Northrop P-61 Black Widow

Template:Infobox Aircraft The Northrop P-61 Black Widow was an all-metal, twin-engine, twin-boom, monoplane night fighter and night intruder aircraft flown by the United States Army Air Forces during World War II. It was the first American aircraft designed specifically as a radar-equipped night fighter.

Origins

During the few last years of peace leading up to World War II, both the Royal Air Force and the German Luftwaffe made the first and largest steps in developing and refining the theories of waging war with aircraft by night. The complexity and challenge of nocturnal airborne combat must be briefly explained to be fully appreciated, and to comprehend the undertaking and achievement the P-61 entailed.

First signs of the Black Widow

In August 1940 – a full 16 months before the United States entered the war – the U.S. air officer in London, Lt. Gen. Delos C. Emmons, was briefed on British research in RADAR (Radio Detection and Ranging), which had been underway since 1936 and had played an important role in the nation's defense against the Luftwaffe during the Battle of Britain. Gen. 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 on many aspects including radar 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 night bombers attacking London. The aircraft would need to patrol continuously over the city throughout the night, demanding at least an eight-hour loiter capability, and it would obviously be equipped with one of the early, and heavy, AI radar units. Specified armament was to be carried in ‘multiple gun turrets’. The RAF’s Boulton Paul Defiant had proved capable as a radar equipped night fighter but was discarded in 1942, and the night-adapted variants of the de Havilland Mosquito, while excellent performers, needed strengthening. The needs were conveyed to all the aircraft designers and manufacturers the British 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.

Gen. Emmons returned to the US with details of the British night-fighter requirements, and in his report said that US aircraft design bureaus 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, Wright Field. 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, like Jack Northrop, realized the aircraft would need the considerable power and resulting size of twin engines, and recommended such parameters.

Vladimir H. Pavlecka, Northrop Chief of Research, was present on unrelated business at Wright Field. On October 21, 1940, Col. Lawrence Craigie of the ATSC phoned Pavlecka, explaining the USAAC's specifications, but told him to ‘not take any notes, “Just try and keep this in your memory!”’ (Davis & Menard, 4). 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 November 5, 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 most deadly pursuit-class aircraft flown by the US during the war.

Jack Northrop's first proposal was a long fuselage gondola between two engine nacelles and tail booms. Engines were Pratt & Whitney R2800-10 Double Wasp 18-cylinder radials, producing 2000 horsepower (1.5 MW) each. The fuselage housed the three-man crew, the radar, and two four-gun turrets. The guns were heavy, belt-fed .50 cal (12.7 mm) Browning M2s fitted with 36 inch '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" (named after Northrop engineer Edward Zap) in the wings.

The aircraft was huge, as Northrop had suspected. While far heavier and larger two, three, and four (or more) engine bombers existed, the 45 foot 6 inch length, 66 foot wingspan, and projected 22,600 lb full-load weight were unheard of for a fighter and hard for many to accept as less than ridiculous.

Changes to the plan

Some alternative design features were investigated before finalization. Among them was conversion to a single vertical stabilizer/rudder but this was dismissed. One change kept was 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, the belly turret was abandoned and four 20 mm Hispano M2 cannons were mounted in the wings. The P-61 therefore became one of the few US designed aircraft to have 20 mm cannons as factory-standard. Others were the P-38, the F4U-1C, a limited production Corsair sub-variant, and the A-36 Apache dive-bomber, an early form of the P-51 Mustang. While some F6F Hellcats and repossessed British lend-lease P-39s (renamed as P-400s) were also fitted with 20 mm cannons, it was not standard practice.

Northrop Specification 8A was formally submitted to Army Air Material Command at Wright Field, on December 5, 1940. Following a few small changes, Northrop's NS-8A fulfilled all USAAC requirements. Northrop was issued a Letter of Authority For Purchase on December 17. 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 January 10, 1941. Northrop Specification 8A became, by designation of the Department of Defense, the XP-61.

Physical characteristics

It featured a crew of three: pilot, gunner, and radar operator. It was armed with four 20 mm Hispano M2 cannons, two in each wing, and four Browning M2 .50 cal (12.7 mm) machine guns, lined up horizontally with the two middle guns slightly offset upwards in a remotely-aimed turret, dorsally mounted. 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 the aiming control and gyroscopic collimator sight assembly posts attached to their swiveling seats.

