Of all the piston-engined fighters of the Second World War, the Spitfire was the subject of the greatest level of development. The later variants had a top speed of approximately 460 mph and a climb rate of 5000 ft/min, which represented performance improvements over the Spitfire I of 28 per cent and 100 per cent respectively. Such advances were largely the result of vastly increased power, the Rolls-Royce Griffon 61 of the Spitfire F.21/24 developing 2050 hp, slightly more than twice the output of the Merlin C type as fitted to the prototype. In addition, there was a four-fold increase in the Spitfire’s weight of fire (four 20 mm Hispano cannon in comparison with eight 0.303-in Browning machine-guns) and a doubling of its loaded weight. The prodigious rise in engine power, together with increases in propeller size and blade area to transmit that power, eventually led to handling problems, which the testing regime was able to identify so that modifications could be effected before it became a service problem. In the early days, however, the Spitfire was beyond criticism and the prototype (K5054) was delivered to A&AEE at Martlesham Heath in May 1936 for an initial assessment of its performance capabilities, and for handling trials.
Although the Spitfire had significantly better performance than contemporary biplane fighters, it was found to be extremely easy to fly and had no vices at all. It was stable laterally and if one wing was lowered and the control column released, it would return to a level keel in a reasonable amount of time. The aircraft was also stable directionally under all conditions of flight with the engine on or off. Longitudinally, the Spitfire was neutrally stable with the engine on but stable with the engine off, although it tended to be unstable in the glide with the flaps and undercarriage down. During take-off, there was a slight tendency to swing, but this was not as pronounced as on a Hawker Fury and it could easily be corrected by use of rudder. Landing was also straightforward and if the engine had to be opened up, as in the case of a go-around, the aircraft could easily be held with the stick. A number of aerobatic manoeuvres were flown including loops, half rolls off loops, slow rolls and stall turns and the aircraft was pleasant to handle throughout.
In the air, the ailerons were light to handle in the climb and although they were noticeably heavier with increased speed, they were considered to be not unduly heavy, even during dives up to 380 mph IAS. The ailerons were effective down to stalling speeds and response was rapid at all times. No snatch or vibration was experienced at any time. The rudder was generally heavier than the ailerons, but not excessively so and it was felt that pedal pressures were in line with expectations for a single-seat fighter. The elevator control was also light but, again, tended to become heavier with an increase in speed. A rapid response was obtained for very little control input and on landing the control column did not need to be pulled all the way back to obtain the correct three-point attitude. As it was felt that this particular characteristic could catch out inexperienced pilots, it was recommended that the gearing of the elevator control be adjusted accordingly.
Owing to its very effective elevator control, it was not necessary to pull the control column right back to bring about a stall. When it occurred, the straight stall was normal and there was no snatch. In tight turns of around 3 g at speeds below 140 mph IAS, a distinct juddering was felt by the pilot, but this stopped as soon as back pressure on the stick was released. In a fully developed stalled glide with the flaps and undercarriage up, the aircraft tended to wallow from side to side and some snatch was felt from the elevators, but this was eased when the flaps and landing gear were lowered. Although a wing could be raised when close to the stall, eventually there was a tendency for the aircraft to take over, and although this was rather disconcerting for the pilot, at no time did a spin result. One of the reasons for the Spitfire’s controllability at low speeds was that washout had been incorporated into the wing, whereby the angle of incidence at the tip was 2 degrees less than at the root. This allowed aileron control at very low speeds as the wing stalled from root to tip, and the progressive aerodynamic buffet experienced as speed was reduced gave pilots ample warning that a stall was imminent. The stall speed with flaps and undercarriage up was 64 mph IAS, reducing to 58 mph IAS with the flaps and undercarriage down.
In its summary, the A&AEE report concluded that the Spitfire’s controls were entirely satisfactory and no modification was required except to the elevator circuits as mentioned above. All the controls were well harmonised and resulted in an excellent compromise between the contrasting requirements of manoeuvrability and a stable gun platform. Although take-off and landing were easy it was felt that the aircraft had a rather flat glide, even when the undercarriage and flaps were down, which resulted in a prolonged float if the approach speed was a little high. To improve this situation it was recommended that the flaps be modified to move through an angle of 90 degrees instead of the 57 degrees as tested (this modification was applied to the Spitfire I).
In October 1938 a more comprehensive series of tests were carried out to determine the spinning and diving characteristics of Spitfire I K9787. The first spins were carried out from a straight stall. During these it was noticed that there was considerable unevenness, especially when spinning to the right, when the rotational speed showed a pronounced variation during each turn, together with a rising and falling of the nose and large changes of sideslip. After three turns the spin became smoother, except in spins to the right at aft CG, where it remained rough throughout. Some snatching of the rudder and aileron was also noticed during spins to the right, together with much buffeting and vibration, the spin also appearing to be flatter.
