Analysis of flying qualities for an F/A-18 aircraft

The stability and control derivatives were calculated at NASA Dryden Flight Research Center DFRC laboratories for a number of 52 flight test conditions characterized by combinations of Mach numbers, altitudes and angles of attack for the longitudinal and lateral F/A-18 aircraft motion. More precisely, these different flight tests were considered at Mach numbers situated between 0.3 and 1.3, at altitudes between 1,000 ft and 40,000 ft and at angles of attack between 1° and 10°. Following two methods were used to calculate the characteristic system eigenvalues: the approximate method and the exact method. From these eigenvalues, by use of the classic vibration equation, the natural frequencies and damping were determined for the longitudinal and for the lateral aircraft motion. In case of the longitudinal aircraft motion, the flying qualities were evaluated for the long and short period of motion, while for the lateral aircraft motion, the flying qualities were evaluated for the roll, Dutch roll and spiral motions. From the values of frequencies and damping, based on aircraft class and phase, the flying qualities for the F/A-18 aircraft were determined. In this paper, is shown, for the first time in the literature, that relationships exist between flying qualities levels 1, 2 and 3 and flight conditions expressed in terms of Mach numbers, altitudes and angles of attack for the F/A-18 SRA (System Research Aircraft). These relationships are extremely helpful in detecting if intermediate stability and control derivatives are well calculated for flight tests intermediate conditions.