Curtiss P-36 Hawk

The Curtiss P-36 Hawk also known as the Curtiss Hawk Model 75, was an American-designed and built fighter aircraft of the 1930s and 40s. A contemporary of both the Hawker Hurricane and Messerschmitt Bf 109, it was one of the first of a new generation of combat aircraft—a sleek monoplane design making extensive use of metal in its construction and powered by a powerful radial engine. Obsolete at the onset of World War II and best known as the predecessor of the Curtiss P-40 Warhawk, the P-36 saw only limited combat with the United States Army Air Forces.

The P-36 was used more extensively by the French Air Force, both during the Battle of France and by the Vichy French; and was used against French forces in the Franco-Thai War (October 1940–9 May 1941.) It was also used by the British Commonwealth (where it was known as the Mohawk), and by Chinese air units. Several dozen also fought in the Finnish Air Force against the Soviet Air Forces. With around 1,000 aircraft built, the P-36 was a major commercial success for Curtiss. This article also covers the YP-37 and the XP-42 prototypes based on the P-36.

Design and development
The Curtiss Model 75 was a private venture by the company, designed by former Northrop Aircraft Company engineer Don R. Berlin. The first prototype constructed in 1934, featured all-metal construction with fabric-covered control surfaces, a Wright XR-1670-5 radial engine developing 900 hp, and typical United States Army Air Corps armament of one .30 in and one .50 in machine guns firing through the propeller arc. Also typical of the time was the total absence of cockpit armor or self-sealing fuel tanks. The distinctive landing gear which rotated 90° to fold the main wheels flat into the thin trailing portion of the wing, resting atop the lower ends of the maingear struts when retracted, was actually a Boeing-patented design for which Curtiss had to pay royalties.

The prototype first flew on 6 May 1935, reaching 281 mph at 10000 ft during early test flights. On 27 May 1935, the prototype was flown to Wright Field, Ohio, to compete in the USAAC fly-off for a new single-seat fighter but the contest was delayed because the Seversky entry crashed on the way to the contest. Curtiss took advantage of the delay to replace the unreliable engine with a Wright XR-1820-39 Cyclone producing 950 hp and to rework the fuselage, adding the distinctive scalloped rear windows to improve rear visibility. The new prototype was designated Model 75B with the R-1670 version retroactively designated Model 75D. The fly-off finally took place in April 1936. Unfortunately, the new engine failed to deliver its rated power and the aircraft attained only 285 mph.

Although its competitor, the Seversky P-35, also underperformed and was more expensive, it was still declared the winner and awarded a contract for 77 aircraft. However, on June 16, 1936, Curtiss received an order from USAAC for three prototypes designated Y1P-36. The USAAC was concerned about political turmoil in Europe and about Seversky's ability to deliver P-35s in a timely matter, and therefore wanted a backup fighter. The Y1P-36 (Model 75E) was powered by a 900 hp Pratt & Whitney R-1830-13 Twin Wasp engine and further enlarged scalloped rear canopy. The new aircraft performed so well that it won the 1937 USAAC competition with an order for 210 P-36A fighters.

Its extremely low wing loading of just 23.9 lb/ft² gave it outstanding turning performance, and its high power-to-weight ratio of 0.186 hp/lb gave it superb climbing performance as well, especially for the time, although its lack of an engine supercharger handicapped it at high altitudes. Compared to the Allison-engined P-40, the P-36 shared the later P-40's traits of excellent high-speed handling, of roll rate that improved at high speed and of relatively light controls at high speed. However, it was underpowered affecting its acceleration and top speed and it did not accelerate in a dive as well as the P-40.

Curtiss YP-37


In early 1937, the USAAC ordered Curtiss to adapt one P-36 to the new liquid-cooled turbo-supercharged Allison V-1710 engine with 1150 hp. Designated XP-37, the aircraft used the original Model 75 airframe with radiators mounted on the sides of the fuselage around the engine. The cockpit was moved far to the rear to make room for the radiators and the bulky turbocharger system, and to balance the aircraft. The aircraft flew in April 1937, reaching 340 mph at 20000 ft. Although the turbo-supercharger was extremely unreliable and visibility from the cockpit on takeoff and landing was virtually nonexistent, the USAAC was sufficiently intrigued by the promised performance to order 13 service test YP-37s. Featuring improved aerodynamics and a more reliable turbo-supercharger, the aircraft first flew in June 1939. However, the powerplant remained unreliable and the project was cancelled in favor of another Curtiss design, the P-40.

