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When turboprops shut down, they feather the propellers. If piston aircraft did this, would it ruin the oil system? Also, why do pilots feather very quickly when doing the engine run-up?

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    $\begingroup$ It is wrong to say “When turbines shut down, they feather the propellers”. Not all turboprops are feathered during shutdown. $\endgroup$ Jan 27, 2023 at 5:25
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    $\begingroup$ "On many variable pitch propellers, the blade pitch can be increased to the point that the chord line of the blade is approximately parallel to the on-coming airflow. This process is referred to as feathering." $\endgroup$
    – Mazura
    Jan 28, 2023 at 2:51

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Hydraulically operated propellers feather the same way on both recip and turboprop airplanes, by dumping the oil supply that drives the blades in the fine pitch direction to the sump so that coarse pitch pressure moves the blades to feather. There is often a supplemental hydraulic pump to make sure there is pressure to do that following an engine failure, although in normal operation it isn't needed (counterweight props will rely on the counterweights + a spring that helps the blades move to feather).

Turboprops are feathered at shutdown mostly to stop them from windmilling in the breeze while not running, there being little resistance to rotation coming from within the engine. They'll rotate a bit if there is a breeze from the side pushing on the flat of the blade, but nothing like the pinwheel you'd have if they were set at normal operating pitch when stopped and there was a decent breeze coming from in front or behind. You don't want this happening when the gas generator part of the engine is not producing oil pressure for the turbine shaft bearings.

A piston engine doesn't need to do this for obvious reasons so the feathering system isn't implemented at shutdown.

Pilots won't work the feathering system on a piston engine run up; they just run the propeller control levers between max RPM and min RPM to exercise the pitch change system and get warm oil into the propeller hub, same as a non-feathering propeller. It may sound like it's feathering with the big downward whoosh, but it's just running from normal fine pitch to normal coarse pitch.

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