North American Mustang

One of the finest fighter aircraft of all time, the P-51 Mustang was designed to a British requirement in 1940 by a relatively new company, North American Aviation Inc (NAA) of Mines Field, Southern California. Desperate for fighter aircraft of any sort, members of the British Purchasing Mission had been touring US manufacturers placing orders, but it was obvious that most of the products on offer would be of limited use and would soon become obsolescent. However, James H. ‘Dutch’ Kindelberger, President of North American, whose company was about to commence production of the Tomahawk under licence, managed to persuade the British that this would be a waste of time and that his designers could come up with something much better. As a young organisation, North American was forward thinking. Although committed to using the Allison engine like many of its competitors, the company’s creation of a low-drag airframe, employing a radical laminar flow wing, raised performance expectations. The clean lines of the aircraft were also enhanced by the particularly neat design of the radiator, which was set well back under the fuselage centre section.

With many aspects of the design already established, contracts were finalised on 23 May 1940. The prototype NA-73X was flown for the first time on 26 October 1940 by Vance Breese. The performance was way ahead of any other contemporary American fighter, and large contracts for production aircraft, to be named Mustang I, were soon placed. The first machine for the RAF (AG345) was flown on 1 May 1941 and the initial examples arrived by sea at Liverpool docks in November 1941. After re-assembly, the aircraft were test flown at nearby Speke before delivery to the RAF.

Pilots at A&AEE were soon able to get their hands on the new machine, as AG351 and AG383 were delivered for trials in early 1942. Entry to the cockpit involved the usual climb up the port wing with the help of a recessed handhold in the fuselage. Once on the wing, however, access was relatively easy, as the hood and side panel folded up and down respectively, leaving a large area for entry and a low ledge to step over. In the cockpit, the first thing the pilot noticed was a distinct lack of headroom, even with the seat fully down. The view ahead was better than in a Spitfire, due to the narrowness of the nose, and although there was no clear view panel, the side panels were fitted with sliding windows. A rear-view mirror was mounted on the inside of the hood, but this proved to be of limited use.

The control column was of the plain stick type, but it was considered to be too long and would have benefited from being 3–4 in shorter. The rudder pedals were individual pendulum types with toe-operated brakes and could be adjusted fore-and-aft through five different positions. The control for the elevator trim tab consisted of a 6 in diameter plain rimmed wheel with a large cut in the rim, which indicated the neutral position when at the top. It was positioned near the pilot’s left knee and fell readily to hand. The rudder and aileron trim controls were conveniently located on a ledge on the left-hand side of the cockpit.

To operate the flaps, undercarriage and radiator, a hydraulic pressure control knob on the left of the panel was pushed in, and when the appropriate selector lever was moved to the required position, the operation commenced. The undercarriage selector lever was located on the left side of the cockpit floor and was not easy to reach. Three positions, ‘UP’, ‘DOWN’ and ‘EMERGENCY’ were provided, the latter only being used when the undercarriage had not locked correctly in the down position. When the lever was placed in this position, the locking pins were mechanically forced into place. In the event of the engine-driven hydraulic pump failing, a hand pump was located to the right of the seat. Should there be a complete failure of the hydraulic system, the undercarriage could be lowered by pulling an emergency knob on the left-hand side of the panel, which allowed the wheels to come down under their own weight.

The wing flaps were controlled by a yellow-handled lever on the left of the cockpit, and the amount of air entering or exiting the radiator was controlled by another lever just forward of that for the flaps. The undercarriage, wing flap and radiator shutter positions were shown by mechanical indicators sliding in calibrated grooves on the left-hand side of the cockpit. The usual red and green indicator lights and a warning horn were fitted for the undercarriage system. The tailwheel was steerable to the extent of rudder pedal movement and could be locked for take-off and landing. Throttle and mixture controls were positioned on the top left-hand side of the cockpit panelling, the propeller pitch control being just below.

