P-61

When air combat ceased to be a day-time operation and the Nazis began using London as an all night target, pursuits and light bombers were converted by the British into night fighters, but were unsatisfactory. In the fall of 1940 top U.S. Army officials returned from England with a secret list of arms requirements for both Britain and the US — and a night-fighter was on that list.

As a result, on Nov 5, 1940, the first version of Northrop's specifically built night-fighter, the XP-61, was laid out. Today's P-61 Black Widow is an all-metal, midwing, night interceptor pursuit monoplane. It has twin engines, two tail booms, retractable tricycle landing gear. Located on the wing between the two engine nacelles is the crew nacelle fully covering two or three crewmen. Early P-61A models carried pilot, radio-man and gunner, while later P-61As and P-61Bs accommodate only pilot and radio operator.

Deriving its name from its deadly destructive qualities and sorties under cover of darkness, the Black Widow's primary tactical function is the interception and destruction of hostile aircraft in flight during the night hours or under conditions of poor visibility. As large as a medium bomber, the P-61 weighs more than 25,000 pounds, has a speed of over 350 mph, an overall length of forty-nine feet, wing span of sixty-six feet. Her four-blade Curtiss Electric, full-feathering propellers are powered by two Pratt & Whitney R-2800 eighteen cylinder, double-row, radial, aircooled engines that can develop the 2000 hp needed for takeoff.

To protect the Widow against anything the enemy could send up into the air are four 50-cal machine guns, able to fire 5600 bullets and shells a minute, housed in the remote control turret atop the crew nacelle, and four 20-mm cannon in the belly. Because night landings on war-time fields are hazardous for fighters, new flaps and ailerons have been incorporated into the plane. It has full-span landing flaps which run almost the whole length of the outer wing panels, leaving no room for conventional ailerons. Hence, a unique system of retractable ailerons, which actually are small gates lifting up out of the top of the wing, break the airflow, thus controlling the lift. In profile, the P-61's fuselage is very narrow and the nose section sticks out in front several feet ahead of the propeller hubs, making identification easy. The fuselage tapers down almost on a line with the trailing section of the big wing, while her low-slung engine nacelles taper off with streamline effect into the thin tail-booms supporting the elevator and rudder surfaces. All go to achieve good climb, high maneuverability, and a comparatively short takeoff run. Crew members, protected by armor-plate deflector shields and bullet-resistant glass, can fly the P-61 (whose existence was kept secret until January, 1944) with assurance of safety. Other safeguards are self-sealing gas and oil tanks and lines.

A semi-monocoque type structure, the crew nacelle completely encloses the crew of the P-61. Constructed of stressed skin reinforced by stiffening members, the body has wing front and rear spars extending through the center of the nacelle in front of and behind the gun turret. A door in the nose-wheel well provides entrance to the pilot's, gunner's and turret compartments, while that of the radio operator is entered through a door on the bottom of the aft portion of the crew nacelle. On some models a "clam shell" enclosure over the radio operator's station permits access to the top of the wing for servicing the airplane; on other models access to the wing is gained through a hinged section of the enclosure to the left of his station. Mounted between the two wing spars and protruding through the top of the crew nacelle is the flexible gun turret, the area beneath the nacelle structure being covered with a fairing which encloses the retracted nose wheel and four 20-mm cannons.

