Flying Qualities Analysis and Comparison Between an Airliner and a Fighter Jet Aircraft

Aircraft operations mostly take place in stabilized cruise flight. It is therefore important that an aircraft is designed in such a way that disturbances are as low as possible. and have less impact on its flight. Many factors can lead to effects, such as a loss of aircraft stability, and onboard instruments play an essential role in countering these effects. Flight operations first rely on aircraft static stability, where no control input nor command signal from the pilot is involved. Therefore, the flying qualities between two different types of aircraft, namely a fighter jet Douglas A-4 and an airliner Douglas DC-8 are compared in this paper. To this end, the aircraft longitudinal and lateral flight dynamics were modeled using MATLAB/Simulink by using linear state-space representation. Perturbations were introduced on the control surfaces, such as elevators, ailerons and rudders. Aircraft state responses were visualized and compared. Different oscillatory modes affecting the longitudinal and lateral stabilities were identified using linear analysis. For this identification, the poles of the longitudinal and lateral dynamics were extracted, along with the damping and the frequency of oscillatory modes. Two longitudinal modes were compared for both aircraft, the short period mode, corresponding to rapid pitch oscillations, and the phugoid mode corresponding to a low-frequency pitch oscillations. In the lateral dynamics, axis, three different modes were compared for both aircraft, including the roll mode, which correspond to a first order motion, and a rotation around the longitudinal axis, the spiral mode, also a first order low-frequency motion which occurs when the aircraft enters a turn which gradually becomes uncontrolled, and finally a Dutch roll, which is an oscillatory mode, which corresponds to a mixed motion between yaw and roll. Introducing small perturbations allows us to compare the flight qualities between both aircraft. In fact, the flight qualities of an aircraft make it possible to evaluate its dynamic performance during flight, particularly in terms of stability, maneuverability and comfort. The flight qualities were assessed using the MIL-STD 1797A requirements, which are used by the FAA to evaluate aircraft performance. Results have shown that fighter aircraft has a better maneuverability at high speeds, rather than stability nor pilot comfort. On the contrary, airliners aim for passenger safety with good comfort, thanks to excellent stability performance. Finally, a short review of control techniques is presented, which are the main features to improve the aircraft’s flying qualities. These control techniques include classic control systems, modern control systems, and nowadays intelligent control techniques, such as neural networks and fuzzy systems, reinforcement learning and other techniques.