CEM: noob question time!

[Babi]

to keep it short:
in planes with variable prop pitch, what exactly am i controlling? the angle at which the blades hit the air OR the engine RPM?

I know that the first influences the latter, but i want to know if i just have to set a specific RPM and then the propeller governor changes the pitch accordingly; or if i have too keep the RPM constant myself, using the pitch.

[huckster]

yes, adjusting the pitch alters the angle at which the blades attacks the air and in turn affects rpm.

this vid explains it in detail

]

[Blackdog_kt]

The Luftwaffe aircraft use variable pitch props. These are not constant speed props (there's no governor) so you directly control the blade angle to influence the RPM. This means that whenever your airspeed, throttle or attitude changes you will also have to make adjustments to the prop to keep the RPM where you want it.

The RAF ones have a lot of different props, ranging from fixed pitch (tiger moth), to two-position props (early spits and hurricanes) to constant speed props.

With a fixed pitch prop, the RPM is controlled by your airspeed and throttle settings. With a two-position prop it's like having two fixed pitch props, one for best cruise efficienty (the low RPM setting) and one for maximum power (the high RPM setting).

Finally, with a constant speed prop you just move the lever until you get the RPM where you want it and it will take care of things on its own from that point on. It's still possible to be unable to maintain the chosen RPM when the blades reach their movement limits, eg you will notice that even with the lever full forward you're not making a lot of RPM when idling on the ground, or in a similar fashion you might be able to overspeed the prop in a prolonged high speed dive.

[Babi]

Originally Posted by Blackdog_kt

The Luftwaffe aircraft use variable pitch props. These are not constant speed props (there's no governor) so you directly control the blade angle to influence the RPM. This means that whenever your airspeed, throttle or attitude changes you will also have to make adjustments to the prop to keep the RPM where you want it.

The RAF ones have a lot of different props, ranging from fixed pitch (tiger moth), to two-position props (early spits and hurricanes) to constant speed props.

With a fixed pitch prop, the RPM is controlled by your airspeed and throttle settings. With a two-position prop it's like having two fixed pitch props, one for best cruise efficienty (the low RPM setting) and one for maximum power (the high RPM setting).

Finally, with a constant speed prop you just move the lever until you get the RPM where you want it and it will take care of things on its own from that point on. It's still possible to be unable to maintain the chosen RPM when the blades reach their movement limits, eg you will notice that even with the lever full forward you're not making a lot of RPM when idling on the ground, or in a similar fashion you might be able to overspeed the prop in a prolonged high speed dive.

Thank you very much! it's what i wanted to know! it's also explained in the video (great one btw, thx huckster!) from 12:40. I guess flying the bf will be a little more demanding than the spit II for this reason.
So now i just need to know what is the RPM for max power and what is better for cruise condition / fuel economy on different planes. Do you have this data?

[Blackdog_kt]

Some of the aircraft are detailed in the manual. Even if you don't have the printed manual that comes with the collector's edition, you have the PDF manuals of all the available languages in the following folder:
c:\(your program files or games folder)\steam\steamapps\common\IL2 sturmovik cliffs of dover\manual

The data starts with the Spitfire at page 83. It is by no means complete and some aircraft are missing altogether, but you can probably use the settings from one aircraft on another one that uses the same engine.

From what i can remember, the Stuka, Ju88 and He-111H all have Jumo engines, so the data for the Stuka would probably be the one to use. The Ju88 also has the limits clearly marked on the instrument bezels of the RPM and manifold pressure/ata indicators: green is maximum continuous, yellow is probably climb power (this is usually restricted to a certain amount of time but i don't know how much, i just watch my temperatures) and red is emergency and/or take-off power.

The hurricane and spit also have similar engines, so i use the same settings on both. According to the Spit manual in my collector's edition the advised power setting during combat is climb power which is 6 1/4 lbs of boost at 2600 RPM for 30 minutes. The next lowest setting (which is probably the continuous one) is cruise with rich mixture, which is 4 1/4 lbs of boost at 2600 RPM. I think this is also the setting marked with a little triangle on the instrument and labeled as "boost for level flight" if you zoom in close to the instruments.

In practice i just drop my RPMs to 2600 and keep my boost on the little triangle marker, then i add extra power in short bursts whenever i need to instead of running at higher settings all the time. This way the engine cools down between sessions of increased power and i don't have to worry about the 30 minute time limit.

Finally, the 109, 110 and He-111P use daimler benz engines, but i think that the one used by the 109 is geared differently and has different limits (the manual says max continuous is 1.35 ata and 2300-2400 RPM).

However, if you fly the 110 you will see that the operating limits are clearly marked on the instrument bezels of the RPM and manifold pressure/ata gauges and they are slightly lower than the ones used on the 109: a red "d" letter is the maximum continuous setting, a red "30" is allowed for 30 minutes and is probably climb power and a red "1" is allowed for 1 minute and is probably emergency and/or take-off power.

I haven't tried the He-111P a lot, but i would probably start by using the 110's settings on it.

Hope this helps a bit

[bDAWg]

Blackdog_kt... you've given some well thought out advice there...thanks for taking the time to do so...you obviously know your WWII prop planes sir

would you enlighten us as to what "ATA" represents...I'm guessing something to do with the boost?...for example the manual says to cruise the 109 @ 2,326 rpm / 1.20 ata...how do I check the ata?

[Babi]

Thank you very much Blackdog for the info.. didn't know there was a pdf manual in the game folder!

one last thing: i know ATA is a measure of pressure (in atmospheres?), i guess of the air coming into the engine cylinders. In the british planes, is the "boost" the same thing? And how do we control this pressure/boost? I see that if i throttle back, the pressure drops, is this the only way?

thank you again for your help!

