Oct 10, 2025
Every time you look up and see an aircraft gracefully soaring through the sky, what you are actually witnessing is a perfectly balanced act of physics. For what is basically a heavy piece of machinery weighing a ton to defy gravitational pull, it must constantly manage four fundamental forces. These are Lift, Weight, Thrust, and Drag.
Lift, Weight, Thrust, and Drag are the fundamental underlying concepts in aerodynamics, they are all what’s known as vector quantities, meaning they have both a magnitude (strength) and a direction. For an aeroplane to maintain a constant speed and altitude (straight-and-level flight), these four forces must be balanced. As aspiring pilots, understanding these fundamentals isn’t just academic, it’s essential for safe, efficient, and precise control of your aircraft.
1. Lift
Direction: ⬆️ Acts perpendicular to the direction of the relative wind (the air flowing past the wing).
Lift is the upward-acting force that opposes Weight and is produced by the effect of air acting on the wing. It is the force that literally enables flight.
Lift is created by forcing a difference in air pressure between the top and the bottom of the wing. Put simply, the curved shape of an aeroplane wing causes air to travel faster over the top surface than the bottom. The faster-moving air creates lower pressure above the wing (a suction), and higher pressure beneath the wing, resulting in the net upward force we call Lift.
The amount of lift generated is primarily controlled by the pilot adjusting the Angle of Attack (the angle between the wing’s chord line and the relative wind) and the airspeed. Increasing the Angle of Attack increases lift.
The principles of Lift are a consequence of Newton’s Third Law (for every action, there is an equal and opposite reaction) and the Bernoulli Principle (faster-moving air has lower pressure). You can learn more about these scientific principles here.
2. Weight
Direction: ⬇️ Always acts straight downward toward the Earth’s centre through the aircraft’s Centre of Gravity.
Weight is the natural, downward-acting force caused by the pull of gravity on the aircraft toward the centre of the Earth. Weight is made up of the total mass of the aircraft, including the structure, occupants, fuel, and baggage. The weight of the aircraft must be overcome by Lift for the aircraft to fly or maintain altitude. Weight is what’s known as a constant force, but it does change during flight (as fuel is burned). For pilots, proper load planning and weight distribution are an important part of flight planning. If the centre of gravity moves too far forward or back during flight, the aircraft can become unstable. For an aircraft to maintain altitude in level flight, the Lift generated by the wings must exactly equal the total Weight of the aircraft.
3. Thrust
Direction: ➡️ Forward, Thrust must overcome Drag to accelerate the aircraft or maintain forward motion.
Thrust is the forward-acting mechanical force generated by the aircraft’s propulsion system (engine and propeller or jet engine). It is the force that opposes Drag.
Thrust is the forward-moving force that propels the aircraft through the air. It is produced by the aircraft’s propulsion system (engine and propeller or jet engine). Thrust is a direct result of Newton’s Third Law of Motion, where the engine accelerates a mass of air backward, and the resulting equal and opposite ‘reaction’ is a force pushing the aircraft forward.
The propeller blades act like tiny rotating air foils, taking a large mass of air and accelerating it backward. For jet engines, air is taken in, compressed, mixed with fuel, ignited, and violently expelled out the rear, creating forward thrust.
4. Drag
Direction: ⬅️ Backwards or rearward force that opposes the motion of an aircraft through the air.
Drag is the backward-acting force that resists the aircraft’s movement through the air or opposes the direction of flight. It is air resistance or the aviation equivalent of friction. Drag is caused by the disruption of airflow over the aircraft surfaces and acts in parallel to and in the same direction as the relative wind, opposing Thrust.
Pilots must be aware of two major types of drag, Parasite Drag, caused by the shape and surface of non-lifting parts (like the fuselage, antennas, landing gear) which increases with speed. And Induced Drag, an unavoidable effect when the wings create lift, as they also create wingtip vortices, which essentially “drag” the wing backward. Induced drag is highest at slow airspeeds and high Angles of Attack.
The Ultimate Balancing Act
In aviation, the state of an aeroplane’s flight depends on the relationship between the opposing forces. The goal is to achieve and maintain equilibrium between all four forces. This applies even in a steady-state climb or descent where the forces are not perfectly opposed but are balanced along the flight path.
- Straight-And-Level Flight. Lift = Weight AND Thrust = Drag. The aircraft maintains constant speed and altitude.
- During Climb. Thrust > Drag AND Lift > Weight (initially). The net upward and forward forces cause the aircraft to accelerate up and forward.
- During Descent. Weight > Lift AND Drag > Thrust (often). The net downward and rearward forces cause the aircraft to lose altitude and typically slow down.
- Acceleration. Thrust > Drag. The aircraft speeds up.
- Deceleration. Drag > Thrust. The aircraft slows down.
Forces Of Flight Quick FAQs
What are some quick questions and answers about the four forces of flight you can take away? Her we go!
Q: What Are The Four Forces Of Flight?
A: The four fundamental forces acting on an aircraft in flight are Lift – The upward force that opposes weight, Weight – The downward force pulling the aircraft toward the centre of the Earth, Thrust – The forward force created by the engines that opposes drag and lastly, Drag – The backward force caused by air resistance that opposes thrust.
Q: How Does An Aircraft Fly Straight And Level At A Constant Speed?
A: Commonly referred to as un-accelerated or steady flight, this is where the forces are all in equilibrium (balanced), so Lift equals Weight and Thrust equals Drag.
Q: How Is Lift Created?
A: Lift is primarily created by the shape of the wings (air foils). Their shape and angle of attack cause air to flow faster over the top surface than the bottom, resulting in a pressure difference (lower pressure above, higher pressure below) that pushes the wing upward.
Q: How Does An Aircraft Speed Up?
A: To speed up, the pilot must increase Thrust so that it is greater than Drag. This creates a net forward force, causing acceleration.
Q: How Does An Aircraft Climb?
A: To climb, the pilot typically increases Thrust and/or changes the aircraft’s Angle of Attack (pitching the nose up). This increases both Lift and Thrust, so that Lift is greater than Weight, and Thrust is greater than Drag (if accelerating).
Q: Which Forces Are Created By The Aircraft, And Which Are Natural?
A: Natural forces acting on the aircraft are Weight and Drag, artificially created forced are Lift and Thrust.
Q: What Happens If Lift Becomes Less Than Weight?
A: If Lift is less than Weight, the aircraft will descend or lose altitude. If this happens suddenly due to the wing’s angle of attack being too high, the wing can stop producing enough lift, leading to an aerodynamic stall.
Every adjustment you make as a pilot, whether it’s a nudge of the throttle, a little movement of the yoke, or something else, all shifts the delicate balance that demands a corresponding adjustment to the other forces. Mastering the manipulation of these forces is what separates a passenger from a pilot! Interested in learning to fly in New Zealand – get in touch with Southern Wings Flight School for more information today!