نشریه دانش و فناوری هوافضا
سال هفتم شماره 2 (پاییز و زمستان 1397)

In many cases of rotating systems, like jet engines, coaxial rotor system is used for power transmission between a high/low pressure turbine and a compressor. In this paper, chaos analysis of a magnetically supported coaxial rotor system in auxiliary bearings is presented, which includes gyroscopic moments of disks and geometric coupling of the magnetic actuators. The nonlinear equations of motion are developed using the Lagrange’s equations and solved by the Rung-Kutta method. Then, the effects of friction and clearance in auxiliary bearings on the chaotic vibration of the coaxial rotor-AMB system are investigated by the dynamic trajectories, power spectra, Poincare´ maps, bifurcation diagrams and maximum Lyapunov exponents. The results show that dynamics of the system can be significantly affected by varying these parameters, so that the system responses reveal a rich variety of nonlinear dynamical phenomena including chaos and jump. Also according to the results of analysis, some threshold values can be obtained with regard to the design of appropriate parameters for this system.

In this paper, the amount of spring-back after 3- roll bending and the effect of different factors such as friction, lateral roller position, roll speed, and thickness reduction percentage of rolling sheet prior to bending on the spring back ratio of aluminum alloy sheet in ambient temperature have been investigated. For this purpose, simulation and finite element roll forming model is done by ABAQUS. In order to validate, a comparison between simulation data and experimental results was performed. To obtain the equation for precise prediction of the values of bending radius after spring-back using SPSS software, regression analysis of the radius values after the spring return obtained from finite element simulation has been studied. Then, the effects of factors affecting bending radius and spring-back ratio have been discussed. The results show that with the method presented in this paper precise prediction of the effect of roll bending parameters changes on spring-back ratio can be obtained. In the following, this technique is used to produce shells with two different curvatures. This method can be a very suitable alternative for bending in the mold or stretch forming in the production of a weld shell and the body of the aircraft.

In this paper, the energy absorption and delamination damage in thin composite plates reinforced with nanoparticles under ballistic impact are investigated analytically. During perforation process in the nanocomposite plates considered different regions such as: fracture region, elastic deformation region, delamination region. Mechanical properties like tensile modulus, fracture strain, shear modulus, strain energy release rate specification of projectile and target were used as input to the analytical model. Then, by using of analytical relations and input data are achieved the deflection, strains and tensions around the impact point of the target. Also the amount of energy absorbed by different failure modes and the kinetic energy variation of projectile and target at small time intervals, the radius and energy of the delamination and residual velocity of the projectile is estimated. Finally, the results of analytical models on the nanocomposite plates compared with experimental results under ballistic impact and a good agreement was found. According to the results obtained the effect of nanoparticles on the improved mechanical properties of composite materials and also investigation of various failure modes is discussed.

Accompany with technology improvement, traditional systems have been superseded with complex engineering systems. Operation in uncertain and dynamic environments is one of the most important characteristics of these types of systems. The perturbations, which exist in uncertain and dynamic environments, can extremely affect the value delivery of complex engineering systems to their stakeholders. So improving the ability of these systems against uncertainty seems inevitable. Therefore, in this study we have proposed some design principles and rules which when added to system architecture leads to have more viable systems against perturbations in uncertain and dynamic environments. Also, a 7 step mathematical model is presented for analyzing the amount of design principles’ impact on systems ability against uncertainty. The main characteristics of proposed model are: describing the uncertainty in the operational environment, analyzing how the uncertainty will affect the functional and physical characteristics of the system and finally representing the regions in the system architecture that are mostly impacted by the operational uncertainties   The applicability of the proposed model is presented by using a descriptive example of synthetic aperture radar satellite as a complex engineering system and finally the results of that have been analyzed by more details.

In this paper a new method for guidance of solid propellant and aerodynamic control missile is introduced. In proposed method after achieving maximum acceleration, flight path angle of missile changes in order to impact to target. In this method flight information such as time history of magnitude of velocity and position vectors are saved in the table before firing. For calculating difference of nominal and disturbed parameters new concepts are used. Corrected values of nominal parameters are used in guidance after firing to guaranty impact to target. The flight simulation of proposed method is compared with functional guidance method and the effect of the proposed method is shown. Design verification is done in base of simulation in presence of flight disturbances.

