randomTOTEN

'Magic' Superchargers

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This really isn't a big issue for me, but I thought I would include it for the sake of completeness.

In testing a 'competitor' propliner **coughcough** some users discovered that the superchargers are not terribly accurate. Specifically, their output (MAP increase) has no relation to engine RPM. The aircraft flies as though it is equipped with turbos equipped with automatic waste gate control.

Unfortunately, it appears the PMDG DC-6A/B suffers from the same limitation. In our opinion, this is due to the base Microsoft code, and how it handles forced induction.

At the critical altitude... in low blower.... I pulled the propeller synchronizer from 2400 to 1500 RPM and noticed no drop in MAP. In fact, doing this caused the BMEP to greatly increase. The aircraft behaves as though the supercharger compressors are spinning at their maximum RPM at all times, regardless of crankshaft RPM.

Thanks,

Robert Toten

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On 29/07/2017 at 10:17 AM, randomTOTEN said:

This really isn't a big issue for me, but I thought I would include it for the sake of completeness.

In testing a 'competitor' propliner **coughcough** some users discovered that the superchargers are not terribly accurate. Specifically, their output (MAP increase) has no relation to engine RPM. The aircraft flies as though it is equipped with turbos equipped with automatic waste gate control.

Unfortunately, it appears the PMDG DC-6A/B suffers from the same limitation. In our opinion, this is due to the base Microsoft code, and how it handles forced induction.

At the critical altitude... in low blower.... I pulled the propeller synchronizer from 2400 to 1500 RPM and noticed no drop in MAP. In fact, doing this caused the BMEP to greatly increase. The aircraft behaves as though the supercharger compressors are spinning at their maximum RPM at all times, regardless of crankshaft RPM.

Thanks,

Robert Toten

Your best bet is to submit a ticket and the tech team will look into it for you.

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On 7/28/2017 at 9:17 PM, randomTOTEN said:

I pulled the propeller synchronizer from 2400 to 1500 RPM and noticed no drop in MAP. In fact, doing this caused the BMEP to greatly increase.

I hope the BMEP increased.  You just significantly loaded the engine, and to be fair you provided no numbers for the MP or BMEP with which to do an analysis for you.  In simple terms, BMEP = (BHP * 283) / RPM and you just made the denominator much smaller.  What did you determine to be the critical altitude?  Your logic is that since the engine (and prop) is turning slower the MP should drop because the supercharger is turning slower and is providing less air by weight... but you are ignoring that when the engine turns slower it requires less air by weight.  I'm not sure exactly how the MP should respond and you don't have an argument in favor of it decreasing yet that convinces me.

 

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58 minutes ago, downscc said:

I hope the BMEP increased.  You just significantly loaded the engine, and to be fair you provided no numbers for the MP or BMEP with which to do an analysis for you.  In simple terms, BMEP = (BHP * 283) / RPM and you just made the denominator much smaller.  What did you determine to be the critical altitude?  Your logic is that since the engine (and prop) is turning slower the MP should drop because the supercharger is turning slower and is providing less air by weight... but you are ignoring that when the engine turns slower it requires less air by weight.  I'm not sure exactly how the MP should respond and you don't have an argument in favor of it decreasing yet that convinces me.

 

Well one argument is that boost curve of a centrifugal supercharger in it's efficient range follows a square rule (don't remember the exact formula but is was something like output flow=supercharger displacement*rotation speed²*something else related to efficiency IIRC), while the cfm that the engine needs is linear (basically displacement*rpm)

Any mustang pilot could tell you about this effect !

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2 minutes ago, Genista said:

Well one argument is that boost curve of a centrifugal supercharger in it's efficient range follows a square curve, while the cfm that the engine needs is linear (basically displacement*rpm)

Any mustang pilot could tell you about this effect !

Good try, I've been around pumps of all kinds for long time as an engineer and I do know that Pratt & Whitney spent months on Pikes Peak (the highest place you could access in N Amer from a road) with their superchargers and radial engines working out the best match between pump and engine.  Again, I do not know what the MP should do but I've not seen an argument that compels me to accept your position that it is wrong.  I accept that it might be wrong, but I would need something a little more meaty maybe a R2800 pilot weighing in on this.  We had several DC6 pilots on our team and the guy that did the flight engine and matched power to performance has a pretty solid reputation in our sim world.  Need more meat on your argument.

