فصلنامه مکانیک هوافضا
سال نوزدهم شماره 2 (پیاپی 72، تابستان 1402)

The main objective of this article is to apply the control allocation approach for the landing phase of the F/A-18 aircraft. For this purpose, the non-linear three-degree-of-freedom model of the aircraft is used, and the intelligent control allocation approach, based on fuzzy logic, is utilized to design the longitudinal flight control system. The actuators involved in the aircraft landing process are the elevator angle and the thrust vector control angle of the aircraft engine. By allocating control signals between the two mentioned actuators, the plane starts the process of lowering the height and finally reaches the ground level. To improve the efficiency of the fuzzy controller, reduce the control effort and increase the accuracy and quality of the landing, the genetic algorithm based on the NSGA-II method is used and the variables of the fuzzy controller are modified. The results obtained from the simulation show that the proposed control allocation approach has a high ability to control and stabilize the aircraft in the landing process. Also, the output variables converge to a desired value and the aircraft completes the landing process with proper accuracy and low control effort.

This study deals with experimental and numerical response of trapezoidal corrugated core sandwich panels made of ST37 subjected to oblique blast loading. Sandwich target plate is 270*270 mm that consist of front/ back plate with 1 mm and trapezoidal corrugated core with 0.7mm thickness. The experimental explosive was done at the end of four different blast tube with angle of 0°, 15°, 30° and 45°respect to sandwich target plate. The mass of explosive (C4) was 10g and the stand-off distance was constant at 300 mm. The results of numerical simulation, obtained using coupled Eulerian–Lagrangian (CEL) model at commercial ABAQUS/Explicit software. The results show good agreement between the numerical and the experimental values. The results show that with increasing angle of tilt from 0° up to 45°, the maximum displacement decreases. At the higher masses of explosive, this reduction in deformation will be greater than the lower masses of explosive. Also, due to directing a large part of the blast wave to out of the blast tube, the amount of plastic dissipation energy at the angle of 45 degrees is significantly reduced compared to other angles.

In this study, finite element method is applied to simulate thermal spraying of Stellite-6 microparticles on steel substrate. The effects of particle size, substrate pre-heating temperature and spraying angle are investigated on the process output parameters including stress, equivalent plastic strain, penetration depth, and temperature distribution. The simulations are designed and performed based on the Design of Experiments (DOE). Response surface methodology (RSM) is used to explore the relationships between the input factors and responses. The simulation results revealed that penetration depth as the main factor, affecting the bonding strength of coating, is highly dependent on the particle size. Spraying angle is also found to be a significant and effective parameter on the penetration depth. On the other hand, the pre-heating temperature of the substrate is observed to have no substantial effect on the penetration depth.The depth of penetration increases with increasing the particle size and spraying angle, and reaches its maximum value in spraying angle of 90˚.

Small vibrations (Shimmy) are lateral and torsional vibrations in the aircraft wheel that excites itself and cause instability in fast functions; which can damage the aircraft's landing gear and its fuselage, as well as, in commercial aviation, this may harm passengers. Therefore, controlling and damping these vibrations is essential. For this purpose, the present study has designed a controller using the NARMA-L2 type neural method to prevent small vibrations in the aircraft's landing gear. The controller as mentioned above has a high capability against parameter uncertainties and external disturbances. To check the performance of the proposed controller, the vibration response of the system was simulated by MATLAB software, and its efficiency was measured by comparing the results obtained from RCTM and Proportional-Integral-Derivative (PID) controllers. The obtained results show a significant improvement in the performance of the closed-loop system with an effective reduction of vibrations as a result of using the proposed controller.

The additive manufacturing method, as the newest way of producing parts in medicinal and industrial fields, has made significant advancements in recent years. Among the existing methods, digital light processing (DLP) using flexible membranes is one of the most extensively utilized polymer-based approaches. Dimensional accuracy along the vertical and plane axis of the printed parts is one of the key issues in manufacturing polymer parts, both layer by layer and layerless. In this article, utilizing the system designed and manufactured in this faculty's laboratory, as well as three flexible fluorinated ethylene propylene membranes of varying thicknesses and Provision and Anycubic resins, the influence of the calibration technique and initial distance on the dimensional accuracy of printed parts in the DLP method have been investigated experimentally. By assessing the compressive forces caused by the initial distance, it was discovered that the compressive force increases when the initial distance was determined in membranes with thicknesses of 100, 150, and 200 microns in both resins, resulting in a 17.5% - 34.1% decrease of the part's thickness is caused by the fact that by positioning it at the appropriate initial distance, the resulting error can be greatly reduced. Furthermore, for both of the resins stated, reducing the thickness of the flexible membrane has a direct link with increasing the dimensional accuracy in the printed parts.

