by Colin Downes
During the Second World War the results of air combat changed the parameters of fighter design in becoming not just concerned with manoeuvrability and speed, but also the need for increased range and endurance. Air, unlike water, can be compressed and as fighter speeds increased the compressibility of the air affected the control of the aircraft as it approached a critical speed in relation to the speed of sound when the air flow over the wing and control surfaces became disturbed and turbulent air flow around the fuselage resulted in an increased drag factor. Generally, in the conventional wing design of fighters at the time, the thickest part in the cross-section of the wing or main plane occurred somewhere between the leading edge and the centre line of the wing. The design of a laminar flow wing on the Mustang, the first fighter to have such a wing, moved the thickest section of the wing further aft, to produce a non-turbulent air flow in parallel layers over the surface of the wing. This resulted in better control and reduced aerodynamic drag at high speed, giving increased range and endurance. In assessing the Spitfire, the Mustang, the Messerschmitt BF-109 and the Focke-Wulf FW-190, there was no clear overall winner but the Merlin Mustang became generally acknowledged to be the best all-round fighter of the Second World War.
Until the advent of the Mustang the RAF were unable to escort the heavy bombers to their targets, as even with a 90 gallon (Imp.) ventral drop tank on the Spitfire IX its radius of action was around 400 miles. Fighter Command chose to reject the early version of the Mustang powered by the Allison engine and concentrate on improved Spitfire models for the interceptor role. In this respect good high altitude performance was imperative and although the Mustang provided advantages in range and speed its deficiencies at high altitude made it unacceptable. For this reason the RAF’s Mustang Is went to Army Cooperation Command where its speed and manoeuvrability were essential in its role of fighter-reconnaissance. By mid 1943 the Mustang squadrons attached to Army Cooperation Command transferred to Fighter Command and after D-day to 2nd Tactical Air Force. The Allison engine Mustang I was faster at low and medium altitude than the Merlin engine Spitfire V but the arrival of the Spitfire IX changed this advantage. The Mustang was heavier by 2,000 lb than the Spitfire and consequently could not climb as quickly. Above 15,000 feet the Mustang’s performance deteriorated rapidly due to the lack of supercharging on the Allison engine.
The installation of the Merlin 61 engine with a two-stage, two-speed supercharger in the Mustang III addressed many of the Mustang I deficiencies compared with the Spitfire. The USAAC designation for the aircraft was P-51 B, followed after further improvements by the P-51 C. The later aircraft arrived for the RAF under the US-UK Lend-Lease arrangement as the Mustang III. However, before going into service with the RAF the Mustang III had the cockpit canopy modified with a bulbous one-piece sliding hood, known as the Malcolm hood after the designer, and this greatly improved pilot visibility. The arrival of the Mustang IV with a full tear-drop canopy improved the pilot visibility even more and was far superior to that of the Spitfire. A second major modification to the Mustang III increased the armament of four 0.5 in Browning machine-guns to six. The two inner guns carried 400 rounds and the two centre and outer guns 270 rounds, for a total of 1,800 rounds. The 0.5 calibre Browning gun had a muzzle velocity of 2,900 ft/sec and was very reliable when geared to 800 rounds per minute. The only problem associated with the gun occurred under high G-turns when a high centrifugal force held back the ammunition belt while firing the guns, causing the breech mechanism to block the feed of the rounds. The Mustang had a reliable K-14 gyro gun sight developed from a British design that enabled proper tracking and deflection shooting. This meant that pilots were more likely to hit the target than miss it with a deflection shot. The USAAC P-51Cs and Ds had a Berger G-suit that was not in the RAF Mustangs. The absence of a G-suit in the RAF continued well into the jet era and it was not until after the Korean War when the Hunter entered service that RAF fighter pilots had this facility in a British built fighter. A third major modification involved the installation of an 85 US gallon (70 Imp.) fuselage fuel tank directly aft of the pilot’s seat and below the radio equipment. This affected the aircraft’s centre of gravity and therefore its directional stability when full. The effect of this in a tight turn was the aircraft tended to pitch up or tuck into the turn. With the additional fuel tank of 85 US gallon (70 Imp.), two internal wing tanks of 90 US gallon (75 Imp.) each, and two external wing drop tanks of 108 US gallon (90 Imp.) each; the Mustang had a total fuel load of 480 US gallon (400 Imp.). This was sufficient fuel to give the Mustang a round trip of 1,200 miles and an endurance of seven to seven and half hours. The installation of the Merlin 61 engine with a four bladed propeller and a two-stage, two-speed supercharger increased the aircraft weight but resulted in more power and a better performance. The US Packard-built Merlin engine was reliable although not as smooth running as the Allison, and it was hard on plugs that would lead-up at low engine rev settings.
