C-54 Skymaster
Engineering Design Analysis

By Edward F Burton
Chief Engineer, Douglas Aircraft Company

The largest land transport in volume production, the C-54 is engaged in active military service in most war theaters

Engineered for efficiency and economy, the Douglas Skymaster today is flying the north and south Atlantic, the Pacific, India and China and into Alaska. The largest land transport in the world actually in mass production, this long range personnel and cargo carrier is a four-engined low-wing airplane equipped with fully retracting tricycle landing gear.

Manufactured at the Douglas factory at Santa Monica, California, and the new $33,000,000 Douglas factory at Chicago in ever-increasing numbers, it is credited from an engineering standpoint with being the only large production aircraft which did not have a prototype. It went directly from blueprints into production. Genealogically, it can be called the offspring of the original triple-tailed DC-4, which was a larger airplane in every respect.

The wing of the C-54 reveals aerodynamic advancement through its smallness in relation to the size of the fuselage and the load it carries. In shape it resembles previous Douglas products, for it has the characteristic taper, but the resemblance ends there. It is of full-cantilever all-metal construction and is composed of a center section and two outer wing panels. Smooth skin covering of Alclad sheets, incorporating cover plates, is applied to the structure with flush-type rivets.

The center wing panel of three spar Wagner type beam construction, composed of front, center and rear spars, extends completely through the fuselage to outer wing attachment points and is permanently attached to the fuselage. Four fuel tanks treated with sealant are built integrally within the wing structure.

The spars incorporate .128 gauge aluminum alloy spar caps and webs of .032 to .081 gauge aluminum alloy sheet stiffened by vertical bulb angles. The lightened formed sheet metal ribs run from .025 to .081 gauge. The upper and lower spanwise stringers are rolled from .040 to .072 gauge 24SRTAL and 24STAL sheets. The skin, of 24STAL sheet, runs in gauge from .020 to .081. Aft of the rear web, the center section incorporates NACA single slotted flaps from the sides of the fuselage to the inboard ends of the ailerons. Wing flaps are hydraulically operated.

Flaps are of all-metal construction. Each incorporates a single spar running its entire span. The spar is a continuous sheet web with lightening holes and is reinforced by cap angles of ..032 bent sheet and vertical stiffening angles riveted to the web. The nose and aft ribs, also provided with lightening holes, are riveted to the spar web. Extruded bulb angles extending spanwise the length of the flap are used as stringers. The flaps are covered with .025 Alclad skin riveted to the ribs, stringers, and cap angles. Inspection doors are provided on the under side of the covering.

Flaps are hinged at four points by supporting links which give the desired back and downward motion when the hydraulic actuating struts are extended. Two struts are used to actuate each flap and are connected at the piston ends to lever arms extending forward from the forward support link of the flap.

There are three flap doors riveted to the underwing skin aft of the rear spar which provide a smooth air flow through the slot between the flap and wing when the flap is down. These flap doors are overlapping flexible sections of skin, with Neoprene rub strips to minimize friction. As the flap approaches the middle of its downward travel, mechanism actuated by the flap supporting links causes the flap doors to retract automatically into the open position where they remain during the remainder of the flap travel.

Synchronizing valves in the hydraulic system prevent one flap from leading the other when the flaps are in motion. An autosyn type wing flap position indicator is provided on the pilots' instrument panel.

The four engine nacelles in the center section are similar in construction to the fuselage. The two inboard nacelles house the main landing gear, and are provided with doors that completely enclose the gear when retracted.

Each complete outer wing panel is composed of a main panel, aileron and a wing tip, the latter easily removable for repair or replacement. The ailerons are balanced statically and dynamically by lead weights in the leading edge. The main panel, of single spar construction, is attached to the center wing panel by heat-treated steel bolts.

Outer wings are spaced farther and farther apart as the tip is approached, and the gauge of the shear web drops from .051 at the root to .025 at the tip. The upper and lower spar caps both taper, the lower being the most affected. Rib gauges run from .032 to .051. Stringers generally lighten, both upper and lower, as stresses are reduced along the span. The bulk of stringers are .025 and .032 gauge but run to .064. The upper and lower wing skins run from .025 to .051 gauge.

The aileron is of single-spar design. It is attached to the outer wing panel at five points, and is braced throughout by ribs. The right aileron has a trim tab in the trailing edge, which is controlled from the flight compartment.

