I have not yet had an engine quit in flight, so I cannot provide any first hand experience here. However, in the simulator, 200-220rpm have been observed in flight with a feathered propeller driven by an idling engine. Also, our documentation is rather clear about this; it contains the following figures as an aid to determine the state of a malfunctioning engine/propeller assembly:
- engine running, propeller feathered: 200rpm
- engine dead, propeller feathered: 0 rpm
- engine dead, propeller unfeathered: above 0 rpm; the sim shows about 150-170 rpm constantly in this condition after a V1 cut
So I have no reason to assume that a feathered propeller will not be spun by a still running engine. Another thought: A running engine provides a few percent of torque. Were the propeller to stand still despite this, the torque would have to be countered by something. That "something" can only be aerodynamic force; likely created by airflow interacting with the feathered prop blades. Assuming this to be true, what happens when one shuts down the gas generator? The engine provides no more torque, but the aerodynamic forces in the opposite direction are still there. So the propeller would start to spin in the direction opposite to its normal operation.
On the DH8, this is definitely not the case. Is it on Your type? If not, is a prop brake installed and activated in such a situation?