# Can birds generate buoyancy from their bodies

## Flying with propulsion

We humans have long dreamed of flying as we know it from birds and insects. Nowadays that dream has more or less become a reality. Even if our human bodies cannot fly on their own, we have invented many means of transport such as airplanes and helicopters in order to be able to do so with their help. But how do they work?

### Why does an airplane fly?

Airplanes can move very quickly thanks to their large engines. The fast movement of the aircraft wings relative to the air leads to a lift force that can be canceled out by the entire aircraft. This power arises from the Flow of air around the wings. In this case one speaks of dynamic lift. But how does this buoyancy come about?

Air flowing horizontally is through the Shape of the airplane wing diverted down. To compensate for this, the wing experiences dynamic lift and can fly when the lift is large enough to offset the weight of the aircraft. Of course, the aircraft must also be traveling fast enough for the air to flow over the wing very quickly.

The following two physical effects essentially play together for the phenomenon of flying: low air pressure over the wing (created by the so-called Bernoulli effect) and the Momentum flow the air.

### Air pressure and Bernoulli effect

Due to the special shape and inclination of the wing, the air particles move faster at the top of the wing. Fast moving air has a lower pressure. This negative pressure creates a buoyancy force.

experiment

#### Negative pressure by blowing

Cut a narrow strip of paper, for example from an A4 sheet of paper. Hold one side to your lower lip and blow straight out very hard! Do you notice how it pulls the strip in the direction of the airflow?

However, the pressure difference alone is not enough to keep an aircraft in the air. Flying is only possible in combination with the following effect.

### Momentum flow

The air particles change their flight direction when they flow over the wings. With perfectly constructed wings, the airflow is directed downwards by the flow over the wing. So that the movements of air and wing remain balanced overall, this leads to an upward movement of the wing, a lift.

The effect of the wing on the airflow depends very much on its angle of inclination. However, if the incline is too steep, the resistance increases and the aircraft is braked. The effect of the angle of inclination on the air flow can be felt particularly well if you hold your hand out of the window while the car is moving and alternately hold it flat and at a slight angle. It works similarly in water, as in this experiment:

experiment

#### Momentum flow of water

Try moving your hand in water in two ways:

1. Hold your hand flat and move it quickly towards the thumb.
2. Hold on hand a bit angled and quickly move them in the same direction again.

What do you feel What happens if you hold the palm at right angles to the direction of movement?

### Why does a bird fly?

The flight ability of birds is based on the same principles as that of airplanes. However, bird flight is much more complicated. There are two phases in bird flight: the Climb and the Descentwhich are of different lengths depending on the type of bird. When climbing, the bird flaps its wings downwards strongly and thus generates a large amount of lift. When descending, the shape of the bird's wing directs the air coming from the front downwards. Buoyancy is created, but it is less than the weight of the bird. Nevertheless, when descending, the bird sinks only slowly in flight due to its aerodynamic shape.