Reasons why i severely lowered my BoS flight time lately [from a BoS enthusiast]

February 10, 2015

Are you aware you just said minus one? -1

Are you aware that (-i)² = 1, so I said : +1 !

February 10, 2015

Since when does an aneroid that is explicitely evacuated to exclude influences of temperature react to density?

That aneroid (pressure measuring capsule) is used to maintain a calibration point on the control linkage. It is part of the "magic" of the BMW801 Kommandogerät. It keeps the power lever (single lever power control in the cockpit) producing the same engine power at the same position under an infinite number of atmospheric conditions.

Today, that system is mostly digital and the Kommandogerät is similar to an Analog FADEC system.

Let's take a look at the pressure capsules for the supercharger gear control, manifold pressure regulator, and mixture control to see the function IAW the BMW manual and the NACA report. Both are correct and good information on the system.

Supercharger Gear control:

The NACA clearly states the purpose of the supercharger gear is to change the gears. It is a stupid mechanism in and of itself. It does not decide when to change gears....it changes them when told too by the change in engine speed which is a function of density altitude and the maintenance of a calibrated point on the linkage spindle by that pressure capsule.

The purpose of the pressure capsule is clearly stated. Because the sum of the variable datum(engine speed changes) and that calibrated point on the linkage spindle is a constant distance. The linkage spindle is geared and allowed to move as a function of atmospheric pressure. This keeps that constant distance so that the supercharger can distinguish between an engine speed change caused by the pilot moving the power lever or one caused by the correct density altitude for a gear change being reached. It does not command the gear change based on pressure altitude...it commands the gear change based on a change in engine speed without a change in the power lever setting.

The other systems are linked into that "variable datum" through hydraulic and electrical signals.

It is a fact the pressure capsule simply maintains a fixed distance relationship with the control linkage...nothing more.

That cause of those engine speed changes without pilot input is density altitude. When the variable datum can no longer be maintained as a function of density altitude that constant distance can no longer be maintained and without a change in power lever position, the supercharger changes gears.

It is not the pressure capsule that commands the gear change....it is change in engine speed upon reaching the proper density altitude.

What the NACA says:

Here is what the BMW Kommandgerät manual says....

Adjust the pressure capsule to maintain a constant distance relationship with the linkage spindle!!!!!

No gear change is commanded by pressure altitude. Gear changes are commanded by engine speed which as we will see is a function of density altitude and calibrated point on the control linkage.

So the supercharger gear control is stupid....sitting and waiting to be told when to change gears.

Will be Continued.....

February 10, 2015

Manifold Pressure regulator Pressure Capsule:

The only purpose of the manifold pressure control is to maintain the required amount of air coming into the engine to keep a constant ratio of fuel to air. It only provides the air. The actual ratio of fuel to air is controlled by the fuel metering system. We will get to that.

Now lets look at the function of the pressure capsule in the manifold pressure regulator.

What does it do?

Ladedruckregular%20function%20BMW801.png

Read the adjustment instructions....

Manifold%20pressure%20and%20propeller%20

Manifold%20pressure%20to%20Power%20Lever

It too adjust's the linkage spindle to maintain a fixed relationship with the power lever control.

All that pressure capsule does is maintain a linkage relationship so that the power lever position always corresponds to the same manifold pressure of the engine. Nothing else.

We will continue.....

Now let's get to the heart of the matter.

Mixture Regulator....or How Does the Kommandogerät find the correct density altitude?

The mixture regulator is the conductor of this engine orchestra! It's job is to maintain a fixed fuel to air ratio that the engine needs to achieve power.

The Mixture control adjustment:

There is a specific fuel flow required to maintain a specific manifold pressure at the fixed fuel to air ratio under a specific density altitude. Each specific density altitude requires it's own unique fuel flow to maintain that ratio.

The mixture control tells the fuel pump how much fuel to deliver and it controls the butterflies to adjust the ratio of air to keep that specific fuel flow required under any density altitude.

It is reading charge air density and controlling engine speed!!

When it cannot maintain that fuel to air ratio because it has reached the density altitude the engine needs to change gears...engine speed changes.....the supercharger gear control sees the speed change....looks for the calibrated point on the linkage...at changes gears at the proper density altitude!!!

That is why the gear change will happen at a constant density ratio under an infinite number of atmospheric condition combinations!!!

It is a fact, the supercharger gear change occurs as a function of density altitude.....not pressure altitude!!

Edited February 10, 2015 by Crump

February 10, 2015

OTOH, this high air density, and the impact it has in dynamic pressure, is taking me to accept some flight characteristics, I really don't know how to justify, given that I do believe there is indeed a very sophisticated flight model behind BoS, that's for sure ! I believe one would then be able to notice the difference when a Summer / Spring map becomes available ( ? )

It all changes as a function of the density ratio.

That is why that NACA chart is so important. If the density ratio is constant for the gear change then it will change gears at whatever density altitude equals that ratio over an infinite number of atmospheric conditions.

Your aircraft performance curve will simply shift to the equivalent density altitude on a standard day..assuming lapse rate is standard dry adiabatic.