Curtiss XP-42
In an attempt to improve the aerodynamics of the air-cooled piston engines, the fourth production P-36A (serial 38-004), designated the XP-42, was equipped with a long streamlined cowling resembling that of a liquid-cooled engine. Twelve different designs were tried with little success; although the aircraft was faster than a standard P-36A, engine cooling problems were never resolved. Since the new P-40 was faster, the project was canceled. Late in its service life, the sole XP-42 was fitted with a stabilator and used to study that control configuration.

Argentina
Argentina bought a number of the simplified, fixed landing gear Hawk 75Os, (intended for rough-field operations and ease of maintenance) and purchased a manufacturing license for the type. These aircraft used the same engine, Wright Cyclone R-1820-G5 as the Martin 139WAA's and Northrop 8A-2s used by the Argentine Army Aviation at the time. Usually armed with one 11.35 mm Madsen machine gun and three 7.65 mm Madsen light machine guns, there was provision for up to 10 30 lb bombs on underwing pylons. The last Argentinian Hawks remained in service until November 1954.

Brazil
In March 1942, 10 USAAC P-36As were transferred to Brazil.

British Commonwealth


The Royal Air Force (RAF) also displayed interest in the aircraft. Comparison of a borrowed French Hawk 75A-2 with a Supermarine Spitfire Mk I revealed that the Hawk had several advantages over the early variant of the iconic British fighter. The Hawk was found to have lighter controls than the Spitfire at speeds over 300 mph, especially in diving attacks, and was easier to maneuver in a dogfight (thanks to the less sensitive elevator) and better all-around visibility. The Hawk was also easier to control on takeoff and landing. Not surprisingly, the Spitfire's superior acceleration and top speed ultimately gave it the advantage of being able to engage and leave combat at will.

Although Britain decided not to purchase the aircraft, they soon came in possession of 229 Hawks by way of diverted shipments to occupied France and aircraft flown by escaping French pilots. The aircraft received the designations Mohawk I through IV, mirroring French Hawk 75A-1 through A-4, and were fitted with 0.303-cal. Vickers K machine guns and conventional throttles (forward to increase power).

Although they were considered obsolete, a number saw service with the RAF and Royal Indian Air Force (RIAF) in India and Burma. In April 1941, the British government of India ordered 48 Cyclone-powered Mohawk IVz (Hawk 75A) for the RIAF, to be built by Hindustan Aircraft. The first such aircraft completed was test flown on 31 July 1942. However, only four additional aircraft were completed before the project was abandoned. The Indian-built series were used by RAF/RIAF units. Similarly, Chinese license production of the Hawk 75A-5 was moved to India, and these aircraft were also absorbed into RAF as Mohawk IVs. These aircraft were supplemented by 10 Hawk 75A-9s captured during the Anglo-Soviet invasion of Iran in August 1941, while 74 ex-French Mohawk IVs were shipped to India from the United Kingdom. The only RAF units to see combat in Mohawks were No. 5 Squadron RAF and No. 155 Squadron RAF, using the type mainly for Bomber escort and ground attack. The type was retired by the RAF/RIAF in 1944.

The South African Air Force received 72 Mohawks. Its first Mohawks were delivered to East Africa in mid-1941, where they were used by 3 Squadron SAAF to support operations in the East African Campaign, taking part in the Battle of Gondar which ended the campaign, and helping to patrol the border with Vichy French held Djibouti. These Mohawks were then sent to South Africa, where, supplemented by fresh deliveries, they were used for training and for home defence.

China
The prototype of the Hawk 75H—a simplified version with fixed landing gear, like the 75O—was eventually sold to the Chinese Nationalist government who presented it to Claire L. Chennault for personal use. China also received two similar demonstrators, the Hawk 75Q. They also used a number of simplified Hawk 75Ms against the Japanese. The Hawk 75A-5 was built under license in China, but production was later moved to India, and these aircraft were absorbed into the RAF as the Mohawk IV.

Finland


After the fall of France, Germany agreed to sell captured Curtiss Hawk fighters to Finland in October 1940. In total, 44 captured aircraft of five subtypes were sold to Finland with three deliveries from 23 June 1941-5 January 1944. Not all were from the French stocks, but some were initially sold to Norway and captured in their wooden crates when the Germans conquered the country. The aircraft were given serial codes CU-501 to CU-507 (A-4 submodel with Cyclone) and CU-551 to CU-587 (all other submodels with Twin Wasp).

