RPM increase when cycling the prop? Why??

[gear goes down]

I came across an interesting question the other day and I (sadly) don’t know the answer, I have ideas, but don’t think they are right.

Anyways, why does not RPM increase when cycling the prop in the run-up?

 

[Workin'Stiff]

The RPM should be slowing down and increasing when you exercise the governor...

 

[wt219200]

Do you mean why does manifold pressure increase? If so it is because as you pull the prop lever to high pitch low RPM it creates more pressure on the engine because the blades are creating more resistance to air flow.

 

[GravityHater]

I think it (MP falls as pitch is coarsened) happens because when the blades meet more resistance (presented broadside to the air)...............

the engine is no longer sucking as much air - simply because it is slowing down.

 

[avbug]

Retarding the propeller RPM when cycling the propeller does not cause an RPM increase. It causes an RPM decrease, because by retarding the propeller toward "coarse" you are increasing the propeller angle of attack, increasing the drag on the propeller, and slowing it down.

The engine is a suction machine. It sucks air in. It acts just like your vacum at home. Put your hand over the vacum cleaner hose (shopvac) and what happens to the air pressure in the hose? It drops; it decreases. Your hand is the throtttle.

Remove your hand from the hose a little at a time, and as you increase the opening and allow more and more air to flow through the opening to the vacum cleaner hose, the air pressure rises in the hose. Your hand is the throttle, and the throttle is an air valve. It's function is to let in more air. At sea level on a standard day, the air pressure in that hose can't rise above 29.92" of mercury...it reaches it's highest value when the engine is shut down.

Why does manifold pressure increase when you retard the engine RPM? As the engine slows, it isn't drawing as hard, just as slowing down the vacum cleaner with your hand covering the hose would have an identical effect; pressure in the hose would increase because as the vacum cleaner slows down, it's not drawing as hard (creating as much suction), and subsequently the air pressure increases. The same thing occurs with the engine. Close the throttle, retard propeller RPM, and see a rise in manifold pressure because the engíne isn't turning as quickly, and therefore producing as much suction.

 

[siucavflight]

Um, yeah. The RPM does not increase when cycling the prop. The M.P. does, as the engine is sucking less air in. But if your aircraft is increasing RPMs as you are cycling the prop, you better have MX look at it.

 

[gear goes down]

Yeah, I typed that fast. I actually meant M.P. increase.

"IM SOFA KING WE TODD DID." Say that slow, that’s how i feel right now.

 

[svcta]

the engine is no longer sucking as much air - simply because it is slowing down.

Therefore M.P increases. Perefectly correct.

 

[time builder]

Your engine can only draw in the amount of air pressure available, you lose manifold pressure with friction in the air intake system. The higher the RPM, the faster moving air, the less pressure available. Hence, the highest amount of manifold pressure available is when the engine is not running.

 

[Princedietrich]

Maybe someone installed his prop governor upside down!

:laugh:

 

[svcta]

Your engine can only draw in the amount of air pressure available, you lose manifold pressure with friction in the air intake system. The higher the RPM, the faster moving air, the less pressure available. Hence, the highest amount of manifold pressure available is when the engine is not running.

Say huh?

By this line of reasoning you would make lowest M.P. at takeoff power. The engine will make ambient pressure in the intake manifold when the flow is unrestricted by the throttle. The atmosphere is pushing as much air as it can without restriction in to the engine(all the way into the cylinders).

At a constant RPM you reduce the M.P. by restricting the ambient pressure's access in to the intake manifold by blocking it with the throttle plate. The engine keeps a suckin', but the atmospehere(ambient press.) cannot reach the cylinders. Therefore you create a vacuum, or lower M.P.

SO! If you maintain a constant throttle(M.P.) setting and change the RPM, in this case reduce the RPM, the manifold pressure has to increase because your big sucker(the engine) is asking for less air(not sucking so much) and not vacuuming the M.P. down. Simplest terms: The RPM decreased at a given Throttle position will result in increased Manifold Pressure.

I hope this will settle it.

 

[VNugget]

Say huh?

By this line of reasoning you would make lowest M.P. at takeoff power.

He said RPM, not power.

