Northrop P-61 Black Widow
The Northrop P-61 Black Widow is a twin-engine United States Army Air Forces fighter aircraft of World War II. It was the first U.S. aircraft conceived from the outset and deployed operationally specifically as a night fighter, incorporating onboard radar as part of the original airframe design.
Named for the North American spider Latrodectus mactans, it was an all-metal, twin-engine, twin-boom design armed with four forward-firing 20 mm Hispano M2 autocannon in the lower fuselage, and four M2 Browning machine guns in a dorsal gun turret. It was developed during the war, and the first test flight was made on 26 May 1942. The first production aircraft rolled off the assembly line in October 1943.
Although not produced in the large numbers of its contemporaries, the Black Widow was operated effectively as a night fighter by United States Army Air Forces squadrons in the European Theater, Pacific Theater, China Burma India Theater, and Mediterranean Theater during World War II. It replaced earlier British-designed night-fighter aircraft that had been updated to incorporate radar when it became available. After the war, the P-61 was redesignated as the F-61, and served in the United States Air Force as a long-range, all-weather, day/night interceptor for Air Defense Command until 1948, and for the Fifth Air Force until 1950. The last aircraft was retired from government service in 1954.
On the night of 14 August 1945, a P-61B of the 548th Night Fighter Squadron named Lady in the Dark was unofficially credited with the last Allied air victory before VJ Day. The P-61 was also modified to create the F-15 Reporter photo-reconnaissance aircraft for the United States Army Air Forces and subsequently the United States Air Force.
Development
Origins
In August 1940, sixteen months before the United States entered the war, the U.S. Air Officer in London, Lieutenant General Delos C. Emmons, was briefed on British research in radar, which had been underway since 1935, and had played an important role in the nation's defense against the Luftwaffe during the Battle of Britain. General Emmons was informed of the new Airborne Intercept radar, a self-contained unit that could be installed in aircraft, and operated independently of ground stations. In September 1940, the Tizard Mission traded British research, including the cavity magnetron that would make self-contained interception radar installations practicable, for American production.Simultaneously, the British Purchasing Commission tasked with evaluating US aircraft declared their urgent need for a high-altitude, high-speed aircraft to intercept the Luftwaffe bombers attacking London at night. The aircraft would need to patrol continuously over the city throughout the night, requiring at least an eight-hour loiter capability. The aircraft would carry one of the early, heavy AI radar units, and mount its specified armament in "multiple-gun turrets." The British conveyed the requirements for a new fighter to all of the aircraft designers and manufacturers they were working with. Among those contacted by the British was Jack Northrop, who realized that the speed, altitude, fuel load, and multiple-turret requirements demanded a large aircraft with multiple engines.
General Emmons returned to the U.S. with details of the British night-fighter requirements, and stated in his report that the design departments of the American aviation industry's firms could possibly produce such an aircraft. The Emmons Board developed basic requirements and specifications, and handed them over to the Air Technical Service Command at Wright Field, Ohio towards the end of 1940. After considering the two biggest challenges—the heavy weight of the AI radar, and the very long loiter time of eight hours minimum—the board, including Jack Northrop, realized the aircraft would need the considerable power and resulting size of twin engines, and recommended such parameters. The United States had two twin-row radials of at least 46 liters displacement in development since the late 1930s: the Double Wasp and the Duplex Cyclone. These engines had been airborne for their initial flight tests by the 1940/41 timeframe, and were each capable, with more development, of exceeding.
Vladimir Pavlecka, Northrop Chief of Research, was present on unrelated business at Wright Field. On 21 October 1940, Colonel Laurence Craigie of the ATSC phoned Pavlecka, explaining the U.S. Army Air Corps' specifications, but told him to “not take any notes, ‘Just try and keep this in your memory!’” What Pavlecka did not learn was radar's part in the aircraft; Craigie described the then top-secret radar as a “device which would locate enemy aircraft in the dark,” and which had the ability to “see and distinguish other airplanes. “The mission, Craigie explained, was ‘the interception and destruction of hostile aircraft in flight during periods of darkness or under conditions of poor visibility.’”
Pavlecka met with Jack Northrop the next day, and gave him the USAAC specifications. Northrop compared his notes with those of Pavlecka, saw the similarity between the USAAC's requirements and those issued by the RAF, and pulled out the work he had been doing on the British aircraft's requirements. He was already a month along, and a week later, Northrop pounced on the USAAC proposal.
On 5 November, Northrop and Pavlecka met at Wright Field with Air Materiel Command officers, and presented them with Northrop's preliminary design. The Douglas XA-26A night fighter proposal was the only competition; Northrop's design was selected.
