نشریه مهندسی هوانوردی
سال هجدهم شماره 1 (بهار و تابستان 1395)

In this research¡ the free vibration analysis of cylindrical shells with longitudinal stiffeners¡ i.e. stringer with two methods of analytical and experimental for different boundary condition is investigated. Ritz method is applied in analytical solution while stiffeners treated as discrete elements. Also¡ in experimental method the modal testing is performed. Different types of simply supported¡ different types of clamped and also free boundary conditions are considered. Some analytical results for natural frequencies are compared with other’s experimental and analytical results¡ and hence with experimental results of this research¡ which showed good agreement. Furthermore¡ variations of natural frequencies¡ fundamental frequency and natural mode shapes for different types of boundary conditions are considered.

The present study aims to explain entrepreneurship at the air University. In terms of data collection the research method is descriptive. The population consisted of all faculty members experienced in Academic entrepreneurship. Using purposive sampling, 29 samples were selected by snowball technique. To measure the "entrepreneurial orientation" five components of innovativeness, proactiveness, risk-taking, competitive aggressiveness and autonomy and to measure "academic entrepreneurship" three components of novelty- seeking, advantage-seeking and opportunity- seeking was considered. In determining the validity of questionnaire of this study was greater emphasis on content validity. Cranach’s alpha reliability coefficient was used to determine reliability of questionnaire. In order to analyze data t- test was used to check the status of existing components. The chi-square test to check the current status of indices and to examine the relationship between the variables, structural equations and path analysis with PLS software is used. The total average score of risk-taking 2.61, competitive aggressiveness 2.69 and autonomy 2.89, innovativeness component 2.81, proactiveness 3.16 ,novelty- seeking 2.30, advantage-seeking 2.72 and opportunity- seeking 2.59. The result of structural equations analysis indicates that entrepreneurial orientation has a positive effect on academic entrepreneurship.

In this paper, the miss distance analysis of proportional navigation for high-order binomial guidance and control system is presented in presence of seeker noise using normalized adjoint method. In this analysis, the effects of effective navigation ratio and system order up to 30 are studied due to heading error, target maneuvers, and seeker noise. Numerical solutions for the cases of initial heading error, target with constant, linear, and parabolic maneuvers show a quasi-asymptotic behavior with respect to system order. In addition, the quasi-asymptotic behavior occurs for higher system order due to seeker noise. The values for the infinity's order system are obtained numerically and analytically. Based on the analysis and its assumptions, an order of 8 to 10 is suggested for miss distance analysis of a binomial guidance and control system.

In this study a two-stage linear adaptive filtering is designed in order to remove the single-frequency Jammer from the broadband message signal. The result of this suggested approach is a system that despite the Doppler Effect, obtains the unknown interference in inputs automatically and removes it. The design of the adaptive system is based on a version of the interference signal, in which an unknown amount of Doppler shift occurred. The proposed system is implemented based on NLMS two linear adaptive filtering due to the low complexity of NLMS algorithm on the one hand and the possibility of practical implementation on the other hand. Given the parameter µ indicates the step size of the algorithm implementation, is a key factor in signal estimation based on adaptive filter. In the first step, the NLMS algorithm with a large step size and a variable behavior over time is used to initial estimates of the interference signal. Afterwards, the NLMS algorithm with a small step size is used as an approximation of the Wiener filter to correct the reference signal amplitude and the phase shift. The simulation of the proposed interference removal system has been done for the broadband QPSK signal which is mixed with strong interferences. The results have demonstrated that the proposed system has improved the QPSK broadband message signal, which affected by the single-frequency jammer as a strong interference factor, by 60 dB in SJR. Thus, despite a strong interference, the proposed system illustrates significant improvements in signal detection (bit error).

The ring rolling process is widely used to manufacture seamless ring-shaped parts for automotive, aerospace and nuclear industry such as aero-engines and nuclear reactors parts. In addition, the savings in time, energy and material during the production of ring shaped parts have been instrumental in reducing the overall cost. Due to the complexity and high nonlinearity of the process, it is obviously difficult to investigate the process purely by analytical and experimental methods, more attention is paid to the finite-element method to explore and develop the advanced ring rolling technologies. In this research, simulation of hot ring rolling of titanium alloy (Ti6Al4V) has been investigated. For the sake of time, the effects of rolls dimension, their velocities and some assumptions in rolling process with two dimensional models were conducted. The results of 2-D and 3-D models might agree or disagree, depending on the key parameters. In this paper, according to the values of the key parameters, the 2-D models have been used as an appropriate case for simulation and analysis. The effects of some relaxing assumptions on ring rolling process, like material properties and guide rolls existence, have been studied. The results provide guidelines for hot ring rolling simulation of Ti6Al4V alloy.

This paper introduces a novel strategy for the specification of certification of airworthiness (CoA) categories for unmanned aircraft systems (UAS). Logical approach in this classification, based on determination of allowable risk for an UAS supposing of operational scenario that using from this parameter as alone criteria for segregation of manned and unmanned airworthiness. In this research, for issuance of CoA in scope of UAS, is proposed regular matrix according to risk table of mil-std-882 that based on, level of airworthiness control on the design & construction of UAS’s with various missions, is specified. Referral point of the present work, is validation on proposed matrix with statistical work upon greater than 500 kinds of military and civil UAS that has been accomplish in two stage and with computation of two criteria as maximum kinetic energy and hazard area from physical, performance and aerodynamic specification each UAS with aim of determination of risk. For this intent in first step, attention to introduced various level of injury in references, is specified limits for kinetic energy that respect to their current UAS in archive is categorized and is conformed with introduced categories in proposed matrix. In next step, with creation of statistical algorithm is defined limits for hazard area parameter that in beside limits of kinetic energy, meanwhile increase the precision in proposed categorization for UAS in certification matrix, this matrix is validated again. Such after determination of certification category on the proposed matrix, this certification issue on what limitation of requirements and design & construction regulations, is subject that will implement in next issuance. Important point correspond to this technique, is degeneration and no confliction of introduced methodology for determination of CoA category of an UAV with regulations framework of part 21.