Supercharger output seems inconsistant

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Aymi
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Re: Supercharger output seems inconsistant

Post by Aymi »

Indeed Esa, good eye. Starter-Generator is "coaxial" and connected to N2 (As you know the starter would be useless on N1). See details here:

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Now for the pics :)

Engine core separated (Compressor + heat chamber + N1 and N2 turbines). On the right you can see the epicyclic gears coupling N1 turbine to the FAN

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Casing removed, showing the centrifugal compressor (Not the worst looking part I must admit)

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This one is for you Esa :)

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Can't wait for the runs tomorrow.
Maintenance, Repair and Overhaul

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AKar
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Re: Supercharger output seems inconsistant

Post by AKar »

Aymi wrote:This one is for you Esa :)
Thank you! :mrgreen:

That pic indeed shows better how the stator windings look like, the cutout pic (as usual) is not too exact.

There are a couple of schemes how this could be made up; if I had to take a wild guess (also, based on how it looks like), I'd expect that the rotor uses permanent magnets, so that no slip rings for external excitation nor on-axis pilot-exciter are required. Due to difficulty of access into the generator for maintenance, I wouldn't expect to have slip rings for the field current here, and because of lack of space, I wouldn't expect to have a pilot exciter here either. Also, because it is directly mounted on the N2 spool, the generator is necessarily of "wild frequency" kind. I'd guess this engine is from VLJ of some sort...I don't know those in person at all, but I'd figure they use 28 V DC in their primary electrical power buses - because the electrical output from the generator is rectified and the output DC voltage controlled by power electronics anyway, the difficulties in controlling the output voltage of a PMG itself would not be that much of an issue here.

-Esa

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Re: Supercharger output seems inconsistant

Post by flapman »

This really is a issue that needs to be addressed IMHO, and I hope we can get some response from the developers.

There is a certain **coughcough** freeware version of the L-049 that has variable supercharger output based on engine RPM. IMO this is a realistic feature, as the supercharger is driven by the engines at a defined (but variable) ratio. Thus, depending on the blower selection, manifold pressure is dependent on RPM.

Here's a scenario to describe what we (IMVHO) should be seeing.

Per the F.A.A. Type Certificate Data Sheet A-763, the Lockheed model 49-46 has "4 Wright Cyclones 745C18BA-3 with 16:7 reduction gear ratio." In high blower, the superchargers turn with an impeller ratio of 8.67:1. I can't find out for sure, but since the supercharger is attached to the Accessory Gearbox of the Cyclone, and propeller reduction is in the front, that the supercharger ratio is in regards to the crankshaft RPM.

Thus, lets consider a mathematical example. Suppose we were to climb a L-49 in high blower up to FL250, and then adjust RPM to a comfortable cruise RPM. I can't find definitive numbers from the A2A manual, and I don't want to fire up FSX to get the numbers off the pilot notes, so I'm using numbers from the **freeware version** (Note: I use these notes when flying the A2A aircraft, and because both are so well modeled, the aircraft delivers equivalent climb and speed performance.)

At FL250 ambient air pressure is 11 inHg (as calculated from an online standard atmosphere calculator). This is the value that would be displayed on the MAP gauge of an engine that is not turning.

Per the handling notes, climb RPM is 2300. Thus, the crankshaft is turning at 5,257 RPM, and the supercharger in HIGH blower is turning at 45,579 RPM. The handling notes specify a climb map of approximately 33" MAP for the climb. I imagine at service ceiling this is not a achievable MAP (the throttles are full forward), but 33" works for the math.

33-11= 22 inHg which is the sole product of the spinning supercharger. We can subtract maybe 2" for compression of the inlet if you want (I would hope that's modeled, although the Connie climbs slow so this might be a mute point 8) ).

Upon reaching cruise altitude, we set a cruise RPM of 2,000, with the crankshaft turning 4,571 RPM and the same supercharger turning 39,634 RPM.

