Boeing XB-52/YB-52 Stratofortress

Last revised June 30, 2000



Once the contracts were let, work on the two XB-52 prototypes proceeded rapidly and they were ready for rollout by late 1951.

The aircraft that emerged had a shoulder-mounted wing with a sweepback angle of 35 degrees. The wingspan was 185 feet, with an area of 4000 square feet. The wing was set at an angle of incidence of six degrees. This was necessary because of the tandem undercarriage layout, which did not permit the aircraft to rotate on takeoff.

According to the standards of the day, the wings were quite thin. On the center line of the of the fuselage, the wing structure had a thickness ratio of 16.2 percent, declining gradually to a thickness ratio of only 8 percent at the tip. Although they were quite thin, the wings carried bladder-type cells for fuel. The thin wings had a considerable amount of flexibility, and could move up or down through a 32 foot arc at the tip without failing. When sitting on the ground with no fuel load, the wings sat high enough so that the outrigger wheels did not actually touch the ground. However, when fully loaded with fuel, the wheels always touched the ground, only rising off the runway after sufficient lifting force has been generated during takeoff. When airborne, the wings generally have an upward curvature.

The eight Pratt & Whitney J57 turbojets were carried two each in four underwing pods. The pods were suspended underneath the wings on pylons. The engines were situated beneath and ahead of the forward edge of the wing. Careful positioning of the engine pods helped to limit the drag rise at high speed and also served to alleviate load factors. The pylons also doubled as wing fences and helped to delay the onset of the stall.

The wings were fitted with four segments of Fowler-type flaps, two on the trailing edge of each wing. Total flap area was 797 square feet. Only two settings were available, fully up or fully down, with the down angle being 35 degrees.

Aerodynamic surfaces on the B-52 wing consisted of a combination of ailerons and spoilers. The ailerons were located on the midwing trailing edges between the inner and outer flap sections, while the seven-segment spoilers were located somewhat further out on the upper wing surface. When operated asymmetrically, the ailerons provided adequate roll control during most normal flight operations, but an additional measure of control could be obtained by using the spoilers during landing or inflight refuelling. When deployed symmetrically, the spoilers could act as airbrakes, making a deceleration chute (such as that used by the B-47) during final approach unnecessary.

The vertical fin was 48 feet 3 inches tall, and incorporated a nearly full-span rudder of rather narrow chord. The entire vertical fin assembly could be folded sideways to allow the aircraft to be wheeled into standard hangars. The horizontal tail surfaces had a span of 52 feet and an area of 900 square feet. The horizontal tail was of the fully-variable type, pivoting through an arc of 13 degrees (9 up, 4 down).

A lot of space in the fuselage was taken up by fuel tanks, with the upper sections from just behind the cockpit to just aft of the rear main undercarriage members being used almost exclusively for fuel. The weapons bay occupied almost the entire section of the lower fuselage between the forward and rear undercarriage members. It was 28 feet long and 6 feet wide, and was enclosed by double-panel doors. Three interconnected and hydraulically-actuated lower panels on each side made up the section of the bomb bay doors that could be opened in flight. While on the ground, the hinged upper panels could be swung back to provide additional clearance for loading and unloading of weapons.

Defensive armament was limited to four 0.50-inch machine guns in a manned tail turret.

The landing gear used double twin-wheeled units mounted side by side underneath the fuselage, one forward and one to the rear. To prevent the wingtips from dragging on the ground during takeoffs or landings, there were small outrigger wheels which retracted into the outer wing. However, the mainwheels gave the aircraft enough ground stability so that it could stand by itself without the need for the outrigger wheels. The main landing gear retraction process was fairly complicated, with the wheels swiveling through almost 90 degrees before folding to lie flat within the storage bays. The retraction was asymmetric, with the port units folding forward and the starboard units folding aft. Any one of the four main units could be lowered independently. A unique feature of the landing gear was the ability of the main units to rotate up to 20 degrees left or right of the line of flight. This facilitated crosswind landings and takeoffs by permitting the aircraft to point directly into the wind while the wheels remain aligned with the runway.

Normal crew was five, with pilot and copilot seated in tandem under a bubble-type canopy in the forward nose. The navigator and radar operator sat side-by-side on a lower deck in the forward nose. The tail gunner sat in a separate cockpit in the extreme tail. In an emergency, the pilot and copilot ejected upward and the navigator and radar operator ejected downward. The tail gunner jettisoned the turret by firing four explosive bolts, and he dived after it.

The wing spoilers made the approach chute used by the B-47 unnecessary for the B-52. However, a 44-foot braking parachute was still required to shorten the landing roll. This chute was stowed in a hatch underneath the rear fuselage.

An unusual feature of the B-52 was the use of a pneumatic system as the primary power source in the operation of all auxiliary functions aboard the aircraft. High pressure, high temperature air was bled from the second stage compressor of each jet engine, and carried by ducts to the desired locality in the aircraft where it was transformed into electrical or hydraulic energy by air turbine-driven power packs. There were ten turbine-driven hydraulic pumps which supplied pressure at 3000 pounds per square inch to drive the brakes, steering mechanism, spoilers, bomb bay doors, and the adjustable stabilizer. The pneumatic system also drove air turbine alternators which provided the electrical power for the aircraft.

