RPM increase when cycling the prop? Why??

[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..