Flight data gathered by use of the F-18 System Research Aircraft (SRA) based at Dryden Flight Research Center have been used to estimate stability and control derivatives for a baseline F-18 airplane. The data were obtained in the high-dynamic-pressure range of the F-18 flight envelope in an experiment performed in support of a future F-18 program to be devoted to the concept of the active aeroelastic wing (AAW). The AAW technology is intended to integrate aerodynamics, active controls, and aeroelasticity in such a way as to maximize the performance of the airplane. More specifically, the goal of the AAW project will be to maximize the contribution of a reduced-stiffness F-18 wing to roll-rate performance.
In order to support the AAW technology, changes in flight-control computers and software will be required, and an understanding of the effectiveness of each control surface under various conditions is essential. The experiment on the SRA was performed to obtain this understanding. The results of the experiment can be used to update a mathematical model of the aerodynamics of the F-18 airplane, which model can be used to improve the control laws under development for the AAW version of the F-18 airplane.
In the experiment, an onboard excitation system (OBES) was used to provide uncorrelated single-surface input (SSI) doublet sequences. Longitudinal maneuvers included leading-edge flap (LEF), trailing-edge flap (TEF), symmetric aileron, and symmetric horizontaltail SSIs. Lateral-directional maneuvers included rudder, differential LEF, differential TEF, aileron, and differential tail SSIs. The pilot initiated each sequence of maneuvers from the cockpit and the OBES commanded the SSI doublets. During some maneuvers, the control surfaces were moved in combinations not used by the basic F-18 control laws: these included symmetric LEF, TEF, and aileron deflections at high speeds.
The data acquired during flight were analyzed afterward by use of an outputerror parameter-estimation algorithm. A complete set of experimental stability and control derivatives was obtained for tests performed at 20 different combinations of mach number and altitude. A complete set of nominal control derivatives for these test conditions was also obtained from computational simulations. For example, the top part of the figure shows the effect of symmetric aileron deflection on pitching moment. In this plot, the magnitude of the experimental pitching-moment effectiveness can be seen to exceed that obtained by computational simulation, which is represented by lines for various altitudes between 5,000 and 25,000 ft (1,524 and 7,620 m), especially at low mach numbers.
Control-surface rolling-moment “reversal” was of special interest to the project. The middle and lower parts of the figure show TEF and aileron rolling-moment derivatives. For example TEF reversal was revealed by the flight data acquired at mach 0.95 at altitudes below 10,000 ft (3,048 m). The flight data did not reveal aileron reversal, but did show that aileron effectiveness was reduced by increasing the mach number (especially in the subsonic range) and reducing the altitude.
This work was done by Tim Moes and Gregg Noffz of Dryden Flight Research Center.