Allison and Merlin in a P-51

[wuzak]

The Army Cooperation Command did not exist when the Mustang was first ordered. It started on December 1st 1940, according to Wiki.

Pretty hard to order an aircraft for a command that didn't exist.

 

[ColFord]

For the most recent, probably one of the best researched and currently the most accurate books on the early development of the Mustang, including how the British came to order the Mustang from North American Aviation, I can commend "P-51B Mustang: North American's Bastard Stepchild that Saved the Eighth Air Force" by Bill Marshall and Lowell Ford, Osprey Publishing, UK, 2020. Bill is a member of this forum and engaged with a lot of Mustang researchers and operators globally in researching this book, and a number of members of this forum also are noted in the acknowledgements. Bill and Lowell went back to the basics, original period source documentation, including a lot of material from the Boeing Archives, who hold the original North American Aviation documentation quoted in the book, as well as a deep dive into the USAF Archives.

The North American Mustang Mk.I was ordered as a fighter. It ended up, in some ways, via a set of circumstances to be the right aircraft available at the time when it eventually arrived in the UK to meet an emerging need for a modern, high performance, single seat, fighter type aircraft to be used in the fighter reconnaissance role by the Squadrons in Army Co-operation Command. They had already proven the fighter reconnaissance concept via use of Curtiss Tomahawks, but the serviceability and performance of the Tomahawks left something to be desired and the Mustang looked to be a way better option. (Or another way to look at it is that there was great pressure coming from the War Office for the RAF to get dive bombers - the Stuka fixation - and orders had been placed for a range of dive bomber types such as the Vultee Vengeance and Brewster Bermuda, which it had been proposed to equip the RAF ACC Squadrons with, and that would not have ended well..................). In some ways there were politics at play, and Fighter Command, passing the Mustang on to Army Co-operation Command and being presented to the War Office as "ACC being given the most advanced US fighter produced to date to arrive in the UK" (which it was at the time) went a long way to stifling any negative comments from the War Office about the RAF not giving ACC suitable aircraft to operate. (There are a heap of files in UK Archives covering the procurement of the Mustang and the deliberations and inter-service politics and inter-command politics of how the Mustangs ended up with ACC). Fighter Command also at that time had a certain fixation with high altitude performance for fighters based around potential German use of high altitude bombers with pressurised cabins for the crew to conduct a bombing campaign against the UK, plus high rate of climb for the point interceptor home defence fighter role. So more than just Fighter Command experience from the BoB at play in their assessment of the Mustang.

By VE-Day, there were FOUR RAF Squadrons operating the North American Mustang as one of their primary types.

No.516 Squadron was using an enlarged flight of Mustang Mk.I aircraft in a non-operational role as was No.285 (AA Support) Squadron.
No.516 provided the air component, particularly artillery and naval gunfire direction support to the Joint Operations Training School at Dundonald in Scotland.
No.285 (AA Support) Squadron provided target aircraft for AA practice using Mustang Mk.I in March to July 1945 at Weston Zoyland.

No.26 Squadron had re-equipped with the Mustang Mk.I in January 1945, having operated the Spitfire Vb since early 1944, and were flying operationally providing naval gunnery direction to units of the RN Home Fleet and Free French Navy against German hold out ports along the French, Belgian and Dutch Coasts. They re-equipped with Spitfires again when they joined BAFO in June 1945.

No.268 Squadron RAF was equipped with Mustang Mk.IA and Mk.II aircraft and was using them operationally up to and beyond VE-Day. On VE-Day they were in the process of re-equipping with the Spitfire FR.XIVe, and had a mix of both types used on operations. No.268 Squadron relinquished the last of its operational Mustang Mk.II aircraft (reluctantly) in late August 1945.

A couple of Mustang Mk.I and Mk.II aircraft continued in use until early 1946, mainly attached to Group Support Units and various Training Establishments, primarily used as personal "hacks" for senior officers with those units.

Of the Allison engine variants of the Mustang, the RAF used operationally the Mustang Mk.I, Mustang Mk.IA, Mustang Mk.II and A-36A

Squadrons to use the Mustang Mk.IA: No.II(AC), No.63, No.168, No.170, No.268
Squadrons to use the Mustang Mk.II: No.II(AC), No.268.

No.225 Squadron RAF initially used Mustang Mk.I aircraft in the UK, leaving their Mustangs there when the unit was posted to the MTO and re-equipped with Hurricanes and Spitfires in late 1942. They 'borrowed' for a short period commencing in April 1943, four P-51 aircraft (equivalent to the Mustang Mk.IA) from the USAAF to give them a longer-range capability until Spitfires with fittings for long range slipper tanks arrived.

