By PETER M. BOWERS.
The Curtiss Hawk line of fighters for the U.S. Army, U.S. Navy and export was one of the best-known single-seat biplanes in the years between the two world wars and is still a favorite with model builders. The many configuration changes that the Hawk displayed over its very long — for those days — production life from 1923 through 1936 adds to the technical interest shown in the line.
It started as a private venture by the Curtiss Aeroplane and Motor Co. of Buffalo, N.Y. In 1922, the U.S. Army stated it was not in the market for a new pursuit plane. Curtiss thought that it could produce a new design that would be so superior to the wooden wartime designs the Army was using that the Army would have to buy. Curtiss was right.
The new model, powered with a 435-hp Curtiss D-12 water-cooled V-12 engine and using welded steel tube fuselage construction, was far superior to the 300-hp models in service. Rather reluctantly, the Army bought three for evaluation and assigned the designation PW-8, for Pursuit, Water-cooled, eighth model.
The first two PW-8s proved Curtiss’ point and the Army then ordered 25. The third prototype was completed as the PW-8A with revised wings and was later fitted with entirely new tapered wings as the PW-8B. At this time, 1924, the Army adopted the prefix letter X for experimental planes, so the new taperwing model became the XPW-8B.
This one really hit the jackpot, and the Army ordered 98 over the next few years under the new designation P-1 through P-1C, with a few others given different designations (P-2, P-3, P-5) because of different test engine installations. Another 71 were ordered as AT-4 and AT-5 advanced trainers with lower power (180-220 hp) engines. This concept didn’t work out very well, so all the ATs were refitted with D-12 engines and became P-1D, E and F.
The U.S. Navy bought equivalent examples of the P-1 as the F6C-1, but subsequent development was along different lines.
The Army Hawk line followed the normal line of development that many fighters still follow – adding bigger engines and more equipment to the point where the increased power was almost offset by the increased weight. When the 600-hp Curtiss Conqueror engine came along, it was tested in a P-2 airframe. The combination worked, so 64 more were ordered in a new P-6 series. In those days, a change of engine model called for a new airplane model even though the planes were otherwise identical aft of the firewall. The P-6s were delivered from 1929 into 1932. Experimental conversions carried the designations to XP-6H. Other major conversions became YP-20 and YP-22: the XP-17 was the first P-1, fitted with an experimental inverted V-12 engine.
By 1932 the biplane fighter, and with it the now classic Hawk, was clearly on the way out. Curtiss made one last effort to extend the Hawk line by redesigning the last and still undelivered P-6E into the almost entirely new XP-23. Only the wing, which changed hardly at all from the 1924 XPW-8B, was retained intact. The XP-23 had a turbosupercharger for high-altitude work, and was the fastest of the entire Hawk line. However, the turbo was removed and the XP-23 became the YP-23 (the prefix letter Y identified a service test model).
The Navy, citing the higher landing speed of monoplanes as being unsuitable for aircraft carriers, was slower to abandon the biplane fighter, so its Hawk line continued for a few more years after Army production ended in 1932.
THE PICTURES
THE BEGINNING: The first of the Curtiss Hawks was the PW-8 of 1923-24. An unusual design detail was the use of radiators that formed part of the upper and lower surfaces of the upper wing. These were a mechanical headache, as well as being vulnerable to damage from gunfire.
IMPROVEMENTS: The PW-8A had improved wings. At first, a new core-type radiator was built into the center section of the upper wing, but it was soon replaced by the “funnel” type beneath the engine as shown here. This became standard on the production models through P-6D.
EVOLUTION: The P-1 Hawks featured the tapered wings and revised radiator of the XPW-8B and were outwardly identical, through the P-1F. The engine was the 435-hp Curtiss D-12, a water-cooled V-12 that evolved from the famous Curtiss racing engines of 1920-23.
THE FASTEST: The P-6 series featured the 600-hp Curtiss Conqueror engine of 1,570 cubic inches. The fuselage lines were rounded out to fair with the larger engine. Early P-6s used water cooling; later ones used ethylene glycol (Prestone) that reduced drag by using smaller radiators.
THREE’S THE CHARM: The third P-6A was fitted with a new nose and radiator and the single-leg landing gear developed for the Navy XF6C-6 of 1930, but retained the fat fuselage lines of the P-6. The 46 production versions ordered in 1931 were to have been Y1P-22s, but were redesignated P-6E.