Quest for Performance: The Evolution of Modern Aircraft
Chapter 13: Jet Transports
First-Generation Transports
[415] In this section, three families of transports that were configured and sized to more fully exploit the unique capabilities of jet propulsion in commercial aircraft are discussed. These aircraft were responsible for the beginnings of the revolution in air transportation caused by the jet transport, and the configuration concepts of these designs have had a lasting influence on jet transport aircraft.
Boeing 707
The Boeing 707 transport was the first of the long-range and, for its day, high-passenger-capacity aircraft that marked the real beginning of the revolutionary jet age in air transportation. Even today, many people consider the terms 707 and jet transport to be synonymous. The prototype of this remarkable aircraft first flew in July 1954, and an early production version first entered airline service in the fall of 1958. Over 900 Boeing 707 commercial transports have been built, but by 1980 the 707 was no longer in production as a commercial transport. A tanker version of the aircraft, the KC-135, has been built in large numbers for the USAF; and the Airborne Warning and Control System aircraft (AWACS) now being delivered to the Air Force utilizes the basic 707 airplane.
The prototype of the 707 was known in the Boeing Company as the model 367-80, and within the company it has always been referred to as the Dash-Eighty. The aircraft served as a test vehicle for the exploration and development of new ideas for many years. Finally retired in 1972, it was presented to the Smithsonian Institution. The aircraft is shown in figure 13.5, and a few of its characteristics are given in table VII.
A fully developed Boeing 707-320B is shown in figure 13.6, and a three-view drawing of this version is given in figure 13.7 The 707-320B is the last version of the aircraft built solely for passenger use. The last variant produced was the 707-320C, which is similar in most respects to the B model but is fitted with a cargo door and strengthened floor structure; the aircraft may therefore be used for cargo or mixed cargo and passenger service. Data for the 707-320B are given in table VII. Specifications and performance data quoted below are for this version of the aircraft.
The wing of the Boeing 707 is mounted in the low position at the bottom of the fuselage; this wing location has been preferred on transports designed for passenger use since the Boeing 247 and Douglas...

Boeing 707 taking off
[416] Figure 13.5 - Boeing model 367-80, prototype of Boeing 707 series transports. [mfr]

....of the early 1930's. The wing has an aspect ratio of 7.1 and employs a 35° sweepback angle. This wing geometry provides a combination of good cruising efficiency at high subsonic speeds, low structural weight, and large internal volume for fuel. The main landing gear consists of two struts to which are mounted four-wheel bogies. The landing gear is attached to the wing and is retracted inboard into the thickened juncture of the wing and fuselage. The nearly straight trailing edge of the wing near the fuselage is dictated by the required storage space for the landing gear in the retracted position. The two-wheel nose gear retracts forward into the fuselage.
The four engines are mounted similarly to the manner pioneered by the B-47 bomber described in chapter 12. Each engine is contained in a single nacelle that is attached to the bottom of the wing by a sweptforward pylon. According to reference 182, consideration was given to mounting two engines in each of two nacelles; such an arrangement was employed in mounting the four inboard engines of the B-47. This engine configuration was abandoned on the transport because of the possibility that disintegration of one engine might cause failure of an adjacent engine. This possibility was apparently not acceptable on a passenger-carrying transport. Early versions of the 707 were powered with turbojet engines. Several different engines were used, but most of these early aircraft employed the Pratt & Whitney JT3C engine, which is basically a civil version of the military J-57 turbo jet used by such aircraft as the Boeing B-52 bomber and the North American F-100 fighter. Most 707 aircraft manufactured since the early....

aerial view of a Boeing 707-320B
[417] Figure 13.6 - Boeing 707-320B airliner. [mfr]

....1960's, however, have been powered with a turbofan version of this engine. The Pratt & Whitney JT3D turbofan engine utilizes the same basic gas generator as the J-57 but has a front-mounted two-stage fan with a pressure ratio of about 1.8. The bypass ratio is 1.43, and the sea-level static thrust is about 19 000 pounds. The fan discharges through a short duct that appears somewhat similar to a NACA cowling of the type employed on many radial-type piston engines. The short duct can be seen in figure 13.6. Thrust reversers are employed to assist in stopping the aircraft on its landing rollout. Reverse thrust may also be used to increase the rate of descent. The aerodynamic efficiency of the 707-320B may be judged by the value of the maximum lift-drag ratio, which is estimated to be in the range from 19 to 19.5. This value of (L/D)max , is just slightly lower than the value of 20.0 given for the B-47 in table VI, primarily because of the lower aspect ratio of the wing employed on the 707.
The 707-320B's wing loading is a relatively high 111.6 pounds per square foot; however, the stalling speed is maintained at an acceptably low 121 miles per hour by the use of trailing-edge slotted flaps and leading-edge flaps. The lateral control system of the aircraft consists of a combination of spoilers and ailerons that are mixed in their use according to the speed regime in which the aircraft is flying. (See the section on high-lift systems in chapter 10.) The spoilers are also used for reducing the stopping distance of the aircraft on landing and for rapid descents in flight. Descent rates of as high as 15 000 feet per minute can be achieved by deployment of the spoilers and the use of reverse engine thrust.

