The 4 Forces Of An Aircraft
Thrust, Drag, Lift, and Weight are the 4 forces an aircraft needs to fly. Maybe you ask yourself now “Why the hell are these 4 forces so important?”.
If you want to become a pilot, the first theory course will be about principles of flight. This subject covers nearly everything you have to know about the aerodynamics of an airplane. Without these 4 forces, the aerodynamics of an airplane wouldn’t exist.
What are all these forces?
Thrust
Let’s start with Thrust. Thrust is created by the engine(s) and makes it possible that an airplane moves forward. The engine(s) push the incoming air backward with the same force as the air moves the airplane forward. That’s also called “force-pair” and is always equal and opposite (Newton’s 3rd Law). Thrust opposes drag, this means that when Thrust is greater than Drag an acceleration exists and the airplane moves forward (Newton’s 2nd Law). When an airplane flies at a constant altitude and velocity (speed), then:
- DRAG = THRUST
- LIFT = WEIGHT
That means in short that an airplane can’t move forward without thrust!
Drag
Drag opposes Thrust. Imagine you’re sticking your hand out of the window of your car. The force that pushes your hand back is called “Drag”. You push your hand into the wind, so the wind also tries to push your hand back. That means in short that Drag slows an airplane down. Flaps, Spoilers or gears are examples of how Drag is produced on airplanes.
Lift
Lift opposes Weight. An airplane which is situated on the ground, doesn’t have any Lift but it has Weight. Lift is proportional to the square of the velocity of an airplane. This means that if the velocity of the airplane increases, the lift increases too. Because of the Thrust, the airplane moves forward until the lift is equal to the Weight. Without this, an airplane wouldn’t be able to fly!
More about LIFT in another post, feel free to check it out here.
Weight
Weight opposes Lift. In my opinion, it’s the simplest force to explain because we all know what Weight is. Weight is the force pulling down on an aircraft this is due to its gravity.
WEIGHT = MASS x GRAVITY (9.81 m/s2)
Big airplanes often need more lift because they have more Weight than lighter planes. That’s the reason why they have larger wings!
Short summary
If Thrust is greater than Drag, the plane moves forward.
If Drag is greater than Thrust, the plane moves backwards.
If Lift is greater than Weight, the plane moves upwards.
If Weight is greater than Lift, the plane stays on the ground and can’t fly.
The following books helped me a lot to get a better understanding of the forces of an aircraft and much more. I can only recommend them:
