Manifold Propeller Check

JeppUpdater said:
Should you one day fly a turboprop, you'll note torque (a measure of power which is basically the turboprop equivalent of manifold pressure) will do the same thing - rise with a reduction on RPM.

In the 600-hp Cessna Caravan for example, the torque limit for takeoff is 1658 ft/lbs at a prop RPM of 1900. That combo produces 600 shaft horsepower.
Once in climb/cruise, the prop RPM can be reduced as low as 1600 RPM, with the torque limit rising linearly as prop RPM decreases, until it hits a minimum RPM of 1600, where the torque limit is 1970 ft/lbs...which also gives you 600 shaft horsepower.

The common car formula works for turboprops as well (you just have to know the actual torque rating, not a percentage):
Horsepower = (torque x speed) / 5252

There's also a little extra power from the jet thrust of a turboprop, but that's another discussion. ;)
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You're right as far as the coorilation in the power setting increase with an RPM reduction. But the reasons why this happens is very different between the two power plants.

They're both self propelling air pumps. But MP is more of a potential power indication with RPM being the limitor, while torque more of a final output of power being generated by a given RPM.
 
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belizepilot said:
Reason why three times is because you are looking to note three separate things the rise in MP, drop in Rpm, change in oil pressure etc. If you could note all those in one prop cylce then by all means, but for most students/pilots tend to follow the three cycle and note each individually. Once the habit is developed then it stays with them, and so the next guy.
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Genius, except once is still enough.

KatanaHawk said:
Of course it doesn't help if, as an instructor, your hands are tied. The SOPs of my last job required us to cycle it 3 times every time we started the engine, and 1 additional time for a feather check. Any less and your student would probably fail their stage check.
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Spare us all from bad procedures in the name of 'learning' as well, please.
 
If any of you have ever had the pleasure of driving an old car or truck in the rain with vacuum powered windshield wipers, this should be easy to relate to.

For a naturally aspirated piston engine just think of manifold pressure as manifold vacuum, because that's what it really is. There are two ways engine RPM can change. One is by throttle position, the other is by engine load.

As RPM increases by throttle adjustment, manifold vaccuum drops because the throttle valve is opening to allow a less restricted/higher velocity flow of air into the engine. Think Bernoulli. At WOT the manifold vaccuum should drop to the ambient pressure. If you're reading this as manifold pressure, then you will see a numerical increase on the gauge. Technically your WOT MP stetting should be very close (within an inch or so) to what the MP gauge reads before engine start. This is because your MP gauge reads ambient pressure with the engine off.

The prop affects RPM through engine load, making RPM changes possible with out changing throttle position. MP increases when Prop/Engine RPM decreases because the engine is directly connected to the prop. When you make a prop adjustment to reduce engine RPM, you are changing the amount of air flowing through the engine without changing the throttle valve position. MP increases because when RPM is reduced the total volume of air being moved through the engine over a period of time has been reduced. This is shown as a drop in manifold vacuum, or an increase in MP if read on the gauge.

For cycling the prop, conduct a governor and/or feather check per POH. To "warm it up" mainly on piston twins, after engine start I would just pull the prop controls back just clear of feather for taxi and engine warm up. Doing that on cold mornings I definatly noticed a difference in being able to sync the props smoothly after takeoff.
 
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Autothrust Blue said:
Genius, except once is still enough.


Spare us all from bad procedures in the name of 'learning' as well, please.
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If the prop actually cycles those three things happen, I guarantee it.

I do it until it responds quickly... rarely takes more than twice, as in the first one is usually slow, and the second one is usually perfect. If the engine is warm already once. I look at the windshield for oil mist. If I'm doing a maintenance run I set a certain RPM and increase manifold pressure to ensure that the governor is governing.

Once a person brought their airplane in and told us "The prop has cycled extremely slow ever since I had the propeller overhauled."

Want to take a guess at what the problem was? ;)
 
trafficinsight said:
If the prop actually cycles those three things happen, I guarantee it.

I do it until it responds quickly... rarely takes more than twice, as in the first one is usually slow, and the second one is usually perfect. If the engine is warm already once. I look at the windshield for oil mist. If I'm doing a maintenance run I set a certain RPM and increase manifold pressure to ensure that the governor is governing.

Once a person brought their airplane in and told us "The prop has cycled extremely slow ever since I had the propeller overhauled."

Want to take a guess at what the problem was? ;)
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Did the governor get overhauled at the same time?
 
A very simplified explanation of this that might make it easier to picture what others have said is this. Picture a room (manifold) that has a door on either side. The room is full of people but there is a steady flow of people coming in the door on the left and an equal amount leaving through the door on the right. Now when we decrease the RPM we are restricting the flow of air through the engine. This is as if we partially closed the outflow door on the right thus backing up the flow through the room and increasing the number of people in the room. Just a visualization that really helped me picture it
 
thevideographer said:
John Deakin doesn't live in North Dakota where you have to do it 6 times to warm up the prop hub.
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Nor does he live in Wyoming!

I would like to show the naysayers the difference in smoothness in between cycling it once and three times when flying -20.

Our ops, Cold engine 3 times. Warm day or engine one time.
 
hattrick said:
A very simplified explanation of this that might make it easier to picture what others have said is this. Picture a room (manifold) that has a door on either side. The room is full of people but there is a steady flow of people coming in the door on the left and an equal amount leaving through the door on the right. Now when we decrease the RPM we are restricting the flow of air through th engine. This is as if we partially closed the outflow door on the right thus backing up the flow through the room and increasing the number of people in the room. Just a visualization that really helped me picture it
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I like this. It helps. Especially if you think of the doors as revolving doors and you slow them down by reducing the door (RPM) on the right.

Cool analogy. Thanks.
 
killbilly said:
I like this. It helps. Especially if you think of the doors as revolving doors and you slow them down by reducing the door (RPM) on the right.

Cool analogy. Thanks.
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And the throttle controlls the revolving door on the left. I will have to use this one!
 
And you can also add that the room immediately following the right revolving door is a sweat shop floor and they don't need as many people working there as before (decrease in RPM) so the intake of people through the right revolving door leading to it is slowed, thus increasing the piling of people in the first room (increase in MP) . . . that's a neat analogy for sure!
 
Nobody here knows a plumber? Air, water and in some respects electricity all kind of follow the same path. Basic physics aren't that hard to teach.
 
In some aircraft the POH and checklists call for the prop to be cycled 3 times while I believe once is enough for aircraft that don't say otherwise why would you teach anything other than what the manufacturer asks for? I don't know why it would help to cycle it more but I didn't engineer or build it so who knows why but the OEM spent time writing it out that way so they must have had some thoughts on it.
 
Regarding the rag in the crankshaft, I'd like to know if it was the owner and his "A/P friend" that installed it. Unless a pilot is certificated as a mechanic as well (see how I used lower case for both) they make everyone look bad. There are plenty of bad mechanics, and some pilots if you actually had to deal with them daily might make you want try to bail and see if you can make a RedBox franchise work.
 
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