Fuselage

The cigar-shaped fuselage of the C-54 is typically Douglas in that it is engineered to carry bulk loads and at the same time to reduce drag. It is a semi-monocoque all-metal structure. Flush riveted skin plating is reinforced by vertical transverse frames of rolled hats and rolled channels formed in sections into circular rings. It is permanently attached to the center wing section.

Generally, a channel frame is used where the frame is part of a bulkhead, transmitting loads into the skin. Wing spar supporting frames are made of heavy extruded channel sections and heavy rolled sheet channel sections.

Longitudinal stringers are spaced around the circumference of the fuselage and are attached to transverse frames wherever they intersect. The stringers are predominantly rolled-sheet hat sections. Heavier sections of extruded hats are used in regions where the nose wheel and the wing cause highly concentrated loads.

Floor sections are panels of dural sheet reinforced with rolled corrugations and rolled hats. These panels are supported by the longitudinal floor beams and by transverse beams which are part of the bulkhead frame sections below the floor. The floors in the flight compartment are screwed down and may be removed to gain access to installations beneath; those in the main cabin are an integral part of the fuselage structure.

In the nose section stringers range from .020 to .040 gauge, the bulk being .020 with .040 being used on the upper section above the pilots' windshield. Stringers in the main fuselage section run from .032 to .125 gauge, with greatest strength concentrated at the top of the structure above the wing, the sides above the wing, around the crew door on the forward right hand side and the main cabin doors, rear left hand side, and the belly beneath the wing. Stringers in the tail section run from .020 to .064 gauge. Fuselage stringers in the tail cone range from .040 to .064, the bulk being .051 gauge.

Fuselage frames run from .025 to .064 gauge, with the exception of the wing spar supporting frames which vary from .188 to .670 gauge, and those in the center section which range from .032 to .091 to withstand added stresses at this point.

Fuselage floor beams, longitudinal and transverse, run from .020 to .093, the .040 gauge predominating. Floor panels run from .025 to .040 gauge.

The fuselage skin ranges from .020 gauge in the tail cone assembly to .064, the use of .025 and .032 predominating. Heaviest gauges are found directly above and below the wing on both sides and the belly of the fuselage to withstand the most extreme stresses. To protect the fuselage from ice slung by propellers .040 gauge is used, backed by reinforced frames and stringers.

The interior of the fuselage is divided into flight compartment, relief crew quarters, fuel tank compartment, cabin, and belly cargo compartments.

Accommodations are provided for a crew of six: pilot, co-pilot, navigator, radio operator and two relief crew members. Two cargo compartments in the belly of the fuselage provide approximately 290 cu ft of stowage space.

The flight compartment accommodates the pilot and the copilot, seated side by side with the control pedestal between them; a radio operator and a navigator. The pilot's and copilot's seats are adjustable vertically and fore and aft and will move backward to facilitate entry. They are far more comfortable, and the pilots have more head, leg and arm freedom than in previous commercial transports, which aids in relieving fatigue on long flights. The flight compartment enclosure provides a single pane windshield with a removable inner panel; heated air is passed between the panels to defrost or de-ice the windshield. Corner windows are hinged at the forward end. Handles located on the aft edge of the windows lock them by engaging a slot in the locking plate. Neoprene sealing strips are cemented to the fuselage structure around the window openings to make the enclosure watertight when the windows are locked. Windshield and windows are arranged in conjunction with the pilots' seats to give maximum visibility vertically and horizontally. Soundproofing is accomplished by the use of combinations of such materials as stonefelt, kapok, felt and mica in walls and ceilings.

The flight compartment door is located on the right hand side of the flight compartment behind the copilot's seat. The door opens inward and is hinged on the forward edge.

Flight compartment equipment includes a folding chart table for the navigator, with two table lamps; radio operator's table and desk lamp with rheostat; log book and map containers; rag containers; navigator's map case and storage box; drift signal chute; containers for the day and night drift signals; astro compass, and signal pistol and flares. A light-tight curtain may be installed across the flight compartment to separate the pilots from the other crew members.

The crew compartment, located aft of the flight compartment, contains accommodations for two relief crew members. Entrance is gained from both the flight and the main cabin.

The compartment is lined with canvas and doped airplane cloth. The ceiling consists of removable panels fitted with Dzus fasteners or screws. The panels over the passageway from the flight compartment provide access to the filler valve and sight gauge on a 10-gal water tank supplying the crew compartment wash stand. A crew's toilet is installed in the forward corner of the compartment.

