Originally Posted by
lomapaseo
the forward force is "lift" and like a leaf that goes to zero in much less than a second as it stalls. The forward speed of the blade is then matched by the forward speed of the aircraft and that leaves the unbalanced force of fore and aft drag relative to the aircraft
Yes, but the momentum of a propeller blade is significant, and what if you don't have a meaningful forward speed when it lets go (e.g. takeoff power setting)? A prop can move a long way in that fraction of a second before drag takes over. If you're going to try to protect the fuselage, you need to assume it might go forward from the plane of rotation.
For uncontained engine failures I think the assumed scatter trajectory was +10/-10 degrees for high energy debris (-30 degrees for low energy debris IIRC). For a released propeller blade think you'd need assume something like +10/-30 degrees - given the distance from the engine to the fuselage that's a pretty large chunk of fuselage you'd need to protect.