Floors in the crew nacelle body are of plywood and metal construction, attached to the side frames approximately fifteen inches below the horizontal reference line, and are supported by the frames and lower bulkheads. Of four of these floors, one in each compartment, the gunner's floor forms the upper housing for the cannons, the pilot's is an integral part of the forward structure, the turret access floor is a five-ply board bolted to the bulkheads forward of the turret structure, and the radio operator's floor is aft of the turret structure. Equally important with floors are doors, of which there are four on the Black Widow. The front entrance door is located aft of the pilot's seat and is part of the nose wheel well, with the door frame and folding ladder forming a welded steel structure, hinged at the forward end. A formed aluminum alloy wheel well is riveted to the door structure, neoprene tubing making the door weathertight when closed. With the nose wheel down, the door may be lowered from within the crew nacelle or from the ground. Free fall is checked by a hydraulic cylinder and piston dampener. The door may be closed manually from the outside or the inside, and a key lock is provided for ground use. Located just forward of the tail cone on the lower side of the crew nacelle is the rear entrance door. A formed aluminum alloy structure with a steel tube ladder bolted to the side channel stiffeners, it is, with the omission of the wheel well, the same as the front entrance door. The two nose gear doors form part of the lower forward crew nacelle and enclose the nose gear and the front entrance door while the plane is in flight. Weather sealing is provided where the two doors, which operate on piano hinges, fit together. Opened and closed by two actuating arms operated by the nose gear lower strut cylinder, the nose gear doors close tight when the gear is retracted and strikes the upper actuating arms hard enough to pull the linkage off, center. The cannon doors, of Alclad sheet and frame construction, form the lower central crew nacelle skin and are attached by three hinges on each door. Manually operated, the cannon doors are unlocked with the cannon door locking handle. They are held in the open position by a link hook attached to the forward end of the door which hooks into the crew nacelle side above the door hinge.

Technically termed enclosure panels, the pilot's, radio operator's and gunner's glass houses are made up of molded Lucite sheets, bound by extruded aluminum alloy frames. The section directly over the gunner is supported by square steel tubing. Weathertight cork and neoprene seals are provided around all frames and window openings, with the entire enclosure attached to the cockpit coaming, cockpit rail, and upper skin of the crew nacelle. Giving adequate protection to pilot and gunner are windshields incorporating three-inch plates of armor glass. A hinged window on each side is provided for the gunner, an adjustable sliding window on each side for the pilot. A hinged canopy, opening from the inside only, is located above the pilot's seat. Means for emergency escape for the gunner is a kickout panel to the right of his seat which can be opened from the inside or outside.

Flight controls on the Black Widow make for easy maneuvers. Ailerons, retractable ailerons and elevators are controlled in the conventional manner by the torque tube type control column in the cockpit. The torque tube at the base of the column extends from one side of the cockpit to the other, with four control cranks at the extreme ends of the tube attached to the four elevator control cables. Ailerons and retractable ailerons are controlled by the wheel on the control column by means of an enclosed sprocket and chain attached to the aileron cables. Conventional, adjustable foot pedals control the rudders. Used on the P-61 are extra-flexible, preformed, carbon-tinned, corrosion-resistant steel control cables. Each assembly is composed of seven strands of seven wires, or seven strands of nineteen wires and Northrop cable terminals.

The P-61's automatic flight controls, consisting of an automatic pilot (gyroscope units, a hydraulic servo unit, followup cables, an oil pressure regulator, an oil filter and an air filter, are capable of taking over the ship, giving relief to the pilot on long tedious hops. Automatic pilot gyroscopes are actuated by air drawn in over the diaphragm which operates valves that control the flow of hydraulic fluid. Pressure of the air drawn over the diaphragms is governed by the action of the gyroscopes, while hydraulic fluid actuates the pistons of the servo-control unit. The pistons are connected to the flight control cables.

The instrument panel, mounted on four rubber vibration absorber mounts, has dial numerals, scales and pointers marked with fluorescent material, making possible night readings of the instruments with the aid of fluorescent lamps. Behind most of the vital instruments on the Black Widow is the pitot static system which uses an electrically heated pitot head, type G-2, and flush static tubes. Mounted on a mast located on the bottom side of the crew nacelle nose, the pitot head measures impact pressure, transmitting it instantaneously by airtight tubing to the pitot pressure side of the air speed indicator. The static tubes are mounted inside the crew nacelle nose with the two end nipples connected flush with the outside surface of the nose, one nipple located on each lower side of the nose. Except in still air at normal sea level, the indicated airspeed is different from the ground speed. The airspeed indicator mechanism responds to small differential pressure changes in the difference between impact (pitot) and static pressures.

