ch12-7

Quest for Performance: The Evolution of Modern Aircraft


Part II: THE JET AGE


Chapter 12: Jet Bomber and Attack Aircraft


Three Navy Attack Aircraft



[393] Described next are three Navy attack aircraft (one is also used by the USAF) that appeared between 1954 and 1964. The three aircraft relied heavily on fighter technology of the 1950's, but in contrast with fighters of that period, none had a supersonic capability. Hence, none incorporated area ruling and none was equipped with afterburning engines. All three aircraft were equipped with power-operated controls and arresting hooks for carrier operation; all had pressurized cockpits and ejection seats for crew escape. Designed under the restraints imposed by aircraft-carrier operation, the three differed primarily in their range-payload characteristics and the conditions under which they were expected to operate. Yet, just as with the fighters of the mid-1940's, they differed markedly in appearance.

Douglas A-4 Skyhawk

Designed as a light naval attack aircraft capable of delivering a nuclear weapon, the Douglas A-4 Skyhawk (A4D in the old Navy nomenclature) made its initial flight on June 22, 1954. When the last aircraft of this type rolled off the production line in February 1979, a total of 2096 units had been produced in a remarkably long 25-year production history. These ubiquitous aircraft have been in the military inventories of nine different countries, one being the United States, and have seen action in numerous world trouble spots including Vietnam, the Middle East, and the Falkland Islands where they served with the Argentine forces. Although retired from United States Navy fleet use in 1976, the aircraft is still used extensively by various elements of the Navy and Marine Corps for training and utility purposes. The A-4 is also currently [394] flown by the famous Navy Blue Angels exhibition team. Approximately 15 variants of the A-4 were produced during its long production life. Although primarily a single-place aircraft, the Skyhawk has also been produced in a two-place trainer version known as the TA-4.

Shown in figures 12.22 and 12.23 are two versions of the Skyhawk, and data for the A-4E are given in table VI. Configuration of the aircraft is characterized by a cropped delta wing of 33° sweepback mounted in the low position on a short, stubby fuselage with a large vertical tail and dorsal surface. The horizontal tail is mounted part way up the vertical fin just above the wing chord plane extended and consists of an electrically actuated stabilizer and hydraulically powered elevator. The hydraulically actuated rudder is of a unique design, consisting of a single central skin with external riblike stiffeners. (See figs. 12.22 and 12.23.) According to the interesting account of the development of the A-4 given in reference 164, this design feature was found to offer a solution to problems of rudder buffet or buzz.

Two inlets mounted high on the fuselage just ahead of the wing supply air to the single Pratt & Whitney turbojet engine of 8500....




Figure 12.22 - Douglas A-4E Skyhawk attack aircraft. [Peter C. Boisseaul




[395] Figure 12.23 - Douglas A-4 Skyhawk as used by the Navy Blue Angels aerobatic team. [Peter C. Boisseau]

....pounds thrust. Clearly shown in figure 12.23 are the boundary-layer diverters located Just ahead of the inlets. The long landing-gear struts were dictated by clearance requirements for large stores carried beneath the wings on either side and between the main landing-gear legs. A braking chute was provided for use at shore-based landing facilities.

For simplicity, the wing of aspect ratio 2.91 was built as a single unit with continuous top and bottom skins. it was so small that no folding was necessary for storage and movement on the aircraft carrier, an unusual feature in a Navy aircraft. Automatic leading-edge slats and trailing-edge split flaps were provided for lift augmentation. Outwardly extending speed brakes were mounted on either side of the rear of the fuselage.

The Skyhawk is capable of carrying literally hundreds of combinations of external stores including fuel tanks, both conventional and nuclear bombs, rockets, and Sidewinder missiles. Armament consists of two 20-mm cannons, one of which is mounted in each wing root.

The data in table VI show that the Skyhawk has a remarkably low gross weight of 18 311 pounds but can deliver a weapons load of 2040 [396] pounds at a mission radius of 680 miles with two 300-gallon external tanks; unrefueled ferry range is 2130 miles. Maximum speed of the aircraft is 673 miles per hour (Mach 0.88) at sea level, and cruising speed is 498 miles per hour. Ceiling is just over 40 000 feet. Certainly, the aircraft has an impressive performance for a lightweight attack aircraft.

The Skyhawk was designed at a time when the complexity, weight, and cost of combat aircraft were escalating at an alarming rate. Under the supervision of designer Edward H. Heinemann, an intensive effort was made during the design and development of the aircraft to keep it light and uncomplicated (ref. 164). The effort paid off so well that the first version of the Skyhawk had a gross weight of only about 15 000 pounds. Growth in capability resulted in the increase in gross weight to 18 311 pounds shown in table VI for the A-4E. The light weight of the aircraft caused it to have at least two appellations: "The Bantam Bomber" and "Heinemann's Hotrod." The long production life of the Skyhawk, together with its widespread use in a variety of roles, attests to the basic soundness of the original design and its potential for growth and adaption to differing requirements. The Skyhawk should continue to be a familiar sight for many years.

Grumman A-6 Intruder

The Grumman A-6 Intruder was designed in response to a Navy requirement for a true all-weather attack aircraft capable of precision weapons delivery at night under conditions of zero visibility. These difficult mission specifications dictated a relatively heavy, twin-engine aircraft manned by a crew of two, consisting of a pilot and a weapons-systems operator, and equipped with a complex array of electronic gear.

First flight of the A-6 (A2F-1 in the old Navy designation system) took place in April 1960, and the first squadron to use the aircraft was formed in early 1963. Operational sorties against North Vietnam were begun in 1965 from carriers located off the coast. The A-6 and its derivatives have been in continuous production since 1960 and are still rolling off the production line at this time. A close relative of the A-6 is the EA-6B Prowler electronic warfare aircraft, which first flew in May 1968. The Prowler differs from the Intruder mainly in the amount and type of equipment carried and in the use of a four-man instead of a two-man crew.

The A-6A is shown in figures 12.24 and 12.25, and physical and performance data are given in table VI for the more advanced A-6E.




[397] Figure 12.24 - Grumman A-6A Intruder all-weather attack aircraft. [Arthur L. Schoem via AAHS]

Configuration of the midwing subsonic aircraft features a 5.31-aspectratio wing of moderate sweepback (25°) and one turbojet engine nestled on either side of the fuselage in the intersection of the lower wing surface and the fuselage side. Exhaust nozzles are located just behind the wing trailing edge, and, as can be seen in the figures, side-mounted inlets are low and far forward on the fuselage. A side-by-side seating arrangement accommodates the crew in the A-6. Clearly seen in the photographs is the refueling probe located on top of the fuselage just ahead of the cockpit canopy. To provide the lift augmentation necessary for carrier operations, nearly full-span leading-edge and trailing-edge high-lift devices are installed. The deployed leading-edge slat can be clearly seen in figure 12.24. The trailing edge of each wingtip outboard of the fold line splits to form speed brakes that deflect above and below the wing when deployed. As seen in figure 12.25, the outer portion of the wing folds upward to facilitate carrier storage. Also evident in the figure is one of the two short flow-control fences located on each wing. Spoilers are used for lateral control, and the longitudinal control surface is an all-moving horizontal tail.

No guns of any kind are carried aboard the A-6, and the aircraft has no internal bomb bay. A wide variety of stores, however, can be....




[398] Figure 12.25 - Grumman A-6A Intruder with wings folded. [Arthur L. Schoeni via AAHS]

....mounted externally; these include both conventional and nuclear bombs, fuel tanks, and an assortment of rockets and missiles. Two of the store-mounting stations are visible in figure 12.25.

As shown in table VI, the A-6E is capable of a maximum speed of 653 miles per hour (Mach 0.86) at sea level and a cruising speed of 390 miles per hour. Although the wing loading of the A-6E is over 40 percent higher than that of the A-4E, the stalling speeds of the two aircraft are nearly the same, which attests to the effectiveness of the highlift devices on the A-6. The zero-lift drag coefficient of the A-6E is about 20 percent higher than that of the B-57B; however, the maximum lift-drag ratios of the two aircraft are about the same. The higher-aspect-ratio wing of the A-6E compensates for its higher zero-lift drag coefficient in determining the maximum lift-drag ratio.

As with all versatile attack aircraft, many combinations of payload and mission radius are available to the A-6E. For example, a weapons load of 2080 Pounds consisting of a Mark 43 nuclear bomb can be delivered at a mission radius of' 890 miles. For that mission, four 300-gallon external tanks are carried. Alternatively, 10 296 pounds can be delivered at a mission radius of 450 miles with two 300-gallon external [399] tanks. Unrefueled ferry range is 3300 miles. Normal gross weight aircraft is 54 393 pounds, nearly three times that of the A-4E.

The subsonic A-6 has no spectacular performance or design features but is superbly suited to the particular attack role for which it so carefully tailored. The A-6 Intruder and its close relative the EA-6 Prowler will no doubt continue in service with the U.S. Navy and Marine Corps for the foreseeable future.

Vought A-7 Corsair II

As applied to an aircraft, the name Corsair has its origins in a series of famous biplanes built for the Navy by the Vought Corporation between World Wars I and II. Later, the name was applied to the famous Vought F4U series of fighters flown by Navy and Marine pilots during World War II. Discussed next is the modern-day descendant of these historic aircraft - the Vought A-7 Corsair II.

The Corsair II was developed in response to a Navy requirement for a single-place, fair-weather subsonic attack aircraft capable of carrying a much heavier weapons load than the Douglas A-4 Skyhawk. First flight of the new aircraft (Vought A-7D) took place on September 27, 1965, and it has been in continuous production since then with a total of 1534 units produced by mid-1980. In addition to the Navy and Marine Corps, the USAF as well as air forces of two other nations operate the A-7. The definitive versions of the aircraft are the USAF A-7D and the closely related Navy A-7E. An in-flight view of an A-7D is shown in figure 12.26, and a close-up view of an A-7E is pictured in figure 12.27. Physical and performance data are given in table VI for the A-7D.

That the lineage of the A-7 can be traced directly to the Vought F-8 Crusader fighter is obvious from a comparison of figures 12.26 and 12.27 with figures 11.19 and 11.20. Like the F-8, the configuration of the A-7 is characterized by a high wing, low horizontal tail, chin inlet, and short landing-gear legs that retract into the fuselage. Since the A-7 is a subsonic aircraft, however, no area ruling is incorporated in the fuselage, which is also shorter and deeper than that of the supersonic F-8. Because of the larger mass flow of the turbofan engine employed in the A-7, the size of the chin inlet is somewhat larger than that of the turbojet-powered F-8. These differences make the A-7 appear shorter and more stubby than the earlier fighter. The A-7...




[400] Figure 12.26 - Vought A-7D USAF attack aircraft. [Arthur L. Schoeni via AAHS]




Figure 12.27 - Close-up view of Vought A-7E Navy attack aircraft. [Peter C. Boisseau]

[401]...is sometimes unofficially called the SLUF (Short Little Ugly Fella) USAF crews.

The wing of the A-7 is closely related in geometry and physical size to that of the F-8. (Compare data in tables V and VI.) Leading flaps and single-slotted trailing-edge flaps are fitted to the wing, as are upper surface spoilers located ahead of the flaps. Not used on the A-7 is the unique variable-incidence feature of the F-8 wing. The shorter length of the fuselage together with the slight "upsweep" of the underside of the afterbody (fig. 12.27) allow the A-7 to be rotated to a significantly higher pitch angle on takeoff and landing, without tail scrape, than was possible for the F-8. The higher available ground pitch attitude, together with the good augmentation capability of the high-lift system, no doubt played a large part in obviating the need for a variable-incidence capability in the wing. Speed brakes are located on the bottom of the fuselage about midway between the nose and the tail. A braking chute is provided for use in shore-based operations.

Original power plant of the A-7 was a nonafterburning version of the Pratt & Whitney TF30 turbofan. This is the same engine that, equipped with an afterburner, powers both the F-111 and the F-14. Beginning with the A-71), however, the more powerful Allison TF41-A-1 turbofan was installed. An American-made version of the British Rolls-Royce Spey, the TF41-A-1 has a bypass ratio of 0.7 and uses a five-stage fan.

A wide assortment of external stores can be accommodated on the A-7. Eight store-mounting positions are provided. There are three pylons under each wing, and a single mounting station is located on each side of the fuselage. Two of the underwing pylons and one of the fuselage mounting stations are visible in figure 12.27. A total of 15 000 pounds of stores can be carried. Table VI shows that 6560 pounds can be carried on a typical mission with a radius of 556 miles. A six-barrel 20-mm Vulcan cannon is located on the left side of the fuselage near the bottom of the aircraft. A portion of the muzzle of the gun is visible below the word "intake" in figure 12.27.

Although the A-7 was originally intended as a fair-weather aircraft, later versions (beginning with the A-7D) were equipped with extensive electronic gear for all-weather operations. The various modes in which this equipment may be utilized in attack operations, along with many other aspects of the A-7 story, are discussed in reference 136.

According to the data in table VI, the gross weight of the A-7D is a little more than twice that of the A-4E but is significantly less than that of the A-6E. Maximum speed of the A-7D is listed as 663 miles [402] per hour (Mach 0.89) at an altitude of 7000 feet. Stalling speed at maximum gross weight is a very high 174 miles per hour. For landing on an aircraft carrier following a mission, however, a weight much less than the maximum value, along with a reduction in stalling speed, would normally be expected. For example, with a weight of 23 000 pounds, the stalling speed would be about 135 miles per hour.

The A-7 is one of those aircraft with a demonstrated capability of performing well in a wide variety of missions. Other aircraft are faster or have a greater range-payload capability or have a faster rate of climb; sometimes, certain of these characteristics is deemed so important that it dominates the entire design. What results is a "point design" aircraft that can perform one mission extremely well but is relatively much less effective in any other mission. The design parameters of the A-7 were chosen so that the aircraft has great mission versatility. It was successfully employed in just about every conceivable attack role during the Vietnam conflict where it first saw action in 1967.