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Before I ask "What does Feathering mean when it comes to Jet/Propeller Engines" I have to say what I think I know:

Feathering jet/prop engines mean achieving a sufficient turbine rotation speed at which, the general functionalities such as VFG drives (elec), pumps, etc. could start operation.

I would very much appreciate an explanation with any anomalies.

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    $\begingroup$ I think "feathering" means disabling the propeller on a failed engine. If you think it means something else, would you like to quote/reference the paragraph in which you've seen that word used? $\endgroup$ – ChrisW Dec 13 '14 at 20:52
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Feathering is not engine starting, and also is not used in turbine (excluding turboprop) aircraft. In a piston or turbo prop aircraft, in the event of an engine failure, to decrease drag so you can either glide farther or have better performance on the remaining engine(s), you can set the prop pitch lever so instead of facing at a right angle to drive air backwards and produce thrust, the propeller will instead turn edge-first into the airstream, reducing drag. Not all propeller airplanes have this capability, but it's an added safety feature.

In motorgliders (both light sport aircraft and primary) without retracting blades, the feathering position turns the prop so that the edge faces into the airstream and also locks the propeller so it no longer turns.

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    $\begingroup$ Illegal? Do you have a reference for that? $\endgroup$ – Steve V. Dec 13 '14 at 21:29
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    $\begingroup$ @NathanG: That's not really illegal though. It just means that aircraft with variable pitch propeller(s) does not fit in the light sport aircraft category. $\endgroup$ – Jan Hudec Dec 14 '14 at 20:57
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    $\begingroup$ Yes glide and performance with a feathered prop is better than one spinning in the air like a kid with a pinwheel. But those are happy side effects due to the significant reduction in drag and adverse yaw caused by a free-spinning prop. Moreover a runaway prop can be deadly. The prop can far exceed its normal RPM limits and come apart! Runaway prop blades have known to stretch due to the centripetal forces. $\endgroup$ – radarbob Dec 15 '14 at 5:08
  • $\begingroup$ I'll change it. $\endgroup$ – ptgflyer Dec 15 '14 at 12:19
  • $\begingroup$ motorglider LSAs in the glider classification can have feathering props. $\endgroup$ – rbp Dec 22 '14 at 20:53
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Feathering is only possible with variable pitch propellers and means that the blades are turned such that their mid-to-outer section is aligned with airflow and they create minimal air resistance. This is done when the engine is shut down and the propeller should create minimal drag. This means also that all accessories on this engine will not be powered anymore.

Due to the normal twist of a propeller blade, some sections are still not completely aligned with airflow and produce more normal force and drag than they would if all sections could be aligned with airflow at the same time, but in total those forces will cancel out, and the propeller stops windmilling.

Feathered C-130 prop

This picture of a C-130 engine with feathered prop should help to illustrate the concept.

On pure jets no equivalent setting is possible (we have to wait for variable-pitch fans to add this in the future).

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    $\begingroup$ RE: C-130 feathered prop... The feathered position is such that the prop actually wants to turn backwards. This keeps the prop tight against the prop brake to ensure that the thing does not turn. $\endgroup$ – radarbob Dec 15 '14 at 4:53
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Feathering means to align the propeller blades such that the pressure difference between the camber face and back face are almost equal and hence they produce no thrust or drag.

This feature is mostly incorporated in Multi Engine TurboProp Aircrafts such that if there is an engine failure (any one of the engine); the propeller blades of that engine are feathered so that they produce the minimum drag and also to prevent the windmilling (a condition where the air flow rotates the propeller and in turn the engine, this is a situation where engine is rotating without lubrication and is a very serious concern).

So, when an engine fails the singe acting propeller blades which are fitted with counterweight, have a tendency to rotate toward the high pitch or coarser pitch (toward feather) which is also assisted by a spring inside the pitch change cylinder. If the propeller is a double acting and without counterweight they are usually provided with Manual Feather Pump which works even if the engine is not running and help turn the propeller blade by oil pressure in the cylinder and helps in minimizing the drag or prevent the windmilling condition.

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