When the plane flies normally at high altitude, the wing looks like a whole. In fact, the leading edge and trailing edge of the wing are equipped with wings with different lengths and widths, some of which can be deflected downward, some can be extended forward, and some can be slid backward, which can be described as various. Because these wings are appendages of wings and can be deflected, just like the clothes we wear, scientists have given these wings a very vivid name-flaps. Usually, when an airplane is parked at an airport or flying at high altitude, the flaps are folded at the leading edge or trailing edge of the wing. Once the aircraft enters the take-off or landing phase, their prototypes are revealed.

Why do planes have flaps? Please see below.

1, the secret of flap is to improve lift.

The function of the wing is to generate enough lift to make the plane fly into the sky. If the wing is a whole, when the wing area, airfoil and aspect ratio are determined, its maximum lift will be constant. If the total weight of the aircraft is 50 tons, the wing must generate more than 490 kn of lift to fly. We know that the larger the wing area, the greater the lift; The greater the speed, the greater the lift. In other words, the larger the wing area, the smaller the takeoff speed can be under the condition of constant lift; The larger the takeoff speed, the smaller the wing area can be. Therefore, in order to get this 50-ton plane into the sky, there are two ways: one is to choose a wing with a small area to improve the take-off speed, so that the lift can exceed 490 kn; the other is to maintain a low take-off speed, and use a wing with a large area to generate the lift above 490 kn.

Are these two methods feasible? The first method is that the wing area is smaller and the aircraft structure is lighter, which is an advantage, but the take-off speed is very unfavorable. On the one hand, the airport runway is required to be very long, which is not economical and is not conducive to carrier-based aircraft. On the other hand, high running speed poses a great threat to security. The second method has a low takeoff speed, which is beneficial to shorten the taxiing distance. However, when the speed of the aircraft increases after taking off, the large wing becomes a burden, which not only greatly reduces the weight of the aircraft, but also dramatically increases the resistance and fuel consumption of the aircraft. This problem of small lift at low speed and large resistance at high speed is called the contradiction between high and low speed of aircraft. How to solve this problem? This is achieved by flaps.

One of the main functions of flaps is to coordinate this contradiction. It does not need a large and heavy wing, and can generate enough lift at a low take-off and landing speed, so that the load, speed, resistance and fuel consumption are comprehensively optimized. The overall design of the wing can not meet the requirements of large load, low take-off and landing speed, low drag and low fuel consumption at the same time. Because the specific function of the flap is to greatly improve the lift in the low-speed stages such as take-off and landing of the aircraft, it is collectively called the high-lift device.

Why can flaps increase lift? There are four main methods to improve the lift at a certain speed: one is to change the shape of the wing section and increase the camber of the airfoil; The second is to increase the wing area; The third is to keep laminar flow as much as possible; The fourth is to add jets to the airflow around the wing. Flaps can increase the lift by changing the camber of the airfoil, increasing the wing area and maintaining laminar flow.

2. There are many kinds of airplane flaps.

The concept of skin flap appeared very early. Before the First World War, due to the increase of aircraft speed, it was required that the aircraft could generate enough lift at low speed, so someone started the simplest experimental exploration of trailing edge flap.

Why do planes have flaps?

A simple flap is part of the trailing edge of a wing. It can bend, which will change the camber of the wing and improve the lift. Soon, the split flap reappeared. When lowering, on the one hand, it can bend the airfoil, on the other hand, it will form a low pressure at the trailing edge of the wing, both of which are used to increase the lift. Generally speaking, split flaps can increase the lift coefficient by 75% ~ 85%. At the same time, the split flap can also increase the resistance, which is beneficial to the safe and slow landing of the aircraft.

In the 1920s, the famous British designer Handley Page and the German aerodynamicist Rahman invented the slotted flap. It is one or several movable wings attached to the trailing edge of the wing, which is integrated with the wing at ordinary times and put down when the plane takes off or lands. Flaps can increase the area of the wing, change the camber of the wing, and form one or several gaps at the same time. Increasing the area can improve the lift force, and forming the gap can make the airflow on the lower surface flow to the upper surface through the gap, thus improving the airflow speed on the upper surface, maintaining laminar flow in a large range, increasing the lift force and reducing the stall phenomenon. Slotted flap is a very important flap. It can also be installed at the front edge of the aircraft, usually one. At present, large aircraft, especially passenger aircraft, can improve the lift coefficient by 85% ~ 95% by installing double-slit or three-slit flaps, and the effect is very remarkable.

There are two kinds of flaps that are more common, one is Fuller flap and the other is Kruger flap.

Fuller flap is a movable wing surface installed at the trailing edge of the wing, usually close to the lower surface of the wing. When in use, the flap retreats along the slide rail installed on the lower wing surface and deflects downward at the same time. Using fuller flap can increase the camber of wing section and greatly increase the wing area. The lift coefficient can be increased by 85% ~ 95%, and the lift coefficient of some large-area fuller flaps can be increased by 1 10% ~ 140%. This kind of flap has a complex structure, and is mostly used in large and medium-sized aircraft, which can greatly improve the take-off and landing performance.

Kruger flap is located at the leading edge of the wing. Its shape is equivalent to a part of the leading edge of the wing. When in use, the Kruger flap is extended forward and downward by using the hydraulic actuator, which not only changes the airfoil, but also increases the wing area, resulting in better lift effect.

3. Aircraft flaps are under development.

The development of skin flap is not over yet. The above-mentioned flap device is relatively mature, and another concept of flap was put forward long ago, but it is still not perfect until now, that is, jet flap. There are many design schemes, the basic idea of which is to extract gas from engine or high-pressure gas cylinder and suck it to the surface of wing or flap to increase lift, delay separation, reduce resistance and improve stall characteristics. Due to the complexity of jet flaps, only a few aircraft, such as Harrier VTOL aircraft and F-4, MIG -2 1 light fighters, use jet flaps. Its experimental work is still in progress.