Click here for full page printable version

The Forces

To understand how a plane works you need to look at the forces it encounters.

Thrust:

This is the force that pushes the plane forward. Examples of thrust are:

• If you walk along, your legs move you forward.
• An engine pushes a car forward
• In a plane a propeller or a jet engine pushes the plane forward.

Weight:

This is the force that pulls the plane towards the ground. The heavier something is, the greater weight it has. For a plane to fly it needs a force pushing it up that will equal the weight pulling it down to the ground.

Drag

This is the force that tries to stop the plane moving through the air. You can feel drag in the air when cycling.

• On a calm day when cycling fast, you feel wind in your face and that wind is pushing you backwards, that ‘pushing force’ is the drag.
• On a very windy day when you are on your bike but NOT moving, you also feel the wind on your face and feel the wind pushing you backwards, this force is also drag.
• If you are not moving and there is no wind, there is no drag.

Wind is just another word for moving air. This shows that for drag to occur an object has to be moving through the air OR the air has to be moving past the object. This means that as a plane flies through the air, it experiences drag.

Lift

Lift is the force that keeps an aircraft in the air. On a plane the wings produce the lift and with a helicopter the lift is produced by the rotor blades (see helicopter section). Here we are going to look at wings.
Lift can only be produced when air is moving over a wing. This means that for a plane to fly it’s wings needs to be moving through the air. A wing produces lift because it is a certain shape. If you cut through a wing and look at it side ways you will see the shape that is called an aerofoil.

As air flows over a wing the air on the top of the wing travels faster than the air crossing the bottom of the wing. When the air goes past the wing, the shape of the airfoil turns the air downwards. The difference in speed of the air and the turning of the air means that there is a pressure difference between the top and bottom surface of the wing. There is low pressure on the top of the wing and high pressure underneath the wing. This causes the wing to be ‘sucked up’ causing lift. This action also causes some of the drag that we talked about above.

Air pressure Air pressure is caused by all the air particles moving around and pushing against each other and against other things. Low pressure is when you have a smaller number of air particles and high pressure is when you have a large number of these air particles. Pressure acts in all direction. This means that the air particles push against an object in the air from the top, side and underneath. In the diagram below, arrows represent the air particles pushing against the wing. On top of the wing a few air particles show the low pressure. Underneath the wing the large number of air particles show the high pressure. There are more air particles pushing against the underneath of the wing. This means that there is high pressure under the wing and the wing is forced upwards as shown by the red arrow.

When air flows over the wing, lift is produced acting at right angles to the direction of the airflow and drag is created acting in the same direction as the airflow. In the diagram below you can see the lift and drag produced with the net force shown in red.

What affects lift and drag?

1. Speed:

• Drag: The faster the plane travels through the air, the more drag it produces. For example; the faster you go on your bike the more wind you have in your face, therefore the higher the drag.
  
  
• Lift: The faster the air moves over the wing the greater the lift.

2. The density of the air

Density is how ‘thick’ a fluid is such as air or water. You can imagine different densities by thinking of syrup and water. Syrup is very thick and so has a high density where as water is thinner and so has a low density.

The denser the air the more air particles it has in a fixed space.

Small Number of air particles. Therefore, LOW density air

Large number of air particles. Therefore, HIGH density air

• Drag: A high density fluid like syrup would be hard to swim through showing there is high drag. However, water, which has a lower density, is easier to swim through so lower drag. It is the same with air. At low levels the air is thicker so there is more drag but high up the air is thinner so the drag is lower. Airliners fly very high (about 33,000 feet) in the sky so that they have low drag.



• Lift: The higher the density the more air particles there are that flow over the wing, this means that the lift is higher when the air density is greater.

3. The size and shape of an aircraft.

• Drag: A long thin narrow object like a plane cuts through the air easily while a wide shape takes longer to go through the air. An object that goes easily though air is called streamlined. A plane is streamlined so that it has low drag and cuts through the air easily. This is the reason why landing gear (the wheels that a plane lands on) is retracted into the plane after take off. When the wheels stick out they cause high drag, as they are not streamlined.

• Lift: The wings on a plane are a certain shape depending on the type of aircraft.
• A glider needs a very high lift to drag ratio. The ratio of lift to drag defines the glide angle of the plane. The higher the ratio, the further the plane will fly for any loss of height. Gliders today can have a glide angle of 60 to 1. This means that the glider can travel 60km horizontally for every 1km it descends. A glider has very long thin wings to produce the high lift to drag ratio needed.
  


• A fighter jet needs to maneuver (turn, climb and dive) very quickly and fly at very high speeds. It has very powerful engines and so has shorter fatter wings.
• The wing area and shape affect how much lift it produces. If a wing has a larger area it will be able to produce more lift at slower speeds.

4. The shape and angle of the wing

Angle of Attack: This is the angle that the wing is to the direction of air flow. If the wing is angled upwards, the lift increases. However, after a certain angle the wing will stall. This is when the lift produced by the wing suddenly reduces.

1		At 0 angle of attack the wing has low lift
2		As the wing is tilted up the lift increases
3		The wing now has the greatest lift as the angle of attack is as high as possible
4		The angle of attack is too high so the wing has now stalled which means the lift is suddenly reduced.

The shape of the aerofoil:

Aerofoils come in many different shapes. One way to increase the lift is to have a more curved aerofoil.

A problem with this is that a curved aerofoil like this can produce more drag which is not wanted. Big planes need a large amount of lift at takeoff but not while cruising. Therefore, these planes have what is called flaps. The flaps are extended from the wing at takeoff and landing when the plane is traveling at lower speeds but need a larger amount of lift. When cruising the flap are retracted into the wing so that the drag is reduced.

Flying the Plane

Control Surfaces

A pilot flies a plane by moving ‘control surfaces’. Put your mouse over different parts of the plane to see the control surfaces.

Pitch: The elevators make the plane pitch up or down The elevators are on the tail of the aircraft. If you raise the elevator the tail drops down and the plane pitches up and if you lower the elevator the tail comes up and the plane pitches down. Roll: The ailerons make the plane roll There is an aileron on the back edge of each wing. By moving the ailerons in opposite directions, the plane will roll. Yaw: The rudder makes the plane yaw. By moving the rudder to the right, the plane yaws the right and by moving the rudder to the left the plane yaw to the left. When a plane turns, the plane will roll and yaw at the same time to get a balanced turn.

How the forces work

Now that we have looked at all the forces, how do they actually work together to make a plane fly?

We need to look at the relationships between the weight, lift, thrust and drag. At different times during a flight the relationship between these changes. Please note ' > ' means 'greater than'.