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I didn't want to really hash this out again.. but if it makes the case I guess we can do this again. :biggrin: Hope you don't mind me using some passive language!

Quote

Good try, I've been around pumps of all kinds for long time as an engineer

You ever work with centrifugal air compressors as an engineer? Do they produce the same pressure output when you run them at half speed?

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I do know that Pratt & Whitney spent months on Pikes Peak (the highest place you could access in N Amer from a road) with their superchargers and radial engines working out the best match between pump and engine. 

Not really relevant to the discussion at hand. This is about how the pump behaves, not if it's the right size for the engine.

Quote

We had several DC6 pilots on our team and the guy that did the flight engine and matched power to performance has a pretty solid reputation in our sim world.

Nothing we're saying will invalidate their skills and observation. Again, the problem is not engine performance.. it's how it's achieved.

I'll reiterate my claim from the previous thread:

I contend they (the throttle levers) are currently too low for any given MAP, at altitude. This is because the simulation is not accounting for loss in boost pressure due to decreased supercharger RPM at any prop RPM setting less than TOGA.
 

Quote

I hope the BMEP increased.  You just significantly loaded the engine,

The only way BMEP would increase is if I decreased RPM while maintaining the same power output. My argument is that power output decreases with a reduction of engine RPM due to the design nature of a supercharger as opposed to turbosupercharging.

Quote

to be fair you provided no numbers for the MP or BMEP with which to do an analysis for you.

I can do that in another flight if it pleases you. However, I don't think that's relevant. If the manifold pressure stays constant at 34" or 41" doesn't change the fact that the static response is not appropriate in my contention. That BMEP is at 166 PSI when the RPM reduction sent it above 225 PSI doesn't matter if I had instead started the reduction with 196 PSI. We're talking relationships between values.

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In simple terms, BMEP = (BHP * 283) / RPM and you just made the denominator much smaller.

True, but you forgot the second half of the equation! How do we calculate BHP? My Aerodynamics for Naval Aviators instructs that BHP = Torque *RPM/5255. If I decreased the denominator, I have also decreased the numerator!

Again, to quote Aerodynamics.

"The actual power output of any reciprocating engine is a direct function of the combination of engine torque and rotative speed... If all other factors are constant, the engine power output is directly related to the engine airflow [because fuel meters with air, this must be true]... The pressure received by the supercharger is magnified by the supercharger in some proportion depending on impeller speed.... With the exception of near closed throttle position, an increase in engine speed will produce an increase in manifold pressure." (pg. 137-138)

Quote

but you are ignoring that when the engine turns slower it requires less air by weight.

So then why is an engine with an automatic mixture adjustment producing more (or equal) power when being provided with less air.. by weight?

 

Thanks all,

Robert Toten

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Yeah, been around some pretty darn big air compressors, the fluidic flow catalitic converter unit in a oil refinery that uses one, and many smaller ones in virtually every unit.  My favorites were the wet gas compressors.. those are beasts.

Anyway, I never implied you were incorrect and I find it interesting that you are trying so hard to convince me with logic (I know the BHP decreases, but I don't have the compressor curves for the supercharger or torque and BHP curves for the engine to make a conclusion).  I also said I think you might be right, but PMDG has had several DC6 pilots involved in the design and testing of this product so I accept was they created.  However, on the other hand if you could provide technical data to support your argument, or qualified individuals able to collaborate then I cannot agree that the MP behavior with change in RPM is wrong.  Aerodynamics is a good source, but it is providing generalized information.

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As I said, I'm confident this is a default MS code issue, and asking it to be resolved is akin to asking for a new feature.. not a bug report. We've had to hash this out on another forum for another payware aircraft, and I didn't really want to go through the argument again.

Anyways, I did submit a support ticket to PMDG.

Also, the newest update v1.20.8418 has not resolved this issue. The only reference to the engines was, "[Flight Model] Reduced excessive torque at high engine RPM," which had no effect on the supercharger effect. The only difference I noted was the BMEP mysteriously stopped climbing as the RPM reduced somewhere below 1800 RPM (I didn't record exactly where the pause happened, or at what data points.. the point was the trend was still inappropriate IMO). Still zero change of MAP with RPM change at the critical altitude.

