Basic Flight Principles: The Four Forces

Virtual Flight Simulation Course
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The four forces of flight are weight, lift, thrust, and drag. These forces work together to allow an aircraft to fly and maintain controlled flight. Understanding these forces is essential for pilots in order to properly operate an aircraft.


Weight is the force that pulls the aircraft downward due to gravity. It is determined by the mass of the aircraft and its contents, as well as the acceleration due to gravity.

  • Centre of gravity (CG): The CG is the point at which the aircraft’s weight is evenly distributed. It is important to maintain the CG within the acceptable range specified by the manufacturer in order to ensure stable flight.
  • Lateral vs longitudinal CG: The lateral CG refers to the balance of the aircraft from side to side, while the longitudinal CG refers to the balance from front to back. If the CG is too far forward, the aircraft may be unstable and difficult to control. If it is too far aft, the aircraft may become nose heavy and potentially stall.


Lift is the force that allows the aircraft to rise off the ground and into the air. It is created by the difference in air pressure on the top and bottom of the wings.

  • Angle of attack: The angle of attack is the angle at which the wing meets the air. A higher angle of attack will create more lift, but also increases the risk of stalling.
  • Bernoulli’s principle: This principle states that as the speed of the air moving over the wing increases, the pressure decreases. This difference in pressure creates lift.
  • Factors that affect lift: The shape and size of the wings, the density and velocity of the air, and the angle of attack all play a role in the amount of lift generated by the wings.


Thrust is the force that propels the aircraft forward. It is created by the engine or engines of the aircraft.

  • Types of propulsion: There are several types of propulsion systems used in aircraft, including jet engines, propellers, and rockets.
  • Factors that affect thrust: The type of propulsion system, the size and power of the engine, and the speed and direction of the air being expelled all affect the amount of thrust generated.


Drag is the force that opposes the motion of the aircraft. It is created by the resistance of the air against the aircraft as it moves through it.

  • Types of drag: There are two types of drag: parasitic drag and induced drag. Parasitic drag is caused by the resistance of the air against the surface of the aircraft, such as the fuselage, wings, and tail. Induced drag is caused by the lift being generated by the wings.
  • Factors that affect drag: The shape and size of the aircraft, the speed and direction of the air, and the type of surface materials all contribute to the amount of drag experienced by the aircraft.
  • Reducing drag: Pilots can reduce drag by flying at a lower altitude, where the air is denser, and by maintaining a smooth, streamlined shape.

In order to maintain controlled flight, it is important for pilots to understand and manage the balance of these four forces. Weight and lift must be balanced in order to maintain level flight, while thrust must be greater than drag in order to move the aircraft forward. By understanding these forces and how they interact, pilots can safely and effectively operate their aircraft.

It is also important for pilots to understand the limitations and characteristics of their specific aircraft. Each aircraft has its own unique weight, lift, thrust, and drag characteristics, and it is important for pilots to be familiar with these in order to operate the aircraft safely and efficiently.

Understanding and managing these forces is crucial for all pilots, whether they are flying a small single-engine aircraft or a large commercial airliner. By learning about the four forces of flight, pilots can gain a deeper understanding of how their aircraft operates and how to safely and effectively control it.

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