45,579/39,634=1.15, or we lost 15% supercharger performance as a result of the Prop RPM reduction. 1/1.15=.87 (get the fraction right)x22=19+11(ambient)=30. With this basic calculation, we should lose a minimum of 3" MAP just by changing the RPM... and nothing else.

This assumes a linear pressure reduction with supercharger RPM reduction, if this calculation is exponential, then the MAP decrease would be 6" (probably most accurate). Still, the trend is for reducing MAP with reducing RPM.
This is normal because we're talking about a supercharger here which creates a FIXED increase in airflow mass.
I also disagree. A supercharger will only create a fixed increase in airflow at a steady RPM. When we change the supercharger speed, we change the airflow increase (hence a transmission for changing blower gear).
When RPM decreases, MP increases because the engine is doing less work.
How is MAP going to increase because of an engine doing less work, when the ambient air pressure is only 11 inHg? This scenario only works on the ground, where MAP is reduced by restriction of the throttle plate, and ambient air pressure is 30 inHg! Less sucking on the ground means higher pressure, less sucking in thin air can't suddenly result in thick air.

If A2A is modelling an aircraft with superchargers, then an engine doing less work is a supercharger doing less work. MAP cannot increase above the current RPM because the dynamic is different now, Instead of the MAP gauge being in front of an air pump.. it is now between 2 air pumps connected together. If the output of the supercharger is not dependent on the speed at which it is being driven by the crankshaft.. then we are flying Turbocharged Constellations boys!

And IMHO, that's exactly how the current version behaves. :mrgreen:

EDIT: I forgot why this is important for the user experience. When the freeware offering first equipped this feature... power changes with high compression became completely different. The usual "normally aspirated" procedure is reducing MAP (and leaving it there), then reducing RPM (while MAP is unchanged). The supercharged procedure is completely different. Reduce MAP 1-2" (based on experience), then reduce RPM. When RPM reduces, you see the MAP needles move at the same time. The procedure and observations are completely different.

When increasing RPM (say for approach), first increase RPM. As you do this, watch the MAP needles increase, then make a slight adjustment to the new MAP setting, and you're done!
Last edited by flapman on 06 Jun 2017, 19:55, edited 1 time in total.

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DC3
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Re: Supercharger output seems inconsistant

Post by DC3 »

My 2 cents worth.... based on why the supercharger will not enable space travel in the connie.... :)

We all know the connie will not climb into orbit. The supercharger simply can not provide enough compressed air because the source air is too thin. Also mechanically a finite amount of time is needed to fill the scoop with air and the faster the supercharger turns the less likely it is to fill each individual scoop fully with air. The fact that the supercharger is mechanically turning faster does not mean it is scooping a correspondingly larger volume of air. In fact at a certain point the cylinder volume is going to require more air volume than the supercharger can provide causing a drop in manifold pressure with an increase in rpm. It would be worth trying the experiment at 18000 ft to see if the manifold pressure decreases with an increase in rpm. At that altitude manifold pressure should not drop as this is well within the operating range of the connie.

I don't know how A2A modeled the supercharger on the connie, but I do know the output of the supercharger is not infinite or even in a linear relationship with rpm after a certain altitude. Therefore it would be proper to see a drop off in manifold pressure at a certain altitude as the speed of the supercharge goes beyond it's physical ability to fully capture all the air it is capable of capturing. If A2A modeled this I am amazed.