No.14 Squadron RAF also borrowed some P-51s (equivalent to the Mustang Mk.IA) aircraft from the USAAF which they used for about 4 weeks in 1943 to do some long range recce work, primarily into areas where the units usual Martin B-26 Marauders had been encountering regular enemy fighter opposition and a number of losses had occurred.

No.1437 Strategic Reconnaissance Flight borrowed six A-36A Mustangs from the USAAF in the MTO which were used in a fighter reconnaissance role in the absence of other suitable aircraft from RAF sources in the MTO at the time. After a period of heavy use, the surviving A-36s were passed on and used by a number of RAF P-40 units to assist them in conversion training getting ready to re-equip with Merlin engine Mustang Mk.III aircraft.

One A-36A flown in trials in the UK by the Aeroplane and Armament Experimental Establishment.

 

[emu27]

Not really. The command by name didn't exist at the time but the role did. Do you think that the RAF suddenly decided in the 1st​ December 1941 to form a new command. The command formation was the result of events that had already passed under the bridge, when formation of the command was proffered would have obviously been on a preceding date. The completion of the Dunkirk evacuation on the 4th​ June 1940 no doubt had an impact on the thinking of the RAF, among everyone else, as to the way forward. Aircraft with the ability to strafe, rocket or bomb European targets would have been high on the list, also with the ability to fight the Bf 109 and FW 190. The P-40 had shown it lacked the ability and was withdraw on arrival of the -51.

The RAF had squadrons fulfilling the Army Cooperation role long before the formation of the command, the Westland Lysander entered RAF squadrons to fulfill the Army Cooperation role in June 1938 for example. It is a role that aircraft have been fulfilling since WWI at least. In my time of combat the army cooperation was provided by helicopter, F-4, F-5, A-37, A-1, F-100, in Iraq and Afganistan it was F-14, F-18, Harrier, helicopter etc etc.

In the fifth month (1st​ February 1940) of the war Britain realised its need for US fighters, not for Fighter Command as the US didn't have any fighters that could match the Spitfire at the time. What they needed was an aircraft of greater range than the Spitfire, an ability available US aircraft had, and take on the low altitude role. Over 1,000 fighter were deemed necessary by 1941 to provide assets to it forces around the world.

It wasn't until 25th​ March 1940 that the US government Aircraft Policy allowing the Allies to purchase aircraft was completed, with permission being given to the Allies on the 10th​ April 1940, preceding that date, 25th​ February 1940, Dutch Kindelberger had been asked by the British if he could take on building P-40's. In March Dutch asked Ed Schmued if he wanted to build P-40's. Ed replied that he could design a better aircraft than that and Dutch asked him to come up with a multitude of details for a new type that he could take to England in two weeks time for presentation. Sir Henry Self signed a letter of intent on 11th​ April for 400 aircraft. Cost was not to exceed $40,000, lend lease was to arrive later, estimated cost was $37,590.45 per aircraft.

First flight of the aircraft was 26th​ October 1940, first production 23rd​ April 1941, first flight in England 11th​ November 1941, first allotted to 26 Army Cooperation Squadron January 1942, first operational sortie 10th​ May 1942.

 

[Reluctant Poster]

Every engine has strengths and weaknesses:

As noted, a Merlin supercharger was mounted remotely to an Allison and basically performed the same. However the supercharger failed shortly into the test, the exact cause was never confirmed but the speculation was that by being remote it wasn't kept up to same temperatures that it would have it was directly mounted to the crankcase.

Early aircraft engines used brass floats to control the fuel level in the carburetor – rising to stop fuel flow, sinking to allow fuel in. If the floars weren't properly soldered shut or developed leaks in service, they would sink and fuel would flow overflow the carb. With an updraft carb, the excess fuel simply falls to the ground. With a downdraft carb, it usually flows somewhere bad – into the intake flooding the engine, over the red hot exhaust causing fires, etc.

The American auto industry, with engines like the flat head v-8, were making better downdraft carbs. And let's not forget Allison is a subsidiary of GMC, an automotive company.

Aside: Peregrine and Vulture, RR latest 2 engines pre-War and the post war Merlin 130s were downdraft. Downdraft is a better configuration, but sometimes you have to stick with what is working.