To come out of the spin the application of full opposite rudder, together with a forward movement of the control column, brought recovery within 1–2 turns. However, when rotation ceased the aircraft was in a stalled condition and there was a strong tendency to flick into another spin in either direction. Moving the stick forward too soon or too quickly could also result in a delayed recovery and considerable height loss. Spins were also induced from turning flight and although entry was fairly benign off a gentle gliding turn, it was a different matter when entering a spin from a stalled turn. In this case the aircraft carried out a most violent series of evolutions before settling down into a steady spin after 2–3 turns. Violent pitching was experienced, during which the pilot was thrown about the cockpit, and it was felt that the aircraft might turn on its back at any time.
Diving trials showed that the Spitfire was steady in the dive when its Merlin engine was running correctly, but the aircraft showed a tendency for the engine to cut in and out and the resulting intermittent loss of power caused a certain amount of longitudinal pitching. Vibration was also experienced between 350–380 mph IAS and above 400 mph IAS. Slight control movements were made at maximum speed without any sign of control instability. As speed increased, all controls became heavier, especially the ailerons and rudder, the ailerons eventually becoming almost immovable. The aircraft also exhibited tail heaviness in high-speed dives and it had to be held into the dive, a situation that was most marked at normal CG when the force required to maintain the correct dive angle was considerable. Recovery was straightforward, but care had to be taken not to allow excessive accelerations to develop as the control column had a tendency to come back strongly. Another aspect noted in the dive was that the canopy could not be opened at speeds above 300 mph IAS (later aircraft had a small break-out panel incorporated in the hood to equalise the pressure inside and outside the cockpit).
The development process was constant throughout the Spitfire’s life and from the seventy-eighth production Mark I the original two-blade wooden propeller was replaced by a three-blade, two-speed de Havilland propeller. With ejector exhausts, the top speed was officially quoted as 367 mph, but increases in all-up weight had reduced the climb performance. By the end of the Battle of Britain, most Spitfires had been fitted with constant-speed propellers. The Spitfire II was virtually identical to the Mark I, apart from having a Merlin XII and Coffman cartridge start. The next major production variant was the Spitfire V, which was powered by a 1440 hp Merlin 45 and was to be the mainstay of Fighter Command from 1941–3. Its problems with the Focke-Wulf Fw 190 (see Chapter 9) led to the hurried introduction of the Spitfire IX in June 1942.
Although it possessed the same basic airframe as its predecessor, the Mark IX introduced the two-speed, two-stage supercharged Merlin 61, which brought about a significant improvement in performance. In April 1942 AB505 (a converted Spitfire V) was tested by the Air Fighting Development Unit (AFDU) at Duxford. These trials included a comparative assessment with a Spitfire V and Typhoon I. On take-off the Spitfire IX was similar to the V but, due to its increased weight, the landing speed was slightly higher. During dives the aircraft felt more stable and showed less tendency to yaw, which was put down to the fact that there was now a radiator under both wings. The elevator control was slightly heavier, but this was considered to be an improvement, as it tended to result in better harmonisation.
The speed of the Spitfire IX was measured at 386 mph TAS in MS gear at 16,300 ft and 409 mph TAS at 28,000 ft in FS gear, figures that were vastly superior to the Spitfire V. Two speed runs were made to compare the Spitfire IX with the Typhoon. At 15,000 ft the Spitfire IX was around 10 mph faster and at 18,000 ft it was 2 mph faster. Comparative climbs were also carried out and the Spitfire IX was superior to the Mark V and the Typhoon at all heights. Under maximum continuous climbing conditions the Spitfire IX was taken up to a height of 39,500 ft at which point its rate of climb was still 700 ft/min. The operational ceiling was considered to be 38,000 ft (the height at which the climb rate fell to 1000 ft/min) and this was achieved 18½ minutes after take-off. The Mark IX proved to be easy to fly at high altitudes, although occasionally the trimming tabs tended to freeze up, which could be a little embarrassing if the aircraft was still trimmed for the climb.
In dogfights there was little to choose between the Spitfire V and IX at 15,000 ft, although the superior speed and climb of the Mark IX allowed it to disengage by climbing away and then attack again in a dive. The Spitfire IX also had the advantage of being fitted with a negative-g carburettor, which allowed its pilot more freedom and lessened the risk of power loss due to fuel starvation. At 30,000 ft the two aircraft were evenly matched in terms of manoeuvrability, but at this height the superiority of the Mark IX with regard to speed and rate of climb was decisive. The pilot of the Spitfire V had great difficulty in maintaining height during steep turns, whereas his counterpart in the Mark IX was able to retain height without difficulty because of the large reserve of power at his disposal. During a simulated dogfight with a Typhoon, the Spitfire IX was found to be more manoeuvrable and superior in the climb, although it tended to lose out in a dive. In a turning competition at 18,000 ft, the Spitfire out-turned the Typhoon and was on its tail after 1½ turns.
