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A Hawk Between Two Wars

By Robert Guttman
5/16/2018 • Aviation History Magazine

Curtiss Hawk biplanes became the classic embodiment of interwar fighter aviation.

The classic single-seat biplane fighters produced by the Curtiss Airplane and Motor Company were among the best-known military airplanes of the 1920s and ’30s. Popularly known as Hawks, they were flown by both the U.S. Army and Navy, as well as by a dozen foreign air forces, for more than a decade. During that period both Army and Navy Hawks made frequent appearances at airshows throughout the United States, putting on spectacular flying displays. Their distinctive tapered wings and brilliant markings made Curtiss Hawks instantly recognizable, and they became famous all over the country, inspiring innumerable youngsters to learn to fly. Modified Hawks, flown by military pilots, were conspicuous competitors in the National Air Races. Though never used in combat by the U.S. military, Hawks saw action in at least four foreign wars. A few examples were still in service at the end of World War II.

The Hawk was not a single type of airplane but rather a whole series of fighters built during a time of remarkably rapid developments in aviation technology. The evolving Hawk mirrored those developments to a greater degree than any other plane of the period. The initial variants bore such a strong resemblance to World War I fighters that they often stood in for them in movies. On the other hand, final production Hawks incorporated all the technological improvements of the mid-1930s—all-metal structure, light-alloy metal wings, low-drag engine cowling, supercharged engine, three-bladed adjustable-pitch propeller, enclosed cockpit and retractable landing gear. Nevertheless, Curtiss’ ultimate biplane fighters were still clearly recognizable as developments of the original mid-1920s Hawk.

Glenn Hammond Curtiss was among the very earliest aviators to achieve heavier-than-air flight, making the first public flight in the United States in 1908. His early pusher biplanes established an enviable reputation for speed and efficiency. By the time WWI began, Curtiss had switched to more efficient tractor engine designs, and in 1916 the company became America’s first major aircraft corporation.

Curtiss built thousands of JN-4 “Jenny” trainers for the U.S. Army and Navy, as well as for use by the British in Canada. Of the pilots trained in the United States during WWI, 95 percent trained on JN-4s. A JN-4H, powered by a 150-hp Hispano-Suiza engine, also made the first scheduled airmail flight in the U.S., and Jennies became the mainstay of postwar barnstormers.

The Hawks originated from a series of post–WWI military racing biplanes designed for Curtiss by William Gilmore and powered by a 450-hp water-cooled engine, the Curtiss D-12. The D-12 was a development of the revolutionary K-12 aero engine, designed during WWI by Charles L. Kirkham, then head of Curtiss’ engine department. Combining high performance with low drag, Kirkham’s brainchild had been one of the world’s most efficient aero engines in 1918.

Flown by both Army and Navy pilots, the Curtiss racers dominated air racing from 1921 until 1926. Bert Acosta won the 1921 Pulitzer Race in a Curtiss CR-2 at a speed of 176 mph. Lieutenant Russell Maughan won the Pulitzer for the Army in a similar plane the following year, when Curtiss racers came in first, second and third. In 1923 a float-equipped Curtiss CR-3 of the U.S. Navy won the Schneider Trophy Race for the United States for the first time, and Curtiss racers swept the Pulitzer for the second year in a row. That November one of them set a world speed record of 266 mph. In 1924 Cyrus Bettis won the Pulitzer Trophy in another Curtiss, and Jimmy Doolittle took the 1925 Schneider Trophy in the Curtiss R3C-2 floatplane.

Although the Curtiss racers were extremely successful, it was clear that the firm could never thrive simply on high-performance racing planes. Gilmore was therefore directed to create a single-seat fighter based upon his racer design. The result was a substantially more robust biplane incorporating the same basic lines as the Curtiss racing biplanes. Designated PW-8 (pursuit, water-cooled) by the Army, the fighter utilized the same D-12 engine as the racers.

The Curtiss PW-8 was a two-bay biplane with straight, equal span wings. In order to reduce drag, flush-fitting radiators were built into the top of its upper wing, just as they had been in the racers. First flown in 1923, the PW-8 demonstrated a top speed of 168 mph, 27 mph faster than the Army Air Corps’ then-current fighter, the Boeing M.B.3A. In 1924 the Army procured 28 PW-8s, a substantial order considering the stringent budgetary restrictions of the 1920s.

