Shulou(Shulou.com)11/24 Report--

When you are usually on a plane, have you noticed a strange phenomenon that the plane will be "delayed" on the runway for a long time before landing? why?

In fact, this is a famous physical phenomenon and is also used to build military aircraft called "monsters". Today let's take a look at the "ground effect" (ground effect) that every passenger has encountered, but few people know about it.

Ground effect: it is difficult for aircraft to land on landing and lift off during take-off, and it is often necessary to "float" on the runway for a long time. According to the US Naval Academy (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 the ground effect? The ground effect mainly comes from the decrease of resistance and the increase of lift.

One of the major sources of resistance encountered by a wingtip swirl aircraft in flight is the wingtip swirl at the end of the wing. According to the National Aeronautics and Space Administration (NASA), when the aircraft is close to the ground or the surface of the water, the wingtip swirl is damaged and the drag is reduced. This drag reduction caused by wingtip eddy current damage is called wingspan-dominated ground effect (span dominated ground effect).

When approaching the ground, the wingtip vortex (the ring in the picture) is destroyed, so the resistance decreases. Photo Source: US Naval Institute in addition, the plane will encounter an air cushion when flying very close to the ground, also known as the invisible "hand of God", which provides a powerful lift for the aircraft. This air skill enhancement gained when close to the ground is also called the stamping effect (ram effect).

As early as the 1920s, pilots learned about the ground effect through first-hand flight experience. In 1934, Technical Memorandum 771, a technical report of the National Aviation Advisory Board (NACA), pointed out the impact of ground effect on aircraft take-off and landing.

Some pilots even deliberately use ground effect tactics to save fuel and fly low. In 1930, for example, the German giant airship Dornier DO-X took advantage of the ground effect when flying across the Atlantic. During World War II, when the plane ran out of fuel or the engine broke down, some pilots would fly low in the sea, using the ground effect to make the plane fly longer.

German giant airship Dornier DO-X. Photo source: wikipedia such an attractive law of physics with blood and magic, the military is naturally reluctant to let it go. In the 1960s, both the former Soviet Union and Germany were actively developing ground effect aircraft, which make use of ground effect.

In 1967, when a prototype of the current Soviet ground effect aircraft, Ekranoplan, appeared in the Caspian Sea, American intelligence agencies were shocked.

When CIA analysts saw satellite photos of the Ekranoplan, they didn't know whether it was a ship or an aircraft, because it was as big as a ship, more than any aircraft known at the time, but had small wings and could fly at a high speed (with a top speed of 560,740km / h). Therefore, they gave this "Yuanzu" ground effect aircraft a nickname-"Caspian Sea Monster" (the Caspian Sea Monster).

Developed and manufactured by the former Soviet Union, the London-class ground effect aircraft Ekranoplan, numbered MD-160,Ekranoplan, is often referred to as the "Caspian Sea monster". Source: the advantage of wikipedia Caspian Monster is obvious. It can run at the speed of an airplane, but its load is the same as a ship, so Caspian Monster ground effect aircraft is often called "killer mace of aircraft carrier" and "ideal tool for surprise landing".

Images of the Caspian Sea Monster Ekranoplan tested on the sea surface. So why don't we see mature military or civilian ground effect aircraft now?

First of all, the advantage of the ground effect is also its disadvantage: the ground effect aircraft can only fly at an altitude close to the ground or water surface, which is generally less than 15% of the length of the wing, and the ground effect aircraft cannot resist when they encounter obstacles. Usually it can only be bypassed, and it is easy to fall to the ground because of the abnormal bulge of the ground or water.

In addition, as early as 1926, the National Aviation Advisory Committee (NACA) pointed out in its report No. 265 that another major technical difficulty of ground-effect aircraft is that once it flies off the ground or water surface, it is easy to hold its head high, that is, its longitudinal stability is relatively poor (longitudinal instability). This problem still haunts R & D personnel.

The Caspian Sea monster MD-160 has been in disrepair for 30 years and is now stranded on the edge of the Caspian Sea. It is these two disadvantages that limit the spread of ground-effect aircraft. At present, in the civilian field, only a very small number of ground effect aircraft "land" in the market.

A prototype of Airfish 8, a modern ground effect aircraft capable of carrying 6-8 passengers, was developed by Wigetworks of Singapore. However, a reverse "twin" of the ground effect has long been used in the competitive field.

As mentioned just now, it will be 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.

Some cars bulge at the bottom, forming a Venturi tube, creating a low-pressure area (red). Photo Source: steemkr when the lower part of the wing is protruding, the plane may be "sucked" by the earth. This is because a Venturi tube is formed between the convex wing and the ground, and a low pressure zone appears under the wing because of Bernoulli's law.

The narrow part of the Venturi tube has a higher gas velocity and a lower air pressure. Source: wikiwand, also known as the "ground effect" (ground effect), was discovered by Formula one racing clubs in the 1970s.

We know that the car should slow down when turning, otherwise it is easy to overturn. However, if the car can be "sucked" on the ground, there will be no need to slow down when turning. Racing cars that can be sucked on the ground can leave behind opponents with weak grip at the corner and easily overtake at the corner.

In the 1970s, the Lotus F1 team began to develop and use the accelerated "skin" of the Venturi at the bottom of the car. As can be seen from the picture, the bottom of the Lotus 78 is convex, forming a Venturi tube with the ground, creating a low-pressure area that is sucked on the ground.

Lotus 79 and 80 bottom Venturi tube shape. Photo Source: formular1's later improved and upgraded version 79 of Lotus 78 won the championship in 1978. When other teams saw this, they followed suit, and the F1 car immediately evolved into a ground-effect equipment game.

However, similar to the airplane version of the ground effect, the racing car's ground effect is also fraught with crisis. When the air tightness of the Venturi tube is broken, the "grip" will suddenly loose and cause the vehicle to get out of control. At the 1982 Dutch Grand Prix, for example, the failure of the Venturi at the bottom of the car caused the famous driver Ren é Arnoux's car to lose control and fall into the tire wall.

Ren é Arnoux's car accident at the 1982 Dutch Grand Prix. So in 1983, the F1 governing body (FISA) decided to limit the ground effect, stipulating that the chassis of each team must be flat and that "kryptonite skin" should no longer be used to give racing cars a ground effect.

All in all, although these two "approachable" ground effects have been played by high-end players, they can also kill people.

-what kind of magic plane is that?

-Voldemort.

This article comes from the official Wechat account: take Science Home (ID:steamforkids), author: Qijun

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