Allison will have marginally better economy as it has ~10% higher compression ratio (CR). The Merlin with 10% lower CR, can build ~10% higher boost. The higher boost comes at a cost as it takes more power to drive the supercharger. But the end result is the Merlin more than makes up for the 60 in^3 it is short on displacement with more power.

The engineers at Allison developed very sophisticated intake manifolds that "rammed" air into the cylinders at optimum rpm. The engine was also developed for turbocharging. The combination of the 2 meant the camshaft timing was fairly mild allowing the engine to run smoothly at less than 50% power, again allowing it to be more economical.

The engineers at RR, concentrated on a better supercharger and used a "sewer pipe" to deliver air to the cylinders. As the Merlin wasn't designed for supercharging, they used pretty radical cam timing (Merlin idling sounds angry and when at operating speed mean). The use of radical cam timing meant the Merlin was prone to backfiring. As RR didn't have a fancy ram air manifold, they had not issue installing "flame traps" in intake ports to stop the issue. (Flame trap is a number of thin plates which stop the backfire from entering the engine).

For the Allison engines in the P-82 had 2 stage superchargers and more radical cam timing. The result was backfires, which without the flame traps, went through the entire intake, intercooler and supercharger. (Think AA Fueler blower explosion). Allison engineers didn't want to give up their fancy intake manifolds for flame traps, so Allison got a bad rap.

As you might recall from high school chemistry, it takes a huge amount of energy to boil water*. RR engineers used this fact by allowing very limited, very local boiling in their engines around the exhaust valves. Once mixed with coolant in the engine, the water vapour is return to water again but the mixture is very hot. As the mixture is very hot, you need a smaller radiator because there is greater temperature difference with outside air. So, Merlin gets away with less than ½ the cooling water of the Allison.

It's also why you can't turbocharge a Merlin – the exhaust is running so close to thermal limits that the extra heat from restricting exhaust exit melts the head.

Early P-38s issue was Lockheed didn't anticipate flying for extended periods of time at high altitude. The decision to use the outer wing leading edge as an intercooler appeared to be stroke of genius. But unfortunately, in conditions that 8th​ AF found themselves in, planes were flying at cruise speed for hours in -30° cold. And the load on the turbocharger wasn't enough to have them creating enough warm air to justify the intercooling but there was no way to limit it. The carb/engine on the Allison wasn't designed for operating in that environment for extended periods.

Adding to the problem, the leading edge intercooler didn't have enough cooling for full power, and the available fuel, while it met the 100 octane requirement lean, didn't meet requirement under boost at rich setting. Add in some sluggish controls as engines are out in wings, oil used in controls congealed in cold and you get engine failures.

Westland had similar issues with the hydraulic throttle on the Whirlwind – great idea, you don't need to worry about cables changing length as plane flexes, etc. Unfortunately, the oil got thick at altitude and pilots couldn't pull throttle back after going full power, resulting in blown up Peregrines. As a result, the engines get a rap for bad altitude performance. But it wasn't the RR engine, it was the implementation.

For the Allison, the late P-38J and L is the sweet spot, not too much power to cause the backfire issue, enough and controllable intercoolers, so as keep engine working like top. Unfortunately, twins use 2X the fuel as single, so why use up your limited logistics when the P-51 is right there. Same thing happens in Korea – P51 is used as ground attack plane even though P-47s and F4Us are available and better planes for that role as the R-2800 burns 2X the fuel as the Merlin.

Typhoon wing and engine both weren't good at attitude. But it was a beast down low.

I do wonder why Allison/GE never pushed for a turbocharged Allison Mustang. If you remove the fuselage tank, there would be room for the turbo. Routing of intake and exhaust wouldn't be any different from Thunderbolt in concept.

*While I am saying water, I know it is a water/glycol mix.

The ramming manifold was a myth. The lengths were too short. The name seems to have come from the shape resembling a rams horns.
The leading edge intercooler was under sized for the task. Air is a crappy medium for heat exchange, you need lots of surface area. That is why the chin type HE was a matrix. Also for good heat exchange you need turbulence. The leading edge is a poor choice for heat exchange as the flow is laminar.
When the P-38s stared flying long range escorts missions with the 8th AF in Nov 1943 the heat exchanger was the chin type.
The Typhon wing was crap at all altitudes, the Tempest was far faster on the same HP. Camm didn't believe the aerodynamicist until too late for the Typhoon.
The bad fuel myth has been discussed ad nauseum. The P-38 was using the same 100/130 made in the USA fuel as the rest of the 8th AF.
The Allison didn't use float carbs by the time except for very early models. It used the Bendix pressure carb like most US engines of WWII. Packard used Bendix pressure carbs and Rolls Royce switched to Bendix for their Merlin 66 and later 2 stage engines.
The Allison was not developed for turbocharging. It was the first models were naturally aspirated engines for airships.
The maybe room for a turbo in the back of a P51 but there is no room to get the exhaust duct to the back and the air duct to the front. Thats is the reason the P-47 fuselage is huge. Compare its profile to a F4U.