In Finnish service, the Hawk was well liked, affectionately called Sussu ("Sweetheart"). The Finnish Air Force enjoyed success with the type, credited with 190⅓ kills by 58 pilots, between 16 July 1941 and 27 July 1944, for the loss of 15 of their own. Finnish ace Kyösti Karhila scored 13¼ of his 32 victories in the Hawk, while the top Hawk ace K. Tervo scored 15¾ victories. The Hawks were flown by Lentolaivue 32 throughout their wartime operational service.

The Finnish Hawks were initially armed with either four or six 7.5mm machine guns. While sufficient during the early phase of the Continuation War, the increasing speeds and armor of Soviet aircraft soon showed this armament was not powerful enough. From 1942, the State Aircraft Factory replaced the fuselage machine guns with either one or two .50 in Colt machine guns and installed two or four .303 in Browning machine guns in each wing. The 12.7mm Berezin UB or LKk/42 heavy machine guns were also used. The installation of heavier armament did not change the very good flying characteristics of the fighter, but the armament was much more effective against Soviet aircraft. The Finnish Hawks were also equipped with Revi 3D or C/12D gunsight.

Surviving Finnish aircraft remained in service with the FAF aviation units HLeLv 13, HLeLv 11 and LeSK until 1948.

France
Even before the P-36A entered production, the French Air Force entered negotiations with Curtiss for delivery of 300 aircraft. The negotiating process ended up being very drawn-out because the cost of the Curtiss fighters was double that of the French Morane-Saulnier M.S.406 and Bloch MB.150, and the delivery schedule was deemed too slow. Since the USAAC was unhappy with the rate of domestic deliveries and believed that export aircraft would slow things down even more, it actively opposed the sale. Eventually, it took direct intervention from U.S. President Franklin Roosevelt to give the French test pilot Michel Detroyat a chance to fly the Y1P-36.

Detroyat's enthusiasm, problems with the MB.150, and the pressure of continuing German rearmament finally forced France to purchase 100 aircraft and 173 engines. The first Hawk 75A-1 arrived in France in December 1938 and began entering service in March 1939. After the first few examples, aircraft were delivered in pieces and assembled in France by the Société Nationale de Constructions Aéronautiques du Centre. Officially designated Curtiss H75-C1 (the "Hawk" name was not used in France), the aircraft were powered by Pratt & Whitney R-1830-SC-G engines with 900 hp and had instruments calibrated for the metric system, a seat for French dorsal parachutes, a French-style throttle which operated in reverse from U.S. and British aircraft (full throttle was to the rear rather than to the front) and armament of four (i.e. later models had six with two firing through the prop and four in the wings) 7.5 mm FN-Browning machine guns, aimed with a French supplied Baille-Lemaire gun sight. The aircraft evolved through several modifications, the most significant being the installation of the Wright R-1820 Cyclone engine. This variant, designated as Curtiss H751-C1, saw little operational use due to its late delivery and reliability problems with the new engine. A total of 316 H75s were delivered to France before the German occupation.

On 8 September 1939, aircraft from Groupe de Chasse II/4 were credited with shooting down two Luftwaffe Messerschmitt Bf 109Es, the first Allied air victory of World War II on the Western front. During 1939–1940, French H75 pilots claimed 230 air to air kills (of a total of 1,009 air to air kills by the French Air Force during the 1939-40 time period) and 81 probable victories in H75s against only 29 aircraft lost in aerial combat. While only 12.6% of the French Air Force single-seater fighter force the H75 accounted for almost a third of air-to-air kills during the 1940 Battle of France. Of the 11 French aces of the early part of the war, seven flew H75s. The leading ace of the time was Lieutenant Edmond Marin la Meslée with 15 confirmed and five probable victories in the type. H75-equipped squadrons were evacuated to French North Africa before the Armistice to avoid capture by the Germans. While under the Vichy government, these units clashed with British aircraft over Mers el-Kébir and Dakar. During Operation Torch in North Africa, French H75s fought against U.S. Navy F4F Wildcats, losing 15 aircraft while shooting down seven American aircraft. From late 1942 on, the Allies started re-equipping the formerly Vichy-controlled French units with the H75s replaced by P-40s and P-39s.

Iran
A total of 10 Hawk 75A-9s were delivered to Persia, but were captured by the British while still in crates. These were then used by the RAF in India as Mohawk IVs.