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Re: Supercharger output seems inconsistant

Post by flapman »

DC3 wrote:My 2 cents worth.... based on why the supercharger will not enable space travel in the connie....
I'm going to completely disagree with your post, please don't take it in a negative light. Starting with this first statement. We are not expecting "space travel" performance from the constellation. Quite the opposite, we want realistic engine performance based on limitations of the real installed equipment. As currently modeled, high MAP at low RPM is "space travel" performance. A real constellation crew carefully chooses cruise RPM because they know it also affects their MAP... therefore fuel flow.. therefore HP. Current A2A Connie pilots can choose whatever RPM they want at altitude and face no real world consequences for their actions.
DC3 wrote:The supercharger simply can not provide enough compressed air because the source air is too thin.
This is literally the job of a supercharger, it's hard to declare that it is not able to perform it's designed job! :roll:
DC3 wrote: mechanically a finite amount of time is needed to fill the scoop with air and the faster the supercharger turns the less likely it is to fill each individual scoop fully with air. The fact that the supercharger is mechanically turning faster does not mean it is scooping a correspondingly larger volume of air.
This isn't quite an accurate description of what a supercharger does.. but it's close enough (I guess). A supercharger inlet doesn't "scoop" air like ice cream. Air is a compressible fluid, not a solid. A supercharger accelerates air, and then transforms that increased motion into increased pressure (via Bernoulli's principle).

What you're describing is a loss of supercharger effectiveness. My Aerodynamics for Naval Aviators describes this occurring as the tips of the centrifugal compressor approach the local speed of sound.
DC3 wrote:In fact at a certain point the cylinder volume is going to require more air volume than the supercharger can provide causing a drop in manifold pressure with an increase in rpm.
Surely the engineers at Wright Aero Engines know of this phenomenon, and designed their compressors to be at maximum efficiency at TOGA RPM, so that their supercharger is creating maximum flow exactly when the engine has maximum demand. The high impeller ratio of 8.67 turns per crankshaft rotation is not randomly chosen, and we can't assume they are incompetent.
DC3 wrote:It would be worth trying the experiment at 18000 ft to see if the manifold pressure decreases with an increase in rpm. At that altitude manifold pressure should not drop as this is well within the operating range of the connie.
I just tried this experiment. When I pulled the RPM back from 2400 to 2000 , the MAP increased 2"... this is a failed test!! :x

I've also tested other A2A aircraft. The T-6 supercharger functions properly at 10,000'

Here's a test for you. If you have the A2A P-51D Accusim, take it on a flight up to FL300. At that point, you will be in high blower. If you climbed at 47" MAP you'll start to see the pressure decrease, the regulator cannot adjust to keep the throttle demanded boost setting. You can set the throttle lever against the safety wire (don't break it), and there will be no change. The automatic regulator is now bypassed, as it will always be working to increase MAP beyond what is available.

When I pull the RPM back from 2700 I see an immediate and drastic drop in MAP. This is because I am forcing the supercharger to spin slower, which reduces it's output.
DC3 wrote:I don't know how A2A modeled the supercharger on the connie,
IMHO They didn't. This release has still has some issues which need resolution (like the electrically driven cabin superchargers, or a manual which discusses the R-4360, which is not installed on the L-049 :mrgreen: ) that still need to be resolved.

I still maintain that this is a BIG ONE.

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Re: Supercharger output seems inconsistant

Post by CAPFlyer »

