Curtiss PW-8/P-1 Hawk

The Curtiss "Hawk" series of fighter aircraft was developed directly from a line of specialized racing planes that the Curtiss Aeroplane and Motor Company of Buffalo and Garden City, New York had built for the Army and Navy between the years 1921 and 1925. The powerplant for these racers was a Curtiss-developed compact, water-cooled, direct- drive V-12 design with a displacement of 1150 cubic inches and a power of 435 hp. This engine bore the manufacturer's designation of D-12, but in the middle 1920s the US military services adopted a system in which the type and displacement were used as the basis of the designation for engines. The D-12 engine was given the designation V-1150, V for the basic Vee design and 1150 for the amount of displacement as measured in cubic inches.

The first fighter based on the new 435 hp Curtiss D-12 engine originated in 1922 as a private venture by Curtiss. The design was given the company designation of Model 33. Three prototypes were ordered by the Army Air Service on April 27, 1923 under the designation PW-8. Serial numbers were 23-1201, 1202, and 1203. Examples of a basically similar competing Boeing design were also ordered by the Army, and were given the designation PW-9.

The designation PW-8 stood for "Pursuit, Water-cooled, Model 8". This Army designation scheme had been introduced in 1920. There were seven separate Pursuit categories, chosen according to the role of the aircraft and the type of engine which powered it--PA (Pursuit, Air- cooled), PG (Pursuit, Ground Attack), PN (Pursuit, Night), PS (Pursuit, Special Alert), PW (Pursuit, Water-cooled), R (Racer), and TP (Two-seat, Pursuit). The PW-8 prototypes were redesignated XPW-8 in 1924 when the X-for-experimental prefix was adopted.

The first PW-8 prototype was delivered to the Army on May 14, 1923. The fuselage of the PW-8 was of welded steel tube construction with fabric covering. The undercarriage was of a divided-axle design. The wings were entirely of wood and were of a very thin section which required two bays of interplane struts for stiffening. The cooling system for the D-12 engine consisted of a set of wing-surface radiators that had been pioneered on Curtiss racers in 1922. These radiators were mounted flush with the upper and lower surfaces of the top wing, resulting in an extremely well-streamlined wing surface.

In the flyoff between the XPW-8 and the competing Boeing XPW-9 at McCook Field, the PW-8 proved faster, but the PW-9 was found to be more maneuverable, tougher, and more reliable. The primary problem that the Army found with the PW-8 was in its unique surface radiator cooling system. Although these radiators improved streamlining, they turned out to be a maintenance headache and were prone to constant leaks. In addition, the Army concluded that such a cooling system would probably be extremely vulnerable to damage by gunfire were the Hawk to be used in combat.

The second XPW-8 prototype (Ser No 23-1202) differed from the first in having a divided type of landing gear with reduced drag. The streamlining of the cowling was improved, and strut-connected ailerons and unbalanced elevators were provided. Gross weight increased from 2768 lbs. to 3151 lb

Although the Army favored the Boeing design, the Curtiss company nevertheless did get an order from the Army for 25 production PW-8 fighters. This order was given to Curtiss in return for the company's agreement to collaborate on a pet scheme of General Billy Mitchell, which involved an attempt a coast-to-coast flight across the USA to be completed between dawn and dusk on the same day.

The prototype XPW-8 23-1201 was stripped of all military equipment and used in two unsuccessful attempts piloted by Lt. Russell Maughan in July of 1923 to cross the USA in a dawn-to-dusk flight. This aircraft was later fitted with a second cockpit, temporarily given a spurious designation of CO-X (for Corps Observation, Experimental) and entered in the 1923 Liberty Engine Builders Trophy race for military two- seaters. It was withdrawn before the race because of objections from the Navy.

The 25 production PW-8s (Ser Nos 24-201/225) that had been ordered in September 1923 began to be delivered to the Army in June 1924. These aircraft were in the configuration of the second XPW-8 (Ser No 23- 1202), which differed from 23-1201 by having a different undercarriage. Most of the production PW-8s served with the 17th Pursuit Squadron, although several production PW-8s were sent to McCook Field for experimental work.