 

[Shortround6]

The Allison may well have been developed with turbocharging in mind.

However the amount of turbocharging (pressure going into the carb) or total boost in the manifolds is certainly subject to question.

It was certainly NOT anywhere near what was used even in 1940-41.

The first build of one of the V-1710-A1 (some of these early engines were rebuilt several times) used unrecorded boost or no boost and ran on 80 octane fuel.
The 2nd build used 34in of MAP and also use 87 octane fuel.
Allison engines during the 30s generally used 91-92 octane fuel.

The V-1710-B1R used no supercharger and higher compression in the cylinders and used 83 octane fuel.

There was a lot of playing around with different cylinder compressions and different gear ratios on the engine supercharger and there even a couple of different supercharger impeller diameters.

Please remember that the several score of Curtiss Conqueror engines that were turbo-ed in the early/mid 30s had NO engine driven supercharger (single stage using the turbo) and the intent at the time was preserve sea level performance to somewhere between 15,000-20,000ft.

All the turbo was supposed to do was maintain sea level air pressure at the carb intake. There wasn't supposed to be any additional strain on the engine.

 

[pbehn]

The RAF was only formed in 1918, prior to that the Royal Flying Corps was basically an army cooperation force. Dowding and Park came from fighter backgrounds but Leigh Mallory did bombing recon and photographic missions.

The RAF tried the P-40 in the Army Cooperation role, it wasnt good enough and was needed elsewhere. The P-51/Mustang was just designed to be a good fighter better than the P-40, it wasnt designed for an army cooperation role just as it wasnt designed as a long range escort.

 

[Shortround6]

The French and British Hawk 81s (Tomahawks) were before/during the Battle of France.

The French first order for 230 planes was placed Oct 9th 1939 but first delivery was June 6th 1940.
The Anglo-French purchasing commission placed a letter of intent for 500 planes, 285 to France and 215 to Britain before France fell.

These planes were purchased/ordered as first line fighter aircraft.

However by they time they were being delivered in quantity the BoB had been fought and the daylight part was over. With the invasion scare receding the use in first line squadrons of planes without self sealing tanks or armor (or much armor) was no longer needed and the Tomahawks were sent to the army co-operation squadrons.

How they managed the all important duties of message pick up with a hook and string or ferrying high ranking officers to staff meetings has not been revealed.

The British experienced quite a bit of trouble with the cowl .50 cal guns and if they jammed the Tomahawk was left with about 1/2 the firepower of an eight gun Spitfire or Hurricane.

The British had a lot of trouble with Allison engines in the early Tomahawks but the later ones seem to have been much better.

They went into service with the British army co-operation squadrons anywhere for a year to a year and half after being ordered so where they wound up is not a good indication of what mission they were ordered for.

 

[ThomasP]

Hey don4331,

re "As the Merlin wasn't designed for supercharging, they used pretty radical cam timing (Merlin idling sounds angry and when at operating speed mean)."

The Merlin was designed for compatibility with supercharging from the start. The PV12/Merlin prototype was rated at 740 BHP at 12,000 ft. It was intended to be the next step up in power from the Kestrel.

re "As you might recall from high school chemistry, it takes a huge amount of energy to boil water*. RR engineers used this fact by allowing very limited, very local boiling in their engines around the exhaust valves. Once mixed with coolant in the engine, the water vapour is return to water again but the mixture is very hot. As the mixture is very hot, you need a smaller radiator because there is greater temperature difference with outside air. So, Merlin gets away with less than ½ the cooling water of the Allison."

All well designed engine cooling systems, that are run at the limits of the temperature of vaporization for the coolant, have very limited local boiling in the engine. The level of pressurization of the system will determine how much can be allowed - a higher pressurization will allow more effective control of the amount of vaporization duration. However, if the coolant was boiling/vaporizing around the exhaust valves any more than it was around any other high performance engine's exhaust valves it was not intentional, and would be unwanted. That kind of vaporization is what causes engines to overheat in critical areas, leading to thermal and structural failure.

