Extract from a USAAF report dated August 1943 utilising information provided by the RAF on their use of Mustang Mk.I and Mk.IA aircraft and some basic information on state of modification at that time of engines in RAF use - although touched on very lightly. The list of engine modifications required for operational status Allison engined Mustangs in RAF service, comprised over 50 different modifications, including specific modifications to the supercharger impellor and a number of ancilliaries. In developing these modifications, the RAF made good use of a number of specialist firms in the UK, including ROOTES, who had even then and still have quite a degree of experience and success in supercharging a wide range of engines. The RAF continued its program of refining and improving the performance and life they were getting out of the Allison engines in the Mustangs until late 1944. SO a number of the performance figures quotes in the USAAF report below, were exceeded by later RAF modifications.
Performance of the Mustang I and IA
32. The record of the Mustang I is excellent. The pilots all like to fly it and its success has been due to its reliability, simplicity and the fact that it is faster than any contemporary aircraft at low and medium altitudes.
33. This aircraft is powered with the Allison 1710-39 engine having a rated power of 1150 H.P. at 3000 R.P.M. and 44" Hg. at 12,000 ft. The engine was originally equipped with an automatic boost control limiting the manifold pressure at the lower altitudes to 44". The British remove this so as to get the vastly increased performance at lower altitudes thru the judicious use of over-boost. As has been mentioned before, they have had exceptionally good service out of these engines and due to its smoothness at low RPM's, they
are able to operate it so as to obtain a remarkably low fuel consumption giving them an operational range greater than any single engine fighter they possess (the fact that the Merlin engine will not run well below 1600 prevents them from obtaining an equivalent low fuel consumption and therefore limits its usefulness for similar operations).
34. Actual combat has proven that the aircraft can run away from anything the Germans have. It's only inferior points are that it can't climb as well as the ME-109 and FW-190 and that at the slower speeds of close combat it loses effectiveness of aileron control and therefore has a poor rate of roll – but its turning radius with a slight amount of flap is shorter than either of the German aircraft.
35. In view of the British experience, it is felt that we have a plane excellently fitted and suited for long range, low altitude daylight intrusion and for a medium altitude escort fighter to accompany our medium bombers. It must be realized that an aircraft will fulfil the conditions for a medium bombardment escort fighter might not be completely suitable for a long range intruder due to the inability on the part of the engine to run at the exceptionally low R.P.M. necessary for such long range operation. This is also assuming an operation which will allow a major portion of such missions to be made over waters where interception would be unlikely, such as from North Africa or the Mediterranean Islands to the mainland.
36. In view of the British operation and the fact that we have an approved war emergency rating on the 1710-39 engine of 56", it is suggested that immediate steps be taken to remove the automatic boost controls from our P-51 airplanes in this theatre and that the instrument dials be marked with the proper lights. The British have operated at full throttle at sea level (72" Hg) for as much as 20 min. at a time without hurting the engines. According to them, the Allison is averaging 1500 hours between bearing failures as compared to 500 to 600 hours for the Merlin. The Allison, they have found, will drag them home even with the bearing ruined.
37. It is suggested that the Allison powered P-51A may lend itself better to a combination low altitude fighter-intruder and a medium bombardment escorter than will the Merlin powered P-51B due to the inherent difficulty of operating the Merlin engine at the low RPM's necessary for a low fuel consumption. It is felt that definite engineering and flight information should be secured in these two aircraft immediately.
CHARLES F BORN Brigadier General, CSC, Asst Chief of Staff, A-3.