For defining 'critical altitude,' I define that as the point at which the induction system is no longer able to deliver desired MAP. Aerodynamics agrees with this definition, although it's not the best terminology for a conventional centrifugal supercharger. A more appropriate term (and sorry for not using it) is "full throttle height." It is the height at which the restriction of the throttle plate is completely removed from the manifold pressure equation.

Again, I can understand not modelling this characteristic. If it wasn't for an unnamed freeware propliner, I would never know the immersion aspect of this minute detail. In short, it completely changes the relationship of the BMEP, MAP, and RPM gauges.. and gives an astonishingly different experience with managing the power of these engines. You treat the engines almost completely differently as opposed to any turbo or naturally aspirated engine you've had experience with. It would go a long ways to answering the many questions of "what power setting should I use in cruise?" because there are very apparent consequences for the choices of MAP/RPM that the pilot chooses.

Robert Toten

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10 hours ago, randomTOTEN said:

the point was the trend was still inappropriate IMO

I get that you finished with "in my opinion," but please provide references, data, and background if you're going to make statements like this.

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Exactly what I've been asking for, you have not established yourself as expert, you are referring to an unnamed freeware propliner as an example of a correct design, and you started this discussion by saying you reduced RPM from 2400 to 1500 without reducing MP which is absurd.  You cannot do that to these engines and I'm not sure that absurd treatment is modeled.  I'm glad you submitted the ticket and I look forward to some definite data or professional consulting.

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So the official statement is, that unless the OP has real world DC-6 experience or has the complete data sheets of this specific engine at hand, he is wrong..?

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Matthias he has not provided any concrete data, he might be correct but without data backing up what he says It wouldn't be sound to blindly follow his statements as true...

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14 minutes ago, MatzeH84 said:

So the official statement is, that unless the OP has real world DC-6 experience or has the complete data sheets of this specific engine at hand, he is wrong..?

I don't recall ever saying that. Where did I say that? Where was this official statement that I missed?

 

All the same, if we're stuffing random assertions into my mouth and running with it:

The sun is powered by completely by gold. In the morning and evening, the sunlight appears golden, and it is very bright in the day, almost like when you shine a bright light at something gold. I assert this as fact. I have no sources to back this information up.

---------- OR ----------

The sun is basically hyrdogen and helium.

---------- OR ----------

As someone who appreciates science, I offer up the following information on the sun. It is powered primarily by hyrogen and helium. My sources are as follows:

 

 

 

 

Now...which one of these statements is true? Moreover, which one of these statements is verifiable as true? If you were going to spend a lot of time re-working something, which one of these statements would you likely find most actionable?

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2 hours ago, scandinavian13 said:

All the same, if we're stuffing random assertions into my mouth and running with it:

Well, obviously you do, because I never said you did an official statement. I was asking the question if this actually was an official statement.

Let's state the obvious: Switching the superchargers from high to low will decrease the manifold pressure. Why is that? Because the impeller turns less fast of course, thus producing less MP. I guess everybody is fine with that. Now what happens if you reduce the engine RPM- of course you will reduce the manifold pressure as well, because the impeller will also turn slower.

What makes it really difficult is the fact that charts and diagrams for this engine are hard to get on the internet for the regular user. As a partner of Boeing, I could imagine that PMDG has better access to the according charts.

This chart here is taken from the official maintenance manual, which goes the other way around, but in my eyes can be used to show the principle.

29966178pe.jpg

In the upper right corner you would see that a decrease of 100RPM would cause a drop of 2 inches MP. In this chart it's the other way around, because the intend of this manual is to calibrate the engine to fixed values. However, it shows the relationship.

This topic was also discussed at A2A's forum, as both their competing propliners show the exactly same behaviour as the DC-6 regarding supercharger performance and MP reaction. The developers admitted they used the supercharger modeling implemented by Microsoft, which exactly reproduces the current MP reaction. Their custom coded warbirds circumnavigate this issue and feature the correct MP reaction, where MP is influenced by RPM; for example the P-47, which in turn is equipped with earlier versions of the R-2800.

