The Boeing 720

DESIGN ORIGINS: The Boeing 720, closely resembling its 707 predecessor, but intended for slightly lower- capacity, intermediate-range routes, was the result of three concurrent commercial aviation forces. 1). Competition and hence fear of market share loss. 2). A realization that passenger acceptance of jet travel led to its demand on all routes, regardless of length. […]

The Boeing 720

DESIGN ORIGINS:

The Boeing 720, closely resembling its 707 predecessor, but intended for slightly lower- capacity, intermediate-range routes, was the result of three concurrent commercial aviation forces.

1). Competition and hence fear of market share loss.

2). A realization that passenger acceptance of jet travel led to its demand on all routes, regardless of length.

3). The refusal of its launch customer to bridge the piston and pure-jet gap with the faster, but still propeller-driven turboprop powerplant, despite the fact that a US aircraft manufacturer offered a suitable design.

United Airlines, that launch customer, originally foresaw a mixed fleet, consisting of piston Douglas DC-3s, Convair CV-340s, and Douglas DC-7s for short- to medium-range service and pure-jet Douglas DC-8s for its higher-capacity, longer-range, transcontinental and Hawaiian routes. But its vision, at the dawn of the jet age, proved too nearsighted: passengers did not want to view propellers through their windows and United realized that their use on some sectors while that of pure-turbines on others would not have provided a consistent ravel experience.

Replacements, at least for the DC-7s that had previously operated form coast-to-coast until they themselves were usurped by superior performance DC-8-10s, were initially viewed as Convair CV-990s, a highly-swept, quad-engine jetliner that sported four upper wing surface, area-rule conforming conical fairings that gave it ultimate speed and promised unsurpassable competition to carriers that failed to order it.

Boeing, fearing lost market share, responded with its own intermediate-range 720 based upon its 707. Of reduced capacity, but offering higher speed to compete with the CV-990, it was seen as operating one-stop transcontinental sectors, while its 707-120s would do so without the intermediate landing and its slightly larger-capacity, but longer-range 707-320s would operate intercontinentally, such as between North America and Europe.

Lockheed, along with Vickers in the UK with its Viscount, viewed the turboprop, which was itself turbine powered, but turned its propellers by means of reduction gear, as the intermediate technological evolution between the piston and pure-jet, and US L-188 Electra operators American, Braniff, Eastern, and Western apparently agreed.

Although the type’s figures were lower than the Convair 990’s, it could still accommodate up to 100 passengers and connect cities separated by medium distances at some 400 mph, a significant increase over the speed of the piston-powered Lockheed Constellation and Douglas DC-6 and DC-7.

Aside from the Convair 990, which itself evolved from the slower Convair 880, and Boeing’s potential 720 counterpart to it, no other intermediate-range jetliners existed. While Douglas proposed a scaled-down, four-engine DC-8 derivative designated DC-9, it never proceeded beyond the concept stage.

Of the two contenders, United felt that the Convair design was too narrow, only able to accommodate five-abreast coach seating.

Announced in July of 1957, it then bore the “717” model number, or the same one shared by the military KC-135. Closely resembling its 707-120, it was briefly designated 707-020 until it was shortened to its definitive “720” to secure United’s all-important launch order. Since it considered the DC-8-10 its mainline aircraft type, it refused to operate anything that bore even a numerical 707 similarity to what was considered Douglas’s competition.

Although it differed in its intended route application and was not designed to serve any sector greater than 1,500 miles, it was otherwise little more than a mirror image of its larger brother. Its differences, including thinner skin gauges, lighter undercarriage forgings, reduced fuel tankage, and wing aerodynamic modifications, were only apparent upon closer inspection.

DESIGN FEATURES:

Sharing the same across-section as the 707-120, -138, -220, -320, and -420, the Boeing 720 introduced a fuselage 7.9-foot shorter than its original 707-120’s, giving it a 136.2-foot overall length.

