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Microlight » Aircraft Tech Knowledge » Forces acting on the aircraft

Four Forces

There are four forces which act upon an aircraft in flight.

Lift, weight, thrust and drag.


Lift is produced when air flows over an aerofoil, be it a wing or rotor blade.

Lift acts at 90 degrees to the Relative Airflow.


The price we pay for the lifting force on a wing or rotor blade is drag.

Drag is the aerodynamic force parallel to the relative airflow.


The weight of the aircraft may be represented by a single vector acting towards the centre of the earth, through the aircraft's centre of gravity (CoG). 


In an Aeroplane or Gyrocopter thrust is provided by the propeller or jet airflow and is the force to overcome drag.

In a helicopter, the thrust is provided by the rotor blades.

Straight and Level

For an aircraft to fly straight and level, it must maintain a constant height, speed and direction.

Thrust must equal drag; and

lift must equal weight.

A state of equilibrium or zero acceleration .


A body will remain at rest or in uniform motion unless acted upon by an external force.
To get something to turn, or follow a curved path, a force must be applied to make it turn.
A component of lift is used to achieve this.
That force is called Centripetal Force.
To achieve this we bank the aircraft.
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