Quest for Performance: The Evolution of Modern Aircraft
 
 
Part II: THE JET AGE
 
 
Chapter 13: Jet Transports
 
 
Wide-Body Transports
 
 
 
[437] Four families of aircraft make up the fleet of wide-body transports that began operation on airlines throughout the world in the 1970's. These aircraft are the Boeing 747, the McDonnell Douglas DC-10, and the Lockheed L-1011, which are manufactured in the United States, and the Airbus A-300, which is produced by a consortium of European countries. All aircraft are still in production,2 and all are expected to continue in service for the foreseeable future. In addition to these aircraft, the Soviet Union has developed a large four-engine wide-body transport. This aircraft, the Ilyushin 86, first flew on December 22, 1976, and airline operations began in 1980.
 
The use of the term "wide body" in describing these aircraft is derived from the interior arrangement of the passenger cabin. Consider first the arrangement of the cabin of a "narrow-body" transport such as the 707 or 727, as shown in figure 13.19. The cabin is divided into a small first-class compartment with four-abreast seating and a large tourist-class cabin with six-abreast seating. A single aisle runs the entire length of the cabin with three seats located on either side. For an aircraft of large passenger capacity, the fuselage of the narrow-body type tends to become very long, which, in turn, may dictate a long, heavy landing gear in order to permit the desired rotation angle on takeoff without scraping the rear end of the fuselage on the runway. The long aisle also causes lengthy delays in passenger loading and difficulty for the cabin attendants in serving meals and refreshments.
 
A schematic drawing of the interior cabin arrangement of a conceptual wide-body transport is shown in figure 13.20. The first-class cabin consists of a small four-abreast compartment in the forward part of the fuselage and a large seven-abreast tourist cabin. The tourist cabin is divided by two longitudinal aisles that run the length of the cabin. In the particular arrangement shown, two seats are located on either side of the aircraft next to the windows, and three seats are disposed about the centerline of the cabin with an aisle on either side.
 
 

cut away drawing of an airliner
 
[438] Figure 13.19 - Interior arrangement of narrow-body, single-aisle jet transport aircraft.

 

cut away drawing of an airliner
 
Figure 13.20 - Interior arrangement of wide-body, double-aisle jet transport aircraft.

 
 
Some wide-body aircraft are designed to accommodate as many as 10-abreast seats. Current high-density versions of the Boeing 747, for example, may seat as many as 550 passengers in a 10-abreast arrangement.
 
For large-capacity aircraft, the double-aisle arrangement offers easy passenger loading and simplifies the serving problem for the cabin attendents. [439] The design may also offer the passenger somewhat wider seats and a feeling of greater spaciousness. The landing-gear problem previously referred to is alleviated by the relatively short fuselage offered by the wide-body design for a given passenger capacity.
 
The large-diameter fuselage of the wide-body aircraft is often cited as a source of increased skin friction drag. The bulky appearance of these aircraft is no doubt responsible for this viewpoint. Actually, the ratio of wetted area to wing area for wide- and narrow-body aircraft of the same passenger capacity and wing loading may not be greatly different because of the shorter length of the wide-body aircraft. The trade-offs between fuselage length and diameter can be assessed with the use of figure 3.11 in reference 176.
 
The wide-body jet transports introduced in the 1970's are characterized by two other distinguishing features. First, these aircraft are very large in comparison with earlier jet transports. For example, one version of the Boeing 747, the largest of the wide-body aircraft, is certified at a maximum takeoff gross weight in excess of 800 000 pounds. Although the wide-body concept was originally applied only to very large aircraft, new designs for use in the 1980's utilize the wide-body concept in aircraft of the 707 weight category.
 
A second distinguishing feature of the wide-body transports is the type of engines used to power them. All the aircraft are powered by very large engines of high bypass ratio. Because of the high bypass ratio and high compressor pressure ratio of these engines, the values of their cruise-specific fuel consumption are about 20 percent lower than earlier low-bypass-ratio engines such as the Pratt & Whitney JT3D. Another outstanding feature of these engines is their relatively low noise levels, as compared with earlier engines, even though the thrust produced by the new engines is significantly higher than values typical of the earlier ones. The low-noise-level characteristic of the high-bypass-ratio engines results from an improved understanding of the mechanism of noise generation, as influenced by engine design, and through the use of new sound-absorbing materials in various parts of the inlets and other flow passages. (See the section on engine noise in chapter 10.)
 