Dutch East Indies
In October 1939, The Netherlands ordered 24 Hawk 75A-7s for their Oost Indië possessions or the Dutch East Indies. These planes were powered by 1,200 hp Cyclones. Factory armament was one .50 and one .303 MG in the cowl with two .303 MGs in the wings. After delivery, the .50 cowl MGs were replaced to standardize parts and ammo. The plane could carry six 23 kg bombs. The fighters were shipped in 1940 and almost rerouted to the Netherlands when Germany invaded. But as the mainland surrendered, the aircraft came to the colonies where they were used extensively against the Japanese attack on the Far Eastern part of the kingdom. By that time, the aircraft had flown so many hours that the engines were showing serious wear and tear. These Dutch Hawks formed 1-VlG IV, or 1ste JachtVliegAfdeling (1ste JaVA) - VliegtuigGroep IV of the ML-KNIL; some flew with 1-VlG V. They saw action over Malakka, Soematra and Java, successfully bombing the railroad and intercepting bombers and participated in the extensive dogfights over Soerabaja, where USAAC, RAF and ML aircraft fought Japanese bombers and fighters together.

Norway
Norway ordered 24 Twin Wasp-powered Hawk 75A-6s, of which 19 were delivered and seven assembled at the time of the German invasion. None of the aircraft were combat-ready. The disassembled aircraft were disabled by a single customs employee who smashed the instruments and cut all the wires he could reach. Norwegian Hawks captured by the Germans were part of the batch sent to Finland. Norway also ordered 36 Cyclone-powered Hawk 75A-8s. Most of this batch (a total of 30) were delivered as advanced trainers to "Little Norway" near Toronto, Ontario, Canada, a Norwegian training base established by the London based government-in-exile. Still later, they were resold to the U.S. and redesignated the P-36G model.

Peru
In 1943, the U.S. sent 28 Hawks to Peru under the Lend-Lease agreement. These were ex-Norwegian P-36Gs that had served in Canada.

Portugal
Portugal was officially neutral during World War II, although the Allies were allowed to use or establish ports and airfields on various Portuguese territories. One result of these friendly relations was the transfer by the British government of 12 Hawk 75A variants to the Força Aérea Portuguesa (FAP), which assigned them to air defense duties in the Azores.

Thailand
A few Hawk 75Ns were used by Thailand during the French-Thai War. They also fought at the Battle of Prachuab Khirikhan against Japanese forces during the Japanese Invasion of Thailand. On 28 January 1941, the Royal Thai Air Force (RTAF) dispatched nine Ki-30 Nagoyas, escorted by three Hawk 75s, to bomb Pailin and Sisophon in French Indochina. Thailand was perhaps the only country operating both Japanese and American aircraft just before World War II.

United States
The first production P-36As were delivered to the 20th Pursuit Group at Barksdale Field in Louisiana in April 1938. The aircraft's service history was marred by numerous teething problems with the engine exhaust, skin buckling over landing gear, and weak points in the airframe, severely restricting the performance envelope. By the time these issues were resolved, the P-36 was considered obsolete and was relegated to training units and overseas detachments at Albrook Field in the Canal Zone, Elmendorf Field in Alaska, and Wheeler Field in Hawaii.

The P-36s had been delivered to Hawaii in February 1941 by being loaded on the carrier the USS Enterprise in California, then in a first for the USAAC, flown off the carrier's deck by the P-36's U.S. Army Air Corps pilots when the Enterprise neared the coast of Hawaii. This saved considerable time over the traditional shipping method of having the fighters first disassembled, crated and then loaded by crane in the hold of a freighter, then unloaded and reassembled in Hawaii.

The only combat by U.S.-operated P-36s took place during the Pearl Harbor attack. Five of the 39 P-36A Mohawks at Pearl Harbor, delivered previously by the USS Enterprise, were able to take off during the attack and were credited with shooting down two Japanese Mitsubishi A6M2 Zeros for the loss of one P-36, among the first U.S. aerial victories of World War II.

Variants

 * Model 75A
 * Company-owned demonstrator aircraft flown with several engine fits


 * Model 75B
 * Prototype with Wright R-1820 radial engine


 * Model 75D
 * First prototype, Wright Whirlwind R-1670 radial


 * Model 75I
 * Company designation for the P-37.


 * Model 75H
 * Internal company designation for a simplified export version with fixed landing gear, two slightly differing aircraft built, first sold to China, second to Argentina


 * Model 75J
 * Company-owned 75A temporarily fitted with an external supercharger


 * Model 75K
 * Unbuilt version, intended to use the Pratt & Whitney R-2180 Twin Hornet radial.


