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!

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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.

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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.
 

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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.

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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)

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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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