Northrop P-61/F-15 Black Widow

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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 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. 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, two 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.

John W. Myers, Chief test pliot for Northrop, told of a little `show-off' flight that I had practiced to impress new pilots, on the versatility of the P-61;     "It took about three minutes: a very short takeoff, back across the deck at red-line 420mph, loop down to deck again and do an Immelmann, come out of Immelmann, feather one engine on the way down to deck, two slow rolls off the deck into the dead engine, approach and land short.
This made true believers of all the pilots who witnessed it."

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The first protoype, designated XP-61 

This aircraft was the direct result of an American visit to Great Britain in the end of 1940. Similarities to the British night-fighters can clearly be discerned, but the twin-boom and pod construction was reasonably novell. The more or less only problem was the reliability of the engine, which could be a reason for concern. Later aircraft would be fitted with a more reliable engine as a result of this concern. 
Number built: 2 XP-61, 13 YP-61 Service Trials aircraft.

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Northrop P-61A Black Widow

The YP-61 trials aircraft revealed that the barbettes caused considerable buffeting problems when the guns were elevated or traversed. As a stopgap measure pending a definite solution the two inner guns were removed, and the construction was reinforced. 
Because production was well underway when the problems were identified, the first 37 P-61A's were delivered with a barbette that was locked to fire straight ahead, and fitted with 4 guns. The rest of the batch, 163 aircraft, were delivered as a two-seat aircraft without the barbette. Some of these were later re-engineered with the barbette when the problem was solved.
Also the P-61A series distinguished themselves in the field of different 
powerplants. The first 45 examples had a powerplant of 2 Pratt & Whitney R-2800-10 radial, rated at 2,000 hp (1.491 kW) each, next came a batch of 35 aircraft powered by 2 Pratt & Whitney R-2800-65 radial rated at 2,250 hp (1.667 kW) at War Emergency Power/Combat Contingency Power each. the other aircraft were fitted with 2 Pratt & Whitney R-2800-65 Double Wasp 18-cylinder radial modified with a water injection system. 
Number built: 200

Northrop P-61B Black Widow

Before the P-61A Black Widow entered operational service, Northrop had started deliveries in July 1944 of the P-61B with a number of improvements including an 8 in (0.203 m) lengthening of the nose. Deliveries amounted to 450 aircraft, and progressive improvements 
effected during the course of the P-61B’s production run resulted in a  number of production blocks:
Number built 450

Northrop P-61B Black Widow

Experience under War conditions with the P-61 showed that although the Black Widow posessed a good general performance, agility and firepower, the speed and climb rate were insufficient. 
Therefor Northrop installed a powerplant consisting of 2 Pratt & Whitney R-2800-73 Double Wasp radial, rated at 2,800 hp (2.088 kW) each, with General Electric CH-5 turbochargers and driving Curtiss Electric propellers with four hollow blades. This uprated powerplant resulted in a maximum level speed of 430 mph (692 km/h) at 30,000 ft (9.145 m) and a service ceiling of 41,000 ft (12.495 m) even though the normal and maximum take-off weights had increased to 32,200 and 40,300 lb (14.606 and 18.280 kg) respectively. The increase in maximum speed was particularly notable, and raised fears that the P-61C would overhaul its targets too quickly and therefore be unable to fire before it had passed the enemy aircraft, so the type was fitted with air brakes above and below the wings. Some 41 of this version had been completed by the time of Japan’s defeat. The remaining 476 of the same order were canceled. 
Number built: 41

Northrop P-61B Black Widow

The P-61C was the last version of the P-61 to enter production, but several other versions were in the prototype stage or in project stage. The XP-61D was powered with 2 R-2800-77 turbocharged radials, the XP-61E version of the P-61B had it's nose radar replaced by 4 0.5 inch (12,7 mm) Browning machine guns in place of the (then deleted) dorsal barbette, the XP-61F version of the P-61C to XP-61E standard (armament of the XP-61E, powerplant of the P-61C).
The P-61G was a conversion of the P-61C that was meant for unarmed weather reconnaissance duties. 
Number converted: 12

Northrop P-61B Black Widow

12 P-61B Black Widows were transferred to the US Marine Corps, which used them as night-fighter trainers with the revised designation F2T-1N 
Number transferred: 12