The Curtiss P-36 was the first of the new generation of monoplane fighters to enter service with the USAAC. It was a contemporary of the Supermarine Spitfire, the Hawker Hurricane, and the Messerschmitt Bf 109, all of which were introduced within a few months of each other in the mid 1930s. Even though the P-36 owed very little to previous Curtiss biplane pursuits, the name Hawk was still generally applied to the aircraft.
The P-36 pursuit had its origin in the Model 75 project which was originally developed as the Curtiss entry in the US Army pursuit aircraft competition scheduled for May 1935. The Model 75 owed relatively little to previous Curtiss designs. It was an all-metal low-wing monoplane, with the metal-frame moveable control surfaces being fabric covered. The cockpit was enclosed by a sliding canopy, with the canopy being faired into a high rear turtledeck. Both the main undercarriage units and the tailwheel retracted, the main legs rotating backward 90 degrees and turning 90 degrees on their axes simultaneously to lay the wheels flat in the thin rear portion of the wing. Initial armament was the standard US fighter armament of the time—one 0.30-in and one 0.50-in machine guns under the forward fuselage deck, firing through openings in the top of the cowling. No armor protection or self-sealing fuel tanks were fitted.
Prototype construction began in November 1934. Initially, the aircraft was powered by the unfortunate 900 hp Wright XR-1670-5 (SCR-1670-G5) twin-row air-cooled radial. The first flight of the Model 75 took place in May of 1935. During early tests, the prototype had demonstrated a maximum speed of 281 mph at 10,000 feet, a service ceiling of 30,000 feet, and a range of 537 miles. Weights were 3760 lbs empty, 4843 lbs gross. Length was 28 feet 3 1/2 inches, wingspan was 37 feet 0 inches, and wing area was 237 square feet.
During the early flight tests, the XR-1670-5 engine which powered the Model 75 had proven itself to be totally unsatisfactory. Don Berlin took the opportunity afforded by the delay to replace this engine by a 700 hp Pratt & Whitney R-1535. Since this engine model had passed its peak of development, a nine-cylinder single-row Wright XR-1820-39 (G5) Cyclone radial was quickly substituted. This engine was rated at 950 hp for takeoff and at 850 hp normal maximum output. The new Cyclone radial of the Model 75B proved to be almost as unsatisfactory as its R-1670 predecessor, and failed to deliver its full rated power. On June 16, 1936, Curtiss got a consolation order from the Material Division for three examples of the Model 75B under the designation Y1P-36, perhaps because the USAAC was getting nervous about the inability of Seversky to meet its delivery schedules and was therefore hedging its bets. At Army direction, they were to be powered with the Pratt & Whitney R-1830-13 Twin Wasp radial, virtually the same type of engine that was used by the P-35. The Twin Wasp was rated at 900 hp at 2550 rpm at 12,000 feet, having been de-rated from 1050 to 950 hp for takeoff. The engine drove a hydraulically-operated, constant-speed three-bladed Hamilton Standard propeller. Armament was the Army standard of the day, one 0.30-inch and one 0.50-inch machine gun under the cowling and synchronized to fire through the propeller arc.
The first Y1P-36 was delivered to the Army in March of 1937, and was tested at Wright Field in June of that year. The Wright Field test pilots were uniformly enthusiastic about the new Curtiss plane, commenting favorably about its maneuverability. The effectiveness and operation of all controls throughout the speed range of the fighter were excellent, and stability and ground handling were quite favorably rated. However, there was some criticism of the location of the undercarriage and flap controls, some complaints about the cabin ventilation, and some unfavorable comments about the curvature of the windshield which resulted in some distortion of vision during landing. With the R-1830 engine, the Y1P-36 did so well that it won a 1937 Army competition, and on July 7, 1937, the Army ordered 210 P-36As, the largest single US military aircraft order since the First World War. Curtiss's private venture had finally paid off.
Curtiss P-36A Hawk
The principal difference between the P-36A and the Y1P-36 was the addition of engine cowl flaps and the addition of bulging "frog's eye" covers over the machine gun ports in the engine cowling. The first production P-36A was delivered to Wright Field in April of 1938. The 20th Pursuit Group at Barksdale Field, Louisiana, comprising the 55th, 77th, and 79th Pursuit Squadrons, had been designated as the first recipients of the new Curtiss fighter, and they had relinquished their Boeing P-26s in anticipation of the deliveries of the new fighter. However, the new Curtiss fighters began to encounter an extensive series of teething troubles almost as soon as they reached the field. Severe skin buckling in the vicinity of the landing gear wells had appeared, dictating increased skin thicknesses and reinforcing webs. Engine exhaust difficulties and some weaknesses in the fuselage structure were also encountered. Despite both production line and field fixes, the P-36As were grounded again and again. At one time, the 20th Pursuit Group was down to six serviceable P-36As, and even these planes had to be flown under severe limitations on their speed, aerobatics, and combat maneuvers.
