[GUIDE] The Four Forces Of Flight, How Do Planes Fly?

How do planes fly?

You are probably thinking Yetty, its obvious you push the throttle forward and the plane takes off. Well yes, but its important to understand the actual basic principles of the forces of the plane to understand why it flies, also understanding these forces will help you become a better pilot as you will understand why your plane does certain stuff in certain scenarios.

The Basic Forces

There is for difference forces which react on a plane which effects how it flies. The plane is always in a state of tug at war with these four forces, and if not managed correctly it can end in disaster.

  • Weight – Is simply gravity pulling down on the plane and depends on the weight, making it want to crash into the ground.
  • Lift – It is the force which acts primarily on the plane’s wings, it is created by air molecules hitting the wings at speed and counter-acts gravity pulling it down.
  • Thrust – Is the force which pulls the plane forward, this is created by the engines of the planes.
  • Drag – Is the opposite to thrust, air molecules hitting he plane causes friction which causes drag. Drag is also affected by air pressure differences.

Basically, If the air molecules push the plane up enough to counter-act gravity, then the plane will stay in the air.

How air molecules interact with the plane and generate lift

In the air is loads of little air molecules which bounce around all over, when you have a surface like a wing loads of air molecules will hit the wing.


Imagine each one of these black arrows is an air molecule, when the wing is at a neutral or slight elevation not many air molecules are hitting the wing and therefore no air is pushing up on the wing to generate lift. However, if we increase our Angle of Attack of the wing then more air molecules are hitting the underside of the wing and generating more lift until it counter acts the Weight of the plane and takes off. The more Thrust you give the more air molecules hit the wings and generate more lift.

But also be aware air hitting the wings also generates Drag which can slow down the plane, so you need the perfect amount of speed, the right amount of air molecules hitting the wing at the right angle and you will take off.


On most airplane there is a piece of the wing called Flaps they are usually attached to the wing but can move separately to the wing, the flaps move down and increase the angle of attack of part of the wing, this generates more lift. The more flaps you apply the greater the lift generated but the more Drag is created therefore more Thrust is needed to stop the plane from falling out the sky.

Air Pressure & Altitude

As I mentioned earlier Air Pressure can affect the lift generated and this is quite important. At higher altitudes there is less air pressure then there is at sea level. Because the air pressure is less, effectively there is less air particles to cause lift , this means to generate lift and take off you need more speed generated by Thrust.

What this means is if your taking off from an Airfield with a higher altitude you will probably need more runway to achieve a successful take-off then you would at sea level. Also due to the decreased density of the air at higher altitudes, aircraft engines and propellers are less efficient, which means you will get less lift and a slower rate of climb. If you’re not careful this can be deadly as you can overestimate getting clear of obstacles such as trees and hills.


The last thing to touch on is Temperature. Heat creates thinner air which means less air molecules to create lift, which means you need more speed and thrust to take-off. So the hotter the area the more runway you will need to take-off. The heat will also limit your ability to climb as quick so obstacles can cause deadly situations as suddenly you cannot clear them.