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flyinmick
01-03-2005, 08:18 PM
Elsewhere on this board, someone asked about this. I thought I'd post my reply here as well, just in case anyone else was interested.
One thing that is inaccurate in this sim is the relationship between Manifold pressure and RPM. The pressure as measured in the engine intake manifold is static air pressure measured at the inside. If the engine is not running then the MP guage should show local atmospheric pressure. This does not happen in some of the maps on IL2. Atmospheric pressure drops 1" per thousand feet of altitude. Standard pressure at sea level is 29.92". Therefore, an aircraft sitting at an airfield 2000' above sea level should indicate 28" or so while at rest.
Start the engine, and you will see a drop in MP. Some of the energy in the atmosphere (experienced as pressure) in the manifold is now being directed along the manifold toward the engine. This moving air has increased DYNAMIC pressure and, in consequence, reduced static pressure since the energy in the air is the same, just being distributed differently. Open the throttle (which is choking or thottling the air supply to the engine) and more air is admitted into the manifold. MP will increase as the throttle is opened even more until MP will almost, though never quite, equal atmospheric pressure. As the aircraft climbs, MP will be reduced 1" per thousand.
Let's do a "run up" and check the prop. Advance the engine to 1700 RPM. You'll probably see about 21"MP or so at sea level. Pull the prop lever all the way back. You are looking for a reduction in RPM and an INCREASE in MP. The air in the manifold has slowed down, reducing it's dynamic and increasing it's static pressures.
In IL2, this effect is reversed and needs to be fixed!! Advance the prop again to High RPM and Low pitch and you should see the origional RPM/MP settings. Now time for a Magneto and a power check. Open the throttle and set MP to local atmospheric pressure. You did note that before you cranked, didn't you? On a P&W R2800 and all the other radials that I'm familiar with, you should get 2100 RPM. This means that all the cylinders are firing. If they weren't, the engine would not generate sufficient power to turn the prop that fast. radials are very smooth, even when they drop a jug, so this is the only way to really tell. Check the mags, one at a time, looking for a drop in RPM of no more than 125 RPM. Look at each actual engine when you do this and make sure there is no vibration. Reduce power and look at the ground under each engine to make sure that an oil cooler or line has not burst; the oil is at it's coolest and thickest and it's pressure highest during run up.
A constant speed prop governer works by balancing the drag of the rotating blades against the torque created by the engine. When the prop blade creates thrust, it is actually creating lift. The creation of lift creates induced drag in turn. As you advance the throttle, the engine increases it's RPM and so increases the airspeed of the prop blade. At some point, the RPM reaches the maximum permissable for the installation and, at sea level, this will happen well below Max MP, especially if supercharged. At this point the prop governer meters engine oil, under pressure, into the prop hub and forces the blades out of fine pitch. As MP, and therefore engine torque, increases the blades increase their pitch or angle of attack thus generating more lift or thrust.
In cruise, MP is reduced followed by RPM and a cruise power setting is set. 30" and 1800RPM is reasonable for an R2800. Mixture is reduced untill it begins to affect MP and then is increased slightly. There should be an Auto Rich and Auto Lean setting but it's not in IL2. Let's say you start do a loop. As you pitch up, airspeed decreases and the angle of attack of each prop blade is increased. The prop begins to slow, the governer senses this, allows oil to port from the prop hub back to the engine oil tank, pitch is reduced, angle of attack (and therefore drag) is reduced and RPM maintained. On the downhill side, the opposite occurs; AOA decrease for the prop blades reducing drag and tending to increase RPM. The governer pumps oil into the dome forcing the blades into a coarse pitch restoring AOA and drag and maintaining RPM. At some very high airspeed, of course, the prop blades will be on the coarse pitch stops and it will no longer govern, resuting in an overspeed if power is not reduced.
At no time should the prop be allowed to drive the engine. For reasons of lubrication, MP in inches should ALWAYS equal or exceed RPM in hundreds e.g. 30" & 1800 or 30/18. This does not apply at low airspeeds such as for landing. At high airspeeds, operating "under square" will trash an engine inside of a minute.
Hope all this helps.
I'm still trying to figure out how that bleedin' clock lookin' thing on Jerry aircraft worked!