If the lapse rate is not standard...the performance curves will still conform to the equivalent density altitude (density ratio) on a standard day.

February 10, 2015

I think Zak can pass this info to the Dev team - if they look at it carefully, they'll glimpse what is presently being done wrong, and fix it for an upcoming patch :-)

Yeah, I'm passing it to the right ppl all every time. The only potential problem here can be a correct translation of all the technical details

February 10, 2015

You are welcome, jcomm!!

I hope it does get fixed.

February 10, 2015

Sorry Crump, I was asking a rhetoric question. I am not interested in more of your nonsense.

February 10, 2015

Sorry you feel that way. There is no need to comment then in the future!

February 10, 2015

Crump is definitely right: The air density is very relevant to the performance of any aeroplane. In both terms: engine power output and aerodynamics. More so than static pressure.

Unfortunately that does not necessarily imply that the BMW 801D2 KG changes the supercharger gear directly dependent on the density. In fact, everything in the NACA report seems to imply completely opposite. Well not opposite, but more towards that other things are relevant in triggering the gear change. Namely static pressure and control lever position. It does explicitly state that the gear change altitude is NOT the critical altitude for the engine. Critical altitude -- if I'm interpreting the NACA report correctly -- is in that report defined as the altitude in which the supercharger can no longer hold the desired (as in set by the pilot via the control lever) manifold pressure. The gear change might then be or not be the optimal solution, but maybe it was the most feasible to implement at the time (considering robustness, technology, cost, maximum performance vs. economic performance, etc).

IF Crump's assumption is indeed correct (I'm still looking for other sources on this, but so far it does not look promissing) and the supercharger should change gears in a higher pressure altitude, then I'm afraid this would not increase the performance of the currently modelled FW 190 engine (at least if the performance is at all related to the manifold pressure). The manifold pressure drops already well before the current modelled gear change altitude, which would mean longer climbs with reduced manifold pressure if the gear change altitude would actually be higher. Considering this, the bigger question is, in my opinion, if the manifold pressure is in fact correctly modelled in the simulation (I'm not saying if it is or it isn't.. just that nobody has in to best of my knowledge considered this so far, at least in the forums.. the engineers actually developing the game may well have and the model may indeed be correct as it is).

- PitbullVicious

Edited February 10, 2015 by PitbullVicious

February 10, 2015

Crump,

Very impressive stuff !

You had lost me since a while yet, and most of the people I guess... ^^

Absolutely no idea of what's all this engineer stuff of course, but thank you for your in-depth study ! I absolutely can't understand (and even am too much lasy to read and study all what you bring to us), but at least, I see you seem to know what you talk about and above all you bring a lot detailled sources. Perhaps, as jcomm said, it will help the dev team to make this sim even more realistic than it is.

February 10, 2015

Critical altitude -- if I'm interpreting the NACA report correctly -- is in that report defined as the altitude in which the supercharger can no longer hold the desired (as in set by the pilot via the control lever) manifold pressure.

Hi Pitbullvicious,

Glad you could join the discussion.

The issue is critical altitude is a density altitude. It is NEVER a pressure altitude.

It does not change gears right at critical altitude because that is not the optimum point for the gear ratio of the supercharger. It is designed to maximize the power curve that gearing ratio offers.

To better illustrate what it is doing lets look at a common experience base. Shifting a car transmission manually.... If you change gears too soon or too late, you will not optimize the power transfer to the next gear which robs the car of performance.

The engine identifies the correct critical altitude. It changes gear based on density altitude as a function of the set gear ratio to maximize power transfer.

Crump,

Very impressive stuff !

You had lost me since a while yet, and most of the people I guess... ^^

Absolutely no idea of what's all this engineer stuff of course, but thank you for your in-depth study ! I absolutely can't understand (and even am too much lasy to read and study all what you bring to us), but at least, I see you seem to know what you talk about and above all you bring a lot detailled sources. Perhaps, as jcomm said, it will help the dev team to make this sim even more realistic than it is.

Thank you!! Yes...lots of people are lost. The devs should not be lost at all in this when given the proper information. They just saw a mechanism that measures pressure in the system and concluded it is commanding the gear changes happen based on pressure altitude.

It is not like the system is complicated or anything......

The manifold pressure drops already well before the current modelled gear change altitude, which would mean longer climbs with reduced manifold pressure if the gear change altitude would actually be higher.

That should not be the case. You can see the density ratio for both critical altitude and the gear change are constant at a specific manifold pressure.

If you raise the gear change altitude......the critical altitude also rises. Both are density and not pressure.

Edited February 10, 2015 by Crump

February 10, 2015

Yeah, I'm passing it to the right ppl all every time. The only potential problem here can be a correct translation of all the technical details

I'm very interested what will come from this discussion, will devs will acknowledge this? For now i hope devs will make comment on this subject.

February 10, 2015

Hi Pitbullvicious,

Glad you could join the discussion.

Thank you I sincerely respect all the effort you are putting towards the community.

