Il Northrop P-61 Black Widow (vedova nera) era un caccia notturno bimotore ad ala media sviluppato dall'azienda aeronautica statunitense Northrop Corporation

Il Northrop P-61 Black Widow (vedova nera) era un caccia notturno bimotore ad ala media sviluppato dall'azienda aeronautica statunitense Northrop Corporation nei primi anni quaranta ed utilizzato dalla United States Army Air Forces (USAAF) durante la seconda guerra mondiale, l'unico concepito per questo scopo da parte degli USA.

Storia del progetto

Lo sviluppo del P-61 si basò su due esigenze parallele, entrambe emerse nel 1940: quella per un caccia di scorta a lungo raggio per l'US Army e quella di un intercettatore notturno per la RAF. Il primo requisito fu sollecitato dal Colonnello Elliott Roosevelt, figlio del presidente e pilota militare che ispirò diverse specifiche del Matériel Command dell'US Army, soprattutto relative ad aerei a grande autonomia. I suoi suggerimenti furono accolti e il 12 aprile 1940 fu emessa una lettera d'invito a presentare proposte cui risposero diverse ditte tra le quali Curtiss-Wright, Hughes e Lockheed; la Northrop apparentemente non fece in tempo a rispondere entro i termini del bando, così come la Beech, che stava lavorando a un progetto del genere e, quando il requisito si trasformò nella richiesta di un "distruttore", risultò vincitrice con l'XA-38 Grizzly che, comunque, rimase senza seguito.

La Curtiss aveva portato avanti i suoi studi su un caccia veloce bimotore caratterizzato dalla sistemazione dei propulsori in posizione spingente, due Pratt & Whitney R-4360-13 Wasp Major radiali a 28 cilindri, raffreddati ad aria, da 3450HP collegati ad eliche controrotanti; l'USAAF ne richiese due prototipi come XP-71 ma, alla fine, cancellò il programma il 26 agosto 1943. Nella specifica cercò di inserirsi la Hughes con il suo D-2A o D-3 XP-73 Convoy Protector del quale fu costruito un solo prototipo, che l'USAAF rifiutò. A questo tema la Lockheed dedicò due progetti, l'L-121 e il similare L-134-3. La Northrop stava lavorando al progetto NS-8, sulla base di uno studio di Vladimir Pavlecka, quando, nell'agosto del 1940, la British Purchasing Commission inglese richiese un intercettatore dotato di radar con cui fronteggiare gli attacchi notturni dei bombardieri tedeschi. Il Tenente Generale Delos Emmons, ufficiale di collegamento con la RAF, stilò una specifica relativa a un bimotore pesante, in grado di rimanere in aria per 8 ore. Il 22 ottobre, Jack Northrop e Pavlecka si incontrarono per discutere la specifica e nacque quello che, provvisoriamente, fu chiamato British Night Fighter.

Contemporaneamente la Douglas sviluppava l'XA-26A, la versione da caccia notturna dell'Invader che, però, non ebbe seguito. Un disegno di massima fu presentato il 14 novembre 1940, poi affinato in una forma vicina a quella definitiva il 22 novembre, per arrivare alla stesura finale NS-8A il 5 dicembre 1940, secondo le direttive di Jack Northrop, Walter Cerny e William Sears. Il 17 del mese la Northrop presentò al Matériel Command il preventivo per due prototipi e due modelli per galleria del vento, accettato l'11 gennaio 1941 e formalizzato il 30 gennaio per 1.367.000 dollari. Il 10 marzo 1941, con un ulteriore contratto, furono ordinati 13 esemplari di pre-serie e una cellula per prove statiche. Il simulacro in legno in scala 1:1 fu esaminato il 2 aprile 1941 e, nei mesi successivi, procedette la messa a punto del radar Western Electric SCR-520B e dell'armamento. Il 24 dicembre 1941 la Northrop ricevette una lettera d'intenzione per 100 esemplari di serie, designati P-61, aumentati a 150 il 17 gennaio 1942 e a 410 il 12 febbraio, con la previsione di "girarne" 50 alla RAF.