Despite its good performance, the PW-8 was regarded as only a limited success. The radiators in the upper wing had an unfortunate tendency to leak scalding hot water onto the pilot, and maneuverability left something to be desired.

At the Army’s request, Curtiss engineer George Page redesigned the plane to rectify the PW-8’s deficiencies. The resulting fighter was the first true Curtiss Hawk. It was designated P-1, in keeping with the Army Air Corps’ new simplified system of assigning all fighters the prefix “P” for pursuit. The Curtiss P-1 replaced the troublesome wing-mounted radiators with a single radiator underneath the engine. The most obvious new feature of the P-1, however, was its distinctive new set of wings. Fitted with only a single set of interplane struts, the lower wing was now smaller than the upper wing. The chord of the P-1’s wings also tapered elegantly toward the tips, a hallmark of the subsequent Hawks produced during the next decade.

The P-1 had both substantially improved performance and serviceability compared to the PW-8. Between 1925 and 1929 the Air Corps bought a total of 93 P-1s. The fighter had a wingspan of 31 feet 3 inches and a length of 23 feet. Although the fabric-covered fuselage was built over a welded steel tube framework, the wing structure was made of wood. The P-1 had a top speed of 163 mph and a range of 300 miles. Its service ceiling was 22,000 feet, and it could climb 1,800 feet per minute. Armament consisted of either a pair of .30-caliber Browning machine guns or one .30-caliber and one .50-caliber weapon.

At the time Air Corps planners believed that a first-line fighter equipped with a low-powered engine would make an ideal training aircraft. Consequently, in 1927 they ordered 36 P-1 fighter-trainers, equipped with 180-hp Wright V-720 water-cooled engines, under the designation AT-4 (advanced trainer). The following year the Air Corps ordered 36 additional AT-5 fighter-trainers, P-1s with 225-hp Wright R-790 air-cooled radial engines. But the idea of using low-powered fighters as trainers turned out to be a mistake. The underpowered planes handled so poorly that they proved hazardous to the inexperienced pilots assigned to fly them. Eventually 52 of the surviving trainers were rebuilt with Curtiss D-12 engines and added to the P-1 fleet.

In addition to filling the Army’s orders for the P-1, Curtiss built 44 similar fighters for the Navy, designated F6Cs. The performance of the F6C fell slightly below that of its Army counterpart due to the weight of additional deck landing and floatation equipment, as well as the structural strengthening required for operation from the Navy’s first aircraft carrier, Langley.

Although the initial batches of Navy Hawks differed little in appearance from Army Hawks, the succeeding F6C-4 version was easily distinguishable due to its 420-hp Pratt & Whitney R-1340 Wasp radial engine. By the mid-1920s the Navy had come to recognize that air-cooled radial engines offered distinct advantages in the naval environment. Although their configuration increased aerodynamic drag, the radials’ lack of a cooling system rendered them lighter in weight and less complex to maintain than the liquid-cooled engines favored by the Army. Radial engines also took up less space, and were thus easier to stow in the confined hangar space of an aircraft carrier. The first F6C-4 flew in August 1926, and proved so successful that the Navy placed an order for 31 production models.

The next big improvement to the Hawk came from the introduction of a larger and more powerful version of the Curtiss D-12 engine, the 600-hp V-1570 Conqueror. The V-1570 was 36 percent larger than the engines used on the P-1s and liquid-cooled F-6Cs, and produced 38 percent more horsepower. The Hawk’s nose became noticeably more bulbous to accommodate the larger engine, and its fuselage was faired into a rounder shape to match. The improved fighter also featured a more massive landing gear with oleo-pneumatic shock absorbers. In 1929 the Army ordered 18 of the improved Hawks under the designation P-6. Some were used as test-beds to develop improved versions of the engine, including superchargers to enhance performance at high altitudes.

The last eight P-6s, redesignated P-6As, incorporated an important change that was not externally visible, the introduction of ethylene glycol (antifreeze) to the engine’s cooling system. The use of glycol not only reduced freezing and boiling over, it enabled engine designers to incorporate smaller, lighter and more efficient radiators and cooling systems. While the design of the P-6A was finalized too late to take full advantage of the drag-reducing possibilities of the glycol cooling system, its successor was not.

