WW2: Why was the 9th Air Force given P51 ins6tead of giving them to the 8th Air Force in 1943? (3 Viewers)

[skyking1]

But "real" RR Merlins were sometimes used in USAAF P-51's. One of my college professors said the P-51 he flew had a RR engine and that they were much smoother than the Packard Merlins because it was a RR practice to sort through the available pistons and select ones that were as close as they could get to the same weight, leading to lower vibration.

The Packard built Merlins were different in numerous ways from the RR version, both due to a need to adapt the design for true mass production and to make some improvements that RR had not gotten around to doing yet.

Spitfire Mk XVI usually had Packard built Merlins.

they likely did that with the connecting rods too. It takes a little time and another operation to do that but it is otherwise a 'free' way to improve the balance overall.

 

[MIflyer]

they likely did that with the connecting rods too. I

True! I guess you have read of the conversation the Packard people had with Stanley Hooker, saying they could not make the V-1650 components to the same tolerances as those used by RR? Hooker replied, "Are the RR tolerances that much tighter than yours?" and the Packard guys replied, "NO! They are TOO LOOSE for us to build engines!" The American way was to build the components to tolerances so tight that it did not matter which parts the used when engines were assembled on a production line. The RR way was to build them to looser tolerances and have their craftsmen hand select them. Packard had to tighten up the tolerances so that they could run a typical automotive style production run.

One interesting thing that occurred was that the RAF followed the usual government practice of buying the airframes and the engines under separate contracts. When they agreed to give the AVG 100 Tomahawk I's, to be replaced by P-40D, they did NOT agree to give them the engines. Faced with no engines for the AVG Tomahawks, Allison stepped up to the plate and provided a solution. They had plenty of non-compliant out-of-tolerance Allison C series V-1710 engine parts. So they got some engineers together and used them to build 100 engines for the AVG out of "left over" parts. They did special machining or added shims and bushings to get the formerly unusable parts in acceptable condition. As a result they produced 100 "blue printed" engines that produced more power while using less fuel than any other C series V-1710. The Flying Tigers confirmed that their Tomahawks were noticeably faster than any other early P-40 around.

A friend of mine who was working with an airline on some RR engine problems in the 1960's said he saw RR custom build fan blade assemblies by making little tweaks to them, even marking them specially.

And the USAF used the TF-41 version of the RR Spey engine in the A-7D, with horrible results. The engine overhaul line at Tinker AFB just dismantled all the engines and sent the parts out for cleaning, inspection, and repair. But the TF-41's had been custom assembled in RR fashion, using match drilled assemblies and just would not go back properly.

 

[MIflyer]

There were a number of changes internally and externally, which required the swap out and replacement of a number of ancillary items if you wanted to change from a R-R Merlin to a Packard Merlin or vice versa.

I read where in early 1944 a P-51B went down, ditching in a French river, when the engine failed. Problem was that the standard spark plugs used by the USAAF were not suitable for the V-1650. I wonder what those spark plug differences were?

 

[ColFord]

I read where in early 1944 a P-51B went down, ditching in a French river, when the engine failed. Problem was that the standard spark plugs used by the USAAF were not suitable for the V-1650. I wonder what those spark plug differences were?

RAF also had a number of losses early on in their experience with the Mustang III (early block P-51B) due to loss of engine power, engine failure after takeoff, in flight engine roughness and other engine related issues. When the RAF did their investigations a range of issues were identified. Majority of the pilots in the RAF Squadrons were converting over from Spitfire V or IX fitted with R-R Merlins and were aware of engine handling requirements. A couple of the early RAF losses were soon after takeoff, with a longer than usual wait by the aircraft taking off, aircraft losing power or engine failure during takeoff run or soon after takeoff. In those cases, on examination of the aircraft wreckage including engine, it was thought that the spark plugs had become fouled - period between engine run up and actual takeoff excessive without opportunity to clear plugs before commencing takeoff run. In a couple of the accidents during approach and landing where extra power was required and engine did not respond or failed, again spark plug fouling or 'over cooling' of engine leading to oil thickening were cited as possible contributing causes. A couple of cases of failing spark plug insulator cores in US supplied spark plugs that came with the engines was identified. They also found on examination of the wiring harnesses on a number of aircraft, issues with some materials - was allied with issues on some aircraft of excessive electrical noise/static on radios. Feedback to NAA, action taken to fix the issues on wiring harnesses and supply of replacements.