flapman wrote:
DC3 wrote:My 2 cents worth.... based on why the supercharger will not enable space travel in the connie....
I'm going to completely disagree with your post, please don't take it in a negative light. Starting with this first statement. We are not expecting "space travel" performance from the constellation. Quite the opposite, we want realistic engine performance based on limitations of the real installed equipment. As currently modeled, high MAP at low RPM is "space travel" performance. A real constellation crew carefully chooses cruise RPM because they know it also affects their MAP... therefore fuel flow.. therefore HP. Current A2A Connie pilots can choose whatever RPM they want at altitude and face no real world consequences for their actions.
The power charts for the Constellation, including the ones that were present in the real aircraft which this airplane is based off of gives literally ONE RPM for each altitude and ONE BMEP setting. This is not later aircraft where the engine manufacturer and airframe builder made complex and in-depth testing and charting of various power settings and available combinations to reach certain performance goals. The C-69 and later the L-049 were built based on simple operation of the engines as the concern was with the other, more complex (for the time) operations and systems monitoring. In fact, the manuals chart only one power setting for climb - METO. They don't even specify METO directly, that's a separate chart and is based off a fixed RPM and only varies by MAP and Supercharger setting with height in that chart.
flapman wrote:
DC3 wrote:In fact at a certain point the cylinder volume is going to require more air volume than the supercharger can provide causing a drop in manifold pressure with an increase in rpm.
Surely the engineers at Wright Aero Engines know of this phenomenon, and designed their compressors to be at maximum efficiency at TOGA RPM, so that their supercharger is creating maximum flow exactly when the engine has maximum demand. The high impeller ratio of 8.67 turns per crankshaft rotation is not randomly chosen, and we can't assume they are incompetent.
Actually, a supercharger is designed to provide maximum boost at TOGA RPM, not efficiency. If it happens to be peak efficiency, then great, but having a inefficient inlet or inlet duct shape may dictate that maximum efficiency is gained at a lower RPM. Remember, Lockheed created the inlet and cowling, not Wright Aero. If Wright Aero had designed the cowling, then maybe the B-29's wouldn't have burned down so often. You're working way too much in the realm of theory and not enough in the world of reality. You ignore factors outside the supercharger that limit its capability and that's what makes for major changes in performance.
flapman wrote:IMHO They didn't. This release has still has some issues which need resolution (like the electrically driven cabin superchargers, or a manual which discusses the R-4360, which is not installed on the L-049 :mrgreen: ) that still need to be resolved.
I'm not sure what issues you're talking about. First, the Cabin Superchargers are engine driven. I'm not sure where you got the information that they're electric, but the C-69 and L-049 manuals we used during development both clearly state and show in illustrations engine driven cabin superchargers. And yes, the manual does discuss the R4360, but it also correctly points out that they're not talking about the Connie:
Pratt & Whitney took this even further, creating the R-4360,with 28 Cylinders (this engine is featured in the A2A Boeing 377 Stratocruiser).
I'm sure that Scott and the crew will take a look to make sure the Supercharger logic is operating properly after they finish with the P3Dv4 updates (which they've already stated several times on both the website and notices on the forum are top priority), but you're not helping your argument when you bring up non-issues as examples of them not "resolving" things.
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Re: Supercharger output seems inconsistant

Post by flapman »

Thanks for the reply CAPFlyer,
CAPFlyer wrote: which this airplane is based off of gives literally ONE RPM for each altitude and ONE BMEP setting.
Thanks for the clarification. I only mentioned the "choice of cruise RPM," to highlight the relationship of supercharger output to RPM. As you said, that's more "theory" than real operating life.
CAPFlyer wrote: a supercharger is designed to provide maximum boost at TOGA RPM, not efficiency
You're absolutely right, and I should have used 'boost' when I erroneously mentioned 'efficiency.' My bad. :mrgreen: And yes, this discussion is ignoring a lot of the operating realities of supercharged engines.. because IMHO the problem is theoretical, not practical. In it's current version, the A2A engines are able to deliver all required MAP, RPM, and BMEP targets... with resulting real world aircraft performance. It can be flown with the proper procedures. No complaint there. The problem is the theory of how the engine gets there. That's what this thread is about. If we get right down to it, it's about where the physical throttle levers would be located at each power setting.

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.

My 'issues' comment was related, in particular, to a past bug that has since been resolved. I only included it to demonstrate that there could be some issues (such as this supercharger one IMHO) which could need resolving. I understand that occasional updates will fix certain problems. And yes, you're right that they do correctly mention the R-4360 is not modeled in this aircraft.

I don't mean to step on anybodies toes.. but I am hopeful that the crew will have a look at this issue when they have time. I understand they are focused on P3Dv4, and am very patient. 8) This is the best propliner I have ever seen modeled in a flight simulator (equal to the B-377), and the desire is for as much authenticity which can be had for a design which hasn't been airworthy in decades (TBH I have no idea when an L-49 was last airworthy).