Take-offs were easy, although to obtain the shortest run the tail had to be raised early by positive use of the elevators and the aircraft pulled off the ground. The best flap setting for take-off was recommended as 15 degrees. At flap settings greater than this, there was a strong tendency to drop the left wing, full aileron being needed to check this when using 30 degrees of flap. The aircraft became airborne at 80 mph IAS and flap could be raised when a height of 500 ft had been reached, by which time the IAS was in excess of 120 mph. Slight tail heaviness was noted when the flaps were raised, but there was no tendency to sink. With the undercarriage and flaps up, the best initial climb speed was 160–170 mph IAS.

The controls were tested at speeds up to 500 mph IAS, and although the forces were large for small movements, all were usable. Special attention was paid to the effectiveness of the ailerons and rate of roll tests were carried out up to 400 mph IAS. Although no stick force indicator was fitted, the force on the control column was light enough for full aileron to be applied without undue effort. At 200 mph IAS, aileron control was very light, but the time to roll through 90 degrees was not too fast, due to the aircraft lagging behind control application. The times for a 90-degree roll at this speed were consistently in the region of 1.8 to 2 seconds. The force on the control column was still light at 300 mph IAS, and the times to roll through 90 degrees were virtually identical. By the time that 400 mph IAS had been reached, stick force had increased, but only to around 20 lb and it was still possible to apply full aileron. Rate of roll tests were carried out at normal loadings by rolling from 45 degrees port to 45 degrees to starboard using the left hand to push the control column. All pilots were unanimous that the aileron control of the Mustang was superior to any aircraft previously tested at Boscombe Down.

Generally, the Mustang was stable laterally and directionally at all CG positions, with the flaps and undercarriage up or down. At aft CG it was stable longitudinally at normal flying speeds, engine on, decreasing to neutrally stable at climbing speeds. This was quite acceptable and allowed pleasant manoeuvring qualities. With forward movement of CG the stability was, naturally, increased and the aircraft felt rather heavy longitudinally. Aerobatics were straightforward and there was an improvement in manoeuvrability with a rearward shift of CG. At the aft CG limit the force on the control column in a tight turn was reduced almost to zero, but there was no tendency to tighten up.

Stalling tests were carried out at the extended aft CG limit, as this was the worst condition in which the aircraft could be flown in this respect. At this loading (8600 lb all-up weight and CG 2.7 in aft of datum) the stall speeds were 92 mph IAS with the flaps and undercarriage up and 80 mph IAS with the flaps and undercarriage down.

In the clean configuration there was little warning of the stall, except for the high position of the nose. As speed fell below 100 mph IAS, the aircraft became increasingly left wing low, until at the stall about half aileron had to be applied to keep the wings level. Immediately before the stall, a shudder was felt throughout the airframe and the aircraft started to pitch and rock laterally. The controls were still effective and the aircraft could be controlled by coarse movements, but as the speed continued to drop, the left wing down tendency could not be held and it fell away. There was no tendency to spin. The same sharp shudder before the stall occurred with the flaps and undercarriage down, but in this condition the right wing went down accompanied by the nose. The right wing could not be picked up by using aileron and the aircraft then fell away in a steep spiral, which could, on occasion, lead to a spin.

Dives were made with AL973 at normal CG loadings up to 500 mph IAS and at extended aft CG up to 480 mph IAS, the lower speed being as a result of problems with the hood, which showed distinct signs of breaking away. In full throttle dives at both CG settings, the aircraft was stable. The dives were smooth and steady except for intermittent engine cutting, which had not been experienced with other aircraft. The push force required to hold the aircraft in the dive was very light, and relaxation of this brought about instant recovery. All controls were effective in the dive and although the ailerons became heavier with increase in speed, they were still usable. With one third throttle set, the dives were as smooth as at full throttle, but a much stronger push force was needed to hold the aircraft in the dive. The aircraft also tended to drop its right wing and turn in that direction, requiring aileron and rudder to keep straight. In all dives, hood locking was very unreliable and at high speed a ½-in gap appeared between the hood and the frame. This was most disconcerting for the pilots involved, as the hood showed signs of being sucked out completely .

The best approach speed with the flaps and undercarriage down was 95 mph IAS, in which condition the aircraft was slightly nose-heavy. There was sufficient elevator control to make a tail down landing at all loadings, but at forward CG the limit was just about reached in this respect. The touchdown speed when using full flap was about 80 mph IAS, and there was no tendency to nose over when the brakes were applied, even at the forward CG limit. The climb away from a baulked landing was good. The flaps and undercarriage could be raised at 120 mph IAS without any appreciable sink, and the tail heaviness that became apparent when the engine was opened up could easily be held.