The two Pratt & Whitney R2800-25S Double Wasp radial engines were each mounted approximately one-sixth out on the wing's span. Two-stage, two-speed mechanical superchargers were fitted. No turbo-superchargers were fitted, despite the expected 50 mph top speed and 10,000 ft operational ceiling increases, as they were not felt to be necessary, would have occupied considerable space, and added significant weight.

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 towards 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 degrees. The horizontal stabilizer extended between the inner surfaces of the two vertical stabilizers, and was approximately three-quarters the chord of the wing root, including the elevator. The elevator spanned approximately one third 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 degrees, forming the elevator into a wide, short trapezoid. The horizontal stabilizer and elevator assembly possessed a subtle 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 degrees of dihedral, and were followed by the remainder of the wing which had a dihedral of 2 degrees. 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 degrees 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.

No ailerons were present. Aside from the full-span retractable 'Zap flaps', all control of the aircraft about the roll axis was maintained through the use of curved, tapered spoilerons, of approximately ten feet in length and six inches in width (in overhead plan view) each. They were located outboard of the outer edge of each nacelle in overhead plan view, approximately one-quarter the length of the outer wing (the section of wing outboard of the edge of each nacelle furthest from the aircraft's centerline) and offset towards the wing leading edge approximately one third the wing's chord from the trailing edge, running towards the wing-tip approximately half the length of the outer wing. Operation was as follows: the spoileron in the wing being turned towards rotated out of the wing's upper surface into the airstream, disrupting effects due to Bernoulli's principle and reducing lift over that wing, causing it to drop.

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 (wing root to wing 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 forward crew compartment, seating the pilot and behind him the gunner, the latter elevated approximately six inches. The multiframed 'greenhouse' canopy featured two distinct levels, one for the pilot and a second for the gunner above and behind him. 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, which the pilot and gunner entered and exited the aircraft through. 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 three-fourths the diameter of the main wheels. Nose gear doors were two pieces, split evenly, longitudinally, hinged at each outboard edge.

The center of the gondola housed the main wing spar, fuel storage, fuel piping and control mechanisms, control surface cable sections, propellor and engine controls, and radio/IFF (Identification Friend or Foe) /communications equipment, but was predominantly occupied by the top turret mounting ring, rotation and elevation mechanisms, ammunition storage for the turret's 4 Browning M2 .50 cal (12.7 mm) heavy machine guns, the GE2CFR12A3 gyroscopic fire control computer, and linkages to the gunner and radar operator's turret control columns, forward and aft, respectively.

At the aft of the gondola was the radar operator's station. Entered by way of a small hatch with a built-in ladder on the underside of the aircraft, the SRC-720 radar set was controlled and its display scopes viewed from the isolated rear compartment. Intercom and radio controls, as well as the radar operator's controls and sight for the remote turret, were present in addition to the radar systems themselves. The compartment's canopy followed the curvature of the gondola's rear section, with only a single rounded step to the forwards 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, 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 spinal 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 increasingly taper 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 and shrinking 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.

Wing tips, 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 aircraft possessed very few sharp corners or edges, and the design was overall exceptionally clean and fluid.

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 updraft carburetor standardization decision was later reversed by the committee (the XP-61 program's predicament having little likely influence), preventing a potential setback in the XP-61's development.

The Air Corps Mockup Board met at Northrop on April 2, 1941, to inspect the XP-61 mockup. Several changes were made following the review. Most prominently, the four 20 millimeter Hispano M2 cannon were relocated from the outer wings to the belly of the aircraft, clustered tightly just behind the rear 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 father outboard, eliminated the inherent drawbacks of convergence.

Convergence was a necessity in wing-mounted guns--the specific point or points of range and elevation at which the weapons' projectile paths were calibrated to intersect the aircraft's centerline, preventing the 'safe zone' in front of the aircraft which no projectiles would pass through if wing guns were set to fire straight ahead. Weapons fire at a target beyond the point of convergence crisscrossed behind the target and missed wide; fire at a target closer than the point of convergence either passed on either side or failed to impact in a concentrated point, preventing maximum damage from being inflicted. Both cases limited the cannons' effective ranges to a very small zone on either side of a set distance, and created additional challenges when calculating deflection ('pulling lead') for a moving target.

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 540 to 646 gallons.

Other changes included the provision for external fuel carriage in drop tanks, flame arrestors / dampeners on engine exhausts, and redistribution of some radio equipment. While all beneficial, especially the movement of the cannons, the modifications required over a month of redesign work, and the XP-61 was already behind schedule.

