Guy Norris/Beverly Hills

US Air Force test crews flying the Northrop Grumman B-2A bomber have been unable to pin down the cause of a mysterious residual pitch oscillation which affects the aircraft at low altitude and high speed.

The 1.5Hz pitch oscillation which has been observed with magnitudes of up to ±0.5g, causing the wing tips to "flap" by up to 1.83m (6ft), says the USAF's 412th Test Wing team. The phenomenon occurs only under specific conditions, including a high gross weight, with full payload, full outboard fuel tanks and high speed at low altitude. The oscillation is not severe enough to endanger the aircraft or its mission, but is uncomfortable for the crews, says the team.

At the 41st Society of Experimental Test Pilots meeting in Beverly Hills, California, on 25-26 September, the USAF revealed details of tests conducted in June and July 1995 which attempted to determine if the oscillation was caused by some previously undiscovered flutter, or by the flight controls. Results quickly eliminated flutter and the outboard elevons as the cause.

Investigations focused on observed close relationship between acceleration and the onset of the phenomenon. "We got decreasing damping with increasing Mach numbers," says the team, which also looked into the theory that the oscillation was related to "dithering" of the flight controls. This was tied to a "deadband" in the actuator motion which led to "dithering of control surfaces and reduced damping".

The team also tested a NASA theory that the phenomenon was caused by close coupling between the aircraft's short body and large wingspan in a condition called "body-freedom flutter". This was believed to be a complicated feedback loop which involved the flight-control system, but after tests this was also eliminated.

Investigators believe that the oscillation could be related to interaction between the aircraft's short body and a strong shockwave which forms just aft of the engine intake area at about one-sixth- chord and extends over the exhaust area. The shockwave was revealed by condensation effects captured on video taken of the aircraft on low-level, over-water, runs.

Source: Flight International