Variable Prop question

ProudPilot said:
As the prop slows down, MP rises. Basically the faster the engine turns the stronger the vacuum in the intake manifold. If it slows down, air has a better chance of making it in there, hence greater volume of air and higher MP. If you speed up the engine, it is as if it's gasping for air, it has a hard time getting all that it wants. Hence turbochargers and superchargers... or a bigger intake.

If you change the prop speed and see no change, either some form of charger is doing a really great job, or 99% chance you have a bad MP gauge.
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Actually the higher the engine RPM the lower the manifold vacuum (or higher MP as shown on the gauge).

I used have an old truck with a vacuum operated windshield wiper motor. Every time you'd accelerate the wipers would slow down because of the decrease in vacuum. Made it real fun passing in the rain...

When you close the throttle you're restricting the air intake opening with the throttle plate. Smaller opening per given cubic feet per minute (CFM) of airflow will accelerate the air and result in lower pressure (think Bernoulli) this creates higher manifold vacuum or lower MP. Open the throttle and the vacuum will drop (MP rise) until the throttle is wide open. A naturally aspirated engine will not be able to flow more air than the ambient pressure available with out some sort of forced induction.

MP rises with RPM decrease because your lowering the CFM moving through the intake by decreasing engine RPM. Decrease in RPM= increase in Vacuum= decrease in MP.



On the constant speed prop side of things. I've found while taxing with the power out of the governing range, pull the props back just shy of feather. On cold mornings this cycles warm oil up into the hubs more efficiently. Obviously push the props full forward for the runup. Props are much easier to sync on departure.
 
trafficinsight said:
And why do I get a manifold pressure rise in a carbureted 182 when I pull the throttle back about 1/4" from full? ;)
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Is the last 1/4'' of throttle travel accelerator pump? Or if it's O-540 powered does it have something to do with it being limited to 24" and 2400RPM?
 
Hammertime said:
BTW, the Dash 8 prop gov is really cranky until you get some warm oil up there...
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I'm curious what you mean by that? Maybe I haven't seen it cold enough yet (mild winter) but I haven't noticed anything with the prop gov in the cold?


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That may have been a slight exaggeration on my part. It's not noticeable unless you are doing a prop heat timer test. The first time you attempt to set 900 rpm, the props wander around, while the second time, it usually goes right to 900. It could be something else, I've just always assumed it was the governor...
 
TwoTwoLeft said:
Is the last 1/4'' of throttle travel accelerator pump? Or if it's O-540 powered does it have something to do with it being limited to 24" and 2400RPM?
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Nah it's the old continental O-470 carbureted one.

Basically when you have the throttle full open the throttle plate is streamlined with the incoming air, when you pull the throttle back just a bit the throttle plate causes turbulence in the intake charge which improves atomization and volumetric efficiency ;)
 
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