Design Details of the Northrop P-61 (Part 1)

Designed primarily as a night fighter, for "the interception and destruction of hostile aircraft in flight during periods of darkness or under conditions of poor visibility," the P-61 Black Widow is an all-metal, twin-engined, mid-wing monoplane.

Produced by Northrop Aircraft, the Black Widow is heavily armed and armored, and is equipped with radar detection and remote gunfire control units.

It carries a crew of three in an enclosed nacelle which divides the center wing panel along the centerline of the airplane. Twin booms extending aft from the engine nacelles support the tail surfaces.

The engines are Pratt & Whitney, Model 2800 series, turbosupercharged, and each rated at well over 2000 hp for takeoff, and each turning a four-bladed Curtiss electric full-feathering "high-activity" propeller.

Principal dimensions of the P-61 are: wing span, 66 ft; length overall, 49 ft 7 in; height, 14 ft 8 in; height to propeller hubs, 84 in. Wing chord at the root is 144 in, and 80 in at the tip attachment. Dihedral of the outer wing is 2°, and of the inner wing 4°.

The wing structure is divided into seven sections: the two wing tips of welded magnesium alloy, two outer wing panels, two inner wing panels, and the spar sections (front and rear) which extend through the crew nacelle. Spars of the inner wing panels are not continuous through the centerline of the airplane, but are bolted to the spars in the crew nacelle a the top and bottom of each spar.

The complete wing assembly, except the wing tips, is fully-cantilever, riveted aluminum alloy, stressed skin construction, with the loads concentrated on the two main spars. The wing skin carries the chord bending and torsional loads, and is supported by chordwise ribs and stiffeners, and such spanwise stiffeners as are necessary to prevent skin wrinkling.

Each inner wing panel contains an engine nacelle, two fuel tanks and a section of the wing flaps. The are built in two sections: the main section which includes the two spars, and the detachable trailing edge section which contains the inboard flap.

The outer wing panels are bolted to the inner wings at the top and bottom of each spar. An oil tank and cooler are installed in each panel. The outer wing also is built in two sections: the main section, containing the two spars, and the detachable trailing edge section which includes wing flaps, spoiler panels and ailerons.

Aluminum Alloy Flaps

There are six flaps of the slotted type mounted on the trailing edges of the wings — two on each outer panel and one on each inner panel. They are of aluminum alloy construction, and are linked so as to move aft and down when extended. Full flap deflection is 60°.

In the extended position there is a gap between the leading edge of the flap and the trailing edge of the wing which permits a flow of air from the lower surface of the wing to the upper surface of the flap. This tends to smooth the air flow over the flap, and to increase its lift at low speeds and high angles of attack. The flaps are hydraulically operated.

Because of the "full-span" landing flaps, which run almost the whole length of the outer wing panels, the ailerons are necessarily small in area. Retractable ailerons, resembling spoiler panels, are installed in the outer wings. They are connected with small conventional ailerons at the wing tips, which serve only to give "feel" to the aileron operation.

The retractable ailerons are curved panels of perforated metal which, in the neutral position, are retracted within the outer wing panel near the trailing edge, inboard of the ailerons. Late models of the Black Widow are equipped with "fighter brakes" — slotted panels which extend from both top and bottom surfaces of the wings to create high drag and slow the plane rapidly without throwing it off its course.

On early models, a combination trim and booster tab, controllable manually from the cockpit, was fitted to the left aileron. A booster tab fitted to the right aileron was adjustable on the ground only. The ailerons of later model airplanes are not equipped with tabs.

The crew nacelle contains the pilot's cockpit and stations for a gunner and a radio operator. Its main structural members are transverse channel-type frames, longitudinal extruded aluminum alloy bulb angles, and the stressed skin. Special stiffeners are used to distribute concentrated stresses at the cockpit enclosures, gun turret, nose wheel well, entrance door cutouts, etc. The wing front and rear spars are bolted to spar extensions which pass through the center of the nacelle in front of and behind the gun turret. The nose section forward of the pilot's compartment is constructed of resin-impregnated Fiberglas. It encloses the spinner and other units of the radar equipment. This section is attached to the crew nacelle by four locating studs and four toggle latches.