Three families of modern, large, high-bypass-ratio turbofan engines are the Pratt & Whitney JT9D, the General Electric CF6 and the Rolls-Royce RB.211. Each of these engines is produced in a number of variants with different capabilities. The maximum takeoff thrust of the various versions lies in the range from 45 000 to 55 000 pounds. The [440] bypass ratios of' the engines are 5:1 for the Pratt & Whitney JT9D, 5.9:1 and 4.4:1 for the General Electric CF6, and 4.4:1 for the Rolls Royce RB.211. As indicated, the C176 engine is available with two different bypass ratios. The compressor pressure ratios of the different engines fall in the range from 24:1 to 30:1. Detailed information on the various versions of the engines may be found in references 130 and 199.
 
In other respects, the wide-body aircraft, as compared with earlier jet transports, have only evolutionally technical refinements. The widespread use of sophisticated, high-speed computational equipment has resulted in more refined aerodynamic and structural design and in improved machine control in manufacturing. As a result of more sophisticated analysis techniques and new developments in transonic aerodynamics, some improvements may be found in wing and airfoil design. Basically, however, the aerodynamic design of the wide-body aircraft is similar to the preceding generation of aircraft. Again, in the area of structural design, no radical innovations are to be found. All the aircraft use fully powered flight control systems, and all employ sophisticated auto pilots and other onboard systems.
 
The combination of large passenger capacity, more efficient and quieter engines, and more sophisticated detail design has resulted in transport aircraft that are safe, reliable, environmentally acceptable, and, from the airlines' viewpoint, profitable. From the passengers' viewpoint, the aircraft are fast, convenient, and relatively comfortable, and they offer reasonable fares.
 
As discussed, most jet transport aircraft actually consist of a series of aircraft of varying characteristics that evolve from a single basic design. Each variant of the series has characteristics that are intended to adapt the aircraft to a particular set of operating requirements. The wide-body transports also follow this trend, and the generic aircraft type of each manufacturer actually represents an entire family of aircraft. In the following, a brief description of a representative model of the Boeing 747, the McDonnell Douglas DC-10, and the Lockheed L-101 1is given. Descriptions of the various versions of the different aircraft may be found in references 129, 130, 150, and 161.
 
Boeing 747
 
The first of the wide-body turbo fan-powered transports to enter airline service was the Boeing 747. Design work on the aircraft was initiated in the 1960's, and the first details were announced in April 1966. Simultaneously, Pan American World Airways announced orders for 25 [441] of the new aircraft. First flight Look place in February 1969, and certification was completed by December of that year. The first passenger were carried on a flight from New York to London on January 22, 1970. The 747 aircraft is utilized by 32 operators throughout the world. Over 595 units have been ordered by mid-1982, and the type will likely remain in production for the forseeable future.
 
A side view of a Boeing 747 in the livery of Pan American World Airways is shown in figure 13.21, and a front view of the aircraft in the landing configuration may be seen in figure 13.22. Some indication of the size of the 747 can be determined from figure 13.23, which shows a young boy standing beneath the nose of the aircraft. Some of the characteristics of the Boeing 747-200B are given in table VII.
 
The appearance of the four-engine 747 is very similar to that of its well-known ancestor, the Boeing 707. In addition to its large size, however, the 747 has two distinguishing features. First, the passenger cabin extends all the way to the forward end of the fuselage. The flight deck, with a small cabin behind it, is mounted on a second level and is reached by a circular stairway from the main cabin. This interior arrangement results in a distinctive hump in the external appearance of the top, forward end of the fuselage. A second distinguishing feature of the 747 is the main landing gear, which is unique for a passenger-carrying aircraft. The main gear consists of four struts, or posts, to which are attached four-wheel bogies. The two rear struts are mounted on the fuselage near the trailing edge of the wing and retract forward into the fuselage. The other two struts are mounted in the wing, farther forward, and retract inward into the wing. The four-post main gear is...
 
 

aerial view of a B747
 
Figure 13.21 - "Boeing 747 wide-body transport. [mfr]

 
 
[442]....required in order to property distribute the massive weight of the aircraft on the runway. The aircraft shown in the approach configuration in figure 13.22 clearly shows the extended four-strut main gear as well as the nose gear that retracts forward into the fuselage.
 
The engines first offered on the 747 were the Pratt & Whitney JT9D turbofans. In addition to these engines, the aircraft is now certified with the General Electric CF6 and the Rolls-Royce RB.211 turbofans. The 747-200B, for which data are given in table VII, is powered with four JT9D-7R4G2 engines of 54 750 pounds thrust each.
 
