What of the Republic P-47 Turbo-Supercharger

[Tony Kambic]

The B-26 Marauder with the R-2800-43 engines had a single stage two speed supercharger. I've spent at with mechanics at American Aero who service B-17s and B-24s, and both aircraft have turbochargers. Since these now-flying aircraft are for rides and shows only, they never go over 10,000 so no true need for supercharging. For the B-17s, they leave the turbochargers in place since they also act as a muffler as without the engine would be very loud. Their waste gates are wide open. I was told though that the B-24 does engage the turbocharger for power on takeoff as it is required due to the different wing loading than the B-17.

Tony

 

[GregP]

Just to set the record straight, Tony, the B-17 and B-24 WERE supercharged since single-stage superchargers were built into the engine just as they were in the Allison V-1710. The B-17 and B-24 had turbochargers because they were expected to fly high in Europe, as was the P-38, also with turbochargers. But their engines WERE supercharged, too.

The single-stage supercharger was for lower altitude boost and the turbocharger was for higher altitude boost. In the case of 2-stage superchargers, as in the Merlin 60 series engines, one stage was for lower altitude boost and cutting in the second stage increased the boost for higher altitude operations when the first stage had reached its critical altitude and was starting to loose power. The second stage helped maintain sea level power past the altitude where the first stage normally dropped off.

 

[Dash119]

I thought both stages of the Supercharger on the Merlin were always engaged. The change in boost came when the second speed was engaged, turning both stages at a higher speed.

 

[Tony Kambic]

Just to set the record straight, Tony, the B-17 and B-24 WERE supercharged since single-stage superchargers were built into the engine just as they were in the Allison V-1710. The B-17 and B-24 had turbochargers because they were expected to fly high in Europe, as was the P-38, also with turbochargers. But their engines WERE supercharged, too.

Never said they were not supercharged. Just adding comments about turbocharging since that seemed to be the topic. I get to work around many aircraft engines and to see the configurations. Recently worked on the Bv155 Hirth Turbo which I believe was the largest I have ever seen. One of my favorites that I have examined is the 3350 with PRTs ( Power Recovery Turbines).

 

[GregP]

Hi Dash 119,

You are correct. Both turn all the time in a 2-stage Merlin.

There is a 24-volt solenoid that engages or holds stationary the sun gears and they either rotate inside the planetary gear or don't rotate, changing the gear ratio of the 2nd stage. There are clutches that accomplish the speed change.

 

[Dash119]

Hi GregP,

So the first stage is single speed and the second stage is two speed?

 

[tomo pauk]

If I may.
Both impellers on both supercharger stages were on the same shaft on 2-stage Merlin (and Griffon, as well as on many German 2-stage supercharged engines). When driving gear was in 1st speed, both impellers were rotating at same angular speed. Or, doing the same RPM if you will. For example, on the Packard Merlin V-1650-3, the impellers were rotating 6.391 times the crankshaft RPM - for 3000 rpm the crankshaft will be making, impellers were making 19173 rpm, providing the boost of 61 in Hg at the rated (low) altitude.
When driving gear was switched up to 2nd gear, again both impellers were rotating at same angular speed (or, on same RPM), this time the ratio was 8.095:1 vs. crankshaft - for 3000 rpm the crankshaft is making, impellers will be making 24285 rpm, again providing the boost of 61 in Hg at the (now much higher) rated altitude.
The faster the impellers rotate, the more compressing work they will do, but also suck up more power (we're talking about gear-driven supercharges, as used on ww2 aero engines,but not just on them).
Low impeller speed (= S/C drive is in low gear; British name is 'MS' - medium supercharge) means the S/C will be using less power to provide required boost, so there is more power left to the prop. It is okay, since at altitudes the low gear is used has thick air around, so there is less of need for the S/C to compress the air. High impeller speed (= S/C drive is in high gear; British name is 'FS' - full supercharge) means the S/C will be using more power, but again that's okay since it will be providing the required boost at high altitudes. More boost = more power; the trade-off favors the 2-speed superchargers.
Some engines were outfitted with 3-speed 2-stage superchargers - yes, the drive gear is outfitted with 3 sets of gears. Such engines were the Jumo 213E and F (late ww2), as well as RR Griffon 100 series (post war)
Widely produced were the 2-speed 1-stage supercharged engines - 2 sets of gears, one impeller.