Development work on the P-6A led to the Army’s ultimate Hawk biplane, the P-6E. Although it retained the basic Hawk configuration, the E model included many advanced features. The engine was fitted with a three-bladed, adjustable-pitch propeller. The P-6’s massive multistrut landing gear gave way to elegant new single-leg units with streamlined wheel fairings. In place of the tailskid found on previous Army fighters, the P-6E featured a steerable tail wheel. In addition, the use of the new glycol cooling system resulted in a slimmer and more elegant fuselage. The P-6E not only looked trimmer than the P-6A, it actually was trimmer, weighing 412 pounds less than its predecessor.

The Army ordered 46 examples of the P-6E in 1931, the largest order that service placed for any version of the Curtiss fighter. The planes were issued to three pursuit squadrons, and remained in service for most of the remainder of the decade. The last P-6Es were withdrawn from service in September 1939, on the very eve of World War II.

By far the most famous P-6Es were those operated by the 17th Pursuit Squadron. Based at Selfridge Field, Mich., the 17th was the only unit equipped solely with the P-6E. The standard finish applied to Army aircraft at that time consisted of an olive-drab fuselage with chrome yellow wings and tail, and red, white and blue tail stripes. The purpose of the yellow wings and tail was to facilitate the location of downed aircraft. To those colorful markings the 17th Squadron added its own unique black-and-white “snow owl” markings, complete with talons painted on the wheel fairings, for its planes’ appearance at the 1932 National Air Races. The result was one of the most spectacular-looking fighter units since Manfred von Richthofen’s Flying Circus, and one that remains a favorite subject with aircraft modelers to this day.

The Army experimented with several additional Hawk prototypes. The XP-6F was a P-6E with an enclosed cockpit and a supercharged engine. Another modified P-6E, designated XP-6H, had the enclosed cockpit as well as new wing panels, each containing a .30-caliber machine gun, for a total of six guns. Despite those attempts to wring the last bit of potential out of the aging design, however, the P-6E would be the final version of the Curtiss Hawk biplane ordered in quantity by the Army.

While the Army persisted with the Conqueror-powered Hawks, Curtiss pursued the development of a separate line of radial engine Hawks for the Navy. In 1932 Curtiss delivered a new prototype, with the cumbersome designation of XF11C-1. It featured the metal-framed wings of the P-6 series, along with the three-bladed propeller, single-leg undercarriage and steerable tail wheel, and was powered by the new 600-hp Wright R-1510 radial engine. The power plant, which suffered from cooling problems, proved to be the new fighter’s Achilles’ heel. In the end the Navy abandoned the XF11C-1 and opted to purchase 28 examples of a navalized version of the then-current export model of the Curtiss fighter, the Hawk II. The Navy designated its new fighter the F11C-2, popularly known as the Goshawk.

Like the Hawk II, the F11C-2 was powered by the far more satisfactory 600-hp Wright R-1820-78 Cyclone radial engine. The Goshawk was, however, a retrograde step from the XF11C-1 in that it reverted back to the wood-framed wings of the earlier Hawks. Nevertheless, the new fighters served successfully with Navy fighter squadron VF-1B, the famous “High-Hats,” assigned to the carrier Saratoga. In 1934 the remaining Goshawks were fitted with bomb displacing gear, enabling them to dive-bomb with 500-pound bombs. Reclassified by the Navy as “bomber-fighters,” the planes were redesignated BFC-2s and continued in service until 1938.

Due to the biplanes’ docile handling characteristics, an important factor for carrier operations, the Navy stayed with them far longer than the Army did. Consequently, in 1934 the Navy ordered 27 examples of one final version of the venerable Hawk, a modified F11C-3 designated the BF2C-1. Fitted with a semi-enclosed cockpit, metal-framed wings and retractable landing gear, the BF2C-1 appeared to represent an improvement over its predecessors. In service, however, pilots soon discovered that the metal wings vibrated excessively at cruising speed. In addition, the airplane suffered from aerodynamic buffeting caused by the reshaping of the forward fuselage to accommodate the retracted landing gear. The new landing gear also proved too weak for aircraft carrier service. The Navy retired the last of its BF2C-1s early in 1936, after barely a year of service.

