• Bernoulli’s Principle:

    Why do aeroplanes fly?

One of the most frequently asked questions amongst those interested in everything related to aviation is: Why do aeroplanes fly? How does such a metallic mass manage to stay up in the air?

To explain an aircraft’s flight we have to look at a series of basic aerodynamic principles – we have already talked about some of these, such as Thrust vectoring or the Sustentation formula.

Today though, we’d like to talk about Bernoulli’s principle, fluid’s dynamics and horizontal and vertical forces that allow an aeroplane to keep flying.

In addition, we will share with you some everyday examples of this theory that will allow you to better understand its applications. Let’s get started!

What is Bernoulli’s Principle?

In the XVIII century, the mathematician, physicist and Swiss doctor, Daniel Bernoulli enunciated the principle that was given his name and that we can find in his works “Hydrodynamics” published in the year 1738.

This principle describes the behaviour of fluids in a closed system. To be precise, Bernoulli’s principle states that any liquid or gas that increases its movement speed will also see its pressure reduced.

In reality, Bernoulli principle is a description of the conservation law of energy, whose definition states that in an ideal fluid, energy will remain constant while it circulates through a closed conduct or circuit.

This ideal situation of fluids is present when there is no friction or viscosity.

Bernoulli’s Principle equation

Before we analyse Bernoulli’s equation, it is important to know about the three components of energy presented in an ideal situation, one in which there is no viscosity or friction.

These are as follows:

  • Kinetic energy: this is the one that appears owing to the velocity of the fluid
  • Potential energy: this is related to the height of a fluid
  • Pressure energy: this arises out of the pressure the fluid itself exerts

The following formula develops the mathematician and Swiss physicist’s principle:

  • V = velocity
  • p = density of fluid (liquid or gas)
  • P = pressure
  • g = gravitational speed
  • z = height in the direction of gravity

As you can see, the equation is made up of three parts that correspond to the energies we mention above. The first one belongs to kinetic energy, the second one is pressure energy and the last part of the formula corresponds to potential energy.

The sum of these three energies has to keep constant. In the case of variation in any of them, there will always be a modification in the others to maintain the constant.

Why do aeroplanes fly with the Bernoulli’s Principle?

The reason aeroplanes can fly are the forces acting upon them when in air. There are four of these: two horizontal ones (the thrust force and its opposite, the applied one) and two vertical ones.

In the vertical forces, the following come into play: the weight of the aircraft pulling downwards and opposite, the force of sustentation which picks it up.

Here we pay special attention to the shape of aeroplane’s wings. The top part is more curved than the lower one, which is straighter.
This results in air circulation on the top of the wing having a larger area and makes air travel quicker than at the lower part.

The main consequence of this speed change in air circulating above is that a pressure difference is created.

With the definition of Bernoulli’s principle, we have seen in this article that the sum of pressures must remain constant and what occurs in this case is that the lower pressure of the top part of the wing exerts a force beneath it upon which it is thrust upwards.

There is also another time in which this principle intervenes facilitating flight.

When an aircraft rises and wings point upwards, the current of the wind finds an obstacle, the wings themselves, which make the aeroplane brake, increasing pressure.

Bernoulli’s principle produces contrary force which pushes the aeroplane upwards. This is the reason why it is much better for aeroplanes to take off facing the wind.

Three examples of Bernoulli’s Principle in everyday life

Further to taking off and aircraft flight, we can find multiple applications of Bernoulli’s principle in our day-to-day lives.

It isn’t difficult to find examples, following we show three everyday situations based on this theory:

Aerosols and atomisers in perfume

The way these containers work is based on the behaviour of moving fluids. Fluid shoots out increasing its speed when it detects a decrease in pressure.

Pipelines

We have here a classic example where we can see Bernoulli’s principle in action. When a pipeline changes diameter, speed increases whilst pressure decreases.

Swimming

in this case, we can see evidence of Bernoulli’s principle each time a swimmer introduces their hands in water. This decreases pressure and increases the sportsperson’s body being propelled.

Aircraft flight is influenced by many more principles of physics.

In order to explain why aircraft fly, not only do we have to study Bernoulli’s principle but it is also necessary to pay attention to many other physics principles.

After today’s post though, we can better explain the behaviour of aircraft, both at take off and when elevating in height.

We are not only learning about aeronautics but also about physics’ applications, as we saw earlier, we can find multiple applications of physics in our everyday lives.

If you liked this post, we’d like to invite you to take a look at our blog, where we explain topics as curious as why aeroplanes do not fly in a straight line or in which direction an aeroplane flies quicker.