The force of the air
Would you like to make a parachute jump? The force of the air can raise you to a great height and then let you come down safely. Here's how.

Imagine you have your parachute packed on your back. You put on your crash helmet then make your way out onto the runway to climb into an aeroplane. When you are safely strapped into the aeroplane, the pilot makes it taxi to the end of the runway. The aeroplane is turned round and when the all-clear signal is received from the control tower, the pilot increases the power of the engine. When this happens, the propellers spin very fast. Air flows over their curved surfaces and pulls the propellers and the aeroplane through the air. You can think of the propeller as screwing its way through the air like a metal screw pushing itself into a piece of wood. In fact, another name for propeller is airscrew.

As the aeroplane moves along the runway its speed increases and air flows over all its surfaces. The curved surface of the main body or fuselage of the aeroplane allows the air to pass over it without pushing strongly on it. This curved shape is known as a streamlined shape. On the wings the air is flowing in a different way.

If you stand some distance away from an aircraft and look at the end of its wing, you will see that it has an upper surface and a lower surface. The upper surface curves upwards and the lower surface is flat. This shape is called an aerofoil shape and it allows the aeroplane to rise into the air. The aeroplane rises owing to the force of the air.

When the aeroplane moves forwards air flows over the upper surface and lower surface of the wing. When the air has flowed over the surfaces, it meets at the back of the wing, which is called the trailing edge. The curved shape of the upper surface makes the air flowing over it travel further than the air travelling over the flat surface under the wing. Because the air above the wing must meet the air below the wing at the same time at the trailing edge it must travel faster. Air pushes down on every surface that it touches but if it is travelling quickly over a surface it does not have enough time to push hard. This means that the force of the air pushing down on the wing is less than the force of the air pushing up on the wing. The stronger upward force pushes upwards on the wings and makes them lift into the air. As the rest of the aeroplane is attached to the wings, the whole aircraft rises into the air.

Once the aeroplane is in the air, the pilot makes it go round in a circle to gain height. Eventually you are told that you are high enough to make your jump. You unstrap yourself from your seat and make your way towards the door. You push the door open and look down at the fields below you. They appear smaller than postage stamps. You tell the pilot you are ready to jump and when she nods her head you leap into the air.
The air rushes past you as you fall. You gather speed every second then you pull the ripcord on your parachute. As the parachute unfurls the air rushes into it and spreads it out. In seconds a huge amount of air is trapped below it and the ropes holding you to it tighten. As the weight of you and your parachute push downwards an upward force, called air resistance, pushes against the parachute and slows you down. The size of the air resistance is always lower than the size of your weight so you keep moving down. By pulling on the ropes connected to your parachute you can control its direction and bring it down to the airfield. As the ground rushes up to meet you pull on the parachute and it catches a gust of wind to slow you down even more and make a very safe landing.

Why does the aeroplane have to move very fast to get into the air?
The faster the aeroplane goes the faster the air flows over the wing surfaces. The faster the air flows over the wing surfaces the greater the lift on the wings. If an aircraft does not move fast enough it will not get into the air.

How does a jet engine work?
Inside a jet engine kerosene is burnt. This makes a ball of hot gas, which pushes on every surface inside the engine. At the back of the engine is an opening. When the gas pushes on this it passes out into the air. The force with which the gas is pushed out of the engine is balanced by a force which pushes the engine forwards. As the engine is attached to the rest of the 'aircraft, the aircraft is pushed forwards too. Some people think that the gases push on the air to make the aircraft move forwards. This is incorrect. Rocket engines work in the same way as jet engines. They can work perfectly well in space where air is not present.

Is the shape of the wing changed to make the aeroplane land?
Yes, it is. At the trailing edge are flaps, which are lowered when the aeroplane is coming into land. The flaps increase the area of the aeroplane on which the air can push and this slows down the aeroplane so it can land safely.

Do birds have a similar wing shape to aeroplanes?
Yes, they do. They also have an aerofoil shape. When a bird takes to the air it beats downwards and backwards on the air. A force acting upwards and forwards then lifts the bird into the air. When the bird holds its wings out the aerofoil shape creates lift to make the bird rise.

Does a sparrow's wing act just like a gull's wing?
Not entirely. A sparrow's wing is shorter and wider. This is useful for flapping flight. A bird which flaps its wings a great deal can twist and turn easily among trees and bushes. A gull's wing is long and thin. This is used for gliding flight. You may often see gulls gliding in columns of rising air called thermals.

What are the forces which act on an aeroplane as it moves through the sky?
The forward force created by the spinning propeller of the jet engine is called thrust. The force which works in the opposite direction is called drag. This is created by the air moving over the fuselage of the aircraft. The upward force called lift is opposed by the downward force of the aircraft's weight. When an aircraft is moving across the sky the force of the thrust is greater than the drag and the force of lift is greater than the weight. If this was not so the aircraft would drop out of the sky like a stone.

How does a helicopter fly?
Each of the rotor blades has an aerofoil shape. When the engines turn the rotors they behave like spinning wings and lift the helicopter into the air.

Why do skydivers fall faster and faster when they leave the aeroplane?
The force of gravity is pulling on them all the time so this keeps them speeding up or accelerating. As they move faster and faster, the air resistance gets stronger and stronger. Eventually the air resistance becomes so strong that it balances the force pulling the skydiver down and they fall at a steady speed called the terminal velocity.

How does a hovercraft work?
There are fans on the top of a hovercraft which draw air in. The air is pushed below the hovercraft where there is a skirt made of flexible material which prevents most of the air from escaping. As more air is drawn in, it becomes squashed or compressed underneath the hovercraft. The upward force of this compressed air is greater than the weight of the hovercraft so it rises. An air cushion forms beneath the hovercraft. This reduces friction between the hovercraft and the ground. The hovercraft is moved forwards, backwards or sideways by propellers.