12.60 Straight and Level Flight

12.60.2 Define the four main forces acting in flight.

The four main forces acting on an aircraft in flight are lift, weight, thrust, and drag.

Lift
Lift is the aerodynamic force that acts perpendicular to the airflow and opposes weight. It is generated primarily by the wings as air flows over them.

Weight
Weight is the force due to gravity, acting vertically downward through the aircraft’s centre of gravity.

Thrust
Thrust is the force that propels the aircraft forward, produced by the engine and propeller (or jet engine).

Drag
Drag is the aerodynamic force that opposes motion through the air. It acts in the opposite direction to thrust and is caused by factors such as friction and pressure differences.

Summary
In straight and level flight:

• Lift equals weight
• Thrust equals drag

When these forces are balanced, the aircraft maintains a constant altitude and airspeed.

12.60.4 For level flight describe how the four main forces change as IAS is varied.

In straight and level flight, the forces remain in balance:

• Lift = Weight
• Thrust = Drag

When IAS is varied, the forces change as follows: