ch6-2

Quest for Performance: The Evolution of Modern Aircraft


Part I: THE AGE OF PROPELLERS


Chapter 6: Design Maturity, 1945-80


Transport Aircraft



[137] Two families of large, long-range, propeller-driven transports dominated U.S. airlines, as well as many foreign airlines, until the jet transport began to appear in significant numbers toward the end of the 1950's. These families of aircraft, which served on both long-range domestic and international routes, were the Douglas DC-6 and DC-7 series and the Lockheed Constellation series. Both were derived from aircraft developed during World War II; they had four supercharged [138] engines and pressurized cabins, and both series underwent large increases in size, power, and weight during their development history.

Representative of the long-range, four-engine transport is the Lockheed L. 1049G Super Constellation illustrated in figure 6. 1; characteristics of the aircraft are given in table III (appendix A). The prototype Constellation, known by its USAAF designation of C-69, first flew on January 9, 1943, and the model L. 1049G first flew on December 12, 1954. The total number of all models of the Constellation constructed was 856.

The Lockheed L.1049G was powered by four Wright turbocompound engines of about 3250 horsepower each. The Wright 3350 turbocompound engine employed a two-speed gear-driven supercharger and, in addition, was equipped with three exhaust-driven turbines. The three turbines were geared to a single shaft that in turn was hydraulically coupled to the engine crankshaft. Each turbine was driven by the exhaust of six cylinders. About 15 percent of the total power of the engine was obtained from reclamation of exhaust gas energy. The specific fuel consumption was probably the lowest ever achieved in a reciprocating aircraft engine.

The gross weight of the aircraft was 133 000 pounds, which was more than twice that of the Boeing B-17 "heavy" bomber of World War II fame. The wing loading was 80.6 pounds per square foot, and....




Figure 6.1 - Lockheed 1049G Super Constellation 91-passenger four-engine airliner; 1954. [mfr]



[139] ....the corresponding stalling speed was 100 miles per hour. The wings employed very powerful Fowler-type extensible slotted flaps to maintain the landing speed within acceptable limits. The landing gear was of the tricycle type that was standard on most post-World War II transports. The maximum speed of the aircraft was 352 miles per hour, and the normal cruising speed was 331 miles per hour at 23 000 feet. The pressurized cabin was capable of seating 71 first-class passengers or 91 coach passengers. Some versions of the aircraft were capable of carrying an acceptable payload nonstop from the east coast of the United States to the west coast. The zero-lift drag coefficient of 0.0211 and the maximum lift-drag ratio of 16 indicate a highly refined and efficient aerodynamic design.

Today, many Constellations and their Douglas counterparts are in operation in nonscheduled activities in different parts of the world. The use of these aircraft in long-range scheduled operations, however, terminated in this country during the 1960's following the introduction of the high-performance jet-powered transport.

The turbopropeller, or turboprop engine, is basically derived by gearing a conventional propeller to the shaft of a gas generator composed of a compressor, burner, and turbine. The turboprop engine may therefore be thought of as a turbojet engine that transmits power to the air by means of a propeller rather than through the jet exhaust. The turboprop engine is light and relatively simple as compared with the large high-power reciprocating engines. For example, a modern turboprop engine may develop between 2 and 3 horsepower per pound of weight, as compared with a maximum of about 1 horsepower per pound for a reciprocating engine, and has been made in sizes of up to 15 000 horsepower. The specific fuel consumption of the turboprop engine, however, is somewhat higher than that of the best reciprocating engines. The turboprop engine has been used in a number of highly successful transport aircraft and is still in fairly widespread use, particularly for short-haul, commuter-type transports.

The first civil airliner to be equipped with turboprop engines was the Vickers Viscount depicted in figure 6.2. The specifications of the Viscount 700 series are given in table III. The aircraft employed four Rolls-Royce Dart engines of 1600 horsepower each and had a gross weight of about 60 000 pounds. Depending upon the configuration, 40 to 59 passengers could be carried in the pressurized cabin. The cruising speed of the Viscount was 334 miles per hour at 25 000 feet. The aircraft employed double-slotted flaps and was equipped with a tricycle landing gear. The Viscount made its first flight in July 1948 and ....




[140] Figure 6.2 - Vickers Viscount 810 40-passenger turboprop airliner,- 1948. [Peter C.Boisseau]



....subsequently was used by airlines all over the world. A total of 441 Viscounts were built and many are still in use.