Simply an aneroid barometer calibrated to read in feet instead of inches of mercury, the altimeter dial of the P-61 indicates 100 feet for the largest pointer, 1,000 feet for the medium pointer, and 10,000 for the smallest pointer. Rate of climb indicator gives rate of gain or loss of altitude regardless of the altitude of the plane, signifying the change of altitude in feet per minute; its pointer moves clockwise in climb and counterclockwise in descent. Remote-reading compass and turn and bank indicator have their places on the instrument panel along with such necessities as gyro-horizon, clock, manifold pressure gage, tachometer, oil temperature indicator, oil pressure gage, cylinder head temperature indicator, fuel and water pressure gages, carburetor air temperature indicator, and so on down the line of fighter equipment. The vacuum system provides a decreased air pressure for the operation of several of the airplane instruments. Consisting of an engine-driven pump, a relief valve, an oil separator, a check valve and an air filter, a visual indication of the amount of vacuum is given by a gage within the automatic pilot. Each engine is provided with a vacuum pump, of the rotary, four-vane, positive displacement type. The suction side of each of these pumps is connected to the air-operated instruments. The compression side supplies air to inflate deicer boots on the wings and empennage. Either pump is capable of supplying sufficient air pressure to operate the entire de-icer system.

Wings bearing the P-61 into combat comprise a structure divided into seven sections . . . two wing tips, two outer wing panels, two inner wing panels, and the crew nacelle section (spars only). The complete assembly, except wing tips, is of riveted aluminum alloy, stressed skin, full cantilever construction with the loads concentrated on two main spars. Wing tips are of welded magnesium alloy. Each inner wing panel contains an engine nacelle, two fuel tanks, and a section of the wing flaps, while each outer panel holds an oil tank and cooler, wing flaps, spoiler panels, aileron, landing light, provisions for leading edge de-icer boots' attachment and, on right wing tip only, recognition lights. A series of six flaps of the trailing edge slotted type, are mounted two on each outer wing panel and one on each inner panel. Constructed of aluminum alloy and so linked as to move aft and down when extended, the flaps in extended position leave a gap between the leading edge of the flap and the trailing edge of the wing structure. This gap has been sealed by a rubberized, waterproofed and fireproofed canvas strip riveted or fastened with bolts to the flap and trailing section. This canvas strip tends to give the airplane more stability and ease of control when landing. Mechanically operated locks hold the outer wing flaps in the retracted position. Similar in operation to a conventional landing gear up lock, the locks restrict oscillation in both up and down directions. Full flap deflection of sixty degrees may be obtained. A combination of the spoiler and conventional type of aileron is used and termed retractable aileron. Adjustable push-pull rods connect to differential bellcranks which can cause the spoilers on one wing to rise sixty-five degrees while the spoilers on the other wing lower twenty-eight degrees. Worth noting is the fact that retractable ailerons retain effectiveness down to and past the stall. The ship may be held straight during a stall with one engine windmilling, while the other is at takeoff power, and if stalled in an inverted position, either intentionally or as a result of attempting a maneuver with insufficient initial speed, recovery may be made by rolling out, making use of the retractable ailerons.

The empennage of the Black Widow, weighing approximately 650 pounds, has twin interchangeable vertical stabilizers which are attached to the twin booms by internal attaching angles, and are equipped with rudder and trim tabs. There is a dual set of controls for the elevator and rudders. A horizontal stabilizer consisting of the main horizontal stabilizer, elevator and trim tabs, joins together the vertical stabilizers. Each of the Black Widow's tail booms weighs about 100 pounds, and is of monocoque construction.