Although the Spitfire could out-turn almost any other Second World War fighter, it still lost out to others (notably the Focke-Wulf Fw 190) in the speed with which it could initiate rolling manoeuvres. This problem was addressed by clipping the wings of many Spitfires that were likely to see low-altitude combat and involved the removal of the wing tips, which reduced the span from 36 ft 10 in to 32 ft 6 in. During trials at AFDU in late 1942 using a Spitfire VB, it was found that the rate of roll at all heights up to 25,000 ft had been improved considerably and that the response to aileron movements was much quicker than on the standard machine. Dogfights were carried out in which a standard wing Spitfire was put on the tail of another with clipped wings. In several cases the clipped wing aircraft evaded so rapidly that it was able to reverse the positions in about twenty seconds (by this time problems with excessively heavy lateral control during high-speed dives, a characteristic of early Spitfires, had been improved to an extent by the fitting of metal-covered ailerons).
From the middle of the war, Spitfires began to take on a fighter-bomber role and even elderly Mark VBs were pressed into use with a 500-lb bomb on the fuselage centreline. With a strengthened Type ‘C’ or ‘universal’ wing, the Spitfire VC and most Mark IXs were capable of carrying two 250-lb bombs under the wings. AFDU was given the task of devising suitable tactics as Flight Lieutenant Len Thorne recalls:
When they first started to hang bombs on Spitfires, we were given the job of evolving the best way of ensuring accuracy. Obviously, the most effective way was to get enough altitude, point the nose straight at the target in a very steep dive and let the bomb go. However, when we proposed this method of attack to the squadrons, they wouldn’t have it as they were concerned that the bombs would hit the aircraft after release. To find out one way or the other, ‘Wimpy’ Wade [Squadron Leader T.S. Wade DFC, later chief test pilot at Hawker Aircraft] and myself flew two Spitfires, one of which was carrying a bomb, while the other had a camera fitted behind the pilot’s seat, pointing sideways. The camera aircraft had a white dot on the end of the wing and the pilot lined up the dot with the bomb on the other aircraft. By such means we got the actual moment of release on film.
‘Wimpy’ Wade, who was an excellent pilot, did the bomb dropping and I had to remain tucked in tight with whatever he did. We evolved the method that you overflew the target, then looked back behind the trailing edge of the wing and as soon as you could see the target you pulled up into a wing-over to the inverted and then pulled back on the stick until you were heading for the target in a dive at almost exactly 70 degrees. I filmed right through the sequence and it was discovered that the bomb never went anywhere near the aircraft. In the dive you were fairly screaming down at around 480 mph so the first thing you did was to commence your pull out and as a result quickly left the bomb behind. This method was eventually adopted for all high level bombing attacks. I did quite a lot of this type of work over the Holbeach range in the Wash, but I absolutely hated it, in my opinion bombing was a complete misuse of a fighter!
The advent of second-generation piston-engined fighters such as the Hawker Typhoon and Republic P-47 Thunderbolt, and the development of existing types, meant that many service aircraft were now able to exceed the Mach number at which shock waves were produced on the wings during prolonged dives. The effects of compressibility had been known for some time and much work had been carried out in high-speed wind tunnels, but there was little flight data of a sufficiently detailed type to enable the test results obtained on the ground to be checked. From May 1943 an exhaustive series of flight trials were commenced at RAE Farnborough using Spitfire XI EN409 and Mustang I AG393. The pilot throughout the test was Squadron Leader J.R. Tobin AFC.
The trials were carried out from the highest altitude to obtain the greatest possible Mach number while keeping IAS and airframe loads down to the lowest values for flight safety. As the Mustang One used in the trials had an Allison engine that developed maximum power at only 10,000 ft, the flights were made with guns and radio removed to reduce weight but even so dives could only be commenced at 28,000 ft. The Spitfire XI, in contrast, could be dived from 40,000 ft. The dives were started by accelerating to maximum level speed at these heights, before the nose was lowered. At the same time the pilot set the engine controls to a position which would give maximum permissible continuous boost at the end of the dive (MS blower in the case of the Spitfire). The dive angle was usually about 45 degrees, the procedure being to dive steadily until maximum Mach number had been reached (this took about 11,000 ft in all cases) then to continue for a few more seconds before commencing a 2–3 g pull out.
