manifold pressure and rpms

[madrid311]

Ok. I get how a car,motorcycle and boat engines rev and provide power to a degree. but I would love to understand manifold pressure and rpm in regards to our AH warbirds. Earl? thanks in advance.

[Franz Von Werra]

Where's are all the 1000-posts-per-day guys?

Obviously though, a VERY good question!  I ALSO look forward to learning all about this topic, thanks too, in advance. 

[gyrene81]

excellent question and i only knew the automotive application but a google search using the words - "increasing manifold pressure in a turbocharged engine" came back with this little gem.

http://flighttraining.aopa.org/students/solo/special/turbo.html

and this simple explanation of the manifold pressure gauge reading...
http://www.askacfi.com/421/what-is-manifold-pressure.htm

haven't looked at turbo-superchargers...

[earl1937]

Ok. I get how a car,motorcycle and boat engines rev and provide power to a degree. but I would love to understand manifold pressure and rpm in regards to our AH warbirds. Earl? thanks in advance.

Manifold pressure is a unit of fuel pressure, usually measured between the fuel pump and measured somewhere before entering the cylinder itself. Most gravity fed systems, such as used on a lot of civilian aircraft, Cessna, Piper, etc, has a fuel pump to use for starting, takeoff and landings, but is not necessary for normal flight operations. Most of those aircraft are 4 and 6 cylinder, opposed engines.
Most all radial and in line military engines have fuel pumps which are used for starting, takeoffs and landing and emergency situations, but all are not required either for normal flight operations. The reason for this is simple, the engine, when running, creates a vacuum or suction inside the engine fuel manifold system, created by the opening and closing of the intake valves, which pretty much keeps the incoming fuel at a constant pressure. This is why most high performance aircraft engines have a fuel pump which must be turned on for safety reasons, during low engine RPM's, such as for landing and taxiing. As the engine RPM's get lower, the vacuum created by the engine gets lower, hence you have to use the fuel pump to maintain constant fuel pressure. Most people and flight manuals refere to this pump as the "boost pump". Most boost pumps have a high and a low setting which gives the pilot a choice, for example on the ground, low boost is usually good enough for safe engine operation. High boost is usually used for takeoffs, climbouts, combat situations and emergencies.
Engine RPM's, Revelations per minute, is a measurement of how fast the crank shaft of the engine is turning. It has nothing to do with the RPM of the propeller. Some sensors are placed for measuring RPM's at the rear of the crankcase but some have an internal pickup for this info. 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. Inside the cockpit, somewhere close to the throttle, is a handle, usually marked in blue, throttle is black, mixture control is red. When you reduce your engine RPM, such as for cruise, let down and etc, if you will notice on most aircraft, the engine RPM will stay at what ever you have it set for until the manifold pressure is reduced to somewhere around 12 to 15 inches of manifold pressure, then the prop RPM will begin to follow your throttle inputs. When you reduce your RPM's for cruise, for example, what is happening is a governor mounted, usually somewhere on the nose section of the engine, is metering oil pressure from your engine through the prop governor to arrive at what ever value that you are trying to set it at. The prop governor operates off of engine oil pressure and the propeller RPM's are usually lower than the engine RPM's. Since this is the function of a governor it is not something to be concerned with. The aircraft with prop governors are called "constant speed props", used on all the WW2 aircraft, which we have in this game.
The exception to this is a fixed pitch prop, such as we have on the Storch and WW1 aircraft, where the propeller has a fix pitch and when you look at the tach in those aircraft, you are seeing engine and prop RPM.


This is rough pic of the prop governor and where it is mounted normally.

Now, what we have been discussing are called hydraulic operated prop control systems. There is another way of controlling prop RPM and that is an electrical prop control. You can set your RPM with a switch, for high or low or many other settings. Most all the aircraft that I am familiar with in this game are hydraulic operated constant speed props.

Hope this helps out!

[Puma44]

Great reply as usual, Earl.  For some real world, historical reference here is an excerpt from an article about Lindberg 's briefings about P-38 long range cruise power settings.  Interesting reading.....

"Two missions later, on 3 July, the group covered sixteen heavies on a strike against Jefman Island. Lindbergh led Hades Squadron's White Flight as they wove back and forth above the lumbering B-25s. After the attack the Lightnings went barge hunting.

