Finite Element Analysis of Low-Velocity Impact Damage on Stiffened Composite Panels

To understand the factors which affect impact damage on composite structures, particularly the effects of impact position and ribs. In this paper, a finite element model (FEM) of low-velocity impact damage on the composite structure was established via the nonlinear finite element method, combined with the user-defined materials subroutine (VUMAT) of the ABAQUS software. The structural elements chosen for the investigation comprised a series of stiffened composite panels, representative of real aircraft structure. By impacting the panels at different positions relative to the ribs, the effect of relative position of ribs was found out. Then the simulation results and the experiments data were compared. Finally, the factors which affect impact damage on the structures were discussed. The paper was helpful for the design of stiffened composite structures

Unsteady aerodynamic model of flexible flapping wing

Bio-inspired flapping wing has potential application to micro air vehicles (MAV). Due to the nature of lightweight and flexibility of micro flapping wing structures, elastic deformation as a result of aeroelastic coupling is inevitable in flapping motion. This effect can be significant and beneficial to the aerodynamic performance as revealed in the present investigation for a flexible flapping wing of variable camber versus a rigid one. Firstly a two dimensional (2D) unsteady aerodynamic model (UAM) based on potential flow theory has been extended from previous study. Both leading and trailing edge discrete vortices are included in the model with unsteady Kutta condition satisfied to fully characterize the unsteady flow around a flapping wing. A wall function is created to modify the induced velocity of the vortices in the UAM to solve the vortices penetration problem. The modified UAM is then validated by comparing with CFD results of a typical insect-like flapping motion from previous research. Secondly the UAM is further extended for a flexible flapping wing of camber variation. Comparing with a rigid wing in a prescribed plunging and pitching motion, the results show lift increase with positive camber in upstroke by mitigating negative lift. The results also agree well with CFD simulation. Thirdly the 2D UAM is extended to calculate the aerodynamic forces of a 3D wing with camber variation, and validated by CFD results. Finally the model is applied to aerodynamic analysis of a 3D flexible flapping wing with aeroelastic coupling effect. Significant increase of lift coefficient can be achieved for a flexible flapping wing of positive camber and twist in upstroke produced by the structure elastic deformation

A bio-inspired flapping wing rotor of variant frequency driven by ultrasonic motor

By combining the flapping and rotary motion, a bio-inspired flapping wing rotor (FWR) is a unique kinematics of motion. It can produce a significantly greater aerodynamic lift and efficiency than mimicking the insect wings in a vertical take-off and landing (VTOL). To produce the same lift, the FWR’s flapping frequency, twist angle, and self-propelling rotational speed is significantly smaller than the insect-like flapping wings and rotors. Like its opponents, however, the effect of variant flapping frequency (VFF) of a FWR, during a flapping cycle on its aerodynamic characteristics and efficiency, remains to be evaluated. A FWR model is built to carry out experimental work. To be able to vary the flapping frequency rapidly during a stroke, an ultrasonic motor (USM) is used to drive the FWR. Experiment and numerical simulation using computational fluid dynamics (CFD) are performed in a VFF range versus the usual constant flapping frequency (CFF) cases. The measured lifting forces agree very well with the CFD results. Flapping frequency in an up-stroke is smaller than a down-stroke, and the negative lift and inertia forces can be reduced significantly. The average lift of the FWR where the motion in VFF is greater than the CFF, in the same input motor power or equivalent flapping frequency. In other words, the required power for a VFF case to produce a specified lift is less than a CFF case. For this FWR model, the optimal installation angle of the wings for high lift and efficiency is found to be 30° and the Strouhal number of the VFF cases is between 0.3–0.36. View Full-Tex

Analysis of Global Sensitivity of Landing Variables on Landing Loads and Extreme Values of the Loads in Carrier-Based Aircrafts

