Supercharger output seems inconsistant

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DC3
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Re: Supercharger output seems inconsistant

Post by DC3 »

Scott - A2A wrote:The engines in the Connie (and 377) us the Microsoft built-in tech that includes the superchargers. You can see the difference of this depth in the maintenance hangar. As some have mentioned here, the Microsoft superchargers are simple compared to what you would find in our P-51, T-6, P-40, etc. (these were built independently by us with Accu-Sim core). We've been able to produce by-the-book performance using the Microsoft tech, but some of the drivers, behaviors, and capabilities are different.

However, thinking about it.... there may be a way we can bring in some of the Accu-Sim supercharger physics from our Warbirds into the Connie without breaking things, so to speak. I put this on my TO DO list in my office whiteboard, and I will look into this.

Scott.

Thank you Scott for replying. It is a credit to A2A that we even look at and discuss these things. It is a high bar that you have set. :D

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CAPFlyer
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Re: Supercharger output seems inconsistant

Post by CAPFlyer »

AKar wrote:I am not sure I can follow on what principles the supercharger output should decrease in comparison to an increase in engine's intake airflow, if sharing a shaft. Of course, I can make up cases, but then, I'd rather leave the engine naturally aspirated to begin with!
I'm not saying that output decreases. I'm saying that output flat-lines or curves out to nearly flat above a certain RPM at altitude because there simply isn't enough volume of air to fully "fill" the supercharger as it spins.
I'm a bit busy, so I don't rant too much at the time ( :mrgreen: ), but I wouldn't mix up the cavitation into this. Cavitation involves a change in phase not applicable to air compressors (water instantly evaporating due to low local pressure). These mean an almost singular transformation in fluid properties. One runs into these when dealing with high Mach numbers with air for instance, but assuming those remain low....I can't see this applicable. The phenomena are fundamentally different, in case of water (or liquid) having to do with vapor pressure, and in case of air flow, with Mach number.
I'm not mixing them up. I made an analogy to better explain the loss of efficiency that is being discussed. It is a proper analogy as the loss in efficiency, while caused by different phenomena, are the same in effect- you have blades spinning at a greater speed through the fluid in which it is immersed that is faster than what it can fully convert that rotation into useful work to propel that fluid through the spinning disk. This leads to inefficiency while still retaining a level of work being done. The supercharger vanes are *NOT* stalling in this case. They are simply becoming inefficient because there is insufficient pressure and volume of air to fill the void left from the preceding vane moving through the air.

If you like, I can make an analogy using the propeller of the airplane. The more blades I add to a propeller of the same diameter, the less efficient that propeller gets. Not because it has more mass to spin, but because as the blades get closer together, they are more likely to enter the wake of the preceding blade and thus not get a full "bite" on the air. The blade isn't stalled in most cases, but it has a turbulent airflow over its surface and thus additional parasitic drag that then gets multiplied for the next blade and so on. At high RPMs and low airspeed, this *can* reach the point where the prop becomes stalled in its own wake, and in a helicopter, this is the onset of the Vortex Ring State.
A centrifugal compressor is quite resistant against stalling, I actually don't really gather how they would operate if I made the static pressure over them negative while retaining positive flow. This, however, would not be how I operated them, for very obvious reasons.
You're right, but it isn't immune to stalling nor is it impossible to stall the supercharger's vanes. Here again we're not talking about stalling, we're talking about rotating them too quickly for the conditions to be fully efficient. I will say this again - the guys who designed these supercharges had no clue what they were doing. I'm not saying that in a negative way. They simply didn't have the technology to know all of what was going on. That technology wouldn't exist until the 1980s, well after the large piston engine went out of style for powering aircraft. Even the first few generations of turbojets and turbofans suffered from this same issue of inefficiency. They were finding this stuff out simply by trying something out and seeing if it worked, not because they knew how it'd behave or they could model its operation. It really was "trail and error" manufacturing and even the engineers involved would happily tell you they are still amazed to this day that they got anything even close to being right. And every one would tell you they'd have loved to have the kind of technology we had even in the 1970s as far as computing because it would have saved them many hours and not so few lives because of what could be done with a simple computational flow model.
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AKar
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Re: Supercharger output seems inconsistant

Post by AKar »

I think that's unrelated, and again, the principles are all different from the compressor's situation. I can't follow it has much to do with the problem. And at high altitudes, it actually makes sense to increase the compressor RPM (and that was - and is! - done regularly, both in multi-speed superchargers and in the turbine engines).

