A question about the operation of the Kommandogerat

[Wulf]

As everyone who mucks about in the 190 knows, you get a sudden increase in ata/rpm as you climb through the 2.3-4 km altitude mark.  I understand this results from a gear change in the supercharger unit (that may or may not be correct I don't know but that's what I understand is happening). 

 

If this is what's going on, isn't the sudden power surge, from say 1.2 to 1.4 ata, a bit strange?    I'd have thought that, if the Kommandogerat was such a clever bit of kit, it should automatically adjust the prop pitch and fuel mixtures etc to ensure that the existing power setting (manifold pressure etc) were maintained or quickly restored as the aircraft transitioned from one supercharger gear to the next.

 

Does that not seem logical??

Edited June 15, 2015 by Wulf

[Wulf]

Um ... this wasn't a rhetorical question.  Does no one know the answer??

[6./ZG26_5tuka]

I asked a mate of mine who was very familiar with this subject and he told me the simulation of te Kommandogerät was incorrect.

 

Precisely my question was wheter the Kommandogerät adjusted RPM and mixture based on the boost pressure or the throttle position.

 

First one is what we got ingame. Once boost pfessure drops RPM decreases as well. You have to either increase your throttle or use manual prop pitch management to overcome it.

 

A Kommandogerät reading the throttle position only would adjust RPM indipendently from boost pressure, means once boost pressure drops the K.Gerät holds RPM steadily.

 

I could not gather any evidence yet though but from what I have been told, the latter is the correct function.

Edited June 15, 2015 by Stab/JG26_5tuka

[6./ZG26_5tuka]

Also (regarding the second part of your question) the K.Geräts job was not to increase or hold ata. It was a mechanical computer adjusting prop pitch and mixture for optimal performance at different power settings. Boost pressure drops with increase in altitude, which is natural. By leaning your mixture your fuel mix becomes more efficient but you still burn less fuel than in lower altitude, thus ata still drops.

 

This however was countered by the supercharger, not the K.Gerät itself.

Edited June 15, 2015 by Stab/JG26_5tuka

[unreasonable]

If you want to know how the KG worked you could read the attached document (in English). Somewhat technical, but has what you want. Originally posted by Crump, (credit where credit is due).

There is a long thread (or two) about the interpretation of what the document says, which I strongly advise you NOT to read.

naca BMW Bench Test.pdf

[Wulf]

Thanks guys.

 

Obviously, I'm not an aeronautical engineer so I wouldn't really have a clue but the report summary does say, among other things that :

 

" because the supercharger is allowed to remain in low gear when the aircraft is gaining altitude above the critical, maximum performance for any particular engine speed can be obtained and abrupt changes in power output when the gear change ratio changes can be eliminated."  

 

And that:

 

"Undesirable fluctuation between the gear ratios of the supercharger is eliminated by the design of the control valve which, provides a range of altitudes in which the supercharger can operate in either high or low gear.  Because the range of altitudes is excessive, the altitude at which the supercharger changes from high to low gear is much lower than the optimal altitude for the gear change.  Consequently, undesirable abrupt changes in power are experienced and less than maximum performances at any particular engine speed is obtained at all altitudes when the aircraft is losing altitude."

 

 So, if this is all correct, doesn't it suggest that that the 'surge' we currently experience in boost and RPM was in fact effectively eliminated in the climb but would tend to occur when descending?  If that's the case, have the devs got it round the wrong way?

[unreasonable]

I think the easiest way to see what is going on is by looking at figures 14, 6a and 6b.

 

14 shows the altitude at which the supercharger gear changed, measured from bench tests, plotted as altitude against rpm. (The actual experimental nputs were environmental pressure against control lever position).

 

You can see from this firstly that these is a big difference between the change up height and the change down height. The reason given for this is that you do not want to have someone flying around close to a single change over height constantly switching the supercharger gear.

 

Secondly you can see that there is also some input from the control lever position: a forwards position (higher rpm) will trigger the change (either way) at a lower height. I am not sure what the advantage of this arrangement is except that it gives the pilot some control over the change heights.

