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

[gjs238]

Much has been discussed here of multi-stage superchargers, but less of multi-speed single-stage superchargers.

The DB engines used a hydraulic fluid coupling with a barometric control to achieve multi-speed drive with a seamless power curve.
Other engines featured multiple speeds, kinda like a stick/manual/standard shift transmission in a car, with a jagged power curve.
The V-1710 was stuck with one speed.

- Wasn't one reason for using the V-1650 in the P-40 because it had a 2-speed drive?
- Could an excellent multi-speed drive somewhat mitigate the effects of having only one stage?
- Which engines and aircraft had multi-speed drives? Which had only single speed?
- Whatever else comes to mind

 

[Shortround6]

- Wasn't one reason for using the V-1650 in the P-40 because it had a 2-speed drive?
- Could an excellent multi-speed drive somewhat mitigate the effects of having only one stage?

In 1941 (Summer/Fall) the Merlin had a better supercharger than the Allison, period. Packard was building the Merlin XX, not the single speed Merlin 45 so that is what the P-40 got. Advantage of the Merlin XX over the Merlin 45 was it gave 1280hp for take off instead of 1185hp at the same boost and rpm. Difference at altitude was only few dozen HP or around 1000 ft. The most marked difference between a plane equipped with a Merlin XX instead of a Merlin 45 is going to be at the lower altitudes where the Merlin XX offers around 100hp more than the Merlin 45 while in low gear.

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) It was flowing all the air it could at as high a pressure as it could while maintaining decent efficiency. Using a 3 speed drive might give you a bit more take-off power and take out the worst of the "dip" between low gear and high gear it wasn't going to do much, if anything, for altitude.

A two-stage supercharger does NOTHING for very low altitude performance as any even mediocre single stage supercharger can deliver all the manifold pressure an engine can stand at sea level. SO a multi speed (more than 2?) single stage doesn't do anything for low altitudes and doesn't do anything for altitudes over the 16-20,000ft range (or less depending on how good the single stage supercharger is) leaving your extra gears (over two) very little to do on a single stage engine.

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. It doesn't matter how excellent (or how bad) the supercharger drive mechanism is. The two are not related.

You will find that the vast majority of superchargers (and engines) at the beginning of the war had critical altitudes of around 11-14,000ft. This is due in large part to the fuel. with 87 octane there is only so much boost you can use (or how much you can compress the air in the supercharger) even at 12,000ft before detonation sets in. US 100 octane helped but US 100 octane was also only 100 octane when running rich so while it may have helped point the way to needing better superchargers ( Like the P&W two stage or the turbos) but it didn't allow for the over 5 to 1 pressure ratio used by the Melrin 60/61 two stage.
The basic superchargers needed to be improved (better inlets, better impellers, better guide vane set ups and better diffusers) to get better altitude performance before fooling around more complicated supercharger drives.

 

[tomo pauk]

...
- Wasn't one reason for using the V-1650 in the P-40 because it had a 2-speed drive?

Nope, that was not the reason. Main reason was redundancy - USAAF was not 100% sure that Allison will be able to supply all these V-1710s needed for P-38/39/40 in numbers needed. In 1940, the Merlin was a known quantity, the V-1710 less so - that was an insurance in case the V-1710 turned to be a lemon, or a dead end when it is about further development. For same reason, the P-44/47B were started, so not all the eggs are in one basket, engine-wise.

- Could an excellent multi-speed drive somewhat mitigate the effects of having only one stage?

In case the V-1710 is slated for bombers, the 2-speed drive would've helped. Otherwise, no much gain IMO.

- Which engines and aircraft had multi-speed drives? Which had only single speed?

Single speed S/C drive - vast majority of the Mikulin's engines, Italian radial engines, Bristol Mercury Taurus, also some Hercules engines, Merlin I/II/III/VIII/XII/30/45-50 (and 'M' versions), Hispano X12/Y12.

- Whatever else comes to mind

Want a V-1710 to perform above 20000 ft? Crank up the work on the 2-stage variant (and stick it on the P-51 for starters), or design an aircraft with turbo in mind.

