Page 24 - September 24

ASK THE EXPERT
Propeller Restraints: The Good, the Bad and the Ugly
by Tom Clements
As you have experienced, the propeller on the PT6A engine series turns very freely. Even a child using a little finger can spin it quite easily in the hangar or on the ramp, due to the fact the PT6 is a “free turbine” engine in which the propeller shaft is not physically connected to the gas generator or compressor section of the power plant. Wind can also supply the force that causes prop rotation. When parked on a windy ramp, almost always one or both propellers will be rotating, sometimes in the proper clockwise direction, sometimes in the opposite direction, depending on the wind’s direction. Due to the ratio of input to output shaft speed designed into the reduction gearbox (RGB) – N2 to NP – the power turbine is rotating 15 or more times faster than the prop.
22 • KING AIR MAGAZINE
SEPTEMBER 2024