Engineers, specialists from Rolls-Royce and NAA were consulted. R-R suggested using different heat range spark plugs, pilots undergo some specific training on proper engine handling for the Packard Merlin, and for a period, more frequent checks by ground crew of spark plugs with records being kept on plug types, condition, type of flying conducted eg short range/local flights vs longer range flights with longer period of economical cruise engine settings. More frequent replacement of spark plugs was also recommended in the interim. A supply of R-R specification/standard plugs being used in R-R Merlin 60 series engines were supplied and largely used to replace the US made spark plugs.

By the time the third of the RAF squadrons start converting over to the Mustang III, entries appear in unit records about specialist parties visiting the airfield to lecture and train pilots and ground crew about special operating and maintenance requirements for the Packard Merlins - this was over and above training that ground crew for example had already done in preparation for the conversion from Spitfire to Mustang III. Pilots in particular were given extra targetted sessions on operating the Packard Merlin for extended periods in economical cruise settings and ways to avoid engine issues.

USAAF at same time had similar issues and were also supplied with R-R Merlin specification spark plugs until new spark plug supply could be sent from US. Issue with the insulators referred back to manufacturers for rectification and monitoring.

Interesting to note that when the RAF got the first of their original Allison engined NA-73 Mustang I aircraft, similar issues with spark plugs and wiring harnesses, engine failures etc happened and were also relatively quickly resolved.

 

[SaparotRob]

They were not. There were a number of changes internally and externally, which required the swap out and replacement of a number of ancillary items if you wanted to change from a R-R Merlin to a Packard Merlin or vice versa. Hence, why the Air Ministry and RAF in their wisdom created a separate designation - Spitfire XVI - for the variant of the Spitfire IX powered by the Packard Merlin 266 rather than the Roll-Royce Merlin and ensured all aircraft within a Squadron were all of the same sub-type IX or XVI. They had separate AM issued Pilot's Manuals which in part for the XVI described the differences in the operation of the engine and a number of the engine related ancillaries compared to the IX. Also all the ground crews on Squadron's equipped with the Spitfire XVI had to undergo specific training on the differences with the Packard built Merlin, and also ensured the supply chain for parts and maintenance tools directed the correct parts and tools to units depending on the version of Merlin fitted.

Similar for Lancasters fitted with Packard Merlins vs R-R Merlins. Different AM Pilot's Notes for the Packard Merlin engined sub-variants, concentration of aircraft in squadrons and also on bases. When the Canadian Lancaster did its visit to the UK some years ago and had an engine problem, they couldn't just 'slap a spare BBMF R-R Merlin on it' as it took longer to sort out all the engine peripherals to make a R-R Merlin work on it as an interim measure so they could complete some of the key commitments in the UK, until they could get a Packard Merlin of the right type for it to make the return flight to Canada at the end of the UK tour. At the time there was a lot of online commentary about why was it taking so long for the BBMF spare to be fitted to it, but the BBMF and Canadian technical people provided some very clear statements about the degree of work to make the BBMF R-R Merlin 'spare' fit and be usable on the Canadian Lancaster which was configured for the Packard Merlin.

Also why the CAC built Mustangs that used the 'recycled' Spitfire VIII R-R Merlin engines, were designated as a different sub-mark of CAC Mustang and required a bit of work by CAC to do all the adaption work to allow the use of the R-R built Merlin in that particular batch of CAC built Mustangs rather than US supplied Packard Merlins (especially by then Lend Lease had finished and to purchase extra engines from the US would have prohibitive in the economy of that time).

