Fighter key to development of quiet supersonic aircraft, with shockwave signature modified by redesigned airframe

Northrop Grumman has begun flight testing an F-5E modified to reshape the sonic boom, as part of the US Defense Advanced Research Projects Agency's Quiet Supersonic Platform (QSP) programme. The tests are key to development of low-boom supersonic aircraft.

The US Navy F-5 first flew on 24 July after installation of a reshaped, extended forward fuselage at Northrop Grumman's St Augustine, Florida, facility. The aircraft was subsequently ferried to Edwards AFB, California, where the first of eight supersonic test runs is scheduled for this week.

Under the QSP programme, the shaped sonic boom demonstration is intended to prove the shock-wave signature can be modified by shaping the aircraft. Instead of the traditional N-wave "double bang" signature of a supersonic aircraft, with an initial overpressure peak, the modified F-5 is designed to produce a flat-top sonic boom with lower overpressure, says Northrop Grumman QSP programme manager Charles Boccadoro.

A 10.7m (35ft)-long fuselage fairing made up of 22 composite panels has been installed. The nose forward of the cockpit has been replaced and the aircraft extended by 1.2m overall. The reshaped forward fuselage produces a series of compression and expansion waves designed to counteract shock waves from the F-5's unmodified cockpit, inlets and wing and prevent them coalescing to produce the traditional N-wave signature.

Boccadoro says back-to-back flights will be conducted at Mach 1.4 and 30,000ft, first with an unmodified US Navy F-5E and then, a few minutes later, with the shaped sonic boom demonstrator. A ground array will measure initial overpressures and boom signatures.

In later flights, a NASA Boeing F-15, Gulfstream V and Raytheon Premier I will take in-flight measurements close to the aircraft, at 20,000ft and 10,000ft to see how the signature develops. A shaped sonic boom that persists to the ground has never been demonstrated in flight, Boccadoro says. "We are on the verge of making history," he adds.

Sonic boom reduction through airframe shaping is key to both Gulfstream's design for a quiet supersonic business jet and work by Lockheed Martin and Northrop Grumman on long-range strike aircraft concepts under the nearlycomplete QSP programme.

Source: Flight International