manifold pressure and rpms

[earl1937]

So you're saying that the the rpm gauge in aces high is actually rpm as in an engine tachometer?

No, it is showing prop RPM. There is not a instrument to show engine RPM. If you set your RPM at, say, 2650 RPM in the P-51, it will stay at that RPM value form 18 inches of MP or at 50 inches of manifold pressure. The prop governor limits the RPM from going above 2750, unless you change it with your key board. It does that by changing the angle of the blades to maintain that constant RPM which you selected. IN the Storch, the RPM instrument is showing prop RPM and engine RPM as the same, because it is a fixed pitch prop. The angle of the blades cannot be changed, so therefore the Prop RPM and engine RPM will always be the same.
There are fixed pitch props for climbing, or lifting heavier than normal weights and you also have a fixed pitch prop which is for cruise use, but suffers on takeoff and climb performance, but they are 2 different props.

[earl1937]

I agree with colmbo.

For props with the variable pitch option the angle of the prop blades is adjusted to keep the engine and prop RPM's at a constant speed.

I think there is some confusion because prop RPM may or may not the same as engine RPM. The prop and engine are mechanically linked and sometimes linked through a gear for optimal performance.

I do have a question about manifold pressure. Are the gauges measuring relative or absolute pressure?
 
Relative pressure is the difference between outside pressure vs. intake pressure. I believe our AH planes use this to gauge pressure because at engine off the manifold gauge falls to the lowest indicated (or zero) pressure.

Absolute pressure wouldn't compare the difference in pressure outside vs. intake pressure. If measuring absolute pressure the gauge would indicate around 32psi at sea level with engine off.

The standard atmospheric pressure at sea level is 14.7 pounds per square inch. I am not sure if we humans could survive at 32PSI!

There are many "geared" engines in use today and just remember this: "If the gear box has a failure, the aircraft is going to come down, no matter what engine RPM that you have".

Simple answer on Manifold gauge is that it shows the amount of fuel and air mix which is supplying the engine, through the manifold system. The manifold pressure gauge shows the atmospheric pressure entering the engine. When the engine is stopped, the manifold gauge will point to the local barometric pressure.

[hitech]

The RPM gauge is Engine RPM in all planes and helicopters I have flown. With out looking up a gear ratio the prop rpm is not a know factor.

Also concerning fuel/air mixture and the manifold pressure gauge. It needs to be understood that it is a measure of gas (not gasoline but meaning something not liquid I.E. air).

You can set your fuel mixture to ideal cut off (no fuel will be going to the engine), and the manifold pressure will remain the same (with the exception of a possible change in pressure do to temperature drop cause buy the fuel evaporation). I.E it is measuring a gas pressure and not a liquid pressure as a fuel pressure instrument does.

HiTech

[smoe]

The standard atmospheric pressure at sea level is 14.7 pounds per square inch. I am not sure if we humans could survive at 32PSI!

I must of been thinking of my car's tire pressure. Thanks for catching that.

One atmosphere (101 kPa or 14.7 psi)

[doright]

I do have a question about manifold pressure. Are the gauges measuring relative or absolute pressure?
 
Relative pressure is the difference between outside pressure vs. intake pressure. I believe our AH planes use this to gauge pressure because at engine off the manifold gauge falls to the lowest indicated (or zero) pressure.

Absolute pressure wouldn't compare the difference in pressure outside vs. intake pressure. If measuring absolute pressure the gauge would indicate around 32psi at sea level with engine off.

The way to think of Relative and Absolute pressure is this:
You have a very solid tank containing gas under pressure. An absolute pressure gauge will measure the pressure of that gas. Its reading will remain the same on the surface of the earth, 100ft below water, or on the moon. It is measuring the pressure of the gas alone hench the absolute name.

Take the same tank and put an relative pressure gauge on. Now you are measuring the difference in pressure between the inside and outside the tank. On the surface of the earth you will get one reading. 100ft below water you will get a significantly lower reading. On the moon you'll get a significantly higher reading (which happens to be the absolute pressure also).

The manifold pressure gauge is 'usually' an absolute pressure gauge, measuring the pressure of the air (not fuel) in the intake manifold. Setting on the ramp with engine not rotating the pressure gauge will read local barometric pressure. With the engine idling (throttle butterfly value mostly closed) it will read significantly below ambient. Throttle wide open on the ramp on a non-boosted engine it will still read below ambient because the throat of the throttle body acts as a restriction. Add a boosting supercharger or turbo-supercharger and the pressure gauge will start reading above ambient pressure that will vary with engine rpm (will also vary with throttle setting when not wide open).

I said 'usually' above because in most things aircraft related absolute statements should be avoided. Some aircraft have a boost gauge which is a relative gauge.

Relative pressure is the difference between outside pressure vs. intake pressure. I believe our AH planes use this to gauge pressure because at engine off the manifold gauge falls to the lowest indicated (or zero) pressure.