 

[svcta]

It took me a while, but I think I understand the point Time Builder was trying to make. At a given throttle setting(with the prop in govern but less than full power) an increase in RPM would certainly decrease MP. But the engine will make equal MP while @ full power (combination of RPM and MP) or while shut down. It has to. Friction? I think if you suffered a slow down in the air flow after the throttle that would cause an INCREASE in MP. This because the world is still trying to force more air in to the manifold regardless of throttle position, and if friction slowed it down before the cylinders press. would have to increase because you would have more air trapped in the manifold. This does not happen. MP is controlled by choking the engine, literally, and then compensating for the lack of air with reduced fuel flow(accomplished at the metering device) to keep the engine running properly. Clear as mud?

 

[time builder]

But the engine will make equal MP while @ full power

The point is that the engine doesn't "make" MP unless you're turbocharged. You can suck as much as you want, but you'll still only get the atmoshperic pressure minus whatever you lose in friction.

It's the same thing with your lungs, breathing faster won't increase the amount of air pressure, instead, its better to take slow deep breaths.

 

[svcta]

okay, so you're stuck on the semantic of "making" power. It's just an expression, don't get wrapped around the axel. In so much you are correct, and it has been my point all along if you read at all that you will only get what good old Ma Nature is throwing at you. But I think we need to visit this idea of friction. How do you lose anything because of friction? Again, just to entertain the theory if you had a slowing down of air inside the manifold you would get an increase in MP. I don't get it.

 

[time builder]

All you have to do is look at your MP guage when your engine is shut off, then with the engine running full power take a look again, you lose an inch or two.

I'm not theorizing, I'm just remembering what I learned in college. And you're right, you slow the air down, you maximize the pressure, hasn't this been the point all along?

Here's another example of fluid dynamics (after all, air is a fluid). When a doctor takes the medicine out with the syringe (kind of like an engine cylinder drawing in air), they do it slowly, otherwise you get bubbles. The reason is because the fluid can only move so fast, and why? Again its friction. If you widen the intake on the syringe, you can draw it in faster. The motivating force is the pressure of the fluid vs. the vacuum on the other end, this does not change, what changes is the amount of friction due to the width of the intake. Its a little harder to see with air, because its less dense and thus faster moving.

 

[svcta]

Well, you shouldn't lose anything if the powerplant is properly set up. Granted I haven't flown a recip with MP gauge that wasn't supercharged in a really long time, but I can't remember seeing that. Admitedly I could be wrong. Anyone?

That having been said I would still contend that if you slow down the air inside your intake manifold you would get an increase in MP, not a decrease. And their is one key difference between air(gas) and any liquid, fluids or not: Air(gas) is compressible, liquid is not. So fixed volumes of air are able to squeeze through smaller places than liquids with greater ease, and the kinds of pressures we are dealing with are fractional. What 14.7 PSI? That's really not much pressure considering the volume of space we're moving it through so I doubt you'd see a drop in ambient pressure of almost 4-7% just through that kind of friction. Again, I could be wrong. It's just not consistent with my experience or knowledge of powerplants. And I would contend that if you're giving up "an inch or two" you should have something checked out. Man, that's 1000-2000' of altitude right off the bat. Maybe up to an inch for mechanical wiggle room, but I would be suspicious of much more than that. Of course, all of this is taking airport elevation in to consideration.

 

[time builder]

I've flown one recently, bear in mind this is a Part 135 operated plane (so the power plant should be "properly set up"): takeoff MP was about 25" at 2500 feet and 23" at 4500 feet.
Man, if I'm the only one reading these forums that has seen this, I'm amazed to say the least.

 

[svcta]

bear in mind this is a Part 135 operated plane (so the power plant should be "properly set up")

Well, I'll leave that one alone.

Otherwise I might be able to wrap my head around those #s if there were some density altitude to consider. Losing 1''/1000' plus whatever you're going to lose for D.A. might add up. A while ago I used to haul cargo in 310s out in the southwest(ELP) and I'll never forget how much I used to love getting to sea level in those things. It was like a whole other machine VS. 3 or 4000' elevations. Especially with 100 degree heat.

 

[RemoveB4Flght]

I was just pointing out the behavior of the MP guage on our 172RG today to one of my new commercial students...

When the engine is not running, it registers the current barometric pressure (well as close as it can, as it doesn't read tenths and hundreths of inches of mercury as accurately). After start at 1000 rpm idle we get about 12-13 inches MP and at a static run up at full rpm (2700 ish) MP is about 27 inches... not even close the 30 inches (30.09 baro @ S.L.) it was before start. So Time builder isn't that far off..