Also, the Merlin did not use 1/2 the coolant the the Allison did. Some coolant system capacities are listed below:

Allison engined airframes, 97% glycol coolant
P-38E_____ 25.0 USgal per engine (the unusually large volume of coolant is due to the distance between the radiators and the engine)
P-39N_____16.0 USgal
P-40E_____15.5 USgal
P-51A_____20.4 USgal (some sources say 19 USgal)
A-36A_____20.4 USgal

Merlin engined airframes,
Battle Mk I________ 18.0 USgal 100% glycol
Hurricane Mk I_____18.0 USgal 100% glycol
Spitfire Mk II_______16.2 USgal 70%/30% water/glycol mix
Mosquito Mk I-VI__ 18.6-19.2 USgal 70%/30% water/glycol mix
P-51B/C/D________16.7 USgal (+4.8 USgal for aftercooler) 70%/30% water/glycol mix

 

[ColFord]

How they managed the all important duties of message pick up with a hook and string or ferrying high ranking officers to staff meetings has not been revealed.

Simples, the RAF ACC Squadrons when they were re-equipped with the Curtiss Tomahawk, each retained two or three Lysanders to perform the tasks that the single seat Tomahwak could not. It was also a reason why not all RAF Squadrons were re-equipped with the Tomahawk. Some like No.16 Squadron and No.309 (Polish) Squadron stayed with the Lysander until the Mustangs arrived and went directly from Lysander to Mustang without operating the Tomahawk. The ACC Tomahawk Squadrons did still practice and use in exercises with the Army such old school arcane skills as message dropping using a weighted bag with streamers attached which certainly had its origins in WW1. Also why a number of the ACC Squadrons and the Wings to which they were attached in this timeframe were given communications aircraft, types like the Percival Proctor, d.h. Dominie and even some impressed small civilian 'tourer' type aircraft to ferry high rankers around.

If people want to dive into Royal Air Force Operations and Strategy, from the RFC in WW1 through the interwar years and currently up until 1943, I can commend the series of currently five books written by Greg Baughen. He has taken the path of going back to the original files held in the UK Archives, RAFHB, RAFM and multiple other sources including personal papers and memoirs of many of the key players. From this research he has produced an insightful analysis of the policies, strategies, politics (at the Government, Service and inter-Service levels) and especially the personalities (with their individual backgrounds and biases) that shaped the direction of the RAF over those years.

 

[pbehn]

Simples, the RAF ACC Squadrons when they were re-equipped with the Curtiss Tomahawk, each retained two or three Lysanders to perform the tasks that the single seat Tomahwak could not. It was also a reason why not all RAF Squadrons were re-equipped with the Tomahawk. Some like No.16 Squadron and No.309 (Polish) Squadron stayed with the Lysander until the Mustangs arrived and went directly from Lysander to Mustang without operating the Tomahawk. The ACC Tomahawk Squadrons did still practice and use in exercises with the Army such old school arcane skills as message dropping using a weighted bag with streamers attached which certainly had its origins in WW1. Also why a number of the ACC Squadrons and the Wings to which they were attached in this timeframe were given communications aircraft, types like the Percival Proctor, d.h. Dominie and even some impressed small civilian 'tourer' type aircraft to ferry high rankers around.

If people want to dive into Royal Air Force Operations and Strategy, from the RFC in WW1 through the interwar years and currently up until 1943, I can commend the series of currently five books written by Greg Baughen. He has taken the path of going back to the original files held in the UK Archives, RAFHB, RAFM and multiple other sources including personal papers and memoirs of many of the key players. From this research he has produced an insightful analysis of the policies, strategies, politics (at the Government, Service and inter-Service levels) and especially the personalities (with their individual backgrounds and biases) that shaped the direction of the RAF over those years.

Thankfully the RAF didnt add dropping off and picking up spies and operatives into the Mustang design brief. The role of army cooperation and tactical recon changed with the fall of France. It changed again after D-Day when the Spitfire Mk XIV was found to be a good aeroplane. An army involved in a fight on a land front has an interest in what is going on behind the front, but is most interested day to day in what is were a few miles away, half of Spitfire Mk XIVs carried a camera.

 

[Shortround6]

I have two of Greg Baughen books now. Most are available on Amazon/Kindle.

The one on the Battle makes for some depressing reading and may have a few passages that are misleading. As we are going over in another thread the dynamics of "maneuverability" are complicated.

 

[Shortround6]

Thankfully the RAF didnt add dropping off and picking up spies and operatives into the Mustang design brief

Why would they?
They were buying fighters in the Spring/summer of 1940, NA said they could build a better fighter than the P-40. The British said "go for it, you have 120 days".
The state of the art in fighters and fighter engines changed between the summer of 1940 and the fall of 1941 When Mustang Is were coming of the production line, let alone April of 1942 when they are issued to a service squadron.

