Effects of multi-speed superchargers (or lack of the same)

[wuzak]

Page 129 or Hermann's book on the Ta 152H specifically states that the Jumo 213E had a two stage 3 speed supercharger. I suppose there is the possibility that this consisted of a single speed first stage that could be declutched combined with a two speed second stage but that sounds like a complicated arrangement. It was used on the PW R2800 Corsair/Hellcat engines.

If the two stages were on different shafts, with one having two speed ratios and the other having one speed ratio the supercharging system would still only have 2 speeds. If both shafts had 2 speeds then they would have 4 speeds all up.

The P&W R2800 used in teh Corsair and Hellcat had the supercharger impellers on different shafts as you say, with the engine supercharger having a single, fixed, ratio, while the auxiliary supercharger had two ratios (Hi and Lo) and a neutral (impeller not spinning).

 

[wuzak]

Given the figures on the chart, it seems like that 3rd speed is a bit beyond the really useful performance (and efficiency range) particularly given the high critical altitude. (that or lowing all 3 speeds, but it's the high gear that seems problematic) Targeting the 3rd stage crit alt 1-2km lower seems like it'd make a lot more sense while still giving a smoother power curve than the contemporary 2-stage Merlin series. Granted, the same shift occurred between the V-1650-3 and -7, except A. the -3 still wasn't tuned quite as high as the 213 appears to be here, and B. the 3 speeds means it could better compromise and avoid the larger altitude performance gap between the -3 and 7. (ie more like if the Merlin could be arranged to use the -7's MS gear as low, middle gear between the -7's FS and 3's MS gear range, and high gear similar to the -3's FS gear)

You have to look at what they wanted the high altitude engine for. Mostly it was to combat USAAF bombers, which often flew 25-30,000ft) and their escorts (which usually flew higher).

The V-1650-7 was introduced to the Mustang to give it better performance and middling altitudes, especially concerning climb rates. When the P-51H was introduced it had the -9, with the critical altitude raised to the same as the -3, if not higher.

Of course the boost affects the critical altitude. The higher the boost, the lower the critical altitude.

Perhaps the Griffon is a better comparison. It is the same capacity, and later versions were fitted with a 3 speed supercharger. In the Griffon's case, the extra gear was for low altitude. The supercharger was redesigned to slightly increase critical altitude in full supercharger gear (S). Can't recall if medium supercharge gear increased as well.

In Rolls-Royce rating terms all the two speed 2 stage engines were rated RM-##SM, where S and M stood for fully supercharged and Medium supercharged.

Similarly 2 speed 2 stage Griffons were RG-##SM.

The three speed Griffons were RG-##SML, where L stood for low supercharged.

 

[tomo pauk]

Are you sure the second stage was always active? Your comment sounds like it was. Then it would require a higher gear to get high alt charging.
I'm a bit suspicious about this but I don't have exact sources about this system.

The comparative drawing shows that 1st stage is always feeding the 2nd stage with compressed air - in case the 2nd stage is not clutched in, the internal S/C losses would be huge.

Given the figures on the chart, it seems like that 3rd speed is a bit beyond the really useful performance (and efficiency range) particularly given the high critical altitude. ...
Or in short, just optimized so high gear was in the ~8 km range rather than over 9.5km.
...

Lets remember that S/C gearing was calculated/designed with MW 50 operation in mind, as such it makes plenty of sense to tailor the 3rd gear to 9.5 km rated height. With MW 50 in use, the rated height would be at around 8-8.5 km, but it took a while to introduce the version of 213E that would allow for MW 50 operation also in 3rd gear.

@ Koopernic:

Post edit, given Tommo's picture its clear it used a single combined shaft for both impellors. The second impellor looks most unusual.

The second impeller looks like the carbon copy of the DVL-designed impeller, used on Jumo 211F and subsequent.

 

[kool kitty89]

The second impeller looks like the carbon copy of the DVL-designed impeller, used on Jumo 211F and subsequent.