But that's sidetracking we've taken too long here: what makes more sense is your initial argument about induction system throttling losses. What we must actually note is that the pressure ratio of a compressor can be rather linear over the RPM range whereas the air mass flow rate takes over. We actually measure only the static pressure of the compressor rim, not the mass flow of the air pushed into the engine, which is assumed by the general square law. Assuming the MP measured at the impeller rim was close to the total pressure of the airflow because of the relatively low velocity there, it would follow that we can make up the MP indication system to actually measure the inlet drop - resulting in this kind of relationship. I think it could perhaps occur at realistic cruise values. But it hasn't much anything to do with the supercharger dropping off faster than the piston's induction capacity.

Perhaps Aymi or someone could bracket out some scenarios quicker?

It would really make sense to have some actual engineering data or flight test measurements to split the road.

-Esa

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Re: Supercharger output seems inconsistant

Post by flapman »

Scott - A2A wrote:The engines in the Connie (and 377) us the Microsoft built-in tech that includes the superchargers. You can see the difference of this depth in the maintenance hangar.

Scott.
Thank you for your answer Scott,

What you've described is what I experienced with the 'supercharger' behavior. It's barely fair to call what Microsoft has modeled a supercharger.. but it's probably more complexity than was demanded by the vast majority of their users. It seems that Microsoft views a supercharger as a ratio of increased boost output over the standard turbo coding. That's what's happening when we switch to high blower.

This default engine behavior is well hidden by the turbocharger management in the B-377, and that's probably why nobody recognized it until the release of the supercharger only Constellation. I realize now that the different product lines have different levels of fidelity, not just user interface (eg, the maintenance hangar). After thinking about the problem I can see why the decision would be made; a 4-engine propliner may need 4 times the computing power of a high quality single engine offering.

With your information in mind, I made some tests of my A2A hangar. Here are the results:

L-049 = Default FSX engine behavior. During my tests I also realized that you can change the propeller pitch using the control on the ground with the engines stopped. This isn't possible in a constant speed propeller, but is default FS behavior.
B-377 = Default FSX engine behavior under the turbo regulator. I discovered this by disconnecting all turbos then performing the RPM change at 10,000'. Again, since so much of the boost comes from actual turbos the supercharger modeling is very academic. Also, moving the propeller control with engines stopped does not change prop pitch (which is realistic and not default FS behavior)
B-17 Wings of Power = A2A Supercharger modeling with an MAP dependent on RPM. This is modeled even though this aircraft uses turbochargers. Your clue about the maintenance hangar helps clarify the difference.. as the flying fortress hangar is more complex.

Since you talked about "default FSX supercharger behavior," I checked to see if indeed Microsoft modeled a supercharger. So I took the Acceleration (the last pure MS product and update to FSX) up to 15,000' and cycled the prop. No change in MAP despite large changes in the RPM of the "supposed" supercharger impeller wheel. So we know there is nothing functional. I know the default DC-3 functions the same, and imagine the G-21A goose handles this issue the same. I believe those are the only default aircraft equipped with centrifugal superchargers.

I still think the new supercharger code would be a drastic improvement to the L-49. But since you've discussed why it isn't present, I accept that this isn't an 'issue' with the aircraft. I am more than happy with the explanation.

Would still be overjoyed to find one day that it's been include at some distant point in the future :mrgreen:

Thanks again for the beautiful aircraft!

