The C-5A is a high-wing monoplane with the wing mounted at the top of the fuselage. The aircraft is equipped with four engines mounted in pods that are attached to the lower surface of the wings in much the same fashion as that employed on the 707 and DC-8 aircraft. The General Electric TF-39 engines that power the aircraft develop a takeoff thrust of 41 000 pounds each and have a bypass ratio of 8.0. The gas generator of this engine serves as the basis of the General Electric CF6 commercial engine.
Features unique to the C-5 include the forward cargo door (visor) and ramp and the aft cargo door system and ramp. These features allow drive-on/drive-off loading and unloading as well as loading and unloading from either end of the cargo compartment. The C-5's kneeling capability also facilitates and expedites these operations by lowering the cargo compartment floor by about 10 feet to 3 feet off the ground. This position lowers cargo ramps for truck bed and ground loading and reduces ramp angles for loading and unloading vehicles. The C-5's floor does not have treadways. The "floor-bearing pressure" is the same over the entire floor. The C-5A/B can carry up to thirty-six 463L pallets. The C-5 has carried special loads, such as large missiles, that would require extra time, manpower and dollars to transport via ship, rail or flatbed truck.
The aspect ratio 8.0 wing is swept 25° at the quarter chord and is equipped with Fowler-type single-slotted flaps and leading-edge slats. Lateral control is provided through a combination of ailerons and spoilers. The ailerons arc also used to reduce wing bending moments when the aircraft undergoes normal acceleration as a result of maneuvers or gusts. In this technique, called active load distribution control system (ALDCS), the ailerons are symmetrically deflected in response to signals received from accelerometers located in various parts of the aircraft. For a positive acceleration, the ailerons are deflected upward whereby the load Is shifted inboard and thus the wing-root bending moments are reduced. This technique is expected to find application in many new aircraft designs.
The empennage consists of a horizontal tall mounted in the T-position at the top of the swept vertical fin. This arrangement, rather than a low-tall arrangement, results in a weight savings. The horizontal tail consists of elevators and an adjustable stabilizer. No trim tabs are provided.
The high-wing position employed on the C-5A is advantageous for a cargo aircraft because it allows trucks and other types of equipment to move beneath the wing, and the bottom of the fuselage can be brought close to the ground for easy cargo loading without causing interference with the engines. A rear door, which serves as a loading ramp when lowered, is deployed from the bottom of the upswept aft portion of the fuselage. The proximity of the bottom of the fuselage to the ground results in a ramp with only a small inclination to the ground; thus, vehicles can be readily driven or pushed into the aircraft. The rear door is also used for aerial deployment of vehicles and equipment by parachute. The fuselage is provided with a forward loading door in the nose of the aircraft. The door is like a visor and lifts up and over the flight deck. The entire cross-section of the cargo compartment is exposed when the nose visor is raised.
The length of the C-5A cargo deck, excluding the loading ramps, is about 121 feet, and the maximum width is 19 feet. The height of the cargo compartment is 13.5 feet. In addition to the lower cargo compartment, the fuselage also has an upper deck divided into three sections. The forward section contains the flight deck and is followed by a rest area for 15 people that is usually occupied by relief crews. The flight crew of five persons consists of the pilot, copilot, flight engineer, navigator, and load master. Behind the rest area is a passenger compartment that will accommodate 75 fully equipped troops. The lower cargo compartment may also be utilized for troop transport; for this  purpose, the aircraft can carry 270 soldiers. Both the lower cargo compartment and the upper deck are fully pressurized.
Four turbofan engines mounted on pylons under the wings power the C-5. Each engine pod is nearly 27 feet (8.2 meters) long, weighs 7,900 pounds (3,555 kilograms) and has an air intake diameter of more than 8 1/2 feet (2.6 meters). The aircraft's four General Electric TF39-1A axial flow turbofan engines, each with a diameter of 16 feet and capable of generating 41,000 pounds of thrust, can produce enough electricity for a city of 50,000 people. With maximum allowable payload and without air refueling, the C-5A could fly 3,250 nautical miles, and its ferry range was more than twice that distance. With one aerial refueling, when that technique was permitted, aircraft carrying its maximum designed load could reach almost any point on the globe.
With a gross weight of 769,000 pounds, the C-5A is a very large aircraft by any standard. The C-5A, in comparison with the 747-200B, has a larger wing span and area and a greater fuselage length. The 747 is, however, somewhat heavier than the C-5A.
The Galaxy has 12 integral wing tanks with a capacity of 51,150 gallons (194,370 liters) of fuel - enough to fill 6 1/2 regular-size railroad tank cars. The fuel weighs 322,500 pounds (145,125 kilograms) and permits the C-5, carrying a 204,904-pound (92,207-kilogram) payload, to fly 2,150 nautical miles (3,440 kilometers), off-load, and fly another 500 miles (800 kilometers) without aerial refueling.
In order to minimize weight and complexity, the landing gear is retracted into blisters located on either side of the fuselage rather than into the high-mounted wing. The Galaxy's weight is distributed on its high flotation landing gear, which has 28 wheels. The landing gear system can raise each set of wheels individually for simplified tire changes or brake maintenance. The capability for operation from semiprepared runways was one of the specified design requirements of the C-5A cargo transport. The achievement of a relatively low unit loading on the runway surface was therefore necessary. In order to meet this design requirement, the main landing gear is equipped with 24 wheels, and the nose gear has 4 wheels. The main gear consists of four struts to which are attached six-wheel bogies. Each bogie has a two-wheel truck forward of the strut and two two-wheel trucks located side by side behind the strut. In order to provide further flexibility in rapidly adapting the aircraft to various runway-bearing capabilities, the pressure in the tires can either be increased or decreased while the aircraft is in flight. The landing gear bogies may be set at an angle of as much as ±20° from the centerline of the aircraft to simplify takeoff and landing operations in various crosswind conditions. Ground maneuverability is enhanced by allowing the front two bogies to castor freely while the aircraft is being taxied. The landing gear is also provided with a kneeling capability to lower the floor of the main deck for ease of transferring cargo from a truck to the aircraft. With the landing gear in the kneeling position, the lower deck is just over 4 feet from the ground at the front loading door and just over 5 feet from the ground at the rear door.
Except for emergencies or unusual circumstances, the C-5 does not carry troops in the lower-deck cargo compartment; but 73 seats are available in the rear compartment of the upper deck for personnel and operators of equipment being airlifted. The troop compartment is located in the aircraft's upper deck. It is self-contained with a galley, two lavatories, and 73 available passenger seats. Another 267 airline seats may be installed on the cargo compartment floor (maximum combined total of 329 troops including air crew over water).
The forward upper deck accommodates a crew of six, a relief crew of seven, and eight mail or message couriers. The flight deck has work stations for the pilot, co-pilot, two flight engineers and two loadmasters. The upper deck's forward and rear compartments have galleys for food preparation, as well as lavatories.
The electrical system has four engine-driven generators, each powerful enough to supply the aircraft sufficient electricity. Each of the two main landing gear pods carries an auxiliary power unit to supply electric and pneumatic power for engine starts and ground air conditioning, heating, cooling and ventilation. Air turbine motors in the landing gear pods also can power the hydraulic systems and the main landing gear kneeling motors.
The Galaxy has sophisticated communications equipment and a triple inertial navigation system, making it nearly self-sufficient. It can operate without using ground-based navigational aids. An automatic trouble-shooting system constantly monitors more than 800 test points in the various subsystems of the C-5. The Malfunction Detection Analysis and Recording System uses a digital computer to identify malfunctions in replaceable units. Failure and trend information is recorded on magnetic tape for analysis.
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