Looking at that diagram, it also appears the impellers for the 2 stages were identical or nearly identical in diameter, but the first stage used a taller/broader pitch for the blades/vanes of the impeller. (so larger area and should be getting higher mass flow)

 

[Koopernic]

Technically it might even be 3 stages, note the axial flow fan in front of the two centrifugal stages. This probably functioned in a similar manner to the inductor fan in front of Heinkel Jet engines providing a low level of compression to smooth the airflow. In other words high volume low pressure.

The first stage of the centrifugal compressor looks unshrouded, the second stage seems to have integral shrouding on the impellor.

 

[kool kitty89]

Technically it might even be 3 stages, note the axial flow fan in front of the two centrifugal stages. This probably functioned in a similar manner to the inductor fan in front of Heinkel Jet engines providing a low level of compression to smooth the airflow. In other words high volume low pressure.

I believe that would actually be the variable inlet guide vanes used in leu of butterfly type throttle plates. (I thought it was an impeller too, the first time I saw it, but this feature was discussed in a few other threads and also explains the smooth power curves for each of the speeds -the swirl inlet was much more aerodynamically efficient than simple butterfly valves)

The first stage of the centrifugal compressor looks unshrouded, the second stage seems to have integral shrouding on the impellor.

I thought so too, at a glance, but looking closer, I think that first stage impeller is fully shrouded as well (the shroud projecting out of the cut-away casing/bell housing)

Junkers tended towards shrouded supercharger designs, including to the detriment of the odd 'spouted' impeller used on the Jumo 210 and 211 prior to the F model. (the versions shown above from the F and later models is a vast improvement, possibly with more real advantages over unshrouded impellers)

 

[Koopernic]

Yes, you can see the pitch adjustment mechanism.

 

[kool kitty89]

Hmm, looking over all the V-1710 charts again (especially those on Peril's P-40 site), it looks like all the pressure altitude charts for actual power at altitude for given manifold pressure and RPM are plotted entirely with throttle wide-open and power only being limited by RPM and atmospheric pressure (and ram for charts plotting for ram effect). They also specifically list take-off power at much lower RPM (but identical power and slightly higher boost) than the "Specific Engline Flight Chart" does. (in the case of the V-1710-39/F3R with 8.8:1 superhcarger, that lists 1150 hp at 3000 RPM 45.5" Hg while the pressure-altitude chart plots take-off power at 2200 RPM with 46" Hg)

Wouldn't controlling the RPM setting on a constant speed propeller give a good deal of automatic control for setting engine speed? (obviously this wouldn't apply to 2-pitch or maualy controlled variable-pitch propellers) And couldn't a barometrically regulated prop speed governor be implemented similar to the supercharger regulator used on the DB-601/605/603 engines?

 

[Shortround6]

The more "automatic" controls you put on an engine the easier a pilots job is (and the harder the maintenance personnel have to work). You also have Murphy's law

One of the bigger problems P-38s had in early/mid WW II was mis-rigged turbo controls. Whirlwinds had a lot of trouble with the engine controls (which was often lumped into blaming the basic engine), and the story goes it became a court marshal offence for British "erks" to openup and mess with the control boxes on Sabre engines.

The US finally went to single lever controls on some planes and a lot of progress was made from 1941/42 to 1945 but trying to get too tricky too early in the war with engine controls just may backfire.