It should be noted that the Navy purchased two additional types of Curtiss biplane fighters during the late ’20s and early ’30s, the F7C and F9C. Although these two aircraft were christened Seahawk and Sparrowhawk, neither was actually a development of the family of taper-winged Hawk biplanes.

In addition to marketing improved versions of its classic biplane fighter to the U.S. Army and Navy for a decade, Curtiss succeeded in selling nearly as many of them abroad. The Army and Navy purchased a total of 370 Hawks, compared with 301 sold to foreign air arms. Curtiss began aggressively marketing its fighters overseas in 1924, when the company sold a PW-8 to Japan. The company also sold 21 P-1s to foreign customers: one to Japan, four to Bolivia and 16 to Chile. Curtiss began marketing an export version of the P-6, known as the Hawk I, in 1929. The company employed the legendary Jimmy Doolittle to demonstrate the Hawk throughout Europe, Asia and South America. As a result, small batches of Hawk Is were purchased by Cuba and Japan, while the Netherlands built eight under license for use in the Dutch East Indies. A well-known civil Hawk, the Gulfhawk was a modified Curtiss I-A owned by the Gulf Oil Company and flown in airshows by Alford J. “Al” Williams.

Introduced in 1932, the Wright Cyclone–engine Hawk II formed the basis for the U.S. Navy’s F11C-2, but lacked aircraft carrier arrester gear. Curtiss sold batches of Hawk II fighters to the air arms of Turkey, Bolivia, Thailand, Chile, Cuba, Colombia and Norway. Len Povey, an American mercenary pilot serving in the Cuban air force, invented the “Cuban Eight” aerobatic maneuver flying one of the Cuban Hawk IIs. The renowned German World War I fighter ace and future Luftwaffe general Ernst Udet brought a pair of Hawk IIs back to Germany in 1934 to demonstrate the possibilities of dive-bombing. The performance of Udet’s Hawk IIs inspired the Germans to develop the infamous Junkers Ju-87 Stuka dive bomber. In all, Curtiss exported a total of 96 Hawk IIs.

The Hawk III featured an improved 700-hp Cyclone engine with a three-bladed propeller, a semi-enclosed cockpit and retractable landing gear. Although it looked similar to the BF2C-1, the Hawk III retained the wooden wings of the Hawk II and consequently did not suffer from the vibration problems that plagued the Navy’s BF2C-1. The Hawk III proved to be an even bigger seller than the Hawk II. Curtiss sold a total of 138 Hawk IIIs to five different countries, most of them going to China.

First flown in 1935, the last of Curtiss’ biplane fighter series, the Hawk IV, differed from the Hawk III mainly in having a cockpit fully enclosed under a sliding canopy. Only one was built and sold to the Argentine air force, along with 10 Hawk IIIs.

Although they never entered combat in U.S. Army or Navy service, Curtiss Hawks saw a great deal of action overseas. The type first flew combat missions during two little-known South American wars.

The so-called Leticia War erupted between Peru and Colombia in 1932. It was caused by a dispute over a remote section of territory at the headwaters of the Amazon River, centered on the town of Leticia. The two nations had already agreed that the region should belong to Colombia, but the Peruvian government fell to a military coup before the treaty could be ratified. The new regime, refusing to accept its predecessor’s agreement with Colombia, dispatched troops to occupy the region, instigating a war.

Determined to regain its national honor, Colombia retaliated with a military expedition that sailed up the Amazon from Brazil to retake Leticia. A trio of Hawk II floatplane fighters accompanied the force. On February 14, 1933, the Hawks broke up an attack on the Colombian flotilla by three Peruvian Vought Corsair biplanes, in spite of the fact that the Hawks’ guns had jammed due to faulty ammunition.

Curtiss also sold aircraft to Bolivia for use against Paraguay during the Chaco War, which lasted from 1928 to 1935. The air forces of both countries operated a hodgepodge of aircraft acquired from several countries, but the Bolivians favored Curtiss planes, eventually acquiring 20 two-seat Curtiss Ospreys, nine two-seat Falcons and nine Hawk II fighters. Bolivia also purchased four Curtiss Condor twin-engine bomber/transports, but those were interned in Peru before they could be delivered. The Bolivian Hawk IIs were regarded as far and away the best fighters deployed by either side during the Chaco War. In spite of their small numbers, and handicapped by operating on substandard fuel, they were widely used as fighter-bombers.