Performance tests were carried out using AG351 and AP222 and showed that at the highest boost setting (56 in Hg) the Mustang could attain 392 mph TAS at a full throttle height of 7900 ft. The full results were as follows:



The maximum rate of climb was 1890 ft/min at 11,500 ft. The rates of climb and times to height were as follows:



One uncomfortable aspect of flying the early Mustangs was excessive heat in the cockpit. Hot and cold air could be admitted to the cockpit area and two louvres were situated behind the pilot to remove ventilated air. The hot air supply proved to be totally unnecessary, however, due to heat coming from the radiator unit, as the top of the radiator shell was exposed to the interior of the fuselage. Air that leaked through the fairing joint and various holes for the coolant pipes, combined with convection currents from the radiator, tended to sweep upwards, striking the pilot on the back of the neck before passing out through the louvres. It was recommended that a false fuselage floor be fitted, which would also protect the pilot from coolant fumes should the radiator be damaged in combat.

At the same time as Mustang I testing was taking place at Boscombe Down, AFDU were carrying out tactical trials and armament tests using AG360 and AG365, which had been delivered to Duxford on 28 January 1942. During stop butt trials on AG365, however, a wing gun jumped off its mounting and fired through the wing, which led to this aircraft being replaced by AG422.

The Mustang I was compared with a Spitfire VB and was found to be 30–35 mph faster up to 15,000 ft, reducing to 1–2 mph faster at 25,000 ft. Its rate of climb at all heights was not as good as the Spitfire. At low altitudes the difference was only slight, but was more marked with height. From 20,000 ft, the Mustang took one minute longer than the Spitfire to reach 25,000 ft. This was considered to be the aircraft’s operational ceiling, as at this point the rate of climb had fallen below 1000 ft/min. The Mustang was climbed, rather laboriously, to 30,000 ft, at which height the controls were relatively sloppy and accurate flying was necessary to avoid losing height in turns. In contrast, it was very fast in the dive, the initial acceleration being particularly good, and it was easily able to leave the Spitfire behind. Recovery was straightforward, even at an indicated speed of 500 mph. During prolonged dives, it was necessary to lower a deflector plate in front of the radiator to maintain the correct glycol temperature. This tended to affect trim slightly and cause some minor vibration.

The Mustang was compared to a Spitfire VB as regards turning circles and dogfighting at all heights up to 25,000 ft. At that height, there was little to choose between the two, but at lower altitudes the Spitfire held the advantage, being able to turn marginally tighter. One tactic tried on the Mustang was to lower partial flap to improve the rate of turn. Although it was quite effective, the Spitfire was still able to out-manoeuvre its rival. Up to 15 degrees of flap could be used in this situation, but lowering flap could prove to be an embarrassment for any Mustang pilot should he suddenly be required to break away in a fast dive. The clean design of the Mustang, together with its weight, led to high speeds being generated in the dive, and this speed could be retained for a long time after levelling out. This characteristic, together with its superior speed below 25,000 ft and the ability to dive with full power from straight and level flight by applying negative ‘g’, allowed the Mustang to break off combat or re-engage at any time. One aspect that emerged during this part of the trial was that it was much more difficult to bring about a high-speed stall in a Mustang than it was in a Spitfire, mainly because of the latter’s light elevator control.

With an inferior rate of climb to that of the Spitfire, the Mustang could not make use of climbing turns to obtain a tactical advantage, unless it had already dived from a higher level. The best tactic for the Mustang was to engage from above and to use the speed gained in the dive to zoom up out of range for another attack. One difficulty encountered during dogfights was the lack of an automatic boost control, which meant that the pilot had to constantly check his boost gauge, especially during dives below 15,000 ft. In such circumstances, the boost limitations of the engine could easily be exceeded during combat. With a total fuel capacity of 140 gallons, the endurance of the Mustang was considerably better than the Spitfire and at economical cruise (1800 rpm, 25 in Hg) it was around four hours. Even at maximum continuous cruise (2600 rpm, 37 in Hg), the endurance was 1 hour 40 minutes.