top, side and front view drawing of a Boeing 707-320B
[418] Figure 13.7 - Three-view drawing of Boeing 707-320B airliner. [mfr]

The elevators and ailerons are aerodynamically balanced and are manually operated by aerodynamic servotabs. In this type of control system, the pilot's primary flight controls deflect tabs on the main control surfaces. The hinge moment of the control surface is altered by deflection of the tab, and, consequently, the floating angle of the surface is altered. This change in angle of the main surface provides the necessary control moments for the aircraft. The spoilers and rudder on the 707 aircraft are operated hydraulically. Small changes in longitudinal [419] trim are made with the use of trim tabs on the elevators. La changes in trim, such as those caused by flap deflection, are balanced, by adjusting the angle of the horizontal stabilizer. Movement of this surface is power actuated.
The gross weight of the Boeing 707-320B is 336000 pounds, nearly three times the weight of the Comet IA. The cabin can be configured to carry a mix of first-class and tourist-class passengers or an all-tourist arrangement. In the all-tourist configuration, 189 passengers can be accommodated. With a maximum payload of 53 900 pounds, the aircraft has a range, without reserves, of 6240 miles; with full fuel tanks and a payload of 33 350 pounds, the range is 7975 miles. With this range capability, the aircraft is capable of connecting many of the important population centers of the world. The aircraft has a maximum cruising speed of 593 miles per hour at 30 000 feet and a cost-economical cruising speed of 550 miles per hour at 35 000 feet; the corresponding cruising Mach numbers are 0.87 and 0.83, respectively. The takeoff field length on a standard day is a relatively long 10 000 feet, which can be directly related to the low thrust loading of 0.23 and the high wing loading of 111.6 pounds per square foot. (See chapter 3 of ref. 176.)
By any measure, the 707 series of aircraft must be ranked as one of the most successful transports ever produced. The present fleet of aircraft will no doubt fly on for many years in different parts of the world. Concluding this brief discussion of the Boeing 707 is the presentation in figure 13.8 of one of the Boeing 707 aircraft used by the President of the United States - perhaps one of the best-known aircraft in the world.
McDonnell Douglas DC-8 and Otber Four-Engine Transports
The second long-range, high-passenger-capacity transport that, along with the Boeing 707, initiated the jet revolution in air transportation was the McDonnell Douglas DC-8 (originally the Douglas DC-8). This aircraft was ordered by Pan American World Airlines in 1955, and first flight was made in 1958. The aircraft entered airline service in August 1959. The DC-8 was built in many different versions; one of the principal modifications incorporated in the aircraft was a stretched fuselage to provide increased passenger capacity. Over 550 DC-8 aircraft were built before production was terminated in 1972.
In most essential respects, the basic configuration of the McDonnell Douglas DC-8 is the same as that of the Boeing 707. Early versions of the two aircraft were virtually indistinguishable except to...

aerial view of B707 with presidential markings
[420] Figure 13.8 - Boeing 707 used ky the President of the United States. [mfr] [Original photo was in color, Chris Gamble, html editor]

....someone very familiar with them. There were, of' course, many differences in the detailed aerodynamic and structural design and in the systems employed on the aircraft. The McDonnell Douglas DC-8 Super 63 is shown in figures 13.9 and 13.10, and some of the characteristics of the aircraft are given in table VII. As compared with earlier versions of the DC-8, the fuselage of the Super 63 has been stretched by the addition of a 20-foot section ahead of the wing and a 17.8-foot section aft of the wing. Also, the wing span of the aircraft has been increased 6 feet over that of the original DC-8. The wing and engine locations are similar to those used on the 707 however, the aspect ratio and sweepback angle are slightly different. The main landing gear consists of two struts to which are mounted four-wheel bogies; the two rear wheels of each bogie can be put in a free swiveling mode to assist in making sharp turns on the ground. The main landing gear is mounted on the wing and retracts inward into the fuselage. The two-wheel nose gear retracts in a forward direction.

aerial view of a DC-8 Super 63
[421] Figure 13.9 - McDonnell Douglas DC-8 Super 63 Airliner . [mfr]


photo of a DC-8
Figure 13. 10 - Underneath view of McDonnell Douglas DC-8 Super 63 airliner. [mfr]

