# Why can helicopters hover in the air?

In addition to normal flight, helicopters can also hover in the air for operations, which is also a unique flight state that distinguishes helicopters from ordinary fixed-wing aircraft. Why can a helicopter stay in the air? This starts with its flight principle.
Anything that wants to leave the ground needs to overcome the attractive force from the earth, which is “gravity”. The direction of gravity is straight down, and it pulls objects on Earth tightly. If the plane wants to lift into the air, it must have an upward force to overcome the gravity, which is equivalent to saying that there is a force “lift” to pull it up. When the lift is greater than the gravity, the plane can leave the ground and rise into the air.
The development of helicopters originated from the bamboo dragonfly in China: two pieces of bamboo cut into uneven thicknesses are mounted on a wooden stick, and when you rub your hands together, the bamboo pieces can fly into the sky by using inertia and air pressure as power. The rotor on the top of the helicopter is similar to that of a bamboo dragonfly, and its take-off lift comes from the continuous rotation of the rotor. From the general shape of the cross-section of the rotor blade, it can be found that the upper surface of the rotor is curved, and the lower surface is relatively flat. When rotating, the air velocity on the upper surface is fast and the pressure is low, while the air velocity on the lower surface is slow and the pressure is strong. A pressure difference will be formed between the upper and lower surfaces, thereby generating upward lift.
When the force of lift is equal to the force of gravity, the vertical position of the helicopter in the air remains the same. The paddle disc plane formed when the rotor is running can resist the surrounding airflow when tilted in different directions, so that the horizontal position of the helicopter in the air remains unchanged. The tail rotor of the helicopter counteracts the reverse moment of the main rotor to prevent the body from spinning. As a result, the helicopter can hover in the air stably.
That is to say, the hovering of the helicopter mainly depends on the pressure difference between the upper and lower rotors to generate lift and the gravity of the helicopter itself to achieve balance.
Moreover, due to the ability of helicopters to fly and hover, it is very useful both in the military and in life.
In life, we often see the news of helicopter rescue. For example, using a rescue helicopter to rescue people on a cliff, even if there are many obstacles such as trees and steep walls, tourists and an accompanying person can be transferred to the helicopter by hoisting.