 

[Shortround6]

I would add as far as the P-40 goes that it was the only logical choice. The Original contract for the Merlin engine with Packard was for 9,000 engines of which the British were to get 6,000 and the US 3,000. They had to go in something and they sure weren't going to work in P-39s. Much as members here fantasize about P-38s with Merlin engines that wasn't going to happen either in early 1942. A a pair of engines offering 1100hp at 20,000ft (Merlin XXs ) or a pair of engines offering 1150hp at 25,000ft (turbo Allison's). Please remember it took around 6 months from Prototype P-40F to first production example. Messing around with the P-38 in the fall of 1941 could have meant hundreds fewer P-38s built in 1942.
Leaves the P-40 to use up the Merlins by default.

 

[SpicyJuan11]

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?

 

[gjs238]

No multi-speed single-stage R-1830 or R-2800?

 

[Shortround6]

You had two speed R-1830s and two speed R-2800s. what is the 3rd gear going to do?

The engine in the early B-26 used an 11in impeller and used a 10.0:1 high gear. If I am doing the math right that means the impeller tip speed was 1247fps. The impeller tip speed in a Merlin XX (V-1650-1) was 1272fps. 2% faster?

Military rating was 1500hp at 14,000ft, and you can't spin the impeller much faster. At the 2400rpm max continuous (or climb?) the critical altitudes were 7500ft in low gear (1500hp) and 13,000ft in high gear (1450hp). A 3rd gear would smooth out the dip at 10,000ft?

R-1830s also used an 11in impeller but I have no idea if it used the same number of blades or if it was even near the same thickness. ( thicker means more volume per revolution of the impeller) high gear was usually 8. 47. and most R-1830s were good for 2700rpm.

Late war R-1830s were rated at 1350hp for take-off at 2800rpm and used the same gear ratios and same diameter impeller, I don't know if there were other changes. High gear Military rating went from 1050hp at 13,100ft to 1100hp at 13700ft with the impeller spinning 847rpm faster due to the higher rpm limit. Most of the power was coming from the extra rpm. Part of the R-1830s altitude problem came from the fact that it was using 48in (9lbs boost) for it's take-off and low altitude (low gear) military ratings. It needed more boost to get similar power to the Allison. Low gear was used to get better take-off performance but a single stage R-1830 needed more than spinning the impeller faster to make power at altitude. Impeller tip speed was 1097fpm.
Power needed by the supercharger is proportional to the sq of the tip speed. An impeller running at 1250fpm tip speed needs just under 30% more power than one running at 1100fpm, every thing else being equal.

 

[gjs238]

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

 

[kool kitty89]

I would add as far as the P-40 goes that it was the only logical choice. The Original contract for the Merlin engine with Packard was for 9,000 engines of which the British were to get 6,000 and the US 3,000. They had to go in something and they sure weren't going to work in P-39s. Much as members here fantasize about P-38s with Merlin engines that wasn't going to happen either in early 1942. A a pair of engines offering 1100hp at 20,000ft (Merlin XXs ) or a pair of engines offering 1150hp at 25,000ft (turbo Allison's). Please remember it took around 6 months from Prototype P-40F to first production example. Messing around with the P-38 in the fall of 1941 could have meant hundreds fewer P-38s built in 1942.
Leaves the P-40 to use up the Merlins by default.

The only other sensible option would have been the Mustang, and for that to work, the USAAF would have needed to take interest sooner AND North American would have to put emphasis on adopting the V-1650 from the start (at very least in parallel with the Allison powered prototypes). With the existing British interest in that design, importing a British (or Canadian) Merlin XX for prototype expedience may have made sense as well.

 

[gjs238]

I would add as far as the P-40 goes that it was the only logical choice. The Original contract for the Merlin engine with Packard was for 9,000 engines of which the British were to get 6,000 and the US 3,000. They had to go in something and they sure weren't going to work in P-39s. Much as members here fantasize about P-38s with Merlin engines that wasn't going to happen either in early 1942. A a pair of engines offering 1100hp at 20,000ft (Merlin XXs ) or a pair of engines offering 1150hp at 25,000ft (turbo Allison's). Please remember it took around 6 months from Prototype P-40F to first production example. Messing around with the P-38 in the fall of 1941 could have meant hundreds fewer P-38s built in 1942.
Leaves the P-40 to use up the Merlins by default.