Due to all these informations, it seems to the experienced and technically interested customer, that the current supercharger modeling uses the standard model, which neglects the dependence of MP from RPM.

No offense with anything intended, english isn't my mother tongue.

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I like that graph but I have a question regarding CARB UPPER DECK PRESSURE.  Upper deck pressure is normally associated with turbocharging and is the pressure output of the compressor stage at the throttle plate. In the supercharged engine, the carburetor is on the suction side of the compressor so I assume the carb upper deck pressure is at the intake to the carburator, the rise from 25 inHg to 56 inHg MP is the 31 inHg of boost provided by the compressor and includes the loss through the carburetor. Correct me if I've made the wrong assumption.  I understand the variance in MAP due to RPM is essentially a variance in compressor boost.  This implies that the upper deck pressure is held constant and the throttle plates are wide open (unobstructed).  This is where my engineer thinking gets in the way.  In your example, using a load generated by the props to reduce RPM, I don't think its likely that the carb upper deck pressure remains the same because of the decrease air flow.  So we are back to the basic problem in that we are lacking data for a sufficient analysis.  The curve you submit as evidence is valid evidence but only  for a specific set of conditions, which are not maintained in your decreasing RPM scenario.

Very interesting topic, I sincerely appreciate the effort you are displaying in presenting your thesis.

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11 hours ago, scandinavian13 said:

I get that you finished with "in my opinion," but please provide references, data, and background if you're going to make statements like this.

Hi,

Please see the post above the one you just quoted. Specifically, "The only way BMEP would increase is if I decreased RPM while maintaining the same power output. My argument is that power output decreases with a reduction of engine RPM due to the design nature of a supercharger as opposed to turbosupercharging." And,

On 7/30/2017 at 6:35 PM, randomTOTEN said:
Quote

but you are ignoring that when the engine turns slower it requires less air by weight.

So then why is an engine with an automatic mixture adjustment producing more (or equal) power when being provided with less air.. by weight?

 Another user has already proved a chart for the R-2800 (including CB-16) Which demonstrates a 2" correction which must be applied for a variation of as little as 100RPM.

I submit the following references for this information:

"The actual power output of any reciprocating engine is a direct function of the combination of engine torque and rotative speed... If all other factors are constant, the engine power output is directly related to the engine airflow [because fuel meters with air, this must be true]... The pressure received by the supercharger is magnified by the supercharger in some proportion depending on impeller speed....

Of course, the engine airflow is a function of the RPM for two reasons. A higher engine speed increases the pumping rate and the volume flow through the engine. Also, with an engine driven supercharger or impeller, and increase in engine speed increases the supercharger pressure ratio. With the exception of near closed throttle position, an increase in engine speed will produce an increase in manifold pressure." (pg. 137-138)

"As altitude is increased with the supercharger or "blower" at low speed, the constant MAP is maintained by opening the throttle and the BHP increases above the sea level value because of the reduced exhaust back pressure. Opening the throttle allows the supercharger inlet to receive the same inlet pressure and produce the same MAP. Finally, the increase of altitude will require full throttle to produce the constant MAP with low blower and this point is termed the "critical altitude" or "full throttle height." If the altitude is increased beyond the critical altitude, the engine MAP, airflow, and BHP decrease.

The critical altitude with a particular supercharger installation is specific to a given combination of MAP and RPM. Obviously, a lower MAP could be maintained to some higher altitude or a lower engine speed would produce less supercharging and a given MAP would require a greater throttle opening." (p. 143)

Hurt Jr., H. H. (1960). Aerodynamics for naval aviators (Report no. NAVWEPS 00-80T-80). Direction of Commander, Naval Air Systems Command United States Navy.

 

Quote

 

There are three cockpit controls that affect supercharger operations:

The throttle lever.

The propeller control lever.

The supercharger gear-selection lever.

1. The throttle lever position determines the boost pressure that is delivered by the supercharger and maintained constant up to the rated height by the boost control unit. It is a boost selection lever and in conjunction with the propeller control lever determines the power output of the engine. In addition its position largely controls the amount of fuel entering the engine and both power and enrichment jets are interconnected with it.
2. On propellers fitted to most engines the blade angle can, within limits, be adjusted by the pilot to increase or decrease speed, and the selected speed will be maintained by the action of the constant speed governor unit. The propeller control lever hence becomes in effect an engine speed control lever.
3. By means of the supercharger gear lever, the pilot can select high or low speed and it is very important to understand the effect of the two speeds on the power output of the engine. In either gear ratio, the boost pressure recorded on the gauge is identical and depends on the throttle lever position. But the power output of the engine is very different.