In the Summer of 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 B-17s, B-24s, B-25s, A-20s, and other bombers. Following this, turret itself became unavailable, as operational aircraft--in this case, the B-29--were ahead of experimental aircraft in line for the high-demand component. A dummy turret was used for flight testing.

During February, 1942, Northrop was notified by manufacturer Curtiss that the C5424-A10 four-bladed, automatic, full-feathering propellor 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 propellors were used in lieu of the Curtiss props until the originally planned type became available.

The XP-61's weight rose during construction of the prototype, to 22,392 lb empty and 29,673 lb at takeoff. Engines were R-2900-25S Double Wasp radials, turning 12 foot 2 inch Curtiss C5425-A10 four blade propellors, both rotating clockwise 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.

SCR-720 radar

The production model of the SCR-720A mounted a scanning radio transmitter in the aircraft nose; in Airborne Intercept mode, it was capable of a range of nearly 5 miles. The unit could also function as an airborne beacon / homing service, navigational aid, or in concert with interrogator-responder IFF (Identification Friend or Foe) 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 remote turret could be aimed and fired by any person of the 3 man crew, or could be locked forward to be fired by the pilot in addition to the 20 mm cannons. The radar operator could rotate the turret to face to the rear, in order to engage targets behind the aircraft. Capable of a full 360 degrees rotation and 90 degrees elevation, the turret could conceivably be used to engage any target in the entire hemisphere above and to the sides of the XP-61.

Variants

XP-61: The first two prototypes.
YP-61: Pre-production series; 13 built.
P-61A-1: First production version, R-2800-10 engines with 2,000 hp (1,490 kW); 45 built. The last 7 aircraft were built without the turret.
P-61A-5: No turret, R-2800-65 engines producing 2,250 hp (1,680 kW), 35 built.
P-61A-10: Water injection to increase duration of maximum power output; 100 built.
P-61A-11: One hardpoint under each wing for bombs or fuel tanks; 20 built.
P-61B-1: Nose stretched 8 inches (20.3 cm), SCR-695 tail warning radar; 62 built.
P-61B-2: Reinstated underwing hardpoints like on P-61A-11; 38 built.
P-61B-10: Four underwing hardpoints; 46 built.
P-61B-11: Reinstated turret with two 0.50 cal (12.7 mm) machine guns; 5 built.
P-61B-15: Turret with four 0.50 cal (12.7 mm) machine guns; 153 built.
P-61B-16: Turret armament reduced to two machine guns; 6 built.
P-61B-20: New General Electric turret with four machine guns; 84 built.
P-61B-25: Turret automatically aimed and fired by the APG-1 gun-laying radar connected to an analogue computer; 6 built.
P-61C: Turbosupercharged R-2800-73 engines producing 2,800 hp (2,090 kW), top speed increased to 374 knots (430 mph, 692 km/h) at 30,000 feet (9,145 m). However, the aircraft suffered from longitudinal instability at weights above 35,000 pounds (15,875 kg), and from excessive takeoff runs — up to 3 miles (4,830 m) at a 40,000 pound (18,143 kg) takeoff weight; 41 built, 476 more cancelled after the end of the war.
TP-61C: P-61Cs converted to dual-control training aircraft.
XP-61D: One P-61A-5 (number 42-5559) and one P-61A-10 (number 42-5587) fitted with turbosupercharged R-2800-14 engines, cancelled when P-61C entered production.
XP-61E: Two P-61B-10s (numbers 42-49549 and 42-39557) converted to daytime long-range escort fighters. Tandem crew sat under a blown canopy which replaced the turret, additional fuel tanks were installed in place of the radar operator's cockpit in the rear of the fuselage pod, and four 0.50 cal machine guns took place of the radar in the nose (the 20 mm ventral cannon were retained as well). First flight 1944-11-20, cancelled after the war ended. The first prototype was converted to an XF-15, the second lost in an accident in 1945.
XP-61F: Conversion similar to XP-61E applied to a single P-61C (number 43-8338).
P-61G: Sixteen P-61B converted for meteorological research.
F-15A Reporter: Photoreconnaissance variant with a new center pod with pilot and camera operator seated in tandem under a single bubble canopy, and six cameras taking place of radar in the nose. Powered by the same turbosupercharged R-2800-73 engines as the P-61C. The first prototype XF-15 was converted from the first XP-61E prototype, the second XF-15A was converted from a P-61C (number 43-8335). The aircraft had a takeoff weight of 32,145 pounds (14,580 kg) and a top speed of 382 knots (440 mph, 708 km/h). Only 36 of the 175 ordered F-15As were built before the end of the war. After formation of United States Air Force in 1947, F-15A was redesignated RF-61C. F-15As were responsible for most of the aerial maps of North Korea used at the start of the Korean War.
F2T-1: Two P-61As tested by the United States Navy.

