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The Four Forces

Picture of Antonio Rodríguez-Laiz

Antonio Rodríguez-Laiz

AERTEC / Marketing & Communication


Understanding the principles that keep an aircraft in flight is easy, it is a matter of basic physics. During the phases of design, manufacture and operation of an aircraft, it is an entirely different thing to achieve the desired balance so that it complies with all required features. But, as I say, the principles of physics that enable flight are basic and can be specified as the action of four forces: weight, lift, thrust and drag.

Weight, lift, thrust and drag are the keys to keeping any aircraft in flight.

Straight, level flight is facilitated when the four forces are kept in balance; that is, when thrust balances drag, and lift does the same with weight.

Weight is the force created by the attraction of the earth’s gravity on the aircraft. It is a force that is applied to the aircraft’s centre of gravity (where all the forces of all the weights of the aircraft are in balance), and always vertically through an imaginary line that connects the plane with the earth.

Lift is the main element that keeps an aircraft in flight and can be defined as the force that is created by an aerodynamic shape moving through the air. It is exercised upwards from below, and is perpendicular to the relative wind, and parallel to the flight path.

Lift is affected by several factors such as the density of the air, the wing area, the curvature of the upper wing surface, the angle of attack and the relative wind speed.

Drag is the aerodynamic force that opposes the forward motion of the aircraft through the air mass. It depends on several factors such as the size of the surface area, the speed of movement and the density of the air, among others. The total drag influencing an aircraft can be of two types. Parasitic drag, which increases with speed, is generated by the friction of all parts of the aircraft exposed to the wind (fuselage, engines, landing gear, antennas, slots, moving part, etc.). Induced drag is produced on generating lift, and arises because of the difference in pressure between the air flowing below the wing (high pressure) and above the wing (low pressure). A curious effect produced by this air circulation are the striking wing-tip vortices.

Finally, thrust is provided by the engine (or engines) of the aircraft. This allows the aircraft to move through the air mass in opposition to resistance.

These four forces are key to the understanding of aeronautics and, without a doubt, were present in one way or another in the minds of the pioneers of aircraft design. From the 9th century, when the Andalusian scientist Ibn Abbas Firnas made the first recorded flight in history, to the 16th century, when Leonardo da Vinci designed in luxurious detail artefacts that could have flown, the challenge was always to achieve the perfect balance between these forces.


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