Northrop N-2B / XP-56
Northrop Aircraft Inc was invited to submit proposals for a new pursuit aircraft (in the R-40C informal Army competition), allotted the designation XP-56, for a single engined pusher. An informal competition initiated late in 1939, the winning contractors being Vultee (XP-54), Curtiss (XP-55) and Northrop (XP-56).
Northrop began to scheme the N-2B in August 1941. A wing very similar to the N-1M, though thinner, was selected for the fighter. On each trailing edge was a single, large elevon, combining the, functions of elevator and aileron. Above each tip was a hinged spoiler for creating drag for yaw control. The fuselage was just a minimal nacelle, with a ventral fin to keep the propeller from hitting the ground.
Armament was to comprise two M-2 20mm cannon and four 0.5in Brownings, grouped in the nose, although this was never fitted. Tricycle landing gear was inevitable, the main-wheels being housed in. the lee of the large ducts that served the engine. Power was provided by a 2,000hp Pratt & Whitney R-2800-29 Double Wasp 18-cylinder radial engine buried in the rear fuselage and driving contra-rotating pusher propellers. The canopy hinged to the right, and for emergency escape the pilot was to jettison not only the canopy but also the propeller, Pratt & Whitney designing a jettison system with explosive cord surrounding the gear-box. The propeller eventually used was a Curtiss, Electric contraprop, with two three-blade units. The primary structure was entirely magnesium, welded by the company's patented Heliarc process (which sur-rounded the are with inert helium) perfected during construction of this aircraft.
As far as possible it was based on the experience gained with the N-1M. It even retained the down-sloping wing tips, though the angle was not acute and the control system was of the latest type with bellows- opening split "trim rudders" (i.e., ailerons) used as single surfaces for lateral control and opened into upper and lower spoilers for making properly banked turns. The actuation of these surfaces was novel. Air was rammed in through a forward-facing intake on each tip, taken through a duct and diverted by valves to pressurise the bellows when required. In straight and level flight the duct was open at both ends, creating little drag.
At first Northrop tried to stay as close as possible to the pure all-wing concept, and planned for the pilot to lie prone, but when the Army began to talk about a prototype contract they made it clear they wanted a regular body with a conventional cockpit. Eventually they purchased two prototypes, ordered on 26 September 1940 and 13 February 1942 respectively, each having quite a fat body of symmetrical streamline form containing a 2,400 h.p. Pratt & Whitney R-2800 Double Wasp engine. Cooling the engine was, as in the N-1M, a major problem. Air was rammed in through large wing-root intakes, ducted sharply in to the engine bay and allowed to escape past the baffled cylinders and out via the ejector-assisted exhaust ducts. Cooling airflow was assisted by a high-speed fan geared up from the engine, but to perfect the installation would have been a very large task. The unpressurised cockpit was immediately ahead of the engine, and the nose was planned for an eventual armament of two 20mm and four 0.50in guns. Elevators were fitted inboard of the tip droops, there was a large delta fin above and below the rear fuselage and the main wheels of the tricycle landing gear were housed in bays immediately behind the cooling ducts.
According to Northrop two proto-types were ordered on September 26, 1940, at a price of $411,000; other observers insist the second was not signed for until February 13, 1942. The first XP-56 was delayed by the decision of Pratt & Whitney to drop the X-1800. The R-2800 Double Wasp was already giving 2,000 h.p. while the, X-1800 did not look like maturing until after the war. The decision, was taken on, July 21, 1941, and Pratt & Whitney had to make a Double Wasp with concen-tric drive shafts and the jettison device (the R-2800-29), while Northrop had to rearrange, the, engine bay and central wing structure to pick up the big 18-cylinder radial and supply it with air, the latter augmented by a fan and discharged via gills ahead of the spinner. Another change was to alter the anhedral of the outer wing as a result of N-1M testing. Eventually the XP-56 emerged in March 1943. In April test pilot John Myers began taxiing tests and found directional stability unsatisfactory. Especially as speed was increased, the XP-56-by now for some - reason dubbed "The Black Bullet" - tended to swerve violently, skidding the, tyres and rocking laterally. At least part of the trouble was due to the; brakes, and a new hydraulic brake system, was fitted to give smoother differential action.
After further cooling and system difficulties, the XP-56 first flew on September 30, 1943. It had been expected that the ventral fin added, to the contraprop side area would prove adequate for weathercock stability, but it was clear that directional stability was almost non-existent. A larger upper fin was quickly added over the original, and from then on the handling seemed to be satisfac-tory. No photographs are known of the modified aircraft. In any case, USAAF 41-786 was soon written off. According to one set of writers it suffered a burst mainwheel tyre. According to another it suffered nose-wheel shimmy, leading to failure of the nose leg. For whichever reason, the aircraft somersaulted and was demolished. Myers broke his back, though his head was saved by the fact he habitually wore his polo helmet.
When the second aircraft, 42-38353, emerged the upper fin was even larger than on the modified No.1 aircraft. A further modification was that the, wingtips now carried bellows-type split ailerons for lateral and yaw control, replacing the upper and lower surface spoilers. Each tip was formed by a large venturi duct which normally sucked the split surfaces closed. For yaw control the surfaces were blown open by a bel-lows to which ram air from the tip duct was admitted via a diverter valve.
The second Bullet was flown by Harry Crosby from Hawthorne, on March 23, 1944. Though flight charac-teristics were better than for the first aircraft the engine did not deliver full power, the nosewheel would not lift off until an airspeed of 160 m.p.h. had been reached, and the flight was ter-minated after 7½ min. On the second flight, with gear retracted, the trim was normal but speeds were below prediction. So concerned were, North-rop at the failure to reach design speeds (the objective was 465 mph at 25,000ft), that they booked a place in the queue for the giant open-jet wind tunnel of the NACA at Molfett Field. Meanwhile, testing continued, but on the tenth flight the pilot logged so many shortcomings that it was decided to discontinue further flying.
By this time the XP-56 had been outclassed by such conventional fighters as the P-51, and overtaken by the jet engine. In 1979 No 2 was still intact, held by the Smithsonian.
Max take-off weight: 5148 kg / 11349 lb
Empty weight: 3946 kg / 8699 lb
Wingspan: 12.98 m / 42 ft 7 in
Length: 8.38 m / 27 ft 6 in
Height: 2.94 m / 9 ft 8 in
Wing area: 28.52 sq.m / 306.99 sq ft
Max. speed: 671 km/h / 417 mph
Range: 1062 km / 660 miles
Northrop XP-56 Black Bullet