Atmospheric Flight

Important Discoveries

Around 1783 Daniel Bernoulli, a Swiss mathematician, discovered that if you increase the speed at which water (a fluid) flows, the pressure decreases. This discovery became known as Bernoulli's Principle and was later used to describe how airflow (air is also considered a fluid) and air pressure are related. When applied to flight, Bernoulli's Principle works like this:

Air flowing over the top of a wing moves faster than air flowing underneath the wing. The difference in speed of airflow causes a difference in air pressure between air over the top of the wing and air underneath the wing. Air pressure over the wing is lower than the air pressure under the wing.

A picture of airflow over a wing
shape
showing
that airflow is faster over the top of the wing, and airflow is slower
underneath the wing

This causes the wing to rise into the pocket of lower air pressure. This is how lift happens.

In the late 1700's, Sir Isaac Newton wondered about the forces that move people and objects around the Earth. He experimented and recorded what he found. His writings are now known as "Newton's Laws". His "Third Law" states that "for every action there is an equal and opposite reaction." For example, when you are swimming and you want to move forward in the water, you move your arms so that you push the water (water molecules) back behind and away from you. By pushing the water back behind you (the action), you thrust your body forward (the reaction). When applied to flight, Newton's Third Law works like this:

A jet engine expels hot, high pressure air out behind it (the action) which causes the airplane to move in the opposite direction or forward (the reaction). This gives the airplane thrust. two balloons showing Newton's Third
Law

Newton also described the principles of gravity. He correctly hypothesized that all objects in the universe exert an attractive force on each other. This force is related to each object's mass, that is the greater the mass, the greater the attraction (or force of gravity). This attraction between the Earth and other objects is measured by weight. Weight is measured in pounds or newtons (That's right, named after Sir Isaac Newton!).

You probably know how much you weigh. To get off the ground, you need to pull away from the Earth's surface with more force than the Earth is using to hold you onto its surface. This is where the forces of weight and lift are used to overcome gravity for flight.

It is this same gravitational pull that holds the Earth's atmosphere close to the Earth. The atmosphere is a thick layer of air molecules that remain close to the surface of a planet or satellite (moon). The greater the mass of the planet, the stronger the gravitational pull. But there is more to gravity and atmosphere that helps airplanes to fly.

Table of Contents