Northrop YB-49 - Design and Development

Design and Development

With the XB-35 program seriously behind schedule by 1944, and the end of piston-engined combat aircraft in sight, the production contract for this propeller driven type was cancelled in May of that year. Nevertheless, the Flying Wing design was still sufficiently interesting to the Air Force that work was continued on testing a single YB-35A production aircraft.

Among the aircraft later completed were two airframes that the Air Force ordered be fitted with jet propulsion and designated as YB-49s. The first of these new YB-49 jet-powered aircraft flew on 21 October 1947 and immediately proved more promising than its piston-engined counterpart. The YB-49 set an unofficial endurance record of staying continually above 40,000 ft (12,200 m) for 6.5 hours.

The second YB-49 was lost on 5 June 1948, killing its pilot, Major Daniel Forbes (for whom Forbes Air Force Base was named), Captain Glen Edwards, co-pilot (after whom Edwards Air Force Base is named), and three other crew members, one of whom, Lt. Edward Lee Swindell was a crew member on the Boeing B-29 that assisted Chuck Yeager in breaking the sound barrier in the Bell X-1 aircraft. Their aircraft suffered structural failure, with both outer wing sections becoming detached from the center section. Speculation at the time was that the YB-49 was lost due to excessive pullout loads imposed on the heavy airframe when a scheduled flight test of the large bomber's stall recovery resulted in a sudden and dramatic high speed, nose-over dive. The post-stall high speed dive resulted from the clean, low-drag, all-wing design, which gave the YB-49 a rapid speed increase in any type of dive. Fellow YB-49 test pilot Robert Cardenas later claimed that the YB-49 rotated backwards in stall, and that he warned Edwards about it. Jack Northrop later countered that such a behavior was impossible for the all-wing design.

On 4 February 1949, the first YB-49 flew from Muroc Air Force Base in California to Andrews Air Force Base near Washington, D.C. in 4 hours 25 minutes, after which President Truman ordered a flyby of Pennsylvania Avenue at rooftop level. The return flight from Andrews was marred when four of the eight engines had to be shut down due to oil starvation. Inspection after a successful emergency landing at Winslow Airport, Arizona revealed no oil had been replaced in these engines at Wright after the Muroc-to-Andrews leg, raising a suspicion of industrial sabotage.

The last operational YB-49 prototype was destroyed on 15 March 1950, during high-speed taxi trials at Muroc Field. The nose wheel began to encounter severe vibration problems and finally collapsed; the aircraft was completely destroyed in the ensuing fire. The taxi trials took place with the YB-49's fuel tanks full, an unusual testing procedure, fanning further speculation of industrial sabotage of the aircraft.

The Air Force ordered the remaining uncompleted YB-35 piston-engined airframes be completed as production B-35B aircraft.

Bombing target tests showed a tendency of flying wings to "hunt" in yaw after turns and when flying in "disturbed" air, degraded bombing accuracy. It was thought that one of the new Honeywell autopilots, with yaw damping, would correct this flaw. Northrop chief test pilot on the YB-35 and YB-49 programs Max Stanley contends in the 1992 Discovery Channel documentary "The Wing Will Fly", the adaptation of the autopilot in this dual function "damped out the directional oscillations to the degree where,...I think you would say it met the (military) specifications." Brig. General Robert Cardenas also flew the YB-49 during many of its test flights, praising the aircraft for its marvelous performance, while also noting the YB-49 required a very long bomb run to dampen out directional oscillations. Many of these challenges would eventually be overcome when fly-by-wire systems and computer generated artificial stability became available in the 1970s, culminating in the development of the all-wing Northrop Grumman B-2 Spirit Stealth Bomber.

The conversion of the long range XB-35 to jet power essentially cut the effective range of the aircraft in half, putting it in the medium range bomber category with Boeing's new swept winged jet bomber the B-47. The B-47 was optimized for high-altitude and high-speed flight and, in an era where speed and altitude were becoming the name of the game, the YB-49's thick airfoil could never be maximized for high-speed performance. In the same Discovery Channel documentary, former Air Force Flight Test Center Historian Dr. James Young states his opinion that while political gamesmanship and back room dealing certainly played a role in the aircraft's demise, the Flying Wing program was ultimately cancelled for solid technological reasons.