The Curtiss Hawk 75A (Mohawk in RAF service) was a close contemporary of the Brewster Buffalo and was similar in several aspects of its design, in particular the fitment of a Wright Cyclone GR-1820-G 205A engine of 1200 hp in the later variants. Despite this, it was to fare somewhat better than the Buffalo in its operational career and was to remain in first-line service in the Far East until January 1944.
The Hawk 75A was the export version of the Curtiss P-36A that had entered US Army Air Corps service in April 1938 and was a development of the Hawk 75, which was outwardly similar, but with a fixed undercarriage. Early aircraft were fitted with a Pratt & Whitney Twin Wasp R-1830-13 of 1050 hp and the type was selected by France to complement the Morane MS.406, Dewoitine D.520 and Bloch 152 fighters of the French Air Force. By the time that war was declared, France had 108 Hawks in service and another 183 had been delivered by the time of the armistice. The most popular version in French service was the H-75A–3, which was armed with six 7.5-mm machine-guns and had slightly improved performance compared with the early variants.
Further orders were placed for the Wright Cyclone-powered H-75A-4, but only six of these had reached Europe by the time of the French collapse. After the fall of France, all outstanding contracts were transferred to Britain and a total of 227 were delivered, plus ten more that were obtained from other sources. Of these, only five were powered by the Twin Wasp (Mohawk III), the rest being equivalent to the H-75A-4 and designated Mohawk IV. Initial deliveries were made to the UK in the summer of 1940. After reassembly, these airframes were held in reserve until the immediate crisis of the Battle of Britain had passed. Having been originally ordered by the French Air Force, there was much work to do to change over to British instrumentation and radios. This work was entrusted to Westland Aircraft, who were to carry out similar work with the Mohawk’s successor, the Curtiss P-40 Tomahawk.
The first example to be tested in the UK was a French Hawk 75A (No. 188), which was also used for comparative trials with a Spitfire I (K9944). The trial was carried out at RAE Farnborough over a two-week period, commencing 29 December 1939, and was brought about by worrying reports of Spitfire ailerons becoming almost immovable at speeds above 300 mph IAS. A Hawk had already been flown by Sam McKenna (a pilot from A&AEE) in France and from this it was known that the American aircraft possessed remarkably good controls. The ailerons in particular were light, which was in marked contrast with the Spitfire. It was hoped that a closer inspection would shed some light on how the problems with the British fighter might be put right.
During the trials at Farnborough, the Hawk was flown at a take-off weight of 6025 lb. The aircraft was found to be unusually manoeuvrable on the ground, owing to its steerable tailwheel. This was coupled to the rudder bar through a spring system and was steerable through 30 degrees in either direction, after which the mechanism automatically declutched, leaving the wheel to castor freely. Although pilots had been warned to be wary of this feature, they experienced no difficulty in doing quite sharp turns, as the turn could be stopped quickly when desired by using the brakes. As the pilot’s seat was quite high, the view ahead was fairly good, but in any event the aircraft could be easily swung from side to side to clear the area ahead when taxying. The brakes were operated from pedals on the rudder bar and were very powerful.
Take-offs were simple and no flap was required. There was a tendency to swing to the left, but this could easily be held by the application of opposite rudder. The acceleration was good and the ground run was free from bouncing or wing drop. As soon as the aircraft was airborne, the pilot had a feeling of complete control, the ailerons being pleasantly crisp and the elevator not unduly light. The initial climb was very good. On raising the undercarriage (which took about twenty seconds to retract completely), the aircraft became slightly tail heavy.
On lowering the flaps prior to landing, the nose tended to pitch down and a slight pull was needed on the control column to maintain the correct speed. The approach was normally flown at 80–85 mph IAS and in this condition all three controls were effective and had positive feel, which was conducive to pilot confidence. When trimmed in the glide at 85 mph IAS, the aircraft was markedly stable (stick-free) and normal gliding turns could be made with no risk of stalling or loss of height. If the engine had to be opened up as in a go-around, the aircraft became tail heavy, but this could be held with one hand while the trim was being adjusted. Care had to be taken not to raise the flaps too quickly, as the nose tended to rise quite rapidly and height could be lost. On landing, there was only a small float and the control column did not have to be pulled right back to produce a three-point touchdown. The aircraft tended to settle on the ground quite firmly and there was no swing until the latter part of the ground run, when the brakes were easily capable of maintaining a straight course. Heavy braking could be applied soon after landing, without fear of lifting the tail.
