Army Pursuits the Biplane Period, 1920-1932
Curtiss P-5, P-6A, XP-6, XP-6B
A contract placed April 30, 1927, purchased five Curtiss P-5 Hawks with turbosuperchargers on V-1150-3 engines. Delivery began in January 1928. Except for cockpit heater, oxygen supply, and longer wheel struts for the enlarged propeller, the P-5 was similar to the P-l. The superchargerís weight reduced sea level speed to 146 mph, but 173 mph was attained at 25,000 feet.
Since these superchargers were driven by hot exhaust gases, and compressed frigid high-altitude air, they required metals able to withstand great extremes of temperature. It was to take another decade of work before turbosuperchargers would be reliable enough for production and service.
When the 600-hp Curtiss V-1570-1 Conqueror became available, two Hawks were rebuilt to try it out at the September 1927 air races. The XP-6 was a P-2 with a Conqueror installed, while the XP-6A had a high-compression Conqueror and a P-lA body with XPW-8A wings and wing radiators. At the races, the XP-6A came first with a speed of 201 mph, leaving the XP-6 second place, but it should be remembered that the XP-6A was strictly a racer whose wing radiators had already been found unsuitable for service. Another conversion was the XP-6B, a P-1C built with a Conqueror and a 250-gallon fuel capacity for a July 1929 flight to Alaska.
Eighteen P-6s were ordered with the P-lC on October 19, 1928, and like previous Hawks, were to have water-cooling for their V-1570-17, but in October 1929 the first had a V-1570-23 cooled by ethylene glycol (commercially sold as Prestone). Chemical cooling permitted smaller radiators, and had been tried on modified P-lB and P-lCs. Eight with Prestone coolers were redesignated P-6A, while ten P-6s delivered with the original engines were modified to P-6A standards and used by the 27th Pursuit Squadron.
Cuba ordered three P-6S Hawks in October 1929, but they were delivered in January 1930 with a 450-hp Pratt & Whitney air-cooled radial. An export P-6 with the usual Conqueror engine was called the Hawk I and flown around Europe in 1930 by Jimmy Doolittle. Eight P-6s bought for the Dutch East Indies in February 1930 were delivered from April to September, and together with six more built in Holland by Aviolande, served the first fighter squadron there until 1937. Mitsubishi purchased another P-6 in March 1930. Incidentally, the Japanese firmís inspector at Curtiss then was Jiro Horikoshi, who later became chief designer of the Zero and earlier Japanese fighters.
No P-6C appeared, but the XP-6D was a P-6A modified by an F-2 turbosupercharger on a V-1570-23. This ship had a two-bladed propeller like most biplanes, but a dozen P-6Ds, converted from February to August 1932 from P-6 and P-6As, had three-bladed propellers whose bite would take better advantage of the turbosuperchargerís high- altitude capabilities. They served with the 37th Pursuit Squadron at Langley Field until 1935.
Boeing continued to give Curtiss stiff competition for fighter contracts during this period. Like other firms, they had failures as well as successes. The Army ordered that the last PW-9D be fitted with a V-1570-1 Conqueror and that aircraft was delivered as the XP-7 on September 4, 1928.