If a WW2 USAAF P-51 had a R-R Merlin fitted, it would have been a very rare occurrence, as it meant that aircraft was then outside the normal support and supply chains set up for the USAAF P-51 units.

So, in this instance the MK XVI was essentially a MK IX with the Packard?

 

[ColFord]

So, in this instance the MK XVI was essentially a MK IX with the Packard?

Yes.

Air Ministry for their own administrative reasons, gave the Packard Merlins an engine type designation with a '2' prefix, with the final two digits being the closest R-R Merlin equivalent to the version of the Packard built engine. RAAF followed the British AM practice during the war, but in the immediate post-War period changed their Mustang documentation to list the V-1650-# designation depending on the engine sub-type and configuration fitted to their US supplied 'kit' P-51Ds.

 

[ColFord]

From Air 40/1096 Statistical Summary of 9th Air Force Operations 16 Oct 1943 to 8 May 1945

MTO operations, B-24 June 1942 to September 1943, B-25 August 1942 to August 1943, P-40 September 1942 to August 1943, plus 316th Troop Carrier Group November 1942 to September 1943.

The departure left the 12th Air Force, North West African Strategic Air Force, North West African Tactical Air Force, North West African Coastal Air Force having USAAF combat units.

When the 15th Air Force was formed on 1 November 1943 it had 4 B-17 and 2 B-24 groups, plus 3 B-26 and 1 B-25 groups, the medium bomber groups left end January 1944, as part of the changes that would see the USAAF in the MTO become only the 12th and 15th Air Forces.

9th Air Force declared operational in England on 16 October 1943, taking the B-26 and troop carrier groups from the 8th Air Force.

During 1943 the Overlord commander was assumed to be either Sir Alan Brooke, or George C. Marshall. To give an idea of the situation when the Washington rumour mill heard Marshall was maybe to be appointed to Overlord it was suggested this was because Marshall had fallen foul of FDR and/or Churchill and/or Harry Hopkins, it was a demotion. The decision was made after the Tehran conference, where Stalin had not been pleased Overlord lacked a commander.

Eisenhower officially took command on 7 December 1943 and while he arrived in England in the middle of January 1944 it was another month before the formal directive from the Combined Chiefs was issued to him, since the US and UK were still discussing terms. Meantime the Mediterranean command team settled in and began recasting tactics and plans, most notably Overlord from 3 to 5 beaches. COSSAC had been too junior to ask for what it decided was needed, instead having to work with what it was allocated, the mixture of landing craft promised "bore little of or no relation as to numbers and types to the actual requirements of the proposed operation".

Leigh-Mallory was appointed Allied Expeditionary Air Force commander at the August 1943 Quebec conference, it became public in September, the unit was to be the 2nd Tactical Air force and an American equivalent. no decision made on the heavy bomber forces yet

Early arriving P-51 groups,
339th FG, designated Fighter Bomber Group in August 1943, using P-39 in US
354th FG, using P-39 in US, stayed with 9th Air Force.
357th FG (transfer from 9th), using P-39 in US.
363rd FG, using P-39 in US, stayed with 9th Air Force.

While lots of things would be nice to end June 1943 a total of 22 Merlin Mustangs had been accepted, 91 in July, 170 in August, however deliveries to the USAAF to end June were 15, then 36 in July and 137 in August, deliveries for the RAF were 22 in July and 52 in August. Meaning the USAAF had enough P-51 end August to fully equip 2 fighter groups except many would be held as attrition replacements in the US or be in the process of being shipped to Britain, the British note 3 Mustang III imports for the RAF in September and 74 in October 1943.

According to the USAAF Statistical Digest P-51 began arriving in Britain in September 1943, with 34 by the end of the month.

