ch12-4

Quest for Performance: The Evolution of Modern Aircraft

Part II: THE JET AGE

Chapter 12: Jet Bomber and Attack Aircraft


Two Pioneering Explorations

[377] Described next are two radically different aircraft, both of which were pioneer explorations into new realms of aeronautical technology. First discussed is the Convair B-58 Hustler. This aircraft was the first United States bomber to have a supersonic dash capability and required the development of much new technology. Although the B-58 [378] was thought by many to be the harbinger of future generations of more advanced supersonic bombers, only about 115 of these unique aircraft were built, and they were quietly withdrawn from the SAC inventory after less than 10 years of service.

The second aircraft discussed, the Martin P6M Seamaster, was hailed as the precursor of a new era in naval aviation. When the program was terminated, however, only 12 aircraft had been produced and these never saw operational service. The Seamaster was the first and perhaps the last large jet-propelled flying boat to be developed in the United States. Perhaps the most advanced flying boat ever developed, the P6M had many advanced technical features worthy of examination.

Convair B-58 Hustler

Mission requirements for the Convair B-58 Hustler called for a subsonic cruise segment of several thousand miles followed by a supersonic dash (Mach 2.0) in the target zone of as much as 500 miles and, finally, a post-strike cruise segment. Diverse requirements such as these call for an aircraft of high aerodynamic efficiency at both subsonic and supersonic speeds, together with a versatile propulsion system capable of efficiently providing the required thrust in the different flight regimes. Today's response to the B-58 mission requirements would no doubt be a variable-sweep configuration employing afterburning turbofan engines. (See discussion of the F-111 in chapter 11 and of the B- I in a later part of this chapter.) Unfortunately, the technology for a practical variable-sweep aircraft did not exist in the early 1950's when the B-58 was being designed - nor did afterburning turbofan engines. The only possible way in which the difficult mission objectives could be met in that time period was with the use of in-flight refueling.

As can be seen from figures 12.14 and 12.15, configuration of the B-58 was characterized by a delta wing and the absence of a horizontal tail. The wing had 60° sweepback at the leading edge, an aspect ratio of 2.09, and airfoil sections that varied in thickness ratio from 3.46 percent at the root to 4.08 percent at the tip. Conical camber was employed in the leading edge to reduce drag at lifting conditions and thus increase cruising efficiency. (A conically cambered wing is one which has a leading-edge camber shape formed from part of the surface of a cone whose apex is located at the longitudinal plane of symmetry of the wing. The amount of camber accordingly increases progressively....




[379] Figure 12.14 - Convair B-58 Hustler bomber without auxiliary pod. [NASA]




Figure 12.15 - Convair B-58 Hustler bomber with auxiliary pod installed. [mfr via David A. Anderton]


[380]...with spanwise distance from the fuselage.) Absence of a horizontal tail for trimming prevented the use of any trailing-edge high-lift devices. Elevons for pitch and roll control extended from the side of the fuselage to the outboard engine nacelles. All the controls were power operated.

The four General Electric J-79 turbojet engines were located in individual nacelles suspended below the wings on sweptforward pylons - an arrangement analogous to that employed on the B-47 and B-52. Area ruling was employed in the high-fineness-ratio fuselage with the single vertical fin and rudder mounted at the rear. Crew members consisting of pilot, bombardier-navigator, and defense-systems operator were housed in a tandem arrangement to aid in maintaining the desired long, narrow shape of the fuselage. Each crew station was an individual rocket-powered escape module capable of providing safe crew egress even at Mach 2.0. The entire crew compartment as well as the wheel wells and electronics bay were pressurized and air-conditioned. Cooling of the tires and electronic equipment was required because of the high temperatures generated by prolonged flight at Mach 2.0. Landing gear consisted of a tricycle design with each main gear having eight wheels arranged in two rows of four. The large number of wheels was used to maintain the landing-gear footprint pressure within acceptable limits while, at the same time, allowing the use of small diameter wheels capable of being stored in the thin wing with only small fairings bulging from the lower wing surfaces. The conventional nose gear had two wheels; a braking chute was provided to assist in stopping the aircraft on landing rollout.