The aerodynamic efficiency of the DC-8 is indicated by the maximum value of the lift-drag ratio, which is estimated to be about 17.9. The value of (L/D)max, is lower for the DC-8 Super 63 than for the 707 because of the DC-8's longer fuselage and consequently increased ratio of wetted area to wing area. The relationship between wetted area, [422] wing area, and (L/D)max, is discussed in chapter 3 of reference 176. The loss in aerodynamic efficiency associated with the long fuselage is more than compensated by the increased passenger-carrying capacity and consequent reduction in direct operating costs per seat mile.
The wing is equipped with trailing-edge double-slotted flaps and slats over the inboard sections of the leading edge. These high-lift devices provide a lift coefficient that gives a stalling speed of 123 miles per hour at the maximum landing wing loading. The maximum landing wing loading is somewhat less than the value given in table VII, which is for maximum takeoff gross weight. The lateral control system consists of inboard and outboard ailerons that are connected by a torque tube that acts as a torsion spring. The inboard sections are power operated. The outboard sections only operate at the lower values of the dynamic pressure where they are needed. As the dynamic pressure increases, the aerodynamic resisting moment of the aileron becomes greater in relation to the torque that can be transmitted through the torsion bar; hence, the aileron deflection is reduced. The amount of deflection of the outboard aileron varies smoothly with variation in dynamic pressure and, therefore, provides the desired variation of aerodynamic control moment with speed and altitude. The rudder is also power operated. Both the rudder and the ailerons have a manual reversion mode in the form of aerodynamic servotabs. Elevator control is manual and makes use of an aerodynamic servotab. The variable-incidence horizontal tail is power operated and is used for longitudinal trim. Wing spoilers are automatically deployed on landing by nose-wheel contact with the runway.
The gross weight of the DC-8 Super 63 is 358 000 pounds; in an all-tourist configuration, the aircraft seats 259 passengers in a 6-abreast arrangement. With a maximum payload of 67 735 pounds, the range is 4882 miles; and with maximum fuel, a payload of 37 101 pounds can be carried for a distance of 6997 miles. As can be seen in table VII, the cruising speeds of the DC-8 are about the same as those of the 707.
Recently, a number of DC-8 aircraft have been retrofitted with modern CFM-56 high-bypass-ratio turbofan engines. Manufactured by GE Snecma, these engines have 22 000 pounds thrust and a bypass ratio of 6.0. The modified aircraft are designated DC-8-71, DC-8-72, and DC-8-73 depending upon the 60-series aircraft from which they were derived. Improved performance and economy together with reduced noise are advantages resulting from installation of the new engines. Characteristics of these modified aircraft may be found in reference 150. With retrofit of the new engines, the DC-8 has indeed received a new lease on life. Along with the 707, the DC-8 has indeed received [423] a new lease on life. Along with the 707, the DC-8 has been a workhorse of great productivity for many years; and although out of production, it will continue to be operated for a long time to come.
Two other aircraft of this first generation of large jet transports are nearly the same in configuration as the Boeing 707 a the McDonnell Douglas DC-8. In fact, when seen at the airport, the Convair 880 and 990 are often confused with one or the other of the more familiar 707 or DC-8 aircraft. The Convair 880 first flew in 1959, and the first flight of the more advanced Convair 990 was in 1961. The maximum cruising Mach number of the 990 is 0.89, which is the highest of any of the subsonic jet transports. The high cruising Mach number of the aircraft is due in part to the Whitcomb bumps on the trailing edge of the wing. The two pods mounted on each wing at the trailing edge make the aircraft readily identifiable and are used to increase the critical Mach number as well as to augment fuel volume.
Both the 880 and the 990 are somewhat smaller and lighter in weight than are the 707 and the DC-8. The gross weight of the 880 is 192 700 pounds and that of the 990 is 253 000 pounds. The range of neither aircraft is really intercontinental, and the payloads are lower than those of the Boeing and Douglas aircraft. For these reasons, perhaps, and because both aircraft became available to the airlines somewhat later than the 707 and the DC-8, only a relatively small number of Convair jet transports were built. Total production of the 880 was 65, and 37 examples of the 990 were built. At this time, neither type is used in scheduled airline service in the United States.
Sud-Aviation Caravelle
The French Sud-Aviation Caravelle was the first really successful short-range jet transport to be developed in the western world. First flight of the prototype took place in May 1955, and the aircraft entered airline service in Europe in April 1959. As with most successful jet transports, the Caravelle was produced in a number of versions; a total of 280 aircraft of all versions were produced before production was terminated in the early 1970's. Many are still in operation in various parts of the world. A Sud-Aviation Caravelle model VI is depicted in figure 13.11. The aircraft shown carries the markings of United Airlines, which operated a fleet of 20 Caravelles for a number of years. Characteristics of the Caravelle Vl-R are given in table VII.
The primary technical significance of the Caravelle was its pioneering use of an entirely new and innovative approach in the integration....

