The question is about why the effective airflow velocity of a rotor blade element changes with its distance from the rotor shaft axis. The correct answer is that peripheral speed increases if the displacement to the rotor blade tip decreases.

As you move from the rotor shaft towards the tip of the rotor blade, the distance from the center increases. This distance is known as the radius (r). The peripheral speed (also known as tangential velocity) at any point on the blade is given by the formula:

• V = Ω x r

where Ω (omega) is the angular velocity (constant for a given rotor speed) and ( r ) is the radius from the rotor shaft.

Since Ω is constant, the peripheral speed ( v ) increases linearly with the radius ( r ). Therefore, as you move closer to the blade tip (where the radius is larger), the peripheral speed increases. Conversely, if you move towards the rotor shaft axis (where the radius is smaller), the peripheral speed decreases. This change in peripheral speed with radius directly affects the effective airflow velocity experienced by the rotor blade elements at various points along the blade.