JAES Learning

Jaes Sponsor - Basket

How do airplanes fly - Bernoulli principle or Coandă effect?

An airliner is typically defined as an aircraft intended for carrying multiple passengers or cargo in commercial service. When the Wright brothers made the world’s first sustained heavier-than-air flight, they laid the foundation for what would become a major transport industry.

Listing every single component of an airplane is almost impossible. The modern airliners in fact are composed of huge variety of mechanical, hydraulic and electronic components.

JAES, besides being a qualified partner for some of the most important aircraft manufacturers, is constantly engaged in the supply of all those spare parts necessary for the production, assembly, repair and maintenance of aircraft. Such as:

But have you ever wondered how such a large object, made up of all these components, can lift off the ground and fly hundreds of people at an altitude of thousands of feet?

In this video we will explain how an airplane flies and how the pilots are able to control it, in a simple but very specific way.

First, let’s take a closer look at the wing of the plane.

We notice that it is not composed of a single solid piece. The wings, as well as the tails of airplanes, have many moving elements.

The most curious thing about the wing is its own specific shape, that follow the basic principles of fluid mechanics. In this section we are able to see the shape of the wing.

We notice that the wing has a teardrop shape and when the airplane starts moving in this direction, the airflow will hit the wing following these trajectories.

But how does the wing rise when it is hit by the air flow?

First of all, it is necessary to say that the wing is placed in a slightly inclined position to improve the air flow. In this way, the angle of attack, which is the angle formed between the chord line of the wing and the air flow direction, is increased.

We can notice now that the air flow entering at point A should have the same speed as the air flow at point B. In this situation we are led to believe that the upper air flow is faster because it has to travel more airfoil surface than the lower air flow. This difference in speed causes a difference in pressure between the two air flows. In the upper air flow in fact there will be lower pressure, while in the lower air flow there will be higher pressure.

This phenomenon is in accordance with Bernoulli principle. It is a common idea in fact to relate the Bernoulli’s equation to this situation. The Bernoulli principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid’s potential energy.

This principle is absolutely right, the problem is not this specific physical law, but it’s the application of this law to this phenomenon. To say that the speed difference between the two air flows is caused by the fact that the upper air flow has to travel more airfoil surface to be reunited with the lower air flow at the end of the wing is just wrong. So the fact that the upper air flow is faster than the lower one is true, but based on a completely wrong conclusion. The upper air flow is not interested in rejoining the lower air flow at the exit of the airfoil. So, the difference in pressure phenomenon is explained by a completely false fact.
Another theory describes this phenomenon not as a difference in speed but as a difference in density between the upper and the lower air flow. In other words, the molecules of the upper air flow are more distant from each other while they travel across the wing surface. While the molecules of the lower air flow, instead, are closer to each other.

The lower air flow would have more molecules than the upper air flow. This would create a difference in pressure between the two parts. The low pressure on the top would create an upward suction effect that allows the wing, and consequently the whole aircraft, to lift up.
In addition, the numerous molecules of the lower air flow would create an high pressure effect, that allows the upward lifting of the wing.

The difference in pressure seems not to be the only phenomenon that causes the lift of the aircraft. You probably wonder why the air molecules, coming in straight line and hitting the wing in this way, instead of deflecting their direction, they follow the entire airfoil surface all the way out. This phenomenon is related by many to the COANDĂ EFFECT, which is the tendency of a fluid jet to stay attached to a nearby surface.
This effect can clearly be shown by placing a simple spoon under running water. Or a common glass jar.

The fluid, or in this case the air flow, moving along the wing surface causes friction, which tends to slow it down. This slowing only affects the molecules in direct contact with the wing.
The air flow coming in a straight line, hit the wing and follows its shape all the way out.
This would contribute to lift the plane.

After all these considerations, we can certainly say that the greater the air flow investing the wing, the greater the lift. This is why planes to take off must reach high ground speed as well as high air speed.

