The emergence of the new generation of fighters in Europe, epitomised by the Supermarine Spitfire and Messerschmitt Bf 109, caused certain US aircraft manufacturers to re-evaluate some of the more traditional aspects of American pursuit aircraft. The simplicity and ruggedness of the air-cooled radial engine had been major factors in its dominance in the inter-war years, but the apparent advantages of the high-performance, water-cooled inline engines that had been developed on the other side of the Atlantic encouraged some US firms to adopt the same philosophy. Even so, the US Army Air Corps (USAAC) would have to make do with a fighter whose performance was still best suited to ground attack and for aerial combat at low to medium levels.
With its P-36 fighter already in production, Curtiss began to look at developments with an inline engine. After experimenting with a 12-cylinder Allison V-1710 in the XP-37 of 1938, the same type of engine was fitted to the tenth production P-36A, which became the XP-40 prototype. It was flown for the first time on 14 October 1938 and was ordered in quantity for the USAAC the following year under the Curtiss model designation Hawk 81A. An export model (the H-81A-1) was ordered by France but, like the Airacobra, no deliveries had been made before the German occupation. Once again, the contract was taken over by Britain, and the aircraft was given the name Tomahawk. The RAF eventually took delivery of 140 Tomahawk Is (equivalent to the H-81A-1/P-40A), 110 Tomahawk IIAs (H-81A-2/P-40B) and 635 Tomahawk IIBs (H-81A-3/P-40C).
The first Tomahawk Is arrived in the UK in September 1940, devoid of bullet-proof windscreens, protective armour and self-sealing fuel tanks – all items that were specified in subsequent batches, some of which were dispatched to Takoradi in West Africa to be flown to Egypt for service in the Middle East. The armament was eventually standardised at six machine-guns – four 0.303-in Brownings in the wings and two 0.5-in or 0.303-in guns in the forward fuselage synchronised to fire through the propeller arc.
The P-40 was of all-metal construction, the fuselage monocoque being built in two halves and divided horizontally. The wings were multi-spar and the individual sections were formed into a single unit before being joined with the fuselage. The high-speed aerofoil was of NACA 2215 section at the root, becoming NACA 2209 at the tip. Split flaps extended between the ailerons and all control surfaces were fabric-covered with individual trim tabs. Fuel could be carried in two tanks in each inner wing section and an auxiliary tank in the fuselage behind the pilot’s seat. The undercarriage was similar to that of the Mohawk, the main oleo-pneumatic legs retracting backwards while turning through 90 degrees to lie flush with the underside of the wing. Although the main wheels were left exposed, the retractable tailwheel was enclosed by two small doors. The radiator was initially located in a ventrally mounted duct aft of the wing trailing edge, but disappointing level-speed performance with the XP-40 prototype in this configuration soon led to it being moved to a position under the nose.
British testing of the Tomahawk soon showed that it would be no match for the Messerschmitt Bf 109E/F in air-to-air combat. This was mainly due to its low-altitude rated Allison engine which performed best at heights up to 15,000 ft. This was at a time when combats between Spitfires and Bf 109s were beginning to take place at heights of around 30,000 ft.
Tomahawk IIB AK176 (Allison V-1710-C15) was one of several machines to pass through Boscombe Down and was reported on in September 1941. Entry to the cockpit was made by climbing onto the port wing root, using a handhold provided on the side of the fuselage. There was no further assistance for the pilot and as the walkway was rather steep; it was felt that a non-slip coating would have been a big improvement. Once in the cockpit, there was insufficient downward travel of the seat, and the pilot’s head was well above the top of the bullet-proof windscreen (this point had not been noticed during the testing of earlier Tomahawks). Like the Mohawk before it, the position of the control column was too far forward when in its central position, and the pilot’s arm was always at full stretch. The Sutton harness was also attached to the seat too low down, and it was considered that this would not prevent the pilot’s shoulders moving forward in the event of a crash-landing. The cockpit had the benefit of heating, but this was not particularly effective as a result of draughts from holes in the hood, which provided access to the fuselage fuel and oil tanks. This problem was eventually remedied by using special covers designed by A&AEE.
Like most fighters of the period, the view forwards when taxying was considerably impaired by the long nose. No clear-view panel was fitted. In flight, the view was good and icing was prevented by the inclusion of a space between the windscreen and the bullet-proof safety glass, through which warm air was passed. Prior to the fitting of the draught covers, misting on the internal face of the bullet-proof glass occurred during rapid descents, but this was completely eliminated with the covers in place. Occasionally, a film of moisture formed between the two screens, but there was always a clear area in the centre where it was swept by the ducted hot air. Sighting through the reflector gunsight was not impeded, but the use of the ring and bead sight was often impossible.