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DC3
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Re: Supercharger output seems inconsistant

Post by DC3 »

Flapman, excellent research, excellent comment! I give Scott a huge amount of credit for replying when most others would've remained silent. Given the trade-off of working within the FSX system and adding extra code to that system to provide a more true to life experience, and still maintaining something that will run on a PC, I believe some of these trade-offs need to be accepted. But, should there be improvements in the future, I am sure no one will complain. Likewise and unfortunately, I don't think most people will notice either. My sense is most people attempt to fly the aircraft within a narrow historic range given by historic tables and so they never will experience the fix if it is made.

I feel this is the best and most satisfying discussion I have participated in since I have been on this board.

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CAPFlyer
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Re: Supercharger output seems inconsistant

Post by CAPFlyer »

flapman wrote:L-049 = Default FSX engine behavior. During my tests I also realized that you can change the propeller pitch using the control on the ground with the engines stopped. This isn't possible in a constant speed propeller, but is default FS behavior.
One quick correction here - on electrically actuated props the prop pitch MAY change when actuated while on the ground and the engine stopped. This is because some electric props directly pump fluid in/out of the prop dome and governor instead of moving the governor control only. The prop on the L-049 and B-29 are this type. I believe (but can't remember as it's been too long) that the props on the Convair 240/340/440 are the same. Can't remember for the DC-6 and DC-7 which also have electric props.

Also, FSX does not have a "default" prop pitch animation. It has to be coded as part of a gauge and thus its movement is based on the programmer's design not FSX. Most of the models where you see the prop moving incorrectly is because the gauge the designer used simply reads the prop pitch axis and turns it into an animation instead of reading the the actual commanded prop pitch from the sim (don't remember what the value is called).
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Re: Supercharger output seems inconsistant

Post by flapman »

CAPFlyer wrote:Also, FSX does not have a "default" prop pitch animation. It has to be coded as part of a gauge and thus its movement is based on the programmer's design not FSX. Most of the models where you see the prop moving incorrectly is because the gauge the designer used simply reads the prop pitch axis and turns it into an animation instead of reading the the actual commanded prop pitch from the sim (don't remember what the value is called).
Interesting, I never knew that.

I've continued the technical details of our discussion at Propeller Governor Logic., since we changed topic and now I think it's outside the category of 'tech support.'

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Re: Supercharger output seems inconsistant

Post by flapman »

So I've tried these tests with the newly released PMDG DC-6B (very nice BTW), and I can report that their "superchargers" have the exact same behavior.

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Re: Supercharger output seems inconsistant

Post by MatzeH84 »

...only that there is no real discussion possible there. It was a joy to read through this thread and even get an official statement. Kudos to Scott and the community. That's why I always like being on this forums.
Kind regards, Matthias

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Aymi
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Re: Supercharger output seems inconsistant

Post by Aymi »

Finally, Thanks Scott ! :mrgreen:
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severniae
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Re: Supercharger output seems inconsistant

Post by severniae »

Out of interest, did this one ever get fixed? I think I read Scott was planning to make it part of an update?

flapman
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Re: Supercharger output seems inconsistant

Post by flapman »

severniae wrote:Out of interest, did this one ever get fixed? I think I read Scott was planning to make it part of an update?
I'm hearing that it's been decided not to add this feature. As it's default FSX forced induction behavior, it's hard to call it a bug.

MatzeH84
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Re: Supercharger output seems inconsistant

Post by MatzeH84 »

A pity to hear it. A2A usually is the last company to point their fingers to FS related bugs saying: We cannot do it, because FS doesn't let us. They always found a way to circumnavigate these lacks to give us the best simulation possible..
Kind regards, Matthias

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CAPFlyer
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Re: Supercharger output seems inconsistant

Post by CAPFlyer »

The issue here isn't that A2A is saying we can't fix it, it's more that they don't think the amount of effort to fix it is feasible given that the B377 is the first COTS airplane and its module only has a passing resemblance to all the other aircraft since released. The better solution is to release a new version of the B377, but that would mean more cost for us and that's where a decision has to be made at some point once their current backlog of projects is completed.
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Re: Supercharger output seems inconsistant

Post by MatzeH84 »

True, but this also happens in the Connie, which is one of the latest releases.

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Kind regards, Matthias

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