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The Curtiss P-6 Hawk was the most advanced version of the Hawk biplane fighter to serve with the USAAF, and differed from the earlier P-1 by using a Curtiss V-1570 Conqueror engine.
The Curtiss P-1 had been powered by the Curtiss D-12 (V-1150) 12 cylinder inline engine. When the P-1 had been designed it had been given the ability to take the larger Curtiss V-1400 engine, and this was used in the P-2. The V-1400 engine wasn't a great success, and so attention turned to the Curtiss V-1570 Conqueror, a new 600hp engine that had been developed from the D-12.
The first P-6 was produced by fitting a Conqueror engine into the fourth P-2 (25-423). The aircraft was re-designated as the XP-6 and was used as a racer, finishing second in the unlimited race at the 1927 National Air Races. The aircraft achieved a top speed of 189mph in the closed course.
A second P-6 also took part in the 1927 air races. This was the XP-6A, which used the fuselage of a P-1A, the single bay straight wings of the XPW-8A and surface radiators similar to those used on the PW-8. This aircraft, piloted by 1st Lt Eugune C. Batten, won the 1927 pursuit race with an average speed of 201.2mph. This was faster than the Army's entry in the 1926 Pursuit Race but significantly slower than the purpose-built racers of the last Pulitzer Race of 1925. This XP-6A was purely a racer and with its outdated wings and vulnerable radiators was of no military significance.
The P-6 fighter was developed from the original XP-6. On 3 October 1928 the Air Corps placed an order for eighteen production aircraft, to be powered by a Prestone (ethylene glycol) cooled V-1570 engine. Prestone was a more efficient coolant than water, allowing the engines to produce more power for longer periods without overheating. The P-6 also introduced oleo-hydraulic shock absorbers in the main landing gear struts, a feature tested on two P-1Cs and that became Air Corps standard.
The Prestone-cooled engines were delayed, and so the first eighteen aircraft were delivered in two batches of nine. The first nine were delivered in October 1929 and were powered by water-cooled V-1570-17 engines. The second nine were delivered with Prestone cooling and the designation P-6A.
The new P-6 was similar to the P-1C. It had the same mixed construction, with a welded metal framework for the fuselage, wooden framework for the wings and fabric covering. It used the same tapered single bay wings, with a smaller staggered lower wing. The front of the fuselage was increased in size to match the larger V-1570 engine. The eighteen P-6s were followed into production by the P-6E, which used a 700hp V-1570C Conqueror engine, while the P-6s and P-6As were given turbo-supercharged engines to become the P-6D.
Service
The P-6 entered service with the 27th Pursuit Squadron, 1st Pursuit Group, late in 1929. In the early 1930s the type was also used by the 17th and 94th Pursuit Squadrons of the 1st Fighter Group and the 33rd, 35th and 36th Pursuit Squadrons of the 8th Pursuit Group.
The P-6D was tested by the 37th Pursuit Squadron, part of the 18th Pursuit Group but attached to the 8th Pursuit Group during 1933-35, the period it operated the P-6.
During the 1931 air exercises an air division was formed. The P-6 was present, but in smaller numbers (17) then either the P-1 (58) or the more numerous Boeing P-12 (130 aircraft).
After the end of the five year expansion program of 1926-31 the P-6E was one of three fighters accepted as 'standard' by the Air Corps (with the P-12E and P-12F). This meant that these were the only fighters considered to be suitable for use in front line service squadrons.
In late 1931-early 1932 the P-6E was issued to the 17th and 33rd Pursuit Squadrons (and possibly to the 95th). The 17th was completely equipped with the type while the 33rd kept some of its older aircraft as well.
In May 1933 P-6Es from the 17th Pursuit Squadron took part in experiments to test out a new early warning ground observation system. The tests were successful over land, although they did demonstrate the problems that were caused if bombers had a similar top speed to intercepting fighters when the B-7 proved able to out-run intercepting P-12s despite the two types having a similar top speed.
By 1937 the P-6E was the only version of the aircraft still in squadron service, although most squadrons had moved on to other aircraft. The last surviving P-6s were grounded in 1939, and many were given to civil flying schools where they served as ground trainers.
Variants
XP-6
The fourth P-2 (25-423) was given a Curtiss V-1570 Conqueror engine to become the XP-6. It took second place in the 1927 National Air Races with an average speed on 189mph.
XP-6A No.1
The XP-6A was produced for the 1927 Air Races. It had the fuselage from a P-1A, the single bay straight wings of the XPW-8A and the surface radiators of the standard PW-8. This aircraft was faster than the XP-6 and took first place in the unlimited race, with an average speed of 201mph, but it wasn't designed as a military aircraft and the P-6 family was based on the XP-6.
P-6
In October 1928 the USAAC placed an order for eighteen P-6s, to be powered by a Prestone cooled engine. The first nine had to be delivered with a water-cooled V-1570-17 engine, and the designation P-6.
Two more P-6s were produced by fitting the water-cooled Conqueror engine in two of the P-11s after the failure of the Curtiss Chieftain engine.
The P-6s were later converted into P-6Ds.
P-6A
The second nine aircraft from the first order were completed with the Prestone cooled V-1570-23 engine, and the designation P-6A. Some of the P-6As were later converted to become P-6Ds.
XP-6A No.2
The second XP-6A was the designation given to the fifth P-6A (29-263) when it was used to tests.