 

I actually have no idea whether BoS models either of these mechanisms in detail or if it has been simplified to give a single automatic changeover height. Presumably you can tell?

 

As for the "power" implications: the report states that the power output is not optimized while descending, but I am not sure if this is easy to understand from the graphs.

 

Both 6a and b plot manifold pressure against altitude for a series of lines, each corresponding to an engine rpm. 6a is ascending, 6b is descending. Note the atm shown here is a calculated number from a set of equations.

 

In 6a there is a gradual drop off in pressure above 1st gear critical height. The gear switches at 12-14,000 ft: at this point there is a sudden increase in pressure. Above 2nd gear critical height the pressure drops again. Presumably the KG is juggling prop pitch and mixture to maintain the desired rpm, but I imagine at the moment of changeover there would be a few seconds while the rpm fluctuated. The pressure surge would be immediate but the alteration in prop pitch to compensate might take a couple of seconds.

 

In 6b (descending) the effect is gone and the rpm lines are at almost constant manifold pressure.

 

So that sounds as though it corresponds to what you see in game?

[Wulf]

I think the easiest way to see what is going on is by looking at figures 14, 6a and 6b.

 

14 shows the altitude at which the supercharger gear changed, measured from bench tests, plotted as altitude against rpm. (The actual experimental nputs were environmental pressure against control lever position).

 

You can see from this firstly that these is a big difference between the change up height and the change down height. The reason given for this is that you do not want to have someone flying around close to a single change over height constantly switching the supercharger gear.

 

Secondly you can see that there is also some input from the control lever position: a forwards position (higher rpm) will trigger the change (either way) at a lower height. I am not sure what the advantage of this arrangement is except that it gives the pilot some control over the change heights.

 

I actually have no idea whether BoS models either of these mechanisms in detail or if it has been simplified to give a single automatic changeover height. Presumably you can tell?

 

As for the "power" implications: the report states that the power output is not optimized while descending, but I am not sure if this is easy to understand from the graphs.

 

Both 6a and b plot manifold pressure against altitude for a series of lines, each corresponding to an engine rpm. 6a is ascending, 6b is descending. Note the atm shown here is a calculated number from a set of equations.

 

In 6a there is a gradual drop off in pressure above 1st gear critical height. The gear switches at 12-14,000 ft: at this point there is a sudden increase in pressure. Above 2nd gear critical height the pressure drops again. Presumably the KG is juggling prop pitch and mixture to maintain the desired rpm, but I imagine at the moment of changeover there would be a few seconds while the rpm fluctuated. The pressure surge would be immediate but the alteration in prop pitch to compensate might take a couple of seconds.

 

In 6b (descending) the effect is gone and the rpm lines are at almost constant manifold pressure.

 

So that sounds as though it corresponds to what you see in game?

 

 

I think you're right.  I just carefully noted what I've been doing with the throttle prior to the gear change.  If you leave the aircraft on the post takeoff power setting, (say 1.2 ata), that power setting will be quickly restored after the initial power serge immediately following the gear change. 

[unreasonable]

I would check myself but BoS currently uninstalled awaiting something better to do on the SP side:  really I suppose the question is if you try to replicate the NASA test, in particular graphs 14, 6a and 6b, do you get similar results or something very different.

 

The key thing about the tests is that the control lever ("throttle") is set at a particular place and then not moved for each test run. You control altitude: in the test by changing pressure in the lab, in the plane by moving the elevator. rpm and ata are then outputs produced through the action of the KG.

 

Almost tempted to reinstall to see for myself.... almost

[ZachariasX]

 

 

If this is what's going on, isn't the sudden power surge, from say 1.2 to 1.4 ata, a bit strange?  

 

Does that not seem logical??

 

No, it is not strange. it has to happen to maintain the same power on the propshaft. Putting the blower to the next step bleeds several hundert horsepower of the power you have to crank the prop.

