Design Analysis of the Lockheed Constellation
By Hall Hibbard
Chief Engineer Lockheed Aircraft Corporation
Used by U.S. Army for trans-oceanic transport, and later modified by Lockheed engineers for passenger comfort, the “Constellation” is prepared for world operations.
The Constellation, conceived by TWA, Inc, and built by Lockheed Aircraft Corp, is a 64-passenger, low-wing, all-metal, land transport, powered by four 18-cylinder, twin-row, air-cooled, radial Wright Duplex Cyclone engines, giving the plane a speed of more than 300 mph. The plane is equipped with hydraulically operated flight control boosters and cabin supercharging capable of maintaining an apparent 8000 ft pressure altitude in the cabin while it is flying at 20,000 ft.
The center section, or stub wing, is built integral with the fuselage and is considered as fuselage structure. The inner panels join to this stub and are not designed to be removed for normal service and maintenance operations. The primary structure of the inner panel is a box beam formed of two main beams and stiffened top and bottom skins. The top skin is stiffened internally with corrugations; the bottom skin with extruded “T” sections. Formed ribs at rather frequent intervals maintain contour and stabilize the structure.
Fuel and oil tanks are integral with the inner panel structure. Each inner panel contains two engine nacelles, the structure of which is integral with the wing structure aft of the firewall. Space is provided in the inboard nacelles for completely enclosing the main alighting gear when retracted. A cabin supercharger and a reservoir for anti-icing fluid are housed in each outboard nacelle. The leading edge section and the nacelles are divided into compartments by bulkheads and are drained and ventilated. Drains are 3/8″ holes in the lower surface and vents are tubes which lead to the outside through louvres. Plumbing and wiring and power plant controls are routed through the leading edge directly forward of the front beam. Provisions are made for installing rubber de-icing boots and more than 180 access openings provide for inspection and maintenance in the wing group.
The outer panel mates to the inner panel just outboard of the nacelles and extends to the tip. General construction is the same as the inner panel with internal stiffening of formed “hat” sections at the top and formed “J” sections at the bottom, the whole covered by Alclad skin .032-.064 thickness. Front, rear and auxiliary beams mate with corresponding units of the inner panel and are connected by ribs. Hydraulically boosted ailerons run the full span of the outer panel and the booster mechanism is contained in the panel. A retractable landing light is mounted near the inboard end of each outer panel and retracts flush with the lower surface. Outer panels may be easily removed for maintenance or service.
The wing tip is built up of beams, ribs and internal stiffners and mates to the outer panel outboard of the aileron. At the extreme outboard end is a tip cap which contains a position light. Ailerons run the full span of the outer wing panels and are joined at the top surface of the wing with piano type hinges. The ailerons, with their trim tabs, are of fabric over an aluminum alloy frame. Ailerons are mass balanced and servo action is provided by the tab-operating mechanism connected by linkage to the tabs.
Flap installation is unique in that twelve flaps are provided — one one each side of the airplane under the center sections and five identical flaps outboard of each of these, the latter five covering the full span of the inner panel. All flaps are of the high-lift Fowler type and are constructed of aluminum alloy with forged aluminum alloy fittings riveted to each side at the leading edge. Movement of the flaps is outward and downward actuated by carriages which roll on tracks riveted to the wing ribs. The outward movement actually increases wing area and downward movement alters the airfoil section. The flap-operating linkage is so arranged that during the first half of flap extension, the motion is largely outward, the flaps tilting down only slightly. During the last half of flap extension the rate of tilt increases to approximately 42° at full extension. A chain and cable drive connects the flap carriage to intermediate gear-reducer units which are driven by a drive shaft extending through the wing center section into each inner wing. The drive shaft is driven by hydraulic motors through a main gear-reducer unit. The hydraulic motors are controlled by a selector valve, which is operated by a follow-up control unit. Cables are routed from the control unit in the fuselage to the flap-control lever on the pi1ot’s control stand. Flap positions can be preselected by moving the control lever the required amount. Flaps can be extended by a hand crank and mechanical drive unit in case of hydraulic failure.
Characteristic of Lockheed design is the multiple rudder tail group of the Constellation. Empennage assembly is composed of three rudders, three fins, two elevators and a stabilizer. Rudders and rudder tabs are fabric-covered. The remainder of the tail group is of all-metal construction. Elevators and rudders are mounted on anti-friction bearings and the tabs on piano hinges. All tabs, except the center rudder, are actuated by tab-operating mechanism attached to the fixed surface and by a linkage to give servo action. In the center rudder, the mechanism, contained in the rudder, gives direct tab action. The two outboard rudders are mass balanced and are interchangeable. The center rudder is a single unit, mass balanced, and its lower portion conforms to the upper contour of the tail cone. The all-metal elevators are mass balanced, and being identical are interchangeable. Because of this feature the tab hinge line is on the top surface on the left elevator, and on the bottom surface on the right elevator. Tail cone flaps, one on each side of the tail cone, rotate upward with the elevator and form an extension of the elevator. During downward movement of the elevator the flaps remain in neutral position.