@bDAWg: for "ATA" there is a gauge in german planes, check the instruments!

[bDAWg]

hey Babi
after I posted and asked I decided to check the 109 to see if there was a "ata" gauge and sure enough that big fat gauge above the tach was marked ata...DOAH!!....the last few nites I've been concentrating on the tach,oil and water gauges only...oh well now I know

so this gauge indicates how much air is being introduced into the engine cylinders?.....were these engines turbo charged?

[Fredfetish]

I've been playing the British campaign and for the later part you play mostly with the type Ia Spitfire.
What I've noticed is that the plane is really slow.
So, I read about the AI engine management problem and thought “hmmm great don't want to really fiddle with this every second”, but on further investigation, it seems that there are only 3 different settings required:
Low -slow (Rich mixture and fine prop setting, mostly take off speeds)
Low - fast (Rich mixture and coarse prop setting, from vid at about 120 mph)
High - fast/slow (should be coarse prop fast or slow due to thinness of the air and lean mixture ... doesn't really matter since most combat in the game is at low altitude)
In the vid it shows the plane kicking into higher gear at 200 mph with the optimum rpm speed for the later model automatic prop pitching optimized at 3000rpm... which would be my target as well.
So here is the problem... The engine never spins past 3000 rpm on the fine pitch. Even whilst going at 300 mph, if I switch to a coarse pitch, the rpm's instantly drops to 2200 rpm and starts detonating and a huge bleed of airspeed. I’ve dived straight down from 18000 ft with the coarse setting with full trottle and the rpm's never reached 3000. I'm running patch 1.4072. As a result, I obtain a max straight line speed of 320 mph in the Spitfire Ia with the fine prop pitch. (between 0 and 5000 ft) . I quickly checked the manual now, and it seems that this is the correct-ish speed ) (between 450 km ~ 550 km) for the Spitfire at this height, so it seems to me that CEM is just incorrect. (Question markI’ve quickly played with the Me 109 as well and find that the max prop pitch increase of only 10% is viable before the engine starts detonation.
I thought maybe that since I had radiator management on AI that the rpm’s are kept down to prevent overheating, but this is not the case after checking.
Any comments or suggestions anyone?

[Blackdog_kt]

Bdawg and Babi: The engine gauges on all aircraft provide a measure of the same thing, they just use different units. What they measure is the air pressure in the engine's intake manifold. If i'm not mistaken, this is where the fuel metering valve mixes the fuel with the air according to your chosen mixture settings, if there's a lot of air you get more fuel, the engine "sucks" more air to burn because the throttle valve is more open and you get more power. This is probably just before the fuel-air mix enters the cylinders to burn.

How it's measured in engines that use direct fuel injection inside the cylinders i don't know for sure. However, for most intents and purposes manifold pressure shows you how restricted, unrestricted or even force-fed (in the case of supercharging and/or turbocharging) the air supply is into the engine. This is a measure of power, because the engine is essentially a vacuum pump: without the throttle valve all engines would run on full throttle all the time, but that valve can restrict the amount of air that goes into the engine and thus, it starves it of the needed air to burn fuel and decelerates the rate at which the burn cycle occurs, enabling us to reduce power.

The Luftwaffe ones are metric so they use Ata, which is the atmosphere unit (aka bar). Modern general aviation aircraft and American WWII aircraft use inches of mercury (Hg), while Italian ones use the metric equivalent which is millimeters of mercury.
This unit of measure dates back to Torricelli's experiments. Imagine a vertical lab tube filled with mercury with a repository "dish" below it that holds more mercury...if the atmospheric pressure is high, the air pushes down on the surface of the liquid mercury in the dish and forces it to "climb" inside the tube.

1 Ata is the standard air pressure on sea level for a standard weather day (which is defined by a set of further data like temperature and so on, let's not get even more complicated if we don't have to ).
The rough conversion between units is that 1 Ata (or 1000 millibars) = 760mm Hg = about 30 inches Hg (i think it's actually 29.92 inches, on modern dual-unit altimeters the calibration window shows 29.92 inches when you set it to 1000mBar).

You probably realized i left the RAF dials last. This is because

a) The RAF planes automatically adjust the throttles to maintain your chosen manifold pressure. In a USAF or Luftwaffe aircraft, as you climb higher into thinner air you will need to gradually advance the throttle if you want to maintain the same amount of manifold pressure, if you dive to lower altitude into thicker air you will also need to reduce throttle to prevent the pressure from going too high.
In an RAF aircraft you set it once and it will do its best to maintain it regardless of altitude changes. In that sense the power lever in a Hurricane or Spitfire is not controlling the throttle directly, it's controlling the target value of a system that varies the throttle to maintain the selected manifold pressure.

b) The units are relative and not absolute.

This means that while the boost gauge measures manifold pressure in psi it is not the actual pressure in the manifold. What it measures is the pressure in psi relative to the standard pressure on sea level ie, it shows you how far above or below you are from standard sea level pressure.
So, 1 Ata = 760mm Hg = 29.92 inches Hg = 0 psi boost
I think that 1 psi = roughly 2 inches Hg (or is it the other way around? i'm a bit fuzzy at the moment).

In any case, it should be enough to show you that zero boost is by no means low power. It's actually full power on sea level for non-supercharged engines and probably why almost no current day general aviation aircraft use this measuring convention: it's neither intuitive nor satisfying for a general aviation pilot to have a big fat zero indicating full power, since most of these aircraft are normally aspirated (that is, they lack superchargers/turbochargers)