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How do planes fly?


Have you ever wondered, what is takes to get plane up in the air? If so, here is the explanation.

First of all, we need to understand the forces that work on a plane. There are four types of forces on a plane -

  1. Thrust: Force that pushes the aircraft forward
  2. Weight: Force that pulls the plane downward
  3. Drag: Its kind of friction that tries to stop plane from moving forward
  4. Lift: Upward force that keeps the plane in air

Forces on aircraft

For an airplane to fly, following 2 conditions must be true:

  1. Thrust > Drag
  2. Lift > Weight

Thrust is delivered by engine and is directly proportional to its horse power. Drag is usually a by product of aircraft shape, wings and other add on. It is also directly proportional to the speed of the plane. Pilot can increase the drag by lowering the flaps, increasing angle of attack (increasing rate of climb or pulling the nose up) etc. However, if engines are running, Thrust will always be greater than drag.

Lift is the most important factor in flight and is created by movement of air over wings. 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(or effectively a vaccum) on the top of the wing and high pressure underneath the wing. Due to this vaccum, wings get tend to move up, thus providing the necessary lift to the aircraft.

How can you increase lift?
  1. Lift is directly proportional to speed of the plane. Faster the plane, greater the lift.
  2. Higher the airdensity, higher the lift.
  3. Wing - area and shape plays a pivotal role in generating more lift for an aircraft.

Forces in various stages of flight:

  Aircraft Position Forces Acting Plane Position Control Surfaces and Flaps
1 Stationary Weight > Lift The plane is on the ground  
Lift = Thrust = Drag = 0 All other forces are 0
2 Moving down the runway Weight > Lift
The plane is still on the ground
The flaps are deployed to increase the lift that the wings produce.
Thrust > Drag The plane is accelerating (getting faster)as it goes down the runway
3 Take off Lift = Weight
At the exact moment that the plane leaves the ground
The elevators are raised, pushing the tail down and the nose up for takeoff.
The flaps are still deployed
Thrust > Drag The plane is still accelerating
4 Climb Lift > Weight
The plane ascends

The elevators are still raised, pitching the plane upward for the climb.
The flaps are slowly withdrawn as the plane rises
Thrust > Drag The plane accelerates horizontally
5 Cruise Lift = Weight
The plane stays at the same height
The elevators are returned to their neutral (flat) position.
Thrust = Drag The plane descends at a constant speed vertically
6 Descend Weight > Lift
The plane descends The elevators are lowered
Drag > Thrust The plane is decelerating horizontally
7 Landing Weight = Lift
The plane touches down The elevators are lowered.
The flaps are deployed to give the
plane more lift as it is traveling slowly
Drag > Thrust The plane slows to a stop













Source: www.raes.org.uk, www.d155.org, NASA, www.berkeleyscience.com

Partner sites: Shimply.com