First one, then two pilots reported dwindling fuel and broke off for home. MacDonald ordered the squadron back but because Lindbergh had nursed his fuel, he asked for and received permission to continue the hunt with his wingman. After a few more strafing runs, Lindbergh noticed the other Lightning circling overhead. Nervously the pilot told Lindbergh that he had only 175 gallons of fuel left. The civilian told him to reduce engine rpms, lean out his fuel mixture, and throttle back. When they landed, the 431st driver had seventy gallons left, Lindbergh had 260. They had started the mission with equal amounts of gas.

Lindbergh talked with MacDonald. The colonel then asked the group's pilots to assemble at the recreation hall that evening. The hall was that in name only, packed dirt floors staring up at a palm thatched roof, one ping pong table and some decks of cards completing the decor. Under the glare of unshaded bulbs, MacDonald got down to business. "Mr. Lindbergh" wanted to explain how to gain more range from the P-38s. In a pleasant manner Lindbergh explained cruise control techniques he had worked out for the Lightnings: reduce the standard 2,200 rpm to 1,600, set fuel mixtures to "auto-lean," and slightly increase manifold pressures. This, Lindbergh predicted, would stretch the Lightning's radius by 400 hundred miles, a nine-hour flight. When he concluded his talk half an hour later, the room was silent.

The men mulled over several thoughts in the wake of their guest's presentation. The notion of a nine-hour flight literally did not sit well with them, "bum-busters" thought some. Seven hours in a cramped Lightning cockpit, sitting on a parachute, an emergency raft, and an oar was bad, nine hours was inconceivable. They were right. Later, on 14 October 1944, a 432nd pilot celebrated his twenty-fourth birthday with an eight-hour escort to Balikpapan, Borneo. On touching down, he was so cramped his crew chief had to climb up and help him get out of the cockpit.

The group’s chief concern surfaced quickly, that such procedures would foul sparkplugs and scorch cylinders. Lindbergh methodically gave the answer. The Lightning's technical manual provided all the figures necessary to prove his point; they had been there all along. Nonetheless the 475th remained skeptical. A single factor scotched their reticence.

During their brief encounter, MacDonald had come to respect Lindbergh. Both men pushed hard and had achieved. Both were perfectionists never leaving things half done. And both had inquisitive minds. John Loisel, commanding officer the 432nd, remembered the two men talking for long periods over a multitude of topics beyond aviation. If, as MacDonald had informed his pilots, better aircraft performance meant a shorter war, then increasing the Lightning's range was worth investigating. Lindbergh provided the idea, but it was MacDonald's endorsement, backed by the enormous respect accorded him by the group, that saw the experiment to fruition. The next day, the Fourth of July, Lindbergh accompanied the 433rd on a six-hour, forty-minute flight led by Captain "Parky" Parkansky. Upon landing, the lowest fuel level recorded was 160 gallons. In his journal entry Lindbergh felt ". . . that the talk last night was worthwhile. " The 475th had lengthened its stride."

[earl1937]

Ok. I get how a car,motorcycle and boat engines rev and provide power to a degree. but I would love to understand manifold pressure and rpm in regards to our AH warbirds. Earl? thanks in advance.

Just as a follow up on my reply to your post, here is a cut-a-way of a constant speed prop showing all the different parts:



One of the thing which I did not discuss is the feathering feature, which we do not have here in Aces High. You can shut down your engine by pressing shift + 1 or 2, 3 or 4, if you want to shut down an engine on one of the 4 engine aircraft in the game. Shift + 1 or 2 for the twin engine aircraft.

[earl1937]

Ok. I get how a car,motorcycle and boat engines rev and provide power to a degree. but I would love to understand manifold pressure and rpm in regards to our AH warbirds. Earl? thanks in advance.