Modifications

The 425th Night Fighter Squadron moved their radar operator's position from the rear to the former gunner's station behind the pilot. This improved the chances of spotting enemy aircraft by adding a second set of eyes up front, and the extra forward weight was found to increase the aircraft's overall performance.

Survivors

Four P-61s are known to survive today. A P-61C (s/n 43-8353) is currently on display at the National Museum of the United States Air Force in Dayton, Ohio. It is marked as P-61B-1-NO s/n 42-39468 and is painted to represent the aircraft "Moonlight Serenade" of the 550th Night Fighter Squadron. It recently had a reproduction turret installed, fabricated by the Museum's restoration team. A P-61C (s/n 43-8330) belonging to the National Air and Space Museum is stored at the Paul Garber Facility at Suitland, Maryland awaiting restoration. A P-61A is on display at the Beijing Institute of Aeronautical Engineering in Beijing, China. The aircraft has no s/n but was one of three P-61As belonging to the 427th Night Fighter Squadron that the Chinese seized in 1945. The aircraft is in very poor condition and probably near the point of structural collapse. The Chinese claim to have two additional P-61s in storage which they have offered for sale for $2,000,000. A P-61B (s/n 42-39445) that had crashed on January 10, 1945 on Mt. Cyclops in New Guinea was recovered in 1991 by the Mid-Atlantic Air Museum in Reading, Pennsylvania. The aircraft has been undergoing a slow restoration since then with the intention of eventually returning it to flying condition. It will have the civilian registration N550NF. Its current status is a partially restored central nacelle and two minimally restored engine pods.

Interestingly, the last flying example of the P-61 line was a F-15A Reporter (RF-61C) (s/n 45-59300) that had been converted into a fire bomber operated by TBM, Inc. in California. It was destroyed in a takeoff accident on September 16, 1968.

P-61 In The Media

In the late 1940s RKO Studios planned a film about the 422nd Night Fighter Squadron in Europe. Little information exists about the film, though Howard Hughes, who was a fan of the P-61 design, was to be executive producer and heavily involved in promoting the project, and James Arness was apparently considered for a role. When Hughes and the studio found out that there were very few flying P-61s, and most were based in Alaska, they attempted to overcome the problem by using a C-82 Packet painted black, but the results were unimpressive and the production was finally cancelled. A few articles of clothing from the production were later sold by RKO's wardrobe dept. They resemble actual 422nd NFS patches and uniforms, but careful examination will clearly reveal RKO's studio stamp and markings.

Specifications (P-61B-20)

Data from Jane’s Fighting Aircraft of World War II,^[1] and Northrop P-61 Black Widow.^[2]

General characteristics

Crew: 2-3 (pilot, radar operator, optional gunner)
Main wheel track: 17 ft 2 in (5.24 m)
Internal fuel capacity: 640 US gal (2,423 L) of AN-F-48 100/130-octane rating gasoline
External fuel capacity: Up to four 165 US gal (625 L) or 310 US gal (1,173 L) tanks under the wings
Oil capacity: 22 US gal (83.3 L) of AN-0-8 oil per engine

Performance

Time to altitude: 12 min to 20,000 ft (6,100 m)

Armament

4× 20 mm Hispano M2 cannon in ventral fuselage, 200 rounds per gun
4× 0.50 cal (12.7 mm) Browning M2 machine guns in upper turret, 560 rounds per gun
For ground attack, two bombs of up to 1,600 lb (726 kg) each could be carried along with six 5 inch (127 mm) HVAR unguided rockets 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

References

^ Bridgeman, Leonard. "The Northrop Black Widow." Jane's Fighting Aircraft of World War II. London: Studio, 1946. p. 251-252. ISBN 1 85170 493 0.
^ Musialkowski, PK (1985). Nocny samolot myśliwski Northrop P-61 Black Widow. Wydawnictwo Ministerstwa Oborony Narodowej. ISBN 831107107.

External links

P-61 Restoration Project at the Mid-Atlantic Air Museum in Reading, Pennsylvania.