The aerodynamic configuration of the 747 is very similar to that of the 707. The 747 wing has slightly more sweepback than that of the 707 and is of about the same aspect ratio. An improved airfoil design is also incorporated in the wing of the 747. The maximum lift-drag ratio of the aircraft, (L/D)max, is estimated to be about 18, as compared with a value somewhat over 19 for the 707, The lower value of (L/D)max, results from a higher value of ratio of wetted area to wing area on the 747 than on the 707.
 
The high-lift system employed on the 747 is typical of Boeing practice and consists of trailing-edge triple-slotted flaps, similar to those employed on the 727, and leading-edge flaps. The large trailing edge flaps are clearly visible in figure 13.22. The lateral control system utilizes a combination of spoilers together with inboard and outboard ailerons. The spoilers are also used for lift and drag control when...
 
 

B747 comming in for a landing
 
Figure 13.22 - Boeing 747 in landing configuration. (Note four-post main landing gear and large flaps.) [mfr]

 
[443]...deployed symmetrically. The horizontal tail is located in the conventional low position at the rear of the fuselage. Longitudinal control is provided by an elevator and adjustable stabilizer trim system. No trim tabs are employed. All controls are fully powered.
 
The very large size of the Boeing 747 is the most striking feature of the aircraft. The gross weight of the 747-200B is seen from table VII to be 836 000 pounds, more than that of any other aircraft ever built. The Lockheed C-5A military cargo transport discussed below is the next largest aircraft at a weight of 769 000 pounds. The 747-200B can carry a maximum payload of 144 520 pounds for a distance of 6854 miles and has a cost-economical cruising speed of 564 miles per hour (Mach number of 0.85) at an altitude of approximately 35 000 feet. With a maximum fuel load and a reduced payload of 87 800 pounds, the range is 8706 miles. In a maximum capacity configuration, the aircraft can carry 550 passengers with 10-abreast seating. In this arrangement, four seats are placed in the middle of the aircraft, between the two aisles, and three seats are located against either side of the cabin. Many other seating arrangements for a smaller passenger load are used in the aircraft. The particular seating arrangement utilized is dictated by the airline using, the aircraft and is based on the passenger density anticipated on the various routes served by the aircraft.
 
The Boeing 747 is available in 10 versions adapted to various airline needs. One major variant of the 747 is the 747SP (Special Performance), which is shown in figure 13.24 along with a standard-size 747. The 747SP is lighter and has a smaller fuselage and lower passenger capacity but has a longer range than any other version of the aircraft. Latest version of the 747 to fly is the 747-300, which has an upper deck extended by nearly 24 feet. Sixty-nine passengers can be carried in a six-abreast configuration in the upper deck of this aircraft and total capacity is 624. Overall size and gross weight are not altered by the extended upper deck. New and larger capacity variants of the aircraft will no doubt appear in the future. Currently, the Boeing 747 is considered to be one of the world's outstanding commercial aircraft certainly, it is by far the largest.
 
Lockheed L-1011 and McDonnell Douglas DC-10
 
The Lockheed L-1011 and the McDonnell Douglas DC-10 are wide-body transports in a weight class between that of the 707 and the very heavy 747. Both aircraft are powered by three high-bypass-ratio turbofan engines located in a new configuration arrangement; one engine is mounted under each wing, and the third engine is mounted
 
 

fronta view of a B747 and a man beneath
 
[444] Figure 13.23 - View of Boeing 747 indicating size of aircraft . [mfr]

 

overhead view of a B747 and a B747SF surrounded by people
 
Figure 13.24 - View of Boeing 747 and 747SP. [mfr] [Original photo was in color, Chris Gamble, html editor]

 

aerial view of L-1011 illustrating  a jet engine below each wing and one below the tail rudder
 
[445] Figure 13.25 - Lockheed L- 1011 TriStar three-engine, wide-body transport showing
arrangement of three engines. [mfr]

 
 
....at the rear of the aircraft. The L-1011 and the DC-10 were initially designed to an airline requirement for a high-capacity transport with transcontinental range, but growth versions of each are presently available with intercontinental capability.
 
[446] Initial flights of' both aircraft occurred in 1970. An early version of the DC-10 entered airline operation in 1971, and the L-1011 began service in 1972. Both aircraft are in wide Use throughout the world.
 