The fuselage is of aluminum-alloy, semi-monocoque construction, 90′ 11″ in length and 11′ 7½” maximum diameter. It is circular in cross section except for the windshield area and for the extreme aft area to which the empennage is attached. The structure is made up of bulkhead rings and formed longitudinal stringers and is covered with aluminum-alloy skin.
A pressure bulkhead is provided approximately one foot aft of the extreme nose, and another 12’8″ forward of the tip of the tail cone. Between these bulkheads the entire fuselage area is built as a pressure tank so constructed that a pressure differential of 4.1 psi can be maintained for passenger and crew comfort during high-altitude flights. Skin joints are sealed with sealing compound and all doors, hatches, and openings for control and actuating cables are provided with air seals.
Aft of the forward pressure bulkhead is the flight compartment, which contains the stations for the pilot, co-pilot, flight engineer, and radio operator, together with necessary controls and instruments. The crew door is located on the right side of the fuselage immediately forward of the after partition.
A central door leads aft from the flight compartment to the relief crew and navigator’s compartment. In trans-ocean operations this compartment will undoubtedly be used for its original purpose, but domestic airline models probably will utilize the space for other purposes. To the right are berth type seats for the relief crew, and to the left is the navigator’s station equipped with all modern aids to celestial navigation. An astral dome is provided in the top of the fuselage just forward of the aft partition. Astro compass, drift meter, chart table and pyrotechnic equipment are conveniently mounted for the navigator. A hole in the forward partition makes for easy communication with the radio operator.
From station 342.8 to station 415.4, the fuselage contains the cabin cargo compartment, which is enclosed and occupies the right side, and the radio rack, which occupies the left side. Radio equipment for commercial models of the Constellation is still in process of design and will vary widely among the various airlines planning to use this plane.
One end of the passenger cabin provides for a galley (or steward’s compartment), aft of which are two lounges with lavatory facilities. Two large cargo compartments are contained in the pressurized fuselage under the cabin floor. The forward compartment extends aft from the rear wall of the nose wheel well to the front wing beam. Access is through a door in the wheel well or a removable panel in the cabin floor. The after compartment extends from the wing beam where provision for access is made by a door in the skin on the lower right hand side of the plane, or through a panel in the cabin floor.
The fuselage aft of the rear pressure bulkhead contains a tail bumper and the elevator and rudder booster mechanisms. Cabin doors are designed to withstand cabin pressurization. The main cabin door moves inward and then slides forward on tracks to open. The crew door opens by moving it inward and sliding it on tracks to an overhead position. Both doors are equipped with flush handles and tumbler locks.
The Constellation has a fully retractable, hydraulic-actuated landing gear consisting of two main gears located in the inboard nacelles and a nose gear located in the forward part of the fuselage. The retractable tail bumper, which extends and retracts with the gear, protects the tail in case of accidental tail-down landings.
Mechanical safety locks secure the gear in both extended and retracted positions, and when retracted, gears are fully enclosed by mechanically operated flush doors. Both main and nose gears are of dual-wheel type.
Each main gear has the dual wheels mounted on an axle which is fastened directly to the piston of the single oleo-pneumatic strut. Torque arms keep the shock-strut piston and cylinder in alignment. The upper end of the shock strut fits into a fulcrum forging; keys prevent turning between the two parts. Two side struts fasten the fulcrum forging to the strut. The fulcrum pivots on needle bearings mounted in brackets attached to the wing structure.
A drag-strut assembly consisting of an upper and a lower strut transfers part of the landing load to the wing structure, and also retracts the gear. The hydraulic actuating cylinder rotates the drag strut, thus folding the drag strut assembly and retracting the gear forward and upward into the wheel well. A down-lock strut, pivoted at the lower end to the connection between upper and lower drag struts, prevents accidental folding of the strut. In the locked position a hook on the upper end of the down-lock strut engages with a shaft mounted on the wheel-well structure. A spring-loaded latch in the hook keeps the down lock engaged until the latch is released by the down-lock release hydraulic cylinder.
A stop in the alighting-gear selector-valve linkage prevents accidental retraction of the gear when the airplane is on the ground. The stop is withdrawn by a solenoid, which is energized whenever the torque-arm switch on the left gear is actuated by a cam on the torque-arm shaft. The cam operates the switch only when the weight of the airplane is off the gear.