There are other operating points of the manifold pressure which you also will need to know so that you will understand what is happening to your aircraft as you fly both in RL and in AH.
First thing you need to understand is the aircraft engine fuel system, both induction air and fuel, is designed to operate at "sea level, at 59 degrees F. and 29.92 inches of barometric pressure. This is considered a "standard atmosphere" and is where all designs start from.  When engine designers began to design engines, they found out that as altitude of the aircraft increased, the performance of the engine decreased. So, the engineers begin to fiddle with things to make the engine work as good at 20,000 feet as it did at sea level. After identifying the primary problem, one of less air induction to the carburation system, they came up with a method called "super charging". Now, you have to understand that these carburetor type engines in WW2, didn't have the benefit of fuel injection as most modern aircraft do. The PSI, pounds per inch, measurement at sea level, with standard atmospheric conditions is 14.7 pounds per square inch. First thing they did to improve the induction system air flow was install a super charger, which was gear driven from the back of the engine. Well, that didn't work to good, as it pulled the power down from the engine by the gear driven method. Then someone came up with re-routing exhaust gas from the engine thru a device called a "waste gate", then it regulated how much air was pumped through the induction system.
There are 5 levels in a B-17 engine or a B-24 engine and the "upper deck" level is where the carburetor has to be "fooled" into thinking it is at sea level and 14.7 pounds per square inch. Even with all the bells and whistles that they put on super charging, they soon learned that as the aircraft acended into the upper atmosphere, the density altitude was to great to overcome, so then you have what is called a "design service ceiling". This is the point at which the aircraft cannot sustain a 100 foot per minute climb rate.
You will notice when flying, for example, the B-17 in this game, that at sea level, 50% fuel and 12 500lbers of bombs, the ole bird will climb at 970 feet per minute, but by the time you get to somewhere around 20,000 feet, that climb rate is now down to about 400 feet per minute and as you cont' to climb, it will fall off more, until you get to that service ceiling we talked about.
As long as the induction air system from the carburetor to the cylinders remains at 14.7 pounds per square inch, you will get the maximum performance from your aircraft.
Step climbing: This is a method of climbing an over weight aircraft: after establishing your standard rate of climb, say to 5,000 feet, then level off, leave power at full throttle until you reach what you think is the maximum speed, then, raising the nose slightly and climbing at a higher feet per minute rate of climb until the speed reduces back to the normal climb speed, then level and repeat the process until you get to the altitude which you want to cruise at.
We used to step climb a DC-6B, out of Greer, S.C., hauling cut blue jeans to San Juan, P.R., for sewing and labels, then haul a finished load of jeans back to Greer. With the old dog at about gross weight, and old engines, it didn't want to climb to good, so climb to 5,000 feet, then step climb to 17,000 feet, our assigned cruising altitude.
Here is a list of some of the aircraft in this game with the R-1820 engine which had the single stage supercharger system.

Boeing B-17 Flying Fortress
Boeing 307
Brewster F2A
Curtiss AT-32-A Condor
Curtiss SBC-4 Helldiver
Curtiss P-36 Mohawk
Curtis's SC Seahawk
Curtiss-Wright CW-21
Douglas A-33
Douglas B-18
Douglas DC-2
Douglas DC-3 (DST, G-102 and G-202)
Douglas Super DC-3, R4D-8 / C-117
Douglas DC-5
Douglas SBD Dauntless
FMA AeMB.2 Bombi
General Motors FM-2 Wildcat
Grumman TF-1 / C-1 Trader
Grumman FF-1
Grumman F3F
Grumman HU-16 Albatross
Grumman J2F Duck
Grumman S-2 Tracker
Lockheed 14
Lockheed Lodestar
Lockheed Hudson
Martin B-10
North American NA-44
North American O-47
North American P-64
North American T-28B/C/D Trojan
Northrop YC-125 Raider
Piasecki H-21
Polikarpov I-16
Ryan FR Fireball
Sikorsky S-58/HUS/HSS/H-34

If you are new to the game, come to the Rook country, type 171 in last window of radio bar, hit enter twice, then check to see if I am on line, call me and will be glad to help you get started!

[Oldman731]

These are all great responses.  But I think at a very basic level, so here's how I view it:

- Manifold pressure is what I'd normally think of as the gas pedal on a car.  It controls how much air goes into your motor, and the air draws fuel with it (depending on how you've leaned your fuel-air mixture).

- The rpm control keeps the propeller moving at a particular speed.  It does this by twisting the propeller blades so that they take bigger or smaller bites of air.  Bigger bites mean lower rpm, assuming you don't touch the manifold pressure, because the strain of taking bigger bites slows the motor down.

Plane manufacturers supply tables telling you what the best combinations of manifold pressure and rpm are for given altitudes and different purposes.

You would think that you'd go fastest if you took the biggest bites of air at the highest throttle setting, but what would happen is that you'd burn out your motor.  

Probably much (most?) of the above is technically wrong, but it works out that way.

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

- oldman

[Brooke]

Manifold pressure/throttle is the gas pedal on your car.

Prop pitch/RPM is what gear you are in on your car.  There were some aircraft propellers that had fixed pitch.  That is like a car with one gear.  Some props had two pitches (fine and coarse), like a car with two gears.  Prop pitch is like a continuously variable transmission that can pick, for a given speed of your car, any given RPM of the engine.