By early 1940, the P-36 was already recognized as being obsolescent, and had been largely supplanted in first-line Army Air Force by the Bell P-39 Airacobra and the Curtiss P-40. At home, the P-36s were largely relegated to training units. Other P-36s were transferred overseas. P-36s served with the 24th, 29th, and 43rd Squadrons of the 16th Pursuit Group and with the 51st, 52nd and 53rd Squadrons of the 32nd Pursuit Group, both groups being based at Albrook Field in the Canal Zone, where they flew alongside the now totally-obsolete Boeing P-26. During February of 1941, 20 crated P-36s were delivered to Alaska, and these planes served with the 23rd Squadron at Elmendorf Field in Alaska. At about the same time, 31 P-36s arrived in Hawaii and entered service with the 78th Squadron of the 18th Pursuit Group and with the 46th and 47th Squadrons of the 15th Pursuit Groups, all being based at Wheeler Field, Hawaii.
Specifications of the P-36A
The P-36A had a fully-rated 1050 hp Pratt & Whitney R-1830-13 Twin Wasp engine driving a Curtiss Electric constant speed propeller. Empty and normal loaded weights were 4567 lb and 5470 lbs. Maximum speed was 300 mph at 10,000 feet. Normal range was 825 miles. Initial climb rate was 3400 feet/minute. An altitude of 15,000 feet could be attained in 4.8 minutes, and service ceiling was 33,000 feet. The aircraft was armed with one 0.5 inch and one 0.3 inch machine gun.
Curtiss P-36C Hawk
The P-36B was an experimental variant equipped with an R-1830-25 engine offering 1100 hp for takeoff. It was not pursued. The P-36C was a response to complaints that the P-36A had always been underarmed in comparison with contemporary foreign fighters (e. g. the Spitfire and Hurricane), and supplemented the fuselage guns with a 0.30-in machine gun in each outer wing panel. The P-36C also featured an R-1830-17 (S1C3-G) engine rated at 1200 hp for takeoff. Further experiments saw the wing gun being doubled to two 0.3 inch machine guns in each wing for the XP-36D and then doubled again to four 0.3 inch machine guns per wing in the XP-36E. The latter model had the nose guns deleted. Neither version saw production. Nor did the XP-36F that had a 23mm Madsen cannon in an underwing gondola.
Specifications of the P-36C
Despite the extra drag produced by the wing guns and the increased weight, the increased power of the engine raised the maximum speed of the P-36C to 311 mph, although the range was lowered to 600 miles. Service ceiling was 33,700 feet. Weights were 4620 lbs empty, 5734 lbs loaded. Wing span was 37 feet 4 inches, length was 28 feet 6 inches, and wing area was 236 square feet.
Curtiss P-36G Hawk
In February 1938, two months before the first P-36A had rolled off the Buffalo assembly lines for the USAAC, the French government entered into negotiations with the Curtiss company for the supply of 300 fighters of the Hawk 75A type which Curtiss had offered to the Armee de l'Air. The Hawk 75A was an export version of the P-36A, and was being offered for sale with either the Pratt & Whitney Twin Wasp or the Wright Cyclone engine. On May 17, 1938 the Minister for Air announced that the French would acquire the Curtiss Hawk, and that a French purchasing commission was instructed to order 100 Hawk airframes and 173 Pratt & Whitney R-1830 Twin Wasp engines. The contract stipulated that the first Hawk should be flown at Buffalo by November 25, 1938 and that the 100th plane should be delivered by April 10, 1939.
A further 100 aircraft were ordered on March 8, 1939. These aircraft differed from the A-1 in having an additional 7.5 mm machine gun in each wing, some structural reinforcement of the rear fuselage, and the minor modifications necessary to permit interchangeability between the R-1830-SC-G and the more powerful R-1830-SC2-G, the latter affording 1050 hp for takeoff. A further 135 aircraft were ordered on October 9, 1939, with improved 1200 hp R-1830-S1C3G engines and six 7.5-mm machine guns. The last French order before the Armistice was for 395 aircraft. These were to be fitted with 1200 hp Wright R-1820-G205A Cyclone engines.