Me personally, to be honest, I don't feel very motivated at work at the moment (mostly just basic coding) so I need a vent for my "life long student" personality. And as this is mostly a new field to me, it is most exhilarating! (OK, OK, I can say that in terms of refreshing my mathematical skills this is also beneficial to my job. And it is also about going back to my roots as I was a physics student back in the days).

The issue is critical altitude is a density altitude. It is NEVER a pressure altitude.

To be honest, I hate to use the word altitude at all in this discussion. It is just confusing in the end. The plane or engine does not know what altitude it is at. The instruments in the plane only detect pressure and possibly temperature. Now, these are related to density and altitude, as you well know, if the conditions are known. However, the way that the NACA report seems to define the critical "altitude" for the engine in question is the point where the supercharger (at its current gear) can no longer uphold the set manifold pressure. Do you disagree with this?

The problem still remains that you have not shown any direct physical mechanism that (mechanically) calculates the threshold density and relates that information to the gear change apparatus so that it can use that information. The NACA report does not seem to support anything in the lines of this if you read it carefully (and I'm sorry to say but the points you emphasize in your underlinings of the NACA report are not necessarily valid to this particular question and even more so, some of the quotes that you leave out are. The German sources I am not valid to judge as I'm not confident enough with the language).

Yes, it DOES make sense that the gear change would happen based on density, but that does not necessarily mean that it actually does with the GK. Even though we have the saying in Finland that "No animal is as intelligent as human, except maybe for the German engineer" it does not necessarily mean that they got it exactly optimal with the KG.

If I've understood correctly, the KG represented the highest technology at its time, but that is not to say that it was perfect from our standards (well, to begin with, it didn't have Bluetooth, which may be even considered old by today's standard!). The matter of mechanical complexity and performance is also questionable: Was the engine tuned to give best power output at all possible conditions, or some compromise between best power output, mechanical complexity and fuel economical output. All these choices, while they may seem obvious for online dogfighters like us, needed to be weighted by the engineers back then. WITH the technologies they had at their disposal.

I think that one way you could start to gather more support to your view would be to calculate the critical altitude in different sea level conditions based on the equation on the NACA report and then calculate the manifold pressure above those critical altitudes according to the NACA report. If at first the manifold pressure drop start point does not match the critical altitude based on the equation and above the critical altitude does not match with the one in the simulation, then we might have things to further look in to. Your current strategy of just quoting, or misquoting the same exact tables from the report do not seem to lead anywhere (whether you are correct or not).

It does not change gears right at critical altitude because that is not the optimum point for the gear ratio of the supercharger. It is designed to maximize the power curve that gearing ratio offers.

So how do you calculate the critical altitude and the gear change altitude? How would you express (mathematically) the gear change altitude as a function of density? Numerical solution in discrete temperatures will do for me, as I think that analytical solution is very difficult looking at the report itself (but that may just be my mathematical skills )

To better illustrate what it is doing lets look at a common experience base. Shifting a car transmission manually.... If you change gears too soon or too late, you will not optimize the power transfer to the next gear which robs the car of performance.

Yes. We humans are very good in adaptation and ambiguous situations. As I'm currently mostly struggling with natural language processing, I'm well aware of this The problem is, as the NACA report also clearly states, that the management of all the complexity of mixture, throttle opening, supercharger gear, propeller pitch has a huge cognitive load deteriorating SA, and anything that reduces this load can be considered beneficial. That is not to say that a human being can not manage the engine as good or even better than KG, but that it takes part of the cognitive load off from the pilot (even with a certain loss in absolute engine performance) which is a huge benefit in aerial combat engagement.

The engine identifies the correct critical altitude.

How? What is the exact mechanical process that leads to this?

It changes gear based on density altitude as a function of the set gear ratio to maximize power transfer.

This would be the best option. Nobody denies that. Still the question is HOW does it achieve this physically. The NACA report does not seem to support this.

February 10, 2015

I hope that anybody speaking German can read and see that the charts that according to Crump are supposed to show specific power settings actually show specific "Ladedruck" and that supposed density charts show "Umschaltdruck".

Could all of this mess please be moved to the other messed up topic?

February 10, 2015

Even in IL2 the level of drama over American aircraft never came close to the level of drama related to the 109s and 190s... not even. The term Luftwhiners was not coined in a vaccum. I have no idea why.. but that is how it has been.. for some reason the German aircraft seem to draw the most fanatically analytical folks .. Perhaps it is the engineering...

It was because most Pony/P38 fanatics whined over 109/190 being indestructible and having better weapons than .5cal.

February 10, 2015

However, the way that the NACA report seems to define the critical "altitude" for the engine in question is the point where the supercharger (at its current gear) can no longer uphold the set manifold pressure. Do you disagree with this?

Yes...that is a density altitude.

The problem still remains that you have not shown any direct physical mechanism that (mechanically) calculates the threshold density and relates that information to the gear change apparatus so that it can use that information.

It does not have to calculate the threshold density. It just maintains that fuel to air ratio. That ratio is determined by air density. When the atmospheric density drops to the point it cannot maintain that fuel to air ratio....the engine speed changes!!

Understand?

How? What is the exact mechanical process that leads to this?

