Numerical Investigation Of Aircraft Rock In Multiple Degrees Of Freedom

Rock is a universal problem of modern combat aircraft maneuvering in high angleof attack, it is a complex motion which is dominated by large amplitude free-roll andcoupled with pitch, sideslip and vertical movement. Rock has significant influence oncombat effectiveness and envelope, and is treated as a key problem that should be fixedin designing phase. A lot of researches focus on rock phenomenon and its mechanism,but many problems are still need to be solved. Rock is a motion essentially coupled withmultiple degrees of freedom (DOF), but in literature, researches on the assumption ofsingle DOF are in absolutely dominant, it is seldom investigated with multiple DOFssimultaneously. This paper concerns with dynamic coupling rock of aircraft, solves N-Sequations and Euler rigid body dynamic equations by coupling method, numericallysimulates the unsteady rock motion in multiple DOFs, and investigates the motioncharacteristics and complex flow mechanism of aircraft in high angle of attack.Firstly, high performance numerical simulation method and relevant software areestablished by sequentially solving the flow control equations and rigid body dynamicequations in a coupling way.â‘ In terms of the characteristics of rock motion, analysesof main movement freedoms are conducted, and then the unsteady dynamic model ofcoupling motions is constructed.â‘¡The2nd-order, upwind, finite volume NNDscheme and LU-SGS method with dual time steps are employed to discretize the flowcontrol equations, based on rigid dynamic mesh technology, a coupling method isemployed to solve the flow control equations and rigid body dynamic equations, thedynamic simulation software suitable for multiple DOFs motion of complicate aircraftis developed consequently.â‘¢Parallel algorithm based on MPI environment isdeveloped for higher efficiency.â‘£Typical validating examples are employed toassess the method and software developed in this paper, and the outcomes show goodagreement with relevant experimental or reference results.Secondly, investigation of the coupling characteristics of slender delta wing incombined force-pitch and free-roll motion focus on the the transverse/lateral stabilitiesof combat aircraft pitching maneuver in high angle of attack. Results show: thehysteresis of pitching motion lags the variations of roll amplitudes, no limit cycle rolloscillation is observed; the coupling effect of pitch and roll motion leads to significantlydecrease of lift force, which is harmful to maneuvering safety and worsens thetransverse stabilities of delta wings. Additionally, the investigation shows that severalsensitive factors have profound influence on the characteristics of coupling motion inpitching and rolling: the moment of inertial affects the roll frequency, the smaller rollmoment of inertial, the higher roll frequency; the structural damp of bearing affects the amplitudes, the larger structural damp, the smaller roll amplitudes, enough large dampeven eliminates the rock phenomenon of delta wing; the plateau curve of roll angleobserved in experiment is result of the collaboration of turbulence, moment of inertialand bearing friction, et al.Thirdly, numerical investigation of the characteristics of double DOFs motion incombined free-roll and free-sideslip and three DOFs motion in combined roll, sideslipand vertical movement are conducted focusing on the DOFs similarity in dynamicproblem. Results show: motion in combined free-roll and free-sideslip behaves asperiodic limit cycle oscillation, sideslip is observed to oscillate with roll in the samefrequency and opposite phase, the coupling mechanism of roll and sideslip can bedescribed as right sideslip with left roll and left sideslip with right roll; compared withthe single DOF wing rock, larger roll amplitudes, lower roll frequencies and smallerbifurcation attack angle are observed in double DOFs motion, which indicates apossibility that the delta wing is stable in single DOF wing rock, but unstable incombined roll and sideslip wing rock, therefore, DOFs similarity should be take as animportant similarity rule to be satisfied in dynamic characteristics researches. Theinvestigation of three DOFs motion in combined roll, sideslip and vertical movementshows: when the gravity is obviously lower than the vertical force, the delta wing rockmay eliminate owing to fiercely climb up; when the gravity is obviously larger than thevertical force, the swift sinking of delta wing lead to an increase of nominal attack angle,as a result, the roll amplitudes may significantly decrease with the increasing dampeffect of vortex burst; when the gravity and vertical force are comparative, small rangeof climbing up and sinking may emerge sequentially. In the case of gravity is equivalentto vertical force, the movement in vertical direction could be neglected, then the threeDOFs motion can be simplified to double DOFs motion in combined roll and sideslip, ifa large mass of delta wing is enforced, the coupling motion can be further simplified tosingle DOF wing rock, that is to say single DOF wing rock is a rational simplification ofessentially multiple DOFs motion in some special conditions.Finally, investigation of single DOF wing/body rock phenomenon on the ground ofcomplicate combat aircraft is conducted. The nonlinearly interaction between theunsymmetrical oscillating fore-body vortexes and the burst vortical flow on wings is themechanism which sustains the roll amplitudes of wing/body configuration. Under thestrong interactions among vortexes, no regular roll oscillation (such as slender singledelta wing) is built. The history curves of roll angle are accompanied with amplitudesfluctuation, even with intermittent and abrupt characteristics, thus the averageamplitudes should be concerned with. Additionally, the results show that thecharacteristics of wing/body rock phenomenon are influenced by the local configuration:the influence of fore-body configuration is primary, it has profound influence ontransverse stabilities; to some extent, tail wings enforce the dynamic stabilities of static stabilities of simple delta wings, the influences of leading edge of wing on thewing/body dynamic characteristics are secondary, no obvious impact is observed.