 

In essence: At max. ata in the low blower you have more power on the prop that at max. ata with the blower set on high. Basically the high blower doesn't give you more power, it is sort of like putting the engine in "a longer gear". The spike in ata you are seeing is just to compensate for what you are bleeding additionally as power to the blower. For the prop, not much changed in that moment.

[unreasonable]

Thanks Zacharias: the thing slowing down my comprehension was making a vague mental connection that "ata = power" when it is just one of the variables that contributes. I sort of think I knew that before, then forgot it....As far as I can see there is no graph in the NASA document that explicitly graphs power.

 

I suppose they just understood as engineers (which I am not), that running the supercharger in high altitude gear when the same rpms could be achieved in low altitude gear must necessarily entail a loss of available power at the prop since the high altitude gear takes so much more power to run. [i know this is what you just said - I think - just restating to see if I have got my head around it].

 

This would mean that IF the BoS 190 changes gear at the same altitude close to the first stage supercharger critical altitude, both going up and down, it might be generating less power than the real thing when climbing in the altitude zone of 8-14,000 ft (approx.: depending on rpm) since the BoS version would be changing gear much too early when ascending. Which would be a pity, since this is such a common altitude for engagements....

 

I really am going to have to check it for myself

Edited June 17, 2015 by unreasonable

[ZachariasX]

Thanks Zacharias: the thing slowing down my comprehension was making a vague mental connection that "ata = power" when it is just one of the variables that contributes. I sort of think I knew that before, then forgot it....As far as I can see there is no graph in the NASA document that explicitly graphs power.

 

I suppose they just understood as engineers (which I am not), that running the supercharger in high altitude gear when the same rpms could be achieved in low altitude gear must necessarily entail a loss of available power at the prop since the high altitude gear takes so much more power to run. [i know this is what you just said - I think - just restating to see if I have got my head around it].

 

This would mean that IF the BoS 190 changes gear at the same altitude close to the first stage supercharger critical altitude, both going up and down, it might be generating less power than the real thing when climbing in the altitude zone of 8-14,000 ft (approx.: depending on rpm) since the BoS version would be changing gear much too early when ascending. Which would be a pity, since this is such a common altitude for engagements....

 

I really am going to have to check it for myself

 

The loss of power output on the propshaft is significant. In the Mustang for example with its automatic switch between high and low blower you still have the manual override to select for high and low blower. This is especially useful for flying long distance. What you do is even though you are above critical altitude of the lower blower, you manually switch back to low blower when you are say, 17'000 ft. At full throttle, you get maybe (as an expample) something like 32'' Hg manifold pressure (or about 1 ata) BUT you are in the light gear, which means the given power output on the propshaft is still the same as in high blower, there throttled back. The difference between these 2 configurations, even though identical regarding power output on the prop are about 300 hp the engine has to produce ADDITIONALLY to tun the blower in fast mode, making it less fuel efficient. Even tough you are travelling at the same speed, your milage is less. You don't want "Bingo fuel" when you reach Berlin...

 

Not losing this significant amount of power make the turbocharged engines (for instance in the P-47 or the B-17) so wonderfully effective at altitude. 2000 HP on the shaft up to almost 10'000 meters give the P-47 legs like almost no other up there.

 

Even in cars the supercharger uses up a lot of HP, about 1/5th of the entire power output of a supercharged V8 AMG mercedes. This varies of course with the compression delivered by the supercharger.

[unreasonable]

OK I had not considered the fuel consumption side of it at all but I understand how that works from your post.

 

But going back to the document and reading again I see a previous post of mine was in error: the document discusses using airflow (calculated) as a proxy for available power, shown in graphs 12a and 12b: ascending and descending again. If you were to superimpose the two graphs (beyond my capability to do it and post I am afraid), you would see that there is a range of altitude between the up change and the down change where the airflow when descending is considerably lower at a given altitude than when ascending.

 

This I believe is the source of the comments in the document about the loss of power when descending.

