What is the relationship between blade inertia and rotor kinetic energy?
The relationship between blade inertia and rotor kinetic energy is:

$$K = N\frac{1}{2}I\omega^2$$
$$I = \int mr^2dr$$

Where: 


$N$ is the number of blades
$I$ is the moment of inertia of a blade
$ω$ is the rotational velocity (rotor rpm)
$m$ is mass
$r$ is distance from hub along the blade


For a fixed RPM increasing the moment of inertia of the blade increases kinetic energy of the rotor. An increase of the moment of inertia can be obtained by increasing the mass of the blades (inefficient, rotor gets heavy and requries more power from engine, less payload, etc.) or by placing relatively small masses at the tip of the blade.