Dopo aver considerato svariate configurazioni, l'XP-61 vide la luce con un'architettura bitrave, molto simile a quella dei bimotori Lockheed, e con due motori Pratt & Whitney R-2800-25S Double Wasp da 2000HP; questi propulsori andarono soggetti ad avarie durante le prove e furono sostituiti con gli R-2800-10. Il primo volo si svolse dal Northrop Field di Hawthorne il 26 maggio 1942, con il collaudatore Vance Breese, in grande segretezza. Un anno dopo iniziarono i collaudi degli YP-61 di pre-serie e il primo di essi fu accettato l'8 luglio 1943.In ottobre uscì di fabbrica il primo P-61A-1 di serie, ma il nuovo aereo fu presentato alla stampa solamente l'8 gennaio 1944. Il primo reparto operativo ad essere equipaggiato con i P-61A fu il 422nd Night-Fighter Squadron, basato in Inghilterra, al quale i primi Black Widow furono consegnati il 23 maggio 1944.

Il P-61 fu prodotto con una stessa identica cellula fino all'XP-61E, destinato all'impiego diurno; in questo ruolo il Black Widow non fu costruito, ma la cellula modificata fu alla base della versione da ricognizione F-15 Reporter. Questi bimotori Northrop furono realizzati in 742 esemplari, 706 P-61 e 36 F-15.

Tecnica

Si trattava del più grande e costoso caccia della seconda guerra mondiale. Aveva una doppia trave di coda come il P-38 Lightning, con il quale avrebbe potuto essere scambiato, ma l'equipaggio era di 2 uomini, e nel muso era ospitato un grande radar d'intercettazione, ricoperto da un rivestimento in materiale dielettrico, come sulle macchine moderne. L'armamento era sopra e sotto l'abitacolo: una torretta dorsale telecomandata con 4 mitragliatrici calibro 12,7, mentre i Cannoni erano sistemati in posizione ventrale. I motori erano sulle ali e avevano la Potenza di 4.000 CV complessivi, struttura radiale a doppia stella e turbocompressori.

Impiego operativo

Per ironia della sorte, il Black Widow era stato concepito principalmente per l'impiego da parte della RAF ma, quando fu disponibile, l'aviazione britannica preferì affidarsi alle contemporanee versioni dei caccia notturni nazionali, dotati di un radar simile. La RAF valutò un P-61A-1 tra il 21 marzo 1944 e il 22 febbraio 1945, ma non richiese i 50 aerei che erano stati previsti. I P-61 entrarono in servizio più o meno contemporaneamente in Europa e nel Pacifico e le prime missioni furono compiute dagli aeroporti di Ford e Hurn sulla costa meridionale dell'Inghilterra per l'intercettazione dei missili V1.Successivamente, i Black Widow furono trasferiti in Francia come caccia notturni. Nel Pacifico i P-61 entrarono in servizio il 3 maggio 1944 con il 419th Night-Fighter Squadron, seguito il 1º giugno dal 421st e in settembre dal 418th. Il loro primo successo in combattimento fu il 30 giugno, con l'abbattimento di un bombardiere giapponese G4M. I P-61 continuarono a operare in Europa e nel Pacifico per tutto il conflitto; furono basati anche in Italia, poiché il 422nd NFS aveva il suo comando a Pontedera dove rimase fino al 13 agosto 1945, per poi trasferirsi in Austria. Nel Pacifico i Black Widow operarono più a lungo sia perché la guerra vi terminò più tardi, sia perché rimasero a far parte delle forze americane che presidiavano la regione. Con la fine della Seconda guerra mondiale la produzione fu sospesa, con l'eccezione dei ricognitori F-15A che rimasero in servizio in Estremo oriente fino al 25 marzo 1949. I Black Widow furono dichiarati obsoleti nel 1948, ma per le loro buone caratteristiche alcuni esemplari continuarono ad essere utilizzati in attività sperimentali, come la valutazione di seggiolini eiettabili o il lancio di missili.

Radar SCR-720

Il modello di produzione dell'SCR-720A montava un trasmettitore radio a scansione nel muso dell'aereo; in modalità Airborne Intercept, aveva un raggio d'azione di quasi cinque miglia (8 km). L'unità poteva anche funzionare come segnalatore aereo / dispositivo di homing, aiuto alla navigazione, o di concerto con le unità IFF interrogatore-risponditore. L'operatore radar dell'XP-61 localizzava i bersagli sul suo mirino e guidava l'unità per tracciarli, vettorando e guidando il pilota verso il bersaglio radar tramite istruzioni orali e correzioni. Una volta a portata di tiro, il pilota utilizzava un cannocchiale più piccolo integrato nel pannello strumenti principale per tracciare e chiudere sul bersaglio.

