Draft:Four Forces of Flight

• Comment: In accordance with Wikipedia's Conflict of interest policy, I disclose that I have a conflict of interest regarding the subject of this article. Henry20101 (talk) 02:00, 29 July 2025 (UTC)

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The Four Forces of Flight

Weight

The first of the four forces acting on an aircraft is **weight**. Weight is the force exerted on an object due to gravity. Objects in space, including planets like Earth, exert a gravitational force that pulls objects toward them. On Earth, this means “down” toward the ground.

The force of gravity can be calculated using the equation:

• • F = mg**

Where:

• **F** is the force in newtons (N)
• **m** is the mass of the object in kilograms (kg)
• **g** is the acceleration due to gravity, approximately **9.81 N/kg**

When specifically referring to weight, we often use the symbol **W** instead of F. So, the formula becomes:

• • W = mg**

Lift

Lift is the force that acts opposite to weight and helps an aircraft rise into the air. When air flows over and under the wings of an aircraft, it creates different pressure zones. This difference in pressure helps generate lift.

Deflection

As the aircraft climbs, it creates a **positive angle of attack** — the angle between the wing's chord line and the direction of motion. At this angle, air is deflected downward by the wing, and, in response, lift is generated upward (thanks to Newton’s Third Law).

If the angle of attack is negative (the wing points downward), less lift is produced, and the aircraft may descend.

Pressure Differences

Lift can also be explained by pressure differences above and below the wing. As air flows over the curved upper surface of a wing, it speeds up and its pressure decreases. Below the wing, the air moves more slowly and remains at a higher pressure. This pressure difference results in an upward force — **lift**.

This phenomenon can be explained using **Bernoulli’s Principle**, which states:

> “As the speed of a moving fluid increases, the pressure within the fluid decreases.”

Since the force from high-pressure air (below the wing) is greater than the low-pressure force above, the result is lift pushing the aircraft upward.

Thrust

Thrust is the forward-pushing force that propels an aircraft through the air. Early aviators flying hot air balloons had no way to steer and were at the mercy of the wind. To move in a specific direction, a new force — thrust — was needed.

Thrust is another **reaction force**, explained by Newton’s Third Law: *for every action, there is an equal and opposite reaction*.

Propellers

In 1784, Jean-Pierre Blanchard attached a hand-powered propeller to a balloon — an early form of propulsion. Later, in 1852, Henri Giffard created a steam-powered airship with a motor-driven propeller.

Propellers usually have two or more blades connected at a hub. If the propeller is at the front of an aircraft, it's called a **tractor**; if it's at the back, it's called a **pusher**.

Each blade is shaped like an airfoil. When spinning, the blades pull in air and push it out behind the aircraft, generating thrust to move it forward.

Rotors

Helicopters use **rotors** instead of propellers. A rotor has multiple spinning blades (rotor blades) mounted on a central hub. Helicopters usually have a **main rotor** (on top) and a **tail rotor** (at the back).

Rotors generate both **lift** and **thrust**. By adjusting the pitch of the blades, pilots can move the helicopter forward, backward, or side to side — and even hover in place.

Jet Engines

Modern aircraft often use **jet engines** rather than propellers. Jet engines produce thrust by:

1. Pulling in air 2. Mixing it with fuel 3. Igniting the mixture 4. Expelling the hot gases out the back at high speed

This produces forward motion as the engine pushes air back and the aircraft is pushed forward in reaction.

Drag

The final force is **drag**, also called **air resistance**. Like any fluid, air resists the motion of objects moving through it. Just like swimming through water creates resistance, flying through air creates drag. Drag acts opposite to **thrust**, slowing down the aircraft.

There are two main types of drag: **parasite drag** and **lift-induced drag**.

Parasite Drag

Parasite drag includes **form drag**, which depends on the shape of the object. Streamlined shapes like airfoils cut through the air smoothly, creating little turbulence. In contrast, round or flat shapes disturb airflow more, creating more drag.

Lift-Induced Drag

This type of drag is a byproduct of lift. When the wing has a low angle of attack, air flows smoothly. But as the angle increases, air behind the wing becomes unstable. If the angle becomes too steep, the wing reaches the **critical angle of attack**, and the aircraft **stalls** — it can no longer generate lift.

Modern aircraft often have stall warning systems to alert the pilot when approaching a critical angle.

Summary

To sum it all up: the **four forces of flight** are **weight**, **lift**, **thrust**, and **drag**. Aircraft rise, fall, speed up, and slow down depending on how these forces are balanced. Thanks to scientific innovation, we’ve learned how to control these forces — making powered flight possible and allowing us to travel the skies.

• Written by Henry Avery*

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