Read the part about how the mixture control regulates the fuel flow and air flow maintaining that fuel to air ratio.

http://forum.il2sturmovik.com/topic/14420-reasons-why-i-severely-lowered-my-bos-flight-time-lately-bos/?p=230332

The NACA report does not seem to support this.

The NACA does support it. In fact, they produced a chart that shows the gear changes under an infinite number of atmospheric conditions all with the same density ratio!!

Density Ratio is just a non-dimensional expression of specific Density altitude....

Edited February 10, 2015 by Crump

February 10, 2015

Hey, Crump, mate... if you get the chance could you be so kind to send me all that good stuff via PM?

It's a riveting read. It's a darn shame some people don't appreciate it.

I really wished I had gone into the level of detail you go when I was doing my Bachelor's, especially with prop-driven planes. Too bad I chose to go in Structures instead.

Edited February 10, 2015 by 71st_AH_Chuck

February 10, 2015

I hope that anybody speaking German can read and see that the charts that according to Crump are supposed to show specific power settings actually show specific "Ladedruck" and that supposed density charts show "Umschaltdruck".

I hope that they can read the instructions and cross reference the charts.

Umschaltdruck....the sum of the variable datum and the relative displacement of the pressure capsule is consant distance to the switchover point (calibration point on the linkage)....Why wouldn't it be called the switchover pressure?

Also why wouldn't the adjustment diagram show the boost pressure relationship to the power lever....it is a fixed and constant relationship. You do not fly airplanes so that significance probably escapes you.

February 10, 2015

No flightsim models geopotential height variation with air density. They all assume an ISA lapse rate for temperature and pressure. (*)

(*) Some flightsims do indeed model air density, under the "simplified" form of density altitude, but do it in an incomplete way.

Is this true? Coming from very basic physics background, I found the ISA model as a relatively simple model (much more simple that I thought it would be when I started to look into this!) for pressure and temperature as function of altitude, from which the density for the said altitude can be easily calculated, once the former values are known. Up to around 11km, that is.

I.e. temperature lapse rate is considered mostly constant, with with its magnitude depending on humidity, and the pressure can be calculated from the equation:

I calculated pressure as function of altitude as follows:

P = P0*(1+h*L/T0)^(-g*M/R*L),

where P is the pressure at altitude h, P0 is the pressure at sea level (taken as 101325 Pa, in general the weather changes are around +/-2 % for a "really nice day" to "bloody storm!"), L is the temperature lapse rate (0.0065 Kelvin / m for dry air), M is the molecular weight of 0.0289644 kg / mol, R is the ideal gas constant (8.31432 n*M/mol*K), T0 is the temperature at sea level, and g depends on your shoe size.

As then the pressure for all altitudes is known and the temperature for all altitudes is know, isn't pressure simple to calculate?

That being said, how accurate that model is, I'm not sure (I'm noob with meteorology). I understand that stability, humidity etc have their say in all this, but whether it is relevant in the scope of this simulation, is unclear to me, but just with a quick look it does not seem so...

February 10, 2015

Hey, Crump, mate... if you get the chance could you be so kind to send me all that good stuff via PM?

It's a riveting read. It's a darn shame some people don't appreciate it.

I really wished I had gone into the level of detail you go when I was doing my Bachelor's, especially with prop-driven planes. Too bad I chose to go in Structures instead.

I would be happy too, Chuck! Glad you understand and enjoy it!!

February 10, 2015

As then the pressure for all altitudes is known and the temperature for all altitudes is know, isn't pressure simple to calculate?

Yep...if the density is known under one set of conditions...it is easy to find it given another.

Think of all those "my airplane flies this fast" charts people throw up. Those charts all show density altitude performance at ISA.

When you change atmospheric conditions....you are changing were that equivalent density altitude occurs. The Performance curve shifts but the shape remains the same.

In your game, the atmosphere is just extended below sea level on a standard day to the airplane.

February 10, 2015

Yes...that is a density altitude.

Yes means that you disagree? How do you disagree with the statement?

It does not have to calculate the threshold density. It just maintains that fuel to air ratio. That ratio is determined by air density. When the atmospheric density drops to the point it cannot maintain that fuel to air ratio....the engine speed changes!!

I assume "it" here is the KG as whole. The fuel to air ratio is controlled by the mixture controller. How does the information from the mixture control get related to the gear change (what is the physical process)?

Understand?

Nope, please elaborate the physical connection between the different controllers.

The NACA does support it. In fact, they produced a chart that shows the gear changes under an infinite number of atmospheric conditions all with the same density ratio!!

Which chart exactly are you referring to (reference is good enought, as I have the NACA report)?

February 10, 2015

Crump,

I am completely lost, too I admit, but I'd really like to understand.

