Supercharger output seems inconsistant

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Aymi
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Supercharger output seems inconsistant

Post by Aymi »

Good morning all. So I've been doing some tests with MP. This is one of the things I found.

Conditions :

FL290 (way above critical altitude so any gimmicky mustang-style throttle won't mess with the observations)
Throttle fully opened, high blower.

What I expected:
Since the connie have supercharged engines, the supercharger output depends only of crank rpm. And since it is a centrifugal supercharger, the output flow versus rpm follows a square rule. So if I double the crank rpm while leaving throttle fully opened, I'd expect to gain some MP since the blower is turning twice as fast. This is how it works in the A2A mustang. But instead the connie shows me this results...

The only thing changing here is prop RPM.
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Why does the MP decrease while the supercharger output flow is supposed to be WAY higher since the blower is turning twice as fast ?
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CAPFlyer
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Re: Supercharger output seems inconsistant

Post by CAPFlyer »

1) 29,000 feet is WAY above the service ceiling for the L-049. I'm surprised you're even able to get there.

2) When RPM increases, MP decreases because the engine is doing less work. This is normal because we're talking about a supercharger here which creates a FIXED increase in airflow mass.

3) It's hard to compare the P-51 and Connie as well because the P-51's Packard Merlin has a 2-stage, 2-speed supercharger with much higher critical altitudes than the Connie's single stage, 2-speed supercharger. Even in the Merlin XX (which had a single stage, 2-speed supercharger), the Critical Altitude for the low speed was 14,000 feet. In the Connie, the Critical Altitude for the low speed is 8000.
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LostSoul
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Re: Supercharger output seems inconsistant

Post by LostSoul »

CAPFlyer wrote:1) 29,000 feet is WAY above the service ceiling for the L-049. I'm surprised you're even able to get there.

2) When RPM increases, MP decreases because the engine is doing less work. This is normal because we're talking about a supercharger here which creates a FIXED increase in airflow mass.

3) It's hard to compare the P-51 and Connie as well because the P-51's Packard Merlin has a 2-stage, 2-speed supercharger with much higher critical altitudes than the Connie's single stage, 2-speed supercharger. Even in the Merlin XX (which had a single stage, 2-speed supercharger), the Critical Altitude for the low speed was 14,000 feet. In the Connie, the Critical Altitude for the low speed is 8000.
1) Irrelevant
2) Wrong. Higher RPM creates more airmass over time - as already said, it's a fixed-gear centrifugal supercharger. Whether the increase in airmass over time is following a true sqare rule with all the dynamic losses remains doubtful. I agree however that there should be an increase in MP as the higher RPM produces more airmass that the increased RPM of the engine is consuming. It would be interesting to see this behaviour from a real life readout. A Super Connie should produce similar results.
3) Irrelevant

There is however a question that I wasn't able to resolve. We don't have any system description about the manifold pressure boost regulation/overboost protection. There must be some kind of boost protection/regulation, even when the blowers are fix-geared.

Maybe someone can shed some light into this technical aspect?
Last edited by LostSoul on 01 Mar 2017, 12:31, edited 1 time in total.

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Re: Supercharger output seems inconsistant

Post by AviationAtWar »

I made it to 30,000 the other night with nearly full fuel and no load. I was still maintaining a decent speed and it was still climbing but I gave it up because it was after midnight and I had to be up for work at 6. I think she'd do 35,000 feet with a light fuel load and enough time.

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Re: Supercharger output seems inconsistant

Post by LostSoul »

AviationAtWar wrote:I made it to 30,000 the other night with nearly full fuel and no load. I was still maintaining a decent speed and it was still climbing but I gave it up because it was after midnight and I had to be up for work at 6. I think she'd do 35,000 feet with a light fuel load and enough time.
Everything is "possible" in a simulator :-) However contradictory to its jet counterparts these propliners were cruising very inefficiently at high altitudes - mainly due to steeply decreasing prop efficency. Cabin Altitude would be another interesting item to observe for a hull with max. diff pressure or around 4.8 PSI ....

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Aymi
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Re: Supercharger output seems inconsistant

Post by Aymi »

Thanks all for answering.

I will try to find my fluids mechanics books to get the right equations but I'm pretty sure that in the case of centrifugal supercharger the flow output (or "BOOST") follows a square rule (the squared item is the rotation speed). And the power consumed by the supercharged follows a similar equation but the speed is cubed instead of squared, so there is a specific point where the supercharger "eats" more power that it helps producing. Reno racers found out that there is little to gain between running 100" or 140" MP because the centrifugal supercharger eats all the extra power past the 100" point.
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CAPFlyer
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Re: Supercharger output seems inconsistant

Post by CAPFlyer »