The compartment is soundproofed in the same manner as is the flight compartment.

The main cargo compartment accommodates thirty passengers when used as a troop transport. Under alternate conditions it accommodates cargo, engines and combat equipment. The metal flooring is covered with plywood to prevent damage. Cargo tie-down rings are provided in the floor.

For carrying heavy loads, such as ordnance, the main cargo compartment has extra reinforcement and heavy gauge flooring in the loading area. The cabin is unlined except for removable plywood panels 30" high. Cargo guard rails are installed approximately 15" above the floor and extend the entire length of the cabin on both sides. Additional rails are shipped loose in the airplane for use in the fuel tank compartment when the tanks are removed. Auxiliary guard rails are attached to the fuselage frames approximately 7" above the floor.

Provisions are made to carry combat equipment in the cabin. Eyebolts are inserted into the engine cradle hold-down fittings in the floor for lashing equipment in place on its way to the battle front. Two load distributing channels are furnished for use when a 75-mm gun or 105-mm howitzer is carried. The channels are placed on the floor between the front and center spars of the wing. The weapons are rolled onto the channels and lashed down.

Special provisions are made for rushing replacement engines to advance bases. Engine hold-down fittings are installed on the floor of the cabin for securing engine cradles.

The Skymaster utilizes its own equipment for hoisting cargo into the main cabin. The hoisting equipment consists of a permanently installed hoist just inside the main cargo doors, with hinged rails which can be extended outside when the doors are open; a winch, and a rope-fall block assembly. The winch, installed inside the fuselage opposite the loading door, is on a level with the upper door jamb. The hand chain for operating the winch is attached to a pulley on the aft hoist track, and is connected to the winch by a drive shaft assembly. Two cables, extending fore and aft from the winch, are directed by pulleys to trolleys on the under side of the hoist tracks. From these trolleys the cables pass around the pulleys of a gin block hoist assembly. Removable support tubes are provided to support the outer ends of the hinged portion of the hoist rails. Rope-fall blocks attach to a ring on each of the trolleys on the main hoist tracks and extend downward to the equipment to be hoisted.

For external loading, which is made possible by the height of the belly of the fuselage from the ground, two manually operated hydraulic hoist assemblies may be mounted under the wing, one on each side of the fuselage. Each hoist, located at the junction of the center wing and fuselage, attaches to the front and center spars. Each complete assembly is composed of two hand pumps, a reservoir, two hydraulic hoist struts and two latches. Operating the forward pump handle supplies fluid pressure to the actuating strut controlling the forward hoist cable; operating the aft pump handle supplies pressure to the actuating strut controlling the aft hoist cable.

The external load may be released in flight in an emergency by a control handle in the flight compartment. Folding benches to accommodate 30 passengers are provided in the main cabin. The bench legs are hinged so that the benches may be folded against the fuselage and secured with spring catches. All the benches are designed to accommodate seat type parachutes and are provided with safety belts. The benches are bolted to the guard rails and are in five-man and three-man sections, any of which may be removed to provide space for cargo or combat equipment.

Provisions are made for stowing four six-man life rafts aft of the cabin door. A folding wooden entrance ladder for use in entering or leaving the airplane through the main entrance door or for engine inspection or maintenance, is stowed against the ceiling.

Dome lights, controlled from the cabin switch panel forward of the cargo door, provide general illumination for the cabin.

Twenty-one individual elliptical windows of single pane Plexiglas are located in the cabin walls, ten on the left side and eleven on the right. In each window there is a gun port of approximately 3" diameter with a neoprene seal and a removable plastic plug.

Emergency exit doors having standard cabin windows, are located on each side of the main cabin. These emergency exit door assemblies are composed of a frame divided by a bracing crossmember. The upper part of the frame is for the window proper, the lower part containing the latch mechanism. This exit mechanism is enclosed on the inside of the cabin by a panel containing a transparent sheet covered recess for the emergency exit handle. This is reached by tearing away the cover.

The main cabin doors both open to provide access to the cabin when large equipment is loaded. Only the smaller front door is used for small cargo items or loading personnel. The door locking mechanism within the forward door section is actuated by a series of rods attached to the bell cranks. The bell cranks in turn move four wedges which slide into slots in the door jamb. The aft wedge on the forward side of the door frame contacts a safety switch signaling the pilot that the door is open or closed. An emergency release system, which pulls the hinge pins, allows almost instantaneous egress.