Actually that is a bug. http://bbs.hitechcreations.com/smf/index.php/topic,351960.0.html

[doright]

Also of note. The pressure measured at the fuel manifold (on an aircraft that has one) is not the same thing as the pressure measured at the intake manifold.

The fuel manifold is used on aircraft where fuel is injected, usually into the intake port of each cylinder. Each intake port requires an injector so a manifold is used to distribute the fuel evenly to each nozzle. Fuel pressure is measured at the manifold and displayed to the pilot. Since fuel has to be pumped through the nozzle at sufficient pressure to atomize the fuel spray it is not the same as the air pressure inside the intake manifold.

The intake manifold pressure is the air pressure at some point in the intake manifold. Again a manifold, but in this case, distributing air evenly to each cylinder.

[earl1937]

Also of note. The pressure measured at the fuel manifold (on an aircraft that has one) is not the same thing as the pressure measured at the intake manifold.

The fuel manifold is used on aircraft where fuel is injected, usually into the intake port of each cylinder. Each intake port requires an injector so a manifold is used to distribute the fuel evenly to each nozzle. Fuel pressure is measured at the manifold and displayed to the pilot. Since fuel has to be pumped through the nozzle at sufficient pressure to atomize the fuel spray it is not the same as the air pressure inside the intake manifold.

The intake manifold pressure is the air pressure at some point in the intake manifold. Again a manifold, but in this case, distributing air evenly to each cylinder.

You sir, are correct! Not being a engine design engineer, there are many things which are still somewhat of a mistery to me, but, being a throttle pusher, I just need to know what happens when I push this lever or flip that switch. I have tried to convey the way that I taught students about the operation of the engine, so that they understood what was happening, so they may operate a aircraft in a safe manner. I didn't mean to start a big argument over manifold pressure, engine RPM's and such, but it has been an interesting discussion!

[earl1937]

The RPM gauge is Engine RPM in all planes and helicopters I have flown. With out looking up a gear ratio the prop rpm is not a know factor.

Also concerning fuel/air mixture and the manifold pressure gauge. It needs to be understood that it is a measure of gas (not gasoline but meaning something not liquid I.E. air).

You can set your fuel mixture to ideal cut off (no fuel will be going to the engine), and the manifold pressure will remain the same (with the exception of a possible change in pressure do to temperature drop cause buy the fuel evaporation). I.E it is measuring a gas pressure and not a liquid pressure as a fuel pressure instrument does.

HiTech

A good example of the statement about manifold pressure staying the same, when leaning mixture, using a exhaust gas temperature gauge, you will notice that the manifold pressure does not change, even though you are restricting fuel into the cylinders.

[kvuo75]

both from this same thread:

When you are looking at your Tachometer inside the cockpit, what you are seeing is engine RPM's, and has nothing to do with your prop.

No, it is showing prop RPM. There is not a instrument to show engine RPM.

 

[earl1937]

both from this same thread:



 

I explained that mistake in an earlier reply sir! Sorry about that, your tach shows prop RPM!

[kvuo75]

I explained that mistake in an earlier reply sir! Sorry about that, your tach shows prop RPM!

sorry I missed it inbetween your posts telling me I didn't know what I was talking about..

and as it happens you end up agreeing with me completely.   

[BaldEagl]

your tach shows prop RPM!

The RPM gauge is Engine RPM in all planes and helicopters I have flown.

It seems we have a contradiction.  Who's right?

[kvuo75]

I always assumed it was prop rpm, before I thought about it, because you basically control it with the prop levers, and I'm used to thinking of small stuff where the prop is essentially mounted to the crankshaft..  but the more research I do the more it seems it is engine rpm.  ie. a merlin makes its power at 3000rpm..  all the P51 POH stuff references off of this 3000rpm, but I'm guessing a huge prop like that doesn't actually spin at 3000rpm, so the tach must be engine tach.  (now off to see if I can do the math to figure out the speed of the prop tips of an 11 foot prop at 3000rpm

[kvuo75]

I got 1754 feet per second for prop tips of a 11'2" prop spinning at 3000rpm..

that is very supersonic. which is bad.


my conclusion, hitech is correct, the RPM gauge does show engine rpm

(because there's no way anyone would spin a 11 foot prop at 3000rpm)

[CMan]

In my experience, in aircraft which have fixed pitch propellers, the RPM gauge generally shows engine RPM (which is the same as prop RPM on some aircraft, depending on whether the prop is geared down). On aircraft with constant speed (variable pitch) props however, the RPM gauge generally shows prop RPM (which again may or may not be the same as engine RPM). This is just what I have seen on light civil aircraft which I have flown or ridden in. I don't know for certain whether this is also true of very high performance applications or our AH models.

For me, the easiest way to understand the relationship between manifold pressure and prop RPM is that manifold pressure is an indication of engine power output, and prop RPM is a measure of how efficiently that power is converted to thrust by the prop at a given altitude/airspeed.