Rotors

• The rotors are the blades on top of the helicopter. When rotated at a rapid speed, they are used to generate lift. Think of how a paddle fan blows air around the room. The rotors of a helicopter is essentially a fan pointed down to generate enough thrust for the aircraft to fly.
  
  

Variable Thrust

• Under normal circumstances, the rotors spin in a way that produces downward force. This thrust offsets the weight of the helicopter. If the helicopter rotors were spun in the opposite direction, however, producing an upward thrust, then it could fly upside down. The force used to levitate an inverted helicopter is considered negative pitch, whereas positive pitch makes a helicopter lift off the ground.
  
  

Reversing the Rotors

• Helicopters that offer reverse thrust are capable of flying inverted. While the helicopter is airborne, the pilot increases the aircraft's altitude, then quickly shifts the rotors in reverse. Since the helicopter is temporarily suspended in free fall, the force of the rotors spins it upside down. When the throttle is accelerated in reverse, thrust is applied toward the ground, and the vehicle remains airborne.