Does the USAAF Statistical Digest break down the P-51 by model/sub-type received? 35 P-51A were shipped to the UK in August 1943, intended to equip the 107th TRS of the 68th TRG, but 12 lost in transit after a fire on the ship carrying them after a collision with another vessel in fog. The surviving 23 arrived in the UK in late August, but they didn't arrive at the USAAF aircraft reception centre at Renfrew until September 1943.

 

[Engineman]

True! I guess you have read of the conversation the Packard people had with Stanley Hooker, saying they could not make the V-1650 components to the same tolerances as those used by RR? Hooker replied, "Are the RR tolerances that much tighter than yours?" and the Packard guys replied, "NO! They are TOO LOOSE for us to build engines!" The American way was to build the components to tolerances so tight that it did not matter which parts the used when engines were assembled on a production line. The RR way was to build them to looser tolerances and have their craftsmen hand select them. Packard had to tighten up the tolerances so that they could run a typical automotive style production run.

One interesting thing that occurred was that the RAF followed the usual government practice of buying the airframes and the engines under separate contracts. When they agreed to give the AVG 100 Tomahawk I's, to be replaced by P-40D, they did NOT agree to give them the engines. Faced with no engines for the AVG Tomahawks, Allison stepped up to the plate and provided a solution. They had plenty of non-compliant out-of-tolerance Allison C series V-1710 engine parts. So they got some engineers together and used them to build 100 engines for the AVG out of "left over" parts. They did special machining or added shims and bushings to get the formerly unusable parts in acceptable condition. As a result they produced 100 "blue printed" engines that produced more power while using less fuel than any other C series V-1710. The Flying Tigers confirmed that their Tomahawks were noticeably faster than any other early P-40 around.

A friend of mine who was working with an airline on some RR engine problems in the 1960's said he saw RR custom build fan blade assemblies by making little tweaks to them, even marking them specially.

And the USAF used the TF-41 version of the RR Spey engine in the A-7D, with horrible results. The engine overhaul line at Tinker AFB just dismantled all the engines and sent the parts out for cleaning, inspection, and repair. But the TF-41's had been custom assembled in RR fashion, using match drilled assemblies and just would not go back properly.

There was a whole thread about this in 2019. "An interesting read about the Packard built Merlin engine."
It started on 21 Dec 2019 in the section Forums World War II - Aviation - Technical - Engines


Cheers

Eng

 

[Engineman]

Air Ministry for their own administrative reasons, gave the Packard Merlins an engine type designation with a '2' prefix, with the final two digits being the closest R-R Merlin equivalent to the version of the Packard built engine. RAAF followed the British AM practice during the war, but in the immediate post-War period changed their Mustang documentation to list the V-1650-# designation depending on the engine sub-type and configuration fitted to their US supplied 'kit' P-51Ds.

Hi,

The UK Air Ministry did use the earlier Merlin numbering for the Packard built Merlin 28 and Merlin 38, also a little later the Packard built Merlin 68 and 69. As Merlin diversity got more complicated, the Air Min decided to prefix the Packard Merlin engines for "British" built aircraft with the 200 series numbers to suit their purposes. So, you see the later 224, 225, 228 and 266, where the Packard built engines were very close to a UK Merlin type ie, Merlins 24, 25, 28 and 66, (albeit that the 28 was already a Packard built version).
The British use of the USA designation V-1650/ numbers was for US built aircraft with Packard built engines that conformed with the US production.

Cheers

Eng

 

[Engineman]