Versioni:

• XP-61: due prototipi con motori R-2800-10.
• YP-61: 13 esemplari di pre-serie.
• P-61A: prima versione di serie prodotta in 200 esemplari, dei quali i primi 45 con i motori R-2800-10 e i successivi con gli R-2800-65. I primi 37 aerei avevano la torretta dorsale con le quattro armi, mentre i successivi ne erano privi, ma su alcuni fu montata in seguito, utilizzando parti di ricambio.
• P-61B: seconda versione di serie prodotta in 450 esemplari con modifiche secondarie; la torretta fu reintrodotta dal 201º esemplare, mentre sul P-61B-20 fu montata una versione con un nuovo sistema di puntamento. Il P-61B-2 aveva due punti d'attacco per bombe o serbatoi che dal P-61B-10 diventarono quattro.
• P-61C: terza e ultima versione di serie della quale furono ordinati 517 esemplari, soltanto 41 dei quali effettivamente costruiti. I motori erano gli R-2800-73 con turbocompressori CH-5 da 2800HP e nuove eliche in acciaio.
• XP-61D: due P-61A convertiti in prototipi per la valutazione dei motori R-2800-77 turbocompressi da 2100HP.
• XP-61E: due P-61B ricostruiti come prototipi per una nuova versione diurna per la scorta a lungo raggio; avevano la gondola centrale completamente riprogettata e sistemazione in tandem per due persone di equipaggio. Al posto del radar, nel muso vi erano quattro mitragliatrici da 12,7mm e la capacità di carburante saliva a 4382 litri.
• XP-61F :il 18° P-61C fu destinato ad una trasformazione simile a quella degli Xp-61E, ma non fu completato.
• XP-61G: designazione applicata nel 1945 ai P-61B impiegati come ricognitori meteorologici.
• XF-15 Reporter:ricognitore fotografico strategico basato sulla cellula del P-61E, con muso contenente fino a sei fotocamere, scelte fra 17 tipi differenti in 24 combinazioni. Il prototipo era il primo XP-61E modificato.
• XF-15A: ulteriore prototipo realizzato convertendo un P-61C, riconoscibile per il diverso disegno delle gondole dei motori.
• F-15A:versione di serie del Reporter, con motori R-2800-65, ordinata in 175 esemplari, dei quali soltanto 36 effettivamente costruiti. Nel 1947 fu ridesignato R-15A e, nel 1948, RF-61C.
• F2T-1N: 12 P-61B furono così ridesignati dopo essere stati assegnati ai Marines per addestramento alla caccia notturna.

ENGLISH

The Northrop P-61 Black Widow, named for the North American spider, was the first operational U.S. warplane designed as a night fighter, and the first aircraft designed to use radar. The P-61 had a crew of three: pilot, gunner, and radar operator. It was armed with four 20 mm (.79 in) Hispano M2 forward-firing cannon mounted in the lower fuselage, and four .50 in (12.7 mm) M2 Browning machine guns mounted in a remote-controlled dorsal gun turret.

It was an all-metal, twin-engine, twin-boom design developed during World War II. The first test flight was made on May 26, 1942, with the first production aircraft rolling off the assembly line in October 1943. The last aircraft was retired from government service in 1954.

Although not produced in the large numbers of its contemporaries, the Black Widow was effectively operated 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—redesignated the F-61—served in the United States Air Force as a long-range, all-weather, day/night interceptor for Air Defense Command until 1948, and Fifth Air Force until 1950.

On the night of 14 August 1945, a P-61B of the 548th Night Fight 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 used by the United States Air Force.

Development

Origins

In August 1940, 16 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 (RAdio Detection And Ranging), 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 (AI for short), a self-contained unit that could be installed in an aircraft and allow it to operate 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 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 (and heavy) AI radar units, and mount its specified armament in "multiple-gun turrets". The British conveyed the requirements for a new fighter to all the aircraft designers and manufacturers they were working with. Jack Northrop was among them, and he 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 in his report said that the design departments of the Americans' aviation industry's firms possibly could produce such an aircraft. The Emmons Board developed basic requirements and specifications, handing them over towards the end of 1940 to Air Technical Service Command (ATSC) at Wright Field, Ohio. After considering the two biggest challenges—the high weight of the AI radar and the very long (by fighter standards) 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 2,000 hp (1,491 kW).