 

[kool kitty89]

The more "automatic" controls you put on an engine the easier a pilots job is (and the harder the maintenance personnel have to work). You also have Murphy's law

I was thinking more in line with what was done on the DB-601 early war but applied to the prop controls rather than the supercharger. I suppose the variable speed supercharger control unit itself would increase failure rates and maintenance (and obviously manufacturing cost, complexity, and weight). Let alone the engine computer BMW introduced on the 801. (Bramo had previously used single-lever systems on the 323 -and I assume developed them for the 329- so there was a good degree of background work there already)

Having automatic limiting/control systems oriented towards avoiding catastrophic failure would probably be more important. (I'd imagine the V-1710's automatic boost control helped to some degree in avoiding detonation, at least on the 9.6:1 models -with 100/130 fuel it seemed near impossible to induce detonation even wide-open at SL at 3000 RPM, at least with a rich mixture; damage due to excessive power levels overstressing components would be less immediate and more related to intentional abuse or prolonged pilot error -or calibration error- rather than the rapid onset cylinder damage caused by detonation)

Aside from that, manual or semi-automated control on the pilots end should still be useful for higher power levels and not just cruise. Even prior to clearing engines for WEP ratings, more power (and better specific fuel consumption) at military rating below critical altitude and take-off would seem fairly straighforward. Manually operated variable pitch propellers would make that more difficult, but constant speed propellers should have simplified that a great deal. (2-pitch propellers would be a good deal worse but not really be a factor in USAAF use)

I'm not sure if all newer USAAF aircraft had adopted constant speed props by the start of the war, and I recall references to constant speed propellers on the P-38, P-39, and P-40, but that term is sometimes misused in the more general context of variable-pitch propellers.


It's possible that individual squadrons adopted unofficial ratings and procedures on their aircraft, but it seems far more useful to actually provide official ratings for optimal performance and practical procedures to implement as standard. (having pilots constantly checking boost and RPM during climb/combat wouldn't be practical, but setting lower RPM+high boost take-off procedures along with lower RPM mil/WEP settings in steps similar to those used for supercharger gear changes would seem more practical)

For take-off purposes (at SL), the V-1710-39 could manage 1325 HP at 2400 RPM wide-open at 50" Hg.

The older V-1710-33 chart has slightly lower power levels across the board (and also lists the supercharger gearing at 8.77:1 rather than 8.8:1; that doesn't seem like a rounding error given Allison tended to give values to 2 decimal points when applicable). I haven't seen details on the overall changes between the C and E/F series V-1710s, but I suppose there were at least some modifications to the supercharger installation. (though the V-1710-39 specification sheet also lists the -33 as having 8.8:1 supercharger gearing)

In any case, the V-1710-33's chart puts it at 1260 hp at 2400 RPM at SL pushing something close to 49.5" Hg.

 

[Shortround6]

The US was a bit ahead of the game on constant speed props. Not only was Hamilton Standard an American company but a large number of US airlines had been using the constant speed props for several years before 1939. In fact over 20 airlines had started using fully feathering props by 1939. You also had Curtiss trying to compete with Hamilton Standard. This goes deep, Hamilton Standard was part of the United Aircraft group which included Pratt Whitney. Boeing at one time was part of United Aircraft but split off in the mid 30s. Vought and Sikorsky were also part of United Aircraft. SO Curtiss Wright was competing against them with airframes, engines and their own Curtiss propellers.

AS far as plying games with propellers on take-off, you might want to think about that very carefully. The whole idea of the "constant" speed propeller or variable pitch was to use the optimum pitch of the blades for each flight condition. For take-off you want a shallow pitch even if high rpm to move the the maximum amount of air. Reducing rpm and increasing pitch in an attempt to use the same engine power results in a steep pitch on the blades. Cutting the rpm to 2400rpm is a 20% reduction in prop rpm and you are going to need a steeper pitch. trouble is at slow speed the angle of attack of the blades would be wrong and the blades are part stalling and part thrashing the air instead of producing thrust.

Those old Schneider trophy racers had fantastic power to weight ratios but lousy take-off performance because the high speed props were such a mismatch. Using a constant speed (or variabe pitch ) helps a lot but you are throwing away part of the advantage if you try this trick. Spitfires with 2 pitch props used fine pitch for take off and 2850rpm. which cut about 100rds form the take-off run of the fixed pitch wood prop planes, which were taking off using a lot less than 2850rpm. At 170ASI and 2000ft the prop was shifted into course pitch where it stayed pretty much until the plane landed. Shifting to high gear dropped the engine rpm to 2070rpm and 6 1/4lbs boost. One can only imagine the power drop.

