[Bavafa]

I had a discussion with a friend about the relationship of manifold pressure and RPM with a CS prop. As RPM decreases, manifold pressure increases without touching the knob. In my test, it was actually by a fair amount increase but I don?t understand why. Isn?t that the throttle (Manifold) simply controls how much air is entering into the Cylinders thru the intake pipes?

Much appreciate the breakdown of the process

 

[Mike S]

Sounds reasonable to me.

The engine is always running at lower than ambient pressure----discounting any ram effect. Some folks would say the engine is sucking in air, others would say the higher outside pressure is pushing in the air. Dont really care, as the end result is the same.

Anyway, the faster the engine runs, the more often the manifold sees a vacuum and the slower it runs the less often for a given period of time.

Less vacuum pulses in a fixed period of time will therefore translate to a higher manifold reading.

What would the manifold pressure be if the engine were not running???

The term manifold "Pressure" is quite misleading.

 

[rocketbob]

Bernoulli's principle in the induction system...as velocity increases pressure decreases. A manifold pressure gauge is a pressure gauge in reverse, increase of MAP is actually an increase of vacuum. Or a decrease in pressure compared to ambient.

 

[BobTurner]

Bernoulli's principle in the induction system...as velocity increases pressure decreases. A manifold pressure gauge is a pressure gauge in reverse, increase of MAP is actually an increase of vacuum. Or a decrease in pressure compared to ambient.

A MP gauge IS a pressure gauge. It reads the absolute (referenced to a perfect vacuum, not ambient pressure) pressure.
With the engine off, it should read the ambient air pressure, in mm of Hg, NOT corrected to sea level.
In an ideal system (no air filter, huge diameter pipes) the MP would always read ambient air pressure at wide open throttle, regardless of RPM. In a real system there are some restrictions, and the pressure drops flowing thru them. The harder you "pump" (high RPM), the more the pressure drops. A closed throttle plate is a significant restriction, and gives a very significant drop.

 

[rocketbob]

oops meant to say absolute, not ambient.

 

[BobTurner]

Bob, I don't understand what you wrote at all. Unless you were thinking of the gauge that used to be in some cars, that read the manifold vacuum relative to ambient pressure. Not at all what is in aircraft.

 

[BigD]

I was getting confused, so I looked it up in the Pilot's Handbook of Aeronautical Knowledge:

but that doesn't answer the original question...
Here's one article that explains what's going on:

http://www.avweb.com/news/pelican/Pelicans-Perch-15-Manifold-Pressure-Sucks182081-1.html?redirected=1

==dave==

 

[crabandy]

Just after takeoff and a safe altitude you pull your manifold pressure back to 24 inches, than you pull the Rpms back to 2500 and the manifold pressure has risen to 25 inches again.
With the engine not running the manifold pressure gauge should read standard pressure (29.92 inches) adjusted for nonstandard temp and pressure (roughly 1 inch loss per thousand feet Above sea level I believe, not sure about temp).

When you start the engine the throttle is closed minimizing how much air the engine can "suck" in, I think of manifold pressure as more of a suction gauge. Reads low pressure (high suction) of maybe 10 inches.

At full power ideally the manifold pressure would read the same as when the engine isn't running, but since there is a filter/airbox/bendy tubes/etc it's probably about 1/2 inch lower.

When you pull manifold pressure back after takeoff, you are setting the amount of air allowed to enter the engine at full RPM 2700ish rpm. This setting is generally 25 inches of manifold pressure, or a measure of how much air the engine is sucking. When you pull the rpm's back to 2500 with the throttle in the same position the engine is sucking less air, as in 200 rpm x engine cubic inch /4 (4 stroke). At the lower rpm setting and the same throttle setting the engine is sucking less (less vacuum) and manifold pressure increases slightly.

 

[rzbill]

Just after takeoff and a safe altitude you pull your manifold pressure back to 24 inches

Ummm......why?

, then you pull the Rpms back to 2500

If still climbing and wanting best performance,......why?