The Mustang was also flown at night, but glare from the open exhausts severely affected the pilot’s night vision. Flame dampers, similar to those used on the Airacobra, were then fitted and these resulted in a big improvement. The aircraft’s stability was a particular asset at night, as was its controllability. On the downside, the lack of a sliding hood hindered the pilot’s ability to search the surrounding area. The opening side panels that were provided were not really large enough and the canopy frames tended to restrict the view. If the aircraft was landing with the aid of an airfield floodlight, it was found that the perspex of the canopy tended to produce bright reflections, which were rather distracting. Cockpit lighting was seldom required due to the luminosity of the instruments.

When flying in cloud on instruments, the aircraft could be trimmed to fly ‘hands and feet off’ in level flight, or when climbing or diving. As the view forwards and downwards was better than a Spitfire, low flying was considered easier, except in conditions of bad visibility, when the lack of a clear vision panel was badly missed. Brief trials were also carried out against a Typhoon from 10–15,000 ft. The Mustang proved to be more manoeuvrable, although it tended to be out-climbed. During dives, it was found that the Mustang held the initial advantage due to its excellent acceleration. However, the Typhoon quickly caught up, and if the dive was prolonged, it would begin to draw away.

The Mustang I was quite heavily armed, with two 0.5-in machine-guns mounted in the lower front fuselage, together with two 0.5-in and four 0.303-in machine-guns in the wings. The guns were fired by electric solenoids operated by a trigger on the front of the control column. A switch mounted on the left-hand side of the panel allowed the pilot to select ‘Wings’, ‘Fuselage’ and ‘All’. A comprehensive series of air and ground firing trials were carried out. It was during one of these trials that one of the 0.303-in guns jumped free from its mounting. Shots went through the blast tube and the leading edge of the wing. On examination, it was found that the locking clips were at fault; as a result, revised clips were designed and fitted by AFDU, with no further problems being experienced. In the air, the guns were fired at low altitude under positive and negative ‘g’, with only occasional stoppages of the 0.303-in guns due to a misfed round under negative ‘g’. The guns were tested up to 26,000 ft (OAT –26 degrees C), the only stoppage being caused by a damaged round in the starboard wing 0.50-in gun. No problems were encountered as a result of the guns icing up at high altitude. Pilots were aware of fumes in the cockpit when the fuselage guns were fired, but these quickly dispersed. All who flew the Mustang remarked on the lack of vibration when the guns were fired.

It was noted during the trials that the Allison engine, although easy to start even under the severest winter conditions, took several minutes to warm up after a cold start, as the minimum oil temperature recommended for take-off was far higher than that quoted for the Merlin. If a quick take-off was required, the engine needed to be kept warm, in which case the time from the order being given to the aircraft becoming airborne was around six minutes.

Due to the limitations of its Allison engine, the Mustang I was used to replace the Tomahawk in Army Co-operation Command squadrons, and proved its worth during low-level armed tactical reconnaissance operations. However, the impression made by the Mustang at AFDU was such that the CO, Wing Commander Campbell-Orde, invited Rolls-Royce test pilot Ronnie Harker to fly the aircraft. Harker was astounded by the Mustang’s speed and immediately wondered what the performance would be like if it was re-engined with the latest two-stage supercharged Merlin. Enlisting the assistance of Air Marshal Sir Wilfrid Freeman, the Vice-Chief of the Air Staff, an Allison Mustang was quickly delivered to Rolls-Royce at Hucknall to be converted to Merlin 61 power. Performance predictions were made by Witold Challier, an exiled Polish engineer, who calculated a top speed of 441 mph at 25,600 ft, faster than any other fighter in service at the time.

The first Merlin-engined Mustang (AL975/G) was flown by Rolls-Royce Chief Test Pilot Captain R.T. Shepherd on 13 October 1942 and was unique in featuring a large air intake under the nose. It was referred to as the Mustang X and went on to confound the cynics and prove that Challier’s figures had been correct. The results of initial testing were made available to North American via the US Air Attaché in London and NAA soon designed their own set of modifications to mate the Merlin to the Mustang airframe. An agreement was quickly made whereby the Packard motor company would supply licence-built versions of the Merlin, the subsequent V-1650-3 being rated at 1520 hp. Merlin-powered Mustangs as flown by the RAF were designated the Mustang III (equivalent to the USAAF P-51B and P-51C) and the Mustang IV (P-51D).