 

[GregP]

It is my understanding that both supercharger wheels turn all the time. The engine diagrams definitely show that in any case. The pilot gets to choose M.S. or F.S. gear ratios (low and high or medium-speed and full-speed). They almost always reduced power and rpm to change ratios to decrease the load on the clutches. Sometimes they'd set the change to automatic and let the engine change ratios as it saw fit. There was a hysteresis in the changeover. That is, it might go to high at 15,000 feet when climbing but would wait until 11,000 feet when descending to change to low. Having a single setting would make for a lot of clutch abuse if dogfighting at or about the changeover altitude if the ratios changed at the same altitude back and forth..

I think Tomo said it better above, except for mostly reducing power and rpm to change ratios. If they got bounced, they might just change ratios without reducing power. More stress but less attention required.

 

[Dash119]

There is a 24-volt solenoid that engages or holds stationary the sun gears and they either rotate inside the planetary gear or don't rotate, changing the gear ratio of the '2nd stage'. There are clutches that accomplish the speed change.

So this should have read '2nd Speed', I think that it what led to my confusion and the question.

 

[BiffF15]

Does anyone have a drawing or video that explains the difference between the single stage and 2 stage? I get the multi speeds (works like a transmission - you take off and fly at lower altitudes in the lower blower RPM range then shift to higher RPM range at higher altitudes).

Cheers,
Biff

 

[Tony Kambic]

Here are the remains of the R-2800-10 from MAAM P-61B that sat in the jungle for many years so it is extensively corroded. But you can visualize the two separate impellors. This engine was a 2 stage - 2 speed. From what I read the auxiliary stage would be bypassed until the pilot engaged it at higher altitudes. Cockpit engine controls had supercharger settings for neutral, low, and high speed.

 

[PAT303]

Agreed. Big ass aircraft faster that all at 25K plus.

https://ww2aircraft.net/forum/attac...538624146844_548418836751198474_n-jpg.270108/

 

[PAT303]

Does anyone have a drawing or video that explains the difference between the single stage and 2 stage? I get the multi speeds (works like a transmission - you take off and fly at lower altitudes in the lower blower RPM range then shift to higher RPM range at higher altitudes).

http://legendsintheirowntime.com/LiTOT/Content/1943/Av_4304_Merlin_sk_p187_W.png Does this help?.

 

[GregP]

No it should not have said second speed. It is second stage.

In a single-stage engine, you can have 2 speeds, but you only have the pressure ratio defined by the single compressor. In a two-stage engine, you can also have 2 speeds, but you have the pressure ratios of both stages added to each other.

Single-stage engines usually have a critical altitude somewhere roughly between 8,500 and 16,000 feet, at which point they will begin to lose manifold pressure and, therefore, power as they climb. The decline in power goes exactly as the decline in air density. If you have a second speed, you can regain some power, but you are constrained by the single compressor to some maximum.

In a 2 stage engine, both stages are spinning, but the aux stage (smaller diameter) is spinning relatively slowly in M.S. gear and not compressing much at all. When the critical altitude is reached, you go into F.S. gear and the both stages increase in rpm. The first stage increases a bit (or not ... they don't HAVE to change the ratio of the main stage) and the aux stage starts spinning at optimum rpm, creating it's design compression that gets added to the first stage maximum. You can plainly see sawtooth power steps in the charts for single and 2-stage engines. At some point, the 2-stage engine reaches it's critical altitude and the power starts declining exactly as the air density declines, but will always be greater than a comparable single-stage engine anywhere above the point where the pilot switches into F.S. gear.

