Manifold Pressure

[PatM]

This might be a dumb question, but could someone define for me what is meant by "manifold pressure?" I've read several references to manifold pressure in some Warbird Digest articles, and do not understand what is being measured, and how it relates to engine performance. Does it apply to both radial and in-line engines? Are RPM's monitored in addition to manifold pressure? I hope I am not the only non-flyer on the board who could be enlightened.

Thanks.

[Jack Cook]

A instrument gage that measures engine power by inches of mercury. You only (to my knowledge) see this on an a/c with a constant speed prop.
The RPM is the speed of the prop where the other is of engine power. Your basic bugsmashers just use RPM gage. BTW never let the engine go faster than the prop. Add power... prop up first then throttle. To decrease power.... throttle back first the prop. Very basic so you experts please don't bite my *ss!

[RobC]

Just to clarify a little bit, Manifold Pressure is a measurement of the vacuum, or pressure, in the throat of the throttle body (which is a carburetor for almost all warbirds, since few are fuel injected). In a normally aspirated engine, this would be a vacuum, whereas a boosted engine could be vacuum or pressure, depending upon power setting. There are many references easily available which can provide a thorough explanation if you search a bit.

[Tigercat]

Hey PatM,

Here ya go!!

http://avstop.com/AC/FlightTraingHandbook/Manifold.html

[bdk]

In general (and overly simplified), since there are exceptions to most rules:

Manifold pressure is an indirect measurement of how much power the engine will make at a given RPM. If you increase the throttle position you will inrease manifold pressure and hence the power. Because aircraft fly at various altitudes (which have different air pressures), 50% throttle position will give you different levels of power at different altitudes. A given manifold pressure and engine RPM combination should provide essentially the same power at any altitude (within the limits of the engine and prop- manifold air temperature, supercharger efficiency, etc.). To maintain power in climb, you must continuously advance the throttle as you climb. Some supercharged engines feature boost controllers that do this automatically.

Manifold pressure is normally measured in inches of mercury in the US of A. Since mercury is very dense it was used in the old days in a standing column to measure barometric pressure because inches of water would require a very tall gauge. A manifold pressure gauge is only calibrated to inches of mercury since the gauge doesn't actually have a column of mercury inside and operates using a bourdon tube like an oil pressure gauge.

It is seldom of value to measure manifold pressure in engines with fixed pitch props since it is nearly impossible to get too high of a manifold pressure (full throttle on the ground with the brakes on is the worst case). Normally engine RPM is your limiting factor. With a controllable prop, you can have a high prop pitch that bogs the engine down like trying to drive your car away from a stop sign in 5th gear. This can cause detonation that damages the engine.

[Stoney]

Jack
I have alway thought that the rpm gage show engines rpm, the gearing on a constant speed prop keeps it below supersonic normally. I did make a mistake once, so correct me if I got it wroug.

[Jack Cook]

I did make a mistake once, so correct me if I got it wroug

I've seen ya fly Stoney and you defently know how. The only thing you got wrong was in spelling the word wrong, wrong!

[PatM]

bdk,
Your analogy to the manual car transmission is exactly what I thougt of when I was reading the link provided by Tigercat. Is the pilot is controlling the prop pitch separately from the throttle, or is it automatic?

[eric_stevens]

One slight correction on a " BTW never let the engine go faster than the prop. " Their are times when it is acceptable, You must read the pilots operating handbook. If the book says it is Okay then it is Okay. I think the Seneca has a few power settings that are over Square. I think geared engines go faster then the prop. At least in the Cessna 421 you take off at around 40 inches and I know you don't have 4000 rpm on the prop.

[Stoney]

Jack
I may spell wroug or wrong, but I'm nenver wrong, well maybe once I did make a mistrake:) I'm alway right, wing that is.

[bdk]

bdk,
Your analogy to the manual car transmission is exactly what I thougt of when I was reading the link provided by Tigercat. Is the pilot is controlling the prop pitch separately from the throttle, or is it automatic?

RPM is controlled with the prop control which is seperate from the throttle, unless you have a Sea Fury with the Centaurus which has a single lever system as I recall.

If you look at a typical US throttle quadrant, the throttle is the longest, biggest, and leftmost lever, the mixture control is next, then the prop control.

The prop control actually sets a load in the governor which uses the centrifugal force on some flyweights to maintain equilibrium. If the engine starts turning faster than the set point, the flyweights move outward and oil pressure is either ported to the prop or relieved (depending on the type of prop) to increase the blade pitch (like shifting to a higher gear, loading the engine). If the engine starts turning slower than the set point, the flyweights move inward and do the opposite with the oil pressure to reduce blade pitch (and unload the engine).

Aeroproducts props have their own oil pump & oil supply, and Curtiss Electric props have a motor in the prop dome to move the blades with slip rings and brushes between the engine and the spinning prop.

Most of this is old A&P school recollections and other assorted fuzzy memories, but I think my explanation is substantially correct.

[Chris]

Check me if I'm wrong guys, but I thought you ALWAYS wanted more MP than RPM. This way the engine is driving the prop and you don't supercool the cylinders. For instance T.O. power might be 48" at 2600 RPM. Pull power back to 28" and 2300 for climb and 21" and 1850ish for RPM. If we have a long approach and have to bleed off excess speed by pulling power back, we will shove the props to full decrease RPM.

[PatM]

Guys,
Thanks for the info. I think I have a better understanding now.

[systemofadown1162]

Well as long as you guys know what you are doing.
Then again its 3:30 in the morning and ive been up all night playing on Live so I dont even know what im doing.

[Randy Haskin]

Check me if I'm wrong guys, but I thought you ALWAYS wanted more MP than RPM.

Completely depends on the aircraft. In some smaller GA aircraft, the opposite is true.