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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 causes the rest of the vehicle to spin in the opposite direction of the blades. In 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 applies in angular, or rotating, systems as well. The angular analogy of force is torque. In the same way that a linear force can accelerate a mass linearly, a torque can cause the angular acceleration of a mass. The equal and opposite reaction to a torque is called a torque reaction.
Managing this reaction is a fundamental part of flying a helicopter. A helicopter generates lift by spinning a set of blades, which deflect air downwards. The equal and opposite reaction of this downwards force on the air is the upwards force on the helicopter. The spinning blades involve an equal and opposite reaction as well. As the engine spins the blades in one direction, the rest of the helicopter tends to spin in the other.
The tail rotor on a helicopter counters the torque reaction caused by the engine trying to spin the main blades. It usually consists of a smaller set of blades oriented to blow air horizontally. When the force generated by the tail rotor exactly cancels out the torque reaction, a helicopter can achieve stable flight. Without a tail rotor, an ordinary helicopter would begin to spin 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 comparable sets of blades, called tandem rotors, which spin in opposite directions. Though spinning 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.
The torque reaction is important for terrestrial vehicles as well. A motorcycle operates by applying a torque to spin the rear wheel. At low accelerations, the torque reaction is not sufficient to overcome the weight of the front of the motorcycle. When a rider gives enough throttle, however, 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.