Northrop P-61 Development and Specifications - Page 2

The Northrop P-61 Black Widow was the first operational U.S. military aircraft designed specifically to use radar

 

The P-61 was designed from the start to be a night fighter. By the time it arrived with combat squadrons in mid-1944, targets were rather scarce. Thus, while it didn't pile up a large score of enemy planes destroyed, it was an extremely capable and deadly aircraft.

It originated in the Battle of Britain, when the British urgently needed a night fighter. Because early radars were so heavy and because the british requirement called for a nightfighter that could stay airborne for a long time, only a twin-engined aircraft would work. Northrop began working on the project in late 1940. Northrop's proposal, submitted in November, followed the general outline of Lockheed's P-38: a big, twin-engined fighter, with crew and guns in the fuselage, and two engine nacelles extending back into twin booms connected by a long horizontal stabilizer

The armament was quite different though; the P-61 housed two dorsal turrrets, each with four .50 caliber machine guns.

While there had been primitive efforts to develop night fighters since 1921, by 1940, radar promised to make them practical. The British had first developed Airborne Interception (AI) radar and also developed the cavity magnetron, which permitted short wavelength radars. Using a British cavity magenetron, by early 1941, engineers from MIT and several American electronics companies had built the first microwave radar, the forerunner of the SCR-270 used in the P-61.

Meanwhile, Northrop struggled with the P-61 aircraft, by far the biggest contract it had ever tackled. Meeting the Army's requirement for a three-man crew was one of many challenges faced by the design team. Throughout 1941, indeed throughout the entire war, required engineering changes continually cropped up, delaying the development of the P-61. Guns were relocated; fuel tanks were added; and control surfaces were redesigned. The first XP-61 protoype flew in May, 1942, with test pilot Vance Breese at the controls.

The second prototype flew that November and had radar installed in April, 1943.

Flights with the YP-61's revealed that the dorsal machine gun turret caused severe tail buffeting. Thus it was removed entirely from many early P-61A's, and when added back, only mounted two guns.

Service deliveries started in May, 1944, when the 348th Night Fighter Squadron (NFS) of the 481st Night Fighter Group (NFG) received their Black Widows. While the P-61 was exceptionally maneuverable for such a large plane (thanks to the large and well-designed flaps), it remained troublesome. In June, deliveries increased to three a day. The first P-61 kill was recorded on June 30, 1944 (some sources say July 6), when a Black Widow of the 6th NFS downed a 'Betty" bomber over the Pacific. In Europe, the crews continued training while debates raged over the nightfighting virtues of the Black Widow, the Mosquito, and the Bristol Beaufighter.

Once the Black Widow did get into action in Europe, it found success against a variety of targets: fighter planes, bombers, V-1 buzz bombs, and ground targets like locomotives and truck convoys. Some ETO NF squadrons did not convert until spring of 1945, when the war was almost over. In the Pacific, the 418th and 421st NFS adopted the P-61 in mid-1944, and in the CBI, the 426th and 427th NFS transitioned to the P-61 later that year.

706 P-61's were built in total.

 

In detail

 

The P-61 featured a crew of three: pilot, gunner, and radar operator. It was armed with four 20 mm Hispano M2 forward firing cannons mounted in the lower fuselage, 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 dorsally mounted turret. 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. 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 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 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 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 four-degree dihedral, and were followed by the remainder of the wing which had a dihedral of two 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.

The P-61 did not have ailerons. 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 10 feet in length and 6 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 inside wing rotated out of the wing's upper surface into the airstream, disrupting the effect of 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 (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 forward crew compartment, seating the pilot and behind him the gunner, the latter elevated approximately six inches. The multi-framed "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, 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 three-fourths 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, fuel piping and control mechanisms, control surface cable sections, propeller 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 four Browning M2 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 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 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 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.

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

 

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 cannons 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 farther outboard, eliminated the inherent drawbacks of convergence.

Convergence was a necessity in wing-mounted guns. Convergence is the specific point or points of range and elevation at which arming crews calibrate the weapons' projectile paths to intersect the aircraft's centerline, preventing a "safe zone" in front of the aircraft through which no projectiles would pass if wing guns were set to fire straight ahead. Projectiles fired at a target beyond the point of convergence crisscross before reaching the target and miss wide; projectiles fired at a target closer than the point of convergence either pass on either side or fail to impact at a concentrated point, minimizing the damage inflicted. In practice, both cases limit the cannons' effective ranges to a very small zone on either side of a set distance, and create 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 (.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 movement of the cannons—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 B-17s, B-24s, B-25s, A-20s and other bombers. Following this modification, the 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. 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 XB-35 flying wing 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.

The service life of the P-61C was quite brief, since it was being quickly outclassed by 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 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.

Interesting Facts

• The Northrop P-61 Black Widow was the largest twin USAAF WW II fighter
• Shot down 18 V-1 "buzz bombs" in Europe
• Rejected by British in favor of de Havilland Mosquito
• Directed by ground-based radar
• Introduced night vision binoculars
• Featured cannon fired by pilot
• Modified to become F-15 Reporter
• One civilian P-61 and one F-15 Reporter were modified as air tankers; both crashed.
• Used in thunderstorm research
• Chosen as featured aircraft in a planned, but canceled, Howard Hughes film
• Used by Marine Corps as trainer aircraft.