Elevator trimming was controlled by a 3-in diameter wheel on the pilot’s left. At full throttle with the flaps and undercarriage up, the Hawk was just stable (stick-free) at all speeds from 100–400 mph IAS. Pilots considered the longitudinal stability characteristics to be ideal for a fighter, thus confirming the widespread view that stick-free longitudinal stability for this type of aircraft at climbing speeds and above should be ‘just on the right side of neutral’. The rudder trim tab was operated by a similarly positioned wheel and could be used for take-off, or when steep prolonged climbs or fast dives were being carried out. Directional control was aided by the fin being set at 1½ degrees to the plane of symmetry to counteract propeller slipstream. No lateral trim was provided, except for fixed tabs that could be preset when the aircraft was on the ground.
Tests were carried out in high-speed dives up to 400 mph IAS, whereby each control in turn was given a slight displacement and released. When rudder was applied at the highest speeds attained, the nose dipped downward quite sharply and the aircraft tended to yaw. On releasing this control input, however, the nose came up again and the aircraft swung back quickly with no oscillation. The dipping of the nose was found to be more pronounced when yawing to the left than to the right. At no stage did any vibration, flutter or snaking develop.
The ailerons had exceptionally nice feel over the whole speed range, being light and powerful, but not over-sensitive. At low speeds they were not excessively light, the pilot feeling a definite resistance to stick movement, and were responsive, if a little sluggish near the stall. As speed increased, the ailerons did not harden up unduly and were still light and very effective at 250 mph IAS. An increase in heaviness was apparent above 300 mph IAS, but even at 400 mph IAS (the highest speed attained) well over half aileron could be applied without excessive effort and the aircraft could be banked quickly from side to side. There was no sign of snatching or vibration at any speed. In general, more stick movement was required in lateral manoeuvres than was usual with British aircraft. This was due to the low gearing between the stick and ailerons. It was found that this did not lessen the ease or pleasantness of the Hawk’s lateral control when turning; but rather, as the control was effective and smooth, it made for definite and well-controlled manoeuvres.
The elevators also exhibited a similar balance in control forces throughout the speed range. The gearing between the stick and elevator (2.75 degrees per inch) was fairly low and as a result the elevator was not unpleasantly sensitive. There was also no appreciable alteration in effectiveness when the engine was throttled back. At low speeds with the engine off, the rudder was a little sluggish. However, with the engine on, the rudder response was normal. Heaviness increased with speed, becoming most marked above 300 mph IAS, but it was still effective, even with small displacements. Control harmony was rated as being exceptional and features that pilots praised most highly were the positive feel on all three controls at very low speed and the absence of any significant stiffening up at high speeds.
During mock aerial combat between the Hawk and a Spitfire I, the Curtiss proved to be superior in several respects. The aircraft were flown by two pilots of Aerodynamics Flight, RAE, who alternated the flying tasks to eliminate any differences in piloting skill. If the Spitfire dived on the Hawk with both aircraft flying at 350–400 mph IAS, the pilot of the Hawk could easily avoid being shot down by quickly applying aileron to set up a banked turn that the Spitfire could not follow. This was due to the fact that the Hawk could apply about three-quarters aileron at 400 mph IAS, whereas the Spitfire could only apply around one-fifth aileron at the same speed. As a result, it could not bank as quickly and tended to overshoot the Hawk, leaving it behind and above. If the Hawk dived on the Spitfire and managed to get within gun-firing range, the pilot had no difficulty in following the Spitfire round until the superior performance of the latter allowed it to draw away. The difference between the two aircraft in the amount of force needed to obtain lateral control was most marked.
Although the elevator of the Hawk became fairly heavy at high speeds, this was not considered to be detrimental, as it lessened the danger of inadvertently applying excessive ‘g’. The Spitfire elevator was much lighter at speeds around 300–400 mph IAS and more sensitive to small control displacements, so that the pilot had to concentrate hard during manoeuvring to avoid a high-speed stall or the possibility of blacking out. Both pilots involved in the trials considered the Spitfire elevator to be ‘too touchy’. Dogfights were also carried out at speeds of around 250 mph IAS. Once again, the Hawk came out on top due to its excellent manoeuvrability and superior all-round view from the cockpit. The Spitfire pilot, however, did have the advantage of being able to break off combat at any time due to his mount’s superior top speed as a result of its aerodynamically cleaner design. The Hawk also lost out to the Spitfire in the dive, as it was rather slow to pick up speed and considerable height was lost in the process.
As the Spitfire tested was fitted with a two-pitch propeller, the take-off and initial climb of the Hawk was considerably better than the Spitfire, which was also prone to swing more during its take-off run. During the climb the controls of the Hawk were effective without being too light, in contrast to both the Spitfire and the Hurricane. On the Spitfire, the elevator was over-sensitive and the aileron too light and on the Hurricane the elevator was rather sluggish. It was naturally desirable that the aircraft be kept steady on the climb as the undercarriage was raised, hood closed, propeller adjusted etc, but this was a difficult task in both British aircraft.