I know this is a drift and I am undeservedly jumping on Andy who is trying to set up an example (sorry) but I don't have to deal with the manifold pressure change due to rpm drop. Except in unusual circumstances where I am flying very low altitude (MP above 26 in), my throttle is wide open from TO to destination. Power level is controlled by prop and is (by choice to follow Lyc docs) set to 75% or below at cruise by using the power indication on the GRT engine screen (which reads a mapping I made myself rather than the one provided by GRT which does not represent an IO-360-M1B). The MP probably does rise a bit as the RPM comes down but I don't use it for power setting since the MP and RPM combinations change with altitude.

For those that shiver at oversquare operation, I am operating at all times within the power chart settings published by Lycoming which show considerable oversquare settings.

 

[Jerry Cochran]

HP formula

I'm wondering if knowing the simple formula for horsepower would help this thread. The formula is: HP=Torque x RPM. One can think of torque as a result of manifold pressure (MAP) and RPM is self-evident. If you raise either side or both sides of the equation it will raise HP and the reverse is true for lowering them.

It works every time it's tried...

Jerry

 

[BillL]

I'm wondering if knowing the simple formula for horsepower would help this thread. The formula is: HP=Torque x RPM. One can think of torque as a result of manifold pressure (MAP) and RPM is self-evident. If you raise either side or both sides of the equation it will raise HP and the reverse is true for lowering them.

It works every time it's tried...

Jerry

No. (min length satisfied)

 

[N546RV]

Ummm......why?

I'm pretty sure the practical reason is "because that's how it gets taught." Instructors I've had in the Arrow really drill in that power/RPM reduction, the same way they drill in the whole oversquare stuff.

 

[usmcmech]

There are a lot of misconceptions about MAP, and Deakins Avweb article should be required reading for anyone flying behind a CS prop.


Misconception #1 MAP is the amount of power the engine is producing.

Not true in the least. The highest MAP you will see is when the engine is turned off producing 0 Hp. Even in flight if you pull the mixture to idle cut off or turn the mags off, the engine will keep spinning and MAP will be unchanged.

What MAP is in fact is a crude way to know your throttle setting. This combined with RPM tells you home much air your engine is breathing, nothing more.

Another key is the word "throttle" and it's many definitions. To "throttle" something (or someone) is to choke off it's air supply. "Full throttle" is actually dead idle, while at full power the engine is not being throttled at all.

 

[rzbill]

I'm pretty sure the practical reason is "because that's how it gets taught." Instructors I've had in the Arrow really drill in that power/RPM reduction, the same way they drill in the whole oversquare stuff.

Ha Ha. Exactly! Of course, I should give credit in case it is due. Hartzell could have issued a timed RPM restriction (2700 for 5 min for instance). I don't remember. The Blended Hartzell/M1B combo has no restrictions. I flew Arrows too and it has taken me a long time to quit fooling with the prop knob on initial climb.

 

[Ron]

Fixed Pitch

So I guess MP has no value to us with fixed pitch?
Just curious...
Ron

 

[luddite42]

I'm pretty sure the practical reason is "because that's how it gets taught." Instructors I've had in the Arrow really drill in that power/RPM reduction, the same way they drill in the whole oversquare stuff.

And I've never understood WHY that was taught. If for some reason you want to make less noise on climb out, I can see backing off on the RPM slightly, but not the power. We don't back off on the power with fixed pitched props. Climbing out at full power/2500 RPM is the same thing whether flying FP or CS.

 

[N546RV]

And I've never understood WHY that was taught. If for some reason you want to make less noise on climb out, I can see backing off on the RPM slightly, but not the power. We don't back off on the power with fixed pitched props. Climbing out at full power/2500 RPM is the same thing whether flying FP or CS.

I found it kind of odd when I first starting doing my complex. 500' AGL in a new plane, and you want me to direct my attention to the MAP gauge so I can precisely reduce power to 25"? Okay...

I mean, it's not like it's a particularly huge deal or anything, but it seems like an unnecessary thing to me. But now, after 10 hours dual in the Arrow, I still do it, because it's just ingrained. It got turned into a habit and now I'm probably going to start teaching myself not to do it, and then I'll get used to not doing it and go for a BFR or something and get **** about it.