Or the P-51?
"Messing around" with the P-51 may have been less risky than with the P-40 (and with hindsight, more rewarding.)

 

[Shortround6]

With at least the first 620 Mustangs being built to British orders and being paid for by the British cash, not lend lease, the Americans had very little interest in powering British aircraft with engines form the American allotment of the production total. What the British choose to do with their first 6,000 Packard built Merlins was up to them. Please note it took until April of 1943 to complete the first 9,000 Packard Merlins. I don't know if the US actually took ALL 3,000 but in the Spring, Summer and Fall of 1941 when allocations and production planning for 1942 was going on The Mustang was hardly a blip on the American plans. The first flight (work started when?) of the XP-40F was before a production P-40D was delivered to the Air Force and was, in fact less than 2 months after the last production P-40C was built and less than 2 months after the the first P-40D (prototype?) flies. It is also about 7 weeks before the first XP-51 shows up and Wright Field to be ignored.
So even if Wright Field HAD flown the P-51 extensively in the first week they had it, decided it was the greatest thing since sliced bread and cold beer combined AND figured out the advantage of Sticking a Merlin XX engine in it you are about 4-6 months behind the P-40F in timing. Without commandeering even more of the British aircraft The Americans aren't going to get any P-51s with either engine until the Summer/Fall of 1942. First P-51A (first lend lease aircraft) is delivered in July of 1942. First flight of a P-51 (NA-91) took place back on May 29th 1942 BUT please note that the order for the P-51s (lend lease) was placed on July 7th, 1941, 8 days after the XP-40F first flew.

Now I may have mixed up a date in their (or two?) but I hope you get the idea. Orders were placed and plans of engine (and other resources) allocations were made months if not a year before the aircraft began to come out of the factory doors, let alone make it to service squadrons. Some last minute changes might be made but the people in charge were very hesitant in case something went wrong and one factory or another was left with dozens if not hundreds of airframes waiting for engines. In 1941 the Army was rationing Allisons at times to the 3 manufacturers that needed them and lets remember that in 1943 North American early production of P-51Bs easily out stripped the supply of V-1650-3 engines for a few months.

 

[tomo pauk]

The British can choose to give the Mustang/Merlin a chance, once both Packard and NAA deals are signed. Just like they provided the Merlin XX for the 1st prototype of the P-40F.
Based on experiences of the BoB, they can add two and two together, and conclude that a V-1710 powered Mustang (with 1150 HP at 12000 ft) will not cut it as good as Merlin powered one (1150 HP at 18500 ft) at altitude. The USAF can test the resulting aircraft and then decide it is worth their attention.
The production of 1-stage Packard Merlins amounted to 4850 pcs in second half of 1942, and ~4500 pcs in 1st half of 1943.

Other, non-Merlin options (with caveat that Curtiss produces 'A-40', so fighter funds can be re-allocated to NAA):
- push for Mustang with 9.60:1 supercharged V-1710 instead of A-36
- two stage V-1710 for Mustang

 

[Shortround6]

Other, non-Merlin options (with caveat that Curtiss produces 'A-40', so fighter funds can be re-allocated to NAA):
- push for Mustang with 9.60:1 supercharged V-1710 instead of A-36
- two stage V-1710 for Mustang

Uh, time line check, P-51As got the Allison V-1710-81 with the 9.60 gears, They just don't show up until march of 1943. They were ordered in June of 1942. Now here is the 'funny' thing. P-40s didn't get the Allison V-1710-81 with the 9.60 gears until the M model (deliveries start in Dec of 1942) and the N models, N-1 is the "stripper" and doesn't show up until surprise, March of 1943.
You can play all the 3 card Monte you want with trying to change engines around between P-51s, A-36s, P-40s and A-40s but the 9.60 engines don't show up until the winter of 1942/43. Mustangs got them only a few months after the P-40s.

