Design Analysis of the Northrop Black Widow

by Richard Spencer

Equipped with a revolutionary new air-brake and turbosuperchargers, the P-61C is expected to be used for training and research purposes.

Latest version of the Northrop Black Widow, the P-61C, is faster and more powerful than preceding models, and is equipped with a revolutionary new air-brake which enables the pilot to slow the plane suddenly in flight for steadier aim in firing. New engine and propeller designing place its service ceiling much higher, and its speed in level flight is well over the 375 mph of older models.

The P-61C is powered by two Pratt & Whitney R-2800-C engines equipped with turbosuperchargers, enabling each engine to develop 2800 hp at maximum war emergency rating, with 2100 normal horsepower. Each engine employs two superchargers, one of which is a single stage, single speed unit and the other the conventional GE exhaust supercharger, both of which can work as a unit.

At 35,000 ft, the turbos have an air intake of 270 lb, or 11,250 cfm.

The fighter brakes are the picket type, which consist of reinforced metal grids which open vertically to top and bottom wing surfaces about midway on the wing chord. Brakes, fully extended, provide a slow force equal to 1 G. Brakes are counter-hinged and counter-acting to reduce operating pressures.

The new model Black Widow uses Curtiss electric full-feathering four-bladed paddle propellers. Blade width has been increased to absorb engine thrust.

The only outward change in appearance of the P-61C over the P-61B is the airscoop under the engine nacelles, and the propeller blades.

Basically, the Black Widow is an all-metal, midwing, twin-boom monoplane, designed and built to fly as a night fighter plane, equipped with the latest in radar detecting devices and extremely heavy fire power.

Carrying a crew consisting of a pilot, gunner and radio-operator-gunner, the P-61 differs from the conventional type of airplane because of the twin tail boom empennage assembly, and abnormally large full span wing flaps, the Northrop retractable aileron, which plays from zero to approximately 7” above the top surface of the wing, above the aft spar,and spring type elevator boost tabs.

Span is 66', with an 84" tip chord and 144" root chord. Overall length is 49' 7.2", height is 14' 3", and from wingtip to ground is 9' 2". Gross weight is approximately 29,007 lb exclusive of external wing tanks.

The crew nacelle is 406" long, 50" wide, and 92" in height. The pilot sits behind a Fiberglas snout in an armor glass and Lucite enclosure. The gunner sits slightly higher and aft of the pilot. The radar operator's enclosure is at the rear of the crew nacelle and is similar in construction to the pilot's and gunner's enclosure. Each crew member has an escape hatch at the top of the enclosure.

The crew nacelle is of semi-monocoque construction mainly of 24ST Alclad skin and diaphragms, 24ST stringers and 24ST Alclad frames. The skin carries shear and tension loads as well as heavy loads from the turret gun blast. In this area, the skin is of .064 sheet steel. Flush rivets are used, mainly of A17ST aluminum and employing the dimpling process.

Diaphragms, carrying the main shear load, are located aft of the alighting gear doors, aft of the gunner's enclosure, the pilot's floors and the bottom of the crew nacelle frame. Diaphragms are made mainly of Alcoa aluminum extrusion 24ST and consist of web, doubler, angle and channel materials.

Approximately 80 stringers of extruded bulb angles and former channels primarily carry direct tension and compression loads caused by bending of the crew nacelle. Nacelle frames are of two types; one that carries a relatively heavy concentrated load and the other that serves mainly as a stiffener to hold the structure shape. Frames, mainly of 24ST Alclad, vary in gauge from .025 to .125.

A keel supports the nose landing gear, transmitting its load into the nacelle, and provides torsional rigidity to the forward section of the crew nacelle. The keel is composed of web, stringers, bulkheads, angles, channels and supports.

Flaps

The P-61 owes it ability to fly at low speeds to full-span flaps of the trailing edge slotted type. To achieve full-span construction, engineers had to devise a new type of control to replace the conventional ailerons, hence the spoiler-type aileron.

A series of six aluminum alloy flaps are mounted, two on each outer wing panel and one on each inner wing panel. Each pair is interconnected. Their extension movement is aft and down. A gap between the leading edge of the flap and the trailing edge of the wing structure is sealed by a rubberized, waterproof, fireproof canvas strip that aids in control and landing stability.

