In concept, the Boeing B-47 was as revolutionary as the North American B-45 was conventional. The Stratojet was far ahead of any contemporary bomber in its performance and operational capability. A total of 2041 of these aircraft were manufactured, more than any other United States bomber built under peacetime conditions. As a key element in the Strategic Air Command, the B-47 served in operational squadrons until withdrawn from service in 1966. The aircraft was used for various types of special operations, however, for at least another 10 years.
The B-47 was the first pure jet strategic bomber whose many unique features included six jet engines; a two-engine, pylon-mounted pod under each wing near the fuselage; and a single-engine pod further outboard. The wings were attached high on the fuselage and swept 35. The design incorporated a revolutionary bicycle-type, retractable main landing gear with single, two-wheel struts on the forward and aft fuselage. Outrigger wheels added lateral stability and retracted into the two-engine pod cowling. The B-47 was 107 feet long, 28 feet high at the tail, and had a wing span of 116 feet. The crew consisted of a pilot, copilot, and bombardier. With a maximum gross weight of about 204,000 pounds, it used rocket assist on takeoff. A tail chute was used to slow down the aircraft during landings.
The configuration of the aircraft is characterized by (1) a thin, high-aspect-ratio sweptback wing mounted in the shoulder position near the top of the fuselage, (2) six jet engines mounted in pods beneath the wing, and (3) an unusual bicycle-type landing gear.
Design of the wing featured an average thickness ratio of about 12 percent, an aspect ratio of 9.42, and a sweepback angle of 35°. Single-slotted flaps located at the trailing edge provided high lift for landing, and conventional ailerons were used for lateral control. All control surfaces were hydraulically boosted. Location of the wing near the top of the fuselage allowed the bomb load to be carried in the fuselage, beneath the wing and near the center of gravity, and to be released through doors in the bottom of the fuselage without interference from the structure of' the wing center section. Further, the shoulder position of the wing allowed adequate ground clearance for the engine nacelles.
Design of the landing gear posed a problem that led to a novel solution not seen before on a production airplane. Wing thickness was not large enough to house the gear and, in addition, the high position of the wing would have resulted in long, heavy landing-gear struts. The solution of the problem was found in an unusual bicycle arrangement in which a two-wheel bogie was located along the fuselage centerline in front of and behind the bomb bay. Small, retractable outrigger wheels extended from the inboard nacelles to assist in providing lateral balance while the aircraft was on the ground. The front bogie was steerable to give ground maneuverability.
One of the most innovative features of the B-47 configuration, and one that was to have a marked influence on future civil and military aircraft of large size, was the engine mounting. The nacelles containing the engines were attached to pylons mounted to and extending below the wings. Two engines were mounted in each of two nacelles, one of which was attached through a pylon to each wing well outboard of the fuselage. The other two engines were mounted singly in nacelles nearly flush with the wing and located near the wingtips. A number of advantages may be cited for the engine arrangement pioneered by the B-47; namely:
- The engine nacelles are widely separated from each other and the fuselage. Thus, the danger to the aircraft and other engines that results from the disintegration of one engine is reduced. This advantage is somewhat nullified in the B-47 because two of the nacelles contain two engines.
- The aircraft is easy to balance because the engines can be located near the aircraft center of gravity.
- The weight of the engines mounted outboard on the wing reduces the wing bending moments in flight.
- The engines are easy to maintain and can be readily removed because of their proximity to the ground. Since the engine inlets are usually outboard of the spray pattern from the nose and main landing gear, the outboard wing mounting offers good protection from FOD (foreign object damage) to the engines when the aircraft is operated on the ground.
A number of disadvantages may also be cited for the type of engine arrangement employed on the Boeing B-47, as follows:
- Failure of an engine, particularly during takeoff or climb, may produce large yawing moments that require immediate correction by the pilot. The magnitude of the corrective yawing moments required to counteract the unsymmetrical  thrust in the engine-out condition may determine the necessary size of the rudder.
