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, what I am saying is that most constant speed props do not turn at the same RPM as the engine. Most constant speed props usually turn at a slower RPM than the engine RPM, but this is not shown by any instrument in your aircraft. There are probabley more aircraft with props which turn faster than the engine, but the only one I know of was a Cessna 175, which had a "geared prop", which actually turned faster than the engine, but the cost of this aircraft was to high compared to other aircraft in its class and was discontinued. They are some still around because you see them advertised in Trade-A-Plane.

[earl1937]

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

Opps! Didn't understand what caused the discussion all of a sudden about the "engine RPM"
This is not a correct statement:  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. Your tachometer shows propeller RPM, as I stated in the previous reply, but the engine RPM and prop RPM are two different value's.
Guess that is what not getting enough sleep lately because of the wife's illness will do for you. You don't realize when you make a dumb mistake.

[Puma44]

AND you fly best if you have a magic feather in your trunk at all times!

- oldman

Oldman, it also helps to have a couple jars of swamp gas and a mirror in the trunk also. 

[Puma44]

 What settings do you guys use for combat?  Full out throttle up hill and reduced in dives?

That depends on a lot of variables but, boils down to one basic thing; energy management.  That is accomplished by a combination of BFM, ACM, SA, power management, AND a lot of practice, practice, practice.........  

Also, in regard to the manifold pressure/RPM discussion below, experiment with the E6B settings.   At level flight with the listed cruise settings, try reducing RPM at a set manifold pressure and compare before and after minutes of flight remaining.  Often times an increase in endurance can be achieved, similar to what Lindbergh demonstrated with the P-38s in the Pacific.

[asterix]

Opps! Didn't understand what caused the discussion all of a sudden about the "engine RPM"
This is not a correct statement:  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. Your tachometer shows propeller RPM, as I stated in the previous reply, but the engine RPM and prop RPM are two different value's.
Guess that is what not getting enough sleep lately because of the wife's illness will do for you. You don't realize when you make a dumb mistake.

[earl1937]

What I should have said is, "the tachometer shows the RPM of the engine and manifold pressure gauge shows the amount of fuel and air to the engine. If the engine is shut down, the manifold pressure gauge will show the barometric pressure which is present where the aircraft is shutdown. When you start the engine, the manifold pressure will reflect the amount of air and fuel, currently being introduced to the engine, this value is usually around 12 inches of manifold pressure. The engine may, at the lower RPM's, such as idling, the prop RPM and the engine RPM may be the same at that time, but as you advance the throttle, the engine RPM and prop RPM will become two separate values, but for purposes of flying, the tachometer reflects the RPM of the engine.  

If you will review reply #3, which I made, there is a cutaway of the prop and shows the various parts, one of which is the propeller reduction gear.
Another good source of information is the Federal Aviation Agency "pilots handbook of aeronautical knowledge", which is available to view on line.

[kvuo75]

  The engine may, at the lower RPM's, such as idling, the prop RPM and the engine RPM may be the same at that time, but as you advance the throttle, the engine RPM and prop RPM will become two separate values,

how is this possible when the engine and prop are mechanically linked?


you sure you aren't thinking of turboprops?

[earl1937]

how is this possible when the engine and prop are mechanically linked?


you sure you aren't thinking of turboprops?

Take a look at reply #3, you will note the reduction gearing in the nose section of the engine. The increased brake horsepower delivered by a high-horsepower engine may be the result of increased crankshaft RPM. It is therefore necessary to provide reduction gears to limit the propeller rotation speed to a value at which efficient operation of the propeller is obtained. Whenever the speed of the blade tips approaches the speed of sound, the efficiency of the propeller decreases rapidly.”
That is the best explanation which I can provide for you, it has to do with prop effectiveness and speed of the prop tips. When the prop tips go super-sonic, the performance of the aircraft is greatly reduced.

[kvuo75]

Take a look at reply #3, you will note the reduction gearing in the nose section of the engine. The increased brake horsepower delivered by a high-horsepower engine may be the result of increased crankshaft RPM. It is therefore necessary to provide reduction gears to limit the propeller rotation speed to a value at which efficient operation of the propeller is obtained. Whenever the speed of the blade tips approaches the speed of sound, the efficiency of the propeller decreases rapidly.”
That is the best explanation which I can provide for you, it has to do with prop effectiveness and speed of the prop tips. When the prop tips go super-sonic, the performance of the aircraft is greatly reduced.

