In average every thirty seconds an airplane land on the airstrips of Paris Charles de Gaules. At a global level, at every moment approximately 15 000 aircraft are in fly. However, they still exist a debate on the raisons why does they fly.
In earth, gravity bring us back undeniably on the ground. To be stabilize in the air, it’s necessary to apply a force opposite to gravity of the same intensity. For example, the rockets engine provide a pushing force opposite to gravity to allow it to launch.
In the case of aircraft engines provide a force perpendicular to the gravity. Another force is at stake the lift force. The debate is on the origin of the lift force. Aircraft float in the air that causes lift. From a physical point of view, there are two different explanation.
On the one hand the Bernoulli theory. The air molecules are in permanent agitation, the repeated shocks of molecules with a surface create pressure phenomena. For aircraft, we can draw the air flow lines around a wing :
The image shows the air speed around the wing. The blue area indicates that the air speed is very low and in red that the speed is high. Bernoulli’s work allows us to establish a link between air velocity and pressure. Along a flow line when the velocity increases, the pressure decreases and vice versa. The pressure difference between the underside and the top of the wing causes the lift force. However, it may seem counter-intuitive to link the increase in speed with a decrease in pressure.
For some people, the origin of lift would come from the laws of Newton. To understand the explanations based on Newton’s laws, let’s take the air flow path again.
As we can see, the incident air flow arrives parallel to the ground. In contact with the wing, the flow is diverted from below. According to Newton’s second law, the air has necessarily undergone a force that logically comes from the wing. According to Newton’s principle of reaction action, the wing has therefore been subjected to a force opposite to that of the air. The air being deflected from below, it is subjected to a force coming from the wing towards the ground. The wing then underwent an opposite force of the same intensity upward the lift force.
The two explanations are perfectly equivalent and do not contradict each other from a physical point of view. The Navier-Stokes equations demonstrate the equality between the lift force calculated by Bernoulli and Newton.
For more information electropaedia : https://www.mpoweruk.com/flight_theory.htm
science étonnante : https://sciencetonnante.wordpress.com/2019/09/03/comment-les-avions-volent-ils/