THE INTEGRATED US Navy/contractor test team reports unexpected discoveries in its evaluation of the McDonnell Douglas F-18E/F Super Hornet, including a "distinct wing-drop tendency" during deceleration through 13° to 15°angle of attack (AoA). The drop, described as a "little bit alarming", has been cured by modifying the control scheduling of a vent in the aircraft's large leading-edge extension (LEX). The LEX are 34% larger than those of the C/D, to improve pitch control at high AoA, and incorporate a vent to allow high-energy air to flow over the wing-fuselage blend.
Analysis revealed significant flow separation over the outer wing panel, with "asymmetric bursts" which caused the sudden wing drop. LEX vents are now "up" for all power-approach configurations: this modification appears to have solved the problem.
Rapid progress has been made with four aircraft, including the F-1, the first two-seat F-18F, which underwent more than 400h in nearly 250 sorties at the US Navy's Patuxent River, Maryland, test centre. Overall, the test team reports "clear, level one, flying qualities and good handling", and tests are expected to be completed by December 1998. All seven, test aircraft will be delivered to Patuxent River by the end of December 1996. "Some 2,000 flight test sorties are planned, with around ten a month [per aircraft] and an average of 1.4h/sortie, but we're flying significantly more often than that so far," says the test team.
Another discovery was aileron limit-cycle oscillation, originally generated by flutter excitation. Various solutions to this aero-elastic condition will be evaluated, including changing the aileron bias in the neutral position, and stiffening up the structure of the aileron shroud.
Some changes to the aircraft's more-powerful General Electric F414 engines have also been necessary. Alterations to the flow body were made following the discovery of a 15Hz oscillation in the aircraft and fuel system. This translated into "-dynamic discharges that could be felt in the cockpit and which affected engine performance". The first engines also produced "-significant exhaust smoke, which is not a good thing". Hardware modifications to the combustor cured this problem. Changes have also been made to the software management in the F414's full-authority digital engine-control system, to cure a nozzle-control issue.
Forthcoming tests include all-important carrier-suitability trials, scheduled to begin on board the USS Stennis in January 1997. In preparation for these, one test aircraft is undergoing simulated ship-board arresting trials at the US Navy's Lakehurst test site in New Jersey. Further ground-load tests will also be conducted at Patuxent River before the end of the year. "A major challenge is weapons separation," says the team. This will cover the release of more than 50 types of missiles and gravity weapons, and will be used to evaluate late changes made to pylons after wind tunnel tests predicted collisions of adjacent stores on the inboard pylon.
"Overall, the aircraft is performing well and meeting specification," says the team. "The aircraft is clearly superior [to the C/D] in subsonic acceleration and climb. The jury is still out in the transonic and supersonic acceleration areas, although the requirement was the E/F should be as 'good as, or better than' the C/D."
Source: Flight International