A small door is provided in the rear bulkhead of the main cabin for access to the tail compartment.

A front belly compartment with a capacity of approximately 125 cu ft and a rear belly compartment of approximately 165 cu ft capacity, are provided in the fuselage beneath the floor.

The tail cone assembly is attached to the fuselage stub by bolts screwed into nut plates and basket nuts.

A glider tow release is incorporated in the aft end of the tail cone. This assembly is connected through a cable to a pull handle located beneath the floor of the flight compartment aft of the control pedestal. It is accessible to both pilots through a hinged door in the floor.

A tailskid is mounted under the aft section of the fuselage for protection of the structure under emergency landing conditions. The skid consists of an arm and steel shoe assembly supported by a hydraulic shock strut. The unit is enclosed in a pair of telescopic fairings held in position by coil springs.

Empennage

The rudder, of aluminum alloy fabric covered construction, is statically and dynamically balanced by lead weights attached to the skin. It is hinged by two permanently lubricated dust-sealed bearings and steel bolts to hinge brackets on the vertical stabilizer. The rudder has a single channel-section spar of .020 gauge 24ST alclad reinforced with horizontal angles at each rib. The nose ribs are .025 gauge alclad with lightening holes. The nose structure is stiffened by two vertical stringers of .051 gauge and the entire leading edge from the spar forward is covered with .025 sheet alclad. The ribs aft of the spar are .021, .025 and .032 gauge alclad with lightening holes. The trailing edge is of .040 gauge bent sheet. The entire rudder is covered with Grade "A" airplane cotton cloth. The rudder tab is of alclad sheet fabric-covered construction, incorporating a channel type spar. It is hinged to the rudder at three points by steel bolts.

The vertical stabilizer and nose section are of aluminum alloy construction with alclad covering attached with flush-type rivets. The stabilizer is composed of a front and rear spar interconnected by rib sections of .020 gauge with special ribs of .025 gauge reinforced by extruded bulb angle stiffeners at the hinge attach points.

The front spar web is of .032 and .025 gauge 24ST alclad sheet with flanged lightening holes, and is stiffened by horizontal extruded bulb angles to which the forward ends of the ribs are riveted. The spar caps are angles which have one leg riveted to the spar web; the other leg carries an .051 gauge "tie plate" of joggled sheet to which are riveted nut planes for attachment of the nose section skin, which thus forms a flush joint with the stabilizer skin. The rear spar is of similar construction to the front spar and has a lower web of .051 gauge, an upper web of .032 gauge, and tapered spar caps which are of "T" section at the lower end and angle section at the upper. The covering is of .025 gauge 24ST Alclad. The tip section is removable and is attached to both the vertical stabilizer and the nose section by bolts and nut plates. The tip skin is of .032 gauge. The vertical stabilizer attaches to the fuselage by means of six internal wrenching steel bolts and nuts. Two 14ST aluminum alloy forgings bolted to the rear spar serve as rudder hinge brackets.

Both elevators are of identical construction and are balanced in the same manner as the rudder. Elevators are interchangeable and are hinged at two points to the horizontal stabilizers. The elevator spar web is of .020 gauge Alclad with vertical stiffening angles on both sides for attachment of the nose and trailing edge ribs. The nose ribs are .025 gauge and the ribs aft of the spar are .020 gauge except at the tab hinge points, where they are .025 gauge. The leading edge is reinforced with metal covering of .025 gauge from the spar forward to the nose and .040 gauge on the leading edge proper. The entire elevator is covered with doped cotton cloth. The elevator tab is of Alclad sheet, fabric-covered construction, incorporating a channel-type spar. It is attached to the elevator at three points.

Horizontal stabilizers are of Alclad construction with flush-type rivet skin attachment. They are interchangeable and incorporate removable leading edges and removable tips to permit quick replacement of portions most subject to damage. Each stabilizer is bolted to the fuselage structure at eight points. Inspection plates are incorporated in each stabilizer.

