What does the Manifold pressure show?

[frank3]

We all know the manifold pressure gauge in our cockpits. But what does it show?
What does it do when I in/decrease it?

The manifold pressure changes with the throttle, but why not the rpm's? When do I in/decrease the rpm? What does it do?

Thanks in advance,

Frank

[joeblogs]

Manifold pressure tells you the amount of air going through the throttle (and into the engine) at a point in time. Higher MP means more air. The engine is basically a hot air pump so higher MP means more power.

In the AH gauges, sea level ambient pressure is about 29 inches of mercury.

The throttle control in the cockpit the position of a gate inside the throttle. Push the throttle all the way forward and the gate is wide open, letting as much air though as possible.

RPMs determine how fast a quantity of air is being run through the enigine. As with MP, higher RPMs means more power. In a plane, and unlike in a car, it's the propeller governer that primarily determines RPM. Think of the propeller governer as a continuously variable transmission, subject to limits.

-Blogs

[frank3]

I see I see, but what use have the MP and RMP combined? For instance, why would I want a high MP but a low RMP? Or the other way around?

If I want to cruise, I ofcourse want the most power, for the lowest fuelburn. What function has the MP or RPM got in this?

[joeblogs]

The GROSS output of the engine is the proportional to MPxRPM. The net output is lower due to energy lost in heating up the engine and friction losses. The latter rise (I think) at the rate of the cube of RPM, so you often see engine manuals describe keeping MP up and RPM down.

But you don't want MP too high on a cruise because you want to keep a lean fuel mixture to conserve gas. Once MP gets high enough, the engine can detonate (the vapor explodes rather than burns uniformly). To stave that off requires a richer fuel mixture, which reduces endurance.

Fortunately Aces now essentially tells you what engine settings to use in a table (E6B) on the kneeboard.

-blogs

Originally posted by frank3
I see I see, but what use have the MP and RMP combined? For instance, why would I want a high MP but a low RMP? Or the other way around?

If I want to cruise, I ofcourse want the most power, for the lowest fuelburn. What function has the MP or RPM got in this?

[frank3]

I don't understand the first part, but let me think...

A higher MP takes care of more fuel being burned, why the rpm just takes care of the air-intake?

And reducing MP is only good for not blowing up your engine, thus is useless in Aces High?

[Ecliptik]

I always thought high MP with low RPM was dangerous and bad for the engine.  From what I've heard, proper procedure, in planes with variable pitch props, for increasing power is to increase RPM followed by opening up the throttle, and to decrease power you should close the throttle first, then reduce RPM, so that at no point do you have high MP with low RPM.  Anyone confirm/deny/explain?

[frank3]

In Aces High, the RPM is connected to the MP (both are rising by throttle)

I have played 'B-17', here you could individually raise/lower them, but the plane would not move with only an high MP.
Only after adding abit of RPM the thing would start to taxi.

Maybe this has something to do with it?

[Golfer]

The manifold pressure and rpm gauges are seperate because they are entirely seperate systems on the airplane.

This is even more complex to explain because of the supercharged engines of the WWII military airplanes that are modeled in game, so I'll keep things simple.

First, the RPM gauge is simply a measure of how fast the propeller is spinning.  In AH we have what are known as Constant Speed propellers.  CS props use oil under pressure from the engine to change the propeller pitch in order to keep the engine spinning at a constant RPM setting.  Basically, you have a propeller governer that does exactly that, it governs the amount of oil pressure going into the propeller hub (think of the hub as that little bubble in the middle of a prop on a B-17) through various mechanical devices in the system which sense RPM changes.

The manifold pressure gauge shows the pressure of intake air going into the engine which is a measure of how much 'oomph' the pistons are giving to spin the propeller around.  In a conventional airplane with no turbocharger, the only time you'll have the MP indicating close to outside ambient pressure (your local weatherman reads the barometer of 29.92 during his weather report.  Pilots use this as what is known as an 'altimeter setting') is when you are at sea level.  As you go up in alititude, pressure decreases, so manifold pressure (Pressure of the intake air) can only go down.  With the throttle 'wide open' or at full, at sea level you're making the most power possible with the engine.  Anytime you 'close' the throttle or decrease power, the pressure of the air going into the cylinders is restricted so it will show a lower value (18 inches for example) and thus a lower power setting.

With our supercharged engines (or turbocharged) they can maintain a value ABOVE ambient pressure because they pump compressed air into the engine.  Compressing the air makes a higher pressure, thus a higher power output.  Thats a whole other ball of wax.

Now, to take all these things and put them into practical use in game think of it this way.  RPM is the number of times the propeller is going around, and the MP indicator is how much "kick" the engine is giving to turn the propeller.

If you're still confused...I'd understand

[Casca]

Originally posted by Ecliptik
I always thought high MP with low RPM was dangerous and bad for the engine.  From what I've heard, proper procedure, in planes with variable pitch props, for increasing power is to increase RPM followed by opening up the throttle, and to decrease power you should close the throttle first, then reduce RPM, so that at no point do you have high MP with low RPM.  Anyone confirm/deny/explain?

