Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: madrid311 on November 01, 2013, 09:28:12 AM
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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.
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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. :salute
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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 (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 (http://www.askacfi.com/421/what-is-manifold-pressure.htm)
haven't looked at turbo-superchargers...
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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.
:airplane: 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.
(http://i1346.photobucket.com/albums/p684/earl1937/propeller_oil_plumbing_zpsf250b3ce.jpg)
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!
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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."
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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.
:airplane: 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:
(http://i1346.photobucket.com/albums/p684/earl1937/800px-Propeller_diagram_zps04c1ca46.jpg)
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.
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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.
:airplane: 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!
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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
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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.
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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?
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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.
:headscratch:
engine rpm and prop rpm are linked.. if you reduce prop rpm, engine rpm is reducing also
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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?
:headscratch:
engine rpm and prop rpm are linked.. if you reduce prop rpm, engine rpm is reducing also
:airplane: 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.
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:airplane: 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?
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:airplane: 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
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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 :airplane: 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.
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So you're saying that the the rpm gauge in aces high is actually rpm as in an engine tachometer?
:airplane: 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.
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So you're saying that the the rpm gauge in aces high is actually rpm as in an engine tachometer?
:airplane: 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.
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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.
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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......... :salute
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.
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:airplane: 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.
:headscratch:
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:headscratch:
:airplane: 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.
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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?
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how is this possible when the engine and prop are mechanically linked?
you sure you aren't thinking of turboprops?
:airplane: 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.
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:airplane: 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.
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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.
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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.
:bhead 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.
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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.
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I'm talking about the rpm gauge in aces high.
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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.
:airplane: 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.
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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.
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So you're saying that the the rpm gauge in aces high is actually rpm as in an engine tachometer?
:airplane: 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.
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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.
:airplane: 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.
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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
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:airplane: 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)
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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 (http://bbs.hitechcreations.com/smf/index.php/topic,351960.0.html)
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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.
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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! :salute
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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
:airplane: 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.
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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.
:airplane: No, it is showing prop RPM. There is not a instrument to show engine RPM.
:headscratch:
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both from this same thread:
:headscratch:
:noid I explained that mistake in an earlier reply sir! Sorry about that, your tach shows prop RPM!
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:noid 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. :aok :salute
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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?
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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 :)
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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)
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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.
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Just an idea, are prop RPM's calculated using the number of blades?
I.E.
So, Kvuo75 would need to divide 1754 feet per second by the number of prop blades for the 3000 rpm calculation.
Another Example: A 4-cylinder, 4-stroke engine sharing the same shaft as a 2-blade prop (no gear in between).
In one 720 degree turn (for simplicity) on the shaft, the prop and engine revolutions would count as four (4) each?
Both prop and engine RPM = 4 / (time in minutes for a prop and engine shaft to spin two (2) complete revolutions, aka 720 degrees)
Basically this example is a 1:1 ratio, change the number of prop blades while keeping the same engine and the prop and engine RPM's are no longer 1:1.
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RPM is rpm regardless of the number of blades, or cylinders for that matter. RPM is the number of times the shaft rotates in a minute. Kvuo's calculation has more to do with the props diameter than it does with the number of blades. The larger the diameter the faster the tip must travel at a given rpm because it has to travel the same 360 degrees that the shaft does in the same amount of time while inscribing a larger circle.
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The guy who started the thread was asking about AH planes, and you guys went off on real life planes confusing everyone. First earl says it's engine rpm then he says it's prop rpm, then hitech comes in and says that in every real plane (i'm assuming he meant real) he's been in the rpm guage indicates engine rpm.
So could someone please answer the poster's question. In AH warbirds what does the rpm guage indicate?
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The guy who started the thread was asking about AH planes, and you guys went off on real life planes confusing everyone. First earl says it's engine rpm then he says it's prop rpm, then hitech comes in and says that in every real plane (i'm assuming he meant real) he's been in the rpm guage indicates engine rpm.
So could someone please answer the poster's question. In AH warbirds what does the rpm guage indicate?
:airplane: I did make a mistake to start with, but to answer you own question, next time you fly a B-17, after getting into cruise confg, pull your throttles back to 30 inches of MP and watch your tach and see what it does. Then go back to cruise power, 38 inches, then press your minus key on your key pad and watch your tach. I Think that will answer your question.
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In AH rpm is engine rpm just like the real thing.
HiTech
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It seems we have a contradiction. Who's right?
On real world airplanes the tach is showing engine RPM.
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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.
That is absolutely not correct. The tach shows engine RPM...that is how it's worked on all the aircraft I've flown J3 cub to Cessna to T6 to Mustang to B-24. Tach shows engine RPM.
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Thank you. Earl I hope your wife is better soon.
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:airplane: I did make a mistake to start with, but to answer you own question, next time you fly a B-17, after getting into cruise confg, pull your throttles back to 30 inches of MP and watch your tach and see what it does. Then go back to cruise power, 38 inches, then press your minus key on your key pad and watch your tach. I Think that will answer your question.