The Mustang cockpit layout was excellent, unlike the Spitfire, with all major levers and switches well placed for operation by the pilot’s left hand while the right was free to hold the control stick. The general finish of the aircraft was superior to the Spitfire, rather like comparing an early Jaguar car with a more recent Cadillac. The cockpit, although a close fit, was far more commodious than the Spitfire cockpit. The blind flying instruments followed the RAF’s ‘Basic Six’ panel although they were positioned slightly differently, with the airspeed indicator, the directional heading indicator and the artificial horizon above the altimeter, the turn and slip indicator, and the vertical speed indicator. In similar fashion to the Spitfire, the associated engine gauges were grouped on the right side of the instrument panel. More creature comforts abounded in the Mustang compared with the Spitfire, from the more comfortable seat to the steerable tail wheel controlled by the rudder pedals. This could be disengaged for tight turns by pushing forward on the control column and the differential use of the brakes. There was excellent trim control for all three flying controls, whereas the Spitfire’s were not so precise and there was no trim for the ailerons. The heating and fresh air controls were far more effective in the Mustang than in the Spitfire. A folding armrest on the Mustang behind the throttle quadrant rested the left arm on long duration flights. A ‘pee’ funnel and tube installed below the seat enabled the pilot to relieve himself on long flights. This was a thoughtful feature but proved decidedly problematical in practice. For a pilot to answer a call to nature dressed in full winter flying gear, wearing a Mae West life jacket, strapped tightly into a parachute harness secured to a dinghy pack, while fastened securely in his seat by seat belt and harness presented an almost insurmountable problem. The fact that he was also flying in squadron formation possibly at wave top height, and monitoring his flying controls, flight and engine instruments required the ingenuity and dexterity of Houdini, even with the excellent trim control of the Mustang. There was also the not insignificant distraction of looking out for enemy fighters. Even flying alone over friendly territory at a safe altitude and in ideal weather conditions the satisfactory accomplishment of passing water by the pilot had an additional design hurdle to overcome. The ‘pee’ funnel was about five inches in length tapering from about three inches in diameter to a flexible rubber tube of half to one inch bore. The pipe travelled along the fuselage and exited behind the radiator by the tail. In theory the airflow passing over the airframe applied suction to draw the contents out into the slipstream. Here was where theory and practice diverted, for to establish the correct exit rate of flow along the pipe required a controlled slow intake rate of flow into the funnel that under the circumstances was well nigh impossible. I have gone into some detail in this particular provision of pilot relief in the Mustang as it is not covered or usually discussed during the aircraft conversion; nor was it to be found in the Pilot’s Notes. The first and only occasion I attempted to utilize this facility I was unaware of the technique required and after some considerable difficult
ies, during which I placed my aircraft and others in dire danger, I experienced an overflow condition that made the whole process and equipment quite redundant. I was no better off than the Spitfire pilot on a long high-altitude photo-reconnaissance flight tentatively peeing down the control column. A friend of mine flying with the PRU at Benson and plagued with a small or weak bladder confessed that while flying at 40,000 feet in the freezing cold, cramped and draughty cockpit of a Spitfire he frequently contrived to pee neatly down the control column while scanning the sky for enemy fighters. At the same time rocking the control column to prevent the urine freezing on the elevator and aileron cables: a good case for photo-recce pilots being allocated their own personal aircraft! After my unpleasant experience I made sure to restrict my fluid intake with as many nervous visits to ‘the loo’ as possible before take-off. In discussing the problem with other pilots I found they seldom attempted the facility and, like me, resigned themselves to the alternative arrangement of British designed fighters. Probably a combination of nervous energy, profuse sweating and dehydration during the flight made the use of the equipment superfluous.
