M20J Acrobatics

[Shadrach]

2 minutes ago, N201MKTurbo said:

 

Back when I was a scrawny shirtless kid, I used to do the Vomit Comet thing, until one time the engine went to about 3500 RPM!

All the oil goes to the top of the engine and starves the prop governor. Not good for the prop.

Not good for the ENGINE!

Good to know. I would have thought the governor would be the last thing to lose pressure.  My assumption would have been Oil pressure, Fuel pressure and then Governor.

[RobertGary1]

8 minutes ago, Shadrach said:

Not good for the ENGINE!

Good to know. I would have thought the governor would be the last thing to lose pressure.  My assumption would have been Oil pressure, Fuel pressure and then Governor.

I've had the same experience in a 182. Oil started overheating over the Sierras and the first sign was prop overspeed.

-Robert

[Shadrach]

Just now, RobertGary1 said:

I've had the same experience in a 182. Oil started overheating over the Sierras and the first sign was prop overspeed.

-Robert

I'd love to know more about why that would be, given that the prop is the only dead end circuit in the system. 

[RobertGary1]

6 minutes ago, Shadrach said:

I'd love to know more about why that would be, given that the prop is the only dead end circuit in the system. 

Interesting thinking about it, when my Mooney lost oil pressure the prop did not overspeed and held RPM. The engine continued to run for something like 3 minutes (I have the JPI data somewhere) before it started toss pieces out.

-Robert

[David Lloyd]

I guess many are in agreement that aerobatics in a Mooney is a complicated endeavor if not done with knowing precision.

[RobertGary1]

1 minute ago, David Lloyd said:

I guess many are in agreement that aerobatics in a Mooney is a complicated endeavor if not done with knowing precision.

I missed the beginning of this discussion but as I recall the factory had someone investigate having an acro demo team in the 201. Although it could be done they determined that without speed brakes the margins were too slim. I doubt anyone investigated negative G acro though.

-Robert

[ArtVandelay]

Interesting thinking about it, when my Mooney lost oil pressure the prop did not overspeed and held RPM. The engine continued to run for something like 3 minutes (I have the JPI data somewhere) before it started toss pieces out.
-Robert

The JPI data would be interesting, because it’s always possible the oil pressure sender fails and it’s a false indication.
What’s the second abnormality after lost of OP?

[RobertGary1]

18 minutes ago, ArtVandelay said:

The JPI data would be interesting, because it’s always possible the oil pressure sender fails and it’s a false indication.
What’s the second abnormality after lost of OP?

Pistons departing the engine. 

[Shadrach]

1 hour ago, David Lloyd said:

I guess many are in agreement that aerobatics in a Mooney is a complicated endeavor if not done with knowing precision.

Could say that about aerobatics in general.

[EricJ]

5 hours ago, Shadrach said:

I'd love to know more about why that would be, given that the prop is the only dead end circuit in the system. 

It's not a closed circuit, though.    Some of the oil under pressure from the governor can escape through the main bearing at the crank snout and back to the sump.   If the oil is hot and less viscous than usual it might make that trip more easily than it normally does, and since the prop spring and centrifugal twisting force are constantly trying to put the prop back to fine pitch, if the governor can't keep up with the leakage through the bearing the prop will go to fine pitch and potentially overspeed.

[Shadrach]

Just now, EricJ said:

It's not a closed circuit, though.    Some of the oil under pressure from the governor can escape through the main bearing at the crank snout and back to the sump.   If the oil is hot and less viscous than usual it might make that trip more easily than it normally does, and since the prop spring and centrifugal twisting force are constantly trying to put the prop back to fine pitch, if the governor can't keep up with the leakage through the bearing the prop will go to fine pitch and potentially overspeed.

This has been hotly debated in other threads.  I am not an engine builder so it would be interesting to know for sure how the that circuit operates. I know there is a journal just aft of the crank flange to which the governor supplies pressure.  I assumed a small amount of leak down around the journal but was not sure how much or if there was a seal in place.  I also know that the oil that comes out of the hub of a freshly removed prop is typically, ambient temp. It is weird to feel cold oil come out of the hub of a warm engine.  So the circuit is closed forward of the crank journal?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[carusoam]

The oil used by the prop is not really a dead end...