Landing Gear

In designing the tricycle landing gear, Douglas engineers have exhibited the skill they have acquired down through the years. The feature most interesting to pilots, not found on any other large airplane, is the steerable nose wheel, which permits "turning on a dime" and turning and taxiing if necessary, with the use of only one engine. The large double wheels of the main gear provide superlative traction due to the large tire area in contact with the ground and together with the nose wheel, permit hard braking when landing in short fields, also a boon to pilots. Another feature is that at rest the fuselage is parallel to the ground, the level floor facilitating the loading and unloading of both personnel and cargo.

The main gears retract forward into the inboard engine nacelles by means of hydraulic cylinders. The nose gear is retracted forward into the nose of the fuselage by a hydraulic cylinder. One end of the cylinder is attached to a lever on the shock strut, the other end to a lever on the drag link. The main gears and the nose gear are automatically latched when down. "Up" latches to retain the main and nose gear in the retracted position in case of loss of hydraulic pressure are provided and are manually controlled from the flight compartment through a cable system leading to latch mechanisms in the top of the wheel wells.

When the wheels, both main and nose, are fully retracted, they are enclosed by doors. These doors are in two sections opening downward and outward from the center along lines parallel with the fuselage.

The width of the main gears from tread center to tread center is 296", giving stable ground operation. Each two-wheeled main gear is mounted on a single shock strut. The cylinders are partially filled with oil which under load is forced through holes in the piston head. Above the oil, air under pressure acts as a cushion on which the airplane rests.

The main wheels are made by Goodyear. The main gear tires are Goodyear all-weather tread, size 44" x 15.50-20, with an inflation pressure of 45 psi. Gross weight of each wheel assembly including tire, tube and wheel, is 349.5 lb.

Brakes are Goodyear high pressure disc, size 17-20. Each wheel is equipped with two brakes, one on either side, allowing for positive action and large cooling area. Normally the brakes are actuated by hydraulic fluid, but in an emergency may be operated from the cockpit by compressed air.

The nose gear is a single wheel, single shock strut unit without brakes. The shock strut is similar in operation to those of the main landing gear. The nose wheel is a Goodyear 17-20. The nose gear tire is a Goodyear size 44" smooth contour, with an inflation pressure of 45 psi. Its gross weight is 169.90 lb.

In evaluating power plants, Douglas engineers have used their commercial experience to good advantage. Realizing that the largest engines would give greater speed, they nevertheless sacrificed speed in favor of greater range and more economical operating factors. Nevertheless, because of its aerodynamic efficiency, the cruising speed of the Douglas C-54 Skymaster is exceptionally high. Each of the four demountable power plants consists of the following units: The engine and its accessories; most of the engine section lines; accessory cowling and five point engine mount, and their attaching parts; the auto-drag ring; cowl flaps and inner rings; the propeller and the exhaust system. The oil cooler fairing forms the bottom section of the accessory cowling.

Power is supplied by Pratt and Whitney R-2000 engines incorporating single stage, dual drive, two-speed integral superchargers. The propeller reduction gearing in each engine has a ratio of 2:1. Each is rated at 1350 bhp for takeoff, at 1100 bhp from sea level to 7000 ft at low blower ratio and at 1000 bhp from 9750 to 14,000 ft at high blower ratio. Engines are equipped with PD-12F6 Bendix-Stromberg injection carburetors.

Four Hamilton Standard three-bladed Hydromatic full-feathering constant-speed propellers 13 ft in diameter are installed. The governor, mounted on the engine, is controlled from the flight compartment. A propeller-feathering system that utilizes engine oil is provided and a standpipe in each nacelle oil tank at the engine oil supply outlet assures a reserve supply of oil for feathering. An electrically driven pump mounted on the firewall supplies engine oil to the propeller for the feathering and unfeathering operations.

The governor used in conjunction with the Hamilton Standard Hydromatics automatically maintains the engine rpm constant at any speed selected by the pilot by changing the blade angles to meet new conditions of altitude, airplane altitude and throttle setting. The governor is mechanically operated by a cable control and incorporates an automatic feathering cutout switch.

The throttle, mixture, propeller pitch, carburetor air, and the supercharger units of each engine are operated by a system of cables and push rods from the control pedestal in the flight compartment. To hold at any desired setting, the pedestal also has locks for the pilots' throttle levers and for the propeller pitch controls. Push rods run from the control levers to bellcranks which in turn, transfer the motion to cables. The cables run to pushrods at the firewalls. These rods actuate a second set of cables which run over to the control pulleys on the corresponding units in the engine section.

