How an airplane flies: the four forces, simply explained

Four forces in balance

In flight, an aircraft is constantly subject to four forces. Weight pulls it toward the ground. Lift, produced mainly by the wings, opposes it. Thrust, provided by the engines, moves it forward. Drag, the air's resistance, slows it down.

When these forces balance in pairs, the aircraft flies level at constant speed. To climb, accelerate or slow down, the pilot changes that balance.

Where does lift come from?

A wing is shaped and tilted relative to the airflow — the angle of attack. Air is deflected downward, and pressure below the wing becomes higher than above it. This pressure difference produces lift.

Lift increases with speed, wing area, air density and angle of attack — up to a limit. Beyond a critical angle, the airflow separates from the upper surface: this is a stall, and lift drops sharply.

Thrust and drag

A jet engine or propeller accelerates a mass of air rearward; in reaction, the aircraft is pushed forward. Drag includes skin friction, form drag and induced drag, which is a by-product of generating lift. Wingtip devices (winglets) are designed to reduce the latter.

Key takeaways

• Steady level flight means the four forces are in balance.
• Lift depends on speed, wing area, air density and angle of attack.
• A stall happens when the angle of attack exceeds a critical value, not only at low speed.