Before beginning the dive the pilot was warned of the possibility of large trim changes in the nose-down direction and of the possible ineffectiveness of the elevator trim tab. He therefore trimmed into the dive at the beginning, but when the nose-down change appeared near maximum Mach, he made no attempt to correct on the trimmer, but held it by stick force alone, always assuming that this was physically possible. In general, the drag coefficient rose very gradually in the region of Mach 0.6–0.7 or more, but this was followed by a rapid increase at higher Mach numbers as the shock-stall commenced over the main wing. The most significant fact to come out of the trial was the difference in the Mach number at which the steep drag rise commenced on each aircraft.
With its low thickness/chord ratio wing (13 per cent at the root and 7 per cent at the tip), the Spitfire was easily superior to the Mustang, despite the fact that the latter had a laminar flow wing with maximum thickness at 40 per cent chord. Although this type of wing had been designed to reduce drag, the Mustang’s thickness/chord (t/c) ratio at the root and tip was 16 per cent and 11 per cent respectively and it appeared that at critical Mach numbers, t/c ratio was the dominant factor. The Mustang was dived to a maximum of Mach 0.80, whereas the Spitfire achieved Mach 0.89. Both the Spitfire and the Mustang showed the same tendency, near maximum Mach, to develop a nose-down moment, which had to be countered by applying negative elevator.
The sequence of events involved a push forward on the stick, which was maintained for the first few thousand feet of the dive. The pilot then found it necessary to release this force to maintain the correct angle of dive and finally he had to pull to prevent the dive angle from becoming excessively steep. The pull force reached a maximum upon, or just after, reaching the maximum Mach number. If the pull force was not corrected rapidly, either through unawareness or because the stick load required was too large, the dive steepened and the Mach number increased more rapidly, to the point where the pilot could not exert sufficient force on the elevator to regain control. This had been reported on several types of aircraft, including the P-47 Thunderbolt, which had been tested in terminal velocity dives in the USA. Control would eventually return as the Mach number diminished in the thicker air at lower levels, but this introduced a second danger as the relaxation of the nose-down moment, together with continued use of back stick, could lead to severe accelerations. This was particularly the case if nose-up trim had been applied earlier in the dive to assist in applying the negative elevator angle required near the maximum Mach.
The P-47 Thunderbolt had a relatively thick wing of conventional section (16 per cent and 9 per cent thickness/chord at root and tip) and this produced a steep drag rise at a very low Mach number. Whereas a rough estimate of the pull force required to hold a Spitfire at Mach 0.89 at 30,000 ft was 50–60 lb, a similar calculation showed that a pull of 200 lb, even assuming a pilot was capable of doing this, would still be inadequate to hold a P-47 at 20,000 ft. The maximum Mach achieved in the P-47 was reported as being 0.86, but the aircraft was out of control at this point and remained so until reaching lower altitudes.
With the Merlin engine at the limit of its development potential, the only way for the Spitfire to retain its position at the forefront of fighter technology was for it to utilise another engine of yet greater power. Such an engine already existed in the shape of the 36.7-litre Griffon, which had been developed from the Rolls-Royce ‘R’ engine of the Supermarine S.6B Schneider racer. Although the Griffon was initially rated at around 1500 hp, thanks to its racing pedigree, the frontal area was only marginally greater than the Merlin and it was a mere 3 in longer. It was 600 lb heavier, however, which necessitated a change from the tubular-type dural engine mounting used on Merlin variants to a girder-type steel longeron. The first Griffon-engined Spitfire was the Mark IV (DP845) which was flown for the first time on 27 November 1941. It was redesignated as the Mark XX in early 1942 to avoid confusion with the PR.IV photo-reconnaissance aircraft. Following a further change of designation, the first Griffon-powered Spitfire appeared as the Mark XII and the third production machine (EN223) was delivered to AFDU for testing in December 1942.
Early examples of the Spitfire XII were based on a standard Mark VC airframe, strengthened to accept a Griffon III two-speed supercharged engine optimised to deliver maximum power at low level. Its role as a low-altitude fighter meant that all Spitfire XIIs had clipped wings. Despite weighing in at 7415 lb fully loaded (approximately 1,000 lb more than a Spitfire V) the Mark XII had a ‘normal’ Spitfire feel to it, but the increased power was immediately felt on take-off, as engine torque tended to cause a swing to the right which, if not counteracted quickly, could not be held, even with full rudder. This swing to the right was in the opposite direction to that experienced on Merlin-engined Spitfires, as the propeller on the Griffon rotated the other way (i.e. to the left as seen from the cockpit). In the air the handling of EN223 was far superior to a standard Spitfire VB or IX, in particular its lateral control, which was crisp and light thanks to its clipped wings. Longitudinal stability was found to be better than a Spitfire V, particularly in a dive, and the recovery was not as fierce. The rudder was more sensitive to throttle movements and much re-trimming was needed as pedal pressures were too heavy to be held for long periods. Pilots were also quick to notice that the Griffon ran more roughly than the Merlin engines they had been used to.
