An Allison V-1710 with a two-speed supercharger in 1942 (10 Viewers)

[Steamed_Banana]

The Americans had working turbochargers. There was not all that much pressure to get fancy superchargers on R2800s.

High altitude carrier aircraft were not really required until the Germans showed up with FritzXs in 1943. Even so, the Royal Navy did not install two-stage Merlins in Seafires. Two-stage Griffon Seafires were post war.

FritzX and Hs 293 were used successfully a few times during the invasion of Sicily, launched from Do 217s, but these were all rapidly shot down before they had any real effect, and they weren't being launched from any kind of stratospheric altitude. So I'm not sure it's anything say, Corsairs would have trouble dealing with. I can double check what in fact did shoot them down as this is all in Shores MAW series.

 

[tomo pauk]

To add to my previous post here, there is a lot of manuals for aircraft, engines and whatnot on this very website. Specifically on the engines: click here, and about the power charts and tables: click here.
The avialogs.com - another go-to site - is currently under overhaul, but there is a copy of that site here.

 

[Howard Gibson]

FritzX and Hs 293 were used successfully a few times during the invasion of Sicily, launched from Do 217s, but these were all rapidly shot down before they had any real effect, and they weren't being launched from any kind of stratospheric altitude. So I'm not sure it's anything say, Corsairs would have trouble dealing with. I can double check what in fact did shoot them down as this is all in Shores MAW series.

I did not mention the Hs293 because it was dropped from low altitude, well within the reach of a Seafire_LFIII. The FritzX was dropped from around 20,000ft, which was within the reach of a Corsair. The Germans were working on high altitude bombers. Intercepting a Ju388 at 30,000ft would have been a challenge for most Corsairs.

In the ETO, the Royal Navy operated within the range of land based, two-stage supercharged aircraft like Spitfires and Mustangs.

 

[MIflyer]

US Navy both requested and funded the 2-stage superchargers' development at P&W and Wright.

The USAAC wanted an improved P-35 and as result Seversky produced the XP-41 with a two stage R-1830-19, which was flown to Wright Field in March 1939. Terrified by the Y1B-17 and its turbosupercharged engines that could even outrun the wonderful new Brewster F2A-1, and at 10,000 ft higher, the USN had Grumman rework the XF4F-2 into the XF4F-3, also using a two stage engine, XR-1830-76, it came out in March 1939. So the USN and USAAC were neck and neck in specifying better high altitude performance with two stage mechanically supercharged engines.

But Maj Seversky wanted a better P-35 as well, and so at company expense produced the AP-4, almost identical to the the XP-41 but using a turbo as the first stage of supercharging. And it was the performance of that airplane that led to the USAAC ordering the P-43.

Meanwhile the USN, recognizing that the two stage supercharged F4F was ready to go, became the first service in the world to introduce such high altitude fighters into service.

 

[tomo pauk]

The USAAC wanted an improved P-35 and as result Seversky produced the XP-41 with a two stage R-1830-19, which was flown to Wright Field in March 1939. Terrified by the Y1B-17 and its turbosupercharged engines that could even outrun the wonderful new Brewster F2A-1, and at 10,000 ft higher, the USN had Grumman rework the XF4F-2 into the XF4F-3, also using a two stage engine, XR-1830-76, it came out in March 1939. So the USN and USAAC were neck and neck in specifying better high altitude performance with two stage mechanically supercharged engines.

Navy supported the development of the 2-stage engines at P&W already by 1934, erstwhile for the R-1535 engine. Efforts were redirected to the 2-stage supercharged R-1830 shortly after.
If was rather easy for the Army to ask for such the engine by late 1930s, because someone else actually supported it the 4-5 lean years predating the Army interest.

 

[Shortround6]

P & W may have gone to the two stage supercharger because their single stage supercharger was so bad?
Textbooks of the time said that a two stage supercharger could give the same amount of compression as a single stage supercharger and do it for less power into the superchargers and less temperature rise in the supercharger system.
This assumes that the supercharger compressors are of similar efficiency's and that two 2:1 compressors are going to be more efficient than a single 4:1 compressor.

 

[tomo pauk]

P & W may have gone to the two stage supercharger because their single stage supercharger was so bad?

That is also my understanding.

To be fair, the other people making radial engines in the 1930s also rarely 'landed' on a (very) efficient supercharger either. Probably because of the reasons of packaging? Trying to keep the engine as compact as possible, they often made the intake elbows & passages to narrow, and/or too convoluted - all of the things that mess badly with airflow, and thus with the efficiency of the S/C set-up and with the the altitude power.
Also, the radials of the time (bar what Wright was installing on their engines?) were with straight blades on their impeller, while the V12s moved to the blades with curved or parabolic vanes on the impellers.