aerial view of a Caravelle in flight
[424] Figure 13. 11 - Sud-Aviation Caravelle short-range airliner. [United Air Lines]

...of the engines and airframe. Figure 13.11 shows that one of the two engines is mounted on either side at the aft end of the fuselage. This engine arrangement set the pattern for many future jet transport aircraft of two-, three-, and four-engine design. When the engine location proposed for the Caravelle was first made known, many engineers expressed doubts about the practicality of such an arrangement. For example, questions were raised about the operation of the engines in the wake of the wing as the aircraft approached a stalled condition, or the effect on engine operation of large angles of sideslip. The aft-engine location, however, has proved to be highly workable. Some advantages and disadvantages of this aft-engine arrangement are as follows:
  1. The short lateral distance between the engines results in relatively small yawing moments following the loss of an engine. The required vertical-tail size is accordingly reduced as compared with that of an aircraft with wing-mounted engines, such as the Boeing 707.
  2. The rear location of the engines results in a relatively low engine-noise level through most of the cabin.
  3. Removal of the engines from the wing results in a small [425] increase in the maximum lift coefficient and elimination of wing-pylon-nacelle interference drag. The integration of engines at the aft end of the fuselage, however, requires careful design in order to minimize interference drag in this area.
  4. The location of the engines at the aft end of the fuselage, as compared with the underwing position, reduces the problem of interference between the engines and the ground, a problem that becomes particularly important as the size of the aircraft is reduced.
  5.   Mounting the engines on either side of the aft portion of the fuselage prevents location of the horizontal tail in a low position. In the case of the Caravelle and a number of other aircraft, the tail is mounted at some location between the root and tip of the vertical-tail surface. Other aircraft utilize the T-tail position in which the horizontal tail is mounted at the tip of the vertical surface. The use of a high tail position offers several advantages: If the vertical tail is swept back, the horizontal-tail moment arm is increased as the tail is moved toward the tip of the vertical surface. The horizontal-tail size, and hence the weight of the tail, may therefore be reduced for a given level of static longitudinal stability. In the T-tail arrangement, the horizontal tail acts as an end plate and reduces the required size of the vertical surface for a given level of static directional stability. Again, a reduction in tail weight may be realized. Structural and aeroelastic problems may, however, cause some increases in weight of the vertical tail. Whether the overall empennage weight is reduced by the use of the T-tail arrangement, as compared with the more conventional low tail position, however, is debatable and depends on the detailed design requirements of the particular aircraft.
  6. The high tail position also has some disadvantages. A brief qualitative discussion of the influence of horizontal-tail position on the static longitudinal stability of swept-wing aircraft is given in chapter 10. As indicated there, certain inherent aerodynamic problems are encountered in the design of an aircraft with a high tail location. Careful attention to the detail design of such a configuration is required in order to achieve reasonably acceptable longitudinal aerodynamic characteristics. Lack of proper care in the design process can [426] result in an aircraft with highly undesirable longitudinal aerodynamic characteristics.
  7. The rear engine location results in large concentrated weights that are a long distance behind the aircraft center of gravity. This arrangement, therefore, causes some problems in balancing the aircraft in certain loading configurations. However, these balance problems have been overcome in a large number of highly successful aircraft that employ the aft-engine arrangement.
Other than the engine arrangement, the Caravelle's configuration is conventional, with the 20° swept wing of aspect ratio 8 mounted in the low position on the fuselage. Two large fences can be seen on each wing in figure 13.11. These fences are intended to control the spanwise flow of the boundary layer on the swept wing and thus improve the stalling characteristics of the aircraft. The wing-pylon-engine arrangement on the 707-type configuration serves the same purpose. The high-lift system consists of trailing-edge Fowler flaps. Large airbrakes are mounted ahead of the flaps on the top and bottom surfaces of the wing. All the flying controls are hydraulically actuated. The aircraft is powered with two Rolls-Royce Avon turbojet engines of 12 000 pounds of sea-level static thrust.
A study of the characteristics of the Caravelle given in table VII indicates that the gross weight of the aircraft is a relatively light 114 640 pounds, even lighter than the Comet, and that it is capable of a range of 1829 miles with a maximum payload of 16 800 pounds. Eighty passengers can be accommodated in a five-abreast configuration. The cost-economical cruising speed of 488 miles per hour at 35 000 feet is somewhat lower than the 550 miles per hour given in the table for the Boeing 707. The lower cruising speed of the Caravelle would be expected in a short-range airplane and explains the low sweepback angle of the wing. The relatively short landing and takeoff field lengths indicate that it was designed to operate from the many small airports appropriate to a short - or medium-range airliner. Again, a wing of low sweepback angle is desirable. A highly successful shortrange jet transport, the Caravelle's place in the history of aeronautical development is secure as a result of its pioneering use of the aft-fuselage-engine location.