On the ground, all the controls could be operated without undue friction or play. The rudder and ailerons could be moved fully, but full downward movement of the elevators involved a considerable forward stretch. The control column had a solid, vertical handgrip, incorporating a gun-firing trigger and a button that operated the electro-hydraulic motor for the flaps and undercarriage. The rudder pedals were of the pendulum type and were provided with fore-and-aft adjustment. When not in use, the controls had a locking device consisting of wires that ran from the rudder, round the stick and then onto the seat. It was thus impossible for the pilot to sit in the seat without first unlocking the controls.
The elevator trim control was fitted on the left-hand side of the cockpit about level with the pilot’s thigh. Although it had a small protruding handle, it was of relatively small diameter and due to its awkward position, it could not be rotated quickly. The rudder trimmer was situated just above the elevator trim wheel and was of similar size, but without the handle. The gearing was rather low, but was otherwise satisfactory. Dials behind the wheels showed the positions of the tabs.
The engine throttle, mixture and propeller controls were mounted in the normal position on the left-hand side of the cockpit. However, the throttle lever was rather close to the cockpit side, making it difficult for the pilot to get his gloved hand around it. As in other American aircraft, there was no automatic boost control. The boost therefore had to be hand-adjusted with the throttle all the time, a considerable disadvantage for a fighter aircraft. A gate was provided to limit boost on take-off, but this was quite ineffective as the gate position did not make itself felt when the throttle was moved forward. There were four positions on the mixture control quadrant – fully rich, auto rich, auto weak and idle cut-off. A spring stop ensured that the lever could not be moved to the idle cut-off position accidentally. The electric propeller was controlled both by the lever on the engine controls quadrant and by toggle switches on a panel just below the quadrant. The petrol cock was situated low down on the left of the cockpit, but was too far forward for convenience, as the pilot had to release his harness to reach it.
In contrast, the flap selector lever was on the left side of the cockpit, but too far aft to use conveniently. After moving the selector, the button on top of the control column was depressed to set the electro-hydraulic motor working to move the flaps. Full flap movement could be obtained in 1–2 seconds and the flaps could be stopped in any position. When subject to air loads, the flaps closed automatically when the ‘up’ position was selected. The undercarriage selector lever was situated slightly forward of the flap lever. On AK176 and later aircraft, a latch bolt had to be pulled forward before the lever could be moved. The undercarriage was operated in a similar fashion to the flaps, i.e. movement of the selector followed by activation of the control column button until the movement was complete. In the case of the undercarriage, the selector lever then had to be returned to the neutral position. The wheels retracted backwards, turning through 90 degrees as they did so. Indicators for both flap and undercarriage operation were located on the lower left-hand side of the instrument panel. When the throttle was closed with the undercarriage not locked down, a red warning light appeared and a klaxon sounded. The latter could be switched off, but it re-engaged automatically when the throttle was opened again. A hand-operated hydraulic pump was fitted on the cockpit floor to the right of the pilot, for use in an emergency situation following failure of the electrical system.
The control knob for the hot and cold air intake was situated on the top right of the panel, but was disliked as it was impossible to select hot air above 140 mph IAS. The lever to control the radiator flaps was to the right of the pilot and, again, too low for convenient operation. The brakes were hydraulic and were toe-operated. They could be locked by depressing both pedals, engaging the parking brake beneath the panel and releasing the pedals. The lock could be deactivated by depressing both pedals again. As for instruments, the lack of a gyro horizon caused adverse comment and it was also noted that some instruments were obscured by the reflector gunsight. The opening handle for the hood was of the longer design recommended by A&AEE for the Mohawk, and it was now possible to open the hood at all speeds up to 400 mph IAS. There was also a jettison handle to release the hood, located on the cockpit roof just above the pilot’s head.
For ground handling, the tailwheel was made steerable via the rudder controls about 35 degrees each side of central. Beyond this range it became fully castoring. Because of this arrangement, the rudder loads when taxying were rather heavy, and there was considerable kickback on rough ground. The brakes were efficient, if rather fierce in operation, but were not particularly smooth, nor progressive in action. With CG forward, the tail tended to lift easily, so harsh brake application was to be avoided.