It would be if pressure was maintained with tight seals...

But, it is not... oil pressure in the prop is maintained with a constant flow of oil... provided by the govenor’s gear pump...

Check the oil flow diagram...

It is not really organized flow either... oil is constantly entering and exiting the hollow crank shaft nose...   delivered by the gear pump, exit rate is controlled by the conical shaped control valve attached to the weights, sensing rpm...

Plenty of dirt can accumulated in the system... because the lack of organization of the flow... and centrifugal forces...

So... find the governor’s oil pick-up to know where the oil needs to be to keep the prop from overspeeding... 

If you were to fly aerobics intentionally... you may want to set the prop stops with intention... 

PP thoughts only, not an oil flow scientist... or mechanic...

Best regards,

-a-

[Shadrach]

1 minute ago, carusoam said:

The oil used by the prop is not really a dead end...

It would be if pressure was maintained with tight seals...

But, it is not... oil pressure in the prop is maintained with a constant flow of oil... provided by the govenor’s gear pump...

Check the oil flow diagram...

It is not really organized flow either... oil is constantly entering and exiting the hollow crank shaft nose...   delivered by the gear pump, exit rate is controlled by the conical shaped control valve attached to the weights, sensing rpm...

Plenty of dirt can accumulated in the system... because the lack of organization of the flow... and centrifugal forces...

PP thoughts only, not an oil flow scientist...

Best regards,

-a-

Prop hub oil is typically dark no matter what the sump oil looks like.

[Nokomis449]

If you're going to do aerobatics correctly you might as well get one of these - made in Bentonville, Arkansas and one of the company founders has a last name of Walton (hmmm...):

https://gamecomposites.com/

[Don Heene]

On 2/14/2021 at 9:25 AM, bradp said:

There’s a lot of irony in the OPs username. Just sayin. Heheh.  
 

This is an example of how fast a Mooney can build up airspeed in an insipient spin. 
 

 

Damn, both pilot and passenger have shoulders harnesses and no using them.  

[EricJ]

49 minutes ago, Shadrach said:

This has been hotly debated in other threads.  I am not an engine builder so it would be interesting to know for sure how the that circuit operates. I know there is a journal just aft of the crank flange to which the governor supplies pressure.  I assumed a small amount of leak down around the journal but was not sure how much or if there was a seal in place.  I also know that the oil that comes out of the hub of a freshly removed prop is typically, ambient temp. It is weird to feel cold oil come out of the hub of a warm engine.  So the circuit is closed forward of the crank journal?

The crank snout and prop attached to it are closed, with an inlet exposed to the pressure created by the governor.   The piston inside the prop hub moves to equalize pressure between the input oil and the prop spring and prop forces (mostly the centrifugal twisting force).   If the prop forces pushing back on the piston exceed the input pressure, it'll push the oil back out, and it'll either go out around the bearing journals or back through the governor (or both). 

The inlet to the crank snout is in the same bearing as the normal oil galley feeds to the #1 journal, and the governor feed comes in between the other two galley feeds.   On the passenger side of the front of the engine you can see the hard line that brings the governor output pressure from the accessory case and feeds it to the middle of the main bearing.   There's not any seals between the middle part supplied by the governor and the ends between the normal galley feeds other than the normal bearing clearance.   A groove makes the governor oil prefer the route into the hole in the crank snout to drive the prop, but otherwise it can escape around the bearing clearance, and does.   It is an expected relief path for the prop oil, but if there is additional excess pressure from the prop the oil can return to the prop governor and potentially even back to the main oil pump.

Pic of the split main bearing below.   The end parts are the normal clearanced bearing surfaces fed by the main galley supplied by the oil pump.   The governor pressure comes into the center part, which has much larger clearance and feeds the hole in the crank snout that is in that region.

