The angular momentum of a blade can be calculated by using its mass (m), velocity (v) and length (or radius r) as:
L = m x v x r
Here, only the length of the blade is given ( as 0.5 meters), mass and velocity are not given. In absence of such information, angular momentum can not be calculated. Assuming that the mass of the blade is 1 kg and the velocity is 2 meters per second, angular momentum can be calculated as:
L = 1 kg x 2 m/s x 0.5 m = 1 kg m^2 /s.
Thus, given the mass, velocity and length of a blade, angular momentum can be calculated using the given equation and following the above mentioned example.
Unless there is an external torque applied to the system to change its velocity, angular momentum will remain constant.
Hope this helps.