Overview
This lecture introduces the four fundamental forces acting on an airplane in flight and explains their interactions as a foundation for private pilot training.
The Four Forces of Flight
- The four forces acting on an airplane are lift, weight, thrust, and drag.
- Lift is the upward force caused by wing design (airfoil) and relative wind.
- Weight is the combined load of everything on board the aircraft, acting downward through the center of gravity.
- Thrust is the forward force generated by the aircraft's engine and propeller.
- Drag is the rearward force that resists the aircraft’s forward movement.
- In steady, unaccelerated flight, lift equals weight, and thrust equals drag.
Types of Drag
- There are two main types of drag: parasite drag and induced drag.
- Parasite drag increases with airspeed and includes form drag, interference drag, and skin friction drag.
- Form drag is caused by the shape of the aircraft disrupting airflow.
- Interference drag occurs where different airflow streams meet and create turbulence.
- Skin friction drag results from microscopic roughness on the aircraft's surface.
- Painting or polishing reduces, but does not eliminate, skin friction drag.
- Induced drag is a byproduct of lift and decreases as airspeed increases.
Relationships Between the Forces
- Increasing thrust increases lift, causing the airplane to climb (lift > weight).
- Increasing lift by raising the nose increases induced drag, which can slow the airplane (drag > thrust).
- Adjusting any one force affects the others due to their interdependence.
Lift to Drag Ratio (L/D Max)
- The lift to drag ratio represents the most efficient airspeed with the least total drag.
- Many common airspeeds are based on achieving the optimal lift to drag ratio.
Key Terms & Definitions
- Lift — Upward force produced by the wing's airfoil shape and airflow.
- Weight — Downward force from the mass of the aircraft and its contents.
- Thrust — Forward force created by the engine and propeller system.
- Drag — Rearward force resisting forward motion through the air.
- Parasite Drag — Drag not related to lift, including form, interference, and skin friction drag.
- Induced Drag — Drag produced as a result of generating lift.
- Airfoil — The shape of the wing designed to create lift.
- Center of Gravity — Point where the aircraft's weight is balanced.
- Lift to Drag Ratio (L/D Max) — Airspeed at which lift relative to drag is maximized.
Action Items / Next Steps
- Review lesson on airfoil and induced drag in the next class.
- Start the recommended video playlist to continue with lesson two.