 * Model 75P
 * Production P-36A (serial 38-010) fitted with Allison V-1710 liquid-cooled engine, prototype for Curtiss P-40


 * Model 75R
 * Company-owned 75A temporarily fitted with R-1830-SC2-G with turbo-supercharger, attained 330 mph but proved complex and unreliable


 * Y1P-36 (Model 75E)
 * USAAC prototype, Pratt & Whitney R-1830


 * P-36A (Model 75L)
 * USAAC version, P-36A-3 mounted four .30 in machine guns in the wings in addition to fuselage armament


 * P-36B
 * production P-36A fitted with an R-1830-25 producing 1100 hp, reached 313 mph, returned to original P-36A configuration


 * P-36C
 * An additional 0.30 in machine gun installed in each wing with external ammunition boxes under the wings, R-1830-17 of 1200 hp; last 30 production aircraft were completed as P-36Cs


 * XP-36D
 * Production P-36A modified with two .50 in machine guns in the nose and four 0.30 in machine guns in the wings


 * XP-36E
 * Production P-36A armed with four 0.30 in machine guns in the wings, retained standard fuselage guns


 * XP-36F
 * Production P-36A fitted with two 23 mm Madsen autocannons under the wings, reverted to P-36A because guns imposed an unacceptable performance penalty with top speed of only 265 mph.


 * P-36G
 * Hawk 75A-8 used by Norway for training in Canada; later delivered to Peru. R-1820-G205A of 1,200 hp.


 * Hawk 75A-1
 * First production batch for France, four 7.5 mm machine guns, R-1830-SC-G of 900 hp; 100 built


 * Hawk 75A-2
 * Second production batch for France, either R-1830-SC-G or 1050 hp R-1830-SC3-G, six 7.5 mm machine guns; 100 built


 * Hawk 75A-3
 * Third production batch for France, similar with Hawk 75A-2; 135 built (133 delivered).


 * Hawk 75A-4
 * Last production batch for France, Hawk 75A-2 with Wright R-1820-G205A Cyclone radial with 1,200 hp: 285 built, 81 delivered to France; others to Great Britain as Mohawk IV


 * Hawk 75A-5
 * Similar to Hawk 75A-4. Built under license in China (production was later moved to India), absorbed into RAF as Mohawk IV


 * Hawk 75A-6
 * Version for Norway; aircraft captured during the German invasion were eventually sold to Finland


 * Hawk 75A-7
 * Version for Netherlands East Indies: 1,200 hp Cyclone, one .5 in and one .303 inin cowl and two .303 inin wings; later four .303 in (two in nose, one in each wing) and six 50 lb bombs.


 * Hawk 75A-8
 * Export version for Norway. Later redesignated P-36G.


 * Hawk 75A-9
 * 10 aircraft delivered to Persia, captured still in crates and used by RAF in India as Mohawk IVs


 * Hawk 75M
 * Simplified version with fixed landing gear and Wright R-1820 Cyclone for China, built by both Curtiss and Central Aircraft Manufacturing Company in China


 * Hawk 75N
 * Simplified version for Siam (Thailand) with non-retractable landing gear and wheel pants.


 * Hawk 75O
 * Simplified version for Argentina, 30 built and delivered by Curtiss with additional 200 to be built under license locally by Fabrica Militar de Aviones, however only 20 were completed.


 * Hawk 75Q
 * Two additional simplified demonstrators for China. At least one is reputed to have been given an armament similar to that of the XP-36F and to have engaged in combat over Shanghai during the Japanese attacks in September 1937, reportedly shooting down several bombers before being brought down with the loss of the American pilot.


 * XP-37
 * Allison V-1710 inline, cockpit moved to the rear of the fuselage


 * YP-37
 * Service test version of XP-37, 13 built


 * XP-42 (Model 75S)
 * Testbed for streamlining cowlings around air-cooled engines

Survivors
P-36A (s/n 38-001, the first P-36 to be delivered to the Air Corps) is on display at the National Museum of the United States Air Force in Dayton, Ohio. It is displayed in the markings of the P-36A flown by Lt. Phil Rasmussen during the Japanese attack on Pearl Harbor.

A complete and restored Hawk 75N survives in the Royal Thai Air Force Museum; unknown serial number.

A French H75C-1, has been restored to flying condition and is owned by The Fighter Collection at Duxford, United Kingdom. It has been flown and is shown in French camouflage with markings on either side, for the same example (n°82) at two different periods in its career.