In terms of performance, the Spitfire XII was considerably faster than a Mark V and also out-performed the Spitfire LF.IX (Merlin 66) by 14 mph at sea level and 8 mph at 10,000 ft. Above 20,000 ft, however, it was slower. In the climb it also lost out to the Spitfire LF.IX and was comparable up to 10,000 ft to the LF.VB powered by the ‘cropped blower’ M-series Merlin 45, which had been developed to achieve maximum power at low levels. Dive trials showed it to have a slight edge owing to its cleaner design, except at full throttle when there was no advantage either way. Manoeuvrability was as good as earlier Spitfires and it was considered that the XII would be able to out-pace and out-turn an Fw 190 below 20,000 ft. As the Griffon engine was set marginally lower in the airframe than the Merlin, the sighting view downwards for gun aiming was slightly improved. In its summary, AFDU concluded that the Spitfire XII was highly suited to the role of a low-altitude fighter, being capable of speeds of 372 mph at 5700 ft and 397 mph at 18,000 ft.
In the event, only 100 Spitfire XIIs were produced as this variant was quickly supplanted by the Mark XIV, which was powered by a two-speed, two-stage supercharged Griffon 61/65 of 2050 hp. To cater for the increase in power a five-blade constant-speed Rotol propeller was used and the first of several revisions of the tail surfaces were incorporated to maintain directional stability. The airframe used for the Mark XIV was essentially a strengthened and modified Mark VIII, but with the use of the two-stage Griffon with its intercooler, the length had increased to 32 ft 8 in. The loaded weight was now 8500 lb.
Following performance trials with JF319, in which the maximum speed was measured at 446 mph at 25,400 ft, a full tactical trial was carried out by AFDU in early 1944 using RB179. In most respects, the Mark XIV was similar to the Mark IX, except that the need to re-trim following throttle movements (as noted with the Spitfire XII), was quite marked. Pilots also had to be aware of the additional power on take-off as the aircraft tended to swing strongly to the right and drag its right wing. It was recommended that full power only be selected when nearly airborne; prior to that +6 lb/sq.in boost was quite sufficient. The landing run was a little longer and the aircraft tended to sink more rapidly than a Spitfire IX. When stalling from a tight turn, the Mark XIV tended to give less warning, although the characteristic shuddering was still present.
In comparison with the Spitfire IX, the Mark XIV had slightly reduced endurance due to the fact that it consumed about 25 per cent more fuel, but its range was similar as it tended to cruise at a higher speed. At all heights the Mark XIV was 30–35 mph faster in level flight, its best performance occurring below 15,000 ft and between 25,000 and 32,000 ft. It was also slightly superior in the climb and pulled away from the Spitfire IX in a dive. The turning circles of both aircraft were virtually identical, as was the rate of roll. Owing to its short range, it was considered that the Spitfire XIV would tend to be operated with a 90-gallon long-range tank, rather than the more normal 30- or 45-gallon tanks, and in this condition top speed was reduced by about 20 mph. The climbing performance was also affected and with a half full tank, the climb rate was identical to a Spitfire IX when flown clean. If the tank was more than a third full, acceleration in a dive was not affected, but there was a definite worsening of the turning circle with the tank fitted.
The Spitfire XIV was also compared with a Tempest V, the latter proving to be 20 mph faster up to 10,000 ft. There was then little to choose between the two until 22,000 ft when the Mark XIV held the upper hand, being around 30–40 mph faster above this height. The operational ceiling of the Spitfire XIV was 40,000 ft, which was 10,000 ft more than the Tempest. As regards climb rate, the Spitfire XIV possessed a significant advantage, although the Tempest showed a better zoom climb as it held a higher speed throughout the manoeuvre. As speed diminished, however, the Spitfire soon began to catch up and if the climb was prolonged it quickly pulled ahead. Because of its increased weight, the Tempest accelerated quicker in the dive, but it was easily out-turned by the Spitfire. In terms of rate of roll, the Spitfire held the advantage below 300 mph, whereas the Tempest was slightly superior at higher speeds. The Tempest and Spitfire XIV were two very different aircraft and it was clear that the former was best used for combat below 20,000 ft, whereas the latter was greatly superior above that height. A comparison was also made with a Mustang III, which showed that the maximum speeds of the two aircraft were virtually identical. The Spitfire held the advantage when it came to climb rate, turning circle and rate of roll, but it tended to lose out in the dive. One area in which the Spitfire could not compete, however, was radius of action, as even with a 90-gallon long-range tank, it still only had half the range of a Mustang fitted with two 62½-gallon drop tanks.
