Mustang with the early Merlins

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tomo pauk

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Apr 3, 2008

In order not to clog the current thread

Merlin marks I've had in mind, and the possible V-1710 equivalents/ballparks: III (-39), X, XII (-81); XX and 45 (the -81 is a somewhat worse). Also the -73 and -87 vs. the 45M. Note that the US engines are, historically, lagging in the timeline.
Aircraft equivalents:
P-51/Mustang I (Merlin III replaces the -39)
A-36 (Merlin 45M instead of the -87)
P-51A/Mustang II (Merlin XII (in '40) or 45 (in '41) instead of the -81)

We can leave, for the moment, the X and XX for the bombers and Hurricanes, respectively.

Two main cases of use: interceptor, and long-range fighter.
 
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tomo pauk said:
P-51/Mustang I (Merlin III replaces the -39)
A-36 (Merlin 45M instead of the -87)
P-51A/Mustang II (Merlin XII (in '40) or 45 (in '41) instead of the -81)
Click to expand...
Might I suggest
P-51/Mustang I (Merlin XII replaces the -39)
A-36 (Merlin 30/32 instead of the -87)
P-51A/Mustang II (Merlin 45 (in '41) instead of the -81)
The Merlin III was on it's way out in the summer of 1940 (from the point of view of production. RR had moved on. The MK XII was the last gasp of the older style supercharger and they starting to make the MK XX in the late summer.
Just using the XII numbers just saves us from trying to figure out when each increase in performance they allowed when in 1940 for the Merlin III.

tomo pauk said:
We can leave, for the moment, the X and XX for the bombers and Hurricanes, respectively.
Click to expand...
Not sure why, the British stuck the Merlin XX in the Hurricane to help with the climb/ceiling.
British Merlin XX/20 series was only 30-60lbs heavier than the single speed engines. Granted you have to cool it but if you are going to higher boost for very long you have to figure out how to cool the single speed engines too.
tomo pauk said:
Two main cases of use: interceptor, and long-range fighter.
Click to expand...
I would be a little hesitant on the long range fighter. Just from a range perspective even with a pair of US 75 gal drop tanks you may be limited to 450miles. Except that you will be cruising at lower altitudes than the two stage engines or pushing the engines a bit harder. This is also best case and with no weather allowance.
The Rhine and western Germany may be within reach, but going a lot further?
rear fuselage tanks went in after the engine gained about 250lbs and the prop gained over 100lbs. Some of this may be offset somewhat by the bigger radiator/cooling system. But trying to stick an 85 US gallon rear tank in a single stage Merlin powered Mustang may not work well.
A lot depends on the defending German fighters.
 
Much better than my listing above.

Availability in 1940 might be a problem, and for 1941 the 45 gives almost anything needed. But if we want to have an even greater fighter for 1940, the Merlin XX-powered Mustang is hard to beat.

The main problem is the RAF/AM doctrine, that has no 1-engined LR fighters in it
German fighters defending are the ~350 mph types in 1939-40, 375 mph in 1st half of 1941, and 375-400 mph after mid-1941. The altitude where these fighters excelled was also increasing, talk 4-5 km before mid-BoB, 5-6 km after that, and also much improving at the lowe altitudes with the Fw 190 included.
The 1-stage powered Mustang does not to have to have the 85 US gal tank, already the 29 imp gal (35 US gals) rear fuselage tank gives perhaps another 200 miles of cruise range, at 7 mpg. Such a mustang will be getting the better mileage than the historical Merlin Mustang due to the lower weight, lower power (and a lower drag?).

Granted, if the RAF really wants a proper LR Mustang, all the bits and pieces were available already in 1941. Should've give them a very fast Ki-61, years before that aircraft materialized.
 
In 1940, the Merlin XX is becoming available. This gives you a two-speed supercharger, which is better than what the Allison had. The Mustang gets decent performance at medium altitudes.

The two-speed superchargers were judged to not fit in the available Spitfires. The Merlin XX was installed in Hurricanes, and the Packard equivalent in some P-40s. The Merlin 45 was Stanley Hooker's nifty new impeller with a single speed drive. The Spitfires sacrificed low altitude performance to get medium altitude performance. The Merlin 45M had the "cropped"* impeller for maximum performance at low altitude for aircraft that cannot fit the two-speed supercharger -- Spitfires, Seafires, and Fairey Barracudas for some reason or other. If the two-speed supercharger fits in a Mustang, you use the two-speed supercharger. The "cropped" supercharger Merlins put out something like 1600hp as long as you stayed below 6,000ft. All Allison engines had the equivalent of a "cropped" impeller. How much better would a Merlin 32 or 45M be? Stanley Hooker's basic expertise was supersonic airflow.

