Final answer:
The collective input changes the pitch of all the main rotor blades equally, altering the lift force and allowing a helicopter to ascend or descend.
Step-by-step explanation:
If a collective input is made, all the blades change equally, and as a result, the helicopter increases or decreases its total lift derived from the rotor. This principle is a fundamental aspect of helicopter flight dynamics and involves the understanding of physics concepts such as conservation of angular momentum, lift generation, blade pitch control, and rotational kinetic energy.
Helicopters use their main rotor to create lift by storing large amounts of rotational kinetic energy in the blades, which must be put into the blades before takeoff and maintained until the end of the flight because the engines do not have enough power to simultaneously provide lift and put significant rotational energy into the blades. The small rotor on the tail, or the opposite rotation of two sets of lifting blades in tailless helicopters, counters the conservation of angular momentum to prevent the body from spinning oppositely to the rotor.
Therefore, by adjusting the pitch of the blades equally through the collective input, the helicopter can engage the wind at any angle to vary its efficiency and the total lift force for rising or descending motion.