Chapter 1 · Principles of Flight · PPL
The Four Forces of Flight
5 min read
Every aircraft in flight is acted on by four forces: Lift, Weight, Thrust, and Drag. Understanding how these balance — or fail to balance — is the foundation of all flight theory.
Lift is generated by the wings and acts upward, opposing Weight (gravity pulling the aircraft down). Thrust is produced by the engine and propeller, opposing Drag (air resistance slowing the aircraft).
In straight and level flight at constant speed, all four forces are in equilibrium: Lift = Weight, Thrust = Drag.
In a climb: Thrust exceeds Drag, and Lift slightly exceeds Weight.
In a descent: Drag exceeds Thrust. The aircraft trades altitude for airspeed unless power is managed.
In a turn: The wing must produce more total lift to maintain altitude because the lift vector is tilted. This increases the load factor and raises the stall speed.
Key takeaways
- The four forces are Lift, Weight, Thrust, and Drag.
- In level flight, Lift = Weight and Thrust = Drag.
- A climb requires Thrust to exceed Drag.
- Turns increase load factor and raise stall speed.
- Drag always opposes the direction of motion.
- Weight always acts vertically downward toward Earth's center.
Memory aid
LWTD — Lift opposes Weight, Thrust opposes Drag.
Important terms
- Lift
- Aerodynamic force acting perpendicular to the relative airflow, produced by the wings.
- Weight
- Force due to gravity acting downward through the aircraft's center of gravity.
- Thrust
- Forward force produced by the engine/propeller.
- Drag
- Rearward aerodynamic resistance opposing motion through the air.
- Equilibrium
- Condition where opposing forces are equal — level unaccelerated flight.
- Load Factor
- Ratio of lift to the aircraft's weight — increases in turns and pulls more than 1g.
Common mistakes
What students usually get wrong on this topic — watch out for these.
- 1Thinking lift equals weight only in level flight — load factor changes this in turns.
- 2Assuming more throttle always means climbing — attitude controls climb/descent, power controls airspeed.
- 3Forgetting that drag increases with speed squared — doubling speed quadruples drag.
- 4Confusing thrust with lift — they act in different directions.
- 5Not understanding that weight always acts toward Earth's center, even in a bank.
Quick check (warm-up)
Two short prompts before the main quiz — answer in your head, then expand to confirm.
In straight and level flight at constant speed, what is the relationship between the four forces?
All four forces are in equilibrium: Lift = Weight, and Thrust = Drag.
What happens to stall speed when you bank into a turn?
Stall speed increases because load factor increases — the wing has to produce more lift, which raises the speed at which the critical AoA is reached.
Knowledge Check
Knowledge Check
8 questions · one at a time · feedback and explanation after each answer.