Vladimir H. 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 super-secret radar as a "device which would locate enemy aircraft in the dark" and which had the capability 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 specification. 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 Material Command officers and presented them with Northrop's preliminary design. Douglas' XA-26A night fighter proposal was the only competition, but Northrop's design was selected and the Black Widow was conceived.

Early stages

Following the USAAC acceptance, Northrop began comprehensive design work on the aircraft to become the first to design a dedicated night fighter. The result was the largest and one of the deadliest 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 2,000 hp (1,491 kW) each. The fuselage housed the three-man crew, the radar, and two four-gun turrets. The .50 in (12.7 mm) AN/M2 M2 Browning machine guns were fitted with 36 in (91 cm) long, lightweight "aircraft" barrels with perforated sleeves, without the heavy, breech-end cooling collar of the -HB barrel. 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" (named after aircraft engineer Edward Zaparka) in the wings.

The aircraft was huge, as Northrop had anticipated. While far heavier and larger multi-engine bombers existed, its 45.5 ft (14 m) length, 66 ft (20 m) wingspan and projected 22,600 lb (10,251 kg) full-load weight were unheard of for a fighter, making the P-61 hard 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 and the shifting of the nose and tail gun turrets to the top and bottom of the fuselage along with 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 (.79 in) Hispano M2 cannon in the wings. As the design evolved, the cannon were subsequently repositioned in the belly of the aircraft. The P-61 therefore became one of the few U.S.-designed fighter aircraft to have a quartet of 20 mm (.79 in) cannon — 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.

Northrop Specification 8A was formally submitted to Army Air Material Command at Wright Field, on 5 December 1940. 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 (costs not to exceed $1,367,000), was awarded on 10 January 1941. Northrop Specification 8A became, by designation of the War Department, 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 (the XP-61 program's predicament likely having little influence), preventing a potential setback in the XP-61's development.

The Air Corps Mockup Board met at Northrop on 2 April 1941, to inspect the XP-61 mock-up. They recommended several changes following this review. Most prominently, the four 20 mm (.79 in) M2 cannon 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 placed just behind the rear edge of the nose gear well. The closely spaced, centered installation, with two cannon 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 cannon created a thick stream of 20 mm (.79 in) projectiles. The removal of the guns and ammunition from the wings also cleaned up the wings' airfoil and increased internal fuel capacity from 540 gal (2,044 l) to 646 gal (2,445 l).

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 (especially the relocation of the cannon) 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 Fortresss, 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, in this case the Boeing B-29 Superfortress, were ahead of experimental aircraft in line for the high-demand component. For flight testing, engineers used a dummy turret.

During 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.

The XP-61's weight rose during construction of the prototype, to 22,392 lb (10,157 kg) empty and 29,673 lb (13,459 kg) at takeoff. Engines were R-2800-25S Double Wasp radials; turning 12 ft 2 in 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. Central fire control for the gun turret was similar to that used on the B-29, the General Electric GE2CFR12A3.

P-61C

The P-61C was a high-performance variant designed to rectify some of the combat deficiencies encountered with the A and B variants. Work on the P-61C proceeded quite slowly at Northrop because of the higher priority of the Northrop XB-35 flying wing strategic bomber project. In fact, much of the work on the P-61C was farmed out to Goodyear, which had been a subcontractor for production of Black Widow components. It was not until early 1945 that the first production P-61C-1-NO rolled off the production lines. As promised, the performance was substantially improved in spite of a 2,000 lb (907 kg) increase in empty weight. Maximum speed was 430 mph (690 km/h) at 30,000 ft (9,000 m), service ceiling was 41,000 ft (12,500 m), and an altitude of 30,000 ft (9,000 m) could be attained in 14.6 minutes.

The P-61C was equipped with perforated fighter airbrakes located both below and above the wing surfaces. These were to provide a means of preventing the pilot from overshooting his target during an intercept. For added fuel capacity, the P-61C was equipped with four underwing pylons (two inboard of the nacelles, two outboard) which could carry four 310 gal (1,173 l) drop tanks. The first P-61C aircraft was accepted by the USAAF in July 1945. However, the war in the Pacific ended before any P-61Cs could see combat. The 41st and last P-61C-1-NO was accepted on 28 January 1946. At least 13 more were completed by Northrop, but were scrapped before they could be delivered to the USAAF.

Service life of the P-61C was quite brief, since its performance was being outclassed by newer jet aircraft. Most were used for test and research purposes. By the end of March 1949 most P-61Cs had been scrapped. Two entered the civilian market and two others went to museums.

