The Curtiss P-6 Hawk was the success that other Hawk designs tried to be. It was the first important indigenous American fighter. It was the most numerous of the Curtiss pursuits and the most important biplane until the Boeing P-12 came along. In the early 1930s, it was also the outer limit of stretching potential for the basic Hawk design.
The XP-6 (25-423) and XP-6A (26-295) were actually a converted P-2 and P-1A respectively, re-engined with the 600-hp (447.4-kW) Curtiss V-1570 Conqueror, the long-awaited definitive engine, for the 1927 National Air Races held in Spokane, Washington. The latter airframe boosted the image of US Army aviation by winning the unlimited race at a speed of 201 mph (323.5 km/h), the XP-6 coming in second at 189 mph (304.2 km/h). Although the XP-6A was lost in a crash the following year, the effectiveness of the Conqueror powerplant was proven and on 3 October 1928 the US Army ordered 18 YP-6s, delivered to the 1st Pursuit Group under Lieutenant Colonel Frank M. Andrews at Selfridge Field, Michigan, the following year.
As a time-saving measure, nine of these machines were delivered with water-cooled V-1570-17 engines. Eight were subsequently reengined with the V-1570-23 and redesignated P-6A, some employing three-blade variable-pitch propellers. Two further P-6s were produced by converting three P-11 airframes (following) to the intended V-1570-23 glycol-cooled powerplant. One machine (29-263) was assigned to radiator trials as the XP-6A no. 2, while a later Hawk (29-259) became the XP-6B. Better known as the Hoyt Hawk Special, intended for a long-distance New York-Alaska flight by Captain Ross G. Hoyt, the XP-6B was unsuccessful in the marathon effort and was later relegated to test duties. For a time, this airframe also held the P-6C designation.
The P-6D Hawk series all had turbochargers. The XP-6D (29- 260), a converted P-6A, was fitted with a supercharged V-1570C and attained a speed of 197 mph (317 km/h) at 15,000 ft (4572 m). Nine further P-6Ds, converted from earlier Hawks and delivered to the 8th Pursuit Group at Langley Field, Virginia, under Major Byron Q. Jones in April 1932, differed from the XP-6D in having three-blade propellers.
The all-time favourite Curtiss Hawk fighter, and the most elegant, was the P-6E variant. The prototype YP-6E (29-374) went through various incarnations as an XP-11, a YP-20 and, eventually, with a revised landing gear, as the sole XP-6F. It set the stage for the P-6E, best remembered for its resplendence in yellow wings and tail, white-trimmed khaki fuselage and red-white striped rudder. Perhaps the best-recalled Hawk of all is this P-6E, often illustrated with a short-lived Arctic snow owl painted in white on its rear fuselage and corresponding beak and talons painted up front.
The developed P-6E had tapered wings and fuselage that faired gracefully into the liquid-cooled Conqueror engine with its three-blade variable-pitch propeller. The initial order was for 46 P-6E variants (32-2331278) and most were delivered to Andrews’ 1st Pursuit Group at Selfridge in 1932. Single-leg landing gear and a flush, belly-mounted radiator gave the P-6E a streamlined appearance and distinguished it from other Hawks. Also, the P-6E had a tail wheel rather than the tail skid which, until then, was standard on biplane fighters.
Classic fighter
The P-6E was to be an all-time classic, and some remained in service as late as 1938, but the series did not stop there. The experimental XP-6F, fastest of all Hawks, had turbocharged V-1570-55 Conqueror and a new feature which spoke of things to come, namely an enclosed cockpit. Slightly heavier than other Hawks, in 1933 the XP-6F attained the then phenomenal speed of 232 mph (373.4 km/h) at 18,000 R (5486 m). Engine cooling problems brought an end to XP-6F operations in August 1933.
The sole XP-6G (32-254) was a P-6E with supercharging deleted. It eventually reverted to service in P-6E standard. The XP-6H (32- 233) carried six 0.3-in (7.62-mm) machine-guns in lieu of the usual two, but tests with this ‘flying arsenal’ proved unproductive.