Fuel System

The fuel system is both simple and foolproof. In addition to this, the four integral wing tanks, when empty, will keep the airplane afloat indefinitely in case of a landing on water. Each fuel tank is furnished with a water-collecting sump and a standpipe outlet.

Fuel is supplied to the engines from the four wing tanks by four tank-to-carburetor systems. Cross-feed lines also make it possible for any engine to draw fuel from any tank. The engine-driven fuel pumps normally supply fuel pressure, but electrically driven fuel pumps are also used for takeoff where a reliable fuel supply is imperative, and in an emergency.

Four cross-feed valves, each mounted adjacent to a similar tank selector valve, are controlled by cable linkage from the flight compartment.

Each engine has an electrically operated primer valve to permit an initial charge of fuel to be supplied for starting. Fuel can also be injected into the oil system in anticipation of cold weather starting by means of pilot-controlled oil dilution solenoids before killing the engines. The amount of fuel in each of the four wing tanks is indicated by fuel quantity indicators on the instrument panel. Normal fuel pressure is maintained at from 12 to 15 psi; fuel pressure gauges are provided for the pilots.

Emergency fuel shutoff valves are installed in each main fuel supply line just aft of the firewall to permit the pilots to cut off the flow of fuel to any engine section in case of fire.

Oil System

Each engine section is provided with a complete independently functioning oil system. Means of transferring additional oil from the 50 gal auxiliary tank in the fuselage to any nacelle tank during flight are installed. The auxiliary oil tank selector control valve and pump operating switch are located in the crew compartment. Oil pressure and temperature indicators together with oil quantity warning lights, are mounted on the pilots' main instrument panel.

Nacelle oil tanks are mounted behind the firewall of each engine. The major units of the nacelle oil system are the tank, automatic temperature control, and oil cooler. The oil tank is vented by lines to the engine crankcase, which has a breather line to the atmosphere. A fire-prevention fluid shut-off valve, installed in the main oil supply line just aft of the firewall, is manually controlled from the flight compartment by means of cables, and cuts off the flow of oil to the engine in case of fire in the engine section.

Oil is pumped from the nacelle tank to the engine by the engine-driven oil pump. It is returned by the scavenging pump from the engine to the cooler and then to the tank. Oil enters the cooler through its muff. The oil flows through the coil unless a pressure of 80 psi above normal is reached. In this case, a bypass valve opens and allows the oil to bypass the core by flowing around through the muff. Operation of the oil cooler door is controlled by a bimetallic, spiral valve. When the oil temperature is low, the valve turns so that the oil pressure closes the cooler door. When temperature is too high, the valve turns the opposite way and the oil pressure opens the door.

When the oil level in any nacelle tank drops to one-half capacity during flight an orange light in the flight compartment flashes on. If the tank's capacity falls to within one gallon above the propeller feathering reserve, a red light flashes.

Instruments

All instruments are scientifically arranged for easy visibility and are centered in the pilots' vision lines in accordance with the frequency of their use and importance.

The instrument panel assembly is composed of non-magnetic alloy to prevent deflection of the magnetic compass needle. The instruments are mounted on three separate plates, one in the center and one on either side. The entire panel is supported by shock absorbing units. The electrical instruments on the center panel are enclosed in a metal shielding box to prevent radio interference. The entire panel is grounded to the fuselage structure.

The pilots' upper instrument panel is also of non-magnetic materials. All the units are electrically operated and are enclosed in an instrument box to prevent radio interference. Further protection is provided by a partition placed inside the box, isolating the ignition wires and preventing induction of charge from them to the other wires. The panel is also grounded to the fuselage structure.

The navigator's instrument panel is located on the righthand side of the fuselage above the navigator's table. A metal support box riveted to a bulkhead and the side of the fuselage acts as a radio interference shield. The panel is grounded to dissipate accumulation of static discharges.

To give pilots the story of what is going on in the engines, the Douglas C-54 is equipped with four fuel and oil pressure indicators with red warning lights; four manifold pressure indicators; four tachometers; four engine temperature indicators; four oil and carburetor air temperature indicators.

Aeronautical instruments include two airspeed indicators, two sensitive altimeters for the pilots and an additional one for the navigator; two directional gyros; gyro horizon; two climb indicators; two turn-and-bank indicators; radio compass bearing indicator and magnetic compass. All flight instruments are installed in the pilots' instrument panel with the exception of the magnetic compass, which is located directly above the main instrument panel in the V of the windshield, and is suspended by three rubber bungees.

