Originally Posted by
Ugly Jet Captain
One other thing noted to me was since this a had a free turbine the prop blades would stop on impact and not be driven to curl the blades on the prop as in a direct drive engine like a piston. It very well could be that these blades as pictured were not feathered, rather that is the pitch they were in and they did not display rotational damage due to being a free spooling turbine. That seems likely as the time to impact was so short from controlled flight and feathering the prop wouldn't be an expected reaction to a OCF.
The engine in the Texan II is a version of the Pratt and Whitney of Canada PT-6A which has a gas generator driving a free power turbine which in turn drives the propeller gear box. Considerable power is delivered to the propeller in the form of torque which doesn't go away if the propeller encounters an immovable object; consequently, propeller blades are bent upon impact. Just think of the connection between the gas generator and the free power turbine as a fluid coupling, which, in fact, it is, using combustion gases as the fluid. Power is transmitted whether through a fluid coupling or a direct gear drive, and propeller damage is much the same upon impact. I think it is easier to visualize, if one considers damage to helicopter main rotor blades upon suffering a blade strike, and virtually all modern helicopter engines (to my limited knowledge) use a free power turbine design.
Cheers,
Grog