On June 23, 1924, PW-8 Ser. No. 24-204 was finally able to complete the first successful dawn-to-dusk crossing of the USA. The aircraft, piloted by Lt. Russell Maughan, took off from Mitchell Field on Long Island, with refuelling stops at Dayton, Ohio, St. Joseph, Missouri, Cheyenne, Wyoming, and Saldura, Utah.

The PW-8 was powered by a 435 hp Curtiss D-12 engine. Maximum speed was 171 mph at sea level. Cruising speed was 136 mph. Initial climb rate was 1830 ft/min. Service ceiling was 20,350 ft. Range was 544 miles. Armament consisted of a pair of 0.30 cal machine guns mounted above the engine synchronized to fire through the propeller arc. Weights were 2185 lbs. empty, 3155 lb. gross.

The third prototype XPW-8 (23-1203) from the original order had been held back at the factory for installation of a set of single-bay wings. These new wings had heavier spars which produced a stiffer structure, permitting the installation of only a single bay of struts. The new aircraft was assigned the company designation of Model 34. It was delivered in this form to the Army in September 1924, and was later redesignated XPW-8A. The troublesome surface radiators of the first two prototypes were replaced by a core-type radiator built flat into the center section of the upper wing panel. A modified rudder without balance area was fitted. XPW-8A Ser No 23-1203 was entered into the 1924 Pulitzer Trophy race. When modified for the race, the radiator was installed in a "tunnel" underneath the engine, similar to the installation on the Boeing PW-9. In this guise, 23-1203 was known as XPW-8AA. It came in third in the race.

The new core-type radiator of the XPW-8A proved to be somewhat less temperamental than the surface radiators of the first two XPW-8s, but it was still considered inadequate by the Army. In the meantime, the Army Air Service had been impressed by the performance of the competing Boeing XPW-9 at McCook Field. The XPW-9 was basically similar to the XPW-8, but it had tapered wings and was provided with a tunnel radiator underneath the engine. The Army was impressed with both of these features. Consequently, the Army asked Curtiss to fit tapered wings and a tunnel radiator to its XPW-8A and resubmit the aircraft for consideration. Curtiss agreed to the changes, and the modified 23-1203 was delivered to the Army in March 1925. The changes resulted in a change of designation to XPW-8B.

The Army was satisfied with the improved XPW-8B, and decided on March 7, 1925 to give Curtiss a contract for a production series based on this design. In the meantime, in May 1924 the Army had combined its seven separate pursuit category designations into one single category--P for pursuit. The first pursuit aircraft ordered by the Army under this new designation scheme were the production versions of the XPW-8B, 15 of which were ordered as serial numbers 25-410/424. These were given the designation P-1, the first entry in the new series.

The P-1 (company designation Model 34A) was the first of the Curtiss biplane fighters to carry the name "Hawk", a name which stuck to Curtiss-designed fighters up to and including the P-40 of World War 2. The only external difference between the XPW-8B and the P-1 was the addition of an aerodynamic balance to the rudder of the P-1, plus some minor changes to the single-bay struts. These airframes were fitted with the 435 hp Curtiss V-1150-1 (D-12C) engine, but were provided with engine mounts that would permit the installation of the larger 500 hp Curtiss V-1400 engine. Original plans were for the last five aircraft of the P-1 order to have this V-1400 engine installed at the factory. Wings were again of wooden construction, but were tapered. Fuselage was of metal tube construction with fabric covering. A 55 gallon auxiliary fuel tank could be fitted underneath the belly.

The first P-1s were delivered to the Army in August 1925. Weights were 2058 lb. empty, 2846 lb. gross. Maximum speed was 163 mph at sea level. Cruising speed was 136 mph. The P-1 could climb to 5000 feet in 3.1 minutes. Service ceiling was 22,500 feet. Range was 325 miles. The P-1 was armed with one 0.50-cal and one 0.30-cal machine gun mounted in the upper fuselage deck and firing through the propeller arc.

The first P-1 (Ser No 25-410) was used primarily for test work. It was briefly fitted with an inverted air-cooled "Liberty" engine and was entered in the 1926 National Air Races. Later, it was fitted with an experimental Wright V-1460-3 Tornado inline inverted air-cooled engine and was redesignated XP-17.

