Final answer:
The dominant force affecting helicopter rotors is rotational kinetic energy, which must be stored before takeoff and maintained for lift.
Step-by-step explanation:
The dominant force affecting the rotor system in rotary wing aircraft, such as helicopters, is rotational kinetic energy. Helicopters store large amounts of this energy in their blades, which must be put into the blades before takeoff and maintained throughout the flight. Helicopter engines are typically not powerful enough to both provide lift and significantly increase the rotational energy of the blades simultaneously.
Moreover, due to conservation of angular momentum, the fuselage of the helicopter would rotate in the opposite direction of the blades if not for the stabilizing thrust provided by the small rotor on the tail.
Air pushing upward is an external force that affects the helicopter's motion. This force creates lift by pushing air down, leading to an upward reaction force against the helicopter. If the rotational kinetic energy of the blades drops below a certain level, lift is lost, and the helicopter can potentially crash; therefore, it is crucial to maintain this energy above a critical threshold during flight.