The flaps are hydraulically operated. Mechanically operated locks, similar in operation to conventional landing gear up-locks, hold the outer wing flaps in the retracted position and prevent oscillation in both up and down directions. The latch is spring loaded closed and is released by a single cable connected to an arm on the hydraulic valve.

Flaps make up more than 20% of total wing area, including ailerons, being 124.61 sq ft in area. Outboard and inboard flaps on each outer wing span a total of 16' 7" and the inner wing flaps span 4' 5". Outer wing flaps taper from 37.17" to 23.561" while the inner flap is 37.17". They are of stressed skin monocoque construction, chiefly of 24ST Alclad.

Inner structure consists of a series of ribs, 10 in number in the outboard flap, 13 in the inboard and 7 in the inner wing flap. Web gauge runs from .020 to .081 for the various members.

The retractable ailerons on the P-61 consist of a long and short curved spoiler plate hinged near the upper surface of the wing, just aft of the rear spar, and extending along two thirds of the outer wing. The surface is made up of ¼” magnesium alloy plates (Dow J1-H), perforated for lightness, to which is welded supporting hinge brackets connected to the operating torque tubes. The brackets are of the same materials as the spoiler plate. The torque tubes are of 1½", 1-7/8" and 2¼" OD x .065" (1020) steel tubing on the outboard retractable aileron, and 2¼" steel tubing on the inboard.

Each outer wing panel contains two retractable ailerons working in conjunction with small conventional type, fabric covered ailerons, 40" long, which lend "feel" to the retractable aileron control through interconnecting linkage. The entire system is conventional in operation by the control wheel. The inner construction is mainly of 24ST Alclad.

Wing Group
Wing area (total)……662.36 sq ft
Span……66'
Airfoil section (root)……Zap 15% section
Airfoil section (tip)……Zap 13% section
Root Chord……144"
Tip chord……80"
Incidence (at root)……1°30'
Dihedral (outer wing)……
Dihedral (inner wing)……
Angle of attack (at root)……1°30'
Ailerons and flaps (sq ft)
Aileron area……10.98
Flap area……124.61
Spoilers (total, fully raised)……11.45
Aileron trim tab……2.26

Wing Structure

The airplane has a stressed-skin, 2-spar, full-cantilever wing. The wing structure is divided into seven sections — two wing tips, two outer wing panels, two inner wing panels and crew nacelle section (spars only). The skin is principally made of 24ST Alclad. Corrosion resistant sheet steel is used at some points, adjacent to engine mountings, etc. Wing tips are of magnesium alloy sheet. Skin thickness varies from .025" to .080".

The inner wing panel is fastened to the crew nacelle section at top and bottom of each spar by bolts. The panel contains one engine nacelle, two fuel tanks and a wing flap section. Two spars are in the main section of the panel while a detachable trailing edge section contains the flaps. The whole panel is 167.5" in length, 117.50" in width and 60" in height.

Spars in the inner wing are of the tension field beam type with shear webs stiffened by formed channels and extrusions. Spar caps are 14ST forgings and extrusions. The inner wing structure, unconventional in design, includes three bulkheads, two upper doors, and one lower floor. Inner wing covers are fuel tank covers, both hinged at the front spar to permit removal of the fuel tanks.

Trailing section structure consists of upper and lower skin with reinforcements, two end ribs, rear web and attach angles. The upper skin is reinforced with spanwise corrugations while the lower skin is strengthened with chordwise beads. The end ribs are comparatively heavy, supporting the flap. Intermediate ribs support the upper and lower surfaces. The nose section is made up of skin, stringers, ribs and ducts. Inboard section stringers are of two types: channels and extruded “T” sections. While the pressed ribs vary, most of them are three-piece ribs around an air duct. The outboard nose section consists of two cast ribs, “T” extrusions and ducting.

The outer wing panels have three detachable units: the main section consisting of the structure from the leading edge to the rear spar; the trailing section composed of the fixed structure aft of the rear spar, and the wing tip. The outer wing panel, including the wing tip, is 255.5" in length, 144" in width and 22" in height. The entire assembly is attached to the inner wing section at the upper and lower caps of the front and rear spars. In the wing panel also are an oil or water tank, engine oil cooler and de-icing equipment.

Spars in the outer wing panel are machine tapered toward the tip. They are reinforced with vertical stiffeners and shear web (.081 to .040 gauge). Web consists of a single thickness except for a double web on the front spar where the inner and outer panels join.