...

The propeller control lever has a marked effect on supercharger efficiency, for it will be remembered that the supercharger is geared to the engine crankshaft, and consequently changes in engine speed are reproduced in the supercharger.

 

 

Bristol Superchargers (quoted for theory) http://enginehistory.org/Turbochargers/Superchargers/BristolSuperchargers.shtml

Quote

Centrifugal superchargers add additional boost as the speed of the motor increases. This means the centrifugal supercharger will provide more horsepower at high engine speeds and less boost at lower engine speeds.

https://en.wikipedia.org/wiki/Centrifugal-type_supercharger#Power_output

(quote not directly cited)

Quote

The pilot controls the output of the supercharger with the throttle and indirectly via the propeller governor control

https://en.wikipedia.org/wiki/Supercharger#Altitude_effects

Again,

As MatzeH84 pointed out (before I was able to), if the propeller speed governor has no effect on supercharger speed... THEN the entire purpose of having a dual speed supercharger is redundant. What's the point of changing the gearing ratio on the supercharger, if the speed of the supercharger has no effect on it's boost output?

Just what is happening when an engine is switched to "high blower?" Why is MAP increasing?

Thanks,

Robert Toten

 

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2 hours ago, downscc said:

This is where my engineer thinking gets in the way.  In your example, using a load generated by the props to reduce RPM, I don't think its likely that the carb upper deck pressure remains the same because of the decrease air flow.

By what ratio? Assuming ISA, at 25" the throttle plate is only restricting airflow to reduce pressure 5" (17%) below ambient. Yet so little as a 100 RPM variance results in 2" change of boost pressure? How drastic could that 5" be reduced by a 4% change in RPM? That 4% change results in 2" lost.. or near half of the total restriction caused by the throttle plate at the carburetor...

Remember, this test is still at sea level. Does it apply to an engine operating in ambient pressure of less than 18", but still putting out ~35" to the cylinders?

Robert Toten

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7 hours ago, downscc said:

I like that graph but I have a question regarding CARB UPPER DECK PRESSURE.  Upper deck pressure is normally associated with turbocharging and is the pressure output of the compressor stage at the throttle plate. In the supercharged engine, the carburetor is on the suction side of the compressor so I assume the carb upper deck pressure is at the intake to the carburator, the rise from 25 inHg to 56 inHg MP is the 31 inHg of boost provided by the compressor and includes the loss through the carburetor. Correct me if I've made the wrong assumption.  I understand the variance in MAP due to RPM is essentially a variance in compressor boost.  This implies that the upper deck pressure is held constant and the throttle plates are wide open (unobstructed).  This is where my engineer thinking gets in the way.  In your example, using a load generated by the props to reduce RPM, I don't think its likely that the carb upper deck pressure remains the same because of the decrease air flow.  So we are back to the basic problem in that we are lacking data for a sufficient analysis.  The curve you submit as evidence is valid evidence but only  for a specific set of conditions, which are not maintained in your decreasing RPM scenario.

Very interesting topic, I sincerely appreciate the effort you are displaying in presenting your thesis.

I searched the net several hours yesterday in an attempt to find any graphs showing the relationship between RPM and MP with throttle untouched, however I couldn't find such data. I guess there may be such data available for similar engines, however I think it would be refused as not referring to the R2800-CB16. Thus, the best curve I found was the one I presented.

From my knowledge, I'd say you are fully correct when you state the carb upper deck pressure will drop when the RPM decreases. The chart suggests this as well,assuming MP drops with decreasing RPM (relationship mp-carb upper deck pressure in the chart, assuming constant RPM, but in my eyes the relationship can't change). So assuming we fly at a pressure altitude where we achieve a good MP within engine limits with throttle fully open, reducing the RPM would first cause a torque peak due to engine momentum, then the torque will settle in with the new parameters. At the same time, the impeller speed drops together with engine RPM, producing less MP, and therefore a lower carb upper deck pressure. This is resulting in less fuel being introduced by the carburetor due to lower air demand, thus the output power decreases (less air, less fuel, less RPM). If you would do this with a partial throttle setting, you would then be able to slightly increase the MP again to obtain the desired power setting with the RPM set.