[madrid311]

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?

[kvuo75]

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.

I'm pretty sure that's incorrect.  rpm is prop rpm which usually is engine rpm unless there is a reduction gear between engine and prop. no?

Inside the cockpit, somewhere close to the throttle, is a handle, usually marked in blue, throttle is black, mixture control is red. When you reduce your engine RPM, such as for cruise, let down and etc, if you will notice on most aircraft, the engine RPM will stay at what ever you have it set for until the manifold pressure is reduced to somewhere around 12 to 15 inches of manifold pressure, then the prop RPM will begin to follow your throttle inputs.

 



engine rpm and prop rpm are linked.. if you reduce prop rpm, engine rpm is reducing also

[earl1937]

I'm pretty sure that's incorrect.  rpm is prop rpm which usually is engine rpm unless there is a reduction gear between engine and prop. no?

 



engine rpm and prop rpm are linked.. if you reduce prop rpm, engine rpm is reducing also

Sir, you need to go online or somewhere and learn something about aircraft engines! The PROP does not turn at the RPM that you are looking at in your cockpit. The only time that is true is with a "fixed" pitch prop, such as that on the Stroch!

A constant speed prop, such as that on a P-51, B-17, B-24 or many other aircraft in this game, does excaly that, it is constant at your set RPM as long as you have enough manifold pressure to maintain  the required oil pressure to the prop governor. In most aircraft, that is usually 12 to 15 inches manifold pressure.

[FLOOB]

Sir, you need to go online or somewhere and learn something about aircraft engines! The PROP does not turn at the RPM that you are looking at in your cockpit. The only time that is true is with a "fixed" pitch prop, such as that on the Stroch!

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

[hitech]

Sir, you need to go online or somewhere and learn something about aircraft engines! The PROP does not turn at the RPM that you are looking at in your cockpit. The only time that is true is with a "fixed" pitch prop, such as that on the Stroch!

You may wish to do that your self earl, because manifold pressure is the measure of air pressure at  the air intake manifold. It is not as you stated fuel pressure.

But I do agree RPM is engine RPM not prop rpm which is normally the same or lower then engine rpm.  But for most small aircraft the prop rpm and then engine rpm are a 1 to 1 ratio. A fixed pitch prop has nothing to do with the ratio between the engine and the prop. That will remain constant on all prop aircraft I know of regardless of fixed or constant. On most wwii birds there is a gear reduction of .4 to .5 range.

The throttle  as brook states is just like a car, it simply opens a butter fly valve that lets more air into the engine.

On planes with constant speed/ variable pitch, when you increase the throttle more torque is driving the prop causing a slight speed increase,
the governor adjust the pitch of the prop to bring the rpm back down to a constant rpm which is set with the prop/rpm control.

For American planes the mainfold pressure is measured just like a baramater in inches of mercury. For a plane to have more manifold pressure than the atmosphere it requires either a super or turbo charger to increase the air pressure at the intake manifold.

HiTech

[earl1937]

You may wish to do that your self earl, because manifold pressure is the measure of air pressure at  the air intake manifold. It is not as you stated fuel pressure.

But I do agree RPM is engine RPM not prop rpm which is normally the same or lower then engine rpm.  But for most small aircraft the prop rpm and then engine rpm are a 1 to 1 ratio. A fixed pitch prop has nothing to do with the ratio between the engine and the prop. That will remain constant on all prop aircraft I know of regardless of fixed or constant. On most wwii birds there is a gear reduction of .4 to .5 range.

The throttle  as brook states is just like a car, it simply opens a butter fly valve that lets more air into the engine.

On planes with constant speed/ variable pitch, when you increase the throttle more torque is driving the prop causing a slight speed increase,
the governor adjust the pitch of the prop to bring the rpm back down to a constant rpm which is set with the prop/rpm control.

For American planes the mainfold pressure is measured just like a baramater in inches of mercury. For a plane to have more manifold pressure than the atmosphere it requires either a super or turbo charger to increase the air pressure at the intake manifold.