The role of army cooperation and tactical recon changed with the fall of France.

It changed in British service. It had already changed (or perhaps never changed?) in some other services that always expected at least part of their Air Forces to take part in the land battle. Some of them may not have been very good at it but they hadn't ruled it out.

 

[ColFord]

It changed again after D-Day when the Spitfire Mk XIV was found to be a good aeroplane. An army involved in a fight on a land front has an interest in what is going on behind the front, but is most interested day to day in what is were a few miles away, half of Spitfire Mk XIVs carried a camera.

Which is why the RAF study in 1944 - after D-Day - concluded that the best replacement for the Allison Mustang for the Tac/R role with the RAF, was another Allison Mustang. In particular they wanted what was essentially a Mustang Mk.II, with the 4 x 20mm cannon armament of the Mk.IA, fitted with a Malcolm Hood, and fitted with twin oblique and one vertical camera mount. Given that they could not get what they wanted as NAA had closed the Allison engine Mustang production line when Merlin engine Mustang production commenced they had to look at alternatives.

Out of the potential aircraft types put forward they considered a Mustang III with a 'de-rated' engine for better low level performance. They actually had experience Tac/R pilots fly trials on the Mustang Mk.III as part of that assessment. But they couldn't get any of those as Fighter Command had the priority on those.

The Hawker Typhoon FR.1b was tried in operational service and proved wanting, especially its lack of range , engine vibration effecting the cameras and handling at low level.

The Spitfire FR.IX was an interim solution due to the lack of replacement Allison engine Mustangs, and it suffered due to lack of range and capability.

The Spitfire FR.XIVe was the next best alternative, but it lacked a number of features that were considered to be 'essential' for the Tac/R role, including limited range on internal fuel. Some of the identified shortcomings would get covered off in the Spitfire FR.XVIII which followed later.

That is why, up until effectively 1 January 1944, the few remaining Allison engine Mustangs, a mix of very elderly and well worn by then Mustang Mk.I, along with Mustang Mk.IA and Mk.II were kept going and equipped half the Tac/R Squadrons in 2TAF. There were instance in late 1944 and early 1945 when as the Tac/R Wings of 2TAF moved forward onto various ALGs, there were periods where ALL the Tac/R operations for a number of days had to be flown by the remaining Mustangs because the cross wind on the sole runway on the ALGs prevented the Spitfire FR.IX and FR.XIV from operating - even the Mustangs found the conditions difficult, but they could still operate.

Tac/R did not cover what was just immediately each side of the front line by a couple of miles, but the enemy's rear areas, including areas where he may have reinforcements, supply dumps, transportation nodes, and you also needed to cover the areas where you might be fighting in a week, a few weeks and months from now. It is also why Allison engine Mustang Mk.IA and Mk.II were still operational on front line Tac/R sorties up to and beyond VE-Day.

The state of the art in fighters and fighter engines changed between the summer of 1940 and the fall of 1941 When Mustang Is were coming of the production line, let alone April of 1942 when they are issued to a service squadron.

First Mustang Mk.I issued to No.26 Squadron RAF, 9 January 1942. The Squadron CO had been test flying a Mustang Mk.I since mid-December 1941. A lot of the delay between No.26 Squadron getting the first of their Mustang Mk.I in January 1942 and onwards is around the rate that aircraft were being reassembled after arrival in the UK, then getting the priorities for the identified modifications from the service trials at A&AEE and AFDU, with the wrangling that then went on about issuing unmodified Mustangs to Squadrons and who would be responsible for modifying them once issued to the RAF. Politics between Ministry of Aircraft Production, Ministry of Supply and Air Ministry, plus time taken to consult with NAA and Allison about some of the issues identified from the trials and early use by No.26 Squadron and identify the solutions to those issues. Also determine was it something the responsibility of NAA or Allison to fix (via their contractors in the UK) or was it a MAP, MoS or AM responsibility. By the beginning of April 1942 the solutions to the issues were in train, who was responsible for what had been sorted, manufacturing of parts for the required modifications had been given the go ahead, and the orders were given for the aircraft to be issued to Squadrons - initially unmodified aircraft so Squadrons could commence conversion onto the type. It then took a few weeks for the aircraft that had already been assembled to be prepared for collection by the Squadrons. So from mid-April 1942 the wider issue to Squadrons commences. By June 1942 the second tranche of Mustang Mk.I Squadrons had completed their conversion and work up, had swapped unmodified for modified Mustangs and commenced operations.

