Newbie Question 4 - Can't feather prop!

[ememic99]

In that event, there is little need to feather it anyway.
We feather a prop to stop it from rotating in the airflow and creating drag.

Non-feathered blades create more drag even when prop is not rotating.

Of course, it creates some drag. But if it was a significant amount we would have feathering props on single-engined planes.

Changing prop pitch when simulating engine-out landing with SEP shows significat difference in drag.

[Steve]

Non-feathered blades create more drag even when prop is not rotating.

Of course, it creates some drag. But if it was a significant amount we would have feathering props on single-engined planes.

Changing prop pitch when simulating engine-out landing with SEP shows significat difference in drag.

Very true. It seems to surprise a lot of instructors, though. Pulling the prop back is the second step in my engine out procedure, after pitching for best glide. During simulated engine out during my last 2 Flight Reviews, pulling the prop elicited a “What are you doing”? from the CFI.

It definitely makes a large difference in glide range with a windmilling prop.

[Rich]

Non-feathered blades create more drag even when prop is not rotating.

Of course, it creates some drag. But if it was a significant amount we would have feathering props on single-engined planes.

Changing prop pitch when simulating engine-out landing with SEP shows significat difference in drag.

Maybe. I've demonstrated numerous times that it has no effect in my plane. The governor is not directly a pitch control. It limits the max RPM based on the prop control position. Mine will not limit prop RPM any lower than 1520 RPM. (You can confirm your limit during a run-up.) At idle and best-glide my engine turns about 1200 RPM so the governor is ineffective in this mode. In any case it will only increase pitch as required to prevent it from going beyond whatever current setting is. It doesn't mean it goes to coarsest possible pitch.

Since others here repeatedly attest that their's has an effect I am trying to find out whether or not the ~1500 RPM bound is normal. It has always been that way.

[Steve]

Rich:

Next time I fly I'll shoot some video of the maneuver to get some exact numbers for you.

Steve

[nworthin]

Of course, it creates some drag. But if it was a significant amount we would have feathering props on single-engined planes.

Unfeathered props (with a stopped engine) in a SE piston do add significant drag and consequent reduced glide. Adjusting the prop in a constant speed prop airplane helps but there's still a major drag effect. Pitching up to slow airspeed and stop the windmilling prop can help more so long as, after you pitch down to best glide, the prop stays still. But, the best case by a significant margin is with a feathered prop. Just ask the motor glider guys....

As with all things aviation there are engineering and economic tradeoffs. In certified single engine pistons, safety in the event of an engine loss is based upon the low (less than 61kt configured) stalling speed. There is no such certification requirement for a twin. Less inertia on landing in a certified single, even off field, means a fairly good survival chance as has been proven time and again. (You DA40 drivers can chime in here but I suspect the DA40 configured for landing does even better than that).

Still, that begs the question, why put feathering props on twins and not singles? Well, because an unfeatherable prop on a twin can be, in certain situations, unacceptably unsafe. To overcome that safety issue (by dramatically increasing power to weight, adding a huge rudder, etc), the useful load of the twin would have to be diminished to below a servicable level.

But, still, why not put feathering props on a single? Two reasons:

1. Because, what else do you need if you have a featherable prop? Answer: To be certified, a manufacturer has to be able to show that an undamaged stopped engine can be started in flight, and for that you need an expensive and heavy accumulator (or a complicated and even more expensive and heavy geared governor arrangement) if the prop is featherable. So, economics and performance considerations play a role.

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.

There were a few piston singles made (all turboprops I am aware of do feather) that could feather. But, there were too many negatives (cost/weight) to make them marketable.

[Boatguy]

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.

[erchegyia]

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.

[chili4way]

There are three planes with three different systems being discussed in this thread: Austro single-engine, Austro twin engines, and Lycoming single-engine. How each of these operate normally, and in engine-out scenarios is different.

[nworthin]

There are three planes with three different systems being discussed in this thread: Austro single-engine, Austro twin engines, and Lycoming single-engine. How each of these operate normally, and in engine-out scenarios is different.

Quite right. In my comments directly above, I was comparing "generic" Continental/Lycoming SE and ME piston propeller systems and philosophies of design.

Lot of thread drift since I started this by asking whether there were any edge cases where the prop on a DA62 (and presumably a DA42) could not, in fact, be feathered.

[Boatguy]

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.