I understand that. In fact I mentioned reduction gearing in my first reply.

however, as hitech pointed out, the ratio of prop rpm to engine rpm remains the same, so there's no way the prop rpm and engine rpm can be the same (say, 1000 and 1000), then change as you add power (1500 and 2000), because they are mechanically linked.

[Karnak]

All you guys rock! Earl, thanks for the photo breakdown,I knew you would deliver. The gas pedal, gear explanation was easy to digest for my lizard brain.  What settings do you guys use for combat?  Full out throttle up hill and reduced in dives?

That depends on your energy state and is affected by which airplane you're in.  If you're low on E you may want to keep the throttle at full even going downhill to try to add more E.  The speeds at which you want to add more E or back off on the throttle on the dive vary from airplane to airplane and from combat situation to combat situation.  The P-51D will take more speed than the A6M2 so in a situation where you'd be backing off of your A6M2's throttle you would likely be holding your P-51D's throttle at max.  Experience is the best way to get comprehensive understanding of when to do what in which plane.

[earl1937]

I understand that. In fact I mentioned reduction gearing in my first reply.

however, as hitech pointed out, the ratio of prop rpm to engine rpm remains the same, so there's no way the prop rpm and engine rpm can be the same (say, 1000 and 1000), then change as you add power (1500 and 2000), because they are mechanically linked.

Yes they are linked, but the reduction gearing is there for a reason, to reduce the prop rpm below the engine rpm at higher power settings! The prop is designed to only turn so fast, then the high speed locks come into play and the prop is restricted from turning at a higher RPM. The reason for that has already been pointed out, to restrict the prop tips from going super sonic.
When at high speed and full throttle, the engine is turning much faster than the prop, for the above stated reason. Very few times, if any, will the prop and engine be turning at the same RPM above 15 inches of manifold pressure. According to Hamilton Standard propeller company, the B-17, at 40 inches of manifold pressure and 2400 RPM, the prop was turning at 2,000 RPM, which is the max R's for that application.  As the speed of the aircraft increases, the "pitch" on the blades decrease so as to not exceed that max limit. The reverse occurs when the aircraft starts climbing, now the prop pitch will increase, so the prop can stay at the value you have set it at, 2400 RPM. Now with that said, how do you think the engine and prop stays at the same RPM? I would suggest you look up "Sun and Planetary" gears as they apply to aircraft engines, I think that should clear this discussion up.

[colmbo]

When at high speed and full throttle, the engine is turning much faster than the prop, for the above stated reason.

The difference between engine RPM and prop RPM does not change regardless of the power setting.  It is a fixed ratio, it is not possible to change it -- at least on conventional aircraft engines.

[FLOOB]

I'm talking about the rpm gauge in aces high.

[earl1937]

The difference between engine RPM and prop RPM does not change regardless of the power setting.  It is a fixed ratio, it is not possible to change it -- at least on conventional aircraft engines.

Point is taken! But if I am working with a student on a multi-engine rating, I don't want him thinking that if he pushes the throttles wide open on a go around, that the props are now going to working at their most efficient RPM, because that is not true. I want that student to understand that if he misses the check list on landing where he should go to full increase on the props, if he has to go around because of a failed approach for some reason,  and he pushes the throttle wide open, as he should for go around, if he had the prop RPM's set at 2200 RPM on approach, then that is all the props are going to turn at, even if he goes to full power, assuming the full increase RPM settings with full throttle is, say, 2750RPM. He needs to understand that 550 RPM could get him in a lot of trouble. I want that student to remember and think that the props are independent of engine RPM, that way he will never forget to go to full increase prior to making an approach to landing.

[smoe]

The difference between engine RPM and prop RPM does not change regardless of the power setting.  It is a fixed ratio, it is not possible to change it -- at least on conventional aircraft engines.

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. This does not mean the prop and engine RPM are equal, but correlative.

I think there is some confusion because prop RPM may or may not be 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.