Its 35-degree sweptback wings, with a 130.10-foot span and a 2,433-square-foot area, deviated from the basic 707’s airfoil by incorporating inboard, three-section leading edge Krueger flaps for the first time. Resulting in virtual full-span, high-lift provision only interrupted by the two engine pylons, they collectively increased camber and low-speed lift to augment lower approach and touchdown velocities and thus reduce runway length requirements. These out- and upward-extending Krueger panels were only later applied to larger 707s on the production line.

Each wing also incorporated dual spoiler/speedbrake panels; in- and outboard tabbed ailerons for, respectively high- and low-speed operation; and two-section, double-slotted, trailing edge Fowler flaps that increased both wing camber and area. All devices ere hydraulically actuated.

The 720’s wing was almost exclusively the design feature that provided Convair 990 competitive performance, the very aircraft that served as its catalyst.

Boeing had already experienced such a design disadvantage at the dawn of the jet age. During the first generation “jet buying spree,” it initially offered superior performance over existing pistonliners with its 707-derived 367-80 prototype. But Douglas, with its later-appearing DC-8 counterpart, introduced a wider cabin for a revenue-increasing six-abreast configuration. Devoid of a prototype itself, it could make changes to cater to potential carriers’ needs far more easily and cheaply. Without its decision to increase the 707’s width, Boeing would most likely have lost orders and given the race to Douglas.

It now faced a similar situation with the Convair Division of General Dynamics. Its intermediate-range, albeit five-abreast CV-880 and the CV-990 it was based upon, promised speeds beyond those of either its 707 or Douglas’s DC-8, and TWA, through often-eccentric Howard Hughes’ influence, viewed this performance as an advantage over all existing aircraft, regardless of powerplant type. Similar-minded airlines, without alternative or choice, would have flocked to Convair.

Without its own comparable design, Boeing would have once again conceded this market segment to another aircraft manufacturer.

To achieve its goal, yet reduce development time and cost by basing it on the 707, it installed a leading edge “glove” between the fuselage and the inner engine pylon. Creating a forward extension during this segment of the airfoil, it increased the sweep between these two points and gave the wing’s leading edge a compound one in the process.

The resultant reduction in the thickness-chord ratio facilitated high cruise Mach numbers that made it competitive with the Convair 990, whose own wing sweepback was 39 degrees.

The 720’s reduced, 11,835-US gallon fuel capacity decreased its range, but also its structural weight.

Power was provided by four cowling-encased, pylon-mounted, lighter-weight, thrust reverser-equipped, 12,000 thrust-pound Pratt and Whitney JT3C-7 “straight-pipe” turbojets, which were later uprated to 12,500 pounds of thrust. 13,000 thrust-pound JT3C-12s could also be retrofitted.

Compared to the 707’s outboard engine pylons, the 720’s were straight in configuration. Engine weight reduction was primarily achieved by eliminating the 707’120’s original water injection system.

The aircraft featured an elevator-provisioned horizontal tailplane, and rudder-provisioned vertical fin, both of which were hydraulically operated. Although its overall height was initially 38.5 feet, it was later increased to 41.7 after the fin itself was extended, a modification introduced by the Rolls Royce Conway powered 707-420. An antenna common to all 707 versions was optionally available on the 720.

Its tricycle, hydraulically retractable undercarriage, consisting of a dual-wheeled nose unit and two quad-wheel main ones, gave it a 21.9-foot track, a 50.8-foot wheel base, and a 101-foot turning radius.

Entrance was provided by two port, 72- by 34-inch passenger doors, one located ahead of and the other behind, the wing, and they opposed the starboard galley servicing doors. Unlike the larger 707, there was only a single overwing emergency exit on each side.

Cockpit vision was provided by two forward, four side, and our eyebrow windows. The standard, three-person crew consisted of captain, first officer, and flight engineer, but there was seating for two supplemental crew members or observes.

Cockpit configuration, with primary instruments, such as the altimeter, the artificial horizon, and the vertical speed indicator, were, like those of the short-fuselage 707-120, duplicated before the pilot and copilot. Engine instruments, including revolutions-per-minute, fuel flow, and exhaust gas temperature, were displayed on the center panel. Forward and reverse thrust levers and the flap selection handle were mounted on the center pedestal. And systems and components were identical to those of the 707.