The driftmeter, a telescopic sight, is located in the navigator's compartment. The objective end of the instrument protrudes through the bottom of the fuselage, lenses and prisms making it possible for a sight to be taken in any direction. The pitot static instrument system actually is two separate systems, each incorporating an individual pitot tube. In the left pitot tube system are the instruments mounted on the left hand side of the pilots' instrument panel. The navigator's instruments and those on the right hand side of the instrument panel are connected with the righthand pitot tube.

Automatic Pilot

The airplane is provided with an automatic pilot installation. It comprises units that permit gyroscopic action to control the surfaces and maintain the airplane on a set course. Basically such an installation depends on three distinct forces; an oil pressure system, an air vacuum system and a cable follow-up system. Oil from the main hydraulic system supply tank is supplied to a pump on the left inboard engine which supplies the fluid under pressure to the servo units which operate the surface controls. The air-vacuum system units govern the servo action.

Major units of the system include the directional gyro control unit; the bank and climb gyro control unit; three servo units, and cable follow-up system.

Surface Controls

Complete dual surface controls are provided. Positive stops on the control columns and rudder pedals prevent movement beyond that necessary for control of the airplane.

The system consists of the cables and linkages that actuate the rudder and elevators, the aileron and the wing flap hydraulic units. A gust lock makes the use of external locks on the various surfaces unnecessary, but they may be used as an added precaution under extreme wind conditions. The gust lock lever is on the floor of the flight compartment between pilots' seats.

Aileron, rudder and elevator trim tabs, previously described, are also a part of the system.

In spite of the size of the airplane the control forces have been kept so low that no mechanical assistance is required to operate the controls.

Hydraulics

A comprehensive hydraulics system has been designed to do all the "heavy work" on the Douglas C-54. This system operates the nose wheel steering unit, the wing flaps, the landing gear, the cowl flaps, the automatic pilot, the brakes, and the carburetor air-filter doors.

It consists essentially of the following units:

Electrical System

The extensive use of electricity in the C-54 shows the amazing progress since World War I, when only an ignition system was used. The application of electricity shows that today aviation is a 24-hr job around the world under all sorts of climatic and weather conditions and at the same time reveals how man is winning his battle against the elements in extending aeronautical frontiers. The basic electrical system in the C-54 is of the 24 VDC single-wire type. In addition, a 26 VAC 400 Hz system for the autosyn instruments and the remote-indicating compass is provided; and a 115VAC 400 Hz system for the radio compass, the receiver loop motor, and the driftmeter. The structure of the airplane serves as a ground return for all circuits except those in the vicinity of the magnetic compass.

Two Exide 12-V batteries, connected in series, are located just aft of the nose wheel well. The batteries are mounted on a spring-loaded elevator. When the elevator is lowered, the batteries are easily accessible. In the raised position, connection with the electrical system is made automatically by plugs on the batteries and receptacles on the fuselage structure. Latches are provided for maintaining the batteries in both the up and down position.

Each engine has a 100-Amp generator connected for parallel operation. The generators are equipped with thermal protector switches. When the generator becomes overheated to a point endangering its continued operation, the thermal switch closes and induces a resistance parallel to the field circuit, thereby reducing the output of the generator. An individual generator switch, located on the pilots' electrical panel, permits the generators to be disconnected separately from the electrical system.

Four voltage regulators automatically maintain constant voltage for the electrical system, regardless of generator speed or variations in electrical load.

A voltammeter used in conjunction with a selector switch is located on the pilots' upper instrument panel. The main bus voltage, as well as the individual bus voltage output of each generator, may be read on the voltammeter by setting the desired position on the selector switch. The amperes supplied by the generator may be read by depressing a button located on the lower side of the voltammeter case.

Four reverse current line switches are used; one in each firewall junction box. The relays connect the generators to the electrical system when the voltage output from the generators is sufficiently high. The relays, however, will close only when the individual generator line switches are closed. The relay switches open automatically when the generator voltage becomes lower than the system voltage, thus preventing reverse current flow.

The bus bar system transmits the main 24-28 VDC current from each wing to the forward end of the fuselage.

The main junction box, located in the forward inboard corner of the crew's toilet compartment, is the central distribution point for the electrical system. Located in the main junction box is the main fuse panel containing all the fuses to the various circuits except the generator fuses, located in the nacelle firewall junction boxes, and the landing light fuses, located in the inboard nacelle firewall junction boxes.