The first examples of the new Mustang were made available to the testing establishments as soon as they were delivered and AFDU received FZ107 on 26 December 1943 for a tactical evaluation. Apart from the Merlin engine, the Mustang III differed from the Mustang I in having only four wing-mounted 0.50-in guns, a four-blade propeller, an air intake immediately under the propeller hub (instead of over it), a deeper rear fuselage housing radiators and oil coolers, and a slightly larger fin and rudder. The Mustang III was very similar to fly to the Allison-powered variant, but its increased performance meant that compressibility speeds were much more likely to be encountered during dives. Pilots were warned not to use the elevator trim wheel in an attempt to prevent the nose from dropping, as there was every likelihood of a sudden nose-up change of trim occurring as the aircraft came out of the compressibility range, which could lead to high accelerations and possible structural failure.

One of the most interesting aspects of the trial was a tactical comparison with a Spitfire IX (BS552), as both aircraft were powered by basically the same type of engine. Although it was slightly heavier, the Mustang III was cleaner aerodynamically and had a higher wing loading at 43.8 lb/sq.ft compared with 31 lb/sq.ft for the Spitfire. The Mustang III’s normal fuel capacity was 154 gallons, which gave it endurance up to 175 per cent greater than the Spitfire. It could also carry two 62½ gallon overload tanks under the wings. The Spitfire could carry a ‘slipper’ tank under the centre section of 45- or 90- gallon capacity. The fuel consumption was approximately the same at similar boost and engine rpm settings.

The best performance heights were similar, being between 10–15,000 ft and 25–32,000 ft but for the same engine setting the Mustang was 20–30 mph faster in level flight at all heights. It was also significantly superior in the dive, the Spitfire IX requiring 4–6 lb/ more boost to remain in formation at the same engine rpm. The Mustang could also use its excellent dive performance to good effect in a zoom climb, but the Spitfire held the upper hand during full power climbs, needing 5 lb/ less boost to stay in formation. The Spitfire could also roll more quickly than the Mustang at normal speeds (the aircraft used in the trial had clipped wings) and its lighter wing loading meant that it could turn inside its adversary, even if the Mustang lowered partial flap.

The opportunity was also taken to compare the Mustang III with several other aircraft including a Spitfire XIV (RB141). The results were similar to those obtained with the Spitfire IX, except that there was virtually no difference in maximum speed between the two aircraft and the margin in terms of dive performance was not as great. Against a Tempest V (JN737), the Mustang was slower by 15–20 mph up to 15,000 ft. After this height there was little to choose until 24,000 ft was reached, when the Mustang began to pull ahead, being 30 mph faster at 30,000 ft. Maximum rates of climb compared directly with the results of the speed tests, except that the Tempest had a better zoom climb at all heights. The Tempest was able to leave the Mustang during prolonged dives, but its rate of roll and turn performance were not as good.

Brief comparisons were made between the Mustang III and captured examples of the Focke-Wulf Fw 190A (PM679) and Messerschmitt Bf 109G (RN228). When flown against the Fw 190A, the Mustang was nearly 50 mph faster at all heights, increasing to 70 mph faster above 28,000 ft. It also had a decisive advantage in the dive, but the Fw 190 was able to match the Mustang in climb performance and turning circles. Not surprisingly, the Fw 190 could initiate rolling manoeuvres much quicker than its rival, thanks to its large ailerons and excellent response. This agility meant that it was not a good idea for Mustang pilots to attempt to dogfight with Fw 190s, rather to maintain high speed and regain height after each attack. A good defensive tactic for a Mustang was to carry out a steep turn (an attacking Fw 190 would be carrying more speed and would be unable to turn as quickly) and to follow this with a full power dive, which would rapidly increase range.