I'm still having trouble understanding where this linkage between the pressure capsule and the throttle position (this is what you say, right?) is mentioned here:

"the shift from ground- to high-altitude supercharger gear is triggered by a pressure capsule, whose basic position is brought to a one dependent on charge pressure in a way to chose the most favorable shifting altitudes - in accordance with the altitude performance chart - for the various maifold pressures. The pressure capsule adjusts a control piston reach, which activates an oil pressure switch and by this switching the Ortlinghaus- exchange couplings on or off via pistons, transmissions and shift forks." (translation from Schaltladerumschaltung, http://lespotesajoss...gine Part 1.pdf (page 49)

Doesn't the second sentence explicitely state that the pressure capsule just triggers a mechanical motion which does absolutely nothing but directly shifting the gears? The words in bold print are even "stronger" in german. It is a chain directly and only between pressure capsule and the gear box (exchange couplings)

February 10, 2015

Sorry for the late reply, and I guess Crump already answered, somehow...

But truth is, you can imagine the surfaces formed by points at the same pressure as being 3-dimensional, with deeps and highs. Those are the Geopotential surfaces.

[ see here i. e. ]

When you move from High to Low, watch out bellow, is a well known "adagio" in Aviation, usually interpreted in only one of it's components - atmospheric pressure.

But indeed, it also applies to temperature, because the areas of low temperature on the Globe have above them parcels of the troposphere which are sometimes considerably

shrunken , so... even if two airports at the same height above MSL have the exact same QNH setting, if the temperature difference, and hence the density of the air mass above them

differ considerably, due to the fact that one of them is at a much lower temp ( like the typical weather Winter times of the Battle for Stalingrad ), an

aircraft flying in the colder region with it's altimeter reading 2000m will actually be at a much lower height above ground than an aircraft flying at the very same altimeter reading

on the ISA or even hotter region. They'll be at the very same pressure level, their QNHs are the same ( not even talking QNE here ! ), and yet, the one at the cold region will

risk CFIT if there are mountains around...

Mountain pilot's use to say - the Mountains are higher in the Winter :-)

I believe that this is all is relevant to the pilot when he or she reads the instruments as not to crash to a mountain, and when taking off or landing in terms of engine performance (especially if the instruments are calibrated to the conditions in the departure airfield and the target airfield conditions are considerably different).

However, this does not say anything about the complexity of the atmosphere model in simulators like IL-2 BOS yet. I'd still say that the ISA model (I've understood that the model states not only the standard temperature and pressure at sea level, but also how density and temperature change in function of altitude, taking into consideration sea level deviations from the standard conditions) takes all this into account in a reasonable accuracy when we are talking about the performance of the planes in the time and geological frame of the game.

The problem that you are describing is that the typical instruments in an aeroplane do not adjust to changing conditions dynamically, as they only measure one easily measurable aspect: pressure (e.g. the altimeter is mostly calibrated for the known height from sea level of the departure airfield, and the TAS meter needs to be manually adjusted for temperature and pressure by the pilot and are in general not changed dynamically during flight). I don't know how sophisticated instrumentation modern Cessnas for example have, but I do believe that most military planes during WWII had simple manually calibrated altimeters (barometers) and ram pressure-static pressure based IAS meters and not much more.

If you know more details about atmospheric modelling and it's complexities, I'd be more than glad to hear!

February 10, 2015

"the shift from ground- to high-altitude supercharger gear is triggered by a pressure capsule, whose basic position is brought to a one dependent on charge pressure in a way to chose the most favorable shifting altitudes - in accordance with the altitude performance chart - for the various maifold pressures. The pressure capsule adjusts a control piston reach, which activates an oil pressure switch and by this switching the Ortlinghaus- exchange couplings on or off via pistons, transmissions and shift forks."

Nothing conflicts with anything I have said.

It does all of this. It is triggered by a pressure capsule. That pressure capsule is in turn triggered by looking for an engine speed change and the calibrated point on linkage. The sum of the variable datum and calibrated point reach that fixed distance and control piston reach changes.

That is how it adjusts the control piston.

It is just not based on pressure altitude. It does change "in a way to chose the most favorable shifting altitudes".

That would be the density altitude the engine speed change occurs and the system can no longer maintain the fuel to air ratio.

Edited February 10, 2015 by Crump

February 10, 2015

I believe that this is all is relevant to the pilot when he or she reads the instruments as not to crash to a mountain, and when taking off or landing in terms of engine performance (especially if the instruments are calibrated to the conditions in the departure airfield and the target airfield conditions are considerably different).

jcomm is correct. Keep in mind that is altimeter error. The key is "constant altimeter setting".

That is one of the reason why Class A airspace was established. Aircraft in Class A (FL180+) change altimeter settings to 1013Mb or 29.92inHg. The speeds of these aircraft travel at that altitude means they would changing altimeter settings every few minutes based on the ground based pressure conditions. Somebody forgets and you have a mid-air collision.

The aircraft maintains the same density ratio. It only feels density.

If the density ratio is 1 (sea level on a standard day) then it will stay at that ratio. If the density altitude is -970 feet that means our density ratio is greater than 1 at sea level and the aircraft will climb to stay at 1!

February 10, 2015

Yes means that you disagree? How do you disagree with the statement?

I do not disagree with the statement. The term "critical altitude" is by definition a density altitude just wanted to be clear on that.