In this paper, a satellite attitude control system with the reaction wheels based on suboptimal robust tube based model predictive control is designed. The satellite receives many external bounded disturbances in space. Due to the fact that disturbances have a specific range, it is possible to satellite attitude control by using a robust predictive controller based on a tube. But since the satellite has a system with complex dynamics, the challenge of increasing the volume of calculations arises when calculating the minimal Robust Positive Invariant set. The challenge of increasing the volume of calculations of this set (tube) in complex systems such as satellites is caused by the large number of state variables of the system. The large number of system state variables causes an exponential increase in the volume of calculations due to the formation of multiple Minkowski sums in the tube calculation. In order to solve this challenge, a new solution of robust predictive controller based on suboptimal tube has been presented. This solution reduces the volume of tube calculations. Simulation has been done for the accuracy of the design of the predictive controller based on the suboptimal tube to attitude control of satellite.

Crack nucleation mechanisms, its growth, and the determination of critical failure parameters are of industrial importance. Therefore, it is inevitable to study the mechanical behavior of cracked materials under applied load during severe plastic deformation. The present study studied the mode I fracture behavior, mechanical properties, and microstructure characterization of the 6061 aluminum alloy fabricated by friction stir processing (FSP). For this purpose, a milling machine made a perfect stirring zone to perform the FSP method with the specific non-consumable tool. According to the tensile test results, the yield and ultimate tensile strength of the FSP-processed sample have increased by 39% and 37%, respectively. Based on the three-point bending test, the fracture toughness of the processed aluminum was calculated as 10.86 MPa√m, which shows a 14.3% improvement compared to the as-received annealed state. Eventually, the average grain size of the annealed and processed samples was measured as 35 and 15, respectively, which indicated a 57% reduction in the aluminum grain size after FSP. Note that this grain refinement is associated with improved strength and toughness.

Polymer electrolyte membrane fuel cells with dead-end mode operation are known as an important alternative for achieving a clean and sustainable future in terms of energy supply for the transportation sector, as well as for military and aerospace applications. Their commercial success, however, is dependent on addressing the water management issue that accumulates inside their channels and porous media during dead-end operations. The current work utilized the electrochemical impedance spectroscopy method to evaluate the effect of gradual accumulation of liquid water in the cathode on the impedance diagram of a dead-end fuel cell. Despite the fact that this approach is limited to steady-state systems and dead-end operation is transient, quasi-steady conditions were provided for the test by measuring the impedance of each frequency in a distinct dead-end interval. Furthermore, the effect of relative humidity, operating temperature, and inlet pressure of reacting gases on the impedance of a dead-end fuel cell was examined. The results show that in such a situation, a large amount of water is always present in the fuel cell, which, while it helps to keep the membrane hydrated and reduces ohmic resistance, causes difficulty in the transport of reacting gases (particularly oxygen) to the catalyst layer, increasing mass transport resistance. Moreover, flooding the catalyst layer reduces the kinetics of the reaction and, as a consequence, increases the charge transfer resistance. Therefore, it is required to specify a criterion for the opening time of the purge valve by considering a threshold for the acceptable value of the cell's total resistance to prevent excessive voltage drop, which is the subject of the next step of this research.

The purpose of this analysis using lattice Boltzmann method (LBM) is to investigate the matter with which strategies can be used to control the amount of heat transfer and entropy formation. For this goal, the mixed convection heat transfer of non-Newtonian nanofluid under the impact of the magnetic field inside the square chamber containing the conductor wall has been analyzed. The results depicted that the flow characteristics and heat transfer are strongly affected by changing the position in these places. If the magnetic field is applied at the same location as the speed application position, the effect of increasing the Hartmann number in reducing the average Nusselt number becomes more evident. In order to have the highest value of Nusselt number, CASE3 should be considered, however, the greatest impact of magnetic field on fluid flow was observed for CASE2. By increasing the distance between the conductive wall and the moving wall, the average Nusselt number increases up to 1.5 times. By decreasing the thermal conductivity ratio from 10 to 0.5, the average Nusselt number decreases to about 55%. The entropy value has a direct/inverse relationship with the Richardson number/power-law index.

In this paper, a method for improving the performance of Complementary filter in the Attitude and Heading Reference System for estimating the orientation in accelerated movements is presented. Although existing complementary filters have advantages such as low computational volume, stability in different dynamic conditions, effectiveness at low sampling rates, and simplicity in the parameter setting process; But in the situation where the mobile device is exposed to non-gravitational accelerations, they show inappropriate performance. The proposed algorithm is designed based on the threshold-based path selection method and by adjusting the gain of complementary filters according to the size of the external acceleration, it improves the estimation of angles. In the following, the proposed algorithm is compared with the Extended Kalman filter and its three adaptive versions. The simulation and evaluation results of the proposed method show that the improved complementary filters achieve good performance in accelerated movements compared to the AEKF in the direction and alignment reference system.