The passenger cabin, with a 96.6-foot length, an 11.8-foot maximum width, and a 7.7-foot height, offered a 1,003-square-foot floor area and a 7,030-cubic-foot volume. Forward and aft galley, lavatory, and garment closet installations, along with continuous seat tracks, facilitated four-, five-, and six-abreast configurations, numerous pitches, and curtained class or section divisions.

A single-class, five-abreast, two-three arrangement facilitated a 110-passenger capacity, but this total increased to 140 at a 32-inch pitch. Both incorporated forward and aft two-unit galleys and up to four lavatories. Minor modifications enabled 149 to be carried.

Thirty- and 29-inch pitches respectively took accommodation to 165 and 170. Exit-limited capacity, requiring reductions in galley and lavatory facilities, was 181, or 32 more than the Convair 990’s 149 with its narrower cabin.

Baggage, cargo, and mail were carried in two lower deck heated, lighted, and pressurized holds, which had a collective 1,390-cubic-foot volume and wee accessed by two 48- by 50-inch inward-opening hatches.

FIRST FLIGHT:

Powered by 12,000 thrust-pound JT3C-7 turbojets and featuring an initial, 202,000-pound gross weight, the first Boeing 720 appeared not unlike its 707 predecessor. But a closer look revealed tis differences, including its shorter fuselage, only a dozen or so passenger windows ahead of the wing, the inboard wing leading edge bulge and its compound sweep, and its modified tailplane. Because of these changes and to assuage United’s concerns, it became the first commercial model not to feature the “X-0-X” numerical sequence.

Representing the initial production version, it respectively had 149,000- and 213,000-pound maximum zero-fuel and takeoff weights, resulting in an 88-pound-per-square-foot wing loading. Its landing weight was 175,000 pounds.

Taking to the skies for the first time on November 23, 1959, it launched the type’s flight test program, during which speeds in excess of Mach 0.95 (650 mph) were reached, in the process demonstrating its competitive Convair 990 performance and its 707 differentiating characteristics.

It was awarded FAA Approved Type Certificate 4A-28 the following year, on June 30.

BOEING 720 OPERATORS:

United, which placed its launch order for 11 Boeing 720s in November of 1957, progressively increased it-to 18 two years later, in June, and finally to 29.

Its aircraft featured a Red Carpet lounge that was configured with two facing pairs of seats separated by a table and located in the forward cabin, immediately behind the divider separating it from the forward, left door. Its otherwise dual-class interiors consisted of four-abreast, two-two, first and six-abreast, three-three, coach sections. It also experimentally operated the aircraft in a single-class, five-abreast configuration, assessing fares and offering service that were somewhere between the two traditional classes.

Following delivery of its first 720-022 on April 30, 1960, which sported the smaller vertical tail and the under-fin one, but lacked the antenna, it inaugurated the type into scheduled service three months later, on July 5, between Chicago and Los Angeles with an intermediate stop in Denver. Intended for intermediate range routes, such as those between Chicago and New York and Denver and San Francisco, it served many midwestern destinations, enabling it to offer its passengers the same speed and comfort as that of its DC-8s on these shorter sectors.

American, for which the Convair 990 was designed, ironically also ordered its Boeing competitor, inaugurating the type, as had United, on the Chicago-Denver-Los Angeles route, on July 31, 1960. Lacking both the ventral fin and the vertical tail installed antenna, its aircraft were considered the same as its “707 Astrojets,” the only distinction between them being the inclusion of a zero in their numerical designation-that is, 707-023 as opposed to just 707-23 for the original aircraft.

Eastern, another major 720 operator, received the first of 15 ordered 720s in September of 1961, followed by a further 11 by the end of the year and the remaining three in early 1963, after it found itself at a speed disadvantage. Initially plying the same routes as Delta and National with turboprop L-188 Electras, it had relinquished its first six DC-8-10 delivery positions to wait for higher powered equipment, only acquiring the 720s as a stopgap measure, but found it just as underpowered and was therefore only able to distances as long as those between New York and Houston with them, even with water injection provision.

