I thought I was referring to the ~200kg rotor of the Glasgow crash aircraft, assuming it operates at ~200rpm. Its rotational energy should then have corresponded to ~3s of full power.
However, in post #995 (
http://www.pprune.org/rotorheads/528...ml#post8197441), I'd overestimated the energy in the rotor (at 2MJ).
Trying again: Four 5-m-long 50-kg blades, give a moment of inertia of 4 x 1/3 x 25 x 50 (~1500) kg.m-squared). At 200rpm (~20 radians/s), the energy - 0.5 x 1500 x 400 J ~ 0.3MJ. With up to a MW of power available, that's indeed a very brief reserve. The airflow complicates things, and makes it different from a spinning object without air, but the energy stored in the rotors is substantially less than I'd assumed.