The opportunity was also taken to compare the Spitfire XIV with captured examples of the Fw 190A and Bf 109G. Up to 20,000 ft the Spitfire was 20 mph faster than the Fw 190, but above this height its advantage rose to 60 mph. In the climb the Spitfire held a significant advantage, but its diving performance was only marginally better. Thanks mainly to its large, well-balanced ailerons, the rate of roll of the Fw 190 was superior, but in a sustained turn the Spitfire could easily out-turn the German aircraft. Against the Bf 109G, the Spitfire XIV was 40 mph faster at all heights, except around 16,000 ft when this advantage was reduced to only 10 mph. At this height climb rates were the same, but at all other heights the Spitfire was superior. Zoom climbs were similar except when full throttle was used, when the Spitfire pulled away from the Bf 109 quite easily. Dive performance was evenly matched, with the Bf 109 holding an initial advantage until a speed of 380 mph had been reached when this situation was reversed. The Spitfire XIV could easily out-turn the Bf 109 in either direction and it also rolled much more quickly.
The Spitfire XIV was also much liked by A&AEE pilots who had been among the first to assess the new fighter. One of those involved in the testing carried out at Boscombe Down was Group Captain Jamie Rankin DSO DFC, who wrote a short comparative assessment with the Merlin-powered Spitfire VIII in October 1943.
Brief Description – the aircraft flown in these trials was not equipped with the same supercharger gears as the production model. In the aircraft tested the engine gave maximum speed at 7000 ft and 23,000 ft whereas the production Mark XIV will have the maximum speeds at 12,000 ft and 23,000 ft. This difference involved a slight gain in performance at low levels, but a loss of performance between 8–15,000 ft. The boosts used were + 18lb in both the XIV and VIII.
Performance – comparison was made with the Spitfire VIII at all heights. The XIV was found to be approximately 30 mph faster. In the climb the difference was not so marked, but the XIV was definitely ahead of the VIII at all heights, the difference being more marked above 30,000 ft than below. The most noticeable difference between the aircraft arose when using cruising revs and boost, under these conditions the XIV is considerably faster. Probably owing to its increased weight, the diving speeds of the XIV are higher than those of the VIII.
Handling – the XIV handled quite normally in all respects with a Spitfire VIII or IX except in directional stability when changes of throttle setting were used. In this respect it is similar to the Spitfire XII in that frequent adjustment of the rudder trim must be made. It appeared, however, to be less marked than in a Spitfire XII and after flying the aircraft for approximately four hours the pilot becomes rapidly accustomed to this. Aileron and elevator control were normal. The turning circle is also normal and there is no tactical difference between the XIV and VIII. The slight difference in turning circle is in favour of the XIV. The aircraft handles similarly to the XII on take-off and is naturally not so easy as in a Mark VIII or IX. As in the question of change of rudder trim, however, pilots will rapidly become confident in the aircraft on take-off.
Pilot’s View – the view is much superior on the Mark XIV owing to the lower engine cowlings and will be of great advantage in deflection shooting.
Conclusions – the Mark XIV is preferable to the VIII or IX in all respects. Once the initial awkwardness of opposite torque and change of directional trim at speed has been overcome, the aircraft feels exactly as a normal Spitfire with the advantages of improved climb and a considerable gain in level speed.
Another pilot to fly the Spitfire XIV at this time was Wing Commander A.V.R. Johnson DSO DFC, who commented on the aircraft as follows.
Starting – at present starting of the Griffon 65 is by means of Coffman starter and is most complicated. It is recommended that the process be simplified before this aircraft is put into operational use.
Take-off – the take-off is similar to that of the Spitfire XII. Because of propeller torque the aircraft tends to swing to starboard and the starboard wing is inclined to drop until speed is in excess of 30 mph. Immediately this speed is passed, however, no further peculiarities are encountered.
Climb and Level Flight – this aircraft is undoubtedly faster than any Spitfire yet in operational use. It is both faster than the Mark XII at low altitudes and the Mark IX at high altitudes.
General Flying – the ailerons on the aircraft flown were rather heavy but no doubt these could be lightened by experimenting with various sets of ailerons. A great deal of top rudder has to be applied in tight turns, this being due to the additional weight of the aircraft and the position of the CG. Stalling speed is somewhat higher than the Mark IX and the aircraft spins from a tight sustained turn at slightly higher speed than a Mark IX. The aircraft requires constant trimming in diving, climbing and turning flight, and in this respect is very similar to the Mark XII.