The Mustang III also had a speed advantage over the Bf 109G, although of slightly more modest proportions. It was 30 mph faster below 16,000 ft and above 25,000 ft, becoming 50 mph faster at 30,000 ft. There was little to choose between the two during maximum rate climbs, the Mustang being very slightly better above 25,000 ft but worse below 20,000 ft, and zoom climb performance was also evenly matched. The rate of roll was similar, but the Mustang did hold sway when it came to turning circles and dive performance. The recommended tactics against the Bf 109G were the same as for the Fw 190A.

The combat performance of the Mustang III was also assessed when carrying long-range tanks. There was a serious loss of speed in the order of 40–50 mph at all engine settings and heights. However, it was still faster than the Fw 190A above 25,000 ft, although slower than the Bf 109G. The rate of climb was greatly reduced, and the Mustang could be out-climbed by both the Fw 190A and Bf 109G. However, if the tanks were reasonably full the Mustang was still superior in the dive. The tanks did not make as much difference as might have been expected as regards turning circles, and the Mustang could still turn as tightly as the Fw 190A, and more tightly than the Bf 109G. The general handling and rate of roll were also very little affected. With a greatly reduced performance when carrying drop tanks, AFDU concluded that a half-hearted attack could be evaded by a steep turn, but it would be difficult to avoid a determined attack without losing height.

If there was anyone who still did not believe the transformation that had taken place with the Mustang, the results of performance testing carried out at Boscombe Down left absolutely no doubt that the adoption of the two-speed, two-stage supercharged Merlin in place of the Allison engine had turned an aircraft of limited value into a world beater. Mustang III FX953 was put through its paces in May 1944 at a take-off weight of 9200 lb, without bombs or drop tanks. At a normal engine rating of 2700 rpm and 46 in Hg manifold pressure, the maximum rate of climb in MS gear was 2060 ft/min at 16,700 ft and 1555 ft/min at 30,200 ft in FS gear. At the full combat rating of 3000 rpm and 67 in Hg manifold pressure, the best rate of climb was 3610 ft/min at 10,600 ft in MS gear and 2690 ft/min at 23,400 ft in FS gear. The service ceiling was 42,800 ft (approximately 12,000 ft higher than the Mustang I), with an absolute ceiling of 43,600 ft.

Level speed trials showed the Mustang III to be faster at cruise settings than the Mustang I at full combat power. With 2700 rpm and 46 in Hg manifold pressure set, the Mustang III achieved 406 mph at 20,600 ft in MS gear and 438 mph at 33,000 ft in FS gear. However, at 3000 rpm and 67 in Hg manifold pressure, the maximum speed in MS gear was 424 mph at 15,500 ft, and in FS gear it was 450 mph at 28,000 ft.

Although the early-type hood as fitted to the Mustang I had not been received with much enthusiasm, the later Mustangs were fitted with sliding canopies. These considerably improved lookout, as Len Thorne recalls.

On 8 December 1942 I tested a Mustang at AFDU with a sliding canopy instead of the up-and-over hood and it certainly impressed me. The Americans were very loath to change their design as they thought that the bubble hood would upset the airflow and spoil the aircraft but, in fact, it worked the other way as it made the tail surfaces more effective. The bubble hood (and the later teardrop canopy) were both very pleasant to fly because the all-round visibility was very much better than it was with the slab-sided design, and for me it was something of a revelation because you could actually stick your head over the side and look straight down behind the wing which was a peculiar sensation.

In addition to performance testing at A&AEE and tactical trials at AFDU, the Mustang was also used for aerodynamic trials work at RAE Farnborough, including a comparison of its ailerons with those of the Spitfire. Lateral control problems at high speed on the Spitfire were still causing concern and the excellent aileron effectiveness of the Mustang generated considerable interest. The Mustang’s ailerons were of the plain type with geared tabs, while the Spitfire was fitted with Frise ailerons with a sharp leading edge. An unusual feature of the Mustang’s ailerons was their relatively small range of movement (+/–10 degrees) compared with that of the Spitfire (+24, –20 degrees).