The basic AC system consists of the 115 V 400-cycle AC power, and the 26 VAC 400 Hz power both of which are supplied by an inverter. This inverter is one of the two installed above the battery enclosure at the forward end of the forward cargo compartment. Only one inverter is used at a time.

The dual-magneto-type ignition system is used as each engine is supplied with a left and right magneto. Each magneto is controlled by an ignition switch of the four-position type, mounted on the center of the pilots' upper instrument panel. The ignition system is isolated from all other circuits by separate conduit and shielding. As a safety measure, the ignition plug at the firewall is provided with a device that automatically grounds both magnetos when the plug is disconnected.

A booster coil is used on each engine to aid in starting. The booster coil is energized by the starter meshing switch. A comprehensive warning lights system has been devised.

Six pairs of red fire warning lights located just below the pilots' instrument cowl, and just above their respective fire extinguisher control handles, are illuminated when the thermal switches are closed as a result of excessive heat. The six groups of thermal switches are located as follows:

  1. —Six switches in each nacelle, four on the forward side of the engine inner ring, and two on the forward side of the firewall. All the switches in the engine section are set to operate at a temperature of 500°F.
  2. —Three thermal switches in the forward cargo compartment, set to operate at 250°F.
  3. —Three thermal switches in the rear cargo compartment, set to operate at the same temperature.

Two red warning lights, located on the lower lefthand side of the pilots' upper instrument panel, are illuminated when any one of the main doors is unlatched or open. The lamps are connected in parallel so that one will operate if the other fails.

Warning lights are also used to indicate low oil pressure and low oil quantity.

Two warning lamps, located on the lower face of the pilots' center instrument panel indicate low fuel pressure, flashing when the pressure drops to approximately 11.8 psi.

The landing gear warning system consists of "UP" and "DOWN" switches for each landing gear, switch for the right hand landing gear, retracting locking solenoid, throttle warning switch, warning horn, and three green and one red indicator lights, warning the pilots as to the position of the landing gear.

Three navigation lights are provided, one 27-cp light on each wing tip, and one 3-cp light on the tail. Two switches, one for the tail light and one for the wing tip lights are located on the pilots' switch panel.

The two 600 W landing lights are located in the lower surface of the wing between the inboard and outboard nacelles. Normally flush with the wing skin, they are extended by an electric motor controlled by a switch on the pilots' switch panel. The lights go on automatically when the fully extended position has been reached. There are four recognition lights controlled by a key and a switch box installed on the top of the control pedestal.

Communications

Communications equipment is particularly adaptable for military usage. Emphasis has been placed on operation over geographical areas rather than along the fixed routes of commercial airways.

The command or pilots' radio is a short range set used mainly for plane-to-plane communication. It is remotely controlled by the pilot and copilot.

Two transmitters are installed on shelves above the radio operator's station. Receivers are mounted above the radio operator's station.

Transmitter and receiver control boxes are located on the control pedestal. The dynamotor is located on the modulator unit in the radio operator's compartment and supplies power to either transmitter. The antenna relay is fixed to the rear bulkhead of the flight compartment and switches a single antenna to transmitters or receivers. The command antenna extends from a mast over the radio operator's station to the vertical stabilizer.

The liaison or radio operator's set is a long range plane-to-base radio, and is used primarily for reporting the airplane's position and flight progress.

The transmitter is located adjacent to the radio operator's station. The receiver is a self-contained unit capable of voice, tone, and CW reception. It is located on the radio operator's table.

The seven transmitter tuning units are interchangeable. One is inserted in the transmitter, one is mounted over the radio operator's head, and the other five are stored in the crew compartment. The transmitter dynamotor is located behind the radio operator's seat.

The antenna tuning unit is located forward of the transmitter. A key is mounted on the radio operator's table. The change-over switch is fixed to the fuselage above the transmitter.

This Design Analysis article was originally published in the September, 1944, issue of Industrial Aviation magazine, vol 1, no 4, pp 15-24, 26, 28, 30-32, 35-38.
A ledger-size foldout with a color phantom rendering is missing.
Te original article includes a thumbnail portrait of the author and 6 photos, a three-view rendering and 16 detail drawings, and 3 data tables.
Photos are not credited.