What's Your Descending Procedures (let down)?

[Hank]

27 minutes ago, Kris_Adams said:

I"m betting a lot of us descend at higher manifold pressure for speed also .  Low MP descent doesn't happen for me at least

I descend at whatever my cruise MP is, generally 23" when low, but was recently a little under 20" when cruising at 10,500 (DA > 13,000). I was happy to get that much!

[N201MKTurbo]

2 hours ago, jaylw314 said:

 

Wasn't that another argument from Mike Busch, that the whole "prop driving engine is bad" is another OWT?  IIRC, the idea came from radials, where the main crankshaft journal will be pushed from the compression cycles of each cylinder from the side opposite the oil hole 7 or 9 times in 2 rotations, whereas our typical motors each crankshaft journal will only be pushed from the wrong side once every 2 rotations.  He also referenced some accidents where people crashed because they were trying not to reduce MP because of this idea...

You would have to go to idle cutoff for this to happen. If you are making any power at all it will still be pushing, just not as hard.

Where are you getting this every 2 or 7 or 9 rotations? The relationship of the crank journals to the rods is cast in stone so to speak.

[Hank]

29 minutes ago, N201MKTurbo said:

You would have to go to idle cutoff for this to happen. If you are making any power at all it will still be pushing, just not as hard.

Where are you getting this every 2 or 7 or 9 rotations? The relationship of the crank journals to the rods is cast in stone so to speak.

I'm supposing that the "7 or 9 times in two rotations" would depend if you are talking about a 7-cylinder or 9-cylinder radial engine, while our little flat fours are one power stroke per revolution

[N201MKTurbo]

7 minutes ago, Hank said:

I'm supposing that the "7 or 9 times in two rotations" would depend if you are talking about a 7-cylinder or 9-cylinder radial engine, while our little flat fours are one power stroke per revolution

Wouldn't a 4 cylinder engine have two power strokes per revolution?

[jaylw314]

41 minutes ago, N201MKTurbo said:

You would have to go to idle cutoff for this to happen. If you are making any power at all it will still be pushing, just not as hard.

Where are you getting this every 2 or 7 or 9 rotations? The relationship of the crank journals to the rods is cast in stone so to speak.

 

Just now, N201MKTurbo said:

Wouldn't a 4 cylinder engine have two power strokes per revolution?

In theory, the pistons would only resist the crankshaft during the compression stroke.  Since a radial has 7 or 9 cylinders (usually), that means 7 or 9 compression strokes every 2 rotations on the crankshaft journal

In our non-radial motors, every journal has one piston only, so there would be only one compression stroke every 2 rotations.

[DonMuncy]

50 minutes ago, N201MKTurbo said:

You would have to go to idle cutoff for this to happen. If you are making any power at all it will still be pushing, just not as hard.

Bob Kromer said the factory installed a torque measuring device between the propeller and the crankshaft and found that the place where the engine torque went from positive to negative was around 15 inches MP (if my memory is working correctly).

[N201MKTurbo]

3 hours ago, DonMuncy said:

Bob Kromer said the factory installed a torque measuring device between the propeller and the crankshaft and found that the place where the engine torque went from positive to negative was around 15 inches MP (if my memory is working correctly).

At what airspeed?

[DonMuncy]

2 minutes ago, N201MKTurbo said:

At what airspeed?

I have no idea. I presume that it was descending to land. 

[jetdriven]

High speed  more like 18 inches and at a lower speed it’s more like 15 inches. But at some point the crankshaft goes to negative torque and that is not good

[PT20J]

Radials do indeed have an issue with high rpm, low manifold pressure aka the prop driving the engine. It’s all about keeping the load on the correct side of the master rod bearing for proper lubrication. We generally keep them at least square ( MAP in inches = rpm/100) and usually well over square (they are all supercharged). Horizontally opposed engines don’t have this issue. Lycoming SB 245D admonishes against high rpm, low manifold pressure operation because of the potential for detuning dynamic counterweights but only on geared engines. Not sure such operation harms a direct drive 0-360 or IO-360. I’d be interested if someone had a reference one way or the other from an authoritative source. 
 

Skip
 

 

 

. 

[carusoam]

Skip, put this in your thought flywheel... 

 

One thing to keep in mind.

There is often discussion of pulling or pushing on the prop when putting it in the hangar....

That little clunk you get when you go from pulling to pushing...

In flight... There must be a middle zone when the prop isn’t pulling the plane or pushing back on the plane...

that middle zone may be worth avoiding, to keep the prop from banging back and forth on some bearing somewhere...

 

Something I was thinking about when using the prop as a brake...

Best regards,

-a-

[jetdriven]

its not the crank banging on the thrust bearing, its the fact that under negative torque, the prop is driving the engine, and there is negative pressure on top of the pistons rather than positive pressure.  the rings flutter from the top to the bottom of the ring lands, leads to broken rings etc

 

[jaylw314]

On 10/17/2019 at 9:07 PM, PT20J said:

Radials do indeed have an issue with high rpm, low manifold pressure aka the prop driving the engine. It’s all about keeping the load on the correct side of the master rod bearing for proper lubrication. We generally keep them at least square ( MAP in inches = rpm/100) and usually well over square (they are all supercharged). Horizontally opposed engines don’t have this issue. Lycoming SB 245D admonishes against high rpm, low manifold pressure operation because of the potential for detuning dynamic counterweights but only on geared engines. Not sure such operation harms a direct drive 0-360 or IO-360. I’d be interested if someone had a reference one way or the other from an authoritative source. 
 

Skip

I thought the detuning of the counterweights was the whole reason for the RPM limitation on the IO-360/McCauley combo?

[Bob - S50]

9 hours ago, jetdriven said:

its not the crank banging on the thrust bearing, its the fact that under negative torque, the prop is driving the engine, and there is negative pressure on top of the pistons rather than positive pressure.  the rings flutter from the top to the bottom of the ring lands, leads to broken rings etc

 

There will never be negative pressure above the pistons.  The rings already swap positions at least once under normal operation.  As the piston moves up during the exhaust stroke, friction will push the rings to the bottom of the channel.  During the intake stroke, friction will move the rings to the top of the channel.  Then during the compression stroke, friction will once again move them back to the bottom of the channel.  The only question is what happens during the power stroke?  Is the pressure in the cylinder, acting upon what little area of the rings that are exposed between the piston and the cylinder wall, greater than the friction caused by the piston moving down the cylinder?  If so, the rings will stay in the bottom of the channel.  If not, friction will move the ring to the top of the channel again.

Then the question becomes, will the difference in pressure between normal vs a low MP pressure combustion event make enough difference to allow the ring to 'float' up during the power stroke?

And I'm pretty certain that if the top compression ring does its job, there will not be much exhaust pressure on the lower compression ring in either case, let alone the oil ring.

[N201MKTurbo]

Everything you would ever want to know about piston rings:

http://korihandbook.federalmogul.com/en/index.htm

 

[Gagarin]

On 10/17/2019 at 1:06 PM, Bob - S50 said:

The worst shock cooling I've experienced is when the hot water tank ran out of hot water.

That usually happens when two or more people are taking showers at the same time. Adjust the water heater to higher temp  so less  hot water is drained from the tank.

[Bob - S50]

25 minutes ago, Gagarin said:

That usually happens when two or more people are taking showers at the same time. Adjust the water heater to higher temp  so less  hot water is drained from the tank.

Or shower together.  No wait, the shower would take longer and you would still run out of hot water.