How planes fly – is it Bernoulli’s principle or Newton’s third law?
Believe it or not, the exact mechanism behind aerodynamic lift is still in question today.
Bernoulli's Principle Explanation
Bernoulli’s principle states that as the speed of a fluid increases, its pressure decreases.
With an aeroplane wing, the air moves faster over the curved side. This creates a low pressure zone on the top of the wing. The pressure differential between the top and bottom of the wing then creates lift.
Bernoulli’s principle partially explains the low pressure zone that forms on the top of a wing.
Its main downfall – it does not explain why the air speed along the top of a wing is higher. An air particle that flows over the top has to travel further than one that flows below. But there is nothing to say that it should reach the end of the wing at the same time, and hence have a higher speed.
Bernoulli’s principle also does not explain how planes can fly upside down, or with flat wings – like a paper plane.
Newton's third law explanation
Newton’s third law states that if an object exerts a force on another, it receives an equal and opposite force.
With an aeroplane wing, the air hits the underside and is forced downward. The reactionary force on the wing is upward and creates lift.
This explains how lift can occur when a plane is flying upside down. It also explains how lift can occur with flat wings.
However, Newton’s third law does not explain the low pressure zone that forms on the top of a wing.
Is there a complete explanation?
The above theories are too simplistic to fully explain how planes fly. They only account for individual aspects of aerodynamic lift.
Today, aerospace engineers can predict aerofoil performance with high accuracy. Complex equations and CFD (Computational Fluid Dynamics) make this possible.
Despite this, there is still no complete theory on what causes lift. In other words, we know what happens but not why. The formation of the low pressure zone in particular remains to be a partial mystery. It is a topic that is fervently debated among experts.