Designated 720-025s, they were powered by the 13,000 thrust-pound JT3C-12 engines and were high-density configured for 165 single-class passengers, which required the installation of two additional overwing emergency exits for evacuation purposes.

Pacific Northern operated two 720-062s on its Seattle-Anchorage route, commencing service with them in May of 1962. Powered by 12,000 thrust-pound JT3C-7s, they offered identical performance as that of the 720-022.

A higher gross weight variant, the 720-027, but still powered by JT3C-7s, featured a 218,500-pound gross weight and required varying runway lengths according to operating conditions-that is, 7,400 feet at this weight, 8,300 feet with a 15-degree Celsius temperature increase, and 9,800 feet at a 5,000-foot elevation.

Weights included 36, 960 pounds (maximum payload), 105,040 pounds (empty), and 149,000 (zero-fuel). The variant’s speeds entailed 465 knots (best-cruise at between 35,000 and 40,000 feet), 478 knots (long-range cruise at between 30,000 and 35,000 feet), and 521 knots (maximum cruise at 25,000 feet). Range decreased from 3,700 nautical miles with a 20,820-pound payload and maximum fuel, but without reserves, to 2,760 nautical miles with maximum payload. The type’s maximum landing weight, of 175,000 pounds, required a 5,750-foot runway at a 126-knot approach speed.

Braniff, which operated four 720-027s, received the first of them in February of 1961. One aircraft, construction number 18066, had a maximum takeoff weight of 202.000 pounds.

A still-heavier version was the 720-048, three of which were acquired by Aerlinte Eireann Irish Airlines (later Aer Lingus), itself formed to operate North Atlantic routes. The first diminutive Boeing, which was delivered in October of 1960 and powered by 12,000 thrust-pound JT3C-7s, was inaugurated into service two months later, on December 14, from Ireland to North America. The type featured a maximum takeoff weight of 229,000 pounds, which gave it a coincident 94-pound-per-squar-foot wing loading, and required a 147-knot speed on a 9,400-foot runway. This length progressively increased-to 10,800 feet with a 15-degree Celsius temperature increase at a 226,000-pound gross weight and to 12,240 feet at a 5,000-foot elevation at a 220,000-ppound gross weight.

The variant’s weights included a 28,200-pound maximum payload, a 110,800-pound empty one, and a 149,000-pound zero-fuel one.

Its still-air range with its maximum fuel and an 11,630-pound payload was 4,550 nautical miles, decreasing to 3,680 with its maximum payload.

Aerlinte Eireann later sold one of its 720-048s to Braniff.

The only other Boeing 720 operator was the Federal Aviation Administration, which purchased a single JT3C-7 powered 720-061 for flight-testing purposes. After a long life of rigorous flying, it was finally-and purposefully–crashed in the desert on November 10, 1984 to provide landing impact and passenger safety data.

Sixty-five non-fanned Boeing 720s, whose customer identification suffixes ran from -020 to -099, were, in the end, produced.

THE BOEING 720B:

Like the 707 and Douglas’s DC-8, the 720 straddled the line between the pure-turbine and the turbofan engine, enabling Boeing to significantly increase performance and range, yet reduce its noise footprint and fuel consumption with application of the next-generation powerplant. By installing a front fan to the baseline JT3C engine, Pratt and Whitney was able to produce the JT3D, which, with a one-to-one bypass ratio, permitted half of the air to bypass or circumvent the hot, core section, producing cooler thrust that exited the exhaust nozzle at lower and hence quieter velocities.

The resultant 720B, differing little from the original, non-fanned 720, nevertheless introduced a greater, 14,790-US gallon fuel capacity.

The initial production version, a 720-023B intended for American Airlines (and therefore designated as 707-023B by that carrier), was powered by four 17,000 thrust-pound JT3D-1 turbofans, facilitating higher weights. The empty weight, having increased because of the heavier engines and the increased fuel tankage, for instance, was 109,600 pounds. Payload and takeoff weights respectively became 37,400 and 222,000 pounds.

