سال:1392 | دوره: | شماره: |تعداد مقالات:6

Hover performance of a twin-rotor test model in terms of rotor overlap sweep, blade collective pitch, and blade tip speedwasexaminedexperimentally.The experimental setup consisted of two three-bladed rotors (tandem rotor configuration) with a diameter of1,220 mm and constant chord of 38 mm, giving a blade aspect ratio of 16.05. The blades were of a rectangular planform with NACA 0012 cross-sections with no twist or taper. In this model, the front rotor was fixed on the fuselage and the rearrotor could move longitudinally for tests up to about40% rotor overlap sweep.To accurately examine the hover performance, thrust, power required, power loading (PL), and figure of merit (FM) responses were measured usinga central compositetest plan. Furthermore, four quadratic polynomialswere fitted to all responses, necessary for a better understanding of the main effects and interactions. The results clearlyshowed that significant interactions between variables are evident and therefore overlapping at constant collective pitch reduces thrust much more than reducingthe power required. Moreover, the results showed that, for the twin-rotor system,the maximumefficiency in hover (i.e., maximum power loadingof 14.6kg/kW) is obtainedfor no overlapped rotors at low values of disc loading and blade tip speed.Experimental measurements of thetwin-rotor hover performance based on a central composite plan andinteraction analysis were the main contributionsstated in the current work.Results for the twin-rotor test model can be generalized to actual tandem helicopters through the Reynolds number transformation technique and some modifications.

Spoofing could pose a major threat to Global Positioning System (GPS) navigation, so the GPS users have to gain an in-depth understanding of GPS spoofing. Since spoofing attack can influence position results, spoof compensation is possible through reducing position deviations. In this paper, a novel processing technique is proposed and the wavelet transform is used to eliminate the impact of spoofing on the stationary GPS receivers. We assumed that the spoofing attack was immediately detected, and then the position residuals of the last authentic and new spoofing signals were passed to the statistic wavelet transform at the first level. By denoising in the next step, position deviations due to the spoofing attack can be extracted. Then, the estimated position solution of the received signal is corrected. Finally, the receiver coordinates are calculated by averaging the corrected positions. For validation of the suggested algorithm, five different data sets are investigated. We mitigated the spoofing in all data sets more than 93%. The test results show that the proposed technique supremely improves the performance of the GPS receiver and attenuates the spoofing effect.

Three-axis-magnetometers (TAMs) are widely utilized as a key component of attitude determination subsystems and as such are considered the corner stone of navigation for low Earth orbiting (LEO) space systems. Precise geomagnetic-based navigation demands accurate calibration of the magnetometers. In this regard, a complete online calibration process of TAM is developed in the current research that considers the combined effects of environmental and instrumental errors including biases, non-orthogonally parameters, and the scale factors, without the need for clean room facilities. The sensor characteristics are estimated utilizing Kalman filter for a micro electro-mechanical sensor(MEMS)-based TAM standing on the experimental measured outputs in a noisy laboratory environment. Moreover, the stochastic TAM behavior is identified using the method of Allan variance analysis (AVA) through a six-hour static test. Subsequently, the nonlinear/non-Gaussian problem of attitude estimation, using a set of calibrated strap-down magnetometers is addressed utilizing the unscented particle filter (UPF), developed for the removal of colored-noise. Comparison of the estimated attitude, represented by quaternion parameters, with the true orientations demonstrates an acceptable level of accuracy of the developed calibration technique for small LEO space systems. Analysis of the root mean square error of the estimated attitude illustrates an accuracy of less than one degree for all axes. This is an ideal result, given the fact that MEMS-based magnetometers have been utilized.

Numerous experiments have been conducted on plunging Eppler 361 airfoil in a subsonic wind tunnel. The experimental tests involved measuring the surface pressure distribution over the airfoil at Re=1.5×105. The airfoil was equipped with Gurney flap(heights of 2.6, 3.3 and 5% chord) and plunged at 6cm amplitude. The unsteady aerodynamic loads were calculated from the surface pressure measurements, 51 ports, along with the chord on both upper and lower surfaces of the model. The Gurney flap effects over the loads hysteresis loops of the oscillating airfoil were particularly studied prior to stall, at the stall onset, in light stall, and deep stall conditions. The static results of the flapped and unflapped airfoil were also explored in order to make a reference of comparisonsto the dynamic loads. The results showed that, the addition of the Gurney flap provided no changes in the directions of the Cl, Cd and Cm hysteresis loops for the prior to stall flow conditions, while as a result of the positive camber effects, the lift hysteresis loops shifted upward and the pitching moment’s loops moved vertically downward. Additionally, adding the Gurney flap promoted dynamic stall phenomena.The deep dynamic stall of the flapped airfoil with the height of h/c=5% was seen at ads=13.1deg. This phenomenon was observed at ads=14.8deg for the flapped airfoils of h/c=2.6 and 3.3%.

Optimization of the sectional wing in ground effect (WIG) has been studied using ahigh order numerical procedure and response surface method (RSM). Initially, the effects of the ground clearance, angle of attack, thickness, and camber of wing have been investigated by a high-resolutionscheme, which is highlystrong and accurate. In the numerical simulation, Normalized Variable Diagram (NVD) scheme is applied to the boundedness criteria. In the optimization process, lift to drag ratio (L/D) is considered as an objective function and static conditions and shape parameters are noticeable to be considered as design variables, Ths is because the main factor in the design of WIG vehicles is moving near the ground and the distance to the ground draws attention to the significance of it.Therefore, the static conditions strenuously defend this view that they are irrefutable parameters in the aerodynamic optimization of WIG vehicles. Adaptive Neuro-Fuzzy Interface System (ANFIS) is employed to generate the surface response, because the objective function and constraints are particularlynoisy. Sensitivity analysis is also done and the sensitivity amount of the objective function from design variables is explored.

This paper studies the uncertain nonlinear dynamics of a MEMS optical switch addressing electrical, mechanical and optical subsystems. Recently, MEMS optical switch has had significant merits in reliability, control voltage requirements and power consumption. However, an inherent weakness in designing control for such systems is unavailability of switch position information at all times due to the saturated output characteristics, which is aggravated by considering disturbances. In order to circumvent this problem, two nonlinear observers based on the first order and second order sliding mode approach are designed to estimate the state variables of the device subject to external disturbances. The nonlinear observers are then utilized in the control system to maintain robust stability and tracking performances. The newly invented second order sliding mode controller can remove the chattering phenomena as the main drawback of the first order sliding mode controller. Furthermore, since second order sliding mode control is not robust against disturbances/uncertainties which vary with states, a new time-varying second order sliding mode control is proposed to enhance the robust performance of the controller without estimating any switching time. Simulation results show that the proposed observer and control have good tracking ability and robustness against disturbances.