Sergey Shkarayev

Aerodynamic Design of Micro Air Vehicles for Vertical Flight

The research and development efforts outlined in this paper address the aerodynamic design of micro air vehicles with hovering and vertical takeoff and landing capabilities. The tilt-body configuration of the vertical takeoff and landing micro air vehicle is proposed based on a propulsion system consisting of two coaxial contrarotating motors and propellers. Values of thrust, torque, power, and efficiency of this propulsion system were measured in pusher and tractor arrangements of propellers and compared against single motor-propeller propulsion. With comparable efficiency, the developed propulsion system has very little propeller torque. Hot-wire measurements have been conducted to investigate the velocity profile in slipstream. The lower average velocity and significant decrease in velocity in the core of the slipstream found in the tractor arrangement are mostly due to the parasite drag caused by the motors. It causes the decrease of the thrust force observed for the tractor arrangement in comparison with the pusher arrangement. Wind-tunnel testing was conducted for a motor, a wing, and an arrangement of a wing with a motor. The drag force on the wing is produced by two mixing airflows: freestream and propeller-induced pulsating slipstream. The zero-lift drag coefficient increases by about 4 times with propeller-induced speed increased from 0 to 7.5 m/s. The results of this study were realized in the design of a vertical takeoff and landing micro air vehicle prototype that was successfully flight tested.

Effect of Camber on the Aerodynamics of Adaptive Wing Micro Air Vehicles

Four microair vehicle wind-tunnel models were built with 3, 6, 9, and 12% camber, all based upon the S5010-TOP24C-REF thin, cambered-plate airfoil. These models were tested in the Low Speed Wind Tunnel at angles of attack ranging from 0 to 35 deg and velocities of 5, 7.5, and 10 m/s, corresponding to mean aerodynamic chord Reynolds numbers of 5 × 10 4 , 7.5 × 10 4 , and 1 x 10 5 , respectively. Aerodynamic coefficients C L , C D , C M and lift-to-drag ratio (LID) were obtained and plotted vs angle of attack for all of the cambers at each velocity

Flight Dynamics of a Flapping-Wing Air Vehicle

The research and development efforts presented in this paper address the flight dynamics of a flapping-wing air vehicle (ornithopter). The 74-cm-wing-span ornithopter was equipped with an automatic flight control system that provides stability augmentation and navigation of the vehicle and flight data acquisition. Wind tunnel tests were conducted with the control surfaces fixed in the trimmed position and flapping motion of the wings activated by a motor at a constant throttle setting. Coefficients of vertical and horizontal force, and pitching moment were determined at a free stream velocity of 7.25 m/sec, and the angle of the stroke plane varied from 0 to 40 degrees. A series of flight tests were conducted with fixed controls, demonstrating ornithopter stability in all axes. Proportional control laws were programmed into the autopilot for the closed-loop controls. A number of flights of the autonomous ornithopter were conducted with the telemetry acquisition. During the autonomous flights, the ornithopter performed waypoint and altitude navigation, demonstrating stable performance.

Thermo-mechanical stresses for a triple junction of dissimilar materials : Global-local finite element analysis

Finite element analysis with conventional elements fails to provide convergent stresses in regions where a free edge with a bimaterial interface or a junction of dissimilar materials exists. However, these regions are characteristic of electronic devices, and they are the most critical locations for failure. A finite element analysis with global (special) and local (conventional) elements has been developed to provide an accurate description of the stress field at these locations. The global elements capture the singular nature of the stresses arising from geometric and material discontinuities. With this method, the designer can accurately evaluate the thermo-mechanical integrity of various electronic devices.