LostSoul wrote:1) Irrelevant
Not so. Over the Service Ceiling means you're a test pilot and operating beyond what the simulation was designed and tested for. Additionally, during our testing, I don't think anyone got above FL260 unless extremely light (i.e. almost no fuel) because of aerodynamics. To get to FL290 and still be getting the power settings he was suggests something else is going on that is abnormal, thus negating his results.
2) Wrong. Higher RPM creates more airmass over time - as already said, it's a fixed-gear centrifugal supercharger. Whether the increase in airmass over time is following a true sqare rule with all the dynamic losses remains doubtful. I agree however that there should be an increase in MP as the higher RPM produces more airmass that the increased RPM of the engine is consuming. It would be interesting to see this behaviour from a real life readout. A Super Connie should produce similar results.
Not sure what a Super Connie has to do with this simulation or discussion. This is not a PRT-assisted engine. Anyway, as it is a FIXED increase, above the critical altitude, the FIXED amount of boost decreases until (as said below) you get to the point where increasing RPM actually reduces MP because the air is moving faster through the engine but there is no additional compression to the air simply because the supercharger is so inefficient at that altitude you're loosing power.
3) Irrelevant
See above.
There is however a question that I wasn't able to resolve. We don't have any system description about the manifold pressure boost regulation/overboost protection. There must be some kind of boost protection/regulation, even when the blowers are fix-geared.
[/quote]

Supercharged engines typically don't have any method of boost regulation except the crew's hands. This is true for the Connie and even Super Connie. The Super Connie's "boost regulation" is actually for the PRTs and doesn't effect the Supercharger.
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Aymi
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Re: Supercharger output seems inconsistant

Post by Aymi »

Hello CAP,
Not so. Over the Service Ceiling means you're a test pilot and operating beyond what the simulation was designed and tested for. Additionally, during our testing, I don't think anyone got above FL260 unless extremely light (i.e. almost no fuel) because of aerodynamics. To get to FL290 and still be getting the power settings he was suggests something else is going on that is abnormal, thus negating his results.
Please :roll: this is the bone stock connie with custom paints. No weird weather settings, FL290 is a pressure altitude based on 29.92. Also I am not the only one to be able to go that high with a fat loaded plane. But you recognize that something is wrong with the results compared to the real plane. That's the point, we're on tech support !

If someone knows the displacement of R-3350's supercharger, I can show you the calculations and compare it to engine's demand (in volumes of air moved) to plot the net MP gain versus RPM.

Cap by essence you can't get a steady increase (like, +10inHg) of MP along the whole powerband because a centrifugal supercharger output flow is all but linear. Engine demand on the other hand, is linear (displacement*RPM basically).
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LostSoul
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Re: Supercharger output seems inconsistant

Post by LostSoul »

Aymi wrote: ...If someone knows the displacement of R-3350's supercharger...
R = radial engine
3350 = displacement in cu.in. :D

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Aymi
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Re: Supercharger output seems inconsistant

Post by Aymi »

LostSoul wrote:
Aymi wrote: ...If someone knows the displacement of R-3350's supercharger...
R = radial engine
3350 = displacement in cu.in. :D
:mrgreen: Thanks :mrgreen:

I was talking about the supercharger not the engine itself :mrgreen:

On one rotation our twin cyclone moves 3350 cubic inches of air in through the cylinders. At 2 000 crank RPM, the engine breathed 6 700 000 ci of air in one minute... Or about 3800 CFM.

Now we got a blower. This thing is also breathing, a certain amount of air at each rotation.

It sucks the air, and force it to enter the cylinders. We are forcing more air to enter the same volume... pressure increases, as a side side effect. Keep in mind that the supercharger is nothing more than a pump. It creates a flow. And since the supercharger throw in more air than the engine breathe, and this air has nowhere else to go, pressure increases.

Now the flow of a centrifugal supercharger (unlike volumetric ones) is not linear compared to rpm. It follows a square rule. At low rpm, you'll barely get any flow (that'l later turn into boost). At high RPM, you'll get more than you asked for.

If you like cars and go to a dragstrip, ask the guys about prochargers :D They are belt-driven single stage centrifugal superchargers... With asjustable reduction ratio depending of what you want to do... Sounds familliar ? :)

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CAPFlyer
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Re: Supercharger output seems inconsistant

Post by CAPFlyer »

I'm sure there's some manual somewhere that gives the displacement of the supercharger, but it's pretty small (the whole thing is maybe 35cm Long x 10cm Wide x 10cm Deep?). Don't know if you can figure it out from that, but those dimensions are of the exterior casing, not of the impeller.
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Aymi
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Re: Supercharger output seems inconsistant

Post by Aymi »

Yep.

Next week I will work on this thing, including assembly/disassembly and full power tests. I will come back full of data regarding centrifugal superchargers :D

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AKar
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Re: Supercharger output seems inconsistant

Post by AKar »

What is that assembly on the N1 spool right in front of the core air intake? Looks like a clutch or brake or something.

-Esa

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Aymi
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Re: Supercharger output seems inconsistant

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AKar wrote:What is that assembly on the N1 spool right in front of the core air intake? Looks like a clutch or brake or something.

-Esa
Thanks for your interest Esa

The assembly facing the N2 centrifugal and behind the N1 FAN is a reversible generator. It works both as a starter and an alternator :) and spares you the weight, space and and cost of the gearbox and bleed air plumbing.

And in front of it, tetra-epicyclic gears coupling N1 turbine to the fan.

Assembly is tomorrow and test runs Friday. I will bring back some pics :P
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AKar
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Re: Supercharger output seems inconsistant

Post by AKar »

Ah I see - thanks. It must be on the N2 spool then. What appears like solid blocks of copper-color are depicting the windings in this case, the structure is a bit curious, but it could just be the picture. Starter-generators are common in smaller engines, but I've never seen a 'co-axial' design like this one, built around the shaft itself.

-Esa

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