Flaps were not required for take-off (though a successful take-off was made with full flap) and during the run there was a slight tendency to swing to the left, which could easily be corrected with rudder. With forward CG the tail was reluctant to rise, but even when it was in the correct attitude the view over the nose was poor. The aircraft became airborne at around 80 mph IAS, and the undercarriage could be raised as soon as it was clear of the ground. The retraction time was a very long 40–45 seconds and as the pilot’s thumb had to remain on the button on the control column all this time, he was prevented from doing anything else with his right hand, such as closing the hood. As the wheels retracted the aircraft became slightly tail heavy, but this could be held easily with the stick and it could be trimmed to fly ‘hands and feet off’ when the recommended climb speed of 140 mph IAS had been attained. The Tomahawk was longitudinally stable with normal CG (22.7 in aft of datum), but with the aircraft at an extended aft CG (25.2 in aft of datum) it was generally unstable, and in this condition it also tended to tighten up during turns. The only exception to this instability occurred during glides with the flaps and undercarriage down.
The Tomahawk’s stalling characteristics were explored with CG at the extended aft position to simulate the worst case scenario. With the flaps and undercarriage up, the aircraft tended to become unstable and tail-heavy as it approached the stall, which occurred without any control force at around 85 mph IAS. Below 90 mph IAS the aircraft exhibited strong self-stalling characteristics, and the pressure on the control column had to be relaxed to check the fall in speed. There was little or no warning of the stall, but just before it came there was a tendency to yaw to the right and for the port wing to drop. The position of the control column at the point of stall was approximately central.
With the flaps and undercarriage down, the tail heaviness as the stall was approached was still apparent, but not as much as previously and the actual stall occurred at 77 mph IAS. The aircraft again showed self-stalling characteristics, but on this occasion below 80 mph IAS. There was little warning of the stall, except for a slight tendency for the port wing to drop, which became more severe at the stall itself. Use of aileron to try to pick up the left wing only caused the machine to flick sharply over to the right. Closing the radiator shutter reduced the stall speed by 1–2 mph and the self-stalling characteristic was slightly more pronounced.
All aerobatic manoeuvres could be performed with ease. As the Allison was fitted with a Stromberg Bendix carburettor, the pilot was able to apply as much negative-g as he could withstand, without fear of the engine cutting out. Loops could be flown normally and upward rolls and rolls off the top of loops were also straightforward. Diving trials were carried out up to a limiting speed of 450 mph IAS and 3120 rpm, with an all-up weight of 7370 lb and with CG at the normal setting. The aircraft proved to be steady in the dive and could be kept on a target, provided that left rudder bias had been applied. As speed increased, the rudder tended to become very heavy and there was a pronounced tendency to swing to the right. The ailerons also became heavy and the left wing was prone to dropping. In contrast, the elevators remained light and effective and there was only a slight decrease in lightness with increase in speed. No vibration or instability of the control surfaces was experienced and recovery from the dive was easy.
The best glide speed on the approach to land was 100 mph IAS. The landing was easy, but it was important to ensure that the nose was brought up to achieve a three-point landing. Flap was adequate and the landing run was a little longer than most fighter types of the time. If the landing was baulked, the effect of opening up the engine with the flaps and undercarriage down was to make the aircraft tail-heavy, but this could be easily held with forward stick. There was also a slight tendency to yaw to the left. The climb out was accomplished with ease, although when the flaps were raised this tended to increase the aircraft’s tail heaviness and there was some sink, but at no point did this become a cause for concern.
Performance trials were carried out on AK176, which was fitted with guns but was devoid of blast tubes. No flame dampers or air cleaners were fitted, but a wireless mast was carried forward of the fin. The take-off weight was 7300 lb. The maximum rate of climb was 1960 ft/min from ground level up to 13,500 ft (full throttle height) and the service ceiling was calculated as being 31,400 ft. The results of the climbing trials were as follows:
Level speed trials showed a maximum of 331 mph TAS at 15,500 ft (full throttle height).
Take-off and landing tests were performed on BK853 at a weight of 6366 lb. With zero wind and in ISA conditions, the take-off run was measured at 215 yards, with 440 yards being required to clear 50 ft. The landing run was 350 yards.
Although it was a more effective warplane than the Airacobra, the Tomahawk was powered by the same Allison engine and as a result was wholly unsuited to fighter operations in the European theatre. Consequently, its service with home-based squadrons was mainly in the low-level tactical reconnaissance role, with the first deliveries being made to No. 2 Squadron at Sawbridgeworth in August 1941. The Tomahawk was also operated by the Desert Air Force in the Middle East, mainly for ground attack, from June 1941. Despite the fact that it had been rejected as a fighter, the Tomahawk was to give a good account of itself in aerial combat and several pilots ran up quite large scores. The highest-scoring Tomahawk ‘ace’ was Australian Flight Lieutenant Clive ‘Killer’ Caldwell of No. 250 Squadron, whose claims amounted to seventeen, including eight Bf 109E/Fs. The Tomahawk continued to give valuable service in the Middle East until early 1942, when it was gradually replaced by the Kittyhawk.