XP-6B
The XP-6B was produced by fitting a Conqueror engine to the last P-1C (29-259). It had the same enlarged cowling and front fuselage as the production P-6 and P-6A, but was delivered on 18 July 1928, three months before the first of the P-6s. The aircraft was used for a New York to Alaska flight of July 1929 and was also known as The Hoyt Special after its pilot, a Captain Hoyt. It carried more fuel than a standard P-6 and made the flight in thirty-four hours and twenty minutes (of air time). The return flight was less successful - the XP-6B crashed and returned to Wright Field on a truck.
P-6C
The P-6C was the second designation given to what became the P-6E, replacing Y1P-22. The number was changed from -22 to -6 to simplifying the spare parts inventory, as the new design had much in common with the basic P-6/ P-6A. The change from C to E is less clear, but may have been to ensure that the new aircraft had a higher designation than the P-6D, which was produced by upgrading the P-6/ P-6A.
XP-6D
In April 1931 the first P-6A was given a turbo-supercharged V-1570C Conqueror engine and became the XP-6D. Its speed at sea level dropped by six mph, but its speed at 15,000ft rose to 197mph.
P-6D
The P-6D was produced by fitting all of the P-6s (nine as built plus the two P-11s) and eight of the P-6As with a turbo-supercharged Conqueror engine and a three-bladed propeller. The work was done in March-April 1932.
XP-6E
The XP-6E went through a series of re-designations during its career. It was ordered as the third P-11 (29-374), but was completed as the YP-20. At this stage it was basically a standard P-6 but with a Wright R-1870-9 Cyclone engine.
In the autumn of 1931 the V-1570-23 engine, complete with cowling and three blade propeller, cantilever single-strut undercarriage and new nose from the XP-22 were installed on YP-20. This version of the aircraft was judged to have been a success and the type was ordered into production, first as the Y1P-22, then as the P-6C and finally as the P-6E.
After serving as the XP-6E prototype, aircraft 29-374 underwent further work and became the XP-6F.
P-6E
The P-6E was the last version of the P-6 produced for the USAAF, and was based on the YP-20. This had a new nose, engine cowling and undercarriage. The radiator was moved further back, the machine guns were moved from the top of the nose to its side. The balance area of the rudder was enlarged as were the elevators. The weight of the aircraft was reduced and the available power increased and as a result top speed rose to 197.8mph at sea level. The P-6E was powered by the Curtiss V-1570C Conqueror engine. Forty-six were ordered on 8 July 1931 and deliveries began in December 1931.
The P-6E underwent two changes of designation between being ordered and being delivered. In July 1931 forty-five of the aircraft were ordered. At first it was known as the service test Y1P-22 (Y1 as is used F-1 funds instead of the normal Air Corps budget). Misleadingly Y1 aircraft were full service aircraft, and not service test aircraft as the Y designation would seem to indicate). This was changed to the P-6C as despite the changes the new aircraft had much in common with the basic P-6. This was changed to P-6E before the first aircraft was delivered on 2 December 1931.
The last P-6E was used as the basis of the XP-23. Another P-6E was used as the basis of the XP-6H, a third as the XP-6G/ P-6G. In 1939 seventeen surviving standard P-6Es were transferred to ground training schools.
XP-6F
The XP-6F was the designation given to the XP-6E after it was given a turbo-supercharged V-1570F Conqueror (-44) engine. The aircraft was returned to Curtiss in March 1932 and wasn't returned to the Army until March 1933. The supercharger added 389lb to the weight of the aircraft, reducing its top speed at sea level to 194mph. Speed at 15,000ft increased to 225mph, requiring the installation of a sliding canopy over the cockpit. The turbo-supercharger suffered from cooling problems at higher altitudes.
XP-6G
The XP-6G was the designation given to one P-6E while it was used as a test-bed for the V-1570F Conqueror engine without a supercharger. It later had the V-1570C engine re-installed and became a P-6E again.
XP-6H
The XP-6H was the designation given to the first production P-6E when it was used to test wing mounted machine guns. Four extra guns were installed, two in the upper wing and two in the lower wing. The upper wing guns were mounted just outside the centre section struts while the lower guns were in the lower wing panels (and were thus further apart than the upper guns). All four guns were outside the propeller arc. The aircraft retained its normal guns, giving it a total of six .3in machine guns. The extra weight reduced its top speed to 190mph.
P-6A
Engine: Curtiss V-1570-17
Power: 600hp
Crew: 1
Span: 31ft 6in
Length: 23ft 1in
Height: 8ft 9in
Empty weight: 2,389lb
Loaded weight: 3,172lb
Max speed: 178mph at sea level
Climb Rate: 5.8 mins to 10,000ft
Service ceiling: 27,000ft
Armament: Two .3in machine guns
P-6D
Engine: Curtiss V-1570-23
Power: 600hp
Crew: 1
Span: 31ft 6in
Length: 23ft 1in
Height: 8ft 9in
Empty weight: 2,698lb
Loaded weight: 3,483lb
Max speed: 190mph at 10,000ft, 197mph at 13,000ft
Climb Rate: 5.8 mins to 10,000ft
Service ceiling: 32,000ft
Armament: Two .3in machine guns
P-6E
Engine: Curtiss V-1570-23
Power: 700hp
Crew: 1
Span: 31ft 6in
Length: 22ft 7in
Height: 8ft 11in
Empty weight: 2.715lb
Loaded weight: 3,436lb
Max speed: 193mph at sea level, 180mph at 15,000ft
Cruising speed: 165mph
Climb Rate: 2,460ft/ min
Service ceiling: 23,900ft
Range: 244 miles
Armament: Two .3in machine guns