When a carrier-based aircraft is in arrested landing on deck, the impact loads on landing gears and airframe are closely related to landing states. The distribution and extreme values of the landing loads obtained during life-cycle analysis provide an important basis for buffering parameter design and fatigue design. In this paper, the effect of the multivariate distribution was studied based on military standards and guides. By establishment of a virtual prototype, the extended Fourier amplitude sensitivity test (EFAST) method is applied on sensitivity analysis of landing variables. The results show that sinking speed and rolling angle are the main influencing factors on the landing gear’s course load and vertical load; sinking speed, rolling angle, and yawing angle are the main influencing factors on the landing gear’s lateral load; and sinking speed is the main influencing factor on the barycenter overload. The extreme values of loads show that the typical condition design in the structural strength analysis is safe. The maximum difference value of the vertical load of the main landing gear is 12.0%. This research may provide some reference for structure design of landing gears and compilation of load spectrum for carrier-based aircrafts

Effect of asymmetric feathering angle on the aerodynamic performance of a flyable bionic flapping-wing rotor

The current study involves an experimental as well as numerical study on the aerodynamic behavior of a flapping-wing rotor (FWR) with different feathering amplitudes (−20°–50°, −50°–20°, and −35°–35°). In order to fulfil the experimental test, an FWR which weighs 18.7 g is designed in this manuscript. According to the experimental and numerical results, it was observed that, compared with the cases under a zero average stroke angle, the cases under a positive average stroke angle or negative average stroke angle share a higher rotary speed given the same input voltage. Despite the fact that the negative average stroke angle would facilitate the generation of a higher rotary speed, the negative average stroke angle cases tend to generate the smallest lift-to-power ratio. On the other hand, the cases with a positive average stroke angle tend to share the largest lift-to-power ratio (about 1.25 times those of zero average stroke angle cases and about 1.6 times those of negative average stroke angle cases). The above study indicates that the application of a positive average stroke angle can provide an effective solution to further increase the aerodynamic performance of a bio-inspired FWR

Short landing performance and scale effect of a flapping wing aircraft

An investigation was made into the performance and scale effect of birdlike flapping wing aircraft in short landing. A flapping mechanism is proposed to transform a powered shaft rotation to an optimal kinematics of wing motion combining up-and-down stroke, pitching, and fore-and-back swing. An unsteady aerodynamic method (UAM) was developed based on potential flow theory, including the leading- and trailing-edge vortices generated by a flapping wing. After validation based on computational fluid dynamics (CFD) results, the method is used to calculate the aerodynamic forces of flapping wings. The flight dynamics model of the aircraft is built using Automated Dynamic Analysis of Mechanical Systems (ADAMS) software version 2012 interfacing with the UAM coded in Python. The coupling between the inertial force of the body motion and the aerodynamic forces from flapping wings and tailplane is incorporated into the numerical simulation of the aircraft landing. Taking a 0.196-kg birdlike aircraft model with a prescribed kinematics of flapping wing motion as an example, a parametric study was carried out in a small range of initial tailplane angles and subsequent flapping frequencies. Optimal parameters were obtained to reduce the forward and descending velocities of the aircraft to a minimum value for safe and short landing performance. The study is then extended to aircraft of different geometric scales in a range of 0.5–10 associated with a weight scale of 0.1–1,000. Based on the study, a method is developed to determine the required flapping frequency for birdlike aircraft of different scales to achieve a short landing target with the descending velocity reduced to a specified value. For the aforementioned example aircraft (geometric scale of 1), the flapping frequency is 4 Hz to reduce both descending and forward velocities to 50% of the landing performance in fixed-wing mode, while a birdlike aircraft on a geometric scale of 10 and landing weight of 196 kg requires a minimum flapping frequency of 1.25 Hz to achieve a 50% reduction of the descending and forward velocities compared with the same aircraft landing in fixed-wing mode

Aerodynamic performance of a flyable flapping wing rotor with passive pitching angle variation