The Mustang was an unforgiving aircraft to handle, but once understood and respected it was a superb fighter to fly. The Mustang III possessed advantages in some performance spheres but was at definite disadvantages in others, although not falling short on any one aspect whether flying low-level or above 30,000 feet. Top speed in level flight, rates of climb and turning circle were generally held to be the most important factors in fighter performance and in these qualities the Merlin engine Mustang did not lag. The ability to overhaul an adversary in a dive or to pull away from him proved an even greater advantage and with its low-drag airframe and laminar flow wing design the Mustang had few equals in dive performance and was one of the few propeller fighters to experience compressibility control problems in a dive. In a vertical dive from high altitude the Mustang could exceed the critical Mach number of jet fighters in level flight. The indications of this compressibility problem were airframe vibration with aileron flutter, followed by a complete loss of elevator control until denser air and warmer air temperature at lower altitudes allowed the critical Mach number of the aircraft to rise and the flying controls to respond. Experiencing this phenomenon in complete ignorance of flight compressibility for the first time certainly raised the heart rate until control was regained at medium altitudes.
The Mustang was able to hold its own with any interceptor fighter and its crowning advantage was endurance vastly superior to that of the single-seat fighters of its day. In a comparison with its main adversaries the Mustang III was evenly matched against the Bf-109G and FW-190A. The Bf-109G and the FW-190A could out-accelerate and out-climb the Mustang at low altitude, but the Mustang generally had a superior performance over both German fighters. The main advantage of the German fighters lay in the weight of fire of its armament. Against the Bf-109 armed with one 20 mm (0.8 in) cannon and two 13 mm (0.5 in) machine-guns the German advantage over the Mustang was slight; but against the FW-190 armed with four 20 mm cannon and two 13 mm machine-guns, the kill probability against the Mustang if hit was considerable. With the arrival of the long-nosed FW-190D the combat comparison with the Mustang was more evenly matched, although the FW-190D was superior in acceleration, climb and even dive. The long-nosed FW-190D became the finest propeller driven fighter in squadron service with the Luftwaffe. In a comparison with the Spitfire IX, the Spitfire proved superior in climb and turning performance to the German Bf-109G and FW-190A, although inferior in rate of roll and dive. The same comparison resulted with the Griffon engine Spitfire XIV against the long-nosed FW-190D. The Luftwaffe fighters had an advantage over the RAF fighters in that the Daimler Benz engine in the Bf-109 and the BMW engine in the FW-190 were not only better engineered but had two distinct operational advantages in their design. The fuel injected German engines enabled the German fighters to operate at full power under negative G flight conditions, whereas the fuel flow from the carburettor fed Merlin would stop under negative G conditions. To compensate for this during inverted flight the Merlin had a negative G fuel trap that allowed the engine to operate for six seconds while inverted. A second advantage with the German engines was with the supercharger design that varied the boost automatically with changes of altitude and air pressure to provide continuous maximum power. In early Spitfires the pilot manually controlled the two-stage supercharger, but later Merlin engines switched automatically to the second stage on reaching 18,000 feet. However, the Merlin engines gained more power when using 100 octane aviation fuels while the German engines still operated on 87 octane fuel.
By 1944 the Mustangs were on escort duties with the heavy bombers of the US 8th Air Force attacking V-bomb sites in the Pas de Calais area and with RAF Bomber Command in support of the invasion and advance through France and the Low Countries. After flying the Spitfire I found the Mustang easier to land with its wide undercarriage and to be more comfortable with its roomier cockpit. However, with its greater weight it did not respond to the throttle on take-off as did the Spitfire with the same Merlin engine. With the Spitfire the aircraft accelerated quickly on take-off and as the tail rose the aircraft had a strong tendency to swing left due to the propeller torque, to be corrected by strong opposite rudder. For this reason one never used full throttle on take-off until the aircraft became airborne. Taking-off in the Mustang one allowed the tail wheel to remain on the ground until approaching flying speed so that the steerable tail wheel helped correct the swing. Although I turned down the chance to fly the Allison engine Mustang in order to fly the Spitfire; after I accepted the Merlin engine Mustang for the fine combat fighter it undoubtedly was, I regretted not experiencing the original version of the aircraft. Friends who progressed from the Allison Mustang to the Merlin variant all praised the Allison Mustang as the most pleasant aircraft to fly low level, and considered the Merlin Mustang a distinctly wilder aircraft to fly than its predecessor. The more powerful Merlin engine with its four-blade propeller affected directional stability, particularly in a dive requiring constant trim control. The rasping, crackling, ear-shattering roar of the Merlin and the occasional shudder that it sent through the airframe compared unfavourably with the pilots used to sitting behind the docile and smooth running Allison. Therefore, although the Merlin Mustang was not such a pleasant aircraft to fly its combat potential was unquestionably superior.