The Luftwaffe suffered disaster in the WTO because they had single-stage superchargers. The P-47 Thunderbolts had two-supercharger stages -- the turbocharger and the engine's internal supercharger. The B-17s and B-24s also were turbocharged, so combat took place at 30,000ft, definitely the P-47's happy place. The Merlin Mustangs had the two-speed two-stage supercharged Merlins, extending the range of suicidal combat all the way to Berlin.

Much of the allied success later in the war was due to having killed off the experienced Luftwaffe pilots in 1943, fighting them at altitudes their aircraft were not suitable for. If the Mustangs do not have that superiority at altitude, can they fly to Berlin and take acceptable casualties?

* Single speed Merlin impellers turned at 28,000rpm. "Cropping" makes it sound like they took a nibbler to it. The air at the outside of the impeller was supersonic. The structural integrity of the impeller must have been an engineering challenge. Centrifugal force and the resulting vibration is a function of rpm squared.
 
Howard Gibson said:
In 1940, the Merlin XX is becoming available. This gives you a two-speed supercharger, which is better than what the Allison had. The Mustang gets decent performance at medium altitudes.
Click to expand...
Merlin's impeller being bigger than on the V-1710 was a key thing, ditto for the impeller turning faster (9.50 times the crankshaft speed in high gear for the Mk.XX, and just 8.80 for the V-1710-39 and the like).

Howard Gibson said:
The two-speed superchargers were judged to not fit in the available Spitfires.
Click to expand...
Howard Gibson said:
The Merlin 45M had the "cropped"* impeller for maximum performance at low altitude for aircraft that cannot fit the two-speed supercharger -- Spitfires, Seafires, and Fairey Barracudas for some reason or other.
Click to expand...

They did fit. Even the 2-stage 2-speed superchargers did fit. Even the 2-stage 2-speed Griffons did fit.
Ability of the factories to make Mk.XX was probably what put the brake on the idea of the Mk.XX powered Spitfire, and the need that Hurricane is made competitive didn't helped here.

tl;dr, the myth that 2-speed supercharged Merlins will not fit on the Spitfire should've been buried alt least 50 years ago.

Be it as it was, the thread is about the early Merlins, ie. no 2-stage Merlins do apply.

There is no requirement for the Mustang to fly and duke it out above Berlin in this thread.
 
Howard Gibson said:
The two-speed superchargers were judged to not fit in the available Spitfires.
Click to expand...
And yet, strangely enough, there are reports/accounts of about 50 Spitfire IIs being refitted with Merlin II engines late in the war for Air/Sea rescue duties. From Wiki
"Once the Mk II was taken out of front line service, 50 of them were converted for air-sea rescue work, at first under the designation Mk IIc (type 375) but later referred to as the ASR Mk II. The Merlin XII was replaced by the Mark XX, a "rescue pack" was fitted in the flare chute and smoke marker bombs were carried under the port wing."

This not fitting thing may have been "cover" for the fact that the Spitfire II was still considered considered combat capable while a Hurricane with the Merlin XII may not have been judged to be capable. The Hurricane needed all the help it could get in late 1940 and 1941. A better Spitfire could not be built in numbers to make up for worse Hurricanes.
tomo pauk said:
the myth that 2-speed supercharged Merlins will not fit on the Spitfire should've been buried alt least 50 years ago.
Click to expand...
As shown above, it is not just factory "modified/redesigned" Spitfires, It seems to have been fairly easy to do even on the MK IIs.
 
The Mark_III Spitfire was to take the Merlin_XX. This never made it into production. The Merlin 45 was the old single-speed drive with Stanley Hooker's new more efficient impeller. They needed to upgrade Spitfires immediately, so they used the Merlin_45 and they converted the Mark_II Spitfires into the Mark_Vs, and the Spitfire_IIIs never reached production. Eventually, they stuffed two-speed two-stage supercharged Griffon engines in Spitfires. Meanwhile, the Spitfire_Vs and all the Merlin Seafires struggled with single-speed superchargers. I don't understand why the Fairey Barracuda used the single-speed Merlins. They were designed for the Rolls Royce Exe engines. The two-speed superchargers ought have fit.

In any case, whether or not a two-speed supercharger fits in a Spitfire is not important here beyond the fact that the Merlin 32/45/45M/55/55Ms were intended for Spitfires and Seafires. If I am sticking a Merlin into a Mustang, I want at least two supercharger speeds, and I would prefer two stages as well.