F-15/RF-61C

In mid-1945, the surviving XP-61E was modified into an unarmed photographic reconnaissance aircraft. All the guns were removed, and a new nose was fitted, capable of holding an assortment of aerial cameras. The aircraft, redesignated XF-15, flew for the first time on 3 July 1945. A P-61C was also modified to XF-15 standards. Apart from the turbosupercharged R-2800-C engines, it was identical to the XF-15 and flew for the first time on 17 October 1945. The nose for the F-15A was subcontracted to the Hughes Tool Company of Culver City, California. The F-15A was basically the P-61C with the new bubble-canopy fuselage and the camera-carrying nose, but without the fighter brakes on the wing.

F2T-1N

The United States Marine Corps had planned to acquire 75 Black Widows, but these were canceled in 1944 in favor of the Grumman F7F Tigercat. In September 1945, however, the Marines received a dozen former Air Force P-61Bs to serve as radar trainers until the Tigercats would be available in squadron strength. Designated F2T-1N these aircraft were assigned to shore-based Marine units and served briefly, the last two F2T-1s being withdrawn on 30 August 1947.

Design

The P-61 featured a crew of three: pilot, gunner, and radar operator. It was armed with four 20 mm (.79 in) Hispano M2 forward-firing cannon mounted in the lower fuselage, and four .50 in (12.7 mm) M2 Browning machine guns lined up horizontally with the two middle guns slightly offset upwards in a remotely aimed dorsally mounted turret, a similar arrangement to that used with the B-29 Superfortress using four-gun upper forward remote turrets. The turret was driven by the General Electric GE2CFR12A3 gyroscopic fire control computer, and could be directed by either the gunner or radar operator, who both had aiming control and gyroscopic collimator sight assembly posts attached to their swiveling seats.

The two Pratt & Whitney R-2800-25S Double Wasp engines were each mounted approximately one-sixth out on the wing's span. Two-stage, two-speed mechanical superchargers were fitted. In an effort to save space and weight, no turbo-superchargers were fitted, despite the expected 50 mph (80 km/h) top speed and 10,000 ft (3,048 m) operational ceiling increases.

Main landing gear bays were located at the bottom of each nacelle, directly behind the engine. The two main gear legs were each offset significantly outboard in their nacelles, and retracted towards the tail; oleo scissors faced forwards. Each main wheel was inboard of its gear leg and oleo. Main gear doors were two pieces, split evenly, longitudinally, hinged at inner door's inboard edge and the outer door's outboard edge.

Each engine cowling and nacelle drew back into tail booms that terminated upwards in large vertical stabilizers and their component rudders, each of a shape similar to a rounded right triangle. The leading edge of each vertical stabilizer was faired smoothly from the surface of the tail boom upwards, swept back to 37°. The horizontal stabilizer extended between the inner surfaces of the two vertical stabilizers, and was approximately ¾ the chord of the wing root, including the elevator. The elevator spanned approximately ⅓ of the horizontal stabilizer's width, and in overhead plan view, angled inwards in the horizontal from both corners of leading edge towards the trailing edge approximately 15°, forming the elevator into a wide, short trapezoid. The horizontal stabilizer and elevator assembly possessed a slight airfoil cross-section.

The engines and nacelles were outboard of the wing root and a short "shoulder" section of the wing that possessed a 4° dihedral, and were followed by the remainder of the wing which had a dihedral of 2°. The leading edge of the wing was straight and perpendicular to the aircraft's centerline. The trailing edge was straight and parallel to the leading edge in the shoulder, and tapered forward 15° outboard of the nacelle. Leading edge updraft carburetor intakes were present on the wing shoulder and the root of the outer wing, with a few inches of separation from the engine nacelle itself. They were very similar in appearance to those on the F4U Corsair—thin horizontal rectangles with the ends rounded out to nearly a half-circle, with multiple vertical vanes inside to direct the airstream properly.

The P-61 did not have normal-sized ailerons. Instead, it had small ailerons which allowed wider span flaps and a very low landing speed. These ailerons, known as guide ailerons, gave some roll control and provided acceptable feel for the pilot in rolling manoeuvres. Control of the aircraft about the roll axis was augmented with circular-arc spoilerons which provided about half the roll control at low speeds and most of it at high speeds. The spoilers were located outboard of the nacelle in front of the flaps.