Tests showed there to be little difference in aileron effectiveness (rate of roll per degree aileron) at speeds up to 150 mph, but above this speed the Mustang’s aileron became much more effective. This was mainly due to the greater stiffness of the Mustang wing, which gave much less twisting, the aileron reversal speed (the theoretical speed at which wing twist due to lack of torsional rigidity overrides the effect of the ailerons) for the Mustang being 820 mph, compared with 580 mph for the Spitfire. The aileron angles and stick forces required to generate a steady rate of roll of 45 degrees per second were measured in both aircraft. At 400 mph IAS, the Mustang had an aileron deflection of 4.5 degrees and needed a stick force of 23 lb, whereas figures for the Spitfire were 10.3 degrees and 71 lb respectively.

Although the Mustang I had been used for terminal velocity dives at RAE Farnborough to test its behaviour at compressibility speeds (see Chapter 2), these trials had been severely limited by the aircraft’s lack of altitude performance. As the dives could only be commenced at around 28,000 ft, the maximum Mach number achieved before safety height was reached was 0.80, compared with 0.89 for the Spitfire XI, which was able to dive from a height of 40,000 ft. As the later Mustangs were able to match the Spitfire in terms of service ceiling, this allowed the opportunity to find out more about its handling characteristics at the highest speeds that could be attained. A series of thirty-one dives were carried out at the USAAF test establishment at Wright Field, Dayton, Ohio, commencing on 3 August 1944 using P-51D 44-14134, equivalent to the RAF’s Mustang IV.

The dives were entered by a variety of means, including nosing over from level flight and from a diving turn. A half roll and pull through was also tried, but extreme care had to be taken during this manoeuvre as high Mach numbers could be reached within a few seconds. If this was tried at rated power above 36,000 ft, it could well lead to a structural failure. As speed built up, longitudinal instability or ‘porpoising’ set in. This condition could be induced at Mach 0.70 and above, but it was not unknown for it to be encountered at lower Mach numbers at low altitude. The motion was often pilot-induced, and although not severe, any effort to counteract it was likely to result in the motion increasing in amplitude. The most effective solution was to hold the stick firmly in one position or to trim forward to nearly zero stick force as the dive was entered, thus reducing the amount of forward stick force necessary to maintain the angle of dive.

As the speed increased to Mach 0.75, a slight rolling motion became apparent with simultaneous reduction in aileron sensitivity. This did not become severe and could be easily controlled. At Mach 0.76, however, a steady vibration set in due to compressibility effects on the wing and tailplane, and this became worse with increase in speed, becoming heavy by the time that Mach 0.80 had been reached. Several dives were made to Mach 0.84 (and one to Mach 0.85), and on each occasion the vibration caused some structural damage. This included a buckled leading edge skin of the wing flap, a cracked coolant radiator and a broken hydraulic line. During the period the aircraft was in heavy buffet, even a relatively low acceleration could lead to a primary structural failure.

Recovery had to be gradual and executed with extreme caution, as relatively light stick forces or rapid application of trim could easily result in excessive load factors. At the beginning of the pull-out an increase in vibration could occur, but this would gradually decrease as the recovery was completed. At no time was it necessary to select elevator trim to aid the recovery. The P-51D also showed no tendency to ‘tuck-under’ when power was increased or decreased in the dive. From these dives it was recommended that the Mustang be restricted to a Mach number of 0.80, as difficulties caused by compressibility above this point made it increasingly dangerous for the pilot.

In addition to its duties evaluating and comparing the Allied single-engined fighters and their German counterparts, AFDU was also responsible for testing a wide range of weapons for use against ground targets. In September 1944, the unit was required to carry out tests to determine whether standard aircraft drop tanks could be used offensively as so-called fire bombs. Two types of incendiary mixture were considered, perspex in benzole, and aluminium laurate and creosote in ‘pool’ petrol. The aircraft used for the trials were Typhoon IB MN974 and Mustang III FZ107. Initial tests were carried out with water-filled ‘dead’ bombs. The first ‘live’ drops were made over the Holbeach ranges, the benzole mix proving to have the better qualities. To be able to observe the trials more closely, several trenches and a gun emplacement were dug at Collyweston airfield, complete with straw dummies. By this stage only the Mustang was being used and the tactics recommended by AFDU were to make an approach at 5–8,000 ft, according to cloud conditions, and then to turn through 90 degrees on sighting the target. A descent would be made to a height of 1500 ft about 1500 yards from the target. The attack would be carried out in a shallow dive at limiting speed, which, for the Mustang, was set at 350 mph IAS due to the risk of the tanks hitting the aircraft when released. In the final stages the attack was delivered down to a minimum height of 50 ft, which also allowed full use of the aircraft’s guns.