Northrop's P-61 is equipped with a retractable, tricycle type landing gear which consists of a nose gear and two main gears. All three, retracting aft, are normally operated by hydraulic pressure. An emergency compressed air booster system is provided for use if the hydraulic system fails. The main gear is held in place in the retracted or extended position by mechanically-latching and hydraulically-releasing locks, while nose gear latches are released by cables attached to the landing gear selector valve lever. If the gears should fail to latch in the down position and the manifold pressure drops to approximately fifteen inches Hg, a warning horn will sound. Each main gear, weighing approximately 635 pounds, is comprised of a shock strut, actuating cylinder, side thrust braces, and a drop center wheel incorporating a dual, disc type, hydraulic brake. It mounts a forty-two inch smooth contour type tire. The Widow's nose landing gear, with a weight of about 231 pounds, consists of an air-oil type strut with a swivel base and a centering device. Two micro switches, mounted on the link assembly, operate in conjunction with the downlatch to indicate whether or not the gear has been latched in the down position.

Developing 1675 normal maximum hp for the P-61 are her two Pratt & Whitney R-2800, double-row, radial, aircooled engines, equipped with internal gear-driven, two-speed, two-stage superchargers, mounted ahead of the leading edge at the outboard end of the inner wing panels. The engine group includes the engine, engine mount, cowling, and accessories. The engine mount is constructed of chrome molybdenum steel tubing welded into a single unit, the complete unit being secured to the engine nacelle by four nickel steel bolts. Supports for the engine, whose dry weight including standard accessory equipment is 2480 pounds, are six flexible, vibration absorber brackets which are bolted to six steel clamps welded to the mounting ring.

A PT-13G2 Stromberg injection carburetor meters fuel in proportion to the mass flow of air to the engine. Metered fuel from the fuel control unit is then delivered to the supercharger entrance through a spring-loaded discharge nozzle in the auxiliary blower section. A spring-loaded anti-creep device on the carburetor throttle prevents change of throttle position during flight. At the end of its journey, fuel, in the form of exhaust gases, is dissipated to the air through eighteen short, flame dampening stacks which terminate in flared elliptically shaped ports. Exhaust gases pass between the cowl flaps and the diaphragm, and are cooled sufficiently by the engine cooling air to eliminate exhaust glow aft of the flap trailing edge. These ports form a ring which is inside and approximately six inches for- ward of the cowl flaps trailing edge.

Fuel tanks are carried in the wings of the Black Widow, their capacity giving the ship great range. Actual. figures, however, are still under a veil of secrecy. They are constructed with an inner lining resistant to aromatic fuels, and a sealing material which expands rapidly upon contact with gasoline, thus sealing any holes should the tank be punctured.

Two four-blade, 12-foot 2-inch diameter propellers with a gear ratio of .500 serve the P-61. The blade angle of the propeller is adjustable and is controlled by an electric power unit attached to the hub, while a brake locks the blades in fixed position when no angle change is required. Although the low-pitch limit switch is effective in both constant speed and selective fixed pitch operation, the high-pitch limit switch is effective only in constant speed control. Feathering is accomplished through a separate lead which bypasses the flight-range, high-pitch limit switch. The propeller operates in conjunction with a governor unit, a booster-dynamotor, and a relay switch, and is electrically controlled from the propeller switch panel in the pilot's compartment.

The first XP-61, completed May 8, 1942, was wheeled out of the Experimental Department at Northrop onto the main assembly floor and painted a shiny black with red serial numbers and red inspection door markings. Within a few hours it became popularly known as the Black Widow and the name was later accepted as official by the Army Air Forces. Now, her duty done in the European Theater of Operations, the Widow turns her full concentration on the elimination of Japan in the Pacific, an area in which she has already wrought havoc for many months.

This design-details article was originally published in the September, 1945, issue of Air News with Air Tech magazine, vol 9, no 3, pp 57-61.
The original article includes 6 photos and 8 diagrams. Photos credited to Northrop.