Eight P-6 airframes were exported to the Netherlands East Indies and about a dozen went to Cuba with Pratt & Whitney R-1340 radial engines. A single Hawk was purchased in about 1931 by Mitsubishi in Japan, where the technical inspector was Jiro Horikoshi, later the design genius behind the Mitsubishi A6M5 Zero fighter.
As will be seen in this narrative, there were endless variations on the Curtiss Hawk theme, such as the already-mentioned final P-11 aircraft (which was completed as the YP-20 and became the XP-6E and XP-6F) and the sole XP-22 (ordered into production as the Y1P-22 but delivered as the P-6E). Some of the one-off variations had so many designations, powerplants and changes in nomenclature that no complete history may ever be written about all. The important point is that by the time the principle P-6E production variant (originally YP-20. XP-22) was being delivered in numbers to the US Army Air Corps, the Boeing P-12 was also being supplied. The Curtiss P-6 and Boeing P-12 would form the major biplane fighting force of the American air arm in the years between world wars.
In all, the variants of the P-6 encumbered fewer than 90 individual airframes, including the many which bore other designations at one time or another, yet the P-6 left a mark which far exceeds its numbers. One P-6E Hawk is beautifully restored today at the US Air Force Museum in Dayton, Ohio.
Variants
XP-6
Model 34P, modified from a P-1 with a Curtiss V-1570-17 Conqueror engine
XP-6A
Model 34K, same as XP-6 but with untapered wings and wing radiators to reduce drag
P-6A
18 ordered by the U.S. Army, nine were fitted with Prestone- rather than water-cooled V-1670 engines
XP-6B
P-1 converted to take the V-1670 engine
P-6C
cancelled
XP-6D
XP-6B converted to take a turbocharged V-1570-C
P-6D
P-6As, (six of the seven surviving), re-engined in 1932 with turbocharged V-1570-C installed in 1932
XP-6E
Model 35, also designated Y1P-22, ordered in July 1931 as P-6E prototype
P-6E
46 delivered in 1931–1932, equipped the 17th and 33rd Pursuit Squadrons
XP-6F
Modified XP-6E with a supercharger and an enclosed cockpit
XP-6G
P-6E with a V-1570F
XP-6H
P-6E with 4 × .30 in (7.62 mm) machine guns mounted in the wings
P-6S
Hawk I, three sold to Cuba with the 450 hp (336 kW) Pratt & Whitney R-1340 Wasp radial, and one sold to Japan as the Japan Hawk with the V-1570 inverted Vee piston engine
P-11
Three ordered with the Curtiss H-1640 Chieftain engine of 600 hp (447 kW), two were completed with the V-1570 and redesignated P-6D
XP-17
P-1 used as a testbed for the experimental Wright V-1470 air-cooled inverted vee
YP-20
P-11 converted with a Wright Cyclone radial
XP-21
Two conversions of the XP-3A used to test the 300 hp (224 kW) Pratt & Whitney R-985 Wasp Junior radial, one became the XP-21A when modified with the R-975, and the other was converted to the P-1F standard
XP-22
Temporary designation for a P-6A used to test new radiator installations for the V-1570-23 and converted back to a P-6A
XP-23
Model 63, unfinished P-6E with light alloy monocoque fuselage, improved tail, and a turbocharged G1V-1570C with a geared propellor and the turbocharger removed. Later redesignated YP-23
Specification P4E
Type: single-seat pursuit aircraft
Powerplant: one 600-hp (447.4-kW) Curtiss V-1570-23 liquid-cooled 12-cylinder Vee piston engine driving a three-blade propeller
Performance: maximum speed 197 mph (317 km/h) at sea level; initial climb rate 2,400 ft (732 m) per minute; service ceiling 24,700 ft (7529 m); range 570 miles (917 km)
Weights: empty. 699 lb (1224.3 kg); maximum take-off 3,392 lb, (153g. 6 kg)
Dimensions: span 31 ft 6 in (9.60 m); length 23 ft 2 in (7.06 m); height 8 R 10 in (2.69 m); wing area 252 sq ft (23.41 m2)
Armament: two 0.3-in (7.62-mm) fixed machine-guns with 600 rounds per gun on sides of nose firing through propeller disc