 

Similarly if the BoS 190 always changes gear at what would be the descending gear change altitude in the real 190, it will always suffer this power loss, unless some other variable has been tweaked to compensate.

[ZachariasX]

OK I had not considered the fuel consumption side of it at all but I understand how that works from your post.

 

But going back to the document and reading again I see a previous post of mine was in error: the document discusses using airflow (calculated) as a proxy for available power, shown in graphs 12a and 12b: ascending and descending again. If you were to superimpose the two graphs (beyond my capability to do it and post I am afraid), you would see that there is a range of altitude between the up change and the down change where the airflow when descending is considerably lower at a given altitude than when ascending.

 

This I believe is the source of the comments in the document about the loss of power when descending.

 

Similarly if the BoS 190 always changes gear at what would be the descending gear change altitude in the real 190, it will always suffer this power loss, unless some other variable has been tweaked to compensate.

 

The airflow is an indication of the poweroutput, in both gear settings. However in "high gear" (for thin air) an additional consumer load is added. The power indication by the manifold does not take any of that in consideration.

 

The high blower does nothing more than allow you again to run the engine at full manifold pressure up to a higher altitude. If the engine has an aoutput of, say 1600 hp at sealevel and a geared 2 step blower, you can maintain 1600 hp only on the low blower, at high altitude. As soon you switch to high blower, you have only 1300 hp, even though you have max. manifold. This is a rough approximation and it depends on the blower and specific compression. Only turbocharged engines can maintain full hp up to critical altitude of both blowers.

 

The fact that graphs are ascending with altitude stems from tha fact that the higher you go, the cooler the air. Figure 12 is not a trivial chart to read, as it implys a lot about superchargers and the way the Kommandogreät is handling the input. Keep in mind, with the "power lever" and setting the rpm, you leave it up entirely to this fancy piece of equipment to make the right mix to deliver "what you might ask from it". And in this chart, the reduced airflow seems really a good indication of the power loss by switching to high blower. The more you open your throttle, the more it has to pump and the higher the performance penalty. Subtracting the effect from cold air at altitude (the "rise" of the graph), you get a power level up to critical altitude of the low blower, then a significant drop (depending on rpm) then a power level with the penalty induced by the blower in high gear, then above critical altitude, air flow and power drops off with the decreasing athmosphere.

[unreasonable]

Yes I think I have that: just one minor point - the "ascending" mentioned pertains to the fact that the KG changes gear at a very different altitude when you are climbing than when you are descending (Fig 14): 12a shows the results of the test runs plus airflow calculations when climbing (actually when the pressure in the experimental chamber was reduced). 12b shows the results when descending (increasing the environmental pressure).

 

Each chart shows the power changes (or its airflow proxy) throughout the test - but it is only the difference between 12a and 12b that seems to illustrate graphically the report's criticism that the gear change heights are too far apart. Pity there is no explicit graph of HP through the test including the extra cost of running the second stage, it would be easier to grasp.

 

I think this part of the report summary corresponds to what you are saying (hope it is legible edit: it is if you click on it!):

 

 

 

Thanks again for your patience! This stuff is not trivial to absorb but it is fascinating.

Edited June 18, 2015 by unreasonable

[ZachariasX]

Yes, the text you posted explains this issue (and probably better than I did).

 

I found engine operation with these kind of supercharged engines extremely interessting and I spent a while to get some understanding of it. I really like the A2A Mustang for FSX a lot, because it gives you clear feedback how you run the engine as it states temperatures, flow rates etc. in pop-up panels that you can select. BoS hides these internal things more, as you just get to see the gauges and you hardly fly the plane at altitude and over distance to really learn engine management. But its really lovely to see that kind of stuff included in this combat sim.

[Crump]

Thanks guys.

 

Obviously, I'm not an aeronautical engineer so I wouldn't really have a clue but the report summary does say, among other things that :

 

" because the supercharger is allowed to remain in low gear when the aircraft is gaining altitude above the critical, maximum performance for any particular engine speed can be obtained and abrupt changes in power output when the gear change ratio changes can be eliminated."  