In the fall of 1940, Thailand’s 12 Hawk IIs and 24 Hawk IIIs engaged in combat against French fighters over Southeast Asia. Thailand had had a long-standing border dispute with what was then known as French Indochina, but France’s fall to German invaders in June 1940 made its colonial armed forces seem particularly vulnerable. Although the now-Vichy French air arm in Indochina included 17 modern Morane-Saulnier M.S.406 monoplane fighters, the Thai Hawks gave a good account of themselves in the fighting. On December 1, Hawk IIIs dive-bombed a French flotilla approaching the Thai port of Trat, damaging one ship and compelling the rest to withdraw. On January 17, 1941, another Hawk III struck the French light cruiser Lamotte-Piquet with a 500-pound bomb that failed to explode.

Although the Thais used their Hawk biplanes primarily as fighter-bombers, on the last day of hostilities, January 24, a classic dogfight occurred between a trio of Hawk IIs and three M.S.406s escorting a Potez 25 bomber. Two of the Thais attacked the three Morane fighters while their leader took on the Potez. Despite the qualitative disparity, the Hawks came out unscathed and claimed the destruction of one French aircraft.

By far the largest foreign Curtiss Hawk user, China acquired 50 Hawk IIs and 102 Hawk IIIs during the late 1930s. When the Japanese invaded in 1937, the Chinese Hawks enjoyed some stunning initial successes. On August 14, 18 Mitsubishi G3M2 bombers left Matsuyama airfield on Formosa to bomb two airfields in the Hangchou area. Hawks of the Chinese 4th Pursuit Group, led by Colonel Kao Chihang, swarmed over them and shot down three. A fourth damaged bomber was written off after crash-landing at Matsuyama. When 12 Mitsubishi B2M1 torpedo bombers from the carrier Kaga tried to attack Hangchou the next day, they also received a hot reception from the Hawks, which shot down six and forced two others to ditch in the bay.

Learning their lesson from those stinging defeats, the Japanese began furnishing fighter escorts for their bombers, and their introduction of the Mitsubishi A5M2 monoplane fighter soon afterward tipped the balance decidedly in their favor. In spite of the Curtiss biplanes’ obsolescence, some Chinese Hawk pilots still managed to use them to good effect. Captain Liu Chui-kang of the 24th Pursuit Squadron was credited with seven victories over Japanese aircraft while flying the Hawk III, including two A5Ms. Kao Chi-hang shot down five Japanese, and another Hawk III pilot, Lieutenant Le Yi-Chin, was credited with six victories. The availability of more modern Soviet Polikarpov I-15bis and I-16 fighters soon relegated the Curtisses to a less prominent role, but surviving Hawks continued in Chinese service well into the 1940s, eventually being reduced to second-line duty as fighter-trainers.

The Curtiss Hawk may not have been the best fighter of its day, but it continues to epitomize the classic era of interwar airplane design. Although it was produced in relatively small numbers, few airplanes of the 1920s and ’30s were more widely used or as widely recognized as the Hawk.

Several Curtiss Hawks survive today. Al Williams’ famous orange Gulfhawk has been fully restored, and is on display at the National Air and Space Museum’s Udvar-Hazy Center near Virginia’s Dulles International Airport. In addition, a P-6E has been restored for display, complete with the 17th Pursuit Squadron’s elaborate snow owl markings, at the National Museum of the U.S. Air Force at Wright-Patterson Air Force Base in Ohio. The Thais have also carefully preserved one of their Hawk IIIs, painted in appropriate 1940 markings, which can be seen at the Royal Thai Air Force Museum at Don Muang, near Bangkok.

 

For further reading, longtime Aviation History contributor Robert Guttman recommends: The Curtiss Army Hawks, by Peter M. Bowers; and Curtiss Navy Hawks, by Bowers, Don Greer and Joe Sewell.

Originally published in the November 2008 issue of Aviation History. To subscribe, click here

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