Fast, low and dirty: The Allison Mustang
It is easy to write off the pre-Merlin Mustang as a mediocre performer, but as Matthew Willis explains, there is far more to the story of the Allison-engined P-51 than first meets the eye.
Between July and October 1942, the Aeroplane and Armament Experimental Establishment at Boscombe Down evaluated North American Mustang Mk.I AL997, and concluded that it ‘should prove extremely efficient for its purpose.’
It was clear by late 1942 that the Mustang’s purpose was not the one that it had been intended for, a conventional day fighter. By 1942 air combat was generally taking place between 22,000 ft and 27,000 ft. While the Mustang had superlative performance below 18,000 ft, above that height it could not compete as the engine lost too much power. There were plenty of circumstances where good low-altitude performance was beneficial though, and the fighter fell almost perfectly into the lap of Army Co-operation Command, which was still casting about for a suitable aeroplane for tactical reconnaissance.
During the Battle of France, Army Co-operation aircrews had discovered to their cost that the Westland Lysander was too slow and vulnerable for tactical reconnaissance, and the Fairey Battle was no better at gathering battlefield intelligence. In the face of enemy air superiority, a fighter-type aircraft was clearly needed. The Mustang arrived at just the right time to replace the Curtiss Tomahawk, which was proving troublesome for a multitude of reasons.
The Mustangs just starting to arrive from America might have gone to Russia, India or the Middle East. Fortunately for the RAF’s fighter-reconnaissance squadrons, the decision was taken in January 1942 to earmark the Mustang Mk. I for this purpose. By the end of the month, over 100 Mustangs were in the UK or en route, and Lockheeds at Speke were working furiously to fit F.24 cameras and carry out a range of other modifications. The camera fit for tactical reconnaissance was an important feature. An oblique attachment had been specially designed, which fitted onto a tray behind the pilot’s head. The camera pointed down and to the side through an aperture in the canopy glazing sealed by a rubber gland.
After Pearl Harbour, the US recognised the low-level reconnaissance potential the Mustang had to offer. The USAAF held back 50 Mustang Mk. IA aircraft from an RAF order and converted them to F-6A standard with K.24 cameras behind the pilots head as with the RAF aircraft, and in the rear fuselage for vertical photography.
The RAF’s Mustang squadrons immediately carved out a role for themselves over occupied Europe, mixing visual and photographic reconnaissance with attacks on targets of opportunity. Two Mustang squadrons were heavily involved in the Dieppe landings, focussing on deep, low-level tactical reconnaissance of road networks around and behind the objective. The Mustang pilots were charged with searching for any signs of enemy forces being moved in to repel the assault. The roads were largely clear, and the reconnaissance missions had little to report, though the Mustang’s first air-to-air kill resulted from the operation, showing that the aircraft was not just a passive contributor to the war effort.
As 1942 passed into 1943, the RAF’s 2 Squadron spent much of the period photographing the Dutch coast for 34 (Photographic Reconnaissance) Wing. Missions known as ‘populars’ were essentially photo-reconnaissance with specific targets, usually requested by the Army. They could be offensive or not, depending on the importance of the target and the urgency of the information required. An experienced unit with the right facilities available could plan and launch a mission within an hour and a half of receiving the request, and the resulting photographs could be processed and assessed rapidly by specialist units, giving almost ‘real time’ intelligence.
Initially the AC Mustang Mk. I carried a single, oblique camera, which was later complemented with a vertical camera. These were automatic in operation (thanks to an electric motor) and photography was initiated by the pilot. The aim was to take a series of photographs at fixed intervals, while flying a fixed course to ensure good overlap between images. Images were often taken of the coastal areas, ensuring an up-to-date understanding of defences.
The Fighter Reconnaissance squadrons were clear that the Allison-powered Mustang was the most suitable aircraft for this function. It had become an accepted principle that they should be equipped with the fastest single seater aircraft at heights up to 5,000 ft, with a range of 600 miles. They had found that the high diving speed and stability of the Mustang made it ideal for low level oblique photography, and its performance allowed it to go toe-to-toe with the best German fighters if required. Its Allison engine proved highly reliable, with bearings that lasted much longer between overhauls than the Rolls Royce or Packard Merlin. Frequently engines that had been damaged by gunfire were able to drag aircraft and pilot home.
