Army Pursuits the Biplane Period, 1920-1932
Curtiss P-1 through XP-4
Tests indicated that continuous increments of weight had depressed the Hawk’s performance. Cost of the P-lC, $9,862 each, was very low compared to today’s fighters. Navy Hawks of this type were F6C-3s built at Buffalo, while one P-lA was sold to Japan, and Chile bought eight P-lAs and eight P-lBs in 1927.
In addition, 52 of 71 Hawks built as low-powered AT-4 and AT-5 advanced trainers were converted with D-12 engines in 1929 to P-lD, P-lE, and P-lF fighter trainers used at Kelly Field for advanced students. Standard P-l pursuits equipped the four squadrons (17, 27, 94 and 95) of the First Pursuit Group.
While these Hawks went into service, the same basic airframe tested other engine arrangements. Five Curtiss P-2s bought on the first P-l contract and flown in December 1925, had the 505-hp Curtiss V-1400. Like most previous types, this was a water-cooled inline engine, but the Curtiss XP-3 was the last P-lA set aside in October 1926 for a 400-hp air-cooled Curtiss R-1454 radial.
But that power plant failed, so a 410-hp Pratt & Whitney R-1340-9 radial was substituted for the XP-3A (project XP-451) delivered in October 1927. Five P-3As with the R-1340-3 had been ordered December 27, 1927, and delivery began on October 5, 1928. Early flights showed that the naked ring of cylinders handicapped performance, but in May 1929, Air Corps engineers at Wright Field* added a cowl with front shutters and prop spinner to the first P-3A, while Curtiss in Buffalo modified the XP-3A the same month with a deeper cowl of their own design.
Both were known as XP-3A, but the Army version carried the XP-524 project number, and the Curtiss conversion retained the XP-451 number. The former demonstrated a speed increase of from 153 mph to 171 mph on its May 22, 1929, trials and by June 1930, 190 mph had been achieved. Other P-3As were given simple Townsend ring cowls and the two XP-3As that became temporary test beds for the 300-hp R-985-1 Wasp Jr. in December 1930 were labeled XP-21.
Another path of engine development was the turbosupercharger developed by Dr. Sanford A. Moss (1872-1949) of General Electric. After static tests at Pikes Peak in September 1918, flight trials had been made on a LePere fighter in February 1920, and further tests had been made on an MB-2 bomber, the TP-l, a PW-8, and the first P-2.
Boeing also delivered a turbosupercharged high-altitude fighter, the XP-4, in July 1926. Actually the last PW-9 reworked with a 510-hp Packard, four-bladed propeller, a new airfoil section and an enlarged lower wing, the overweight XP-4 was grounded after only 4.5 flying hours. An unusual feature was provision for two .30-caliber guns in the lower wings.