Landing – stalling speeds with wheels and flaps down is approximately 10–15 mph higher than the Mark IX and consequently the final approach is made at about 115 mph.
Recommendation – the Mark XIV should be brought to operational service as soon as possible. No difficulty should be encountered by pilots changing from the Mark V to the XIV.
The next Spitfire in number sequence (if not in chronology) was the Mark XVIII, which was the first variant to be specifically designed for the Griffon engine, rather than being a modification of an existing airframe. In appearance it was almost indistinguishable from a low-back Mark XIVE with clipped wings – most of the differences being under the skin, in particular its revised wing structure with solid instead of tubular spar booms. The Spitfire XVIII entered service with No. 60 Squadron at Seletar, Singapore shortly after the end of the war and was also used by Nos 11, 28, 32 and 208 Squadrons in the Middle and Far East.
The final major redesign of the Spitfire resulted in the Mark 21. Although the Spitfire had always been able to out-perform the Bf 109 and Fw 190 during sustained turns, it was not as good in terms of roll response. Supermarine had been looking to improve this aspect of the Spitfire’s performance for some time, but it was clear that any increase in roll rate would require a stiffer wing to avoid the possibility of ‘aileron-reversal’ caused by the wing twisting in the opposite direction to aileron deflection, due to a lack of torsional rigidity. The main structural element of the Spitfire wing was the D-shaped torsion box, formed by the single spar and the heavy-duty leading edge skinning. On the Mark 21 this was augmented by a number of torque-boxes behind the spar that increased stiffness by 47 per cent and upped the theoretical aileron reversal speed from 580 mph to 850 mph. The revised ailerons with piano-type hinges and balance tabs imparted much better lateral control and the top speed went up by 10 mph compared with the Spitfire XIV, mainly due to the use of a larger 11 ft diameter propeller, which required the oleo legs to be extended by 4½ in. On the downside, directional and longitudinal control were by now becoming marginal in certain areas of the flight envelope, largely due to the destabilising effect of the new propeller, which was 7 in greater in diameter than that fitted to the Spitfire XIV.
In late 1944 LA201 was delivered to AFDU at Wittering for a tactical trial and for once the Spitfire came in for a fair amount of criticism. Although the aileron control was rated as the best yet encountered on any mark of Spitfire, the aircraft was found to be unstable in the yawing plane, especially at altitude and at high speed. The rudder was very sensitive to small movements and most pilots had difficulty achieving balanced flight as the aircraft was prone to skidding or slipping. The elevator control was positive and the aircraft was stable in pitch, but constant correction was necessary, especially at low speed, and at high altitude at all speeds. Trimmers were provided for the rudder and elevators, but movement was extremely critical, a factor that was of most relevance when accelerating in a dive, the aircraft being difficult to fly accurately. The combination of instability in yaw, an increased stalling speed due to higher wing loading and critical trimming qualities, produced unpleasant handling characteristics that compared unfavourably with other fighters.
The Spitfire F.21’s deficiencies were particularly marked when flying on instruments or at low level. In conditions of bad visibility, it was considered that the feeling of instability, together with the poor forward view of the Griffon-engined Spitfire, made flying the aircraft particularly hazardous. Its sensitivity in pitch meant that it was also easy for the pilot to over-control, leading to potentially dangerous height loss when flying at low level. Aerobatics were less easy than on any other mark of Spitfire, as was formation flying. Despite the fact that the Spitfire F.21 was 10–12 mph faster than the Spitfire XIV and had better aileron control at speeds above 300 mph, its handling was otherwise unacceptable as was made clear in the conclusions to the AFDU report.
The instability in the yawing plane and the critical trimming characteristics of this aircraft make it difficult to fly accurately under the easiest conditions and as a sighting platform it is unsatisfactory both for air-to-air gunnery and ground attack. Its handling qualities compare unfavourably with all earlier marks of Spitfire and with other modern fighters and more than nullify its advantages in performance and fire power. The Spitfire XIV is a better all round fighter than the Spitfire F.21. The handling qualities of successive marks of the basic Spitfire design have gradually deteriorated until, as exemplified in the Spitfire F.21, they prejudice the pilot’s ability to exploit the increased performance.
It is recommended that the Spitfire F.21 be withdrawn from operations until the instability in the yawing plane has been removed and that it be replaced by the Spitfire XIV or Tempest V until this can be done. If it is not possible then it must be emphasised that, although the Spitfire F.21 is not a dangerous aircraft to fly, pilots must be warned of its handling qualities and in its present state it is not likely to prove a satisfactory fighter. No further attempts should be made to perpetuate the Spitfire family.