With the flaps and undercarriage down, the gliding angle of the Hawk was steeper than either the Spitfire or Hurricane, and the view very much better. When trimmed for the approach glide, there was a high degree of longitudinal stability and the elevator control was effective, but not too light. The Hurricane was particularly bad in this respect, as it was about neutrally stable and had a light and sluggish elevator at low speeds with the flaps down. The general feel of the controls during the approach in the Hawk was unusually good, the ailerons remaining crisp and positive right down to the stall. The Spitfire in contrast, tended to lose aileron feel at approach speeds. The Hawk’s landing characteristics were also rated as superior as it felt ‘stuck to the ground’ and heavy braking could be used early in the run.
As well as the trials carried out by Aerodynamics Flight, the Airworthiness and Mechanical Test Departments of the RAE collaborated on the problems associated with the Spitfire’s lateral control. An assessment was made of the loss of aileron effectiveness due to wing twist on both the Hawk and the Spitfire. Although the Spitfire wing complied with all relevant stiffness requirements of the time, it was found that there was a 41 per cent loss in effectiveness owing to wing twist, compared with only 16 per cent on the Hawk. There was little difference between the two in terms of wing loading, the Spitfire having a loading of 24 lb/sq.ft compared to 25.5 lb/sq.ft on the Hawk.
The Hurricane was superior to the Spitfire in terms of lateral control at high speed and a comparison of aileron response and stick force with the Hawk had the following results (for the corresponding figures relating to the Spitfire and Buffalo, see page 122).
Further trials were carried out on the Curtiss fighter at Boscombe Down after it had entered service with the RAF as the Mohawk. One criticism was that on entering the cockpit, the control column was positioned too far forward for comfort. It was also found that the operating lever for the sliding canopy did not provide sufficient leverage to open the hood at speeds above 330 mph IAS. As a result, the length of the lever was doubled, allowing the hood to be opened up to the maximum permissible diving speed of 415 mph IAS.
With the flaps and undercarriage up, the stall occurred at 75 mph IAS at forward CG. There was very little warning and at the stall either wing could drop, followed by the nose. The dropped wing could not be raised by aileron alone. If the control column was brought further back, the aircraft tended to spin when on the aft CG limit, usually to the left. With the aircraft on the forward CG limit, there was no tendency to spin. Recovery was immediate when the control column was eased forward, although a change of direction of up to 40 degrees could occur. With the flaps and undercarriage down, the left wing dropped sharply and once again the aircraft was liable to flick into a spin at aft CG loadings. On the forward limit a spin did not occur, the nose simply dropping away until speed was regained. Recovery was straightforward, but the aircraft was liable to come out in a direction 90 degrees removed from its original course.
Performance testing took place in early 1941 using AR645 and AR678, both Mohawk IVs powered by Wright Cyclones. The top speed was measured as 302 mph at 14,000 ft and the best rate of climb was 2600 ft/min at 8000 ft. The rate of climb did not fall below 1000 ft/min until a height of 27,000 ft had been attained. The greatest height reached was 33,000 ft and it was estimated that the absolute ceiling would have been 34,000 ft. When climbing, the change from MS to FS gear generally took place at around 11,000 ft. The speed trials and times to height were as follows:
Fuel consumption tests were also carried out at 15,000 ft, using AR678. These tests showed that the greatest range was obtained in MS blower, with the propeller controlling at 1500 rpm. At 130 mph IAS, which was the minimum speed for comfortable control, the range was 525 miles with a normal fuel load of 84 gallons, after allowing for 15 minutes at full throttle for take-off and climb. With an overload fuel tank, the range could be extended to 960 miles under the same conditions.
A comparison was also made with a Mohawk III (AR631) powered by a Pratt & Whitney Twin Wasp R-1830-SG3-G. The top speed was slightly lower at 300 mph at 10,000 ft and the maximum rate of climb was only 2260 ft/min at 9600 ft. The climb performance was substantially reduced, the Mohawk III taking 7.3 minutes to reach 15,000 ft and 17.5 minutes to reach 26,000 ft, by which the time rate of climb was only 615 ft/min. The estimated service ceiling was 31,200 ft.
Despite having been rejected for operational flying in the European theatre as a result of the trials carried out at Farnborough and Boscombe Down, the Mohawk was to give long-term service in India and Burma. Against Japanese opposition, a lack of outright speed was not as important a requirement as excellent manoeuvrability, its superbly efficient controls and good all-round view giving pilots great confidence. Roles ranged from air defence to operations in support of the Army, for which a bomb load of up to 400 lb could be carried under the wings. The Mohawk remained in first-line service in small numbers until January 1944, when No. 155 Squadron finally converted to Spitfire VIIIs.