The last 5 P-1s which were destined for the larger Curtiss V-1400 engine were considered sufficiently different that they were redesignated P-2 when they were delivered to the Army. However, the V-1400 engine proved to be completely unsatisfactory in service, and three of these P-2s (25-421, 422, and 424) were converted back to P-1A standards after less than a year of service.

The P-1A (Model 34G) was an improved P-1. It was the first of the Hawks to serve in quantity with the Army Air Corps. 25 were ordered in Sept 1925, with deliveries beginning in April 1926. Serial numbers were 26-276/300. Power was provided by the improved D-12C. The fuselage was 3 inches longer than the P-1, the cowling lines were revised, the fuel system was changed, and the bomb-release system was improved. In addition, some additional service equipment was provided which increased the weight by some 20 pounds and decreased the top speed slightly. Weights were 2041 lb. empty, 2866 lb. gross. Maximum speed was 160 mph at sea level. Cruising speed was 128 mph. The P-1A could climb to 5000 feet in 2.6 minutes. Initial climb rate was 2170 ft/min. Service ceiling was 21,000 feet and range was 342 mi. The P-1A had the same armament as did the P-1. Three additional P-1As resulted from installation of D-12 engines in P-2 airframes 25-421, 422, and 424, as described earlier. Only 23 out of the 25 P-1As originally ordered were delivered as such. 26-296 was later modified as the prototype for the XAT-4 trainer, and 26-300 was transformed first into XP-3, then to XP-21 and XP-21A.

P-1A Ser. No. 26-295 was modified into an Army racer known as XP-6A No. 1. The old XPW-8A wings were installed on 26-295, along with the PW-8-type surface radiators. The new V-1570 Conqueror engine was installed in a PW-8-type nose cowling, and various other minor refinements were made. A really fast aircraft was the result. The XP-6A No. 1 won the 1927 National Air Race at a speed of 201 mph. However, the aircraft was destroyed shortly before the 1928 National Air Race.

The designation XP-1A was applied to a stock P-1A (26-280) diverted to test work. Despite the X-prefix, the aircraft was NOT a prototype.

The P-1B was an improved model ordered in August 1926. Serials were 27-63/87. Deliveries to the Air Corps began on October 28, 1926. The radiator was slightly more rounded and the wheels were slightly larger in diameter. The cowling was redesigned. Flares were added for night landings and controls were improved. Equipment changes increased the weight still further, and reduced the performance still more. First deliveries to the Army began in December 1926. Power was provided by the 435 hp Curtiss V-1150-3 (D-12D) engine. Weights were 2105 lb. empty, 2932 lb. gross. Maximum speed was 160 mph at sea level. Cruising speed was 127 mph. Initial climb rate was 1540 ft/min., service ceiling was 21,400 feet, and maximum range was 600 miles. Armament was identical to that of the P-1 and P-1A. The P-1Bs served with squadrons already flying the earlier Hawks.

The designation XP-1B was applied to a couple of stock P-1Bs (Ser. Nos. 27-71 and 27-73) which were used for test work at Wright Field. 27-73 had machine guns mounted in the wings.

33 improved versions known as P-1C (Model 34O) were ordered in October 1928. This was the largest Army Hawk order to date. Serial numbers were 29-227/259. First deliveries to the Army began in April 1929. The P-1C had larger wheels which were fitted with brakes. The last two P-1Cs were fitted with hydraulic instead of rubber-block shock absorbers. Once again, the weight increased and the performance decreased. The P-1C was powered by the 435 hp Curtiss V-1150-5 (D- 12E). Maximum speed was 154.4 mph at sea level. Cruising speed was 124 mph. Service ceiling was 20,800 feet. Empty weight was 2136 lb. and gross weight was 2973 lbs. The P-1C could climb to 5000 feet in 3.9 minutes. Initial climb rate was 1460 ft/min. Service ceiling was 20,800 feet. The normal range was 328 miles, with the maximum range being 554 miles.

The P-1C Ser. No. 29-259 was completed as the XP-6B, with the Conqueror engine in place of the D-12. It was intended for a long- range flight from New York to Alaska. However, the XP-6B crashed short of its goal and was shipped back to the USA for repair and subsequent test work.