A comparison of figures 12.14 and 12.15 shows a large streamlined pod under the fuselage of the aircraft in the latter but no such pod in the former. The pod served the dual purpose of housing a nuclear warhead (bomb) and several thousand gallons of fuel. Large amounts of fuel were also carried in the wings and fuselage. The pod was divided into two main parts: the portion containing empty fuel tanks was to be jettisoned on the outboard flight to the target, and the other component containing the warhead, as well as additional empty fuel tanks, was then to be dropped at the target. The B-58 might thus be considered as a sort of two-stage system. Armament on the aircraft consisted of a single six-barrel 20-mm rotary cannon controlled by the defense-systems operator.

To give a light, strong, stiff structure for the thin high-fineness-ratio elements of the aircraft, the B-58 made extensive use of aluminum honeycomb panels. Most of the outer covering of the aircraft [381] consisted of such panels having outer and inner aluminum skins bonded to a honeycomb of aluminum and fiber glass. In addition to its light weight, this type of structure had a smooth exterior surface. In service, however, problems were encountered in ascertaining and maintaining the integrity of the bonded joints.

With a gross weight of 163 000 pounds and a maximum speed of 1321 miles per hour (Mach 2.0) at 63 150 feet altitude (table VI), the B-58 was an impressive aircraft by any standard. This performance was dramatically demonstrated in a number of record flights. Perhaps the most notable of these was the May 26, 1961, nonstop flight of 3 hours and 19 minutes from New York to Paris. Average speed for the 3669 mile flight was 1105 miles per hour; three in-flight refuelings by KC-135 aircraft were required. Interestingly, almost 34 years earlier on May 20 and 21, 1927, Charles A Lindbergh required 33.5 hours to make the first nonstop flight from New York to Paris - a remarkable advancement in aeronautical technology during a time period of just a little more than three decades.

In spite of the spectacular records set by the B-58, the data in table VI show that the aircraft was woefully deficient in range performance. Without in-flight refueling, the radius of action, including a 450-mile supersonic dash, was only 1500 miles. With no supersonic dash, the maximum radius increased to 2000 miles, thus indicating the relatively poor supersonic cruising efficiency of the aircraft. Ferry range at subsonic speeds was 4025 miles. With in-flight refueling, a target distance of 4300 miles, including a supersonic dash of 500 miles, was possible. After weapons delivery, the aircraft had a range of 1500 miles - hopefully enough to reach a friendly base but not enough to reach the point of departure. With in-flight refueling, ferry range was 6995 miles.

The limited range capability of the B-58 can be directly traced to the compromises required in its aerodynamic design. The Mach 2.0 dash requirement dictated the use of a delta wing with leading-edge sweep angle of 60° and a low aspect ratio of about 2. As a consequence, the value of the maximum subsonic lift-drag ratio, without the fuel and weapons pod, was only 11.3 (compare this with the value of 21.5 for the B-52G); an even lower value would be expected with the pod attached. The value of (L/D)_max at Mach. 2.0 was slightly greater than 5. Thus, the aircraft was not capable of efficient cruising flight at either subsonic or supersonic speeds. Aircraft configuration design for highly efficient cruising flight at subsonic speeds is well understood, as demonstrated by the B-47 and B-52 as well as by the numerous highly efficient jet transports described in the following chapter. Unfortunately, [382] the design of a highly efficient and practical supersonic cruising aircraft still remains somewhat elusive although much progress has been made since the design of the B-58. Still more elusive is a configuration concept that enjoys high cruising efficiency at both subsonic and supersonic speeds, such as required by a commercial supersonic transport. Variable sweep, however, now offers a means for achieving good subsonic cruising efficiency in combination with a reasonably efficient supersonic dash capability.

With the increased effectiveness of enemy detection and antiaircraft capability discussed previously, the high-altitude Mach 2 method of weapons delivery became increasingly less viable, and an on-the deck method of attack became the preferred mode of operation. For this type of weapons delivery, however, the payload-range characteristics of the B-58 were much inferior to those of the B-52. For whatever reason, the last B-58 was withdrawn from service in January 1970 after about 10 years in the active inventory. First flight of the aircraft took place on November 11, 1956; approximately 115 units were built.

As a final comment, the B-58 represented a significant technical achievement in the 1950's time period, but the mission requirements called for innovations that far exceeded the technical state of the art then available.