 

[svcta]

Understood....what's the elevtation there?

 

[VNugget]

With a good ram air induction setup, not only can you get full ambient pressure at wide open throttle, but a local raceplane actually gets 2-3 inches above ambient.

 

[time builder]

With a good ram air induction setup, not only can you get full ambient pressure at wide open throttle, but a local raceplane actually gets 2-3 inches above ambient.

I was thinking along the same lines last night as I drove home. One of my favorite planes was a 60's model Mooney M20 which had an air filter bypass which not only reduced the drag from the filter, but utilized the Ram Air pressure. You could get an extra 1 or 2" MP in cruise (hence the nickname the "poor man's turbocharger"), but you had to be careful that you were in clean dry air.

I should have mentioned earlier, the biggest culprit in the drag or friction in the intake system is the air filter.

 

[pilotnbr1]

Another key point. The intake system we're looking at has no idea what is taking place on the other side of that intake valve in the combustion chamber. It doesn't matter if there's "fire in the hole," or if the prop is windmilling in the breeze or the airplane is diving with the fuel shut off, engine not even running.
If that engine is turning for any reason, the pistons are hopping up and down, and every time one goes down with the intake valve open, it's sucking more air in. If the throttle plate is closed, it's sucking against resistance, creating suction that shows up as low MP; if the throttle is open, it's not blocking the airflow so manifold pressure remains equal to outside ambient (or perhaps an inch less due to unavoidable restrictions in the induction system).
The rule: Manifold pressure depends on ambient pressure, the position of the throttle plate, and the speed at which the pistons are moving up and down. Manifold pressure does not indicate "power," unless other things are taken into account.
For a silly-but-true example, take an engine that is not running, and lift it from sea level to 18,000 feet. If the MP is 29 inches at sea level, it will be about 14.5 inches at 18,000. The change in MP is entirely due to the reduction in ambient pressure at altitude. Did the engine's power output change when the MP went from 29 inches to 14.5 inches? No, of course not — it's zero either way.
Now a real-world example: Assume you're cruising at some low altitude (say 4,000 feet), throttled well back to about 20 inches MP and 2,000 RPM. (Remember, this means the throttle plate is somewhat cocked, restricting induction airflow.) Now reduce the RPM to 1,200 without changing anything else, and you'll see the MP rise sharply. Why? Simple: The ambient pressure hasn't changed; the throttle plate hasn't changed; the only thing that has changed is the speed at which the pistons are pumping the air. Since they are moving much more slowly at the lower RPM, they are not sucking nearly as hard — not creating as much of a vacuum — so the MP goes up, towards ambient pressure. The natural extension of this experiment is to reduce the RPM to zero, when the MP will rise all the way to outside ambient pressure (about 25 inches at 4,000 feet).
In this example, the RPM has been lowered. The pistons are sucking far less air, the speed of the air going through the intake is less and fuel flow is less. This means there is less power being developed, in spite of a much higher MP! You will also see the airspeed drop off sharply, confirmation of "less power."
Conversely, start once again with our example of cruising at 4,000 feet, 20 inches MP and 2,000 RPM. Now run the RPM up to 2,700, leaving everything else unchanged. Now the pistons are pumping much faster, drawing more air in past the (partially open) throttle plate. That creates more suction — a lower pressure in the induction system — which will show as a lower MP. There will be more fuel flow, and you'll be producing more power at lower MP. (This is complicated by prop efficiency, so give me a little room here.)

To sum up manifold pressure increases in the runup when rpm is decreased because the pistons aren't moving up and down as fast thereby creating less suction against the partially closed throttle plate.
Manifold pressure should increase when you check the magnetos for the same above reason- less power being produced due to a less efficient burn of the fuel air mixture.
Manifold pressure initially on the takoff roll will be about an inch below ambient pressure due to imperfections within the intake and filter which somewhate starve the engine. Once you get some speed ram air effect in most airplanes will cause the manifold pressure to increase somewhat. This is also why you should carefully check the manifold pressure gauge during preflight- notice there is no green arc on a manifold pressure gauge indicating what normal takeoff pressure should be( at least in non-turbo engines). Takeoff manifold pressure will vary with ambient atmospheric pressure- read elevation for all practical purposes.
Knowing all this its alaways fun to question you're student when you are flying a twin and ask them "so why does the feathered engine sill registering some manifold pressure?"