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Re: Supercharger output seems inconsistant

Post by DC3 »

flapman wrote:I'm going to completely disagree with your post, please don't take it in a negative light.
No offense taken. You stuck to the facts, didn't call me any names (at least that I could hear :D ), and presented your case.

I think that we can both agree the higher the altitude the thinner the air, which can be roughly thought of as fewer 'air' molecules per volume unit (square foot, square meter, for example). For normally aspirated engines this limits the altitude they can achieve in flight. The cylinder draws in air through the inlet pipe which even when fully open can not provide enough molecules to maintain the necessary fuel/air ratio to provide the energy for flight.

Adding a turbo or super charger 'boosts' the air density (air molecules per volume) above the current atmospheric density permitting more fuel to be added to the fuel/air mixture thus producing more energy to move the aircraft. The density of air that can be delivered through compression is influenced by the atmospheric density of the uncompressed air. The higher the aircraft goes the less effective the turbo or supercharger will be. The converse is also true, e.g. at sea level it is easy to overboost the engine.

In the case you originally cited the throttles were all the way open, which means no restriction to the incoming air flow. Since the throttles are all the way open they are effectively out of the equation. The only two things driving the MAP equation are the vacuum created by the cylinders, the faster they turn the higher the input vacuum (and the lower the MAP), and the supercharger. In the previous post I 'theorized' the supercharger was already beyond it's limit to provide any more boost. In other words it could not provide more compression because the air was not dense enough and it's physical dimensions were not large enough to permit it to compress more air. The supercharger now running at a higher RPM with the motor has to overcome the higher vacuum of the cylinders to provide the same amount of air density to the cylinders as before the RPMs were increased. I postulated the supercharger could not overcome the higher vacuum produced by the cylinders and this was why the MAP went down.

Of course, in real life, the supercharger may still at that point have been able to overcome the vacuum produced by the cylinders, this I don't know.
Either way, in order to match and model real life A2A would need to know what the parameters are and a comment would be good to reassure the A2A customers that look for 'ultra-realism' in A2A products.

****
All of the above is only a theory to explain what you observed and other experiments could be done to see if this theory has any validity at all.

So I proposed a test at 18000 feet which is well within the operating range of the aircraft.

Your observation when pulling the RPM back from 2400 to 2000 was a 2" increase in MAP.

How do I explain this? More importantly, how does A2A explain this? :)

After thinking about this I wondered (in the actual real world) how linear the supercharger output is versus the demand from the cylinders for air. For example you stated the impeller ratio for the supercharger was 8.67 turns per crankshaft rotation. Each cylinder draws air/fuel every second rotation of the crankshaft. So the engine is requiring half of it's cylinder volume to be filled for every rotation. The supercharger has to fill half the cylinder volume and all connecting hardware with a volume of air that is enough density to equal the boost amount plus atmospheric pressure. For a specific RPM within a specified altitude range this is going to be accomplished by the 8.67 ratio. The limiting device is the throttle which will choke off the air from the atmosphere and supercharger. I was wondering is it possible the supercharger actually produces boost that is not linearly proportional to the vacuum created by the cylinders when the RPM of the engine is varied? For example at low RPMs the amount of extra air volume from the supercharger is much greater than the need of the cylinders while at high RPMs the extra air volume from the supercharger is only greater than the need of the cylinders. Because of the compressibility of air I think it would be difficult to get a linear relationship.

Please make note, I am not proposing A2A has modeled this. In fact this would require some sophisticated modeling.

I would like to join you in asking A2A for some comment and explanation. Also, if anyone has experience on actual supercharged aircraft maybe they can provide more concise answers. I can see reasons for what has been observed, but I would not say my reasoning is any more than theoretical at best and misleading at worst.

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Re: Supercharger output seems inconsistant

Post by DC3 »

flapman wrote:I'm going to completely disagree with your post, please don't take it in a negative light.
No offense taken. You stuck to the facts, didn't call me any names (at least that I could hear :D ), and presented your case.