I suppose you COULD design both a single-stage and a 2-stage engine to have identical power levels at, say, 20,000 feet. In that case, the single-stage engine might have more power at particular altitudes than the 2-stage, but that situation is a "what if." Nobody in WWII wanted to DO that. They wanted more power at a higher altitudes, so they designed high-altitude engines that way.

 

[wuzak]

Does anyone have a drawing or video that explains the difference between the single stage and 2 stage? I get the multi speeds (works like a transmission - you take off and fly at lower altitudes in the lower blower RPM range then shift to higher RPM range at higher altitudes).

Cheers,
Biff

A single stage supercharger has one supercharging stage - that is, one impeller.

A two stage supercharger has two impellers, where one impeller discharges into the inlet of the second. Depending on the layout of the supercharging system, there may be an intercooler between stages.

In turbocharged aircraft of WW2 there were 2 stages of compression. One is at the turbocharger, which typically was used to bring ambient air pressure to about standard sea level pressure, the other is the supercharger connected to the back of the engine.

 

[wuzak]

I would note that all other US turbos just dumped the exhaust gas from the turbo out into open air. In part to expose the turbine blades to cool air to keep the temperature down.

The B-29 was another exception, having an exhaust pipe and flight hood.

DSC_0026-a20 by Fred Roessler, on Flickr

 

[wuzak]

But why a big huge unproven fighter with massive

Turbocharger system?

Note that it wasn't Republic's first attempt at a turbocharged fighter

The Republic P-43 was powered by the smaller R-1830 connected to a B-series turbocharger in a layout very similar to the P-47's.

The B-series turbo was also used in the P-38, B-17, B-24 (also with R-1830) and B-29 (2 turbos per engine).

The P-47 used the larger C-series tirbocharger.

 

[wuzak]

In a 2 stage engine, both stages are spinning, but the aux stage (smaller diameter) is spinning relatively slowly in M.S. gear and not compressing much at all. When the critical altitude is reached, you go into F.S. gear and the both stages increase in rpm. The first stage increases a bit (or not ... they don't HAVE to change the ratio of the main stage) and the aux stage starts spinning at optimum rpm, creating it's design compression that gets added to the first stage maximum. You can plainly see sawtooth power steps in the charts for single and 2-stage engines. At some point, the 2-stage engine reaches it's critical altitude and the power starts declining exactly as the air density declines, but will always be greater than a comparable single-stage engine anywhere above the point where the pilot switches into F.S. gear.

That all depends on which engine you are talking about.

In the Merlin and Griffon, both impellers ran at the same speed as they were on the same shaft, as Tomo explained.

I'm not sure if Rolls-Royce used the terminology of auxiliary stage, but that impeller (the first stage, which takes the ambient air) is the larger diameter, being 12" vs 10.25" for the second stage (which takes air from the first stage and feeds the engine). Early 2 stage Merlins had 11.5" first stage.

The auxiliary stage on the V-1710 was larger as well - 12.0" vs 9.5" (IIRC) for the engine stage. The auxiliary stage was driven by a fluid coupling and the speed ratio could be varied.

Pratt & Whitney 2 stage engines had a gear system that allowed for Lo, High and Neutral. The latter meant that the auxiliary stage did not spin at all.

 

[Dash119]

No it should not have said second speed. It is second stage.

GregP,
Keeping in mind that we were specifically speaking of the two-stage supercharger from the Merlin. I had always assumed that both impellers in that engine were on a common shaft or were at least geared to spin in some fixed ratio to one another. Your post seemed to imply that it was possible to spin each rotor at a different RPM or ratio to one another, that a clutch was placed between the two impellers...

While it may be possible to spin the first stage at a different speed or ratio relative to the second stage in some applications, I don't think that is the case with the Merlin.

Regards,

Kim

 

[wuzak]

While it may be possible to spin the first stage at a different speed or ratio relative to the second stage in some applications, I don't think that is the case with the Merlin.

That is correct.