Physics of Flight

At any one time there are around 8,000 large passenger planes in the air around the world, ferrying up to 1.6 million people. This marvel of engineering is made possible by both overcoming and using various forces of physics. 

There are four forces involved in flight - lift, gravity, thrust and drag. These all work together to achieve flight.  

The wings are a key element to an aircraft’s design. Lift occurs because air flows both over the wing and under it. 

Although we can’t see them the air is full of molecules. When the wing cuts through these molecules it forces some of them to go over the wing and some to go under. 

Wings are generally curved on the top and flat at the bottom. This design forces the air particles to behave in a certain way which creates areas of high and low pressure.

The curved part of the wing creates an area of low pressure, meaning not many air molecules are hitting that part of the wing. The flat part of the wing creates an area of high pressure with many air molecules hitting the wings surface. This creates lift.

Gravity is the force that keeps all objects attracted to the Earth’s surface. This force needs to be overcome in order to achieve flight. Weight is the way we measure the gravitational force of an object. 

The heavier the aircraft, the more gravitational force that needs to be overcome. 

In order for an aircraft to fly lift and thrust are needed to overcome its weight. If a hang glider and a person weighs 250 pounds then the lift created by the wings needs to exceed 250 pounds in order to take flight. 

Of course this idea needs to be scaled up for larger objects. A 900, 000 pound 747 will need a lot more lift and drag in order to overcome that weight. This is why these large planes have very large engines and wingspans.

Launching a car off a cliff (creating thrust) will not be enough to get the car to overcome its weight and start to fly. Not enough thrust would be created and lift is also needed, which is provided by wings.

Thrust is the force that pushes the aircraft forwards. Lift and thrust are both created by the aircraft itself, whereas gravity and drag are forces that the aircraft needs to overcome. 

Thrust is created by either propellers or jet engines (or both). Heavy planes need powerful engines which generates enough thrust to create lift which overcomes gravity. 

The blades of propellers push air through the engine of the plane which moves the plane forward. As the wings begin to cut through the air lift is created.

Lift and thrust work together to achieve flight. A very fast car will be unable to fly due to its inability to create lift. In fact, fast sports cars have inverted wings to keep these cars glued to the road!

Drag is the opposite of thrust. Whereas thrust pushes the aircraft through the air drag is the frictional force that resists this motion. Remember air is full of molecules; drag is the contact between the aircraft and these molecules. 

Placing your hand outside the window of a moving car gives you a demonstration of drag. Changing your hand position can either increase or decrease the amount of drag acting upon your hand. 

Drag is generated on every part of an aircraft (or bird). Factors that will increase or reduce the amount of drag include the body shape and the texture of the outer materials.

Most aircraft (and birds!) have a pointed nose which then tapers out to a streamlined shape. This allows the air to flow over more efficiently. A square shaped object would find it very difficult to overcome drag.

Aircraft designed for speed have very slim lined bodies and use smooth materials. This ensures the air molecules flow over the aircraft smoothly, allowing the aircraft to cut through the air at high speed.

Birds have evolved to reduce drag. Birds that need to move quickly will generally have more streamlined bodies and/or be small in size. 

The wings of an airplane are designed to create lift and help it overcome gravity. The engines and propellers are used to create thrust and overcome drag. But how do other human made flying object fly?

These objects still have the same two forces acting against them - thrust and drag but use different methods to bypass them.

Hot air balloons use warmer air to create lift. Warm air is lighter than cold air because it has less mass per unit of volume. However a lot of warm air is needed to lift even small objects which is why the balloons are so large!

Rockets are not designed to fly like most aircraft. It needs to launch almost straight up and to do this it uses an incredible amount of thrust.

The majority of a rocket’s design is used to store propellant - gas or liquid which will generate the power needed to get the rocket into space.  

The rotors of a helicopter provides lift. Tilting those rotors forward helps provide the thrust needed to move through the air. However due to its design helicopters are unable to glide. 

The physics behind animal flight is the same for human-made flight. Birds, bees, butterflies and other flying animals need to utilise lift and thrust and overcome gravity and drag just like human-made aircraft.

All flying animals have wings to provide lift and lighter bone density compared to non-flying animals to help overcome weight. 

The Quetzalcoatlus was the largest flying animal to ever exist. It was the size of a giraffe and probably weighed around 400 pounds. Its wingspan was a minimum of 36 feet. 

Paleontologists are unsure whether the quetzalcoatlus was a gliding animal like an albatross or a vigorous flier. 

It is a popular urban myth that bees defy physics in order to fly. Like the rest of us, bees obey the same laws. Their wing pattern is unusual though; they flap in a circular motion which creates a low pressure vortex, giving them lift.

Birds have three different types of flight; gliding, soaring and flapping. Gliders like the albatross lock their wings into place and use air currents. Soaring birds like the eagle use warm air to lift themselves higher. Flapping birds like the hummingbird need continual wing movement to stay airborne. 

Dragonflies are possibly the most skilled flying animal on the planet. They can fly in six directions including backwards, somersault and roll in the air and can fly across whole oceans. Their four wings are controlled by separate muscles allowing them to move each wing independently.