 

[11bwmech]

It could be done, the question would be whether it could have been got into production in numbers to make a difference, and if it wasnt better than the Merlin why do it? The P-82 twin Mustang first flew as the war was coming to an end.

"Economy" depends on many things, I have read that the RAF found the the Mustang MkI to be more economical in low altitude low speed conditions because the Allisson engine ran more smoothly at low revs.

Speaking of the P-82 (I remember seeing one at Shepherd AFB when I went to A&E school there), i was told it had counter-rotating props. Do you know how they did that? I'm curious whether they added gearing, reversed the engine, or modified the engine to run the opposite way.

 

[pbehn]

Speaking of the P-82 (I remember seeing one at Shepherd AFB when I went to A&E school there), i was told it had counter-rotating props. Do you know how they did that? I'm curious whether they added gearing, reversed the engine, or modified the engine to run the opposite way.

I presume it had idler gear like those in the P-38.

Later Merlins in the Hornet were also "handed" with idler gear.

 

[wuzak]

I presume it had idler gear like those in the P-38.

Later Merlins in the Hornet were also "handed" with idler gear.

V-1710s reversed the engine and put an idler gear in the supercharger/accessories drive train.

Merlins put an idler gear in the prop reduction gear.

It required a new crankcase and cover in the case of the Merlin.

 

[pbehn]

V-1710s reversed the engine and put an idler gear in the supercharger/accessories drive train.

Merlins put an idler gear in the prop reduction gear.

It required a new crankcase and cover in the case of the Merlin.

My knowledge of the Merlin doesnt stretch that far. Was a new crank case and cover absolutely necessary just to change rotation, the Merlins for the Hornet had downdraught carburettors and re arranged coolant pumps. Wiki simply states it had idler gear in the reduction casing.

• Merlin 130/131

2,060 hp (1,536 kW); redesigned "slimline" versions for the de Havilland Hornet. Engine design modified to decrease frontal area to a minimum and was the first Merlin series to use down-draught induction systems. Coolant pump moved from the bottom of the engine to the starboard side. Two-speed, two-stage supercharger and S.U. injection carburettor. Corliss throttle. Maximum boost was 25 psi (170 kPa gauge; or an absolute pressure of 270 kPa or 2.7 atm). On the Hornet the Merlin 130 was fitted in the port nacelle: the Merlin 131, fitted in the starboard nacelle, was converted to a "reverse" or left-hand tractor engine using an additional idler gear in the reduction gear casing.[110]​

 

[wuzak]

My knowledge of the Merlin doesnt stretch that far. Was a new crank case and cover absolutely necessary just to change rotation, the Merlins for the Hornet had downdraught carburettors and re arranged coolant pumps. Wiki simply states it had idler gear in the reduction casing.

Part of the reduction gear casing was cast into the crankcase.

The idler gear required new bearings and shaft as well as the gear, which had to be supported by the reduction gear housing.

 

[Mike Williams]

For the most recent, probably one of the best researched and currently the most accurate books on the early development of the Mustang, including how the British came to order the Mustang from North American Aviation, I can commend "P-51B Mustang: North American's Bastard Stepchild that Saved the Eighth Air Force" by Bill Marshall and Lowell Ford, Osprey Publishing, UK, 2020. Bill is a member of this forum and engaged with a lot of Mustang researchers and operators globally in researching this book, and a number of members of this forum also are noted in the acknowledgements. Bill and Lowell went back to the basics, original period source documentation, including a lot of material from the Boeing Archives, who hold the original North American Aviation documentation quoted in the book, as well as a deep dive into the USAF Archives.