Martin P6M Seamaster

The evolution of the propeller-driven flying boat in America is traced in chapter 8. Although the U.S. Navy continued to operate a few flying boats as late as the mid-1960's, the era during which this picturesque class of aircraft played an important role in civil and military aviation really ended with the close of World War II As a last effort to prolong the usefulness of the military flying boat, the Navy sponsored development of a large, jet-powered boat for long-range, mine-laying, and reconnaissance duties. Operation from bodies of water in dispersed and remote locations, with minimum support facilities, was envisioned as a means of avoiding the inherent vulnerability of large numbers of aircraft situated at congested air bases. With first flight on July 14, 1955, the Martin P6M Seamaster was developed to fill the prescribed role.

The Martin Seamaster is shown in figures 12.16 and 12.17, and data for the YP6M-1 version of the aircraft are contained in table VI. Configuration of the aircraft featured a sweptback wing mounted near [383] the top of a single-step, high-length-beam-ratio hull. Two afterburning jet engines were located side by side in each of' two nacelles mounted oil top of the wing immediately adjacent to either side of' the fuselage. (The afterburners, an unusual feature for a large subsonic aircraft, were for use oil takeoff.) Inlets swept back at nearly the same angle as the wing leading edge were found to be unsatisfactory and, as shown in figure 12.16, unswept inlets were finally adopted; exhaust nozzles were behind the trailing edge of the wing, as can be seen in both figures. The location of' the engines was, of course, strongly influenced by the necessity of' minimizing spray ingestion during operation on the water. The horizontal tail was positioned atop the vertical fin in a T arrangement and featured a pronounced positive dihedral angle. Impingement of' both jet exhaust and spray was minimized by the tail configuration. Coupled with the large vertical tail, the positive dihedral of the horizontal surface working with the negative wing dihedral gave the proper dihedral effect for the integrated configuration. The negative wing dihedral allowed the lateral balancing floats to be mounted flush...




Figure 12.16 - Martin YP6AI-1 Sea master flying boat moving at high speed on the water. [mfr via Lester Rose]




[384] Figure 12.17 - In-flight view of Martin YP6M-1 Seamaster flying boat. [Ray Wagner via AAHS]
....against the wingtips with neither drag-producing mounting struts or pylons.

The 40° sweptback wing had an aspect ratio of 5.53 and airfoil thickness ratios that varied from 11 percent at the root to 8 percent at the tip. Lift augmentation was achieved with trailing-edge flaps and with slats located over the outer portion of the leading edge; the slats can be seen in the deployed position in figure 12.16. Wing spoilers were used for roll control; elevators together with an adjustable stabilizer were used for pitch control; and a single rudder was provided for control about the yaw axis. Maneuvering on the water was enhanced by hydroflaps located on both sides of the hull afterbody. When opened individually, these flaps served as rudders for directional control while symmetrical deployment provided braking. The hydroflaps may be seen outlined in black in Figure 12.17.

The Seamaster crew consisted of a pilot, copilot, navigator-minelayer, and radio armament-defense operator. All crew quarters were pressurized and each crew member was equipped with an ejection seat. [385] Armament consisted of two remotely operated 20-mm cannons located in the tail. The mine bay had a watertight rotary door, the outside of which served as part of the bottom of the hull. A rack for mounting mines or other types of stores was fastened to the inside of the door. Rotation of the door in flight provided the means for weapons delivery.

With a gross weight of 167 011 pounds, the YP6M- 1was a large aircraft capable of attaining a maximum speed of 646 miles per hour (Mach 0.86) at 5000 feet and cruised at a speed of 540 miles per hour. Even higher performance was shown by the P6M-2, which had engines of higher thrust than those on the YP6M-1. From the data given in reference 200, the mission radius of the aircraft as a minelayer seems to have been about 800 miles with a payload of 30 000 pounds and 1350 miles for the high-altitude reconnaissance role. Ferry range is estimated to have been about 3500 miles.

In spite of the promising characteristics of the P6M, the Navy terminated the program in August 1959 after 12 aircraft had been constructed, including 2 prototypes that had been lost. Shortage of funds coupled with demands of higher priority programs no doubt played a major part in the cancellation. The Seamaster was the last large flying boat developed in the United States, and many viewed its demise with regret and nostalgia.