It was clear that the only real solution to the Spitfire F.21’s handling deficiencies lay in a redesign of its tail surfaces, but with aircraft already coming off the production lines at Castle Bromwich and South Marston, a temporary fix would have to suffice. The modifications carried out were the removal of the balance function of the rudder trim tab, together with slightly smaller elevator horn balances and a reduction in the gearing to the elevator trim tab. Another early production Spitfire F.21 (LA215) was dispatched to AFDU in March 1945 to see if any improvement had been made.
Once airborne, it was apparent that many of the adverse handling characteristics had been eradicated. The ‘hunting’ which had been experienced as a result of the extreme sensitivity of the elevators was no longer apparent and much less trimming was needed to maintain balanced flight. The rudder control was much improved, although the trimmer was still quite sensitive and care had to be taken to prevent a slip or skid from developing. The reduced gearing to the elevator trim tab had a major impact on controllability and made for smooth and accurate flying. The improvement in handling was most noticeable when flying on instruments and the reduced sensitivity of the elevators in particular made the aircraft acceptable for flying in formation during cloud penetration. In such situations, it was recommended that throttle movements be kept to a minimum to reduce the risk of setting up a strong yawing moment. Low flying was also much more pleasant and low level manoeuvring in bad visibility did not cause any difficulty. In marked contrast to what had been written just three months before, AFDU now considered the Spitfire F.21 to be a satisfactory combat aircraft for the average pilot. Enlarged tail surfaces to restore directional and longitudinal stability were eventually introduced on the F.22 and F.24.
One Spitfire development that may have seen widespread use had it not been for the rapid advance of the jet-powered fighter, was the use of contra-rotating propellers. One of the biggest drawbacks of the piston-engined fighter, as typified by the Spitfire F.21, was the difficulty of harnessing the power of the engine while retaining adequate control. As engine power increased, it was necessary to absorb that power by increasing blade area by having larger diameter propellers and more blades (five on the late mark Spitfires), but this made it increasingly difficult to control the torque reaction and slipstream effects that were produced. Contra-rotating propellers did away with these problems as the two airscrews turned in opposite directions.
The first Spitfire to be modified was AB505, a Mark V that was subsequently converted as a Mark IX and fitted with a Merlin 77 and six-blade Rotol contra-prop. Several Spitfire XIVs were tested with contra-rotating propellers, including the sixth prototype JF321 (Griffon 61) and RB144 (Griffon 85), and a number of Spitfire F.21s were also powered by the Griffon 85 driving either a Rotol or de Havilland six-blade contra-prop. One of these aircraft (LA218) was the subject of a brief test by AFDU carried out in May 1945.
Although the take-off run appeared to be a little longer than a standard Spitfire F.21, the handling was much improved. Provided that the rudder had been correctly trimmed, there was no tendency to swing at all. Once in the air, the pilot was able to climb the aircraft with his feet removed from the rudder controls, without fear of a skid or slip developing. If trimming had been applied incorrectly, however, a swing was likely to develop. Aerobatic manoeuvres were transformed with the contra-prop installation and the pilot could virtually forget about the rudder, irrespective of throttle setting and/or speed. The aircraft’s steadiness in the directional sense was particularly useful during simulated ground-attack dives of up to 60 degrees. During these, it was found that the sight could be held on the target with ease and there was not the slightest tendency for the nose to wander. The aircraft did become tail heavy at about 280 mph IAS and a large forward trimming movement was necessary in order to fly ‘hands off’ especially above 400 mph IAS. Once trimmed, the dive was absolutely straight. The light aileron control of the Spitfire F.21 came to the fore during the breakaway manoeuvre, as it was possible to carry out quite violent evasive action without fear of losing control.
Landing was straightforward, with a very marked braking effect with the propeller in the fully fine position. However, the landing run was considerably longer than on previous aircraft. This was most likely owing to the engine idling a little faster than normal, pilots being reluctant to throttle back fully as this was liable to lead to a complete cut-out. The lack of torque generated was particularly noticeable during a simulated go-around when the throttle was advanced at near stalling speed. The stability was much improved and the possibility of an accident considerably reduced. The significant improvements in controllability of the contra-prop Spitfire F.21 did not come at the expense of performance, which was comparable with that of the standard aircraft. Despite its advantages, contra-prop Spitfires did not enter service with the RAF, although the Seafire FR.47, powered by a Griffon 85 with contra-rotating propellers, did see action with the Fleet Air Arm during the Korean War.