Page 16 - Volume 11 Number 10

Ask the Expert
Propeller Restraints:
the Ugly
by Tom Clements
The Good, the Bad, and
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 way, 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.
The lubricating oil supply to the RGB comes from the engine’s oil pressure pump that is driven by the compressor section ... the other shaft in the engine: N1, not N2. See the problem? When the propeller is allowed to turn without the engine running, no lubrication is supplied to the gears and bearings in the RGB except for residual oil that was originally there.
So that is why airplanes with PT6 engines are always supplied with some type of propeller restraint
14 • KING AIR MAGAZINE
OCTOBER 2017