The main fuselage, or gondola, was centered on the aircraft's centerline. It was, from the tip of the nose to the end of the Plexiglas tail-cone, approximately five-sixths the length of one wing (root to tip). The nose housed an evolved form of the SCR-268 Signal Corps Radar, the Western Electric Company's SCR-720A. Immediately behind the radar was the multi-framed "greenhouse" canopy, featuring two distinct levels, one for the pilot and a second for the gunner above and behind him, the latter elevated by approximately 6 in (15 cm). Combined with the nearly flat upper surface of the aircraft's nose, the two-tiered canopy gave the aircraft's nose a distinct appearance of three wide, shallow steps. The forward canopy in the XP-61 featured contiguous, smooth-curved, blown-Plexiglas canopy sections facing forward, in front of the pilot and the gunner. The tops and sides were framed.

Beneath the forward crew compartment was the nose gear wheel well, through which the pilot and gunner entered and exited the aircraft. The forward gear leg retracted to the rear, up against a contoured cover that when closed for flight formed part of the cockpit floor; the gear would not have space to retract with it open. The oleo scissor faced forwards. The nosewheel was centered, with the strut forking to the aircraft's left. The nosewheel was approximately ¾ the diameter of the main wheels. Nose gear doors were two pieces, split evenly longitudinally, and hinged at each outboard edge.

The center of the gondola housed the main wing spar, fuel storage and piping and control mechanisms, control surface cable sections, propeller and engine controls, and radio/IFF (Identification Friend or Foe) equipment, but was predominantly occupied by the top turret mounting ring, rotation and elevation mechanisms, ammunition storage for the turret's machine guns, the GE2CFR12A3 gyroscopic fire control computer, and linkages to the gunner and radar operator's turret control columns, forward and aft, respectively.

The radar operator's station was at the aft end of the gondola. The radar operator controlled the SRC-720 radar set and viewed its display scopes from the isolated rear compartment, which he entered by way of a small hatch with a built-in ladder on the underside of the aircraft. In addition to the radar systems themselves, the radar operator had intercom and radio controls, as well as the controls and sight for the remote turret. The compartment's canopy followed the curvature of the gondola's rear section, with only a single rounded step to the forward canopy's double step. The rear of the gondola was enclosed by a blown Plexiglas cap that tapered quickly in overhead plan view to a barely rounded point; the shape was somewhat taller in side profile than it was in overhead plan view, giving the end of the "cone" a rounded "blade" appearance when viewed in perspective.

The cross-section of the gondola, front to back, was generally rectangular, vertically oriented. The tip of the nose was very rounded to accommodate the main AI radar's dish antenna, merging quickly to a rectangular cross-section that tapered slightly towards the bottom. This cross-section lost its taper but became clearly rounded at the bottom moving back through the forward crew compartment and nose gear well. Height increased at both steps in the forward canopy, with the second step being flush with the top of the aircraft (not counting the dorsal gun turret). At the rear of the forward crew compartment, the cross-section's bottom bulged downwards considerably and continued to do so until just past the midpoint between the rear of the forward crew compartment and the front of the rear crew compartment, where the lower curvature began to recede. Beginning at the front of the rear crew compartment, the top of the cross-section began to taper increasingly inwards above the aircraft's center of gravity when progressing towards the rear of the gondola. The cross-section rounded out considerably by the downward step in the rear canopy, and rapidly became a straight-sided oval, shrinking and terminating in the tip of the blown-Plexiglas "cone" described above.

The cross-section of the nacelles was essentially circular throughout, growing then diminishing in size when moving from the engine cowlings past the wing and gear bay, towards the tail booms and the vertical stabilizers. A bulge on the top of the wing maintained the circular cross-section as the nacelles intersected the wing. The cross-section became slightly egg-shaped around the main gear bays, larger at the bottom but still round. An oblong bulge on the bottom of the main gear doors, oriented longitudinally, accommodated the main wheels when the gear was retracted.

Wingtips, wing-to-nacelle joints, tips and edge of stabilizers and control surfaces (excluding the horizontal stabilizer and elevator) were all smoothly rounded, blended or filleted. The overall design was exceptionally clean and fluid as the aircraft possessed very few sharp corners or edges.

SCR-720 radar

The production model of the SCR-720A mounted a scanning radio transmitter in the aircraft nose; in Airborne Intercept mode, it had a range of nearly five miles (8 km). The unit could also function as an airborne beacon / homing device, navigational aid, or in concert with interrogator-responder IFF units. The XP-61's radar operator located targets on his scope and steered the unit to track them, vectoring and steering the pilot to the radar target via oral instruction and correction. Once within range, the pilot used a smaller scope integrated into the main instrument panel to track and close on the target.