Transparent Plastic Enclosure

The gunner's compartment is directly behind the pilot's cockpit, and is connected with it by an access door. The pilot's and gunner's enclosure is made up of molded Lucite sheets bound by extruded aluminum alloy frames. The section directly over the gunner is supported by square steel tubing. A hinged window on each side is provided for the pilot. An access panel is located over the pilot's seat, and there is an emergency kickout panel to the right of the gunner's seat which can be opened from inside or outside.

The flexible gun turret is mounted between the two wing spars, and protrudes through the top of the crew nacelle.

The radio operator's position is in the aft section of the crew nacelle, and is separated from the other two compartments by the turret and radio equipment. This compartment is formed of molded Lucite sheets bound with extruded aluminum frames. The tail cone is made of two molded sheets of Lucite cemented together at the vertical centerline, and bolted to the aftermost frame of the crew nacelle.

The pilot's and gunner's entrance door is located just aft of the pilot's seat, and is combined with the nose wheel well. The door frame and folding ladder form a welded steel structure hinged at the forward end, and the aluminum alloy wheel well is riveted to the door structure.

Two tail booms extend aft from the engine nacelles to the vertical stabilizer assemblies. The booms are of monocoque construction, and consist of riveted aluminum alloy frames, stiffeners, and skin. Bolts through forged steel brackets attach the boom structures to the empennage and engine nacelle sections. Communications and identification antennae, and the remote indicating compass transmitter are housed in the booms.

Structure of Empennage

The empennage group includes the horizontal stabilizer and elevator, two vertical stabilizers which are faired into the tail booms, and two rudders. The horizontal and vertical stabilizers are of all-metal construction.

Two spanwise spars support the horizontal stabilizer structure, and the contour is formed by aluminum alloy ribs and skin. It is bolted to the vertical stabilizers at the ends of the spars, and faired with light, removable fairing strips.

The closing channel at the trailing edge forms the main spar of the vertical stabilizer. An auxiliary spar is located just aft of the leading edge of the stabilizer, and extends downward from the tip for approximately six feet. The lower portion of each vertical stabilizer forms the aft end of one of the tail booms. It is attached to the boom by an elliptical internal attaching angle.

The elevators and rudders are constructed of fabric-covered aluminum alloy. Each is statically and dynamically balanced. All-metal combination trim and booster tabs, controllable from the pilot's cockpit, are built into the trailing edge of the rudders. A combination trim and de-booster tab, also controllable from the pilot's cockpit, and two all-metal pre-loaded spring tabs which are self-operating at high speeds to provide boost and reduce control forces, are built into the trailing edge of the elevator.

The fuel system of the Black Widow provides for a total of 646 gal, carried in four self-sealing tanks. There is a tank located in each engine nacelle, between the wing spars, and one between the spars in each inner wing. The main nacelle tanks have a capacity of 205 gal each, and the wing tanks each hold 118 gal. The right-hand wing tank is designated as reserve. A cross feed line is provided so that fuel from any tank can be made available to either one or both engines.

The tanks fit the contour of the wing structure on all sides, and are attached at all tank outlet fittings. The top of each nacelle tank is laced to the V brace between the front and rear wing spars.

Each tank contains a Thompson variable speed electric fuel booster pump. The pump on each tank is individually controlled, and is used for transferring fuel between tanks at any altitude, and for supplying fuel to engine-driven pumps under six to eight psi pressure to prevent vapor lock.

A type G-9 engine-driven fuel pump, of the rotary-vane, positive displacement type, is mounted on each engine.

Design Details of the Northrop P-61 (Part 2)

In order to perform its primary function — that of a night interceptor-pursuit plane — the P-61 Black Widow is heavily armed with four .50-cal machine guns and four 20-mm cannon, and carries seven sets of the latest radar equipment — more than any other warplane in the world.

The .50-cal guns are mounted in the top turret, which is remotely controlled, and which can be traversed through 360°. The four 20-mm cannon are mounted in the belly of the airplane, and are installed on supports below the crew nacelle floor structure.

The cannon are controlled by the pilot, and fire forward through a horizontal range of +¼° from dead ahead, and a vertical range of +¼° and -1° from the horizontal, making possible converging, diverging, elevated or depressed lines of fire. There is a hydraulically operated ejection chute door for each cannon. The door is automatically controlled by a solenoid wired into the cannon firing circuit so that the doors open only during the time the cannons are being fired.