When I arrived at the squadron the flight commander asked what experience I had of the Mustang and I replied none, and that my fighter experience was entirely on Spitfires. He responded saying all the available aircraft were on stand-by with full internal fuel and drop tanks. However, one aircraft required an air test to check the drop tanks and he suggested I flew this aircraft. This was a stupid suggestion for someone who had never flown a Mustang, which initially was not the easiest of fighters to fly in a clean configuration. The Mustang loaded with full internal fuel and drop tanks was best described as a flying pig. Not wishing to appear chicken arriving as a new boy on the squadron and being ignorant of the Mustang Mk III, I assumed if I could fly a Spitfire I could fly a Mustang, so I accepted the predicament. Under normal circumstances I should have completed a Mustang conversion course before joining a Mustang squadron, but in the haste to build up the Mustang wings Fighter Command ‘P’ staff just drafted pilots from within the command. The flight commander gave me a copy of the Pilot’s Notes and a senior pilot in the flight showed me over the aircraft. I then sat in the cockpit while he ran over the cockpit layout, engine start up, taxiing, take-off, general flying and landing procedures with the relevant flying and stalling speeds. The first thing I noticed sitting in the Mustang cockpit for the first time was the complication of the circuit breakers situated on a panel on the right side of the cockpit. I asked their function and my ‘instructor’ replied, ‘Well, I have never really figured them out and I just ignore them.’ I asked what he did if one circuit breaker popped out and he answered, ‘Oh, I just give it a bang a
nd hope for the best.’ I asked him if he had done a Mustang conversion and he said he was on Typhoons before joining the squadron. With his guidance I started the engine and my ‘instructor’ repeated engine settings and limitations with emphasis on the recommended approach and landing speeds while carrying full fuel load and drop tanks. ‘Don’t forget to increase speed on your approach turn by around ten mph; I don’t let it get below 125 mph and if there is a good wind down the runway I add another 5 mph for mother. Keep plenty of power on otherwise you’ll fall out of the sky and the Mustang has the nasty habit of flicking onto its back at the stall! Threshold speed should be no lower than 110 mph with power on and if in doubt add 5 mph for someone who cares – Happy landing!’
With this assurance I taxied out, completed my checks, turned onto the runway and started my take-off run. Accustomed to the Spitfire I opened up to about half power and whereas the Spitfire responded to the throttle like a greyhound from the slips, the Mustang responded sluggishly by comparison and I increased the power and pushed forward on the stick to get the tail up to the flying position. The steerable tail wheel initially helped keep the aircraft straight but as the tail raised the aircraft started to swing to the left and increasing more power to get airborne I found that despite using all the strength in my right leg I could not hold the aircraft straight. A transport aircraft uses the terms V1 and V2 to denote take-off commitment speeds, and as in this instance I felt committed not to abandon my take-off and risk a ground loop off the runway with full fuel on board, I continued to pour on the power. The Mustang became airborne in a climbing left turn and at this stage I had insufficient flying speed to straighten my climb away. I am sure that during the next few seconds, although appearing like minutes to me, the viewers in the control tower were laying short odds whether I would clear the high trees bordering the airfield. With a shaking right leg I cleared the trees by a few feet and as I started to build up speed I was able to settle down and trim the aircraft into a climb, waiting for the shakes to subside, the adrenaline flow to return to normal and the sweat to disperse. My reaction at the time to flying a Mustang compared with a Spitfire was not that of riding a wild horse of the American plains but more like riding a hippopotamus compared with a thoroughbred. This was not an auspicious moment to start my vocation as a fighter pilot on the renowned Mustang where due to crass stupidity my first flight on the aircraft was nearly my last ever! I settled down and flew around the area checking out the fuel tanks and burning up the fuel as much as I could. I tried out the undercarriage and flaps, getting a feel at circuit speed and near the stall before calling for a landing. I made a circumspect circuit with a gentle approach turn around 125 mph with a straight approach to the runway. I crossed the threshold at 110 mph to land well into the runway and use up most of the 1,000 yards of tarmac before taxiing to the squadron dispersal. I had by now recovered from my take-off exhibition but was in an angry mood at not being properly briefed on the take-off peculiarities of the Mustang when loaded with over 400 gallons of fuel.