[Shadrach]

15 minutes ago, EricJ said:

The crank snout and prop attached to it are closed, with an inlet exposed to the pressure created by the governor.   The piston inside the prop hub moves to equalize pressure between the input oil and the prop spring and prop forces (mostly the centrifugal twisting force).   If the prop forces pushing back on the piston exceed the input pressure, it'll push the oil back out, and it'll either go out around the bearing journals or back through the governor (or both). 

The inlet to the crank snout is in the same bearing as the normal oil galley feeds to the #1 journal, and the governor feed comes in between the other two galley feeds.   On the passenger side of the front of the engine you can see the hard line that brings the governor output pressure from the accessory case and feeds it to the middle of the main bearing.   There's not any seals between the middle part supplied by the governor and the ends between the normal galley feeds other than the normal bearing clearance.   A groove makes the governor oil prefer the route into the hole in the crank snout to drive the prop, but otherwise it can escape around the bearing clearance, and does.   It is an expected relief path for the prop oil, but if there is additional excess pressure from the prop the oil can return to the prop governor and potentially even back to the main oil pump.

Pic of the split main bearing below.   The end parts are the normal clearanced bearing surfaces fed by the main galley supplied by the oil pump.   The governor pressure comes into the center part, which has much larger clearance and feeds the hole in the crank snout that is in that region.

Thanks for the pics.  It makes sense.  So given your understanding, do you cycle the prop on the ground to get fresh, warm oil into the hub?

Edited February 20, 2021 by Shadrach

[carusoam]

Kind of makes you wonder why that statement is used in all primary training...

Cycling the prop is done for a good reason... just not that one...

-a-

[EricJ]

Just now, Shadrach said:

Thanks for the pics.  It makes sense.  So given your understanding, do you cycle the prop on the ground to get fresh oil into the hub?

I cycle it to make sure it responds, and if it does I think it's fine.   With the new governor, though, I do notice that if I cycle it twice, the second one is a lot quicker than the first.   It didn't used to do that, but as long as it does I cycle it twice.   If it ever gets back to cycling quickly on the first pull, I'll go back to just doing one.

[carusoam]

Some part of the system drains... so, it takes a few seconds to re-fill...

Let’s see if @Cody Stallings is around to discuss oil flow and governors...

Best regards,

-a-

[Shadrach]

I cycle during taxi to verify control

continuity.

[Cody Stallings]

3 hours ago, carusoam said:

Some part of the system drains... so, it takes a few seconds to re-fill...

Let’s see if @Cody Stallings is around to discuss oil flow and governors...

Best regards,

-a-

Lots of good information being exchanged here.

No need to Cycle your prop to get warm oil in the Hub cause it will never get there. Forward of the Front main bearing in the engine the oil system becomes closed to in/out Flow.

There is a known Calibrated leak in the front bearing as Gear driven pumps(governor) has to have some type of continuous flow.

Why prop control was lost with hot oil? I really can’t say.

The Governor could have excessive clearance on the oil pump side or the front main bearings calibrated leak may look more like a busted fire hydrant. Who knows, But something isn’t playing well with hot thin oil.

[PT20J]

I suspect a lot of the lore about cycling props a number of times to get warm oil into the hub is left over from World War II. Many World War II aircraft were equipped with Hamilton Standard Hydromatic propellers. Hydromatic propellers have a large hub with a lot of surface area that must hold a couple of quarts of typically 120W oil. It can get pretty thick when it's cold. These props also have a double acting piston, so oil coming in on one side pushes out the oil on the other. It was standard practice to cycle these props a few times until the response became acceptable. I think it's likely the pilots just carried over the procedures from their military days when they took up civilian flying after the War.

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[Nokomis449]

My instructions from Penn Yan on breaking in my rebuilt 0-360 was to cycle the prop on run up once just enough to get a response and to avoid dropping the RPM to the extent practical.

[PT20J]

Sometimes I think we wear stuff out by testing it 

I flew seaplanes for a while for two different Part 135 carriers. Both ops manuals (which were approved by the FAA) called for doing runups on the first flight of the day only. That was usually a repositioning flight without passengers. Passengers get nervous when they see pilots fiddling with stuff related to the engine 

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