16 December 2011

The Massive Curtiss XP-71 Fighter

Early windtunnel model of the XP-71 with its wider twin-seat cockpit.
Before the entry of the United States into the Second World War, news stories were splashing headlines about the massed bomber attacks the Luftwaffe was conducting against British cities during the Blitz and in the run up to 1941, a concern about the potential of bomber attacks on the United States took root (despite the obvious shortcomings of any enemy bomber attack on US targets protected on each side by large oceans). A number of designs and studies were undertaken to evaluate the problem of intercepting enemy bombers and one of these resulted in a design specification for a large high-altitude fighter to carry a heavy cannon armament to attack bomber formations from ranges that would place it beyond a bomber's defensive guns. In April 1941, the Curtiss Aeroplane Company submitted six proposals with two, more refined, proposals the following November. One configuration met the proscribed needs of the military for a bomber destroyer and this aircraft was assigned the designation XP-71 with a $3.2 million contract for two prototypes which was signed on 28 October 1941. 

The Curtiss design had the internal company designation CW-29 and had it been built, it would have been the largest fighter aircraft ever developed in the United States- with a wingspan of 82.3 feet, a gross weight of 39,950 lbs and a fuel load up to 1,940 gallons, it was to be able to climb to 25,000 feet in just 12.5 minutes with a cruising speed of 428 mph. With an operating ceiling of 40,000 feet and a range of 3,000 miles, the XP-71 would attack enemy bomber formations well before they reached their targets. A heavy armament of two 37mm cannons with 60 rounds each and a 75mm cannon with 20 rounds was located in the nose and equipped with automatic feed systems so the aircraft only needed a minimal crew of just two. Two large Pratt & Whitney R-4360 Wasp Major 28-cylinder radial engines drove a total of nearly 7,000 horsepower to wing-mounted contra-rotating pusher props with a total of eight blades each and a diameter of 13.5 feet. General Electric turbosuperchargers would give the XP-71 the necessary high altitude performance along with a pressurized cockpit.
To put the massive size of the XP-71 into context, let's compare it with one of the larger production American fighters of the war, the Republic P-47 Thunderbolt. The XP-71 would have twice the wingspan, four times the gross weight, nearly four times the combat range, and nearly three times the engine horsepower of the Thunderbolt. In fact, the XP-71 had it been built would have been larger, heavier, faster, and longer ranged than even a late-model North American B-25 Mitchell medium bomber. 

This schematic hints at the immense complexity of the XP-71.
The mockup was inspected at Curtiss' St. Louis facility on 16 November 1942, at which time the design was revised from having two crew to just a single pilot. The original proposal had the two pilots sitting side-by-side. Following the mockup review, detailed design work took place well into 1943, by which time it was becoming clear to both the US Army Air Forces and Curtiss that the XP-71 might well be the most complex aircraft yet built. During ground firing trials of the nose cannon installation in February 1943, the nose structure failed in spectacular fashion, necessitating a redesign. Curtiss also had problems finding a suitable rangefinder for the fire control system and eventually settled on a radio-based model. Cooling of the R-4360 engines in the wing nacelles also required considerable attention with an annular intake with a gearbox-driven cooling fan being developed to insure adequate cooling airflow to the massive radial engines.

By the latter half of 1943 it was becoming apparent that the XP-71 was a plane without a mission as the strategic bombing campaign of Germany got underway. The likelihood of German bomber formations, let alone Japanese bomber formations, approaching US cities was almost nil. The USAAF considered re-roling the XP-71 as a photorecon aircraft, but no solid commitments were forthcoming. On 23 October 1943, the XP-71 program was terminated after the expenditure of $2.3 million with the first flight planned for June 1944. Curtiss attempted to salvage the program by pitching the XP-71 as an antishipping aircraft with its heavy nose cannons, but this role was already being filled by the proven B-25 Mitchell in the Pacific and Curtiss's engineering resources were needed on other projects.

Source: U.S. Experimental and Prototype Aircraft Projects: Fighters 1939-1945 by Bill Norton. Specialty Press, 2008, p138-139. Photos: National Museum of the United States Air Force.

1 comment:

  1. Cool article. Typo: "proscribed" should be "prescribed".