XP-1C was the designation applied to P-1C Ser. No. 29-238 diverted to test work. It was fitted with an experimental Heinrich radiator and Prestone cooling system. Despite its X-prefix, the XP-1C was not a prototype.

In 1924, The US Army decided that it might be a good idea to equip some of its up-to-date pursuit designs with lower-powered engines and use them as advanced trainers. These advanced trainers were all unarmed. However, the concept was not very successful. Since the trainers used the same airframes as did the fighters, the lower- powered trainers were vastly over-stressed for their missions and were overweight for their power and had very poor performances. After a short service, these advanced trainers were converted to full fighter configuration, provided with armament, and were retrofitted with D-12 engines. These converted trainers were then given pursuit designations, and were designated P-1D through P-1F.

The first Curtiss advanced trainer prototype had been created by fitting P-1A Ser No 26-296 with the 180 hp Wright-Hispano E liquid- cooled engine. It was delivered to the Army in July 1926 under the designation XAT-4, where "AT" stood for "Advanced Trainer".

The AT-4 was the production version of XAT-4. Forty AT-4s were ordered in October 1926 under Ser Nos 27-88/97 and 27-213/242. All of these were powered by the Wright-Hispano E (V-720) engine. Maximum speed was 133 mph at sea level. Cruising speed was 107 mph. Initial climb was 950 ft/min. Range was 535 miles. Weights were 1847 lb. empty, 2484 lbs. gross. 35 AT-4s were eventually converted back to fighter configuration by the fitting of the Curtiss V-1150-3 (D-12D) engine and the mounting of a single 0.30-cal machine gun. These converted aircraft were assigned the designation P-1D.

The last five airframes of the AT-4 order (Ser. Nos. 27-238/242) were completed as AT-5s, with the 220 hp Wright J-5 (R-970-1) Whirlwind radial engine in place of the Wright-Hispano liquid-cooled engine. This engine was considerably lighter than the Wright-Hispano, but the disadvantage of lower power was still there. Maximum speed was 125.4 mph at sea level. Cruising speed was 100 mph. Initial climb was 1096 ft/min. Range was 488 miles. Weights were 1802 lbs. empty, 2445 lbs. gross. These AT-5s were later redesignated P-1E when they were re- engined with D-12D engines of 435 hp and fitted with a single 0.30-cal machine gun. Both the P-1D and the P-1E served with the 43rd School Squadron at Kelly Field.

The AT-5A (Model 34M) was an improved AT-5, with the longer fuselage and other structural improvements of the P-1A. 31 examples were ordered by the Army on July 30, 1927. Serial numbers were 28-42/72. In 1929, these AT-5As were all converted to fighter configuration with the switch to the 435 hp D-12D engine and the addition of armament. These aircraft were then redesignated P-1F. One other P-1F (Ser No 28-189) was obtained by converting an XP-21, which in turn had earlier been converted from a P-3A.

There were only a few export sales of the P-1 Hawk. Four P-1s were sold to Bolivia. Eight export models of the P-1A were sold to Chile in 1926. One example was sold to Japan in 1927. Eight export models of the P-1B were sent to Chile in 1927. Some examples are believed to have been built in Chile.

With each successive variant of the P-1, the weight of the fighter increased, leading to a gradual falloff in the top speed and in the climbing performance. P-1s were flown by 27th and 94th Pursuit Squadrons of the 1st Pursuit Group based at Selfridge Field in Michigan and later by the 17th Squadron which kept them in service until 1930 until they were replaced by later types. I don't they ever fired a shot in the defense of American territory.

Sources:

1. United States Military Aircraft Since 1909, Gordon Swanborough and Peter Bowers, Smithsonian Institution Press, 1989.
   
   
2. The American Fighter, Enzo Angellucci and Peter Bowers, Orion Books, 1987.
   
   
3. Curtiss Aircraft: 1907-1947, Peter M. Bowers, Naval Institute Press, 1987.
   
   
4. "The Curtiss Army Hawks", Peter M. Bowers, Aircraft in Profile, Doubleday, 1969.