This not what you are suggesting and a prop than use an infinite number of pitches won't suffer like this but playing games with the rpm and pitch and the wrong air-speeds is NOT going to increase performance.

basic propeller theory says that for transmitting about 1000hp you need a prop about 10-12% bigger in diameter for a 1200rpm prop than for a 1500rpm prop. This may be for fixed pitch props but you get the idea.

 

[kool kitty89]

AS far as plying games with propellers on take-off, you might want to think about that very carefully. The whole idea of the "constant" speed propeller or variable pitch was to use the optimum pitch of the blades for each flight condition. For take-off you want a shallow pitch even if high rpm to move the the maximum amount of air. Reducing rpm and increasing pitch in an attempt to use the same engine power results in a steep pitch on the blades. Cutting the rpm to 2400rpm is a 20% reduction in prop rpm and you are going to need a steeper pitch. trouble is at slow speed the angle of attack of the blades would be wrong and the blades are part stalling and part thrashing the air instead of producing thrust.

Right, prop pitch control is a bit like gearing in an auto transmission (and constant speed props are a bit like using a torque converter in an automatic transmission while 2-pitch propers would be akin to a very coarse 2-speed manual transmission). I overlooked that issue, so the 2400 RPM case would likely only be useful at high airspeeds.

It still seems like dropping to 2800~2850 RPM for take-off with moderately increased manifold pressure might have advantages for take-off, but more likely for climb. (2400 RPM at high power and coarse pitch would probably still be too low for useful prop efficiency in climb, but something in the 2600-2800 high-boost range seems potentially useful depending on the aircraft's best climb speed)

There's also the issue of fuel burn, and when not in combat but still climbing to cruise altitude (let alone cruising) optimal raw performance and maximum thrust isn't as important as thrust (and drag) relative to fuel consumption. The significantly improved efficiency of very low RPM cruise (1600 RPM) combined with high boost and lean mixture conditions proved ideal for maximizing range on the P-38. And while the high boost aspect would vary a good deal more without the turbos, it should still apply at low altitudes (especially for the 9.6:1 engines) as well as pointing to conditions at modest speeds and relatively high torque/coarse pitch propeller settings. (ie not low-power low-RPM at relatively fine pitch)

 

[Koopernic]

The automatic controls on the German engines did a lot more than control pitch, which I believe was done via vacuum from from the Venturi in allied engines rather than a governor. According to the pilots throttle lever demand the controls selected the optimal rpm, pitch for that airspeed. They even controlled rich/lean mixture. In a dive the propellor would automatically feather. It often took a long time to set up an allied fighter for combat or setup for a dive and I've seen an interview with one P38 pilot who expressed great personnel regret that he had lost a bomber he was escorting due to the time taken changing from cruise to combat settings, I got the somewhat sad impression that he was unnecessarily blaming himself. Selecting emergency power did often require operation of a switch such as MW50, GM1 or rich mixture injection into the supercharger.

Obviously allied aircraft improved their controls but it seems only slowly. Supposedly allied pilots thought the could do better in setting up their engines for lean cruise or in precise control in formation flying but I suspect this is a case of boosting their own moral.

 

[kool kitty89]

Obviously allied aircraft improved their controls but it seems only slowly. Supposedly allied pilots thought the could do better in setting up their engines for lean cruise or in precise control in formation flying but I suspect this is a case of boosting their own moral.

Effectively manually leaning out the mixture to effectively improve maximum range and endurance proved true in the Pacific after Lindberg shared his experience with USAAF pilots. (this was at least true in the Pacific, I'm not sure if the same techniques were applied in the ETO)

I know this specifically applied to the P-38's range and the V-1710, but I wonder if similar attempts were made with the P-47, particularly given (like the P-38 ) it could take advantage of the turbocharger for relatively high manifold pressures at very low RPM.