The USAAF learned a great deal from RAF operations, and its own tactical reconnaissance with the F-6A and F-6B variants were modelled closely on RAF practice. The F-6As did sterling work in the Mediterranean, as did the F-6Bs in North-West Europe, joining RAF Mustangs in building up a detailed photographic record of the French coast before the invasion in June 1944.
As the Mustang Mk. Is and Mk. IAs were gradually replaced during 1944 and 1945, it was arguably not with aircraft that were better-suited to low-level reconnaissance. Indeed, in some cases, (the Typhoon for example), the replacements were a retrograde step in terms of performance. Unfortunately, supplies of the Allison Mustang had all but run out after production switched to the Merlin-engined version.
Matthew Willis is currently working on a book on the Allison Mustang, its development, service and technical details.
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The A-36A Apache variant (a.k.a Invader) deserves a mention. It was a dive bomber similar to the P-51, Allison-powered, with dive brakes and a beefed wing. North American built 500 of them and they gave fine service in the ground attack role for the USAAF.
That is so true, and Matthew Willis’ “Mustang: The Untold Stroy” is a great book. I got it 2 weeks ago and had to “throttle” myself to keep from reading it too quickly! 🙂
And to the earlier post, about the A-36A — it was never officially ANYTHING but “Mustang.” When the comment was made in 2014, no doubt this fact apparently wasn’t well-known. I’m trying to educate the masses, one person at a time, I guess! 😉
Whilst the Tomahawk was getting a little old, it could do things that the Mustang couldn’t, hence you find the Mustang didn’t fully replace the Tomahawks with TacR units until Mid 1943. Additionally Tomahawks did a pretty good job over Europe, losses being very small, whilst coming up against new type fighters. (to many years history books states Tomahawks did no sorties over Europe).
– The last variant of the Allison Mustang, the P-51A (and it’s off-spring: F-6B and RAF Mk II) had a more powerful engine, giving it a gain of 20mph (390 to 410), and a higher operational altitude (fr 17,500 to 22,000 feet), which proved almost ideal in the CBI against the Japanese.
– The RAF claimed that the Allison engine got 1500 hrs use, compared to 400 hrs of the Merlin, before overhaul. Eight AF Merlins often fouled plugs, so maintenance crews began changing plug after every mission.
– Many RAF Allison engine Mustangs MkI, having entered service @ April/May 1942 were still in operational service when the war ended in Europe in May 1945. Find another fighter that could make this claim.
1. These Mustangs could take off at a shorter distance (from hacked out jungle runways for 1st Air Commando Group) with 2 1,000 lb bombs (yes it was not rated for over 2 500 lbs, but the 1,000 pounder was more common, since B-25s in theater used them). P-47s were sent as replacements but just could not do what was need there.
2. Some time in 1944, an officer on leave to the U.S. scoured flight training bases, mostly in Florida for Allison engined Mustangs, finding approximately 30 (all theaters were screaming for long overdue ‘new’ fighters . . . and the wait was taking too long). It is believed that at least one was an A-36A, but it had wing ‘hard points’ for bombs/drop tanks so wasn’t a problem. While the airframes had seen much service, these Mustangs were well maintained at their training bases. All were shipped to the 1st ACG in short order. I have seen a b/w photo of a P-51 in the CBI outfitted with a camera (in the L rear plexiglass). It is unclear if it arrived as an F-6B of a P-51A that was outfitted in the field.
3. If you read much about the use of the A-36A dive bomber (Africa-Sicily-Italy) you will find that it was the preferred single engine bomber over the P-40 and the newer P-47. Squadrons flipped a coin to see who would keep the A-36A, as losses/wear took their toll (as 3 squ became 2, and eventually 1, etc). It was just that good!
The Merlin was needed, but the Allison was also needed. It deserved RESPECT.