What Is Torque Reaction? Definition, Meaning and Concept

The torque reaction is the equivalent of Newton's third law for angular systems. It is a consequence of the law of conservation of angular momentum. In helicopters, it makes the rest of the vehicle turn in the opposite direction of the blades. On bicycles and motorcycles, it allows riders to do a wheelie.

Newton's third law, developed by English mathematician and physicist Isaac Newton, states that for every action, there is an equal and opposite reaction. This law is generally used in the context of linear forces, but it also applies to angular or rotating systems. The angular analogy of force is torque. In the same way that a linear force can accelerate a mass linearly, a torque can cause angular acceleration of a mass. The equal and opposite reaction to a pair is called a pair reaction.

Managing this reaction is a fundamental part of flying a helicopter. A helicopter generates lift by turning a set of blades, which deflect air downward. The equal and opposite reaction of this downward force in the air is the upward force on the helicopter. Rotating blades also involve an equal and opposite reaction. As the motor turns the blades in one direction, the rest of the helicopter tends to turn in the other.

The tail rotor in a helicopter counteracts the torque reaction caused by the engine trying to turn the main blades. It generally consists of a smaller set of blades oriented to blow air horizontally. When the force generated by the tail rotor exactly cancels the torque reaction, a helicopter can achieve stable flight. Without a tail rotor, an ordinary helicopter would start spinning uncontrollably in the opposite direction of the main blades.

The Boeing CH-47 Chinook is a helicopter that solves the torque problem in a different way. It uses two sets of comparable blades, called tandem rotors, that rotate in opposite directions. Although turning each rotor produces a torque reaction of its own, both reactions cancel each other out. The tandem rotor design is used on many helicopters that need to lift heavy loads.

Torque reaction is also important for land vehicles. A motorcycle works by applying torque to turn the rear wheel. At low accelerations, the torque reaction is not enough to overcome the weight of the front end of the motorcycle. However, when a rider gives enough throttle, the reaction can cause the front of the motorcycle to lift off the ground or do a wheelie. Even without contact with the ground, the rest of a motorcycle would tend to spin in the opposite direction of an accelerating rear wheel.