Newbie Question 4 - Can't feather prop!

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erchegyia
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Re: Newbie Question 4 - Can't feather prop!

Post by erchegyia »

Boatguy wrote: Wed Jul 13, 2022 4:50 pm
erchegyia wrote: Wed Jul 13, 2022 6:55 am
Boatguy wrote: Wed Jul 13, 2022 3:32 am FWIW, the prop chart for the DA40NG shows it is "most feathered" at 20% power. The power lever is just a Hall sensor, it's not mechanical. The ECU's tell the governor the desired prop pitch based on the power lever position. In the event of engine failure, with ECU's still powered up (i.e., running on the batteries), setting the power lever at 20% will produce the minimum prop drag. I marked 20% with a piece of tape on my power lever.
Is there a direct connection between the power lever and the feather though? I would assume it will not affect the pitch at all as the stopped engine's parameters will be out of the normal range anyway.
Based on this chart from the AFM which references power lever position I believe there is.
This refers to a running engine.
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Re: Newbie Question 4 - Can't feather prop!

Post by Soareyes »

Thank you for posting on this subject. It motivated me to learn more about props. The DA42 is my first featherable and first FADEC controlled props. I am still a little hazy on the function and failure modes. Check my understanding:
nworthin wrote: Wed Jul 13, 2022 1:24 am 2. Prop governors work by either "pushing or pulling" against springs. They can be set up to fail to fine or coarse. If there is a failure of the propeller governor (and you have a perfectly fine running engine), which way would you want the prop to fail in a SE piston? Would you want it to fail to "fine" feathered mode that provides no propulsion or to the "coarse" unfeathered mode that does? Obviously, coarse mode. In a twin, because of the concerns about asymmetric thrust and other yawing/rolling factors, and the fact that you have that second engine to provide propulsion, you want it to fail to fine feathered mode so that if you ever lose that engine you can continue without undue drama on the backup.
It might be the reverse. A single engine constant speed prop is designed to fail to fine pitch/high rpm mode. Maybe to provide as much thrust as possible if it is a partial power loss or if it is just a governor failure. A twin engine prop fails to coarse pitch/low rpm/less drag mode .

I wondered why twin engine props don't automatically feather when power is lost, either at normal shutdown or in the air. One source said it had to do with engine longevity. If the prop feathered every time we shut it down, when starting up there would be more stress on an engine trying to rotate a feathered prop.
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Rich
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Re: Newbie Question 4 - Can't feather prop!

Post by Rich »

Soareyes wrote: Wed Jul 13, 2022 8:38 pm
nworthin wrote: Wed Jul 13, 2022 1:24 am 2. Prop governors work by either "pushing or pulling" against springs. They can be set up to fail to fine or coarse. If there is a failure of the propeller governor (and you have a perfectly fine running engine), which way would you want the prop to fail in a SE piston? Would you want it to fail to "fine" feathered mode that provides no propulsion or to the "coarse" unfeathered mode that does? Obviously, coarse mode. In a twin, because of the concerns about asymmetric thrust and other yawing/rolling factors, and the fact that you have that second engine to provide propulsion, you want it to fail to fine feathered mode so that if you ever lose that engine you can continue without undue drama on the backup.
It might be the reverse. A single engine constant speed prop is designed to fail to fine pitch/high rpm mode. Maybe to provide as much thrust as possible if it is a partial power loss or if it is just a governor failure. A twin engine prop fails to coarse pitch/low rpm/less drag mode.
An example: MT explains that their propellers for normal single-engine use are built to go to fine (high-RPM) pitch, with the governing force built to restrain it. This helps for things like restart. But for certain applications, such as aerobatic use, they are set to go to coarse pitch with the governor forcing them to fine. This is to prevent severe overspeed during various extreme maneuvers.

For twins it ain't so simple. And FADEC/ECU setups complicate it further.
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