 

[Zipper730]

From what I remember you have a pump which is connected to the engine's shaft, and hydraulic fluid is directed through there and spins it in the process. A stator is located in the middle and better directs the fluid into the turbine, which makes it spin, after which it flows around and back again: By varying the amount of hydraulic fluid, or the spacing between the pump and turbine, one can vary the RPM of the impeller's effective gear-ratio.

This variation was similar to a turbocharger except at higher RPM more horsepower actually *was* taken off the shaft, and there was a smaller gear-ratio range than a turbocharger (similar to a twin-speed supercharger it seemed): I'm curious if there was anyway with the technology of the time to vary this by both varying the amount of fluid *and* increase the spacing between pump and turbine?

Also would it have been beyond the capability of the time to design the stator to vary this? I do remember the USSR developing a swirl-throttle that helped reduce throttling losses to nearly nothing.

S Shortround6 , D Deleted member 68059 , W wuzak

 

[Zipper730]

It would't matter how many gears you used on a Merlin XX/45 supercharger (they were the same for all practical purposes) the inlet/impeller/housing was pretty much maxed out at around 19-20,000ft. (with RAM)

The engine critical altitude was around 16250 right? If that's right then the engine got 2750-3750' of altitude from ram compression. That's actually pretty good for the time.

You can only drive a single stage impeller so fast before the tip speed exceeds the speed of sound in the conditions inside the supercharger (temperature and pressure) and starts up shock waves that interfere with the airflow.

While this might be a slight detour from topic, it's something that I've often been curious about: For propellers and superchargers, it was considered a no-no to have the airflow be supersonic. Then you'd see gas-turbines have tip-velocities that would be supersonic, and root-to-tip conditions at cruise speed, and it supposedly generated superior pressure-ratios than subsonic designs offered.

To make it even better, turbofans also had supersonic fan-stages and they, too, also saw improvements in performance, despite the fact that they acted like propellers.

Forgive me for going a bit off topic, but how do (multiple) turbochargers compare to a 3 speed supercharger (thinking BMW 802 vs BMW P.8011?

I didn't know there were any two-stage turbochargers developed in the war that saw operational use. From what I remember, most turbocharged engine designs had one stage of supercharging and one of turbocharging.

Surely the R-1830 and R-2800 had 3 speed superchargers? LO, HI and Neutral.

Well, not exactly: The main-stage blower only had one speed, and the neutral stage simply had the auxiliary stage unclutched (it didn't spin), with low and high being different speeds.

The Mustang Mk.I/IA did use 8.8:1 supercharged engines (and the Mk.II of course used 9.6) so my previous comments at least still apply there.

I never knew any 9.6's were ever used in the war. I knew the Mustang Mk.II was faster than the Mk.I/IA, but I thought it was due to a change in the radiator design.

 

[MiTasol]

By "multi-speed" I meant more than one speed, not necessarily three-speed.

Do you mean like the late model Allisons with the second supercharger driven by a variable speed drive.

 

[wuzak]

Well, not exactly: The main-stage blower only had one speed, and the neutral stage simply had the auxiliary stage unclutched (it didn't spin), with low and high being different speeds.

Well, 0 Low and High looks like 3 speeds to me.

 

[glennasher]

Which production block of Allisons went to the PT boats, and when did they siphon off those engines to go to the boats? I wonder how much that effected the need for the Merlins to go into P-40s and Mustang B's. Did the US Navy need enough to alter the plans for those aircraft?
That's just a thought, as the Allisons weren't just for aircraft after all.

 

[gjs238]

Which production block of Allisons went to the PT boats, and when did they siphon off those engines to go to the boats? I wonder how much that effected the need for the Merlins to go into P-40s and Mustang B's. Did the US Navy need enough to alter the plans for those aircraft?
That's just a thought, as the Allisons weren't just for aircraft after all.

PT boats didn't use Allisons.
PT boat - Wikipedia