With the collapse of both Britain and France on June 19 and 23rd respectively, the USAAF suddenly found itself with no less than 470 P-36 aircraft with two different engines sitting in factories or on airfields without any owners. One problem could be rectified quickly, the 60 undelivered aircraft in the third batch were ordered equipped with the 1200 hp Wright R-1820-G205A engine bringing all 470 to the same standard. The immediate problem was what to do with them. The first solution was bureaucratic, the aircraft were designated the P-36G.
At this point, fortune crept in. The Commonwealth nations saw themselves as the inheritors of the U.K.s assets that, in their view, had been forfeited by the Halifax-Butler Coup. They put in a bid for the long lines of surplus P-36Gs. 72 were sent to South Africa and 169 to Canada. The remaining 229 were sent to India, arriving in October 1940. Of these, 24 were passed to Thailand as replacements for the P-64 aircraft ordered by Thailand but embargoed by the United States government.
In January 1941, a series of border incidents with the French in Indo-China exploded into full-scale war. Thailand invaded the areas of Cambodia and Laos that had been stolen by the French in 1908 and quickly brought about an almost complete collapse of the French Indo-China Army. With French Government in Indo-China collapsing and the Royal Thai Army heading for the Mekong, the Japanese attempted to “mediate” an armistice that served only Japanese interests. These efforts were rejected and when the Japanese attempted to harrass the advancing Thai columns, their Ki-27 fighters were badly mauled by the P-36Gs. For all its obsolescence, the P-36G had played a decisive role in the Far East that would have major political importance.
Specifications of the P-36G
Maximum speed was 323 mph at 15,100 feet. Initial climb rate was 2820 feet per minute, service ceiling was 32,700 feet, and range was 670 miles. Weights were 4541 lbs empty, 5750 lbs gross. Wingspan was 27 feet 3 1/2 inches and length was 28 feet 10 inches. Armament was six 0.3 inch machine guns.
Curtiss P-36H Hawk
The relative success of the P-36G was reinforced by another factor. Fighter engines were in short supply with the Allison V-1710 liquid-cooled and the R-2600 and R-2800 radials being particularly limited. However, the engines used by the P-36 were in relative abundance and this meant P-36s could be built without affecting the availability of better aircraft. Accordingly, Curtiss received an order for 500 P-36H aircraft on the explicit statement that their construction would not delay deliveries of P-40s. The P-36H was essentially identical to the P-36G and could be equipped with R-1830 or R-1820 engines as determined by availability. It was armed with two .50 caliber machine guns in teh nose and four .30 caliber machine guns in the wings. P-36H aircraft were sent to the Philippines where they equipped the Philippine Air Force and to the Dutch East Indies. Although obsolete by 1942, they were enough to provide at least a pretense of significant local air power. The last P-36Hs were delivered to the Philippines in 1944 and the type remained in service until 1953.
Curtiss Mohawk IV
The Hawk 75s ordered by France and taken over by the Commonwealth were named Mohawk with the various sub-types being designated Mohawk I (Hawk 75A-1), Mohawk II (Hawk 75 A-2) and Mohawk III (Hawk 75A-3). However, the majority of the aircraft taken over were the the last and most capable of the U.S. built Hawk 75s, the Hawk 75A-4 or P-36G. These were designated Mohawk IVs. Mohawk IVs continued to be built in India under license with over a hundred being completed before production shifted to the Mohawk V.
Curtiss Mohawk V
Curtiss had provided a license for the construction of the Hawk 75 to the Chinese Central Aircraft Manufacturing Company (CAMCO) but Japanese advances in China forced the termination of this venture. CAMCO relocated to Bombay in India where it became the core of the state-owned Hindustan Aircraft company. Hindustan Aircraft retained the license to build the Hawk 75. Initially, they started assembling kits supplied by Curtiss but the Indian-built content of the aircraft rose quickly and by the end of 1941, all but the engines were being produced in India. In early 1942, production shifted to the Mohawk V that used the R-1830-94 rated at 1,450 horsepower. The added engine power increased speed by a small amount but the limited gains showed that the upper limit of what could be expected from the Hawk 75 airframe had been reached. Nevertheless, Mohawk V aircraft continued in production until 1944 with 180 aircraft being produced.