Drop in RPM, rise in MP

Well, what happens when the engine is shut down?
Think of it as an "incomplete shutdown". You are reducing mass flow past a restriction. The more mass flow past a restriction, the greater the vacuum. When you reduce the amount of mass that has to be sucked through the restriction, the vacuum will be less.
 
The reason is that as the prop RPM is decreased, it will require more power for the prop to take a bigger bite of air; manifold pressure will rise with that increased power demand.

Conversely, if you adjust manifold pressure to a desired setting with a low prop RPM & subsequently increase the prop RPM the manifold pressure will decrease as the power required to take a smaller bite of air is reduced. I'm not sure that this would be a problem other than the decrease in airspeed or performance, although in some aircraft/engine combinations it may allow the prop to drive the engine--which can cause damage in certain circumstances if I'm correct.
 
DPApilot said:
The reason is that as the prop RPM is decreased, it will require more power for the prop to take a bigger bite of air; manifold pressure will rise with that increased power demand.

Conversely, if you adjust manifold pressure to a desired setting with a low prop RPM & subsequently increase the prop RPM the manifold pressure will decrease as the power required to take a smaller bite of air is reduced. I'm not sure that this would be a problem other than the decrease in airspeed or performance, although in some aircraft/engine combinations it may allow the prop to drive the engine--which can cause damage in certain circumstances if I'm correct.
Click to expand...

Not really. There is no "required power" here. It is all about squeezing mass through a restriction. At lower rpms, less mass flow. The less mass flow, which means it has an "easier time" getting past an orifice (throttle). That translates to the pressure in the manifold being closer to atmospheric pressure.
 
Itchy said:
Not really. There is no "required power" here. It is all about squeezing mass through a restriction. At lower rpms, less mass flow. The less mass flow, which means it has an "easier time" getting past an orifice (throttle). That translates to the pressure in the manifold being closer to atmospheric pressure.
Click to expand...

This is why I'm not a CFI! Thank you for the correction. That was just my dumbed down explanation, but you hit the nail on the head.
 
A couple of key points,

1st, MAP is not pressure, but suction. In a non turbo engine, MAP will always be less than atmospheric pressure.

2nd, MAP does not tell you how much power is being produced. It only tells you how much air the engine is breathing.
 
USMCmech said:
A couple of key points,

1st, MAP is not pressure, but suction. In a non turbo engine, MAP will always be less than atmospheric pressure.
Click to expand...

always,

uh, no.

At zero rpm map = atmospheric pressure,

Some experimental aircraft (Vans RV series) are configured in such a way to recover ram pressure and get a very slight boost. I have seen a map gauge read a little less than 30 on the ramp with the engine off. Airborne at 210 IAS at pattern altitude map gauge was slightly over 30. Perhaps only a .3 or .4 difference but in fact an increase.
 
Engine = Balloon. Assume that you're not changing the rate at which you blow air into the balloon (throttle not moving). The faster you let air out (higher RPMs), the less inflated the balloon will be. When you slow down how much air is being let out, balloon becomes more inflated.
 
ASpilot2be said:
Anybody have a good explanation for why this happens?

I tried reading this article, and it is well written. However I still can't visualize why it happens.
http://www.askacfi.com/433/why-does-manifold-pressure-increase-during-a-prop-check.htm
Click to expand...

I had the same question, and somebody recommended this article, which explains it really well.
http://www.advancedpilot.com/downloads/prep.pdf

when the throttle valve is held in a constant position the only way to change pressure behind it is by creating suction with engine power. With high rpm/high power--think takeoff power, the propellor is sucking lots of air through the engine and lowering pressure behind throttle valve therefore lowering manifold. With a low rpm/high pitch the propellor produces less power and doesnt suck as much air through the engine creating higher manifold pressure. I think the trick is to realize this is only the case with throttle in a fixed position, when you are flying the airplane you are opening and closing the throttle valve to control manifold pressure which isn't the case here. The engine is a big vacuum too, which is an important thing to understand.
 