I think that we can both agree the higher the altitude the thinner the air, which can be roughly thought of as fewer 'air' molecules per volume unit (square foot, square meter, for example). For normally aspirated engines this limits the altitude they can achieve in flight. The cylinder draws in air through the inlet pipe which even when fully open can not provide enough molecules to maintain the necessary fuel/air ratio to provide the energy for flight.

Adding a turbo or super charger 'boosts' the air density (air molecules per volume) above the current atmospheric density permitting more fuel to be added to the fuel/air mixture thus producing more energy to move the aircraft. The density of air that can be delivered through compression is influenced by the atmospheric density of the uncompressed air. The higher the aircraft goes the less effective the turbo or supercharger will be. The converse is also true, e.g. at sea level it is easy to overboost the engine.

In the case you originally cited the throttles were all the way open, which means no restriction to the incoming air flow. Since the throttles are all the way open they are effectively out of the equation. The only two things driving the MAP equation are the vacuum created by the cylinders, the faster they turn the higher the input vacuum (and the lower the MAP), and the supercharger. In the previous post I 'theorized' the supercharger was already beyond it's limit to provide any more boost. In other words it could not provide more compression because the air was not dense enough and it's physical dimensions were not large enough to permit it to compress more air. The supercharger now running at a higher RPM with the motor has to overcome the higher vacuum of the cylinders to provide the same amount of air density to the cylinders as before the RPMs were increased. I postulated the supercharger could not overcome the higher vacuum produced by the cylinders and this was why the MAP went down.

Of course, in real life, the supercharger may still at that point have been able to overcome the vacuum produced by the cylinders, this I don't know.
Either way, in order to match and model real life A2A would need to know what the parameters are and a comment would be good to reassure the A2A customers that look for 'ultra-realism' in A2A products.

****
All of the above is only a theory to explain what you observed and other experiments could be done to see if this theory has any validity at all.

So I proposed a test at 18000 feet which is well within the operating range of the aircraft.

Your observation when pulling the RPM back from 2400 to 2000 was a 2" increase in MAP.

How do I explain this? More importantly, how does A2A explain this? :)

After thinking about this I wondered (in the actual real world) how linear the supercharger output is versus the demand from the cylinders for air. For example you stated the impeller ratio for the supercharger was 8.67 turns per crankshaft rotation. Each cylinder draws air/fuel every second rotation of the crankshaft. So the engine is requiring half of it's cylinder volume to be filled for every rotation. The supercharger has to fill half the cylinder volume and all connecting hardware with a volume of air that is enough density to equal the boost amount plus atmospheric pressure. For a specific RPM within a specified altitude range this is going to be accomplished by the 8.67 ratio. The limiting device is the throttle which will choke off the air from the atmosphere and supercharger. I was wondering is it possible the supercharger actually produces boost that is not linearly proportional to the vacuum created by the cylinders when the RPM of the engine is varied? For example at low RPMs the amount of extra air volume from the supercharger is much greater than the need of the cylinders while at high RPMs the extra air volume from the supercharger is only greater than the need of the cylinders. Because of the compressibility of air I think it would be difficult to get a linear relationship.

Please make note, I am not proposing A2A has modeled this. In fact this would require some sophisticated modeling.

I would like to join you in asking A2A for some comment and explanation. Also, if anyone has experience on actual supercharged aircraft maybe they can provide more concise answers. I can see reasons for what has been observed, but I would not say my reasoning is any more than theoretical at best and misleading at worst.

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Re: Supercharger output seems inconsistant

Post by AKar »

A place to start from is to note that a mechanical supercharger rotates at speed that is fixed to the crankshaft speed. Whereas the amount of air that is passing through the engine due to piston motion increases almost linearly with RPM, the output of a centrifugal supercharger is roughly proportional to the square of its rotational speed. (This is why bleed valves are used in engine designs that combine axial and centrifugal compressor stages: the axial stages would otherwise provide more air than what can be "processed" by the centrifugal stage.)