- A small reduction in maximum lift coefficient may result from unfavorable interference effects in the nacelle-wing juncture and from the impingement of the nacelle wake on the wing at high lift coefficients. The wing-nacelle-pylon relationships must also be carefully tailored, usually in wind-tunnel studies, to eliminate or minimize any interference drag. A positive aerodynamic benefit, however, results from the pylons, which act somewhat like wing fences in alleviating the pitch-up problem so often found in aircraft with sweptback wings.
- The addition of concentrated weights, such as engines or stores, is usually thought to reduce the wing flutter speed. The relationship of the engine center of gravity to the wing elastic axis as well as the dynamic coupling between the engines and the wing strongly influence the effect of the engines on the wing flutter speed. These, as well as other relationships, must be carefully tailored by a detailed process involving mathematical analysis and wind-tunnel tests. By this means, a reduction in flutter speed can usually be avoided.
- The dynamic loads imposed on the wing structure during operations on the ground are usually intensified by the concentrated engine masses mounted on the wings.
The thin, high-aspect-ratio swept wing of the B-47 coupled with its long high-fineness-ratio fuselage contributed to the high aerodynamic efficiency of the aircraft. The maximum lift-drag ratio of about 20 is the highest of any aircraft yet considered in this book, and the zero-lift drag coefficient was a low 0.0148. Maximum speed is given in table VI as 607 miles per hour at 16 300 feet; the corresponding Mach number is 0.85, which is nearly 0.1 higher than that of the B-45.
The very features that contributed to the high performance of the B-47, however, also introduced some new problems that have been present in the development of all subsequent large jet-powered multiengine aircraft.
Aeroelasticity, the interaction of aerodynamic, elastic, and inertial forces, has formed a branch of aeronautical engineering for many years. Because of the flexibility of the long, thin elements of the B-47, however, the need to consider aeroelastic effects in the basic aircraft design process assumed critical importance. For example, in static tests the total deflection of the B-47 wingtip was 17 feet from maximum positive to negative deflection. Areas in which aeroelasticity are important are stability, control, loads, and, of course, flutter.
Flutter is a phenomenon in which an aircraft or one of its components, such as a wing or control surface, extracts energy from the moving airstream and converts it to a harmonic oscillation of the structure that may grow in amplitude until total destruction occurs. Flutter analysis and prediction is an arcane science in which flutter prediction and design for its avoidance have historically been the subject of detailed mathematical analysis. Uncertainties as to the nature of oscillating air forces, however, as well as the complex participation of the entire aircraft in the various structural vibration modes made mandatory the development of new experimental wind-tunnel techniques for studying these phenomena during development of the B-47.
Flutter tests and analyses had usually been limited to individual components of the aircraft such as the wing plus aileron or horizontal and vertical tail surfaces. The aircraft as an entity was usually not considered in the determination of the critical flutter speed, nor was such consideration necessary. However, the concentration of large masses beneath the wings, together with the high degree of flexibility of the wings and other components of the aircraft, required that motions of the complete airplane be considered in determining the critical flutter speeds of the B-47. Both symmetrical and antisymmetrical flutter modes needed to be studied. In a symmetrical mode, each wing deforms in exactly the same way, and the motion of the wings is accompanied by a vertical, up-and-down, and pitching motion of the fuselage. In antisymmetrical flutter, the wings on either side of the fuselage deform in exactly opposite directions, and the wing motion is accompanied by a rolling and yawing of the fuselage.
Wind-tunnel techniques were devised by the Boeing Company to deal with this complex problem. A 3/8-inch rod extended from the floor to the ceiling of the tunnel test section. The model was attached to a gimbal joint located at the center of gravity. The gimbal allowed freedom in pitch and yaw, and was itself attached to the vertical rod by an arrangement of rollers that allowed the model freedom in vertical translation. Snubber lines were used to arrest the vertical motion of the model if it became too large or uncontrollable. At each tunnel speed, the aircraft model was trimmed so that the lift force balanced the weight of the model. Pitch trim was maintained as the tunnel speed varied by remote adjustment of a tab on the horizontal tall. Limited rolling freedom was provided by looseness in the gimbal joint and flexibility in the mounting rod. The model was constructed in such a way as to simulate the stiffness and mass properties of the aircraft and, accordingly, was quite complex and expensive to design and build.
