Abstract
The significant dynamic response under the combined impact of aerodynamic and hydrodynamic forces could be likely to appear because of the structural flexibility, when taxiing on the water surface for amphibious aircraft. Meanwhile, the modal characteristics of the structure are also affected by the additional motion of water. These require that the influence of the structural elasticity and the coupling effect between water and structure should be considered in dynamic response analysis of water-taxiing. According to the peculiarities of the amphibious aircraft, structural dynamics model is based on the distribution of stiffness and mass, Virtual Mass Theory is utilized to solve the wet modes on the water surface, rational function approximations of unsteady aerodynamic force in time-domain are constructed by the Minimum-State Approximation Formula, and loose coupling method is employed to simulate the hydrodynamic elastic response under the encounter of amphibian with single wave and repeated waves, respectively. Analysis of dynamic characteristics during the water-taxiing of the amphibious aircraft has been achieved in this work. The results show that wet natural frequencies of the aircraft have different degrees of decline compared with the dry frequencies because of the influence of added water on the hull, and the response amplitude of dynamic loads obtained by using the wet modes have some certain extent decrease compared with the dry modes. The dynamic amplitude of different locations changes in different degree relatives to the center of gravity position, which reflects the influence of structural elasticity. Due to the excitation of single wave and repeated waves, the structural vibration amplitude will increase rapidly, but the amplitude shows a certain divergence trend under the action of repeated waves with a given oscillation frequency, which is more severe for structural strength design.
Highlights
Amphibious aircrafts possess the characteristics of both hydroplane and landplane and have the advantages of long endurance, strong environmental adaptability to take off and landing, superior flying performance at low altitude, and ultralow altitude
The Virtual Mass Theory is adopted to analyze the wet modes of large amphibious aircraft, the Minimum-State Approximation Formula is used for generalized aerodynamic time-domain fitting of lift surfaces, the hydrodynamic numerical simulation of elastic aircraft is carried out by loose coupling method, and the structural elastic responses are solved by mode superposition method
According to the simplified time history of water load under single wave as shown in Figure 7, the hydrodynamic load factor of the aircraft by empirical formula [1] is nz = 3:75 g, the structural damping coefficient obtained from Ground Vibration Test is g = 0:02, and the dry and wet modes are taken to analyze the transient response of large amphibious aircraft in typical state, respectively; the results are as follows
Summary
Amphibious aircrafts possess the characteristics of both hydroplane and landplane and have the advantages of long endurance, strong environmental adaptability to take off and landing, superior flying performance at low altitude, and ultralow altitude They are widely used in fire fighting, maritime rescue, and other special missions. It is necessary to consider the effects of aerodynamic and hydrodynamic forces as well as the coupling influence of additional water motion on the structures and should employ the wet modes to analyze the dynamic response on the water surface of large amphibious aircraft. The Virtual Mass Theory is adopted to analyze the wet modes of large amphibious aircraft, the Minimum-State Approximation Formula is used for generalized aerodynamic time-domain fitting of lift surfaces, the hydrodynamic numerical simulation of elastic aircraft is carried out by loose coupling method, and the structural elastic responses are solved by mode superposition method. Considering the water load excitation caused by single wave and repeated waves, respectively, the influence of different wave loads on the dynamic response characteristics of the structure is investigated, which can provide a significative reference for the development of the hydroelastic mechanics of amphibious aircraft
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