Which of the following best describes a helicopter's blades during normal operation?

A helicopter's blades are designed to rotate to generate lift, which is essential for both vertical ascent and descent. This rotation creates a difference in air pressure above and below the blades, resulting in lift that allows the helicopter to rise off the ground or descend safely. During normal operation, the rotation of the blades can also be adjusted to change the pitch or angle, enabling the pilot to control the helicopter's movement and direction effectively.

The other options do not accurately describe the operation of helicopter blades. For example, the idea that the blades rotate slowly to increase speed is misleading because while the rotor speed does have to be managed, lift is primarily generated through the angle of attack and rotation speed rather than overly slow rotation. Similarly, the concept of the blades being fixed in place contradicts the fundamental mechanics of helicopter flight, which rely on dynamic movement to create lift. Lastly, while pilot commands do influence blade pitch, the blades always rotate to generate lift; they don't adjust solely based on commands but rather maintain a necessary rotational function to facilitate flight.