The main gear is held in the retracted position by an up-lock assembly bolted to the front wing beam. The up-lock jaws hook around a lug fastened to the shock strut. The up-lock is engaged mechanically and is released hydraulically. Indicators at the flight station show the position of each gear and also indicate whether up locks or down locks are engaged. The position transmitter is operated by an arm on the main-gear fulcrum. Switches on each up lock and down lock control the lock indicators.
The nose gear is mounted on an oleo-pneumatic shock strut and like the main gears the dual-wheel axle is attached directly to the piston. The top of the shock strut cylinder fastens to a fulcrum which is pivoted on spherical bearings mounted on the fuselage structure. A folding drag strut acts also as the retracting linkage and is pivoted on the wheel well structure. To retract the gear a hydraulic cylinder rotates the upper drag strut retracting the gear upward and rearward into the well. Down-lock and up-lock are provided to secure the gear in extended or retracted positions. Tires for the dual wheels are 33″ in diameter. The nose wheel is usually allowed to caster, although steering is provided by two hydraulic steering cylinders mounted by a bracket to the shock strut. Pressure for the cylinders is controlled at the flight station. The cylinders act as shimmy dampers when steering is not being used. When the wheels leave the ground and the shock-strut piston extends, a saddle-shaped cam on the piston engages with a mating cam at the bottom of the shock-strut cylinder. This centers the shock strut piston and insures that the wheels are straight forward when a landing is made.
The Constellation is powered with four 18-cylinder, twin-row, air-cooled, radial Wright Duplex Cyclone engines (model R3350-35). These engines drive Hamilton Standard Hydromatic quick feathering propellers of 15′ 2″ diameter through a propeller shaft reduction gear unit with a 16:7 ratio.
Each engine is rated at 2200 bhp at 2800 rpm at sea level for take-off for 5 min only. At cruising throttle, engines are rated at 1800 bhp each at 2400 rpm at an altitude of 15,000 ft. Dry weight of each engine is 2646 lb. Overall diameter is 55″ and length 7 6″.
Two-stage superchargers are provided and carburetors automatically maintain a constant fuel-air mixture by varying the pressure of the fuel delivered through jets to the nozzle bar in response to engine power demands.
Engine and supercharger controls are cable operated. Corresponding throttle controls are installed at both the flight engineer’s and at the pilot’s stations and are connected by cables passing over transfer pulleys. A carburetor air filter is installed in the right-hand firewall door of each nacelle. A dust door is located at each inlet scoop. Both the dust door and the filter shutter are operated by hydraulic cylinders, and control is through the same lever on the flight engineer’s stand which is used to regulate carburetor’s air temperature. The forward dust door and filter shutter are moved to “Closed” position by hydraulic pressure. They are returned to “Open” position when released, by springs within the cylinders plus rammed-air pressure.
Six panels attached by quick fasteners to a cowl support structure form the nose cowl. The upper panel incorporates the carburetor induction cold air scoop and the lower panel the oil cooler scoop. The lower side panels incorporate shrouds over the front exhaust tail pipes. Cooling air flow is regulated by cowl flaps located immediately behind the rear exhaust collector ring in the mount structure. Small ducts direct air to the accessories.
Each of the four engines is provided with an individual oil system. With the flight engineer’s temperature control selector switch set to “Automatic,” a thermostatic control unit positions the oil cooler flap to regulate cooling. The same switch may be used to control the flap position manually. An oil dilution system is provided for cold weather starts. Propeller feathering pumps are supplied with engine oil. Each engine oil tank is located immediately outboard of its engine nacelle and is integral with the wing. Tanks are formed of the skin of the leading edge, the nacelle skin and leading edge bulkheads.
Tankage is provided in standard models in four tanks. Space between the two parts of the inboard tanks allows for retraction of the main gear. The two parts are connected by a tunnel at their lower aft corners. A surge box at the inner aft corner of each tank guarantees steady supply and a synthetic rubber flapper valve holds a supply of fuel in the surge box in any attitude of the airplane. The fuel outlet is raised to form a sump in the bottom of the surge box.
Cross transfer valves provide for direction of fuel to any engine and, with tank shut-off valves, they are controlled by the flight engineer. Primers are solenoid controlled from the flight engineer’s main electrical panel. Dump valves open into a single dump chute in each wing panel and are controlled from the pilot’s overhead control panel. Movement of the control handle toward the “Open” position first extends the chute so that gas will clear the airplane, then opens the valves.
Surface controls of the Constellation are conventional and are hydraulically boosted. Aileron boosters are located in the outer wing panels and elevator and rudder boosters inside the tail cone. Complete details of the booster system are not yet available for public release, but it is possible to say that the pilot supplies enough of the control forces required so that he still retains a personal “feel” of the airplane in flight and its reaction to control pressures.
