Flight testing of the Boeing F/A-18E/F has reached the half-way point and demonstrated the aircraft's immunity to the "falling-leaf" departure problem which it suffered by earlier models, according to the test team reporting at the 41st Society of Experimental Test Pilots meeting in Beverly Hills, California, on 25-26 September.
The falling-leaf control problem crops up on current F/A-18 models when the aircraft leaves controlled flight at high angles of attack (AoA). The aircraft is known to enter an in-phase yawing-and-rolling motion which reduces pilot-recovery time and has led to several crashes. The F/A-18E/F integrated test team, which notched up its 1,000th flight on 12 September, made investigation of any falling-leaf susceptibility one of its top handling priorities.
F/A-18E/F project pilot Fred Madenwald says that the team knew it had "-got rid of the falling-leaf characteristic" during phase two of a three-phase effort with aircraft E-4, one of seven dedicated test F/A-18E/Fs. Other goals of the high AoA tests with the E-4 included high departure-resistance, roll performance with a centreline tank as good as that of the F/A-18C/D without a tank, and no AoA limits with air-to-air missiles.
Phase one concentrated on single-axis rolls at high positive AoA, demonstrating "good departure resistance and good roll capability above 30-35 degrees". It also revealed sluggish roll rates at 20-25 degrees AoA, however, and a slow reset from large negative-g AoA. Flight-control-software modifications have been developed to solve these problems, and similar solutions are being looked at to cure "poor flying qualities in manual spin-recovery mode" between 25 and 40 degrees AoA.
Phase two concentrated on spins and validating recovery techniques. "We have 53 spins under our belt," says Madenwald, who adds that recovery techniques work and engine performance was good throughout, with the exception of "-pop stalls at negative g which were deemed not serious".
Phase three looked at tactical manoeuvres with aggravated control inputs in attempts to force the aircraft to depart from controlled flight. "With most inputs it was departure resistant, but in one case, with the stick in the aft corner, we went to 85í-yaw rate and a -3.7g spike," says Madenwald.
The flight-control laws have been changed to eliminate this loophole. The team plans to complete "clean" spins this month and will move into spin testing with stores early in 1998.
Source: Flight International
