What forces do aircraft move with in the air?

To understand how aircraft move through the air, it is necessary to first examine the effects of physical forces. An aircraft’s ability to remain airborne and maneuver depends on the interaction of four fundamental forces: lift, weight, thrust, and drag. Among these, thrust and lift are particularly critical for takeoff and sustained flight.

1. Thrust Force

Thrust is the primary force that propels the aircraft forward. Jet engines or propellers draw air in and expel it at high speed rearward. According to Newton’s third law, this action generates an equal and opposite reaction that pushes the aircraft forward. Thrust always opposes drag. If thrust is sufficiently greater than drag, the aircraft accelerates; if not, it decelerates.

2. Lift Force

Lift is generated by the aircraft’s wings. Their specialized shape, known as an airfoil, forces air to flow at different speeds over the upper and lower surfaces. This creates lower pressure above the wing and higher pressure below, resulting in a net upward force. This force is the primary factor that keeps the aircraft airborne. Lift always acts perpendicular to the direction of flight and opposes the force of weight.

3. Weight Force

Weight is the force exerted on the aircraft by gravity, representing the total mass of the airframe, passengers, fuel, and cargo. This force always acts toward the center of the Earth. The center of gravity is the point at which this force is effectively applied. Lift is the force that counteracts weight; if balance is not maintained, the aircraft descends.

4. Drag Force

Drag is the resistance the aircraft encounters as it moves through the air. It acts opposite to the direction of motion and tends to slow the aircraft. Sources of drag include:

• Friction between the air and the aircraft’s surface
• Pressure differences caused by the shape of the fuselage
• Aerodynamic effects such as vortices at the wingtips

Thrust is the force that opposes drag. Modern aircraft designs aim to reduce drag to improve fuel efficiency.

Equilibrium State: Balance of the Four Forces

When the four forces acting on an aircraft are in balance, the aircraft flies at a constant speed. In this equilibrium state:

1. Lift balances the weight force caused by gravity.

2. Thrust balances the drag force caused by air resistance.

This condition can also be explained by Newton’s laws of motion. When all forces acting on a body are balanced, the body remains at rest if it was stationary, or continues moving at a constant velocity if it was in motion. For an aircraft, this means level and stable flight.

^{Note: L = lift, W = weight, T = thrust, D = drag. (> : Greater than) (< : Less than) (= : Equal to).}

Coordination of Wings and Engines

Lift is generated by the aerodynamic design of the aircraft’s wings, which is based on Bernoulli’s principle. According to Bernoulli’s principle, as the speed of a fluid—in this case, air—increases, its pressure decreases; as its speed decreases, pressure increases. The upper surface of an aircraft wing is curved, while the lower surface is relatively flat. This design accelerates the airflow over the top of the wing, reducing pressure above it. The slower-moving air beneath the wing creates higher pressure. As a result, the pressure below the wing exceeds the pressure above it, generating an upward lift force that enables the aircraft to take off.

The aircraft’s forward motion is enabled by the thrust force produced by its engines. However, another force encountered during flight is drag, which arises from air resistance and acts opposite to the direction of motion. For example, the resistance felt when walking against the wind is a simple everyday analogy of drag. The thrust provided by the engines must overcome this drag force for the aircraft to move forward and sustain flight.