Early stages
Following the USAAC acceptance, Northrop began comprehensive design work on what would become the first American dedicated night fighter. The result was the largest pursuit-class aircraft flown by the U.S. during the war.Jack Northrop's first proposal was a long fuselage gondola between two engine nacelles and tail booms. Engines were Pratt & Whitney R-2800-10 Double Wasp 18-cylinder radials, producing each. The fuselage housed the three-man crew, the radar, and two four-gun turrets. The AN/M2 Browning machine guns were fitted with long, lightweight "aircraft" barrels with perforated sleeves. The turrets were located in the nose and rear of the fuselage. It stood on tricycle landing gear, and featured full-span retractable flaps, or "Zap flaps", in the wings.
The aircraft was huge, as Northrop had anticipated. While far larger and heavier multi-engine bombers existed, its length, wingspan, and projected full-load weight were unheard of for a fighter, making the P-61 difficult for many to accept as a feasible fighter aircraft.
Changes to the plan
Some alternative design features were investigated before finalization. Among them were: conversion to a single vertical stabilizer/rudder; the shifting of the nose and tail gun turrets to the top and bottom of the fuselage; and the incorporation of a second gunner.Late in November 1940, Jack Northrop returned to the crew of three and twin tail/rudder assembly. To meet USAAC's request for more firepower, designers abandoned the ventral turret, and mounted four 20 mm Hispano M2 cannons in the wings. As the design evolved, the cannons were repositioned in the belly of the aircraft. Consequently, the P-61 became one of the few U.S.-designed fighter aircraft to have a quartet of 20 mm cannons—along with the NA-91 version of the Mustang, and the U.S. Navy's uprated F4U-1C Corsair—as factory-standard in World War II.
On 5 December 1940, the Northrop Specification 8A was formally submitted to Army Air Materiel Command at Wright Field. Following a few small changes, Northrop's NS-8A fulfilled all USAAC requirements, and the Air Corps issued Northrop a Letter of Authority For Purchase on 17 December. A contract for two prototypes and two scale models to be used for wind tunnel testing was awarded on 10 January 1941. The War Department then designated the Northrop Specification 8A as the XP-61.
XP-61 development
In March 1941, the Army/Navy Standardization Committee decided to standardize use of updraft carburetors across all U.S. military branches. The XP-61, designed with downdraft carburetors, faced an estimated minimum two-month redesign of the engine nacelle to bring the design into compliance. The committee later reversed the updraft carburetor standardization decision and prevented a potential setback in the XP-61's development.On 2 April 1941, the Air Corps Mockup Board met at Northrop to inspect the XP-61 mock-up. They recommended several changes following this review. Most prominently, the four 20 mm M2 cannons were relocated from the outer wings to the belly of the aircraft, clustered tightly with the forward-facing ventral "step" in the fuselage to accommodate them being placed just behind the rear edge of the nose gear well. The closely-spaced, centered installation, with two cannons stacked vertically, slightly outboard of the aircraft's centerline on each side, and the top cannon in each pair only a few inches farther outboard, eliminated the inherent drawbacks of the convergence of wing-mounted guns. Without convergence, aiming was considerably easier and faster, and the tightly grouped cannons created a thick stream of 20 mm projectiles. The removal of the guns and ammunition from the wings also cleaned up the wings' airfoil, and increased internal fuel capacity from.
Other changes included: the provision for external fuel carriage in drop tanks; flame arrestors/dampers on engine exhausts; and redistribution of some radio equipment. While all beneficial from a performance standpoint, the modifications required over a month of redesign work, and the XP-61 was already behind schedule.
In mid-1941, the dorsal turret mount finally proved too difficult to install in the aircraft, and was changed from the General Electric ring mount to a pedestal mount like that used for the upper turrets in Boeing B-17 Flying Fortresses, Consolidated B-24 Liberators, North American B-25 Mitchells, Douglas A-20s, and other American bombers. Following this modification, the turret itself became unavailable, as operational aircraft were ahead of experimental aircraft in line for the high-demand component. For flight testing, engineers used a dummy turret.
In February 1942, subcontracting manufacturer Curtiss notified Northrop that the C5424-A10 four-bladed, automatic, full-feathering propeller Northrop had planned for use in the XP-61 would not be ready for the prototype rollout or the beginning of flight tests. Hamilton Standard propellers were used in lieu of the Curtiss props until the originally planned component became available.
During the construction process of the prototype, the XP-61's weight rose to empty and at takeoff. The XP-61s were then equipped with the following: engines were R-2800-25S Double Wasp radials, turning diameter Curtiss C5425-A10 four-blade propellers, both rotating counterclockwise when viewed from the front; radios included two command radios, SCR-522As, and three other radio sets—the SCR-695A, AN/APG-1, and AN/APG-2; and central fire control for the gun turret was similar to that used on the B-29, the General Electric GE2CFR12A3.