeoff speed, and a higher takeoff speed allows a smaller wing area. Therefore, to get a 50-ton aircraft into the air, two options are available: first, use a smaller wing area and increase takeoff speed to make lift exceed 490 kilonewtons; second, keep takeoff speed low and use a larger wing area to generate more than 490 kilonewtons of lift. Are these two options feasible? The first option has a smaller wing area, resulting in lighter structural weight, which is an advantage, but a high takeoff speed is unfavorable. On one hand, it requires very long runways, which is uneconomical, especially for carrier-based aircraft; on the other hand, high ground speed poses a great safety risk. The second option has a low takeoff speed, which helps shorten the takeoff distance, but as speed increases after takeoff, the large wing becomes a burden. It not only increases weight and significantly reduces payload capacity, but also greatly increases drag, leading to a marked rise in fuel consumption. This problem of low lift at low speed and high drag at high speed is known as the aircraft’s low-high speed contradiction. How to solve this problem? This is where flaps come in. One main function of flaps is to resolve this contradiction, enabling sufficient lift at low takeoff and landing speeds without the need for an overly large and heavy wing, thus achieving an optimal balance of payload, speed, drag and fuel consumption. A single-piece wing cannot simultaneously meet the requirements of high payload, low takeoff and landing speed, low drag and low fuel consumption. Since the specific function of flaps is to greatly increase lift during low-speed phases such as takeoff and landing, they are collectively referred to as high-lift devices. Why can flaps increase lift? At a constant speed, there are four main ways to increase lift: first, change the airfoil shape and increase camber; second, increase wing area; third, maintain laminar flow as much as possible; fourth, inject a jet of air into the airflow around the wing. Flaps increase lift by changing airfoil camber, increasing wing area and maintaining laminar flow.
The concept of flaps emerged early. Before World War I, as aircraft speed increased, there was a demand for sufficient lift at low speeds, leading to early experiments with the simplest trailing-edge flaps. A plain flap is a portion of the wing’s trailing edge that can deflect downward, changing airfoil camber and increasing lift. Soon after, the split flap was developed. When deployed, it bends the airfoil and creates low pressure at the trailing edge, both of which increase lift. Typically, split flaps increase the lift coefficient by 75%–85% and also add drag, which is beneficial for safe, slow landings. In the 1920s, the famous British designer Handley Page and German aerodynamicist Lachmann invented the slotted flap. It consists of one or more movable panels attached to the wing’s trailing edge, retracted in cruise and deployed during takeoff and landing. Slotted flaps increase wing area, change airfoil camber, and create one or more slots. Increasing area boosts lift, while slots channel airflow from the wing’s lower surface to the upper surface, accelerating upper-surface airflow, maintaining laminar flow over a larger area, further increasing lift and delaying stalls. Slotted flaps are a critical type and can also be installed on the leading edge, usually as a single panel. Today’s large aircraft, especially airliners, are equipped with double-slotted or triple-slotted flaps, which increase the lift coefficient by 85%–95% with remarkable effectiveness. Two other common flap types are the Fowler flap and the Krueger flap. The Fowler flap is a movable surface on the wing’s trailing edge, lying flush with the lower wing surface when retracted. When deployed, it slides backward along tracks on the lower wing while deflecting downward. Fowler flaps significantly increase airfoil camber and wing area, delivering excellent lift gains (85%–95% increase in lift coefficient; up to 110%–140% for some large Fowler flaps). Though structurally complex, they are widely used on medium and large aircraft to greatly improve takeoff and landing performance. The Krueger flap is mounted on the wing’s leading edge, shaped like a portion of the leading edge. When deployed, hydraulic actuators extend it forward and downward, modifying the airfoil and increasing wing area for effective lift enhancement.
Flap technology continues to evolve. The designs described above are mature, while another flap concept—proposed early but still underdeveloped—is the jet flap. There are many jet flap designs, all based on bleeding high-pressure air from engines or compressed air bottles to blow over the wing or flap surface, aiming to increase lift, delay airflow separation, reduce drag and improve stall characteristics. Due to its extreme complexity, jet flaps are currently used only on a few aircraft, such as the Harrier vertical/short takeoff and landing aircraft, F-4 Phantom and MiG-21 lightweight fighter, with testing still in progress.
(1) Wingspan: The straight-line distance between the left and right wingtips (including the section passing through the fuselage).
(2) Fuselage Length: The straight-line distance from the foremost to the rearmost point of the model aircraft.
(3) Center of Gravity: The point where the resultant gravitational force acts on all parts of the model aircraft.
(4) Tail Moment Arm: The distance from the center of gravity to the quarter-chord point of the horizontal stabilizer’s leading edge.
(5) Airfoil: The cross-sectional shape of a wing or tail surface.
(6) Leading Edge: The foremost point of an airfoil.
(7) Trailing Edge: The rearmost point of an airfoil.
(8) Chord: The straight line connecting the leading and trailing edges of an airfoil.
(9) Aspect Ratio: The ratio of wingspan to the mean chord length. A high aspect ratio indicates a long, narrow wing.