Other parameters had also increased-to a 91-pound-per-square-foot wing loading and speeds, from a 470-knot long-range cruise at 35,000 to 40,000 feet to a 540-knot maximum cruise at 25,000 feet.

The 720B was the only jetliner able to immediately climb to and cruise at 40,000 feet with its maximum payload, avoiding intermittent fuel burn step-climb profiles, both because of its uprated turbofans and a wingspan that had been designed for the larger, heavier, longer-range 707-120, a distinction it maintained until the 747 first flew in 1969. The 707 itself was unable to achieve this performance.

Still-air range with a maximum fuel uplift, but without reserves, remained the same as that of the 720-048’s 4,550 nautical miles, although this calculation was made with double the payload or 20,600 pounds.

Approaching at 134 knots, the 720B required a 5,750-foot runway at its unchanged, 175,000-pound maximum landing weight.

First flying in pre-production form on June 22, 1960, the aircraft was little more than a 707-120 with the 720’s leading edge glove and three-section Kreuger flap additions, now powered by JT3D turbofans. But the first purposefully built 720B, with 17,000 thrust-pound JT3D-1s, took to the sky four months later, on October 6, and was granted an amendment to the original version’s Approved Type Certificate 4A-28 on March 3, 1961. Although capacity was initially restricted to 149 passengers, it was later increased to 165 with the installation of two additional overwing emergency exits.

BOEING 720B OPERATORS:

American Airlines, the type’s launch customer, had ordered 15 720Bs and inaugurated the type into service on March 12, 1961, soon converting all ten 720-023s (707-023s) to turbofan-powered 720-023Bs (707-023Bs). Now under-fin-equipped, the 25 aircraft made up a significant portion of its fleet.

Continental complemented its Boeing fleet by ordering eight 720-024Bs for its shorter-range, lower-capacity routes in the spring of 1962, operating them alongside its existing 707-120s. These aircraft had a higher gross weight (230,000 pounds) than American’s 720-023Bs with a corresponding increase in maximum wing loading to 94 pounds per square foot, although some later 720-023Bs also offered this weight

The 720-030B, which had the same 229,000-pound takeoff weight, was chosen by Lufthansa, which ordered four in February of 1960 for operation on its inter-European and medium-range Middle Eastern routes, complementing its larger intercontinental 707-420s with which it had inaugurated nonstop trans-Atlantic service from Frankfurt to New York on March 17, 1960. The first 720-030B was delivered in April of 1961 and Lufthansa eventually operated eight of the type, uniquely employing some aircraft on its South Atlantic sectors, on which it inaugurated the type into service on May 20, 1961.

Now turbofan-fitted, the 720B was able to offer ranges comparable to the larger Boeings, but with lower seating capacities. It was thus ideal for long, thin routes insufficiently dense for the 707-320 and -420.

The Boeing 720B was Western’s first pure-jet equipment and was intended for the longest routes presently in its system, such as those to Minneapolis/St. Paul and Mexico City from Los Angeles and a hoped-for nonstop Los Angeles-Honolulu route award. The first of seven 720-047Bs, which had a maximum zero-fuel weight of156,000 pounds, was delivered in April of 1961 on a lease basis.

The definitive 720 version was powered by four 18,000 thrust-pound JT3D-3 turbofans and offered a 234,000-pound gross weight, for which it needed a 6,200-foot, sea level runway. This increased to 6,900 feet during a 15-degree Celsius temperature increase and to 8,200 feet at a 5,000-foot elevation. The heavier gross weight variant employed a wing loading of 96 pounds per square foot, and it featured maximum payload, empty, and zero-fuel weights of, respectively, 43,000, 113,000, and 156,000 pounds.

Its cruise speed at 25,000 feet became 532 knots, while its best-cost cruise speed at 30,000 to 35,000 feet remained the same at 483 knots and its long-range cruise speed at 35,000 to 40,000 feet slightly decreased–to 469 knots. Still-air range with its maximum fuel and 11,930 pounds of payload was the greatest of any previous variant-5,373 nautical miles-yet its still-air range with its maximum payload decreased to 3,608 nautical miles. Maximum landing weight remained the same at 175,000 pounds and required a 6,350-foot runway, which was approached at 132 knots.