Remote turret

The XP-61's spine-mounted dorsal remote turret could be aimed and fired by the gunner or radar operator, who both had aiming control and gyroscopic collimator sighting posts attached to their swiveling seats, or could be locked forward to be fired by the pilot in addition to the 20 mm (.79 in) cannon. The radar operator could rotate the turret to engage targets behind the aircraft. Capable of a full 360° rotation and 90° elevation, the turret could be used to engage any target in the hemisphere above and to the sides of the XP-61. A brief assessment of the turret by the British Aeroplane & Armament Experimental Establishment in 1944 found problems with the aiming and "jerky movement" of the guns.

Operators:

• United Kingdom
• One P-61A delivered under lend-lease for evaluation in 1944.
• United States
• United States Army Air Forces
• United States Air Force.

Surviving aircraft

Four P-61s are known to survive today.

P-61B-1NO c/n 964 AAF Ser. No. 42-39445 is under restoration to flying status by the Mid-Atlantic Air Museum in Reading, Pennsylvania. The aircraft crashed on 10 January 1945 on Mount Cyclops, Papua New Guinea and was recovered in 1989 by the Mid-Atlantic Air Museum of Reading, Pennsylvania. The aircraft has been undergoing a slow restoration since then with the intention of eventually returning it to flying condition, with the civilian registration N550NF. When finished, it is expected it will be over 70% new construction. By May 2011, 80% of the restoration had been completed, with only the installation of the wings and engines remaining. As of June 2019, both engines have been overhauled and two brand new props have been hung. The museum has also started painting the aircraft.

P-61B-15NO c/n 1234 AAF Ser. No. 42-39715 is on static display inside the Beijing Air and Space Museum) at Beihang University in Beijing, China.[38] This aircraft was manufactured by Northrop Aircraft, Hawthorne, California, and accepted by the USAAF on 5 February 1945. It was sent to Newark, New Jersey, on 16 February 1945 and departed the US ten days later for the China Burma India Theater. It was then assigned to the Tenth Air Force, being allotted to the 427th Night Fighter Squadron on 3 March 1945. At the end of the war the Communist Chinese came to one of the forward airfields in Sichuan Province and ordered the Americans out, but instructed them to leave their aircraft. It has been reported that there had been three P-61s taken and sometime later the Chinese wrecked two of them. P-61B-15NO c/n 1234 was stricken off charge by the USAAF on 31 December 1945. P-61B-15NO c/n 1234 was turned over to the Chengdu Institute of Aeronautical Engineering in 1947. When the institute moved to its present location, it did not take this aircraft with them, instead shipping it to BUAA (then called Beijing University of Aeronautics and Astronautics) in 1954 where it was placed on outside display with other aircraft as part of a museum. Sometime in 2008–09 the museum closed and the display aircraft were moved to a parking lot approximately 200 meters south. The outer wing sections of P-61B-15NO c/n 1234 were removed during this transfer. It was confirmed in September 2012 that the museum's display aircraft were no longer at the parking lot. By April 2013 the P-61 had been reassembled and repainted in the new BASM building with the other aircraft that were previously outside.

P-61C-1NO c/n 1376 AF Ser. No. 43-8330, is on display at the Steven F. Udvar-Hazy Center of the National Air and Space Museum in Chantilly, Virginia. The aircraft was delivered to the USAAF on 28 July 1945. By 18 October, it was flying at Ladd Field, in Alaska conducting cold weather tests, where it remained until 30 March 1946. The aircraft was later moved to Pinecastle AAF in Florida for participation in the National Thunderstorm Project. Pinecastle AAF personnel removed the guns and turret from 43-8330 in July 1946 to make room for new equipment. In September the aircraft moved to Clinton County Army Air Base in Ohio, where it remained until January 1948. The Air Force then reassigned the aircraft to the Flight Test Division at Wright-Patterson Air Force Base in Dayton, Ohio. After being declared surplus in 1950 it was donated by the U.S. Air Force to the National Air Museum in Washington, D.C. (which became the National Air and Space Museum in 1966).

