September 19, 2020

Why do planes fly? | Science


Airplanes fly thanks to the performance of a series of forces, both horizontally and vertically. In order for the apparatus to rise, it is essential that the force produced on the vertical axis (lift in aeronautical language) exceeds the weight of the aircraft. Moreover, on the horizontal axis and thanks to the engines that expel gases, the principle of action-reaction that causes a force forward that overcomes the resistance of the air takes place. When the airplane ascends and reaches its cruising height and at a constant speed it is because the balance of forces has been reached both on the vertical axis, in which the lift equals the weight, as on the horizontal axis, in which the Engine thrust equals the resistance that air offers us.

The Magic It occurs when you get that lift. There we have to turn to a series of principles that explain it. Basically, lift is achieved thanks to the wings of the plane. If we cut them we would have what is called the wing profile, the section that has the wing inside. This section has a very efficient way from the aerodynamic point of view. The edge through which the air enters as the plane flies is rounded and the back of the profile is sharp and also curved from the top (in aeronautical language this top part is called extrados and the bottom part is called intrados). This curvature of the wing profile means that when the air flow meets it, it is divided into two paths, a part of the air flow goes above the wing and another part below. Due to the curvature of the wing, the path that has to travel the flow that goes above is longer than the one that goes below. There is theorem, Bernouilli's, which is basically energy conservation and that says that for this to happen, the flow of air that goes above has to go faster. That implies that there is less pressure than in the lower part where it goes at a lower speed and exerts more pressure. That difference in pressure between the air flow above and that below generates a lift. Although this lift due to Bernouilli's principle does not explain all that we need for the plane to rise. To explain the elevation we must resort to another set of physical principles.

The British scientist Isaac Newton (1643-1727)


The British scientist Isaac Newton (1643-1727)

One of them is the third law of Newton. Because of the curved shape of the profile, the air that goes above instead of following a straight path is directed down. This deflection that causes the profile of the wing in the air flow causes that, due to Newton's third law (reaction action principle), a reaction occurs in the opposite direction, that is, up the wing, and that goes to give more support. In addition, this lift is increased by an effect called Coanda that is applied to all viscous fluids. The Coanda effect causes fluids to meet a surface in its path tend to stick on it. There is a boundary layer that is formed between what is the wing profile and the air flow that functions as laminar flow caps, the first one sticks to the wing and drags the rest of the layers that are above. When the air flow is attached to the profile, the effect of Newton's third law is further increased, the air is directed downwards because it remains attached to the profile.

And all this increases with the speed of the air. When you start your take-off run, the plane accelerates progressively, so lift increases as you gain speed. You can understand it better with an example. If we go in a car and take our hand out the window, as the speed increases we notice that the force of the air tends to raise the hand up.

But what definitely makes the plane rise is the nose lift, which is called increasing the angle of attack. The angle of attack is the one that forms the current that affects the profile of the wing with respect to this profile. Once the lift has been increased with the increase in the curvature of the wing profile (extending the surfaces it has: front slats, and flaps rear) the elevators of the horizontal tail stabilizer move. This action causes the nose of the plane to rise. With the nose raised we increase the angle of attack. It is the same effect we would have when we put our hand out the car window, if we turn our hand up in the direction of travel, the hand will go up. And all these joint effects get the plane to rise.

Belén del Cerro She is an aeronautical engineer, technician of the Directorate of Space, Large Facilities and Dual Programs of the CDTI (Center for Technological and Industrial Development)

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