11.   Eyewitnesses said it appeared the main rotor of the helicopter was not moving.

12.   Instead of a tail rotor, the unique NOTAR helicopters use a stream of air to counter the torque produced by the main rotor blades.

13.   Minutes after takeoff, the main rotor began to turn so slowly that soldiers from another helicopter could count the blades as they moved.

14.   The main rotor drives itself.

15.   To fight the tendency of a helicopter to turn in the opposite direction to its spinning rotor system, Savov has removed the engine that spins the main rotors.

16.   Traditional helicopters have a vertical tail rotor to stop the main rotor from spinning the craft endlessly round and round.

17.   Tulloch moved his controls to stop the helicopter from rolling, but in seconds the main rotor hit the ground.

18.   A similar system on the main rotor moves the copter vertically.

19.   Failure of a swashplate assembly on the tail rotor or main rotor could cause the Apache to crash.

20.   The small size, high agility and fast response helicopter uses advanced composite main rotor blades for greater speed and manoeuvrability and lower vibration levels.