Guy Norris / Los Angeles
Crews' reports of "unacceptable lateral accelerations" mean redesign is needed to achieve full envelope clearance
Full flight-envelope clearance of the US Air Force's Boeing YAL-1Aairborne laser (ABL) has been delayed while the Active Ranging System's (ARS) pylon is redesigned following problems with high buffet and lateral accelerations.
The ARS pylon is above the flightdeck roof on the modified747-400F, and is adopted directly from a Lockheed Martin AC-130 system despite the large cruise speed difference. Test crews operating from Wichita, Kansas, reported "unacceptable lateral accelerations" of 1g-plus from the ARS as the aircraft was flown through Mach 0.73. Analysis showed a local airflow of M1.4, although indications of accelerations up to 9gproved false.
Redesign of the pylon is required to complete full envelope expansion. The test team aimed to achieve this "in the first two flights" after an initial sortie on 18 July, but this will not be completed before year-end, says the team. The ARS is intended to provide three-dimensional tracking of ballistic targets, and was removed after the third test flight.
Should the aerodynamic problem prove difficult to solve, the USAF says ARS is not needed for completion of tests. "We don't need it for the shoot-down mission and [the ARS] can track a single missile with the tracking and beacon illuminating lasers," says the USAF.
Flight test targets for the next phase include completion of the flight envelope to M0.85 and 340kt (630km/h) with the ARS installed and the covers removed from around the nose-mounted laser turret. The covers are used to temporarily seal the gaps around the turret to prevent unsteady vortex generation and buffeting on the front fuselage.
Source: Flight International