Police helicopter crashes onto Glasgow pub

[awblain]

jimjim1's note on rotor speed when windmilling

Rotor rpm increasing from 73 to 125 per cent does indeed correspond to a kinetic energy increase of almost 3.

However, the power available to a windmill goes as the airspeed through the disk to the third power, and so the ability to pick up speed is very sensitive to the airflow conditions.

With the right blade angle of attack, a ~2m/s change in the flow speed through the disc could power the rotor as much as the ~1 MW from the engines. (Power ~ disc area * rho * v^3)

The kinetic energy of the rotor is also not large compared with the power flowing through it. A 200-kg rotor, 10-m in diameter has something like 2MJ of energy at 200rpm (~20 radians/s). But, it's driven by about a MW of power, so in the right/wrong conditions, it could stop in a few seconds.

Edit: The energy in the turning rotor's overestimated here.
Moment of inertia for 200kg, 5-m radius, turning at 200rpm is about (1/3)*200*25~1500 kg m-squared. At 20 radians/s, energy is only 750*400J~0.3MJ.