MIflyer
1st Lieutenant
Anybody know if the RAF requested the four 20MM armament of the Mustang Mk1A, or P-51-1? Or was it North American's idea? After all the eight gun armament of the Mk1 was pretty potent for its day - or any day.
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The 4 x 20mm cannon armament was a RAF requirement/desire based on their early war experience and having identified the need for an armament with a heavier weight of fire than that possible with a MG & HMG armament as on the Mustang Mk.I. It is consistent with the direction RAF armament policy was heading as we saw in the later versions of the Spitfire and the Hurricane, and then with the newer RAF types that came later such as the Typhoon and Tempest. The RAF in fact would have preferred to receive their later Merlin engined Mustang Mk.IIIs with the 20mm armament, but in order to speed production, for logistics/support purposes and commonality with Mustangs headed for the USAAF, accepted the USAAF proposed 0.50in HMG armament for the Merlin engined Mustangs. When the RAF in late 1944 into early 1945 was considering using modified Merlin engined Mustangs as replacements for the dwindling numbers of Allison engined Mustangs used in the low level tactical reconniassance role, one of the stated requirements was that the aircraft should have the 4 x 20mm cannon armament fitted for that role. As it was, given the priority for the Merlin engined Mustangs for the fighter and bomber escort roles, the RAF went down the path of the Spitfire FR.XIVe as the replacement - it didn't meet the range requirements by a long mark - but it's mixed armament of 2 x 20mm cannon and 2 x 0.50in HMGs was deemed 'acceptable'. The basic design and dimensions of the armament bays in the P-51, P-51A and P-51B/C could accept with installation of the required mountings and ancillaries, either the 4 x 20mm cannon or 4 x 0.50in HMG armament - they were designed with both RAF and USAAF requirements in mind. But once the decision and production actually took place, the aircraft would be manufactured to meet the planned armament for that production batch. To retrofit either way would have required a degree of rework at Maintenance Unit of Aircraft Modification Depot level, they certainly were not 'swappable' in the field.
Nothing suggested, the Mustang Mk.I (both the NA-73 and NA-83 production batches) had the original armament of 4 x 0.300cal MGs and 4 x 0.50cal MGs. That's a fact.
Mustang Mk.IA (NA-91) had as originally specified by Air MInistry/RAF and contracted under the agreement entered into with NAA by USAAF to provide the Mustang Mk.IA to the UK, an armament of 4 x 20mm cannon - as originally contracted, were supplied with Oldsmobile manufactured Hispano Mk.II. RAF due to issues revealed in testing and in early service made a number of changes to the 20mm mounts in the Mustang Mk.IA, as well as changes to case and link ejection chutes and ports. Key change RAF made to get best reliability out of the armament was to swap out the US manufactured cannon and replace them with UK manufactured 20mm Hispano Mk.II cannon.
The report that you link to was written in end of August 1943. At that stage the RAF was completing working up of Squadrons that had been re-equipped with the Mustang Mk.IA and they had only been operational on the Mustang Mk.IA since mid-August 1943. The RAF officer quoted in the US report, Wing Commander Peter Dudjeon (actually and correctly Wing Commander Peter Dudgeon DFC) only had limited operational experience on the Mustang Mk.I and had no operational experience on the Mustang Mk.IA. He had completed a six month term as Officer Commanding an ACC Squadron equipped with Mustang MkI aircraft in early May 1943 and had been in a largely desk bound HQ role at ACC HQ between then and when he provided his comments to the USAAF at the end of May 1943. (There were better and more experienced people they could have interviewed to get a better appreciation of the strengths and weaknesses of the Mustang Mk.I and the best tactics for using them.) At the time Dudgeon was interviewed, the RAF was just getting ready to issue the first of the Mustang Mk.IA aircraft to squadrons so they could commence re-equipping with the type. So as such, the RAF at that time only had operational experience with the Mustang Mk.I, so any comment made about the Mustang Mk.IA in the report would have been based purely off the early trials conducted by A&AEE before the Mustang Mk.IA was cleared for release to Squadrons for operational use.
The RAF pilots who flew the Allison engined Mustangs in the low level Tac/R role between 1942 and 1945 were generally complimentary regarding the Allison engines - especially once the initial teething and reliability problems were resolved. The RAF conducted a number of modifications on the Allison engines fitted in their Mustangs to get better performance at altitudes below 8,000ft and to allow them to run high levels of boost for longer periods of time than the USAAF and Allison were comfortable with. Pilots with experience on both Merlins and Allisons, complimented the smoother running of the Allison compared to the Merlin and said that it took less 'management' by the pilot compared to the Merlin.
Probably true, but is it a fair comparison, the RR Merlin with twin stage supercharger was not made or configured for running at low speeds at low altitude, generally the higher you tune an engine the lumpier it is at low revs. When discussing three stage superchargers a poster here said that there is a limit to how much you can "choke off" the supercharger output at low level and RPM.Frank Tallman in his book ("Flying the Great Planes"?) said something about being able to throttle the P-40 (Allison) well back at low rpm and loaf along, whereas the Mustang with Merlin wasn't happy doing that. I think I've heard (seen) others make similar comments, too.
When discussing three stage superchargers a poster here said that there is a limit to how much you can "choke off" the supercharger output at low level and RPM.
No, but if a three stage has problems so severe it hardly works perhaps a two stage would be "lumpy". IIRC it was Snowygrouch but I cant remember which thread.(my bold)
A misprint?
Probably true, but is it a fair comparison, the RR Merlin with twin stage supercharger was not made or configured for running at low speeds at low altitude, generally the higher you tune an engine the lumpier it is at low revs. When discussing three stage superchargers a poster here said that there is a limit to how much you can "choke off" the supercharger output at low level and RPM.
Its like using a race horse to pull a plough, from what I have read about the most economical cruising speed of a P-51 B/C or D it wasn't at minimum revs but the criteria is usually distance not time in the air which may be different metrics.A good example of 'strengths & weaknesses' every air force learned to live with.
Its like using a race horse to pull a plough, from what I have read about the most economical cruising speed of a P-51 B/C or D it wasn't at minimum revs but the criteria is usually distance not time in the air which may be different metrics.