Two turboprop aircraft of much larger size were constructed in the United Kingdom. These were the Vickers Vanguard, with a gross weight of 146 500 pounds, and the Bristol Britannia, with a gross weight of 185 000 pounds. Many types of turboprop transport aircraft have been designed and built in Russia, as well. The largest passenger carrying turboprop ever built was the Tupolev Tu-114. This aircraft has a gross weight of 377 000 pounds and is equipped with four 14 795 equivalent shaft horsepower turboprop engines. Each of these engines drives two counterrotating propellers. The wings are sweptback, which is unusual for propeller-driven aircraft; the amount of sweep is 34°. The aircraft carries 220 passengers and cruises at a speed of 478 miles per hour at an altitude of 29 500 feet. The Tu-114 is no longer in airline use, but a version known as the Bear is employed by the Soviet military forces as a reconnaissance aircraft. The Lockheed Electra is the only large turboprop airliner to be developed in the United States. Although the Electra was an efficient high-performance aircraft, it was never produced in large numbers because it was introduced at about the same time as the Boeing 707 jet airliner and could not compete with this aircraft. A few Electras are still in service with the scheduled airlines, and a number are employed in nonscheduled activities. The naval version of the aircraft, known as the P-3 Orion, is employed by the U.S. Navy for antisubmarine patrol work.

[141] A number of highly successful turboprop aircraft have been developed for use as cargo carriers. The largest of these aircraft is the Russian Antonov AN-22, which weighs over 550 000 pounds and is equipped with four 15 000-horsepower engines. The Lockheed C-130 is perhaps the best known of the turbo prop-powered cargo aircraft and the one that has been produced in the greatest numbers. The C-130 is used by all branches of the United States military forces and by the military forces of over 20 foreign governments. A commercial cargo version of the aircraft is also available. The first production contract for the aircraft was placed in 1952; over 1500 models of the C-130 have been built, and the aircraft is still in production.

A Lockheed C-130 is shown in figure 6.3, and specifications are given in table III. Many variations of the C-130 have been produced, and engines of slightly different power ratings have been employed. The specifications in table III are for the C-130E. The aircraft has an unswept wing mounted in the high position at the top of the fuselage and is equipped with four Allison T-56 turboprop engines of 4910 equivalent shaft horsepower each at takeoff. 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 wing or engine nacelles. The high wing position is advantageous for a cargo aircraft because it allows trucks and other types of equipment to move beneath...




Figure 6.3 - Lockheed C-130 turboprop cargo transport; 1955. [mfr]



[142] the wing, and the fuselage can be brought close to the ground without causing interference with the engines and propellers. A rear loading door may be deployed from the bottom of the upswept, aft portion of the underside of the fuselage. The proximity of the forward portion of the fuselage to the ground results in an aft-loading ramp with only a small inclination to the ground so that vehicles can be readily driven or pushed into the aircraft. The Lockheed C-130 has a gross weight of 155 000 pounds and cruises at a speed of 386 miles per hour at 20 000 feet. The wing loading is 88 pounds per square foot, and the landing speed is 115 miles per hour.

A great variety of twin-engine airliners has been developed both in the United States and abroad during the postwar years. These aircraft are smaller than the large, long-range, four-engine aircraft and are employed on short-haul types of operations. Twin-engine airliners have been developed with both reciprocating and turboprop engines. The twin-engine Martin 404 and Convair 440 aircraft and earlier versions of these machines were perhaps the most-used postwar twin-engine transports powered with reciprocating engines. These aircraft are similar in configuration to the Douglas DC-3 but are larger, faster, and are equipped with pressurized cabins; in addition, they both employ the tricycle type of landing gear. The Fairchild F-27 (a Dutch Fokker design built under license by Fairchild in this country) and the Japanese YS-IIA are probably the best known turboprop twins in the United States. The British Hawker Siddeley 748 turboprop-powered twin-engine airliner is widely used in many countries of the world.

Although the long-range propeller-driven transport is essentially a thing of the past, smaller, short-range aircraft of this type are becoming more numerous. Since the advent of airline deregulation in the United States in the latter part of the 1970's, there has been a large growth in short-haul, commuter-type airline operations. Many aircraft employed in this type of service are foreign built, are of high-wing configuration, and are equipped with two turboprop engines. Passenger capacity varies between 20 and 30, and at least one four-engine aircraft of this type carries 50 passengers. Generally speaking, these aircraft have straight wings and employ no new configuration concepts. In fact, some of them have fixed landing gears and strut-braced wings. Since high speed is unimportant and low initial and maintenance costs are critical, these retrogressive technical features are justified on a cost-effectiveness basis. The final forms of the commuter-type transport, however, are yet to emerge.