Principles of flight

For thousands of years, people have wanted to fly. Our legends and fairy tales are full of humans and animals that can fly – effortlessly gliding through the air. In real life, of course, no one can just fly into the air. We don’t have wings and a power source strong enough to keep the wings moving through the air to sustain the lift necessary for flight.

Our attempts to fly have taken us from flimsy paper hot-air balloons and strange-looking gliders to supersonic jet planes. We have learned about the forces of flight, and we know what it takes to keep birds and planes in the air.

Force can be defined as a push or pull. Unbalanced forces produce an acceleration of an object in the direction of the resultant force. Four main forces affect the flight abilities of birds and planes – weight, lift, thrust and drag.

Weight and lift

We all know that gravity is a force that pulls everything towards the Earth’s surface. This pull is called the weight force.

Planes and birds have to be able to provide enough lift force to oppose the weight force. Lift is a force that acts upwards against weight and is caused by the air moving over and under the wings.

Explore these ideas further in our article Wings and lift.

Thrust and drag

The power source of a bird or plane provides the thrust. Thrust is the force that moves the object forward. Thrust is provided by:

• muscles – for birds and other flying animals
• engines – for flying machines
• gravity – for gliders that actually fly by always diving at a very shallow angle (birds do this too when they glide).

The force working against thrust is called drag. It is caused by air resistance and acts in the opposite direction to the motion. The amount of drag depends on the shape of the object, the density of the air and the speed of the object. Thrust can overcome or counteract the force of drag.

How it works

An object in flight is constantly engaging in a tug of war between the opposing forces of lift, weight (gravity), thrust and drag. Flight depends on these forces – whether the lift force is greater than the weight force and whether thrust is greater than drag (friction) forces.

Lift and drag are considered aerodynamic forces because they exist due to the movement of an object (such as a plane) through the air. The weight pulls down on the plane opposing the lift created by air flowing over the wing. Thrust is generated by the propeller (engine) and opposes drag caused by air resistance. During take-off, thrust must counteract drag and lift must counteract the weight before the plane can become airborne.

If a plane or bird flies straight at a constant speed:

• lift force upwards = weight force downwards (so the plane/bird stays at a constant height)
• thrust force forwards = opposing force of drag (so the plane/bird stays at a constant speed).

If the forces are not equal or balanced, the object will speed up, slow down or change direction towards the greatest force.

For example, if a plane’s engine produces more thrust, it will accelerate. The acceleration increases air speed past the wing, which increases lift so the plane gains altitude. Then, because the plane is moving faster, drag (air resistance) is increased, which slows the plane from speeding up as quickly until thrust and drag are equal again. The plane can now remain at a constant but greater height.

A plane can lose altitude by reducing thrust. Drag becomes greater than thrust and the plane slows down. This reduces lift and the plane descends.

Explore the causes of aerodynamic drag as it also relates to cyclists.