Several versions were so configured. The first of these, a 720-040B, was purchased by Pakistan International Airlines, which took delivery of the first of three aircraft in December of 1961 and operated them on medium- and long-range, low-density routes from Karachi and Lahore. The aircraft were powered by JT3D-1s.

Another operator of the type was Western, which leased seven JT3D-1-powered 720-047Bs from Boeing in April of 1961, eventually operating 22.

Still another high gross weight variant was the 720-051B. Of the 18 produced, Northwest Orient purchased 13, of which the first JT3D-1-powered aircraft was delivered in June of 1961; TWA leased four; and the Federal Aviation Administration acquired the remaining one for its flight-testing program.

The 720B proved attractive to foreign carriers, such as El Al, Ethiopian, and Saudia. It was also well-suited to airlines that had not yet purchased pure-jet equipment or for charter and inclusive-tour operators because of its economical medium-range capability. Similar in performance to the VC10, it was able to operate from high-temperature and -elevation fields due to its high power-to-weight ratio.

Eighty-nine Boeing 720Bs were produced, while many non-fanned 720s were converted to this standard. Some of these were as follows.

PROGRAM PERSPECTIVE:

Although the Boeing 720’s collective production, whether it had been in the original pure-turbine or later turbofan version, was only about a third greater than that of the Convair 880’s and 990’s, it served several purposes as a design.

It was, first and foremost, the result of the second US aircraft manufacturer competition and was initiated to retain market share, particularly in the face of the threat posed by the higher performance CV-990.

Because more suitable powerplants were then not available, it retained the four pylon-mounted configuration that had become standard with the first-generation of jets, as evidenced by the 707, the DC-8, the CV-880/-990, and the 720 itself, until Vickers in the UK and Ilyushin in the USSR deviated from it with, respectively their t-tailed VC10s and Il-62s.

This scaling down of existing designs to produce lower-capacity, intermediate-range versions was a strategy employed by de Havilland with its medium-range Comet 4B derivative of its initial, longer-range Comet 4; Tupolev with its Tu-124 version of its larger Tu-104; and was even explored by Douglas with its proposed quad-engine shrink of its DC-8 then designated DC-9.

These strategies resulted in several advantages.

1). They resulted in reduced development periods and costs.

2). They facilitated earlier market entry.

3). In the case of both the Comet 4B and the 720, they featured parts and cockpit crew commonalities.

4). Their operating costs were lower than those of their larger, longer-range counterparts.

Both the 720 and the Convair 880/990 entered service before new engine technology, particularly in the form of the Pratt and Whitney JT8D turbofan, had become available and facilitated engine number reductions and design configuration changes, including aft-mounted powerplants, t-tails, integral air stairs, auxiliary power units, much lower capacities and ranges, significantly improved field performance, and reduced operating costs, as would later occur with the Hawker Siddeley HS.121 Trident, Boeing’s own 727, the Tupolev Tu-154, and the Yakovlev Yak-42.

Nevertheless, because of its cost, capacity, and range reductions, the 720, whose advantage over Convair’s comparable aircraft was the existing Boeing market base and cost-reducing 707 commonalities, proved attractive to many first-time jet operators, especially with its lower acquisition price. For this reason, it was often referred to as the “bargain basement jet.”

Finally, the 720, along with the Convair series, demonstrated that subsonic speeds had essentially been established by the first-generation quad-jets; and, despite their attempts to increase competition by raising their Mach numbers within that envelope, their higher fuel burns and only marginally shorter block times provided inadequate tradeoffs for them.

Nevertheless, along with the original 707-120, the intercontinental 707-320, the “hot-and-high” 707-220 for Braniff, the longer-range 707-138 for Qantas, and the Rolls Royce-powered 707-420 for carriers such as BOAC, the 720 completed Boeing’s first-generation quad-jet family.

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