What exactly is this suppose to prove?
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What exactly is this suppose to prove?
I think he's showing that the prop will maintain RPM with a throttle reduction.
His second example should show a change in manifold pressure with reduction of RPM --- this doesn't always work correctly in AH.
R/L on the B-17, at normal inflight power settings, if you reduce RPM you will get a manifold pressure increase up until 7500 feet or so. Above that altitude a reduction in RPM will result in a reduction of manifold pressure. The engines in the B-17 and B-24 have superchargers built into the engine case, pilot has no control of them. At sea level you can get 44 inches MAP or more without using the turbochargers (normally aspirated engine would get around 27-29 inches MAP at SL). At higher altitudes if you reduce RPM you are also reducing output from the supercharger, therefore you'll get a drop in manifold pressure with RPM reduction.
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What exactly is this suppose to prove?
:airplane: There is no doubt that the "experts" say the tach shows engine RPM's and that is what I have always been taught and for the most part, I guess that is true, but, pull a B-25 or something with full feathering props out on the runway, feather both engines, then push your throttles up and see if you get any increase in RPM's on the tach! (he, he) If the "experts" are right, you would get an increase in RPM's, right? (Seriously, don't actually do that as you probably would blow some jugs off the crankcase). Helluva discussion though, isn't it?
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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 to clarify, are you asking about the theory of it (which has pretty much been discussed) or how to operate and manage manifold pressure and RPM in a realistic manner?
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Speaking of manifold pressure,does anyone know what exactly ATA stands for?
I was asked this question and wasn't sure of the exact meaning so I broke out the trusty google and found that even google didn't have a good answer.
:salute
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If that's German nomenclature, it means atmospheres.
So, 1.42 ATA is 1.42 times atmospheric pressure (at sea level, I believe, but caveat emptor on that).
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they should have gone with 1455 hectopascals :)
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Thanks all for an interesting discussion.
What did the Kommandogeräte do to ease out pilot workload regarding the engine management on German 190's and later 109's, and did they differentiate from each other ?
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Just to clarify, are you asking about the theory of it (which has pretty much been discussed ) or how to operate and manage manifold pressure and RPM in a realistic manner?
Both actually.
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If that's German nomenclature, it means atmospheres.
So, 1.42 ATA is 1.42 times atmospheric pressure (at sea level, I believe, but caveat emptor on that).
Yes I understand that by why ATA and not just TA and I read alot about absolute but for the life of me I cant find out what it actually means.
:salute
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Yes I understand that by why ATA and not just TA and I read alot about absolute but for the life of me I cant find out what it actually means.
:salute
Ah, well then you want the German wikipedia:
http://de.wikipedia.org/wiki/Technische_Atmosph%C3%A4re
Definition[Bearbeiten]Die Technische Atmosphäre wurde genormt als die Größe des Drucks, die 10 Meter Wassersäule verursacht.
1 at:= 10 mWS = 1 kp/cm² = 9,80665 N/cm² = 0,980665 bar = 98.066,5 Pa
Abgeleitete Einheiten[Bearbeiten]Je nach Bezugsniveau wurden aus der Technischen Atmosphäre die folgenden Einheiten abgeleitet:
absoluter Druck (Bezugsniveau: 0): ata (pa)
Druck in at über dem Bezugsniveau: atü (pü)
Druck in at unter dem Bezugsniveau: atu (pu)
[google translate]
Definition [edit] Technical atmosphere was standardized as the amount of pressure that caused 10 meters of water.
1 atm = 10 m wg = 1 kgf / cm ² = 9.80665 N / cm ² = 0.980665 bar = 98066.5 Pa
Derived units [edit] Depending on the reference level derived from the Technical atmosphere the following units:
absolute pressure (reference level: 0): ata (pa)
Pressure at above the reference level: atm (pü)
Under pressure at the reference level: atu (pu)
So, ata is absolute since it is not in reference to any other level of pressure. For example:
atü[Bearbeiten]Die alte Einheit atü (für „Technische Atmosphären über Bezugsniveau“) fand sich z. B. auf den Reifendruckfüllgeräten an Tankstellen. Da der PKW-Reifen das Fahrzeug nur tragen kann, wenn der Reifen im Verhältnis zum Umgebungsdruck (etwa 1 bar) einen Überdruck beinhaltet, zeigen die Reifendruckfüllgeräte nur den Überdruck (atü) an. Wird der Reifen mit 2,2 atü befüllt, beträgt der absolute Druck im Reifen 2,2 + 1,0 bar, also somit ca. 3,2 bar.
