# Why are airliners put under pressure

## Why does an airplane fly?

### The Bernoulli Effect

Humans took the decisive impulses from nature at an early age. It must have something to do with the shape of the wings, why birds soar acrobatically through the air. At least that is what the universal genius Leonardo da Vinci suspected about 500 years ago.

But it would be another 400 years before Otto Lilienthal built the first manned flying machine and successfully sailed down a slope.

In the 18th century, the Swiss physicist Daniel Bernoulli described an effect that became the decisive basis for modern flying: flowing liquids and gases exert less pressure on their surroundings than stationary ones. The higher the speed, the lower the pressure.

A simple experiment that everyone can try at home shows this effect: if you hold a sheet of paper by the corners on one side, the sheet hangs down in an arch.

However, if you blow over the paper strip above, then the speed of the air is greater there than below, where the air does not move. This will reduce the pressure on the upper side. The higher pressure on the underside now pushes the sheet upwards.

### The magic word: asymmetry

If you look at an aircraft wing in profile, you can see that it has an asymmetrical cross-section: the underside of the wing is almost straight, while the upper side is curved. The airflow has a shorter path at the bottom than at the top, that is, the air flows faster at the top. According to Bernoulli, this means nothing other than that the pressure above is lower than below - so there is lift.

Another factor - and for some experts the more decisive one - is the lift that is created by slightly pitching the wings. Due to the slight inclination, the wing pushes the passing air downwards, which creates a corresponding counterforce. The buoyancy generated in this way also allows kites to fly.

And it is also of crucial importance for aerobatic planes whose wings have a symmetrical profile. Here the lift is generated solely by the angle of attack, which also enables upside down flight.

### Minimum speed required

Every aircraft needs a certain minimum speed, depending on its weight and load-bearing surface. If this speed is not reached, the wing is no longer smoothly flown around and the flow breaks off. The aircraft then no longer has enough lift and falls like a stone from the sky.

Sometimes the flow conditions are changed on purpose. This is common during landing, for example, to reduce landing speed. On the one hand, the pilot can practically increase the curvature of the wing by extending the landing flaps and, in some types of aircraft, also enlarge the area of ​​the wing.

Anyone who has ever sat by the window when a jet plane lands knows the hydraulic extension of the wing extension. Both measures increase the lift and thereby enable the landing speed to be reduced.

For a jumbo jet, it is around 270 kilometers per hour. Without aids, the giant bird would have to touch down at almost 400 kilometers per hour.