When you usually take a plane, have you noticed a strange phenomenon, that is, the plane will "delay" on the runway for a long time before landing, why is this? In fact, it's a well-known physical phenomenon and is also used to create military aircraft known as "monsters". Today, let's take a look at the "ground effect" that every passenger has encountered, but few people know about.
According to the US Naval Institute, during take-off and landing, aircraft often "float" on the runway for a long time, which is the role of the ground effect. So, what is the principle of ground effect? The ground effect mainly comes from the reduction of drag and the increase of lift.
A major source of drag encountered by an aircraft in flight is the wingtip vortex at the end of the wing. According to NASA, when the plane is close to the ground or water, the wingtip vortices are disrupted and drag is reduced. This drag reduction caused by wingtip vortex disruption is known as span dominated ground effect. In addition, when flying very close to the ground, the plane encounters an air cushion, also known as the invisible "Hand of God", which provides the plane with a strong lift. This air-based skill enhancement obtained when you are close to the ground is also called the ram effect.
As early as the 1920s, pilots knew about ground effects through first-hand flying experience. In 1934, a technical report of the National Advisory Committee for Aeronautics (NACA) "Technical Memorandum 771" pointed out the impact of ground effect on the take-off and landing of aircraft. Some pilots even deliberately use ground-effect tactics to save fuel and fly low. For example, in 1930, the German giant airship Dornier DO-X used ground effects as it flew over the Atlantic Ocean. In World War II, when the plane ran out of fuel or had an engine failure, some pilots would fly low over the sea, using the ground effect to make the plane fly longer.
The military is naturally unwilling to let go of the tempting physical laws of adding blood and magic. In the 1960s, both the former Soviet Union and Germany were actively developing aircraft that utilize ground effects ground effect aircraft. When a prototype of the current Soviet-developed ground-effect aircraft Ekranoplan appeared in the Caspian Sea in 1967, American intelligence was shocked. When CIA analysts saw the satellite photos of the Ekranoplan, they couldn't tell if it was a boat or a plane, because it was as large as a boat, more than any plane known at the time, but with wings that Small and capable of high-speed flight (top speed 560–740 km/h). Therefore, they nicknamed this "Original" ground-effect vehicle - "the Caspian Sea Monster".
The advantages of the Caspian Sea Monster are obvious. It can run at the speed of an aircraft, but the load is the same as that of a ship. Therefore, the "Caspian Sea Monster" ground-effect vehicle is often called "the killer of aircraft carriers" and "the ideal tool for assault landing". So, why are we not seeing full-fledged military or civilian ground-effect vehicles now?
First of all, the advantage of the ground effect is also its disadvantage: the ground effect aircraft can only fly at a height close to the ground or water surface, which is generally within 15% of the wing length. When encountering obstacles, the ground effect aircraft can fly. It cannot be resisted hard, usually can only be bypassed, and it is easy to fall to the ground due to abnormal protrusions on the ground or water surface. In addition, as early as 1926, the National Advisory Committee on Aeronautics (NACA) pointed out in report No. 265 that another major technical difficulty of ground-effect aircraft is that once it flies off the ground or water, ground-effect aircraft is easy to inexplicably arrogant. Head-up, that is, the longitudinal stability is relatively poor (longitudinal instability). This problem is still plaguing developers.
It is these two disadvantages that limit the proliferation of ground-effect aircraft. Now in the civilian field, only a few ground-effect aircraft have "landed" on the market. However, a reverse "twin" of the ground effect has long been used in the field of competition. As I mentioned earlier, it is difficult for the aircraft to descend when it is close to the ground. In fact, if the shape of the wing is changed, it may be sucked by the ground.
When the lower part of the wing is convex, the aircraft may be "sucked" by the ground. This is because a Venturi tube is formed between the convex wing and the ground at this time. Due to Bernoulli's law, a low-pressure area will appear under the wing at this time.
This phenomenon also happens to be called the "ground effect" and was discovered by the Formula 1 racing club in the 1970s. We know that the car needs to slow down when turning, otherwise it will easily roll over. However, if the car can "suck" to the ground, there is no need to slow down when turning. A car that can suck on the ground can leave opponents with a weak "grip" when cornering and easily achieve corner overtaking.
In the 1970s, the Lotus F1 team began to develop acceleration "skins" using the Venturi on the bottom plate of the car. As you can see from the picture, the bottom of the Lotus 78 race car is convex, forming a venturi with the ground, creating a low-pressure area, which is sucked to the ground.
Later, the improved and upgraded version of the Lotus 78, the 79, won the championship in 1978. Other teams followed suit after seeing it, and F1 racing turned into a ground-effect equipment battle for a while. However, similar to the ground effect of the aircraft version, the ground effect of the racing car is also in danger. When the air tightness of the venturi is broken, the "grip" will suddenly loosen and cause the vehicle to lose control. For example, in the 1982 Dutch Grand Prix, the famous driver René Arnoux's car lost control and fell to the tire wall, which was caused by the failure of the Venturi tube at the bottom of the car.
Therefore, in 1983, the F1 governing body (FISA) decided to limit the ground effect, stipulating that the chassis of each team must be flat, and no longer use "krypton gold skin" to make the car get the ground effect. All in all, although the ground effects of these two "grounding gases" are played out by high-end players, they can also kill people.