However, as you already said, without proper curves it is not possible to provide hard numbers.

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There it is...

See...now we have a discussion based on some sort of supporting facts. Thank you.

World of difference, no?

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I cannot follow that Matthais, I have a hard time coming up with a scenario where I decrease RPM without first decreasing MP so I don't know what to expect.  Always decrease RPM before MP and always increase RPM before MP.  That is how the model is designed too I suspect. The transient behavior of how MP lags the throttle for example is realistic and displays some of the supercharger RPM vs MP pressure dependence that this is all about.

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Dan, the principle stays the same. Of course you would follow throttle->prop->mixture when reducing power. Reduce from climb MP to cruise MP, then reduce to cruise RPM => MP will drop below the cruise setting previously set and has to be reset once more to obtain the cruise rating.

Right now it is, reduce MP, reduce RPM, and you're set.

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7 hours ago, MatzeH84 said:

Dan, the principle stays the same. Of course you would follow throttle->prop->mixture when reducing power. Reduce from climb MP to cruise MP, then reduce to cruise RPM => MP will drop below the cruise setting previously set and has to be reset once more to obtain the cruise rating.

Right now it is, reduce MP, reduce RPM, and you're set.

Now you're making sense to me.  I agree, I wonder how much the drop in MP is in the typical scenario where I reduce MP from 40 to 30 inHg then RPM from 2400 to 2200.  i wonder if the decrease in MP with the 200 RPM drop would be more than 1 inHg?  Your chart indicates a 2 in drop for 100 RPM but at a constant upper deck pressure and in this case upper deck pressure will decrease along with RPM (and the increased throttle plate restriction).  Given that it's a 8.3% drop in RPM will the upper deck pressure drop more or less than 8.3%... I don't know...I'm the wrong kind of engineer (BSEE). Then with a given change in  upper deck pressure how does that add to a 200 RPM change.  I expect a change but is it more or less than 1 inHg (which is 3.3% of 30 inHg).  My guess is that the MP drop would be less than 8.3% but how much less, don't know.  I suspect the drop is minimal because none of the DC6 pilots called it out.  We have a DC3 pilot who says that setting the MP is always a  process, you set it and then set it again and maybe again.  We are not seeing that.

EDIT:  Also just realized upper deck pressure would be less of a vacuum when RPM decreases such that it trends towards ambient pressure.  Those upper deck pressure numbers on your chart probably represent something the A&P would see in a static test at sea level.  I wonder what the upper deck pressure to MP curve looks like at upper deck pressures in the teens? Is it steeper or flatter?  I suspect flatter as we will be on the mid to upper part of the compressor curve.

Good topic.

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I found another source which is relevant to this discussion.

Quote

Gear-Driven Centrifugal Blowers

A centrifugal compressor consists of an impeller and a diffuser housed in a helical casing, or scroll. The diffuser, sometimes called the stator, occupies the annular space between the impeller and scroll. Passages created by the diffuser vanes open wider as they approach the discharge throat. Vanes on the impeller wheel are arranged radially and may be straight or curved. The use of curved vanes came relatively late in the period and improved efficiency.

Air enters at the impeller hub, rotates with the impeller and, under the influence of centrifugal force, moves outward in a path defined by the impeller vanes. Upon contact with the diffuser, the air expands and slows, converting much of its kinetic energy into static pressure.

Because the impeller cannot be allowed to make physical contact with the shroud, there is always some leakage between the vane tips and the scroll. The seal consists of air, an elastic medium. At low rotational speeds the impeller merely flays about delivering little or no output. As tip velocity increases, the air seal becomes more positive and the compressor begins to pump. Unlike Roots blowers that move the same volume of air per revolution, centrifugal compressors are dynamic machines, whose output increases as the square of speed—double the speed and the output theoretically quadruples.

http://enginehistory.org/Piston/InterWarSCdev/InterWarSCdev.shtml

Robert Toten

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