flyinmick
01-03-2005, 08:18 PM
Elsewhere on this board, someone asked about this. I thought I'd post my reply here as well, just in case anyone else was interested.
One thing that is inaccurate in this sim is the relationship between Manifold pressure and RPM. The pressure as measured in the engine intake manifold is static air pressure measured at the inside. If the engine is not running then the MP guage should show local atmospheric pressure. This does not happen in some of the maps on IL2. Atmospheric pressure drops 1" per thousand feet of altitude. Standard pressure at sea level is 29.92". Therefore, an aircraft sitting at an airfield 2000' above sea level should indicate 28" or so while at rest.
Start the engine, and you will see a drop in MP. Some of the energy in the atmosphere (experienced as pressure) in the manifold is now being directed along the manifold toward the engine. This moving air has increased DYNAMIC pressure and, in consequence, reduced static pressure since the energy in the air is the same, just being distributed differently. Open the throttle (which is choking or thottling the air supply to the engine) and more air is admitted into the manifold. MP will increase as the throttle is opened even more until MP will almost, though never quite, equal atmospheric pressure. As the aircraft climbs, MP will be reduced 1" per thousand.
Let's do a "run up" and check the prop. Advance the engine to 1700 RPM. You'll probably see about 21"MP or so at sea level. Pull the prop lever all the way back. You are looking for a reduction in RPM and an INCREASE in MP. The air in the manifold has slowed down, reducing it's dynamic and increasing it's static pressures.
In IL2, this effect is reversed and needs to be fixed!! Advance the prop again to High RPM and Low pitch and you should see the origional RPM/MP settings. Now time for a Magneto and a power check. Open the throttle and set MP to local atmospheric pressure. You did note that before you cranked, didn't you? On a P&W R2800 and all the other radials that I'm familiar with, you should get 2100 RPM. This means that all the cylinders are firing. If they weren't, the engine would not generate sufficient power to turn the prop that fast. radials are very smooth, even when they drop a jug, so this is the only way to really tell. Check the mags, one at a time, looking for a drop in RPM of no more than 125 RPM. Look at each actual engine when you do this and make sure there is no vibration. Reduce power and look at the ground under each engine to make sure that an oil cooler or line has not burst; the oil is at it's coolest and thickest and it's pressure highest during run up.
A constant speed prop governer works by balancing the drag of the rotating blades against the torque created by the engine. When the prop blade creates thrust, it is actually creating lift. The creation of lift creates induced drag in turn. As you advance the throttle, the engine increases it's RPM and so increases the airspeed of the prop blade. At some point, the RPM reaches the maximum permissable for the installation and, at sea level, this will happen well below Max MP, especially if supercharged. At this point the prop governer meters engine oil, under pressure, into the prop hub and forces the blades out of fine pitch. As MP, and therefore engine torque, increases the blades increase their pitch or angle of attack thus generating more lift or thrust.
In cruise, MP is reduced followed by RPM and a cruise power setting is set. 30" and 1800RPM is reasonable for an R2800. Mixture is reduced untill it begins to affect MP and then is increased slightly. There should be an Auto Rich and Auto Lean setting but it's not in IL2. Let's say you start do a loop. As you pitch up, airspeed decreases and the angle of attack of each prop blade is increased. The prop begins to slow, the governer senses this, allows oil to port from the prop hub back to the engine oil tank, pitch is reduced, angle of attack (and therefore drag) is reduced and RPM maintained. On the downhill side, the opposite occurs; AOA decrease for the prop blades reducing drag and tending to increase RPM. The governer pumps oil into the dome forcing the blades into a coarse pitch restoring AOA and drag and maintaining RPM. At some very high airspeed, of course, the prop blades will be on the coarse pitch stops and it will no longer govern, resuting in an overspeed if power is not reduced.
At no time should the prop be allowed to drive the engine. For reasons of lubrication, MP in inches should ALWAYS equal or exceed RPM in hundreds e.g. 30" & 1800 or 30/18. This does not apply at low airspeeds such as for landing. At high airspeeds, operating "under square" will trash an engine inside of a minute.
Hope all this helps.
I'm still trying to figure out how that bleedin' clock lookin' thing on Jerry aircraft worked!

Smokin256
01-03-2005, 09:12 PM
Great post flyinmick!
Finally someone else who likes to talk nuts & bolts! http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif Josf, you still around? http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif. I've been harping about the MP issue to anyone who would listen for years.

The "pitch clock" in the German planes is just a pitch indicator. 12:30 is full fine, 06:30 is full course. The automatic prop controls in the Bf-109s is one of my biggest pet peaves in the game. It should range from 2,000 rpm @ 0% throttle to rated rpm @ full throttle.not 900 rpm @ idle. As it is now Automatic prop control sets the prop at nearly full course when slowing for landing. This is compleatly wrong. Everyone complains that the DB605 series engines accelerate to slowly when in automatic. but no one ever stops to wonder why. This forces everyone who wants to get performance out of their 109s to use manual pitch control. And creates a unhistoric disparity bvetween the performance in Manual & the performance in Auto.
With very little of he pitfalls of using manual mode.

Cheers.........Smokin256

DHC2Pilot
01-03-2005, 09:22 PM
Very well explained....makes me think back to the two years I spent in A&P school, only without the (far too long) lectures on engine theory.

TX-EcoDragon
01-04-2005, 01:00 AM
http://www.avweb.com/news/columns/186778-1.html

http://www.avweb.com/news/columns/182081-1.html

and back to the prop info:

http://www.avweb.com/news/columns/182082-1.html

IL2-chuter
01-04-2005, 01:53 AM
I was taught way back in the mid-seventies that running somewhat (anything can be overdone) undersquare was all right. Never had any problems. Radials, though . . . especially big ones . . . can be so unforgiving concerning overspeeding and windmilling. Did I mention overspeeding? There's a bugger. And where are our featherable Messerschmitt 109 and Focke Wulf 190 props, eh? Still waitin' for that one . . . http://forums.ubi.com/images/smilies/16x16_smiley-indifferent.gif


Carry on, men. http://forums.ubi.com/images/smilies/16x16_smiley-happy.gif

Fliger747
01-04-2005, 06:15 AM
The R2800 is (was?) quite forgiving of high manifold pressures given sufficent cooling. It was not tolerant of overspeed. 2800 RPM was used for takeoff and war emergency power settings, but an overspeed to 3000 RPM would usually wreck the engine. It was for this reason that RPM on carrier aircraft was NOT usually advanced to maximum on approach, as with sudden application of power (waveoff) the engine would likely overspeed before the prop could 'catch up'. The issue has really been confusing for most folks because of the term 'prop pitch' rather than prop/engine RPM is used in the game.

I notice in the game that the maximum takeoff (dry) MP in te F4U is about 54" or so in neutral blower (first stage is still ah blow'n) and then jumps to 63" or so when ADI (Water meth) kicks in. At least they more or less fixed the mixture issue for the R2800, which was generally used in auto modes.

What most digital age people donn'a realize is what a witches art starting a big (analog) radial could be, iss'na just push'n a button!

Have some buddies still flying te R2800 on a daily basis in the venerable DC6.