What to do with Prop in Engine Failure ? Why ?

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epsalant

Filing Flight Plan
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Evan
The POH for my plane (M20J) doesn't say anything about changing the prop control for engine failure in order to maximize glide. The POH just states the prop condition is "Windmilling". The Bonanza F33 POH does mention for best glide "Prop--Low RPM."

I don't quite understand the physics of why low RPM (coarse pitch) will cause less drag than high RPM (fine pitch). Is the difference substantial ?

I understand why feathered is better than windmilling, but not an option on my plane.

Can anyone explain the physics or point me to a good technical article ? I don't understand why a prop spinning at e.g. 1000 RPM should cause less drag than say 1500 RPM.

Thanks !
 
Coarse is just a step closer to feather which is why it may present less drag.
Many will say to expect no change in glide distance etc however a member of our club/chat group did some experimentation with his engine turned off, over the airport and reports numbers showing a greatly increased glide distance with the prop pulled (coarse) even though they are said by some to hit the fine pitch stops when no oil pressure. IO-520 or 550, can't recall.

Now - how much, if any, change in glide distance if the prop is turning,
a) full throttle b) closed throttle? (butterfly plate resistance) :D
 
It takes a force to spin the engine. And, the faster you spin, the more power the engine will absorb. The more power absorbed by the engine, the worse your glide.
 
If the engine is windmilling, odds are, it has oil pressure.

If you've lost all the oil, it will likely seize and not windmill. If you haven't lost all the oil, a turning prop means a turning oil pump.

Most of the engine resistance is in piston rings (not counting compression because it's reversible and you get most of it back in the power stroke). It won't make much difference if the throttle is wide open but no fuel is burning. You will get less resistance if the engine runs slower.
 
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Any technical pubs on this or did the member post his data ?
 
epsalant said:
The POH for my plane (M20J) doesn't say anything about changing the prop control for engine failure in order to maximize glide. The POH just states the prop condition is "Windmilling". The Bonanza F33 POH does mention for best glide "Prop--Low RPM."



Thanks !
Click to expand...

A constant speed prop on a single uses high oil pressure from the governor to push the prop blades to coarse pitch while on a full feathering prop on a twin that pressure drives the blades toward fine pitch.

So, if you have an engine failure on a single the oil pressure reduces and the governor can no longer keep the blades in coarse pitch. As the pressure eases the blades will go to fine pitch and increase the frontal area of the blades.
 
Grab a copy of the Airplane Flying Handbook and look at figure 12-3. You will see that a windmilling prop is major drag....that why feathering was invented.

There is a concept called "flat plate drag." As long as the engine is running and the prop is pulling the airplane through the sky, all is well. Lose that propulsive force and now the airplane, as it inevitably descends, is pushing the prop. As the prop is forced to rotate it turns the crankshaft which makes the pistons go up and down and compress the air in those cylinders when the cam closes both intake and exhaust ports...that's a lot of work going on with no return on investment. The prop is now essentially a flat plate bolted onto the crankshaft. Just consider flying along with an 84-inch flat plate on the front of the airplane.

There is a sweet spot between when the prop is pulling the airplane and when the airplane is pushing the prop...you can feel it in your butt.

Bob Gardner
 
epsalant said:
The POH for my plane (M20J) doesn't say anything about changing the prop control for engine failure in order to maximize glide. The POH just states the prop condition is "Windmilling". The Bonanza F33 POH does mention for best glide "Prop--Low RPM."

I don't quite understand the physics of why low RPM (coarse pitch) will cause less drag than high RPM (fine pitch). Is the difference substantial ?

I understand why feathered is better than windmilling, but not an option on my plane.

Can anyone explain the physics or point me to a good technical article ? I don't understand why a prop spinning at e.g. 1000 RPM should cause less drag than say 1500 RPM.

Thanks !
Click to expand...

Evan, slowing the prop in a power-off glide makes an enormous difference. You can feel it easily - next time you are up, at a safe altitude start a power off glide with the prop at fine pitch. When your glide speed has stabilized, quickly move the prop lever to full coarse pitch. Report back - I am confident you will be surprised at the amount of drag present at fine pitch.
 
But feathering props on engine failure are only on twins, not singles, right?
 
coloradobluesky said:
But feathering props on engine failure are only on twins, not singles, right?
Click to expand...
Pretty much true although a few singles actually do have feathering props, some even have beta (reverse). Also the hydraulic prop conversion for the early Bonanzas which came with electric prop pitch control uses a hub that works like the ones on most twins which means that it goes to coarse pitch if oil pressure is lost.
 
bobmrg said:
Grab a copy of the Airplane Flying Handbook and look at figure 12-3. You will see that a windmilling prop is major drag....that why feathering was invented.

There is a concept called "flat plate drag." As long as the engine is running and the prop is pulling the airplane through the sky, all is well. Lose that propulsive force and now the airplane, as it inevitably descends, is pushing the prop. As the prop is forced to rotate it turns the crankshaft which makes the pistons go up and down and compress the air in those cylinders when the cam closes both intake and exhaust ports...that's a lot of work going on with no return on investment. The prop is now essentially a flat plate bolted onto the crankshaft. Just consider flying along with an 84-inch flat plate on the front of the airplane.

There is a sweet spot between when the prop is pulling the airplane and when the airplane is pushing the prop...you can feel it in your butt.

Bob Gardner
Click to expand...

Yes, I saw that ! (In fact, I read the whole AFH and PHAK and a whole bunch more for my CFI-A last week, it paid off with the oral going very well---practical went fine, too). I understand the difference between feathered and flat plate, I just don't see that much difference between low (say 1500) and high RPM in terms of flat-plate effect. I was wondering if it is the turning of the engine or the negative angle of attack on the prop blades with a rearward lift vector or what.
 
Skip Miller said:
Evan, slowing the prop in a power-off glide makes an enormous difference. You can feel it easily - next time you are up, at a safe altitude start a power off glide with the prop at fine pitch. When your glide speed has stabilized, quickly move the prop lever to full coarse pitch. Report back - I am confident you will be surprised at the amount of drag present at fine pitch.
Click to expand...

Skip, thanks, I will do this. I was also interested in the physics behind this so I could explain it to others.
 
Hank S said:
Mostly. Mooney Rockets have the engine and feathering prop from a Cessna 340.
Click to expand...

Yes, I read about them. Mooneys have great glide ratios as it is, must be amazing with a feathering prop. Practicing my power-off accuracy landings for my CFI and my buddy (a CFI) was pretty sure I'd come up way short and asking why I put full flaps down so early...of course, I overshot my mark on the first attempts...
 
A windmilling prop is easier to restart. It's a continuous crank, resistant to flooding, vapor lock, incorrect priming, and all the other stuff that can go wrong on a stopped engine. If you killed the engine with incombustible mixture, all you have to do is restore the mixture

A coarse prop will still windmill, but slower. A feathered prop will stop.

On a twin, you can usually still fly on one engine, and a bigger concern is reducing the yaw.
 
epsalant said:
Yes, I saw that ! (In fact, I read the whole AFH and PHAK and a whole bunch more for my CFI-A last week, it paid off with the oral going very well---practical went fine, too). I understand the difference between feathered and flat plate, I just don't see that much difference between low (say 1500) and high RPM in terms of flat-plate effect. I was wondering if it is the turning of the engine or the negative angle of attack on the prop blades with a rearward lift vector or what.
Click to expand...

I'm not a mechanic, and I don't play one on TV, but IMHO it is the turning of the engine.

Bob
 
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