How does an aeroplane fly?

To fly an aeroplane, one has to exploit four basic aerodynamics forces: Thrust, Drag, Weight and Lift.

One may think of them as four arms holding the plane in the air, each pushing from different directions. (diagram)

Let us understand these forces:

Thrust, whether caused by a propeller or a jet engine, it is the aerodynamic force that (pulls or) pushes the aeroplane forward through space.  When a substantial amount of air (or gases) is pushed back through the outlets with force, the body that pushes such air (or gases) thrusts forward as a reaction.

Drag is the opposing aerodynamic force to thrust. It is the frication that resists the motion of aeroplane while moving in space.  This is true for even cars that move on the roads. (If one sticks his/ her hand out of a car window while moving, he/ she’ll experience a very simple example of Drag at work. The amount of Drag one’s hand creates depends upon a few factors, such as the size of the hand, the speed of the car and the density of air.)

Weight: Every object on Earth has Weight, a product of both mass and gravity that pulls the object towards the centre of the earth.

Lift is an opposing aerodynamic force to Weight. This feat is accomplished by using wings, also known as an airfoil.

For aeroplane to fly, Thrust must be equal to or greater than Drag. If, for any reason, the amount of Drag becomes greater than Thrust, aeroplane will slow down (in fact it would not move forward). If, the Thrust is increased so that it’s greater than the Drag, aeroplane will speed up. [Try moving against a strong wind to experiment it.] 

Now for speeding aeroplane to fly, it must be lifted up in the air. The actual mechanics for this lifting is that the force occurs when a moving fluid is objected by a solid obstacle. We know that wings are used in an aeroplane for accomplishing Lift. These wings splits the airflow in two directions, up and above the wing and down along the underside of the wing. The wings are shaped and tilted, so that the air moving over it travels faster than the air moving underneath. The part of the air that moves underneath creates an upward force. This force creates ‘Lift’.

When the moving air flows over an object and encounters an obstacle such as a sudden increase in wing angle, its path narrows and the flow speeds up as all the molecules rush through. Once the obstacle is passed, the path widens and flow slows down again. (If you’ve ever pinched a water hose, you have observed this very principle in action. By pinching the hose, you narrow the path of the fluid flow, which speeds up the molecules. Remove the pressure and the water flow returns to its previous state)

As air speeds up, its pressure drops. So the faster-moving air moving over the wings exerts less pressure on it than the slower air moving underneath the wings. This results in an upward push of lift. Thus an aeroplane flies in the air. In the field of fluid dynamics, this is known as “Bernoulli’s Principle”

Pilots deploy flaps (on the back of the wing) and slat (on the front of the wing) on takeoff and landing. The flaps extend downwards from the trailing edge of the wing, which changes shape of the wing, allowing it to divert more air and thus creates more lift. The alteration also increases drag, which helps a landing aeroplane to slow down.