Joint USAF/RAAF trial will compare effect of active separation control on trajectories

Preparations are under way for joint US Air Force/Royal Australian Air Force flight tests of an active separation control (ASC) system intended to bea "virtual" spoiler and improve weapons bay acoustics as well as enhance weapon separation.

An RAAF General Dynamics F-111G modified with an updated version of the ASC, first tested on the aircraft in 2001, will be used for the trial. The test's original objective was to compare separation trajectory differences in generic small smart bombs (SSB) with and without the ASC activated. The ASC is a blowing device which injects nitrogen through a slot nozzle at the leading edge of the weapons bay. This produces a bow shock upstream of the bay cavity that causes flow - and the weapon - to be sucked out of the cavity behind the forward bulkhead.

The US Air Force Research Laboratory (AFRL) has been studying alternatives to spoilers since 1993, and says the system could be an option on the Lockheed Martin F-35 as well as for retrofit. Existing spoilers project into the slipstream and "become a problem as the Mach number increases", says the USAF. It says circulation created by the spoiler also pushes the weapons nose-up towards the top of the bay, "which is also a problem".

AFRL teamed for the tests with the RAAF Aircraft Research and Development Unit because windtunnel and flight-test data with and without the spoiler are available for the supersonicF-111.

Modifications to the ASC include the addition of supersonic slot nozzles to increase the penetration of the gas jets during supersonic flight. The jets' pulsing will also be redesigned to extend the blowing period from 0.3s. Although the ASC uses up to six 210bar (3,000lb/in2) nitrogen bottles, the USAF says future systems could use compressor bleed air or a environmental control system supply.

In the first tests, 16 SSB inert shapes were released from the right side of the modified bomb bay during eight sorties. Results showed the system reduced the tendency of the aft bomb to pitch up by over 8¡ in subsonic launch conditions, while acoustic levels were also reduced. No major differences were seen during supersonic launches, hence the modifications for the next phase.

Source: Flight International