Did I explain the mechanism of how the KG finds density altitude well enough for you? In one sentence...The mixture control is plugged into the fuel pump and manifold butterflies "fine tuning" them to keep a constant fuel to air ratio in the combustion chambers.

When it can't do that, engine speed changes because that fuel to air ratio is not being maintained....and that tells everything else to adjust until it gets that fuel to air ratio back again in equilibrium is re-established.

I assume "it" here is the KG as whole. The fuel to air ratio is controlled by the mixture controller. How does the information from the mixture control get related to the gear change (what is the physical process)?

Yes, IT is the KG as an interconnected system. The information gets from the mixture control to the fuel pump and butterflies through hydraulic pressure, electrical and geared mechanical connections. That is the "variable datum". It really is an engine management system that is using the basic relationship all engines develop power based on, combustion at a specific fuel to air ratio.

The way those pressure capsules are set up with the linkage allows them to adjust the engine power with density altitude changes over a constant Indicated altitude without the pilot moving the power lever. The pilot just changes altimeter setting and maintains that indicated altitude. Crossing a frontal system for example; the engine would maintain whatever power the pilot set without him having to jockey the throttle, constantly adjusting it for changing atmospheric conditions to manually keep his fuel to air ratio where he wants it to be for best power.

jcomm is correct.

I am not correcting you on anything pitbullvicious. I reread and it came out wrong. Just agreeing with jcomm. He gave a very good explanation of density altitude effects on altimeter error.

Edited February 10, 2015 by Crump

February 10, 2015

How would you express (mathematically) the gear change altitude as a function of density?

The fuel to air ratio maintenance is basic relationship:

The air to fuel ratio is the property of fuel and chemical composition of the fuel that defines the value for this ratio. Most of the fuels we use in internal combustion engines are hydrocarbons, and their burning will obviously result in the release of hydrogen and carbon as residuals, along with heat and pressure. Below is an example of the oxidation reaction of methane (natural gas) as a fuel.

CH4 + 2(O2) → CO2 + 2(H20)

If we look up the atomic weights of the atoms that make up octane and oxygen, we get the following numbers:

Carbon © = 12.01

Oxygen (O) = 16

Hydrogen (H) = 1.008

So 1 molecule of methane has a molecular weight of: 1 * 12.01 + 4 * 1.008 = 16.042
One oxygen molecule weighs: 2 * 16 = 32
The oxygen-fuel mass ratio is then: 2 * 32 / 1 * 16.042 = 64 / 16.042
So we need 3.99 kg of oxygen for every 1 kg of fuel
Since 23.2 mass-percent of air is actually oxygen, we need : 3.99 * 100/23.2 = 17.2 kg air for every 1 kg of methane.
So the stoichiometric air-fuel ratio of methane is 17.2.

When the composition of a fuel is known, this method can be used to derive the stoichiometric air-fuel ratio. For the most common fuels, this, however, is not necessary because the ratios are known:

Natural gas: 17.2

Gasoline: 14.7

Propane: 15.5

Ethanol: 9

Methanol: 6.4

Hydrogen: 34

Diesel: 14.6

You may find it interesting that methanol and ethanol both have a very low air-fuel ratio, while the carbon chain length is comparable to methane and ethane. The reason for this is that alcohols like methanol and ethanol already carry oxygen themselves, which reduces the need for oxygen from the air.

When it does not get enough oxygen to maintain...engine speed changes.

For the game....just use density ratio. That is already defined and is a dimensionless ratio. The game has to calculate density ratio if it is going to deviate aircraft performance from standard conditions so I am sure that is already in the code.

The game just needs to plug in the values for the gear change when that ratio is met.

Once that is done, you can put the model in any atmospheric conditions and it will change gear realistically.

There is also Rolls Mass Intake Flow too.

http://www.enginehistory.org/members/articles/ACEnginePerfAnalysisR-R.shtml

February 11, 2015

I expect that they behave like all the historical sources say. And while i don't have to much sources about russian planes, i haven't read a single time about good roling capabilities from russian fighters. Apart that, there are a hundred german comparisons, between the 109 and 190, so its not "how i expect it", it's how it was tested in real life, it's a matter of fact

"Strangely" the FMs in DCS behave like "i expect it to"

I laughed, good reply!

February 11, 2015

Yes means that you disagree? How do you disagree with the statement?

I assume "it" here is the KG as whole. The fuel to air ratio is controlled by the mixture controller. How does the information from the mixture control get related to the gear change (what is the physical process)?

Nope, please elaborate the physical connection between the different controllers.

Which chart exactly are you referring to (reference is good enought, as I have the NACA report)?

You may have missed some of the previous discussion, which encapsulated the problems with Crump's thinking.

I asked him to reconcile the fact that the NACA report specifically states that the gear change occurs well above critical altitude. He responded with this. Note the coloured boxes are Crump overlay on one of the NACA charts.

When I pointed out to him that the gear change altitude is actually the sharp vertical step up well to the right of the arrow he has added, instead of a admitting he has make a mistake, after a day or so he comes back with this:

So you see he has completely shifted his ground on what the gear change altitude actually is, while simultaneously continuing to assert that the change is triggered by density altitude, despite there being no causal chain that could accompany this in the mechanism itself.