 

[BillL]

This has wandered from the OP, but the MAP and RPM reduction right after TO came up from a Bo owner in another thread too. I know that both originated with the engine manufacturers for OEM product. As compression, timing etc were pushed to publish better performance numbers, and achieve the best TO power, the margin for cooling, piston temps, and detonation are reduced relative to cruise. I asked this of the engine engineers (that I worked with) way back and they would give me chapter and verse for why - cylinder pressures etc. for a particular engine model. The engine thrust/fuel flow is better at the lower speed and rpm too. Not absolute performance, but efficiency - as evidenced in what we call peak torque. There was a classic engine power rating schedule of continuous vs TO power as well. The piston speeds (RPM) and power settings (MAP) are lower for lower forces/stresses and lower heat flux (temperatures) on all components at the continuous ratings. This has sort of disappeared over the decades, but had a historical influence on the operating limitations/guidelines. Think how long your engine would last running 2700 - 29" all the time vs. 2400 - 24". Now make an equation for each TBO and time fraction for how the engine is operated. The TBO is somewhat variable by choosing the time at each for a mission profile, maintenance being the same.

Now we, in the RV crowd, are going faster, climbing faster and with better cooling instrumentation than the OEM's. Also, most RV pilots are much more well informed and attentive to these issues than the general population, so temps are not exceeded. For extending life, these lower numbers may well provide more hrs to TBO, or and maybe not. We just don't have good definitive data on limitations of the 4/6 cylinder, normally aspirated, Lycomings.

Sorry, no hard and fast answers here, but some history on why the OEM operating recommendations make some sense even if we don't have the numbers (and I can not remember the numbers either).

 

[agirard7a]

Fixed pitch Map

So I guess MP has no value to us with fixed pitch?
Just curious...
Ron

No it does. To determine percentage of power at a given altitude.
In my 0320. 24 map / 2400 rpm or 24+24equals 48 or 75% power. As you fly
At higher altitude Map decreases let's say to 22 you then can run 2600 Rpm
22+26=48. 48 being the magic number in an 0320 for 75%power.
Just learning this myself. Used it today to determine 75% cruise at various altitudes. Can then lean the engine accordingly. Don't lean at higher percentage of power.

 

[Jesse]

So I guess MP has no value to us with fixed pitch?
Just curious...
Ron

MAP is usually ignored on FP planes because it cannot be controlled independent of rpm. It is good information, though. Even though MaP may not be equal to power, in most situations with the engine running it is a good measure of power. MAP and RPM together help to determine percent power. Fuel flow also helps. Having the information is never bad.

Many people who have only flown fixed pitch probably think that 2500rpm (say, 85% power) at sea level and 2,500rpm at 10,000 feet means they are making the same power. Most don't have fuel flow and don't fly high because they don't realize the benefit of thinner air, even though power is lower. They also often don't have TAS, so IAS is higher at low altitudes, so another reason to stay low.

All that to say, MAP is always a good number to have, even if you can't control it separate from RPM. It helps you understand how hard the engine is working.

 

[RV10inOz]

From down state Louisiana

Enjoy??

http://www.advancedpilot.com/downloads/prep.pdf

 

[BobTurner]

No it does. To determine percentage of power at a given altitude.
In my 0320. 24 map / 2400 rpm or 24+24equals 48 or 75% power. As you fly
At higher altitude Map decreases let's say to 22 you then can run 2600 Rpm
22+26=48. 48 being the magic number in an 0320 for 75%power.
Just learning this myself. Used it today to determine 75% cruise at various altitudes. Can then lean the engine accordingly. Don't lean at higher percentage of power.

If only life were that simple.
You also need the mixture setting, and the intake air temperature. Then go to the Lycoming charts and look up the power.

