What unseen forces help an airplane fly?

Image above: Graphic of a plane with thrust, lift, gravity, and drag identified.

A pull. A tug. A push. Unseen forces, some working against each other, all affect an airplane.

Weight (a result of the force of gravity) pulls a plane toward the ground.Lift, created by the air flowing over a wing, works to overcome gravity’s downward tug. Thrust pushes the plane forward. Propellers, engines, and rockets create this force. Pulling in the opposite direction is drag. Friction, one example of drag, slows down planes. Air resistance causes drag.

There’s a constant “tug-of-war” between these four forces. An airplane can fly when lift is greater than or equal to gravity’s pull and thrust is stronger than drag. Stick your hand out of a moving car window and you can feel these forces. The wind’s force hits the flat surface of your palm, making your hand move up or down. Change the angle of your hand and you change the movement. Your hand has lift. Tilt your hand in different ways and you also feel drag force due to air resistance.

What causes an airplane to have lift? Currently, there are two explanations for this phenomenon. One follows along the thinking of Daniel Bernoulli, an 18th century Swiss physician and mathematician. This explanation, the pro-Bernoulli or Airfoil-Shape explanation, says that wings are “pushed upwards” because air pressure on the bottom of a moving wing is greater than the air pressure on top. “Bernoulli’s Principle” explains that moving air affects air pressure to create lift. His principle shows that the faster air moves, the less pressure it has.

An airplane’s wing is shaped so that the air traveling over the top of the wing travels farther than air beneath the wing. This shape forces the air above the wing to move faster with less pressure than the air on the bottom of the wing. A plane has lift, according to Bernoulli, because the air on the bottom of a wing has more pressure or push than the air on top. This difference in pressure creates lift to hold the plane aloft.

Image above: Side view of an airplane wing with arrows showing air movement.

Another explanation, the pro-Newton version, says that wings are forced upward because they are tilted and deflect air downward and that the plane reacts by being pushed upward. Both the upper and lower surfaces of the wing deflect the air. The upper surface deflects air down because the airflow “sticks” to the wing surface and follows the tilted wing. This interpretation is also called the “Coanda Effect.”

Actually, both the Bernoulli and Newtonian effects contribute to airplane flight. The battle between lift and gravity determines whether airplanes fly or are grounded. Airplanes can fly only when lift and thrust are greater than gravity and drag.

Gravity. Lift. Thrust. Drag. NASA engineers and scientists continue to learn more about these forces testing models of planes in wind tunnels and simulations of these planes on computers. By carefully studying these forces, NASA aeronautics researchers and engineers are able to design and build safer, quieter, and more fuel efficient planes.

Glossary

Bernoulli’s Principle – fluids, including air, moving at greater speeds have less pressure than fluids moving at slower speeds

Drag – the air resistance acting on airplanes. Drag acts in the opposite direction to thrust.

Force – a push or pull used to lift something, start it moving, or hold it in place against another force, such as gravity

Friction – the force that slows down motion when something rubs against another thing

Gravity – the force of attraction that makes objects fall toward the Earth

Lift – a force that acts upward against gravity and makes it possible for airplanes, airships, and balloons to rise in the air

Thrust – a forward force that pushes an airplane through the air