HiTech

quote I am not an internal combustion engineer, so I have to rely on information given to pilots, which is approved by the Federal Aviation Agency. Following are descriptions of the subject matter at hand.
On airplanes that are equipped with a constant-speed propeller, power output is controlled by the throttle and indicated by a manifold pressure gauge. The gauge measures the absolute pressure of the fuel/air mixture inside the intake manifold and is more correctly a measure of manifold absolute pressure (MAP). At a constant r.p.m. and altitude, the amount of power produced is directly related to the fuel/air flow being delivered to the combustion chamber. As you increase the throttle setting, more fuel and air is flowing to the engine; therefore, MAP increases. When the engine is not running, the manifold pressure gauge indicates ambient air pressure (i.e., 29.92 in. Hg). When the engine is started, the manifold pressure indication will decrease to a value less than ambient pressure (i.e., idle at 12 in. Hg). Correspondingly, engine failure or power loss is indicated on the manifold gauge as an increase in manifold pressure to a value corresponding to the ambient air pressure at the altitude where the failure occurred.

Fixed Pitch Propellers:

The pitch of this propeller is set by the manufacturer, and cannot be changed. With this type of propeller, the best efficiency is achieved only at a given combination of airspeed and r.p.m. There are two types of fixed-pitch propellers—the climb propeller and the cruise propeller. Whether the airplane has a climb or cruise propeller installed depends upon its intended use:


•The climb propeller has a lower pitch, therefore less drag. Less drag results in higher r.p.m. and more horsepower capability, which increases performance during takeoffs and climbs, but decreases performance during cruising flight.

•The cruise propeller has a higher pitch, therefore more drag. More drag results in lower r.p.m. and less horsepower capability, which decreases performance during takeoffs and climbs, but increases efficiency during cruising flight.


The propeller is usually mounted on a shaft, which may be an extension of the engine crankshaft. In this case, the r.p.m. of the propeller would be the same as the crankshaft r.p.m. On some engines, the propeller is mounted on a shaft geared to the engine crankshaft. In this type, the r.p.m. of the propeller is different than that of the engine. In a fixed-pitch propeller, the tachometer is the indicator of engine power.

The pitch of this propeller is set by the manufacturer, and cannot be changed. With this type of propeller, the best efficiency is achieved only at a given combination of airspeed and r.p.m. There are two types of fixed-pitch propellers—the climb propeller and the cruise propeller. Whether the airplane has a climb or cruise propeller installed depends upon its intended use:


•The climb propeller has a lower pitch, therefore less drag. Less drag results in higher r.p.m. and more horsepower capability, which increases performance during takeoffs and climbs, but decreases performance during cruising flight.

•The cruise propeller has a higher pitch, therefore more drag. More drag results in lower r.p.m. and less horsepower capability, which decreases performance during takeoffs and climbs, but increases efficiency during cruising flight.


The propeller is usually mounted on a shaft, which may be an extension of the engine crankshaft. In this case, the r.p.m. of the propeller would be the same as the crankshaft r.p.m. On some engines, the propeller is mounted on a shaft geared to the engine crankshaft. In this type, the r.p.m. of the propeller is different than that of the engine. In a fixed-pitch propeller, the tachometer is the indicator of engine power.
Constant speed props:
An airplane with a constant-speed propeller has two controls—the throttle and the propeller control. The throttle controls power output, and the propeller control regulates engine r.p.m. and, in turn, propeller r.p.m., which is registered on the tachometer.
Once a specific r.p.m. is selected, a governor automatically adjusts the propeller blade angle as necessary to maintain the selected r.p.m. For example, after setting the desired r.p.m. during cruising flight, an increase in airspeed or decrease in propeller load will cause the propeller blade angle to increase as necessary to maintain the selected r.p.m. A reduction in airspeed or increase in propeller load will cause the propeller blade angle to decrease.

The range of possible blade angles for a constant-speed propeller is the propeller´s constant-speed range and is defined by the high and low pitch stops. As long as the propeller blade angle is within the constant-speed range and not against either pitch stop, a constant engine r.p.m. will be maintained. However, once the propeller blades contact a pitch stop, the engine r.p.m. will increase or decrease as appropriate, with changes in airspeed and propeller load. For example, once a specific r.p.m. has been selected, if aircraft speed decreases enough to rotate the propeller blades until they contact the low pitch stop, any further decrease in airspeed will cause engine r.p.m. to decrease the same way as if a fixed-pitch propeller were installed. The same holds true when an airplane equipped with a constant-speed propeller accelerates to a faster airspeed. As the aircraft accelerates, the propeller blade angle increases to maintain the selected r.p.m. until the high pitch stop is reached. Once this occurs, the blade angle cannot increase any further and engine r.p.m. increases.