Investigation On Unsteady Aerodynamic Modeling Approach And Application Of (?) Model At High Angles Of Attack

For the requirements of modern fighters which should have the capacity for controllable flight at high angles of attack(AOA),investigations on the experimental technique of wind tunnel,unsteady aerodynamic characteristics caused by angular velocity vector,unsteady aerodynamic modeling,prediction of departure behavior,design of control-augmentation system(CAS),flight simulation and spin characteristics were conducted.Experimental investigations on yaw-roll coupled motions were first conducted in the wind tunnel.Different degrees of yaw-rolling coupling were realized by adjusting the amplitudes of yawing or rolling motions.The effects of frequency and coupling ratio on the damping characteristics of yawing and rolling moment coefficients were studied within the range of the experiments.The difference between the experimental data and the calculated data from the conventional aerodynamic derivative model were exhibited.The comparisons shown that the dynamic derivative method cannot actually reveal the variations of lateral-directional aerodynamic damping characteristics in different coupled motions.Based on the experimental data of yaw-roll coupled motions,an unsteady aerodynamic model named (?) model were proposed.In the (?) model,the unsteady aerodynamic moments produced by the rotation vector were disassembled into two parts,which were severally produced by the module of rotation rate and the time rate of sideslip angle.The aerodynamic moment coefficients in the motions with a higher frequency than the experiments could also be conveniently obtained by this aerodynamic model.The yawing and rolling moment coefficients in different forms of motions(yaw-roll coupled,single degree of freedom forced oscillations superimposed in the rotary motion,pitch-yaw coupled,pitch-roll coupled,etc.)were calculated by (?) model and dynamic derivative model,respectively.The comparisons between the calculated results of these two aerodynamic models showed that the (?) model could reveal the damping characterics of the aerodynamic momennts in different motion more actually.Afterwards,the predictions of lateral-directional departure were obtained by different methods.A new set of stability criteria were obtained from (?) model,based on linearized equations of motion.Compared to the conventional stability criteria,the prediction results of (?) model were much more accurate,and the sensitive area of departure could be further specified to a detailed range of the coupling ratio.With the sufficient consideration of the yaw-roll coupling effects on the unsteady aerodynamic characteristics,the (?) model could actually reproduce the departure behavior appearing in the flight test of F-16XL,which could not be found in the simulation results of the conventional aerodynamic derivative model.According to the prediction results of departure behavior,two different CASs were severally designed by (?) model and aerodynamic derivative method,using pole-placement method.The conventional matrix of feedback gains obtained from aerodynamic derivatives was unique at each AO A.Dissimilarly,a two-segment CAS was obtained from (?) model with respect to the stable and unstable areas of coupling ratios,respectively.In the simulations of straight flight at the AOA which was higher than the critical value of departure,the required control power of conventional CAS was much larger than the one in the two-segment CAS to achieve the same flight quality.Moreover,a violent control would lead to a lateral-directional departure under the action of the conventional CAS.By contrast,the two-segment CAS could provide enough stability in the turning flight,even with drastic steering commands.The period and radius of turn could also be regularly changed with varying deflections of control surfaces.The simulations of the "Cobra" maneuver and "Herbst"maneuver were simplicity realized with the support of two-segment CAS.The simulation of "Herbst"maneuver also revealed that the two-segment CAS was much more effective than the conventional CAS in post-stall maneuvers.The simulations of spin motion were then conducted using (?) model and conventional aerodynamic derivative model,and the simulation results were compared with the results of in the vertical wind tunnel test.The inducements in different stages of the spin were analyzed with the concept of aerodynamic damping characteristics.The simulation result of (?) model reproduced the phenomenon that the aircraft could not be recovered from the spin by only rudder deflection,which was observed in the vertical wind tunnel test.The aerodynamic derivative model could not actually reflect the unsteady aerodynamic characteristics associated with the coupling raito,thus the feature of spin simulated by this model was different from the experimental result.The auto-recovery functions of the two CASs in a spin were also compared,the comparison shown that the two-segment CAS was much more available than the conventional method.The unsteady aerodynamic model proposed in this investigation,which was based on yaw-roll coupled motion tests,could completely reflect the lateral-directional unsteady aerodynamic loads caused by the angular velocity vector,and is obviously more applicative than the aerodynamic derivative model at high AOA,from several aspects of aerodynamic force calculation,prediction of departure,flight simulation,design of CAS,etc.Furthermore,the (?) model can be compatible with the dynamic derivatives which are commonly used in the engineering application of flight dynamics at low AOA,by local linearization.This investigation is of certain worth for theoretical and practical values in the design and dynamic analysis of a maneuverable fighter,which is required the capability of controllable flight at high AOA.