 

[luddite42]

From down state Louisiana

Enjoy……

http://www.advancedpilot.com/downloads/prep.pdf

Good collection of articles. Excerpt from John Deakin -

Remember that this is the chart of operating parameters at 25" and 2,500 RPM. Does that setting sound somehow familiar? Yes, it's the setting many have used for years right after takeoff, for climb power! The high temperatures are the direct result of pulling the MP back to 25", which cuts out the power enrichment feature of the fuel controller, leaning the engine dramatically!

This practice (pulling the throttle back after takeoff) may be the single most damaging thing many people do — with the best of intentions, to "make it easier on the engine!" Forget it, folks. Leave that throttle fully in, unless you need to make a substantial reduction in MP, maybe to 22", or so. (Remember, we're talking about normally-aspirated engines here.)

 

[rvmills]

The throttle does not "control" anything. It sets the throttle plate position which is a resistance to flow. That resistance creates a pressure drop across the plate. That pressure drop changes with flow demand. Flow demand comes from the engine, a pump. If you drop RPM with the governor control ( yes that is a control of speed) then the "pump" lowers demand and the pressure drop across the throttle plate is reduced.

Manifold pressure = Ambient pressure - pressure drop across throttle plate

Make sense now?

Bill,

In a discussion with another very shmart poster here on VAF (like you), we chatted about this "rising MP while decreasing RPM" effect. His comment was that the amount of MP rise is an indication of the efficiency of the induction inlet system. That would seem to follow, and compliment, your comments about the throttle plate?as an inefficient inlet system is just another resistance to flow, so when reducing RPM, the pressure drop across the inlet system and the throttle plate is reduced?thus larger restriction + same pressure drop = more MP rise. Does that seem correct to you?

I did test that a bit: Following that discussion, I changed from a home-grown snorkel + ram air inlet system to a James inlet, and I do believe I see less MP rise when reducing RPM (after level off in cruise) with the more efficient induction system. I'm fabricating another inlet duct now and will test soon. Looking for more MP for racing, but will test to see if the MP rise when reducing RPM gets smaller with this straight pipe, filterless inlet.

Thoughts?

Cheers,
Bob
PS: Been a while since the RV-1 hit Peoria, eh!

 

[BillL]

Bill,

In a discussion with another very shmart poster here on VAF (like you), we chatted about this "rising MP while decreasing RPM" effect. His comment was that the amount of MP rise is an indication of the efficiency of the induction inlet system. That would seem to follow, and compliment, your comments about the throttle plate?as an inefficient inlet system is just another resistance to flow, so when reducing RPM, the pressure drop across the inlet system and the throttle plate is reduced?thus larger restriction + same pressure drop = more MP rise. Does that seem correct to you?

I did test that a bit: Following that discussion, I changed from a home-grown snorkel + ram air inlet system to a James inlet, and I do believe I see less MP rise when reducing RPM (after level off in cruise) with the more efficient induction system. I'm fabricating another inlet duct now and will test soon. Looking for more MP for racing, but will test to see if the MP rise when reducing RPM gets smaller with this straight pipe, filterless inlet.

Thoughts?

Cheers,
Bob
PS: Been a while since the RV-1 hit Peoria, eh!

Hi, Bob, always good hearing from you. Yes, you are spot on. The lower restriction intake system (@WOT) will show a lower MAP change when the rpm is dropped. So piping, edges, and filter all have an effect.

I have to make a filter system with filter bypass for my plane soon and will be dealing with the same issues. I will send a PM later to get that discussion going. Cheers,

PS Sorry for the response delay, I was driving 850 miles today. I gotta get my 7 flying!!

 

[rvmills]

Hi, Bob, always good hearing from you. Yes, you are spot on. The lower restriction intake system (@WOT) will show a lower MAP change when the rpm is dropped. So piping, edges, and filter all have an effect.

I have to make a filter system with filter bypass for my plane soon and will be dealing with the same issues. I will send a PM later to get that discussion going. Cheers,

PS Sorry for the response delay, I was driving 850 miles today. I gotta get my 7 flying!!

Thanks Bill…and yes you do! Look forward to hearing from you! (and read Dan H's thread on his inlet/filter system too, if you haven't already…good stuff!)

Cheers,
Bob