Ribs are of varying types; built-up of extruded caps and webs reinforced with stiffeners and highly stressed to carry the main load (included in this category are hanger ribs of the trailing section which support the flaps, .064 gauge); pressed-up ribs with formed flanges and perforations (.032 gauge), and formers made up of channels used principally to maintain wing contour.

Wing tips are made of Dow J1-A magnesium alloy sheet attached to the main wing structure by 3/16" screws. Three main bulkheads and attaching angles make up the internal structure.

Tail Group
Stabilizers
Horizontal stabilizer (including elevator)……122.54 sq ft
Fins (total)……80.20 sq ft
Vertical fin height……110.2"
Horizontal stabilizer span……200"
Maximum chord……63.7"
Incidence (fixed)……0°45"
Dihedral……
Elevators
Area (including tabs)……43.38 sq ft
Elevator trim tab……2.26 sq ft
Elevator span……200"
Chord……34.10"
Rudders
Area (total, including tabs)……39.04 sq ft
Rudder trim tabs (total)……2 sq ft
Span……108"
Chord……36"

Empennage

The tail group is composed of a horizontal stabilizer, two vertical stabilizers, elevator and two rudders. The horizontal stabilizer is supported between the two vertical stabilizers by two spanwise spars with end fittings. Ribs, stamped from 24ST Alclad (.025) sheet, are perforated and are similar in both the horizontal and vertical stabilizer. Covering of the tail group is 24ST Alclad and varies from .025 to .032 gauge. The horizontal stabilizer is fixed at a positive 0°45' angle of incidence (nose up) relative to the longitudinal axis. It is 200" in length, 63.70" in width and 11" in height. The vertical stabilizer is 105.4" long, 20.20" wide and 110.2" high.

Elevator and rudder are supported at three points, and each is built around a single spar to which stamped nose and trailing section ribs are fitted. The elevator is 200" in length, 34.10" in width and 11" in height. The rudder is 36" long, 7.58" wide and 108" high. Nose skin of the surface is 24ST Alclad sheet and the whole surface is fabric covered.

The tail booms are of monocoque construction. They are 108" long, 38.10" wide and 41.2" in height. Skin, 8 .040 gauge frames and 18 stringers of Northrop standard material comprise the boom structure.

The two main wheels of the landing gear retract into the engine nacelles while the nose wheel pulls up into the crew nacelle just below the pilot. Operated by hydraulic pressure, all three retract aft. Similar in design to the main wheels, the nose gear is equipped with a swivel and shimmy damper. The rear wheels are of single-leg, half-fork, full-cantilever type. They are equipped with Bendix shock struts consisting of a cylinder and piston interconnected by forged chrome-molybdenum steel torsion links. Shock absorption is provided by air and oil. Landing loads are transmitted through the shock strut to the side thrust brace, two trunnions, a downlock and the main supports Two steel castings support the main gear, one on each side of the nacelle. The nose wheel is supported by brackets and is hinged on trunnion pins which pass through bearings and into the trunnion structure. The nose gear is held in an extended position by a folding drag link. Main wheels are drop-center Goodyear operated by disc-type, hydraulic brakes and carry 45.2" smooth contour type, 12 ply, 15.50-20 casings. Inner tubes are dual-sealed type, tread is 17' .69". Nose wheel tire is 33" 8-ply, smooth contour type.

Four self-sealing fuel tanks are built into the wings. Total capacity of the permanent tanks is 630 US gal. Each outboard tank carries 200 gal and inboard 115 gal. Fuel can be used from any tank for either engine and fuel and oil tanks are filled from the top of the wing. An electrically operated two-speed booster pump supplies fuel to the engine-driven pumps in case of an emergency. Tanks are built with an inner liner resistant to aromatic fuels, in addition to a sealing material. In addition to the tanks and booster pumps the fuel system has one cross-flow valve, two strainers, two engine pumps and two electrically operated engine primers. Droppable fuel tanks with a capacity of 165 or 310 gallons are carried under each outer and inner wing panel.

Electrical and Hydraulic Systems

The electrical system is 24 VDC, single-wire, negative grounded, and consists of generator and battery circuits, engine ignition and starter circuits and lighting and auxiliary circuits. Each generator is capable of operating all necessary equipment and batteries are adequate for a short flight if generators fail.