Len Thorne was heavily involved in these trials, but dropping live ordnance was always liable to cause some anxious moments. He recalls one particular occasion when things did not quite go according to plan.

As the war progressed we hung all sorts of things on those lovely fighters: bombs, rockets and drop tanks for longer range. In 1944 thought was given to the problem of winkling the Japanese out of fox holes and slit trenches and the idea of ‘napalm’ was born [from two constituent components naphthalenic and palmitic acids]. One Sunday I had the very doubtful honour of demonstrating this weapon to a group of Army top brass, both American and British. Dummy trenches had been dug at Collyweston, and two 62½-gallon drop tanks filled with jellyfied benzole were hung on my P-51. The idea was to release the tanks in a shallow dive about 100 yards short of the target, the tanks would burst on impact throwing the napalm forward to run down into the trenches. Strapped to the outside of each tank was a white phosphorus grenade attached by a wire to the bomb rack so that when the tank was released it pulled on the wire and made the grenade live. If it was done at the right altitude, when it hit the ground it exploded and ignited the napalm which travelled forward for a distance of about 350 yards.

On the day in question the weather was atrocious as it was blowing half a gale. I said it was ridiculous to try to do a test in such weather but, as everybody was in position, I had no choice. I released the drop tanks at the appropriate moment but one of them hung up. I then pulled round in a fairly steep climbing turn and in the middle of the turn the remaining tank came off and hit an American Army camp a couple of miles to the south of the airfield, falling right in the middle of the parade ground. As I landed I had visions of having killed countless American soldiers but because it was a Sunday afternoon they were all off camp so the napalm didn’t do any serious damage. They insisted I repeat the run and the same thing happened the second time but on this occasion I flew straight on and managed to get to the Wash before it came off.

The Mustang III entered service with the RAF in February 1944 when No. 19 Squadron took delivery of the first aircraft at Ford. The old-type hood was fitted to early production aircraft but this was quickly replaced by the bulged Malcolm hood. As previously stated, this improved the pilot’s search view, but it was not until the arrival of the Mustang IV with its moulded one-piece canopy that the problem of rearward vision in particular was fully addressed. As the profile of the rear fuselage had been reduced slightly by the adoption of the new hood design, a dorsal fin was incorporated to restore directional stability. The armament on the Mustang IV comprised six 0.50-in wing-mounted machine-guns.

The role of the Mustang in the final months of the war ranged from high-level bomber escort to ground attack in support of the Allied armies. From the beginning of 1944 Mustangs began to replace obsolescent Kittyhawks and Hurricanes in Italy, and with the withdrawal of German forces in northern Europe, Mustangs of the 2nd Tactical Air Force ranged far and wide seeking to destroy targets with their 500-lb bombs.

Even UK-based Mustang squadrons could play a big part in the fighting over Europe. On 16 April 1945, No. 611 Squadron became the first RAF unit to encounter Russian aircraft over Berlin, when Shturmovik ground-attack aircraft, escorted by Yak fighters, were seen. Shortly afterwards No. 611 Squadron ran into a gaggle of Fw 190s. Six were shot down, including one that fell to the guns of Pilot Officer Ian Walker flying Mustang IV KH743. At the time No. 611 Squadron was based at Hunsdon in Hertfordshire and their duties that day involved escorting medium bombers to Swinemunde, followed by a sweep of the area around Berlin. Ian Walker’s logbook shows a total flight time for the mission of 5 hours 55 minutes. Such a stark fact may seem of little interest, but it highlights just one aspect of the Mustang’s success, its remarkably long range. When this is added to extremely high performance, excellent handling characteristics and an impressive load-carrying capability, the combination becomes overwhelming, and puts the Mustang not only at the pinnacle of all Second World War fighters, but in a select group of weapons that helped to shorten the course of the war.

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