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Mustang X
The Mustang X (sometimes known as the "Rolls-Royce Mustang") was a variant of the P-51 Mustang utilizing a Rolls-Royce Merlin engine in an experimental program established by the Rolls-Royce company in 1942.
| Mustang X | |
|---|---|
| Mustang X AM203 in the third configuration tested with a high-speed paint finish applied by Sanderson and Holmes, the coachbuilders in Derby, UK. | |
| Type | Experimental aircraft |
| Manufacturer | Rolls-Royce |
| Designed by | Witold Challier |
| Maiden flight | 13 October 1942 |
| Introduced | experimental |
| Primary users | Royal Air Force United States Army Air Force |
| Number built | 5 |
| Variants | P-51 Mustang |
Design and development
In April 1942, the RAF's Air Fighter Development Unit (AFDU) tested the Allison-engined Mustang at higher altitudes and found it wanting, but their CO, Wing Commander Ian Campbell-Orde was so impressed with its maneuverability and low-altitude speed that he invited Ronnie Harker from Rolls-Royce's Flight Test establishment to fly it.
It was quickly evident that performance, although exceptional up to 15,000 feet, was inadequate at higher altitudes. This deficiency was due largely to the mechanically supercharged Allison V-1710 engine, which lacked power at higher altitudes. Still, the Mustang's advanced aerodynamics showed to advantage, as the Mustang I (P-51A) was about 30 mph faster than contemporary Curtiss P-40 fighters using the same Allison powerplant. The Mustang I was 30 mph faster than the Spitfire Mk VC at 5,000 feet and 35 mph faster at 15,000 ft, despite the latter having a significantly more powerful engine than the Mustang's Allison.[1]
Rolls-Royce engineers rapidly realized that the Mustang powered by a Merlin 61 would result in a significant improvement in performance and started converting five Mustangs to Merlin power as the "Mustang Mk 10 or Mustang X." With a minimum of modification to the engine bay, the Merlin engine neatly fitted into the adapted engine formers. A smooth engine cowling with an "chin" radiator was tried out in various configurations. The Merlin 65 series engine was utilized in all the prototypes as it was identical to the Merlin 66 powering the Spitfire Mk IX, allowing for a closer comparison. Due to the speed of the conversions, engines were often swapped from aircraft to aircraft as well as being replaced by newer units.
Testing
The high-altitude performance was astonishing with the Mustang X AM208 reaching 433 mph at 22,000 ft and AL975 tested at an absolute ceiling of 40,600 ft. Air Ministry official, Sir W.R. Freeman lobbied vociferously for Merlin-powered Mustang, insisting two of the five experimental Mustang Xs be handed over to Carl Spaatz for trials and evaluation by the US 8th Air Force in Britain. After sustained lobbying at the highest level, American production started in early 1943 of a North American-designed Mustang patterned after a P-51 Mustang prototype originally designated the XP-78 that utilized the Packard Merlin V-1650-3 engine replacing the Allison engine.
The pairing of the P-51 airframe and Merlin engine was later designated P-51B or P-51C (B (NA-102) being manufactured at Inglewood, California, and C (NA-103) at a new plant in Dallas, Texas, in operation by summer 1943). The RAF named these models Mustang III. In performance tests, the P-51B achieved 441 mph/709.7 km/h at 25,000 ft (7.600 m) and subsequent extended range with the use of drop tanks enabled the Merlin-powered Mustang version to be introduced as a bomber escort.
Mustang X conversions
- AG518: Utilized for engine installation studies, but due to a lack of guns, armour and wireless equipment, it was deemed by Rolls-Royce to be "below" latest production standards and not converted.
- AM121: This aircraft arrived at the Rolls-Royce Flight Test Establishment at Hucknall on 7 June 1942 and was the first to be delivered but the last to be converted. A broader chord fin was installed but the aircraft was not slated for testing at Hucknall and instead was sent to Duxford before being loaned to the 8th Fighter Command USAAF at Bovington along with AL963.
- AL963: First used for performance and handling trials of the Mustang I before conversion on 2 July 1942; its nose contours had a much "sleeker" appearance due to the intercooler radiator being relocated to the main radiator duct. Other changes included a small fin extension and the "blanking" of cowling louvres. This example was able to reach 422 mph at 22,400 ft. It was sent to the USAAF Air Technical Section at Bovington for evaluation.
- AL975-G: First used for performance and handling trials of the Mustang I before conversion on 2 July 1942; flying for the first time on 13 October 1942. The aircraft was identifiable by a bulged lower engine cowling and was also fitted with a four-blade Spitfire Mk IX propeller. In testing, it achieved a top speed of 425 mph at 21,000 ft.
- AM203: The third aircraft was fitted with a four-bladed, 11 ft 4 in wooden propellor and hit 431 mph at 21,000 ft.
- AM208: The second conversion had the front radiator flap sealed permanently giving a six to seven mph boost. The same modification was subsequently made to all test aircraft.
References
- ↑ Birch 1987, p.11.
- Birch, David. Rolls-Royce and the Mustang. Derby, UK: Rolls-Royce Heritage Trust, 1987. ISBN 0-9511710-0-3.
- Delve, Ken. The Mustang Story. London: Cassell & Co., 1999. ISBN 1-85409-259-6.
- Gruenhagen, Robert W. Mustang: The Story of the P-51 Mustang. New York: Arco Publishing Company, Inc., 1969. ISBN 0-668-039124.