Flight station controls are connected to the boosters by cables and the boosters in turn to the control surfaces by push-pull tubes. The booster actuating cylinders act as control surface stops in all cases, limiting aileron travel to 25° up aileron and 10° down aileron, the elevators to 40° up elevator and 20° down elevator and the rudders to 30° travel each side of the centerline.
Servo trim tabs are used on all surfaces, further acting to reduce control forces. All boosters are equipped with bypass and shutoff valves so that in case of failure of the hydraulic system, controls can be manually operated. In addition, the elevator booster mechanism incorporates a shift mechanism which increases the mechanical advantage of the elevator control when the booster is shut off.
Hydraulic power is supplied by engine-driven piston-type variable-displacement pumps which circulate hydraulic fluid through the system at pressure up to 1700 psi. Through either cylinders or hydraulic motors this power operates the surface control boosters, reservoir pressurizing aspirator, alighting gear, brakes, nose wheel steering, wing flaps, automatic pilot, cabin-air circulating fan, and the carburetor-air filter control. Auxiliary power is supplied to the elevator booster by an electric motor-driven pump, and a hand pump provides emergency power to extend the landing gear. The engine-driven pumps are mounted one on each engine. The two left hand engines drive the “primary” pumps and the two right hand engines the “secondary” pumps. Fluid is supplied to the pumps from separate compartments within the same reservoir. The reservoir for the hydraulic fluid is located in the leading edge of the left-hand wing center section. Fluid can be added to the main reservoir if necessary during flight from an auxiliary fluid container by means of a wobble pump and flexible hose.
The major part of the Constellation electrical system is a 24 VDC single-wire installation. In the forward portion of the plane the steady-current loads are carried in a two-wire system to minimize compass deviation. Twenty-six and 115 V, 400 cycle, single-phase AC is supplied by inverters for operation of the autosyn instruments, drift meter, radio compass, and fluorescent light. Power for the DC system is supplied by two 24-V generators and two 24-V batteries. Wherever possible, wiring is carried in harnesses rather than conduit for ease of maintenance. A receptacle at the nose of the airplane permits the connection of an external source of power for distribution on the airplane’s two buses. Such power supply is used for starting the engines and testing electrical equipment.
The main ignition system is independent of the airplane’s battery system and has its own switches and wiring. Cowl flaps are electric-motor driven and controlled from the cockpit and flight engineer”s station. Oil temperature may be automatically controlled through a thermostatic unit. The fuel gauge system consists of six fuel level transmitters and two dual indicators. Two temperature bulbs per engine are used in conjunction with dual indicators to give the temperatures of engine oil “in” and “out.” Engine oil, carburetor air, outside air, cabin air, and heater duct temperatures are measured by means of resistance-type temperature bulbs. Cylinder temperatures are measured by means of thermocouples.
Connected with the electrical system are the various anti-icing devices, windshield and propeller fluid pumps, carburetor anti-icing fluid pump, pitot heater, windshield wipers and wing and tail de-icer boot distributor valve heater. Unit and continuous type fire detectors are provided in the engine nacelles and warning lights are thereby remotely controlled on the flight engineer’s panel with a master light at the pilot’s panel. Two vacuum warning switches are located in each nacelle. Hydraulic pressure warning and cabin pressure warning indications are provided by means of pressure-controlled switches. Position indicator for the alighting gear is installed on the pilot’s panel and a warning horn sounds when any throttle is cut while any one of the gears is not locked in the “down” position.
The navigation lighting circuit is conventional in every way. A 24-VDC outlet is pumped from navigation lights into each lavatory for electric razors, and an electric coffee maker circuit is wired to the galley. A retractable landing light retracts into the bottom surface of each wing. Interior lighting is provided in the main cabin and cargo compartments supplied by the 24 V circuit. Eight 24-V, four-watt fluorescent lights are installed for instrument lighting. Each lamp has a filter shutter which may be left open to provide visible light, or rotated to provide ultraviolet light for instrument markings.
The entire crew, passenger, cargo compartments and cargo area is sealed and pressurized, air being inducted through wing scoops and two centrifugal superchargers driven by the outboard engines.
Pressure control valves maintain pressure altitude by controlling the amount of air exhausted from the cabin. These valves are located in the aft cargo compartment. Two cabin-pressure relief-valves protect the fuselage from excessive loads due to high cabin pressures. Check valves prevent internal pressures from being lower than atmospheric pressure. These check valves are used also as auxiliary ventilation exhaust. Pressure controls are pneumatically operated and are fully automatic. When outside air temperatures are high, all supercharged air is diverted by an electric control system through aftercoolers, and when outside temperatures are low the air passes through the cabin heaters. Supercharged air is delivered through ducts to the passenger and crew compartments and is carried through louvers near the floor to the cargo compartments. A recirculating fan in the rear cargo compartment takes in air through a dust filter and discharges it into the cabin distribution ducts.