ch13-7

Quest for Performance: The Evolution of Modern Aircraft


Part II: THE JET AGE


Chapter 13: Jet Transports


Dedicated Cargo Transport



[454] The jet transport has so far been discussed only in the context of a passenger-carrying aircraft, and those described have indeed been configured with passenger transport as a primary design consideration. Most modern jet airliners, however, have some type of cargo-carrying capability. Even those aircraft configured primarily for passengers usually carry a limited amount of cargo, along with baggage, in the hold below the passenger deck. This cargo space is illustrated by the fuselage cross-section views in figures 13.19 and 13.20. Many jet transports are also available in convertible form and may be changed quickly from a passenger to an all-cargo configuration. In this case, the passenger seats are removed and cargo is carried in the space usually occupied by the passengers as well as in the hold. The floor of the passenger cabin is usually strengthened to allow for the higher unit floor loadings likely to be encountered with cargo. A large cargo door is also provided on convertible passenger-cargo aircraft.

Some jet transports are available in dedicated cargo versions. In addition to special cargo doors and strengthened floors, these aircraft have no facilities for attending to passenger needs and may even be constructed with no cabin windows. A crew rest compartment is sometimes located immediately behind the flight deck. An in-flight view of a dedicated Boeing 747 jet freighter is shown in figure 13.30. (Note the absence of passenger windows.) The visor-type loading door located in the nose of a 747 freighter is shown in figure 13.31, and the large rear cargo door is pictured in figure 13.32.

Another class of cargo aircraft, designed to meet the special needs [455] of certain types of military operations, has evolved since the end of World War II. These aircraft are used to transport various type military cargo such as trucks, tanks, jeeps, and artillery pieces, as well as troops. The size and shape of the fuselage is dictated by the number and type of vehicles to be carried, and some important aspects of the overall configuration are strongly influenced by the requirements for cargo handling and loading. One of the best-known military cargo aircraft is the turboprop-powered Lockheed C-130 shown in figure 6.3. This aircraft is in use by the military services of over 20 countries throughout the world. The Soviet military services also operate turboprop-powered cargo aircraft of the same configuration concept as the C-130. One of these, the Antonov An-22, is very large with a gross weight of about 550 000 pounds.

Two jet-powered cargo aircraft that employ many of the configuration features embodied in the C-130 design have been developed in the United States. These are the Lockheed C-141 StarLifter and the Lockheed C-5A Galaxy. The C-141 has a gross weight of about 317 000 pounds, which places it in the same size class as the Boeing 707-320B, and first flew in December 1963. The very large C-5A has a gross weight of 769 000 pounds, in the same weight class as the Boeing 747, and first flew in 1968. The C-141 and the C-5A are similar in appearance, but the difference in size is very obvious when the aircraft are seen side by side. The Soviet Ilyushin Il-76 military transport is similar in appearance to the two Lockheed aircraft and has a gross...




Figure 13.30 -Boeing 747 freighter showing absence of passenger windows. [mfr]




[456] Figure 13.31 - Cargo being loaded through nose-door of Boeing 747 freighter. [mfr] [Original photo was in color, Chris Gamble, html editor]



....weight of 350 000 pounds. First flight of the Il-76 took place in 1971. As representative of dedicated, jet-powered, military cargo transports, the Lockheed C-5A is briefly described below.

In the early 1960's, several aircraft companies began design studies of a heavy logistic jet transport intended to replace and augment the capabilities of the aging Douglas C-133 and complement the existing fleet of C-141 jet-powered transports. The aircraft was intended to deliver payloads in the range of 100 000 to 200 000 pounds over intercontinental distances and be able to operate from semiprepared runways. Following a design competition, Boeing, Douglas, and Lockheed were given contracts for further development of their designs. Concurrently, General Electric and Pratt & Whimey were given design contracts for high-bypass-ratio turbofan engines to power the new aircraft.




[457] Figure 13.32 - Rear-loading cargo door of Boeing 747 freighter. [mfr] [Original photo was in color, Chris Gamble, html editor]



The weight of the aircraft was expected to be in the 700 000-pound class, and the thrust level required of the new engines was about 40 000 pounds. The selection of the General Electric Company to develop the engine was announced in August 1965, an in October of that same year, the Lockheed Aircraft Corporation was selected to develop the aircraft. First flight of the C-5A took place on June 30, 1968, and the last of a fleet of 81 aircraft was delivered to the USAF in May 1973. An interesting account of the engineering development of the C-5A is given in reference 158. Several views of the aircraft are shown in figures 13.33 to 13.36.

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 discussed previously.

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 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 ...




[458] Figure 13.33 - Lockheed C-5A military freighter. [mfr]




Figure 13.34 - Lockheed C-5A in approach configuration showing 28-wheel landing gear and large fowler flaps. [mfr]




[459] Figure 13.35 - view showing for and aft loading capability of Lockheed C5A. [mfr]




Figure 13.36 - Photograph of man standing on horizontal tail is indicative of large size of Lockheed C-5A. [mfr]

[460] ....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. According to reference 158, 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 (somewhat like that shown in fig. 13.31 for the Boeing 747 freighter). The entire cross-section of the cargo compartment is exposed when the nose visor is raised. Figure 13.35 shows a rear view of the C-5A with both forward and aft doors open and with various types of equipment ready for loading.

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 [461] purpose, the aircraft can carry 270 soldiers. Both the lower cargo compartment and the upper deck are fully pressurized.

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. The highly versathe landing gear may be seen in figures 13.33 and 13.34.

With a gross weight of 769 000 pounds, the C-5A is a very large aircraft by any standard. The data in table VII show that 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. An indication of the large size of the aircraft can be obtained from figure 13.36, which shows a man standing on the horizontal tail. The aircraft is capable of performing missions with various combinations of range and payload. Two mission profiles given in table VII indicate a range of 3744 miles with a payload of 220 967 pounds and 6521 miles with a payload of 112 600 pounds. The aircraft has a maximum cruising speed of 541 miles per hour at an altitude of 25 000 feet. This corresponds to a Mach number of 0.78. The stalling speed at maximum landing weight is 120 miles per hour.

As indicated previously, the final C-5A was delivered to the USAF in May 1973. A recent decision has been made, however, to put the type back in production to augment the United States' heavy air-lift capability. The new aircraft will be designated the C-5B, of which the total number to be procured has not been finally determined.