He also states that NACA concludes this; as you have read the report yourself you already know that this is a blatant untruth.

Arguing with someone like this is similar to arguing with a creationist: they "know" that any evidence you present for evolution "must" be wrong, because... God did it!

This is going to be my last comment on any Crump related stuff, since I have now put him on ignore. I do simply to appeal to readers who are tempted to give him credence under the weight of irrelevant technical detail (aka as flannel) to think for themselves and see what the documents actually say.

Edited February 11, 2015 by unreasonable

February 11, 2015

There are so many more details, but I'm affraid it wouild fall out of the aim of the thread here...

ISA model is a special very particular reference atmosphere, out of which we are everyday, most of the time, allover the World :-)

...

I really don't know what is being used in BoS weather model of course, but I believe it doesn't have to be so complex to serve the purpose of this sim :-)

Yes, I must admit that that was a rather loaded question, but in the end I think that we do agree that in the scope of IL-2 BoS the ISA gives good enough model to calculate density, pressure and atmosphere for all feasible conditions, at all relevant altitudes.

Just as an interesting detail: I was flying on the DED server last weekend, and the FW 190 seemed to change its supercharger gear around indicated altitude of 2,600m, starting from Skvorin (after this and the other discussion in the thread "Does anyone have a clue (FW 190 performance again):)" @ Tutorials and Manuals -> German units -subforum, I started to pay quite a lot of attention to the manifold pressure on the FW 190). Unfortunately, the server did not state (or I didn't find it) the conditions.

I am not correcting you on anything pitbullvicious. I reread and it came out wrong. Just agreeing with jcomm. He gave a very good explanation of density altitude effects on altimeter error.

Well, that's good because I also agree with him and he seems to agree with me

February 11, 2015

As this discussion feels rather out of place on this thread (and seems never ending) I'll reply to Crump in the thread where I originally joined the discussion about the performance of the KG:

http://forum.il2sturmovik.com/topic/14031-does-anyone-have-clue-fw-190-performance-again/page-17

February 11, 2015

Yes, I must admit that that was a rather loaded question, but in the end I think that we do agree that in the scope of IL-2 BoS the ISA gives good enough model to calculate density, pressure and atmosphere for all feasible conditions, at all relevant altitudes.

Just a quick elaboration on the above: I do not of course know what model the game actually uses, but contrary to my initial belief that the model could be quite complex, I actually do not think that it needs necessarily to be after studying the matter a bit further. My impression is that the atmospheric conditions for any one mission are static throughout the mission and in all geographical locations on the map. I also assume that the turbulence and wind effects on the plane are modelled independently of the atmospheric model.

February 11, 2015

So you see he has completely shifted his ground on what the gear change altitude actually is

Actually I did not...you misunderstood the first chart. I realized it was not clear and tried to clarify. Thank you for disparaging my character over it. It speaks to your maturity. There is little to do about it when your mind is closed and has already determined the outcome. That is why I have to ignored you.

As this discussion feels rather out of place on this thread (and seems never ending) I'll reply to Crump in the thread where I originally joined the discussion about the performance of the KG:

Zak is watching this one...let's not make him run all over the boards!

To me it looks like the gear change does indeed happen independent of the critical altitude and dependent of the engine speed (rpm). This can also be seen from the Figure 14. Manifold pressure is clearly dependent on density, and that is what that graph is really about. Unfortunately we do not have the same graph for different atmospheric conditions, so saying anything about the gear change based on that graph alone, is very haphazard.

Let's look at figure 14 and see what it is telling us, Pitbullvicious.

It is not defining the altitude the gear change occurs. That is just people reading into the information presented and misunderstanding what the chart is telling us. Figure 14 is simply showing us the system accounts for RAM Air Effects.

Figure 12 defines the gear change altitude. Density Ratio defines a specific altitude over an infinite number of atmospheric conditions. In other-words, in any atmospheric condition the density ratio defines ONE altitude specifically.

There really is no case to be made that the gear change does not occur based on density altitude.

To further clear up these muddied waters....

Let's take a closer look at the supercharger gear change system.

Pay attention to the function of the supercharger gear changing system. It is NOT to change gears AT critical altitude....it is to identify critical altitude in order to smoothly maximize the transfer of power.

How does it do that? It uses the relationship of a fixed distance across the linkage that represents the sum of variable datum and the pressure capsule.

Think of it like an egg timer. Instead of measuring units of time....it measures the point that variable distance reaches the constant distance required for gear change.

The catalyst to start the timer and close that fixed distance is reaching critical altitude.

Carefully read the NACA report:

Lastly, let's look at the math for determining the gear change and critical altitudes.

Gear change in the BMW801 is a function of density altitude and nothing else. The pressure capsule does not trigger a gear change. It simply maintains a calibrated point in the linkage and is trigger to start the gear change process upon reaching the correct density altitude.

Pitbullvicous asks : What actually triggers the gear shift.

The change in engine speed upon reaching critical altitude.