The North American Mustang Mk.I was ordered as a fighter. It ended up, in some ways, via a set of circumstances to be the right aircraft available at the time when it eventually arrived in the UK to meet an emerging need for a modern, high performance, single seat, fighter type aircraft to be used in the fighter reconnaissance role by the Squadrons in Army Co-operation Command. They had already proven the fighter reconnaissance concept via use of Curtiss Tomahawks, but the serviceability and performance of the Tomahawks left something to be desired and the Mustang looked to be a way better option. (Or another way to look at it is that there was great pressure coming from the War Office for the RAF to get dive bombers - the Stuka fixation - and orders had been placed for a range of dive bomber types such as the Vultee Vengeance and Brewster Bermuda, which it had been proposed to equip the RAF ACC Squadrons with, and that would not have ended well..................). In some ways there were politics at play, and Fighter Command, passing the Mustang on to Army Co-operation Command and being presented to the War Office as "ACC being given the most advanced US fighter produced to date to arrive in the UK" (which it was at the time) went a long way to stifling any negative comments from the War Office about the RAF not giving ACC suitable aircraft to operate. (There are a heap of files in UK Archives covering the procurement of the Mustang and the deliberations and inter-service politics and inter-command politics of how the Mustangs ended up with ACC). Fighter Command also at that time had a certain fixation with high altitude performance for fighters based around potential German use of high altitude bombers with pressurised cabins for the crew to conduct a bombing campaign against the UK, plus high rate of climb for the point interceptor home defence fighter role. So more than just Fighter Command experience from the BoB at play in their assessment of the Mustang.

By VE-Day, there were FOUR RAF Squadrons operating the North American Mustang as one of their primary types.

No.516 Squadron was using an enlarged flight of Mustang Mk.I aircraft in a non-operational role as was No.285 (AA Support) Squadron.
No.516 provided the air component, particularly artillery and naval gunfire direction support to the Joint Operations Training School at Dundonald in Scotland.
No.285 (AA Support) Squadron provided target aircraft for AA practice using Mustang Mk.I in March to July 1945 at Weston Zoyland.

No.26 Squadron had re-equipped with the Mustang Mk.I in January 1945, having operated the Spitfire Vb since early 1944, and were flying operationally providing naval gunnery direction to units of the RN Home Fleet and Free French Navy against German hold out ports along the French, Belgian and Dutch Coasts. They re-equipped with Spitfires again when they joined BAFO in June 1945.

No.268 Squadron RAF was equipped with Mustang Mk.IA and Mk.II aircraft and was using them operationally up to and beyond VE-Day. On VE-Day they were in the process of re-equipping with the Spitfire FR.XIVe, and had a mix of both types used on operations. No.268 Squadron relinquished the last of its operational Mustang Mk.II aircraft (reluctantly) in late August 1945.

A couple of Mustang Mk.I and Mk.II aircraft continued in use until early 1946, mainly attached to Group Support Units and various Training Establishments, primarily used as personal "hacks" for senior officers with those units.

Of the Allison engine variants of the Mustang, the RAF used operationally the Mustang Mk.I, Mustang Mk.IA, Mustang Mk.II and A-36A

Squadrons to use the Mustang Mk.IA: No.II(AC), No.63, No.168, No.170, No.268
Squadrons to use the Mustang Mk.II: No.II(AC), No.268.

No.225 Squadron RAF initially used Mustang Mk.I aircraft in the UK, leaving their Mustangs there when the unit was posted to the MTO and re-equipped with Hurricanes and Spitfires in late 1942. They 'borrowed' for a short period commencing in April 1943, four P-51 aircraft (equivalent to the Mustang Mk.IA) from the USAAF to give them a longer-range capability until Spitfires with fittings for long range slipper tanks arrived.

No.14 Squadron RAF also borrowed some P-51s (equivalent to the Mustang Mk.IA) aircraft from the USAAF which they used for about 4 weeks in 1943 to do some long range recce work, primarily into areas where the units usual Martin B-26 Marauders had been encountering regular enemy fighter opposition and a number of losses had occurred.

No.1437 Strategic Reconnaissance Flight borrowed six A-36A Mustangs from the USAAF in the MTO which were used in a fighter reconnaissance role in the absence of other suitable aircraft from RAF sources in the MTO at the time. After a period of heavy use, the surviving A-36s were passed on and used by a number of RAF P-40 units to assist them in conversion training getting ready to re-equip with Merlin engine Mustang Mk.III aircraft.

One A-36A flown in trials in the UK by the Aeroplane and Armament Experimental Establishment.

Nice post. I wonder if 430 squadron R.C.A.F. doesn't deserve some mention here. They were still equipped with Mustang I when they started converting to Spitfire XIV in Nov 44.

 

[Mike Williams]

The Spitfire FR.XIVe was the next best alternative, but it lacked a number of features that were considered to be 'essential' for the Tac/R role, including limited range on internal fuel. Some of the identified shortcomings would get covered off in the Spitfire FR.XVIII which followed later.

Oh, I think if you read the Operations Record Books of 2 and 430 Squadrons you'll find that the Spitfire XIV equipped units acquitted themselves quite admirably in the Tac/R role. They could also engage enemy aircraft successfully when opportunities presented themselves such as here and here and here and here and here