The Lockheed L-1011, also known as the TriStar, is shown in figure 13.25, and the McDonnell Douglas DC-10 is depicted in figure 13.26. Some of the characteristics of the two aircraft are given in table VII The three-engine configuration employed on both aircraft is clearly, shown. This arrangement, in which two of the engines are located near the aircraft center of gravity, offers ail advantage in aircraft balance over an arrangement in which all three engines are mounted at the rear of the fuselage (Boeing 727, for example). Placement of two of the engines under the wing also allows the horizontal tall to be mounted in the highly desirable low position, as contrasted with the T-tall arrangement. The large lateral distance between the wing-mounted engines, however, causes larger yawing moments following loss of' power of one of these engines as compared with a similar power loss in the rear-mounted engine arrangement.
 
The method of mounting the rear engine is seen to be quite different on the L- 1011 and the DC-10. The L-1011 utilizes a mounting arrangement similar to that of the Boeing 727. The center engine is mounted in the aft end of the fuselage and is connected through an S-shaped duct to the large inlet mounted on top of the fuselage. In contrast, the center engine of the DC-10, including inlet and exhaust...
 
 

aerial view of a DC-10
 
Figure 13.26 - McDonnell Douglas DC-10 three-engine, wide-body transport. [mfr]

 
[447] nozzle, is integrated with the fin above the fuselage. The engine efficiency resulting from this straight inlet-engine-nozzle configuration, as compared with the S-shaped duct arrangement, was thought to more than offset the structural complexity (and probable weight increase) of integrating the engine with the fin. The high performance of both aircraft, however, suggests that either method of engine installation can be made to operate successfully.
 
The Lockheed L-1011-200 is powered with three Rolls-Royce RB.211-524 engines of 48 000 pounds thrust each. The McDonnell Douglas DC-10-30 is powered by three General Electric CF6-50CI engines of 52 500 pounds thrust each but is also available with a version of the Pratt & Whitney JT9D engines.
 
The main landing gear of the L-1011 has two struts to which are attached four-wheel bogies. Early versions of the DC-10 employed a similar arrangement. The heavier DC-10-30, however, employs a third strut, equipped with a two-wheel bogie mounted on the fuselage centerline between the other two main landing-gear struts. This arrangement helps to distribute the weight of the aircraft on the runway and thus keeps the runway-bearing stress within acceptable limits.
 
The aerodynamic design of both of the three-engine jet transports is conventional. The wings of both aircraft have about 35° of sweepback with aspect ratios in the range of 7.0 to 7.5 and feature transonic airfoils of advanced design. The wings have double-slotted trailing edge flaps and leading-edge slats. Lateral control is provided by a combination of ailerons and spoilers. The spoilers are also used to control lift and drag when deployed symmetrically. Longitudinal control of the L-1011 is provided by a variable incidence stabilizer to which the elevator is mechanically linked. The DC-10 employs separately actuated elevators and stabilizers. Neither aircraft employs longitudinal trim tabs. The maximum lift-drag ratio of the two aircraft is estimated to lie in the range between 17.0 and 17.5.
 
The data in table VII indicate that the L-1011-200 and the DC10-30 are very large aircraft. For example, at a gross weight of 468 000 pounds and with a maximum payload of 74 200 pounds, the L-1011 - 200 is capable of flying for a distance of 4884 miles. With a maximum fuel load and a reduced payload of 42 827 pounds, the range is 6204 miles. The aircraft is capable of carrying 400 economy-class passengers in a 10-abreast double-aisle configuration. An interesting feature of the interior design of the L-1011 is the location of the galleys below the passenger deck; food service is provided to the passenger cabin by means of elevators.
 
[448] The economical cruising speed of the L-1011 is 567 miles per hour at 31 000 feet, which corresponds to a Mach number of 0.84. The takeoff field length of 8070 feet is relatively short compared with 10 370 feet for the DC-10-30 and 10 450 feet for the 747. According to the data in table VII, the values of gross weight, payload weight, and range of the DC-10-30 are significantly larger than the corresponding values for the L-1011200. A comparison of the values of the wing loading and thrust loading of the two aircraft clearly shows why the takeoff distance of the DC-10-30 is greater than that of the L-1011 - 200. Methods for rapid estimation of the takeoff distance are discussed in chapter 3 of reference 176. The cost-economical cruising speeds of the two aircraft are comparable.
 
Total orders for the DC-10 number 382, and it remains in production at the rate of 1 aircraft per month. Included in these figures are a small number of tanker versions of the aircraft for the USAF. These carry the Air Force designation of KC-10. Lockheed, however, has announced termination of the TriStar program after completion of aircraft number 250 in the fall of 1983.
 

2 Lockheed L-1011 production has now ended.

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