 

[Shortround6]

Things were changing very fast in the 1930s and the octane scale for fuel is nowhere near accurate,
70 octane is 48.28 PN
77 oct = 54.90 PN
82 oct = 60.87 PN
87 oct = 68.29 PN
90 oct = 73.68 PN
93 oct = 80.00 PN
96 oct = 87.50 PN
98 oct = 93.33 PN
100oct = 100 PN
The PN scale is more linear.

In the early 30s with low PN fuel nobody could use much boost. Put that together with air cooled engines running hotter than liquid cooled engines and not being able to use as much boost before they ran into trouble (things got better in the late 30s but still not equal) and the supercharger design itself was not usually the weak link. Until it was with fuel going into the high 70s PN/low 90s octane lean and the British pushing things with the Aromatics and rich response. Now a bad supercharger design was a weak link. But very few people knew what a good supercharger design was. And a lot of engines were not designed to use a lot of boost in the cylinders (more pressure) without breaking mechanical parts or overheating.
Sticking a Merlin 45 supercharger on a Hispano 12 Y engine might have gotten you another 1000meters or more altitude but even with 100/130 fuel it would not have been a 1300-1400hp engine at any altitude. Something would have broken.
Air cooled engines, on average, were even worse.

Making superchargers that were efficient (65-75%) at providing 3 times or more compression ratio required fuel that would not detonate at those pressure levels.

 

[Steamed_Banana]

Come on guys, I don't think many of us are petrochemical engineers, what is "PN"

 

[Jugman]

Come on guys, I don't think many of us are petrochemical engineers, what is "PN"

Performance Number.
1PN = 1% the knock value of 100 octane of whatever octane scale one is using. So 100PN = 100 octane. 130PN = a 30% increase in knock rating over 100 octane.

 

[Shortround6]

Performance number.

100 octane is fuel that is equal to 100 % iso-octane and 0% N-heptane.
92 octane fuel acts like a fuel that would be 92% iso-octane and 8% N-heptane.
you can blend in other stuff and you can add stuff, like tetraethyllead (lead for short).
But you can't have more than 100 % octane.
You can try to use things like "100 octane + 2 cc's of lead" but that gets awkward.
Using PN for numbers above 100 octane was just easier.

 

[Steamed_Banana]

So what, based on that, is the PN of 100/120 or 100/130 or 100/150 fuel?

 

[MIflyer]

This assumes that the supercharger compressors are of similar efficiency's and that two 2:1 compressors are going to be more efficient than a single 4:1 compressor.

And therefore assumes that there is going to be some kind of heat removal from the charge, inter-or after- cooling or both. Obviously, you can be worse off by heating up the charge too much.

I think that few people indeed realize just how challenging such cooling can be, especially when considering the drag that can be associated with it. But then again I clearly took too much thermodynamics in college. In any case Sir Stanley Hooker's solution for the two stage Merlin had to be exceptionally brilliant because Nobody Else seems to have thought of it.

And as for octane ratings, note that 73 octane was the "normal" aircraft engine fuel in the late 1930's. P&W built the R-2000 version of the R-1830 to ensure equal performance even when using lower octane gasoline.

 

[tomo pauk]

So what, based on that, is the PN of 100/120 or 100/130 or 100/150 fuel?

The very common 100/130 fuel has the performance numbers of 100 for lean mixture, and 130 for rich mixture. 100/150 means 100 PN for lean mixture, 150 PN for rich mixture.

 

[BarnOwlLover]

Since that report on the RAF running Allisons to 72" Hg on their recon Mustangs came from 1943, I don't think that the 150 octane/PN fuel was out yet (wouldn't be for several months to a year from when that report was published). I do believe also that the Merlin could be run to similar boost levels on 100/130 (this was done on dynos/test benches in 1942), but as pointed out in the same report, the Merlins were taking a serious durability hit at the time.

The Allison could run that boost and still run to 1500 hours between rebuilds due to bearing wear issues, on the Merlin at the time, that was typically 500-600 ours on similar boost levels. Also, many racing and warbird Merlins also use Allison connecting rods.

 

[Steamed_Banana]

It's funny, the durability of the Allison V-1710 is legendary and seems widely attested, but I recently read two memoirs by pilots who flew in 450 Sqn RAAF in the Med (one British, one Aussie) and their chief complaint about the P-40 was the Allison engines being highly subject to catastrophic breakdown, usually due to bearing failure. Both guys made a big point of this, one of them even includes a song they sung about it in the squadron. I was wondering if that unit maybe got a lot of rebuilt / refurbished engines from a local workshop, or was the Allison more susceptible to tropical conditions than we assume. Worth pointing out that both of these guys were mostly flying P-40K which had the strengthened V-1710-73.