On 3 October 1950, the P-61C was transferred to Park Ridge, Illinois, where it was stored along with other important aircraft destined for eventual display at the museum. The aircraft was moved temporarily to the museum's storage facility at Chicago's O'Hare International Airport, but before the museum could arrange to ferry the aircraft to Washington, D.C. the National Advisory Committee for Aeronautics asked to borrow it. In a letter to museum director Paul E. Garber dated 30 November 1950, NACA director for research I.H. Abbott described his agency's "urgent" need for the P-61 to use as a high-altitude research craft. Garber agreed to an indefinite loan of the aircraft, and the Black Widow arrived at the Ames Aeronautical Laboratory, at Naval Air Station Moffett Field in California, on 14 February 1951. When NACA returned the aircraft to the Smithsonian in 1954 it had accumulated only 530 total flight hours. From 1951 to 1954 the Black Widow was flown on roughly 50 flights as a mothership, dropping recoverable swept-wing test bodies as part of a National Advisory Committee for Aeronautics program to test swept-wing aerodynamics. NACA test pilot Donovan Heinle made the aircraft's last flight when he ferried it from Moffett Field to Andrews Air Force Base, arriving on 10 August 1954. The aircraft was stored there for seven years before Smithsonian personnel trucked it to the museum's Garber storage facility in Suitland, Maryland. In January 2006 the P-61C was moved into Building 10 so that Garber's 19 restoration specialists, three conservationists and three shop volunteers could work exclusively on the aircraft for its unveiling at the Steven F. Udvar-Hazy Center on 8 June. The aircraft was restored to its configuration as a flight test aircraft for swept-wing aeronautics, so the armament and turret were not replaced. A group of former P-61 air crews were present at the aircraft's unveiling, including former Northrop test pilot John Myers.

P-61C-1NO c/n 1399 AAF Ser. No. 43-8353 is currently on display at the National Museum of the United States Air Force at Wright-Patterson AFB in Dayton, Ohio. It is marked as P-61B-1NO 42-39468 and painted to represent "Moonlight Serenade" of the 550th Night Fighter Squadron. The aircraft was presented to the Boy Scouts of America following World War II and kept at Grimes Field in Urbana, Ohio. On June 20, 1958, it was donated to the museum by the Tecumseh Chapter of the Boy Scouts of America in Springfield, Ohio. The aircraft has had a reproduction turret, fabricated by the Museum's restoration group, installed.

Specifications (P-61B-20-NO)

General characteristics:

• Crew: 2–3 (pilot, radar operator, optional gunner)
• Length: 49 ft 7 in (15.11 m)
• Wingspan: 66 ft 0 in (20.12 m)
• Height: 14 ft 8 in (4.47 m)
• Wing area: 662.36 sq ft (61.535 m2)
• Airfoil: Zaparka
• Empty weight: 23,450 lb (10,637 kg)
• Gross weight: 29,700 lb (13,472 kg)
• Max takeoff weight: 36,200 lb (16,420 kg)
• Fuel capacity: 640 US gal (2,400 l) internal and up to four 165 US gal (625 l) drop tanks
• Powerplant: 2 × Pratt & Whitney R-2800-65W Double Wasp 18-cylinder air-cooled radial piston engines, 2,250 hp (1,680 kW) each
• Propellers: 4-bladed Curtiss Electric constant-speed feathering propellers, 12 ft 2 in (3.72 m) diameter.

Performance:

• Maximum speed: 366 mph (589 km/h, 318 kn) at 20,000 ft (6,100 m)
• Range: 1,350 mi (2,170 km, 1,170 nmi)
• Ferry range: 1,900 mi (3,100 km, 1,700 nmi) with four external fuel tanks
• Service ceiling: 33,100 ft (10,100 m)
• Rate of climb: 2,540 ft/min (12.9 m/s)
• Time to altitude: 20,000 ft (6,100 m) in 12 minutes
• Wing loading: 45 lb/sq ft (220 kg/m2)
• Power/mass: 0.15 hp/lb (0.25 kW/kg).

Armament


• Guns: ** 4 × 20 mm (.79 in) Hispano AN/M2 cannon in ventral fuselage, 200 rounds per gun
• 4 × .50 in (12.7 mm) M2 Browning machine guns in remotely operated, full-traverse upper turret, 560 rpg
• Bombs: for ground attack, four bomb]s of up to 1,600 lb (726 kg) each or six 5-in (127 mm) HVAR unguided rockets could be carried under the wings. Some aircraft could also carry one 1,000 lb (454 kg) bomb under the fuselage.

Avionics


• SCR-720 (AI Mk.X) search radar
• SCR-695 tail warning radar.

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