[google translate]
atü [edit] The old unit atü (for "Technical atmospheres above reference level") was found, for example, on the tyre inflation machines at petrol stations. Since the car tire can only support the vehicle when the tire pressure is higher relative to the ambient pressure (about 1 bar) the tyre inflation machines only show the overpressure (atü). If the tires filled with 2.2 atm, the absolute pressure in the tire is 2.2 + 1.0 bar, so therefore about 3.2 bar.
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Both actually.
General rule when making power changes on an engine with a constant speed prop:
Increase power -- Mixture adjusted as needed, prop RPM increased to desired setting THEN throttle up to desired MP.
Decrease power -- throttle down to desired MP, prop RPM decreased to desired setting, mixture adjusted as needed
For takeoff the prop control will be at the High RPM setting so that you can get maximum HP. Mixture will be rich. Throttle up to the Takeoff Power MP setting. Once in the climb you usually reduce power to a Climb power setting, then when level an additional power reduction. The operating handbook for the aircraft will have a chart showing usuable MP/RPM/mixture combinations for each mode of flight.
On the B17 (as we fly it present day)takeoff power is 42" MP, 2500 RPM with mixture in Auto Rich. For climb power MP is reduced to 36" MP, RPM down to 2300, mixture stays at Auto Rich. Once level we used 30"MP/2000RPM/Auto Lean as our cruise power setting. For doing our rides we would use 28"/2000RPM/Auto Rich. If power went above 31" OR 2100 RPM the mixture must be moved to Auto Rich.
For the B-24 it was same procedure but slight different power settings. We would engage the turbochargers (just ballparked the setting based on experience) then for takeoff would use 42-44" MP/2700 RPM/Auto Rich. After climb established turbos OFF, MP 36"/RPM 2500/Auto Rich. For cruise we used the same 30/2000/Auto Lean as the B-17.
R/L you can damage an engine by running to high of MP with lower RPM....pressures inside the cylinders can be high enough to cause the head/cylinder to fail (and it can be quite impressive when they do let go :D). This is the reason for increasing RPM before MP and decreasing MP before reducing RPM...to avoid overboosting the engine.
In game you don't have to worry about breaking the engine but if the RPM is set to low you won't get the power needed. Try a takeoff with the RPM set to minimum.
You can reduce fuel consumption by reducing RPM.
On the B-17 my setup for bombing when at altitude is I leave the throttle all the way forward but reduce the RPM to 2300 which reduces the MP to 38"(A realistic effect of the supercharger, B-24 doesn't do this but it should). A realistic power setting, it stabilizes the speed for an accurate drop and with the throttle forward you don't have to worry about bumping it and changing your power setting on bomb run.
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General rule when making power changes on an engine with a constant speed prop:
Increase power -- Mixture adjusted as needed, prop RPM increased to desired setting THEN throttle up to desired MP.
Decrease power -- throttle down to desired MP, prop RPM decreased to desired setting, mixture adjusted as needed
For takeoff the prop control will be at the High RPM setting so that you can get maximum HP. Mixture will be rich. Throttle up to the Takeoff Power MP setting. Once in the climb you usually reduce power to a Climb power setting, then when level an additional power reduction. The operating handbook for the aircraft will have a chart showing usuable MP/RPM/mixture combinations for each mode of flight.
On the B17 (as we fly it present day)takeoff power is 42" MP, 2500 RPM with mixture in Auto Rich. For climb power MP is reduced to 36" MP, RPM down to 2300, mixture stays at Auto Rich. Once level we used 30"MP/2000RPM/Auto Lean as our cruise power setting. For doing our rides we would use 28"/2000RPM/Auto Rich. If power went above 31" OR 2100 RPM the mixture must be moved to Auto Rich.
For the B-24 it was same procedure but slight different power settings. We would engage the turbochargers (just ballparked the setting based on experience) then for takeoff would use 42-44" MP/2700 RPM/Auto Rich. After climb established turbos OFF, MP 36"/RPM 2500/Auto Rich. For cruise we used the same 30/2000/Auto Lean as the B-17.
R/L you can damage an engine by running to high of MP with lower RPM....pressures inside the cylinders can be high enough to cause the head/cylinder to fail (and it can be quite impressive when they do let go :D). This is the reason for increasing RPM before MP and decreasing MP before reducing RPM...to avoid overboosting the engine.
In game you don't have to worry about breaking the engine but if the RPM is set to low you won't get the power needed. Try a takeoff with the RPM set to minimum.
You can reduce fuel consumption by reducing RPM.
On the B-17 my setup for bombing when at altitude is I leave the throttle all the way forward but reduce the RPM to 2300 which reduces the MP to 38"(A realistic effect of the supercharger, B-24 doesn't do this but it should). A realistic power setting, it stabilizes the speed for an accurate drop and with the throttle forward ythrottleou don't have to worry about bumping it and changing your power setting on bomb run.
Thanks Columbo, I bow down to your flying real airplanes. great explanation of real world usage of MP and rpm. I'm going to the Pacific air Museum in Hawaii today to see what's up. Thanks for the knowledge sir.