The technique was successfully employed in the development of the B-47 as a means for identifying flutter-critical combinations of speed and altitude and development of design fixes for flutter avoidance. The mounting rod limits the usefulness of the technique to fairly low subsonic speeds because of aerodynamic interference effects associated with the formation of shock waves on the rod at high subsonic Mach numbers. The complete model flutter tests made on the B-47 were carried out in a low-speed wind tunnel, and the results were then adjusted for estimated Mach number effects. Later techniques developed by NACA and NASA allow flutter tests of complete airplane models to be made at high subsonic and transonic Mach numbers in a wind tunnel especially designed for high-speed flutter investigations.
The aluminum skin of the B-47 varied in thickness on different parts of the aircraft and had to be machined carefully to produce the proper taper. The structural members, made of strong, light, heat-resisting metals such as titanium, required extensive machining on high-powered, high-torque, low-speed machines, because such metals were much harder to cut than aluminum. While the techniques of assembling the aircraft had not changed much, the process had returned to the handcrafting methods of the 1930s because the airplanes were so complex and packed with electronic equipment. This process was a major factor in the skyrocketing costs of the new aircraft.
The aircraft was manned by a crew of three. Two pilots sat in a tandem arrangement under a bubble-type canopy in a manner similar to that of a fighter; a bombardier-navigator sat in an enclosed compartment located in the nose of the aircraft. Upward-firing ejection seats were provided for the pilots, and the bombardier was equipped with a downward-firing ejection seat. Crew compartments were heated, ventilated, and pressurized. As fast or faster than most fighters, the Stratojet was equipped with only two 20-mm cannons situated in a remotely controlled turret located in the tail of the aircraft. Aiming and firing of these guns was the duty of the copilot whose seat could be rotated 180° to face rearward.
For assistance in the landing maneuver, the B-47 was equipped with a drag chute that was deployed during the approach. The added drag of the parachute aided in controlling the speed and the flight-path angle during this phase of the landing maneuver. Once on the runway, a large braking chute was deployed to assist in stopping the aircraft. An interesting insight into the airport performance of the B-47 is provided by a comparison of its stalling speed of 175 miles per hour with the cruising speed of 182 miles per hour for the World War II B-17G. Not surprisingly, the length of hard-surface runways at military air fields increased dramatically in the years following World War II.
Although the B-47 was equipped with six 7200-pound-thrust (with water injection) turbojet engines, the thrust-to-weight ratio at maximum gross weight was only 0.22, which, coupled with its high stalling speed, resulted in a long takeoff ground roll. To reduce the takeoff field length, the aircraft was initially equipped with 18 short-duration booster rockets. These units were an integral part of the aircraft and were known by the acronym JATO (jet-assisted takeoff). Nine JATO nozzles were located on each side of the fuselage. On some later versions of the aircraft, weight was saved by replacing the integral JATO units with a jettisonable external rack containing the rockets. In this installation, 33 rockets of 1000 pounds thrust each were provided.
By post-World War II standards, the B-47 was classified as a medium bomber; but with a gross weight of 198,180 pounds, the B-47E was far heavier than any bomber flown in World War II (the gross weight of the B-29 was 120 000 pounds), and it ranked second only to the 357,500-pound B-36D as the heaviest aircraft operated by the USAF in the early 1950's. Designed as a strategic bomber, the B-47 also filled various other roles such as photoreconnaissance. In its design role as a strategic bomber, the B-47 could deliver a 10,845-pound weapons load at a mission radius of 2013 miles. Ferry range was 4,035 miles. With air-to-air refueling, which became standard operating procedure following the close of World War II, both the mission radius and the ferry range were greatly increased, and targets in Eastern Europe could be reached from bases in the United States with sufficient range potential to allow safe return to friendly territory.
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