The present work was based on an experimental study on the aerodynamic performance of a flapping wing rotor (FWR) and enhancement by passive pitching angle variation (PPAV) associated with powered flapping motion. The PPAV (in this study 10o~50o) was realized by a specially designed sleeve-pin unit as part of a U-shape flapping mechanism. Through experiment and analysis, it was found that the average lift produced by an FWR of PPAV was >100% higher than the baseline model, the same FWR of a constant pitching angle 30o under the same input power. It was also noted that the lift-voltage relationship for the FWR of PPAV was almost linear and the aerodynamic efficiency was also over 100% higher than the baseline FWR when the input voltage was under 6V. The aerodynamic lift or efficiency of the FWR of PPAV can be also increased significantly by reducing the weight of the wings. An FWR model was fabricated and achieved vertical take-off and free flight powered by 9V input voltage. The mechanism of PPAV function provides a feasible solution for aerodynamic improvement of a bio-inspired FWR and potential application to micro-air-vehicles (MAVs)

Multi-Scale Nonlinear Progressive Damage and Failure Analysis for Open-Hole Composite Laminates

In order to study the nonlinear behaviors and interactions among the constituents for the composite material structure under the tensile load, multiscale damage model using generalized method of cells (GMC) and a lamina-level progressive damage model were established, respectively, for fiber reinforced composite laminates with a central hole, which were based on the thermodynamic Schapery Theory (ST) at either the micro-level or the lamina level. Once the nonlinear progressive degradation of the matrix material reached the lower limit value for the ST method, matrix failures naturally occurred, the failure of the fiber was determined by the maximum stress failure criterion. For the multiscale progressive damage model, the GMC model consisting of a fiber subcell and three matrix subcells was imposed at each integral point of FEM elements, and the three matrix subcells undergo independent damage evolution. The load versus displacement curves and failure modes of the open-hole laminates were predicted by using the two progressive failure models, and the results were compared with that obtained by the Hashin-Rotem progressive failure model and the experimental results. The results show that the ST based method can obtain the nonlinear progressive damage evolution states and failure states of the composite at both the lamina level and the multiscale level. Finally, the damage contours and failure paths obtained are also presented

Multi-Scale Nonlinear Progressive Damage and Failure Analysis for Open-Hole Composite Laminates

In order to study the nonlinear behaviors and interactions among the constituents for the composite material structure under the tensile load, multiscale damage model using generalized method of cells (GMC) and a lamina-level progressive damage model were established, respectively, for fiber reinforced composite laminates with a central hole, which were based on the thermodynamic Schapery Theory (ST) at either the micro-level or the lamina level. Once the nonlinear progressive degradation of the matrix material reached the lower limit value for the ST method, matrix failures naturally occurred, the failure of the fiber was determined by the maximum stress failure criterion. For the multiscale progressive damage model, the GMC model consisting of a fiber subcell and three matrix subcells was imposed at each integral point of FEM elements, and the three matrix subcells undergo independent damage evolution. The load versus displacement curves and failure modes of the open-hole laminates were predicted by using the two progressive failure models, and the results were compared with that obtained by the Hashin-Rotem progressive failure model and the experimental results. The results show that the ST based method can obtain the nonlinear progressive damage evolution states and failure states of the composite at both the lamina level and the multiscale level. Finally, the damage contours and failure paths obtained are also presented

Finite Element Analysis of Low-Velocity Impact Damage on Stiffened Composite Panels

To understand the factors which affect impact damage on composite structures, particularly the effects of impact position and ribs. In this paper, a finite element model (FEM) of low-velocity impact damage on the composite structure was established via the nonlinear finite element method, combined with the user-defined materials subroutine (VUMAT) of the ABAQUS software. The structural elements chosen for the investigation comprised a series of stiffened composite panels, representative of real aircraft structure. By impacting the panels at different positions relative to the ribs, the effect of relative position of ribs was found out. Then the simulation results and the experiments data were compared. Finally, the factors which affect impact damage on the structures were discussed. The paper was helpful for the design of stiffened composite structures