When the impeller speed (and engine RPM) is high, any reduction in RPM should show in noticeable reduction in MP, unless the supercharger in question is significantly under-dimensioned.

-Esa

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Re: Supercharger output seems inconsistant

Post by CAPFlyer »

Again, this assumes that the inlet is sufficient to supply the supercharger fully at any given time and the supercharger is operating at an efficient speed at all times, and this is simply not possible or reasonable to achieve, even today as that would require either massively oversizing the inlet or having a variable-geometry inlet which expands with altitude to maintain airflow as air density decreases.

Also, let's be real here - these superchargers and turbochargers were very inefficient compared to modern units. While the engineers who designed these units were pretty smart, their understanding of how it all works and ability to model it was woefully incomplete. Many times, these units were actually massively OVERSIZED for what was needed simply because they would rather make it too big than too small. However, that didn't mean there was an excess of power. Poor manufacturing quality, inherent inefficiencies in design, and external factors (like having a big enough surface area for cooling but too little airflow through the cowling) could make it where the design just doesn't work as intended.
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Re: Supercharger output seems inconsistant

Post by AKar »

CAPFlyer wrote:Again, this assumes that the inlet is sufficient to supply the supercharger fully at any given time and the supercharger is operating at an efficient speed at all times, and this is simply not possible or reasonable to achieve, even today as that would require either massively oversizing the inlet or having a variable-geometry inlet which expands with altitude to maintain airflow as air density decreases.
Having this inverse relationship would mean that the supercharger is effectively restricting the induction, not boosting it.

I'd expect the induction system to be of quite good efficiency, actually better than a simple "open pipe" due to ram effect.

-Esa

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Re: Supercharger output seems inconsistant

Post by Scott - A2A »

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.

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Re: Supercharger output seems inconsistant

Post by CAPFlyer »

AKar wrote:
CAPFlyer wrote:Again, this assumes that the inlet is sufficient to supply the supercharger fully at any given time and the supercharger is operating at an efficient speed at all times, and this is simply not possible or reasonable to achieve, even today as that would require either massively oversizing the inlet or having a variable-geometry inlet which expands with altitude to maintain airflow as air density decreases.
Having this inverse relationship would mean that the supercharger is effectively restricting the induction, not boosting it.

I'd expect the induction system to be of quite good efficiency, actually better than a simple "open pipe" due to ram effect.

-Esa
It's the area of that duct that becomes the limiting factor here. If the inlet duct is sized for optimum performance at sea level, it will become restrictive (i.e. not let enough volume in) as height increases. As density decreases exponentially instead of linearly but true airspeed increases on a more linear curve, the actual ram effect will not make up for the lack of additional inlet volume as the airplane climbs. As such, at some point the volume of air coming into the duct will be insufficient for the supercharger to provide efficient operation.

Think of it like a submarine's propulsor. When it's operating efficiently, it spins quietly and leaves very little wake and no bubbles are formed to create cavitation. However, if you spin it too quickly for either the submarine's forward speed through the water or for the blade design itself, it begins to cavitate (create bubbles of air). The sub still moves through the water (and even at high speeds), however it's doing so inefficiently and making a lot of noise while doing it. If you pull back on the power just a little though and get back into the efficient operating range of the propulsor, it stops cavitating and you continue to move at high speed, but do so quietly and efficiently. Same here. When you spin the supercharger too fast, it begins (in effect) cavitating, still providing air, but not efficiently. You back off the speed a little, it becomes efficient again, and you continue to provide the same power (boost), but do so efficiently.
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AKar
A2A Master Mechanic
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Joined: 26 May 2013, 05:03

Re: Supercharger output seems inconsistant

Post by AKar »

Both the supercharger and the engine share the common airflow. And the intake. They form a classic series circuit, if you wish to calculate.

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 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.

Stall is entirely a different phenomenon from cavitation!

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.

-Esa

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