The hydraulic system operates the landing gear, main gear up and down latches, landing gear doors, brakes, wing flaps, engine cowl flaps, intercooler exit flaps, oil cooler air outlet doors, carburetor air heat valves, carburetor air filter, ejection chute doors and automatic pilot. Fluid under pressure from two engine-driven pumps flows through a filter into an unloading valve which maintains 850 to 1000 psi pressure in the main and accumulator systems. In case of failure of the engine pump, enough fluid is stored under pressure in the accumulator for six full applications of landing wheel brakes. Except for the automatic pilot, an emergency system is available to all units. The lower part of the reservoir, below the main pressure fittings, contains fluid for use in the emergency system only which is fed directly from the reservoir through a hand pump filter.

Besides radar, the P-61 carries an unusual amount of radio equipment, both for communication and interception. Each crew position has T-30 throat microphones, HS-33 earphones, A-11 helmet headsets and A-14 oxygen masks with installed T-42 microphones. Eight VHF channels communicate with the ground control and the plane carries a low frequency beam receiver and interphone system. In addition to a radio compass and radio altimeter, there is special radio equipment, SCR-695, which identifies the P-61 in flight.

Four 20-mm cannon are mounted in the belly and are controlled by the pilot. Four .50-cal machine guns are in a remote control turret on top of the crew nacelle. Two hundred rounds of ammunition are carried for each cannon and 500 rounds for each machine gun. The turret is electrically operated from either the forward gunner's compartment or the RO-gunner's aft position, or by the pilot when the turret is locked in the forward position. The turret swings 360° horizontally and 90° vertically. The cannon are installed in supports below the crew nacelle floor structure with a Chatellerault power feed (Type AN-M1) mounted on the receiver body of each cannon. They are fired by the pilot who presses a push button on the right side of the control wheel. A type LY-3N gun sight is mounted on a bracket above the pilot's instrument panel.

Four 1000- or 1600-lb bombs may be carried under each wing, replacing droppable fuel tanks. Released by the pilot, two outboard and two inboard bomb shackles are controlled by selector switches, one salvo switch and the pilot's release switch control. Emergency mechanical release controls are also provided.

Night binoculars permit vision four times as far as normal sight at night. They are a combination of 5.8-power night glasses and optical gunsight and are mounted on vibration-free gimbals.

Power is supplied by two Pratt & Whitney double-row, radial, air-cooled, 18-cylinder engines, equipped with internal gear-driven, two-speed, two-stage superchargers and water injection system.

Engines are mounted ahead of the leading edge and at the outboard end of the inner wing panels. They are secured to mounts made of SAE No X4130 chrome molybdenum steel tubing welded into a single unit, attached to the nacelle by four nickel steel bolts. Six flexible vibration-absorbing brackets holds the power plant in position. The brackets are bolted to six steel clamps welded to the mounting ring.

An anti-drag nose ring section of three 120° segments bolted together and supported by 10 Lord rubber mounts, is linked and bonded to the cylinder head.

Piston stroke is 6" and bore 5.75". Total displacement is 2804 cu in. Compression ratio is 6.65:1; blower gear ratio, main, 7.80:1; blower gear ratios, low auxiliary, 6.46:1 and blower gear ratios, high auxiliary, 7.93:1.

Exhaust gases are dissipated through 18 short flame-dampening stacks which terminate in flared elliptically shaped ports.

Each engine has a complete individual water injection system with a capacity that enables the plane to operate for about 20 min on the fuel, water and 30% alcohol mixture. Each water injection unit consists of a fuel derichment valve, water regulator unit, supercharger reset, solenoid valve, water pumps, tanks and lines.

Propellers are Curtiss-Wright electric, full-feathering, four-bladed, 12' 2" in diameter. Blade angle is adjustable and controlled by an electric power unit attached to the hub. Ground clearance is 11". The propeller operates in conjunction with a governor unit, a booster-dynamotor and a relay switch.

This design analysis article was originally published in the November, 1945, issue of Industrial Aviation magazine, vol 3, no 5, pp 7-8, 10-13, 56-57.
The original article includes 10 photos, a phantom rendering, and 2 data tables.
Photos are not credited; 9 assembly-line photos are certainly from Northrop; in-flight photo may be Northrop or USAAF.