The procedure you describe is correct and commonly taught.  Running the MAP (Manifold Absolute Pressure, which is what the manifold pressure gauge reads) X 100 equal to the RPM is called running "sqare".  If MAP X 100 is greater than the RPM its called "oversquare" and if MAP X 100 is less than RPM its called "undersquare".  The manifold pressure gauge is only an indirect measurement of the power being produced by the engine.  On a naturally aspirated engine it will be at its highest on the ramp before you start the airplane.  It tells us something about the throttle position but that has to be considered in relation to the mass airflow through the engine (the RPM).

Running an engine oversquare causes an increase in the BMEP (Brake Mean Effective Pressure) which is just a fancy way of saying that the cylinder studs have to work harder to hold the cylinder on the engine.   Extremely high BMEPs can result in detonation an/or mechanical failure.

A lot of old timers will tell you that you should never run oversquare but the correct answer, in my opinion, is RTFB (read the fine book).  Depending on the engine the POH will frequently list allowable oversquare settings.  If you are running a turbocharged or supercharged engine you will be running oversquare a majority of the time in any case.

[joeblogs]

Both higher MP and higher RPM increase the amount of air flowing through the engine every second. The carburetor (or an injection system) meters a fixed amount of fuel for every pound of air going through the engine.

Aces models both MP and RPM. They are not the same in the flight model. But you will see there is an effect of RPM on MP and vice versa for pure mechanical reasons.

The amount of RPM the engine can turn for a given power setting is determined by the load on the propeller (the power required to turn it so fast at such a spead). The propeller load curve increases as a power function of RPM (I can't remember if it is a square or a cube). So, if you are running at peak RPM and MP and pull the throttle back far enough, you may not have enough power to maintain the RPM setting.

Conversely, hold the throttle constant for a second and increase RPM a lot. You'll actually see MP drop slightly as more air is literally being sucked through the engine and lowering the pressure measured in the throttle tube. Note this is a bit more complicated when you have a lot of supercharging going on but I'll leave that out for simplicity.

In terms of adjusting MP and RPM, if you look at an engine calibration chart and pilot's instructions, you'll see that pilots are told to adjust RPM and MP in stages. This is to protect the engine and the supercharger. There is always a chart with a grid of RPM and MP settings to deliver a certain amount of power. That grid is the square another person mentioned.

When I suggest for a cruise it is better to have lower RPM and higher MP, remember that we are not talking about a ton of MP. The limiting factor is the desire to have a lean fuel mixture which is not possible for very high MP settings.

Both high pressure and high MP impose loads on the engine, but the enging manufactureres will typically say that higher RPM wears the engine out faster than does higher MP. Of course everything can be taken too far.

-blogs

Originally posted by frank3
I don't understand the first part, but let me think...

A higher MP takes care of more fuel being burned, why the rpm just takes care of the air-intake?

And reducing MP is only good for not blowing up your engine, thus is useless in Aces High?

[gripen]

The idea for using (relatively) high MAP and (relatively) low RPM for economical cruising in the (most) supercharged engines is quite simple; energy losses due to supercharging are smaller than in the case of the (relatively) low MAP and (relatively) high RPM.

Most engines had a mechanical stage in the supercharger system and energy used by this stage is quite linearly proportional with the RPM. So keeping the RPM in the minimum resulted minimum energy used in the supercharging. In addition low rpm resulted less need for throttling ie the MAP generated by the supercharger is used more effectively and the less throttling also results lower heating of charge.

In the case of the (most) turbocharged engines high MAP in the manifolds between the turbo and mechanical stage decrease the load of the mechanical stage ie in a way the turbo (variable speed) is used to "turn" mechanical stage with (mostly) free energy from the exhaust gases which again results better efficiency.

gripen

[frank3]

Thanks guys, I now slightly know how it works, but when do I have to lower my rpm's in flight?

I just use the throttle to slow down right? not the rpm

[joeblogs]

There are separate keys for controlling RPM. They are Keypad + and -.

See http://www.hitechcreations.com/htcindex.html for all the key commands.

You never have to lower RPM, although it is a good idea for increasing endurance. Always remember to put RPM back into max for combat or rapid climb.

-blogs

Originally posted by frank3
Thanks guys, I now slightly know how it works, but when do I have to lower my rpm's in flight?

I just use the throttle to slow down right? not the rpm

[JB73]

don't forget in HA in some instances you will not be able to lower the rpm...

and example is a 190d @18,000 going 400mph

you will be able to bring the MAN back, but if you try to adjust the RPM it will not do anything until you slow down or go lower.

[frank3]

yep, I knew about THOSE things guys, but thanks anyway

JB73, I think that had something to with having a turbo/supercharger or something (like P-47)