RAF also had a number of losses early on in their experience with the Mustang III (early block P-51B) due to loss of engine power, engine failure after takeoff, in flight engine roughness and other engine related issues. When the RAF did their investigations a range of issues were identified. Majority of the pilots in the RAF Squadrons were converting over from Spitfire V or IX fitted with R-R Merlins and were aware of engine handling requirements. A couple of the early RAF losses were soon after takeoff, with a longer than usual wait by the aircraft taking off, aircraft losing power or engine failure during takeoff run or soon after takeoff. In those cases, on examination of the aircraft wreckage including engine, it was thought that the spark plugs had become fouled - period between engine run up and actual takeoff excessive without opportunity to clear plugs before commencing takeoff run. In a couple of the accidents during approach and landing where extra power was required and engine did not respond or failed, again spark plug fouling or 'over cooling' of engine leading to oil thickening were cited as possible contributing causes. A couple of cases of failing spark plug insulator cores in US supplied spark plugs that came with the engines was identified. They also found on examination of the wiring harnesses on a number of aircraft, issues with some materials - was allied with issues on some aircraft of excessive electrical noise/static on radios. Feedback to NAA, action taken to fix the issues on wiring harnesses and supply of replacements.

Engineers, specialists from Rolls-Royce and NAA were consulted. R-R suggested using different heat range spark plugs, pilots undergo some specific training on proper engine handling for the Packard Merlin, and for a period, more frequent checks by ground crew of spark plugs with records being kept on plug types, condition, type of flying conducted eg short range/local flights vs longer range flights with longer period of economical cruise engine settings. More frequent replacement of spark plugs was also recommended in the interim. A supply of R-R specification/standard plugs being used in R-R Merlin 60 series engines were supplied and largely used to replace the US made spark plugs.

By the time the third of the RAF squadrons start converting over to the Mustang III, entries appear in unit records about specialist parties visiting the airfield to lecture and train pilots and ground crew about special operating and maintenance requirements for the Packard Merlins - this was over and above training that ground crew for example had already done in preparation for the conversion from Spitfire to Mustang III. Pilots in particular were given extra targetted sessions on operating the Packard Merlin for extended periods in economical cruise settings and ways to avoid engine issues.

USAAF at same time had similar issues and were also supplied with R-R Merlin specification spark plugs until new spark plug supply could be sent from US. Issue with the insulators referred back to manufacturers for rectification and monitoring.

Interesting to note that when the RAF got the first of their original Allison engined NA-73 Mustang I aircraft, similar issues with spark plugs and wiring harnesses, engine failures etc happened and were also relatively quickly resolved.

The problems with Sparkplugs on the developing Merlin 2-stage engine were mostly due to the extra pressure and heat loads on the plugs at higher Boost levels. Additionally, the high lead load of the high-Boost fuels gave a fouling problem, especially with the harder plugs.
The increasing pressure and heat loads at higher Boost made gas leakage within the ceramic core insulator of the plugs a critical problem. Hot combustion gas leakage would cause the plug to become overheated and cause pre-ignition/detonation. The plug could also burn out the ignition harness/HT wire with the hot gas internal leakage. Such internal gas leakage was a severe problem as Boost levels rose above 16lb/in and sparkplug development was a real issue. It seems each increase in Boost ratings was met with sparkplug development issues.
The problems with harder plugs to allow correct function at higher Boost was partly that the plugs could then run too cool at low power, allowing fouling of the electrodes as the plug was not self cleaning. These problems were partly cured by regular plug cleaning and also by the operation of the engine at high power at regular intervals to self-clean the plugs after running at low power. Plug technology also eventually improved the heat-range that the plugs could self-clean at. In the case of the Mustang using the early V-1650-/3 it was found that the RC5/2 plugs were needed for the engine but, the "Air Corps" needed the latest British RC5/5 plugs for Packard testing at 18lb Boost and this was a problem in early 1943.
Sparkplugs were not the only culprits, early operational experience with the Mustang III found that the fuel mixture control was too rich in Auto-rich in FS gear. However, RR proved that Auto-lean was satisfactory for all operations and the control was changed to delete the Auto-rich position. Unfortunately, it was also found that the mixture control had no gate on the Cut-Off position and this may have been a factor in some after take-off incidents where the pilot accidently moved the selector to cut-off when moving the RPM selector lever back after take-off. This was cured by introducing a gate on the Mixture control lever.

These details mostly from the RRHT Book Rolls-Royce and the Mustang, by David Birch.

Cheers

Eng