The .50-cal machine guns in the turret may be controlled by the gunner, radio operator or the pilot, although the gunner normally retains full control of the turret unless he transfers control to radio operator or pilot.

Most important radar device is the interceptor, a bulky apparatus installed in the nose of the crew nacelle. Heart of the interceptor is a reflector with a slender arm antenna projecting from its center. This antenna whirls constantly while the plane's interceptor equipment is working, emitting radar waves which bounce from aircraft, ground, water, or a surface boat.

Other radar equipment includes the interrogator and responder, which identifies friendly aircraft from among other planes nearby. The tail-warning radar is defensive, ringing a bell if another aircraft approaches withing a certain distance of the Black Widow's tail. The airplane is also equipped with a radio altimeter, which, operating on a principle similar to that of the radar, tells the pilot exactly how high he is above the ground or water..

Electric power for the radar and armament is provided by the plane's electrical system. This consists of two 28 V, 200 A engine-driven generators and two 24 V, 34 Ahr storage batteries. These power all electrical equipment including primers, starters, electrical instruments, lights, selector valves, gun controls, cannon and aiming point camera control, fuel pumps, oil dilution, crew nacelle heaters, anti-icers, de-icers, radar, radio, warning bell, landing gear position warning system, and propeller feathering.

The plane's hydraulic system operates the landing gear, main gear up and down latches and wheel doors, upper and lower engine cowl flaps, carburetor air heat valves, intercooler exit flaps (on earlier models not equipped with turbosupercharged), carburetor air filter, oil cooler air outlet doors, wing flaps, ejection chute doors, and the automatic pilot.

Pressure is maintained in the system by an engine-driven hydraulic pump mounted on each engine accessory drive case. Fluid under pressure from both pumps converges and flows through a filter into an unloading valve, which maintains 850 to 1000 psi pressure in the main and accumulator systems. The valve also supplies fluid through the automatic pilot pressure regulator to the automatic pilot and to the reservoir.

Mounted on a bulkhead aft of the gunner's position re the hydraulic reservoir, accumulator, main pressure regulator, surge chamber and other units of the system.

There are four main subdivisions of the hydraulic system — the main, accumulator, emergency, and automatic pilot systems.

In the main system, pressure from the unloading valve is transmitted through a check valve directly to a manifold from which the fluid flows to the selector valves for landing gear, carburetor air heat valves, oil cooler air outlet doors, engine cowl flaps, and intercooler doors.

Hydraulic pressure from the accumulator system actuates the wing flaps, wheel brakes, ejection chute doors, and the carburetor air filters.

In the event of engine pump failure, or of damage to the surge chamber or accumulator, all hydraulic units except the automatic pilot may be operated by the emergency system. By operating the hand pump of this system, fluid is forced to a selector valve from which it may be directed to the main manifolds or either the main or accumulator systems.

The main pressure regulator is mounted on the aft edge of the reservoir support. This valve is permanently set to maintain a maximum of 1000 psi and an minimum of 850 psi of pressure in the main and accumulator systems. When the maximum pressure is obtained, fluid is directed to the automatic pilot pressure regulator.

The surge chamber in the main system is mounted below the reservoir. A flexible diaphragm divides it into two parts. The upper part is filled with the hydraulic fluid and is directly connected to the pressure line. The lower part contains compressed air which forms a cushion and maintains a back pressure against the main pressure regulator valve to prevent it from constant functioning or pounding because of small changes in fluid volume.

Each engine nacelle is a semi-monocoque structure consisting of stressed skin, longitudinal stringers, and transverse frames. The loads applied directly to the engine mount, main alighting gear, fuel tank and tail boom are transferred to the basic wing structure through the structural strength of the engine nacelle. This load transfer results in a highly stressed structure, both in flight and in landing conditions.

The forward nacelle structure is made up of three sections: inboard, outboard and lower forward sections. The upper and lower ends are built as an integral part, respectively, of the bulkheads and keel structures on each side of the landing gear cutouts.