 

And that:

 

"Undesirable fluctuation between the gear ratios of the supercharger is eliminated by the design of the control valve which, provides a range of altitudes in which the supercharger can operate in either high or low gear.  Because the range of altitudes is excessive, the altitude at which the supercharger changes from high to low gear is much lower than the optimal altitude for the gear change.  Consequently, undesirable abrupt changes in power are experienced and less than maximum performances at any particular engine speed is obtained at all altitudes when the aircraft is losing altitude."

 

 So, if this is all correct, doesn't it suggest that that the 'surge' we currently experience in boost and RPM was in fact effectively eliminated in the climb but would tend to occur when descending?  If that's the case, have the devs got it round the wrong way?

 

 

The surge is only experienced in the descent.  Figure 12 b (descending altitude) shows this quite clearly.

[Crump]

The NACA report simply says the BMW801 Kommandogerät does not find the critical altitude of the engine for a proper gear change when descending.  It tracks the high gear density altitude required for a gear change much better than it does the low gear on descent.

 

It has a little trouble identifying the correct density ratio on the descent and the NACA bench tested KG changes gear at a lower density altitude than optimum.   The system maintains a set fuel mixture which is how it finds the density altitude it needs to change the gear speeds.

 

That is why the gear changes are rough because of the excessive range on the high gear compared to the low gear.  That is an adjustment range and not necessarily indicative of all FW-190A variants.  This was the subject of an investigation and the fuel mixture adjustment regulations were changed to eliminate this characteristic.

 

You can see the range of mixture ratio's is much larger for the High Gear than it is for the low gear:

 

[Crump]

A Kommandogerät reading the throttle position only would adjust RPM indipendently from boost pressure, means once boost pressure drops the K.Gerät holds RPM steadily.

 

 

It does not read throttle position only, that is a fact.   The pressure capsules on the supercharger and manifold pressure regulator do keep the linkage aligned so that the throttle position always corresponds to a manifold pressure/rpm setting no matter what the density altitude.  

 

Throttle position never changes to the pilot and full forward is 1.42ata @ 2700U/min in the summer, winter, and everything in between very much like a modern FADEC system.

 

 

 

The Kommandogerät basically maintains a specific fuel to air ratio which if you know engine performance, is the basis for power production.  It has a "cruise range" where that ratio is maintained for best economy and "high power" range where the ratio changes to maintain best power.

 

That changeover point in fuel to air ratio logic represents the big break in the throttle position chart posted above.

[Crump]

A Kommandogerät reading the throttle position only would adjust RPM indipendently from boost pressure, means once boost pressure drops the K.Gerät holds RPM steadily.

 

 

To be clear, your friend is right.

 

RPM is adjusted independently from manifold pressure by the Kommandogerät system.  The pilot simply selects climb power on the throttle lever.

 

That throttle position is independently telling the VDM propeller governor to maintain a specific RPM.  

 

 

Notice in the following chart, the blade angle opens up at speed increases to maintain a constant 2400U per minute RPM.  

 

 

 

You can clearly see that while manifold pressure is dropping and system must adjust engine speed to maintain the fuel to air ratio,  when the sum of the systems adjustable and fixed linkage reaches the set datum point and a change in engine speed occurs independent of the propeller RPM....the supercharger changes gear.

 

Since it does not directly control RPM it remains constant and the blade angle is increased with speed-altitude as it should.

 

 

 

 

 

First one is what we got ingame. Once boost pfessure drops RPM decreases as well. 

 

 

That is wrong.  The change in engine speed would trigger a supercharger gear change.  That is what the supercharger looks for in its gear change logic.

[Crump]

 

 

engine speed occurs independent of the propeller RPM....the supercharger changes gear.

 

BTW...this reads somewhat confusing.  Engine RPM and propeller RPM are essentially the same thing.  Propeller RPM gauges are actually reading engine RPM thru propeller reduction gear.

 

It is that inability to hold RPM that forms the other half of the supercharger gear logic.