Calculation the rate of change or differentiation of digital signal has always been one of the most important challenges in Digital Signal Processing field. Differentiating digital signals is essential in various applications. In many applications, even well-known methods aren’t useful and cause sharp increase in error and noise up to 10 times. One of the applications that is focused on in this paper is determining velocity from noisy and discrete data. Various aspects of this issue is investigated and the best derivation algorithm for the aforementioned application is designed and proposed. It was seen that Kalman filter method is the best approach for minimizing least square error for determining derivative from GPS data. It was also seen that Kalman filter method has not good transient response. This problem is somewhat improved with modification Kalman filter and using adaptive Kalman filter with appropriate weighting of old data. Another important aspect, which is also discussed in this paper, is adequate classification of various differentiation algorithms that is designed and referring the practical application of each algorithms. In order to depict the advantages of the methods, some practical results are given based on real GPS data for extracting instantaneous velocity vector.

In this paper, performance of communication channel a multiple-antenna high-altitude UAV (MIMO-HALE) with considering atmospheric conditions is analyzed. In this system, orthogonality between sub-channels is established with optimal placing antenna arrays (receiver and sender) and this result in achieving full rank channel and maximum capacity. In this paper, optimal capacity of the channel is mathematically analyzed and simulated by choosing a suitable model channel (statistical-geometrical), and considering the UAV flight parameters, in clear sky conditions and line-of-sight (LOS) channel The effect of rain is also simulated as one of the most important factors of atmospheric conditions on quality of channel and the change of capacity in high-altitude UAV with analysis outage capacity. In the following, statistical specifications (PDF, CDF) of received signal-to-noise ratio (SNR) and average symbol error rate (ASER) with MPSK Modulation is analyzed and simulated. Finally, simulations the results of simulations are compared with the analytical expressions to verify the exactness of the derived theoretical results.

The aim of this paper is to design an Algorithm for damage detection of the open cycle liquid propellant engine which is based on artificial neural networks in combination with stochastic analysis. Damage is simulated as cavitation in pumps (oxidizer or fuel pump) and fouling in some path of engine. The key stone of the method is feed-forward multi-layer neural network with back propagation algorithm. This network uses output signals of unhealthy system to detect place and quantity of damage. It is impossible to obtain appropriate training set for real engine, so stochastic analysis using mathematical model is carried out and dynamic simulation is made to get training set virtually. Result of dynamic simulation of engine is validated with experimental result. In this plan, percentage of variation of output signals of engine such as output pressure of subsystem and revolution of turbine, considered as best input data for neural network. This data is obtained from output parameters of simulated unhealthy engine. Finally, this damage detection approach was carried out using laboratory hot test.

A fin in an aircraft accounts for stability and control which can be in planar or latticed shape. A latticed fin consists of several crossing planes placed in a frame and in contrast to planar fin, is perpendicular to flow direction. In comparison to planar fins, latticed fins have low hinge moment, high stall angel, high axial force and low roll dynamic coefficient. Recently, designers have more focused on latticed fins, because of high hinge moment of them in supersonic flow regime. Due to smaller chord length, latticed fin has lower roll coefficient in comparison to planar fin, which is important in roll stability of aircraft. Therefore, design of a latticed fin with low axial force and high roll dynamic coefficient is very important. In this paper, impact of latticed fin sweep back angle on roll dynamic and static coefficients in supersonic regime is investigated using computational fluid dynamics (CFD). Firstly, simulation of two geometries with known experimental results was done and then, optimum grid and proper turbulence model is chosen. Simulation for obtaining static coefficients was done in steady state and for dynamic coefficients; it was done in unsteady condition. Figures of axial force coefficient, normal force coefficient gradient and roll dynamic coefficient is given for flight Mach of 1.1, 1.5 and 2. Results show that the effect of fin sweep back angle relates to flight Mach. In low flight Mach, sweep back angle improves fin performance, but in high flight Mach, it causes poor aerodynamic performance of fin.

The first solution to increase the efficiency of spacecraft is to decrease the weight of spacecraft which cause an increase in speed and flight range. Since the body of spacecraft tolerates a specific temperature range, the use of thermal protection systems which are designed to increase the weight of the structure in optimal conditions is essential. In this paper, thermal insulation with heat sink as a body boundary condition is considered in order to lower body temperature. For this purpose, transient thermal conduction equations are written in the curvilinear coordinate system and it is developed for a variety of axially symmetric vehicles geometries. The ablative insulation material is graphite and the effects of pyrolysis layer are ignored. The equations are solved with using discrete finite difference method and governing equations are solved with using alternating direction implicit method (ADI). Then the impact of the heatsink with different thickness on body temperature was investigated. The numerical results were compared with the exact solution results and they can see that their difference is less than 2% at all times. To reduce the body temperature, the concept of the heatsink is used and the results show that the heatsink decrease from 10% to 24% of the body temperature (depending on the thickness of the heatsink).