We can compare the Allison P-40s with Merlin P-40s. The Merlin P-40s had better performance at altitude. Otherwise, they were not a whole lot faster.

A really good discussion of Merlins can be found in The Seafire, by David Brown. The low altitude Seafire LIIs and LIIIs were little hot rods below 9,000ft. The "cropped" Merlins hit their critical altitudes at 6,000ft. Even the Allisons could manage 12,000ft.
 
Howard Gibson said:
The "cropped" Merlins hit their critical altitudes at 6,000ft. Even the Allisons could manage 12,000ft.
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A lot depends on what you are measuring (or how?)
One test of a Spitfire VB with a Merlin 50 (cropped) shows it holding 18.2lbs (66 +inches) of boost to 3800ft in a climb at 5900ft in level flight. So close enough. Except that the plane will hold 13.4lbs (just under 57in) in climb to 8000ft and 10,000ft in level flight.
The -81 Allison engine in a P-40N could hold 57in (max official WEP) at 10,550ft in level flight and 8,000ft in a climb. Maybe the Allison did a little bit better as the ceiling climbed higher.
There doesn't seem to be a lot difference in the engines? Some yes (Allison is 3.6% bigger and uses a bit more compression ratio).
Cropped Merlins were allowed high boost level which really helped at low altitude. Unfortunately if we try comparing the the P-40N to the Cropped Spitfire the P-40 is about 1000lbs heavier so climb is going to be way off. This P-40 is one of the early ones with 4 guns, restricted ammo, one fuel tank missing and even the electric starter motor taken out so it is rather useless for comparing performance.
The Allison engines that were boasting the high manifold pressures at low altitudes were the earlier engines with the lower supercharger gears, they are going to fall off quicker at altitude than the -81 engine.
 
I am looking at speed tables. An Allison engined P-39D hits its critical altitude at 12,000ft. A Seafire_LIIIC its it at 6,000ft. Boost pressures are tricky because they are limited by the available fuel. The Seafire is just as fast as a Spitfire_IX up to 6,000ft while using 18psi boost. It is running on 130_octane fuel, almost certainly not available to the P-39 in 1942. I cannot find boost pressure for the P-39. High octane fuel and/or methanol water injection allow higher boost pressures, which result in lower critical supercharger altitudes.
 
P-39 shows that looking at speed results can be flawed. P-40s with basically the same engine always had critical altitude higher than the P-39. In large part because the air intake behind the canopy was not as efficient. The P-39 was faster due to it's smaller size and lower drag but it hit it's peak speed up to several thousand ft lower. P-39 also introduces a number of problems (as do 1940-41 P-40s). The -35 engines used in the P-39 were rated at 42in (6lbs of boost) at 12,000ft but this is without RAM. There also different backfire screens fitted at times which screws things up a little bit. In the Fall of 1942 when the US Army authorized WEP they were allowed to use 56in (13lbs) at 4300ft. What the pilots did may have been different. For the P-39s and P-40s there were a few problems. The engines got new crankcases and new crankshafts in late 1941 and early 1942. The -63 engines in the P-39K & L got the stronger parts and were tested to higher limits. Same supercharger and gears. Same critical altitude, or within 200ft. This can really change depending on source but the changes are always small, except that the WEP rating went to 1580hp at 2500ft using 60in (15lbs). The later engines with the 9.60 supercharger gear were restricted to 57in but they could old that to 9,500ft. The US got 100/130 fuel in 1942. The question/s may be when in 1942 and in what theaters. The US often did not specify any fuel grade except 100 octane on some of their charts/instruction manuals. You need to look up the actual fuel specifications like "AN-VV-F-781 amendment 5" to really figure out fuel they are talking about ( P-39K In May 1942)
By the end of 1942 they were on the 3rd specification (at least) for 100/130 fuel. All 100/130 fuel was far from the same. The 3rd big change was the 2nd time they had changed the allowable lead content. They also changed how much of certain other additives could be used. But changing the specs doesn't change the fuel in tankers or at the airbases for a number of weeks or months. The mechanics and pilots just knew it as 100 octane or 100 grade to differentiate it from 87 or 91 octane. Nobody was driving around a tank truck with AN-VV-F-781 amend. 5 painted on the side
Howard Gibson said:
High octane fuel and/or methanol water injection allow higher boost pressures, which result in lower critical supercharger altitudes.
Click to expand...
That is true but no Allisons used water injection except the ones in the P-63s. The US started with it's own 100 octane fuel in 1940. It was 100/100 but the last could vary a bit.
 
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