Convair B-36 - Design

Design

The B-36 took shape as an aircraft of immense proportions. It was two-thirds longer than the previous "superbomber", the B-29. The wingspan and tail height of the B-36 exceeded those of the Antonov An-22, the largest ever mass-produced propeller-driven aircraft. Only with the advent of the Boeing 747 and the Lockheed C-5 Galaxy, both designed two decades later, did aircraft capable of lifting a heavier payload become commonplace.

The wings of the B-36 were large even when compared with present-day aircraft, exceeding, for example, those of the C-5 Galaxy, and enabled the B-36 to carry enough fuel to fly very long missions without refueling. The widest point around the chord of the wing was 7.5 feet (2.3 m) thick containing a crawlspace that allowed crew access to the engines. The wing area permitted cruising altitudes well above the operating ceiling of any 1940s-era piston and jet-turbine fighters. All versions of the B-36 could cruise at over 40,000 ft (12,000 m). B-36 mission logs commonly recorded mock attacks against U.S. cities while flying at 49,000 ft. In 1954, the turrets and other nonessential equipment were removed, resulting in a "featherweight" configuration believed to have resulted in a top speed of 423 mph (700 km/h), and cruise at 50,000 ft (15,000 m) and dash at over 55,000 ft (16,800 m), perhaps even higher.

The large wing area and the option of starting the four jet engines gave the B-36 a wide margin between stall speed (V_S) and maximum speed (V_max) at these altitudes. This made the B-36 more maneuverable at high altitude than the USAF jet interceptors of the day, which either could not fly above 40,000 ft (12,000 m), or if they did, were likely to stall out when trying to maneuver or fire their guns. However, the U.S. Navy argued that their McDonnell F2H Banshee fighter could intercept the B-36, thanks to its ability to operate at more than 50,000 ft (15,000 m). The USAF declined the invitation from the U.S. Navy for a fly-off between the Banshee and the B-36. Later, the new Secretary of Defense, Louis A. Johnson, who considered the U.S. Navy and naval aviation essentially obsolete in favor of the U.S. Air Force and Strategic Air Command, forbade putting the navy's claim to the test.

The propulsion system alone made the B-36 a very unusual aircraft. All B-36s featured six 28-cylinder Pratt & Whitney R-4360 'Wasp Major' radial engines. Even though the prototype R-4360s delivered a total of 18,000 hp (13 MW), early B-36s were slow and required long takeoff runs. The situation improved with later versions delivering 3,800 hp (2.8 MW) apiece. Each engine drove an immense three-bladed propeller, 19 ft (5.8 m) in diameter, mounted in the pusher configuration. This unusual configuration prevented propeller turbulence from interfering with airflow over the wing, but also led to chronic engine-overheating due to insufficient airflow around the engines, resulting in numerous in-flight engine fires.

Noise at altitude as a consequence of the large, slow-turning propellers clipping high-pressure air behind the wings produced a characteristic very-low-frequency pulse at ground level, approaching infrasound (sound pulses slower than 20 Hz), that betrayed approaching overflights.

Beginning with the B-36D, Convair added a pair of General Electric J47-19 jet engines suspended near the end of each wing; these were also retrofitted to all extant B-36Bs. Thus the B-36 came to have 10 engines ("six turnin' and four burnin' ", as said by American airmen), more than any other mass-produced aircraft. The jet pods greatly improved takeoff performance and dash speed over the target. In normal cruising flight, the jet engines were shut down to conserve fuel. When the jet engines were shut down louvers closed off the front of the pods to reduce drag and to prevent ingestion of sand and dirt on take off. The jet engine louvers were opened and closed by the flight crew in the cockpit, whether the B-36 was on the ground or in the air. The two pods with four turbojets and the six piston engines combined gave the B-36 a total of 40,000 hp for short periods of time.

The B-36 had a crew of 15. As in the B-29, the pressurized flight deck and crew compartment were linked to the rear compartment by a pressurized tunnel through the bomb bay. In the B-36, one rode through the tunnel on a wheeled trolley, by pulling oneself on a rope. The rear compartment featured six bunks and a dining galley, and led to the tail turret. The B-36 also tested the experimental Boston Camera.

The XB-36 featured a single-wheel main landing gear whose tires were the largest ever manufactured up to that time, 9 ft 2 in (2.7 m) tall, 3 ft (1 m) wide, and weighing 1,320 lb (600 kg), with enough rubber for 60 automobile tires. These tires placed so much weight per unit area on runways, the XB-36 was restricted to the Fort Worth airfield adjacent to the plant of manufacture, and to a mere two USAF bases beyond that. At the suggestion of General Henry H. Arnold, the single-wheel gear was soon replaced by a four-wheel bogie. At one point a tank-like tracked landing gear was also tried on the XB-36, but proved heavy and noisy and was quickly abandoned.