Two aircraft were completed by Boeing's Seattle factory to serve as Stratofortress prototypes. Both had been originally ordered as XB-52s, but the second machine was redesignated as YB-52 in the wake of a 1949 Boeing proposal which recommended installing some operational equipment so that it might serve as a production prototype. This idea apparently fell by the wayside, since there was actually very little difference between the two aircraft when they emerged. The XB-52 and YB-52 were both powered by eight Pratt & Whitney YJ57-P-3 axial-flow turbojets delivering 8700 lb.s.t. each.

On the evening of November 29, 1951, the XB-52 prototype (49-230) was rolled out of the assembly hall and into the flight test hangar. It was covered with a tarpaulin to conceal its shape from prying eyes. It was subjected to a series of ground tests and checkouts. Unfortunately, the XB-52's pneumatic system failed during a full-pressure test and the resulting blow-out severely damaged the wing trailing edge, which required that the aircraft be moved back into the production hall for repair. The company and the Air Force decided to keep this news under wraps and attributed the delay to the installation of further equipment. As a result, the XB-52 did not become airborne until nearly a year later.

Consequently, it was the second prototype, the YB-52, that was actually the first to get airborne. The YB-52 (49-231) rolled out of the assembly hangar on March 15, 1952. The Y prefix indicated service test, but it was essentially identical to the XB-52. It had originally been held back in the factory so that some of the changes found desirable during testing of the XB-52 could be incorporated, but the accident which befell the XB-52 moved it up to primary position.

The first flight of the YB-52 took place on April 15, 1952, with company test pilot A. M. "Tex" Johnston and Lt Col Guy Townsend of the USAF Air Research and Development Command on board. The aircraft stayed in the air for 2 hours and 15 minutes and landed at nearby Larson AFB. Only relatively minor problems were encountered--the failure of one of the main landing gears to retract properly, defects in the liquid oxygen system, and a leaking engine oil valve. The Air Force was so paranoid about security that take-off photographs released to the press had the landing gear units censored out.

By the beginning of October 1952, the YB-52 had logged 50 hours in the air and had begun Phase 1 flight trials.

Almost a year later than originally planned, the XB-52 took off on its maiden flight on October 2, 1952. It stayed up in the air for more than two hours.

Phase II tests were accomplished between November 3, 1952 and March 15, 1953. They turned up some problems with engine reliability, the J57 engines being prone to surge when normal throttle movements were undertaken at high altitude with low engine inlet temperatures. There was a tendency to pitch up and roll to starboard when approaching the stall. The braking system was unable to bring the Stratofortress to a halt within the required distance.

The two prototypes continued an extensive series of flight tests in support of the Stratofortress program. Both the XB-52 and the YB-52 ended their days at the Wright Patterson AFB in Ohio. In 1957, the XB-52 was sent to the Wright Air Development Center at Wright Patterson AFB, Ohio. It later flew as a six-engined aircraft, with four J57s inboard and two J75s outboard. After logging 783 flying hours, the YB-52 was donated to the USAF Museum at Wright Patterson AFB in Ohio on January 27, 1958. Unfortunately, both aircraft were scrapped in the mid-1960s. They were victims of Lady Bird Johnson's national beautification program, which sought to remove eyesores such as surplus military hardware from the landscape.

Specification of Boeing YB-52 Stratofortress

Engines: Eight Pratt & Whitney YJ57-P-3 turbojets, each rated at 8700 lb.s.t. Performance: Maximum speed 611 mph at 20,000 feet, 594 mph at 35,000 feet. Cruising speed 519 mph. Stalling speed 146 mph. Initial climb rate 4550 feet per minute. Combat radius 3545 miles with 10,000 pound bombload. Ferry range 7015 miles. Dimensions: Length 152 feet 8 inches, wingspan 185 feet 0 inches, height 48 feet 3.6 inches, wing area 4000 square feet. Weights: 155,200 pounds empty 405,000 pounds gross. Armanent: Not fitted with any defensive armament. Maximum offensive payload 43,000 pounds.

Sources:


  1. American Combat Planes, Third Enlarged Edition, Ray Wagner, Doubleday, 1982.

  2. Post World War II Bombers, Marcelle Size Knaack, Office of Air Force History, 1988.

  3. Boeing Aircraft Since 1916, Peter M. Bowers, Naval Institute Press, 1989.

  4. United States Military Aircraft Since 1909, Gordon Swanborough and Peter M. Bowers, Smithsonian, 1989.

  5. Boeing B-52--A Documentary History, Walter Boyne, Smithsonian Institution Press, 1981.

  6. Boeing's Cold War Warrior--B-52 Stratofortress, Robert F. Dorr and Lindsay Peacock, Osprey Aerospace, 1995.