Are you able to show the actual connection between the mixture controller and the gear change controller on any of the schematics (with a rudimentary notation of actual direction of forces)?

The mixture controller determines engine speed. Change in Engine speed is what tells the Supercharger gear controller to start looking for that constant distance on the linkage.

While this seems an easy and effective (as in terms of the performance of the plane) if it is not actually how the KG does it, wouldn't this be wrong?

It would be exactly right and produce the gear change at the proper density altitude in an infinite number of atmospheric conditions.

The gear change is a density altitude not a pressure altitude.

February 11, 2015

Let's look at figure 14 and see what it is telling us, Pitbullvicious.

It is not defining the altitude the gear change occurs. That is just people reading into the information presented and misunderstanding what the chart is telling us. Figure 14 is simply showing us the system accounts for RAM Air Effects.

It is THE ONLY chart in the entire report that has the words "change in gear ratio" and it further has the words "altitude" and "engine speed" and show the axes of the graph as "altitude" and "engine speed". Furthermore the report that you misquote continuously describes why the engine speed is used as an index. Conversely, nothing on the chart says anything about ram pressure.

Figure 12 defines the gear change altitude. Density Ratio defines a specific altitude over an infinite number of atmospheric conditions. In other-words, in any atmospheric condition the density ratio defines ONE altitude specifically.

The Figure 12. does not have even the word "gear" in its caption. The caption clearly states that it is a chart about air flow. That is relevant for the manifold pressure and to mixture setting as the equations you quoted below and the report itself shows.

Let's take a closer look at the supercharger gear change system.

Pay attention to the function of the supercharger gear changing system. It is NOT to change gears AT critical altitude....it is to identify critical altitude in order to smoothly maximize the transfer of power.

How does it do that? It uses the relationship of a fixed distance across the linkage that represents the sum of variable datum and the pressure capsule.

Yes. The problem only is that you have not shown any mechanism that links the variable datum to anything that does calculate density. Even though I have asked you several times. The report clearly states that "the displacement of the variable datum is caused by a change in the position of the main-servocontrol lever."

Lastly, let's look at the math for determining the gear change and critical altitudes.

The Equation (6) is described as "equation for determining the atmospheric pressure at the critical altitude of the supercharger". Again you are misquoting and adding your own words to the description. And yes, as I've many, many times said, the manifold pressure and the critical altitude are dependent on density.

The Equation (7) is described as "the relation between manifold pressure and altitude at each engine speed above the critical altitude". Again, nothing about gear change.

The mixture controller determines engine speed.

According to the report (pp. 8-9) "The purpose of the propeller-pitch control is to regulate engine speed by control of a variable-pitch propeller in such way that the desired relation between engine speed and manifold pressure (fig. 5) can be maintained up to the critical altitude."

Conversely (pp. 15), "The mixture control is designed to meter fuel to the engine in accordance with some function of manifold pressure, altitude pressure, and charge air temperature."

It is also worth noting that the same section notes: "Laboratory bench tests, however, showed that changed in charge-air temperature had no effect on fuel flow." The report however does not speculate why this is so.

Unfortunately you just misquote over and over the same sections of the report, without saying anything new that supports your case. Unless I can't find any further evidence on the matter myself, or you are not willing to provide it, this discussion is not going anywhere.

February 11, 2015

The report clearly states that "the displacement of the variable datum is caused by a change in the position of the main-servocontrol lever."

Of course it does...it is how the pilot changes power settings. I am not sure why this is even an issue or what it has to do with the fact the KG identifies the density altitude and changes gears appropriately.

"Laboratory bench tests, however, showed that changed in charge-air temperature had no effect on fuel flow."

The NACA is also very clear on what it was designed to do. They were testing a captured piece of equipment so capsule stack may have been compromised. That does not change its function and purpose.

BMW is very clear on the fact it's job is maintain a fixed fuel to air ratio. That is the basis of all engine power.

The NACA did get the principles right on how it operates that fact they could not get it to operate properly does not change the mixture control design purpose.

manifold pressure and the critical altitude are dependent on density.

Ok good, I was not seeing that clearly spelled out. Please do not get frustrated..internet boards are a very poor communication medium.

Now that it is clear.

Answer this question:

Is engine speed at a constant power lever setting going to change at critical altitude?

February 11, 2015

No, it doesn't. That's kind of bad for the stories you are telling, isn't it?

February 11, 2015

No, it doesn't. That's kind of bad for the stories you are telling, isn't it?

ok...

Explain how manifold pressure changes but engine speed does not?

February 11, 2015

Err, the CSP decreases pitch and doesn't give a diddlydoo about manifold pressure? Do you understand the C part of the CSP?

February 11, 2015

Err, the CSP decreases pitch and doesn't give a diddlydoo about manifold pressure?

It does give a "diddly do" about manifold pressure. They are connected man.

Not only that...critical altitude is by defintion the altitude the supercharger can no longer maintain manifold pressure.

So, asking again.

Will there be a change in engine speed at critical altitude?

Edited February 11, 2015 by Crump

Reasons why i severely lowered my BoS flight time lately [from a BoS enthusiast]

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