The "prop driving engine" thing that DPA mentioned is a problem on geared flat pack engines (e.g., not the ones in your Seminole) and on certain radials. At combinations of high RPM and very low manifold pressure, and high airspeeds (say in a screaming descent to get on the glideslope...errrr) you can cause the gearboxes in a geared flat pack to chatter, which will cause damage to the engine over time. You are far more likely to shock cool an engine than reverse-load and damage it when you consider most GA flat pack engines.

(Geared Lycoming engines, though, can be damaged in this manner. As stated, nasty things can occur in the gearboxes, and on a radial you can actually cause severe damage to a master rod bearing as well. Brief reverse loading does not hurt—i.e., when I pull the throttles to idle in the flare, at or near approach speed—but descending at less than 16" at 160 knots will in fact hurt the Twin Bonanza's engines.)

slough said:
I had the same question, and somebody recommended this article, which explains it really well.
http://www.advancedpilot.com/downloads/prep.pdf

when the throttle valve is held in a constant position the only way to change pressure behind it is by creating suction with engine power. With high rpm/high power--think takeoff power, the propellor is sucking lots of air through the engine and lowering pressure behind throttle valve therefore lowering manifold. With a low rpm/high pitch the propellor produces less power and doesnt suck as much air through the engine creating higher manifold pressure. I think the trick is to realize this is only the case with throttle in a fixed position, when you are flying the airplane you are opening and closing the throttle valve to control manifold pressure which isn't the case here. The engine is a big vacuum too, which is an important thing to understand.
Click to expand...

All of Deakin's columns are or should be mandatory reading. (Does ATP still go to 24"/2500 RPM as "climb power?" Rant withheld.) You are also correct.

The change in RPM is itself a result of a change in aerodynamic load, caused by a change in the propeller's angle of attack, caused by the governor, answering the pilot's RPM order, assuming that the prop is within the governing range.
 
its pretty simple once you get your head around it. the throttle valve(throttle position) is fixed on run-up. You pull the propellor lever towards you to cycle prop/run oil through govenor, this raises pitch of propellor. As the pitch raises the propellor turns slower because of increased drag on prop. Therefore the manifold pressure behind the throttle rises becauses the prop is turning slower and there are less intake/compression/power/exhaust strokes of the engine going on. When you push the prop lever forward prop pitch lowers, turns prop faster which increases intake/compression/power/exhaust strokes of engine. More strokes of the engine means more air being sucked through and a lower manifold pressure behind throttle valve.

imagine the engine with a big balloon attached as its air source. With a low pitch/high rpm propellor setting you will suck the ballon flat faster than with a high pitch/low rpm setting. That is basically what is happening on runup when you see the MAP rise as the rpm drop/propellor pitch raises.
 
ASpilot2be said:
Yup. I have tried reading the columns, and there is a lot of good info. But they are so detailed that I can't visualize what is trying to be said. So I will end up confused at the end.
Click to expand...

Damnit, if the engine is rated for blank horsepower continuous, you should run it at blank horsepower until less power is required. (He said as he hopped in the 206 and complied with Company policy requiring 25" and 2,500 RPM for normal climb power, of course.) Rabble rabble!

Power enrichment...it is cool.

(We climb the Twin Bonanza at full throttle and 3,100 RPM - maximum continuous power - all the time, because of power enrichment. We may piss off the neighbors, but yeah, we'll spend less time pissing them off than at 28" and 2,800 RPM—itself not quiet.)
 
As someone already said, Manifold Pressure is actually suction. In a normally aspirated engine that suction is generated by the intake stroke of the piston. Ignoring the throttle plate for a moment as you decrease RPM by manipulating the prop control you're causing the engine to turn slower, and therefore it is sucking less. MP rises because the engine is generating less suction.

That is a seriously oversimplified explanation that overlooks a lot of interrelating factors but it's the simplest explanation that gets to the core of your question.

As was posted above this is required reading. The third article delves mostly into LOP operations, but the first two are invaluable.
 
You must log in or register to reply here.
Back
Top