The aircraft wing are designed to obtain maximum efficiency, maximum speed and minimun drag to reduce fuel consumption during the longest cruise phase of flight. So only for take off and landing phase pilots use slats and flaps to obtain the most efficient aircraft configuration based on the aircraft performances and the airport limitation.
A way to increase the lift is to change the shape of the wings by adding some moving elements such as SLATS and FLAPS, which by opening up they increase the deflection as well as the wing surface.

For take off and landing phase, when SLATS and FLAPS are put in position by the pilots, there is a substantial increase of the wing surface area to obtain more lift at a lower speed.

The aircraft rotates around 3 axes: Longitudinal (Roll), Lateral (Pitch), and Vertical (Yaw) axis.
The ailerons are used to roll the aircraft along its longitudinal axis (Roll left or right).
These elements can move up and down and for this reason the lift force can decrease or increase respectively.

At the tail of the plane you can see two main elements: the RUDDER that moving to the right and left allows control of the horizontal force moving the aircraft around its vertical axis (Yaw to the left or right).
And the ELEVATORS that moving up and down allow control of the vertical force generated on the tail.
So the aircraft is able to rotate around its lateral axis (pitch up or down)

Thanks to these moving parts, it is possible to control the direction of the aircraft, from the take-off to the landing phase.

The 4 forces actig on an airplane are: LIFT, WEIGHT,THRUST and DRAG.

Let’s take a detailed look at the take-off phase: to raise a plane from the ground it is necessary to increase the lift force to counteract the weight of the aircraft.

This is possible by increasing the speed at first,of the aircraft, and increasing the angle of attack.

When FLAPS and SLATS are in position, the pilots increase the thrust of the engines to make the plane speed up. This brings a further increase in lifting force.
When the aircraft enters the take-off phase, the elevators are activated upwards.

The tail force tilts the plane and the angle of attack of the airfoil is increased.
Thanks to these maneuvers, the lift is suddenly increased and causes the lift of the aircraft.

So far we have talked about the engine thrust, but we have not yet said how the engine is able to generate the thrust

Modern airplanes use special types of engines called TURBOFAN, they are combined turbines that use the effect of the turbojet reaction and the fans reaction that provide the necessary thrust.

By burning more fuel the pilot can achieve more thrust.

After the take-off phase, it comes the CLIMB PHASE of the plane. As long as the thrust of the engine is higher than the drag, the airplane’s speed will continue to increase.
..and, as much as the lift is greater than the weight, the aircraft will continue to climb.

When the airplane reaches LEVEL FLIGHT, the pilots will not make any acceleration or change in altitude.

During level flight, in fact, the thrust should be exactly equal to the drag and the lift force should be equal to the weight of the aircraft.

Now the plane can travel its own way.

Unfortunately, the airline routes are not always straight, in fact it may happen to make some changes in direction.

In this case, it’s easy to think that just by adjusting the rudder the pilots are able to change direction.

The rudder produces a horizontal force that can turn the airplane.
However, this maneuver, by itself, is not the best method to change direction because it would cause a sudden movement that will cause a discomfort to passenger.

To make a TURN the pilots, first of all, raise the AILERON of a wing and lower the AILERON of the other wing at the same time.

The difference in lift force will cause the roll of the airplane.

During this turn the lift force is not vertical.

The horizontal component of the lift can provide the required centrifugal force to bank the airplane.
During this phase the vertical component of the lift would be reduced, and the plane would also start descending, so the pilots must be able to properly maintain the altitude.

In this way the pilot can achieve a turn of a certain radius depending upon the angle of roll and the speed of the airplane.

For the landing phase, pilots decrease the engine’s thrust and keep the nose of the plane down.

As the airplane loses speed, it gets ready for landing. At this point SLATS and FLAPS are put in position to increase the wing surface area and to increase the DRAG.

When the aircraft touches the ground other moving elements called GROUND SPOILER are deployed to further increase the drag. These spoilers are opened once the aircraft is on the ground, in order to make the wings lose lift, as well as to achieve a greater grip on the ground.
At this point the pilots use another trick to reduce the stopping distance, that is, the REVERSETHRUST.

The engine covers open and the air which was supposed to go back is pushed forward.
This will generate a reverse thrust to facilitate the stopping of the airplane.