All these engines are basically high performance racing engines, more or less, maybe a little more robust, but very temperamental. Maintenance was hard to keep to the high standard that was needed to keep them running properly, especially in remote tropical war zones where conditions ranged from bad to abysmal. Reading the operational histories, when a dozen fighters took off for a mission, it was normal for one or two to immediately turn back with engine trouble. Sometimes 3 or 4 or even 5 of them. And this was true for just about all fighter types of every nation.

 

[MiTasol]

The Allison engines had three different types of bearings during their production life with the last one entering production in very early 42. Rolls used those bearings under licence from Allison.

These guys might have been flying on engines built with the earlier bearings. Another factor to consider is that no engine likes going to take off power with oil that is not up to operating temperatures (the oil is too thick to flow properly, causes the pressure relief valve to open thus reducing flow to the bearings) and if they were on scramble defensive ops then that may have been all or part of the problem. Likewise all engines do not like to operate with oil that is too hot and therefore too thin to lubricate and remove heat. Long taxi will do that on any engine, liquid or air cooled.

Remember also that the P-38s had a high engine failure rate when operated under the original USAAF instructions and that problem only went away when units started following Lockheed and Allison's recommended procedures. I have never heard of P-40s having that issue but I would also expect that whatever the USAAF said for one aircraft with a specific type of engine they would also say with any other aircraft with the same series engine.

 

[Steamed_Banana]

I only ever saw references to it really with this one unit, but it's an interesting mystery.

 

[don4331]

OK, Occasionally, I don't ask the right questions initially, but the voice in the back of my head keeps prodding me until I do:'

The right question is what is limiting power down low:

RR Merlin III was capable of 1,440 hp (Sea Hurricane w/16psi boost & 100 octane fuel) - that the strength of the engine (based on RAF 100 hr TBO). However with 87 octane fuel, RR has to limit the engine to 6-¼ lb. boost (42.6" HgA) from sea level to 16k'. Where power peaked at 1,030 hp. As the boost control mechanism was limiting power to 6-¼ lb., power for take off was reduced - 880hp...

So, the Merlin X with 2 speed supercharger allows for reduced power going into the supercharger = more power for take off/initial climb.

But the Allison V-1710 is a different beast: Selecting the -39 found in the Mustang Mk.1, the engine can only make 1,150hp (based on USAAF 150 hr TBO). And the boost control mechanism for the Allison is different from the RR one. It allows 45.5" HgA for take off = 1,150hp then boost is decrease as altitude increases to 44.6" HgA at 11,700', but the 1,150hp is maintained. (Above 11,700' the supercharge may not longer create enough boost to maintain the 1,150hp).

So, a 2 speed supercharger on a Mustang Mk. 1 buys us nothing. (It actually hurts us with extra weight and complexity). Allison needs to find the other components which are limiting power and strengthen them. Only after the engine can accept more power (While passing the USAAF 150 hr TBO test) is there an opportunity to take advantage of a 2 speed supercharger.

 

[Steamed_Banana]

OK, Occasionally, I don't ask the right questions initially, but the voice in the back of my head keeps prodding me until I do:'

The right question is what is limiting power down low:

RR Merlin III was capable of 1,440 hp (Sea Hurricane w/16psi boost & 100 octane fuel) - that the strength of the engine (based on RAF 100 hr TBO). However with 87 octane fuel, RR has to limit the engine to 6-¼ lb. boost (42.6" HgA) from sea level to 16k'. Where power peaked at 1,030 hp. As the boost control mechanism was limiting power to 6-¼ lb., power for take off was reduced - 880hp...

So, the Merlin X with 2 speed supercharger allows for reduced power going into the supercharger = more power for take off/initial climb.

But the Allison V-1710 is a different beast: Selecting the -39 found in the Mustang Mk.1, the engine can only make 1,150hp (based on USAAF 150 hr TBO). And the boost control mechanism for the Allison is different from the RR one. It allows 45.5" HgA for take off = 1,150hp then boost is decrease as altitude increases to 44.6" HgA at 11,700', but the 1,150hp is maintained. (Above 11,700' the supercharge may not longer create enough boost to maintain the 1,150hp).

So, a 2 speed supercharger on a Mustang Mk. 1 buys us nothing. (It actually hurts us with extra weight and complexity). Allison needs to find the other components which are limiting power and strengthen them. Only after the engine can accept more power (While passing the USAAF 150 hr TBO test) is there an opportunity to take advantage of a 2 speed supercharger.

The British got that V-1710-39 on that original Allison got up to 72" Hg, which produced well over 1600 + hp just by overboosting, presumably with high octane fuel. The 45.5" Hg limit in the original manual was apparently low-balling it quite a bit. The US manuals for P-40s, P-39s, and P-51As were later revised to allow 54, 56, 57, or 60" Hg (depending on when and the specific sub-variant) as a high setting for WEP etc.

See this war-memo on the subject

E-GEH-16