The aft structure of the nacelle is attached directly to the forward structure by riveted keel and stringer splices, and to the rear spar by two fittings.

All stringers in the nacelle structure are 24ST extruded bulb angles. The nacelle frames are built up or formed single channel sections, or double channel sections riveted back to back.

The Black Widow is equipped with retractable, tricycle-type landing gear. The two units of the main gear extend from each engine nacelle, and the auxiliary nose gear extends from the crew nacelle. When retracted, the alighting gear is completely enclosed by doors which, when closed, form the lower contour of the nacelles.

Each unit of the main landing gear is supported by two steel castings, which are bolted — one on each side of the nacelle — to closed box keel structures built into the nacelle on each side of the cutout which is provided to accommodate the landing gear. Landing loads are transmitted through the shock strut to the side thrust brace, the tow trunnions, the downlock, and the main supports.

The main gear retracting cylinder and uplock, and their fittings, are attached to a diaphragm, or bulkhead structure, installed transversely across the engine nacelle at the aft end of the cutout. The diaphragm serves to reinforce the nacelle frame as well as to support the cylinder and uplock assemblies.

In retracting, the gear hinges on two trunnion pins at the two steel castings. Both pins are secured by lock bolts. One end of the actuating cylinder is attached to the drag arm, or head, of the shock strut. A side thrust brace is mounted between the inboard end of the trunnion and the shock strut to absorb side loads.

The main gear is held in position, both when extended and retracted, by mechanical latching mechanisms. If the gear should fail to latch in the "down" position and the throttle is closed to less than 1000 rpm, a warning horn will sound. An emergency system is installed to extend the gear in the event of hydraulic system failure.

The nose gear is supported by two brackets, one on each side of the crew nacelle, and hinges on two trunnion pins which pass through needle bearings and into the trunnion structure. Both pins are secured in place by lock bolts. A folding drag link assembly also supports the gear when it is in the extended latched position.

A warning horn, similar to that provided for the main gear, sounds when the nose gear is not latched down and the throttle is closed to 1000 rpm. A micro-switch wired to the gear and flap position indicator registers when the gear is latched down. The nose gear is equipped with a shimmy damper to prevent rapid wheel oscillations.

Ailerons, spoilers, and elevator are controlled in the conventional manner by the torque tube type control column in the cockpit. The torque tube is attached at the base of the column, and extends completely across the cockpit. Four control lines at the extreme ends of the tube are attached to the four elevator control cables.

Ailerons and spoilers are controlled by the wheel on the control column by means of an enclosed sprocket and chain attached to the cables. From the chain ends, cables run aft along the left and right walls of the cockpit, and outboard through the wings to the spoiler quadrants and then to the aileron quadrants.

From the spoiler quadrants, adjustable push-pull rods connect to differential bellcranks which cause the spoilers on one wing to rise 65° while those on the other wing lower 28°. Adjustable push-pull rods connect the differential bellcranks to the spoilers. Aileron quadrants connect directly to the ailerons by means of adjustable push-pull rods.

Conventional, adjustable foot pedals control the rudders. Two cables extend aft from each of the two pedals, providing an independent system for each rudder. A buss cable connects the two pedals.

The cables are installed on both sides of the cockpit and through both tail booms to bellcranks in the empennage, from which they extend aft to the rudder horns. The right rudder cable system is connected through the automatic pilot servo unit.

The automatic pilot servo unit consists of three cylinders cast en bloc with piston rods extending at each end. The piston rods are connected directly to the main control cables of the airplane.

There is a manually-operated bypass valve for engaging or disengaging the automatic pilot. Spring-loaded relief valves (overpower valves) are built into each hydraulic surface control, to permit the pilot to overpower the automatic pilot by applying increased force to the controls.

The servo unit is located on the right-hand side of the crew nacelle below the floor of the gunner's compartment. Bank and climb, and turn gyro control units are located on the main instrument panel.

This two-part design analysis article was originally published in the November 1 and December 1, 1945, issues of Aero Digest including Aviation Engineering magazine, vol 51, nos 3 and 5.
These scans were taken from offprints, so page numbers of the originals are not available.
The original article includes 2 photos of the P-61 in flight and 2 detail photos and 11 detail line drawings.
Photos are not credited