Ford and Lord International Pty Ltd

Torque in quadcopters

When a force is applied to an object, it makes the object move in a straight line. This is called linear motion. The force acts in the direction of the motion, causing the whole object to move along a straight path.

Torque, on the other hand, is a force that causes an object to rotate around an axis. Unlike linear motion, where the object moves along a straight path, torque is responsible for causing an object to rotate or spin about an axis. The simplest example is pulling a door handle. When you open a door, you simply apply torque. The applied force causes a rotation of the door around the hinges. The force you exert and the distance from the hinge determine how easily the door opens. Another example is cycling. Cyclists utilize torque by pushing the pedals to generate the rotational force needed to move the bicycle.

Torque is a fundamental concept in the world of physics and engineering. It is typically represented by the
symbol τ (pronounced tau). In case of quadcopters, torque is particularly significant as it directly influences the stability and control of these unmanned aerial vehicles. It plays a crucial role in the dynamics of various mechanical systems associated with quadcopters. There are a few different types of torques in action at the time of flying.

Below are two important types that play a vital role and need to be understood and considered while designing quadcopters.

Electromagnetic Torque

The spinning of the rotors in brushed or brushless DC motors in a quadcopter is the result of the force known as electromagnetic torque. Electromagnetic torque is the twisting force produced by the interaction between the magnetic fields of the electromagnets and the permanent magnets within the motor. Electric motors typically have a stator, which contains electromagnets (coils of wire carrying electric current), and a rotor, which has permanent magnets. When electric current flows through the coils in the stator, it generates a magnetic field. The magnetic field produced by the stator interacts with the permanent magnets on the rotor. Due to the attractive or repulsive forces between the two magnetic fields, the rotor experiences a torque. This torque is responsible for the rotation of the rotor. The rotation of the rotor continues as long as the electric current flows through the coils, maintaining the interaction between the magnetic fields of the stator and rotor. This electromagnetic torque is what drives the rotation of the motor's shaft. It is a fundamental principle in the operation of electric motors and is crucial in various applications, including those found in quadcopters and other types of machinery.

Reactive Torque

Reactive torque is an essential concept in the field of quadcopters, playing a pivotal role in their stability and control. When a propeller on a quadcopter spins, it generates thrust perpendicular to its axis of rotation, located at the centre of the propeller. While the rotor of a motor rotates in one direction, another torque is generated which tends to rotate the whole motor in the opposite direction, following Newton's third law which states that for every action, there is an equal and opposite reaction. This torque is known as reactive torque. Since all motors are attached to the quadcopter's body, reactive torque tries to rotate the quadcopter around its centre in the opposite direction. In simple terms, reactive torque refers to the rotational force exerted in the opposite direction as a consequence of the propellers' thrust on a quadcopter. It is an undesired torque and balancing this torque is crucial for stable and controlled flight and also to prevent unwanted rotation of the entire vehicle along z-axis.

Yawing Movement and Reactive Torque

Yawing movement involves the rotation of the quadcopter around its vertical axis, changing its direction. This movement is a result of the uneven distribution or lack of balancing of the reactive torque produced by the motors. To address uncontrolled yawing motion, quadcopters use counter-rotating pairs of propellers, with two rotating clockwise and two counterclockwise, cancelling out reactive torque. Flight controllers are incorporated to adjust individual rotor speeds, ensuring balanced reactive torque. However, deliberately creating uneven reactive torque can induce a yawing movement in the desired direction. Thus, reactive torque contributes to the overall rotational dynamics and stability of the quadcopter.

The Torque Formula

Mathematically, torque τ is calculated by multiplying the applied force (F) by the distance (r) from the pivot point. The formula is τ = r × F, with torque measured in Newton-meters (Nm), force in Newtons (N), and distance in meters (m).


In conclusion, torque is a fundamental concept that influences a wide array of daily activities and technological marvels. From the simple act of opening a door to the complexities of automotive engines, understanding torque enhances our comprehension of the physical world. As you delve into the intricacies of this twisting force, you'll gain a newfound appreciation for its role in shaping the mechanics of our everyday lives. So, the next time you turn a key, tighten a screw, or pedal a bike, remember that torque is at the heart of the motion.

Authored by: Azwer Alam