 

[tomo pauk]

The AHT lists both P-39 and P-40 with 'faster' superchargers 1st delivered in November 1942 (the P-51A indeed 1st delivered in March 1943), a month after the A-36 is 1st delivered. The A-36 are in N. Africa in April 1943 - so the 'early P-51A' should enter the combat in May 1943, instead in September 1943?

 

[Shortround6]

We have a bit of confusion as to built and/or delivered and/or delivered where. We especially have a bit of a problem with getting aircraft to specific war zones.
1. A Factory can a roll a plane out the door and declare it "built".
2. A plane can be test flown (or acceptance flight) at the factory airfield and upon being signed off on (accepted by government representative) declared "delivered" to the Army or Navy.
3. A plane can be declared "delivered" when it arrives at an Army or Navy depot for any additional work required or for issue to using unit. Once things got going planes rarely went directly from the factory to the service squadron.
4. For delivery of Curtiss and Bell aircraft vs North American Aircraft to the Med or to Britain you have the voyage through the Panama canal just to get to the Atlantic ocean. Figure 3-4 weeks minimum extra travel time if not more (several weeks) depending on convoy schedules. It's either that or "knock down" the plane (remove wing) crate them and ship cross country by train to Atlantic Port.
Now if you are sending planes to the South Pacific the Mustangs should have had an advantage

 

[tomo pauk]

I'm trying to use the 'A-36 rule of the thumb' - counting from the 1st delivered example, it took 6 months for them to be in a war theater (North Africa). With 'early P-51A' 1st delivered in Nov 1942, we add 6 months for them to be in a war theater (be it North Africa, or, maybe UK - the distance from California is similar)- meaning it is May 1943.

 

[Shortround6]

are you really going to get enough to make any difference to the war, not just the Mustang record book?

In the Oct-Dec of 1942 Bell was building just under 300 P-39s a month. Curtiss was building 360-404 P-40s a month. It took NA from Oct to March to build 500 A-36s. NA never exceeded 86 Mustangs a month until April of 1943. Perhaps they could have gained some speed had not the production lines been disrupted by the A-36 (fitting dive brakes and such) but it would take a while to build up any meaningful number of 9.60 gear Mustangs. 2-3 squadrons per month?

 

[kool kitty89]

Unless the 9.6:1 Allison engines enter production sooner than they did historically, wouldn't the V-1650-1 still be the better bet time wise? This applies both to Mustang I/IAs and A-36s (and the few dozen P-51/F-6As taken from the Mustang I production block). Of course, British deliveries would be taking engines otherwise earmarked for British ordered Kittyhawks or Canadian Hurricanes.

 

[tomo pauk]

Agreed all the way.

Re. post # 18:
The NAA produced 86 Mustangs for the 1st time in April 1942, 68 was produced in Dec 1941, 84 next month - a bigger monthly production than the F4F that has 1 year of headstart in production. All of those Mustangs are for the RAF.
What NAA needs is a signed contract with USAF (along with provision for prioritized items/materials), pronto - 8th Dec 1941?, so both NAA and USA can devote more resources for the production of P-51. That way we'd see the increase in production, like it was case for P-38/39/40, whose production doubled from late 1941 to late 1942. The production of F4F went into triple digits in mid 1942, for example.
So we'd see the Mustang production making maybe 150 pcs monthly by the end of 1942, when P-39 was going to 300 pcs (already in Aug 1942 it was 306), the complicated P-38 at ~150, the F4F at almost 200. Even the P-47 was produced in 142 pcs in Dec 1942, despite a bit later start of production than P-51, but 3 factories started producing the Jug by then.

are you really going to get enough to make any difference to the war, not just the Mustang record book?

Better/longer ranged fighter coverage of Italian possessions in mid/late 1943? Earlier introduction of second (and third?) source for Mustang? Freeing more P-38s for ETO and Pacific? Hammering home the fact that P-47 needs both a better drop tank facility and more internal fuel, earlier than historically? No P-63? Earlier increase of internal fuel for mainstream Spitfire variants and the Tempest?

 

[wuzak]

You had two speed R-1830s and two speed R-2800s. what is the 3rd gear going to do?

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