Analysis of composite laminates with multiple fasteners

Fatigue- and fracture-related cracks are to be expected with the large number of fasteners present in aircraft structures. Therefore, contact stresses around the fastener holes and stress intensity factors associated with edge cracks are critical concerns in damage-tolerant designs. Mechanical joints consisting of many fasteners with a staggered pattern further complicate the already rather complex analysis for single-fastener joints. Load distribution among the fasteners significantly influences the failure load of multi-fastener joints. Most existing analyses are confined to single-fastener joints, and the data available for multi-fastener joints are rather limited. Very few experimental and/or analytical/numerical investigations of contact stresses for mechanical joints with staggered fasteners exist in the literature. Therefore, the accurate prediction of contact stresses (load distribution) and stress intensity factors associated with edge cracks is essential for the reliable design of such mechanical joints. This study concerns the development of an analytical methodology, based on the boundary collocation technique, to determine the contact stresses and stress intensity factors required for strength and life prediction of bolted joints with many fasteners. It provides an analytical capability for determining the contact stresses in mechanically fastened composite laminates while capturing the effects of finite geometry, presence of edge cracks, interaction among fasteners, material anisotropy, fastener flexibility, fastener-hole clearance, friction between the pin and the laminate, and by-pass loading. Also, it permits determination of the fastener load distribution, which significantly influences the failure load of a multi-fastener joint.

Aerodynamics and Controls Design for Autonomous Micro Air Vehicles

Due to their small size, micro air vehicles (MAVs) demonstrate intrinsically unsteady behavior with high frequency oscillations, disturbing the usefulness of their applications. An enhanced automatic flight control system is in a great need for the progress of MAV technology. This paper presents an approach for simultaneous aerodynamics and closedloop control design for MAVs including the determination of stability and control derivatives, simulation of flight dynamics of a vehicle with open- and closed-loop control, and analysis of the telemetry from flight tests of autonomous vehicles. Aerodynamic stability and control derivative coefficients of the MAV were determined for various parameters (angle of attack, roll rate, pitch rate, etc.) using the MAV geometry and airfoil characteristics. These coefficients were integrated with the geometric, mass, and inertial data to produce the longitudinal and lateral equations of motion. Closed-loop control laws were determined via root-locus methods and the closed-loop system simulated. The effects of varying the center of gravity and changing dihedral angle on the stability are discussed in detail. The proposed approach was applied in the development and evaluation of two autonomous micro air vehicles with wingspans of 12 and 23 inches.

Effects of Propulsive-Induced Flow on the Aerodynamics of Micro Air Vehicles

A propulsion system (DC electric motor and propeller) was installed on micro air vehicle wind tunnel models of 3, 6 and 9% camber. In one set of tests the motor was mounted in its typical location; in the next test set the motor was angled both toward and then away from the leading edge; and in the final set of tests the motor was extended Ω inch away from the leading edge from its usual location. All models had wingspans of 9” and wing areas of 60 in 2 . The models were tested in the Low Speed Wind Tunnel, with the propulsion system activated (motor-on testing), at angles of attack ranging from 4 to 43° or 0 to 35.1°, depending on the test set, at velocities of 5, 7.5 and 10 m/s, corresponding to mean aerodynamic chord Reynolds numbers of 5◊10 4 , 7.5◊10 4 , and 1◊10 5 , respectively. CL, CD, CM-c/4 and L/D were obtained and plotted versus angle of attack at each velocity. The aerodynamic coefficients obtained from the motor-on testing were compared to those obtained from a previous study that were completed without a propulsion system installed. In general, it was found that the propulsive-induced flow had a positive effect on the aerodynamics of the micro air vehicle models at higher angles of attack, particularly for low Reynolds number tests, and motors angled away from the leading edge. It was also found that relocating the motor forward from its usual mounting position increased both the lift and drag coefficients. However, due to the way that the aerodynamic coefficients were calculated, the effects in the aerodynamic coefficients are partly a mathematical phenomenon. In other words, at low Reynolds numbers there is an increase in the lift coefficient, but it is due to the way that the lift coefficient is calculated. Because this reduction in the Reynolds number is due to a reduction in speed, the actual lift produced at these low speeds may decrease faster than the lift coefficient increases.

Computational modelling of shot-peening effects on crack propagation under fretting fatigue

Recent experimental studies have demonstrated fretting fatigue life enhancement of titanium alloy Ti-6A1–4V specimens after treatment by shot-peening. Because of complexities in tracking crack growth under fretting conditions experimentally, the present work describes computational modelling for crack propagation behaviour in specimens with and without shot-peening. A crack growth model is combined with a finite element submodelling technique to assess the crack trajectory and crack propagation life in the specimens under fretting fatigue. A parametric numerical analysis has been performed to investigate crack trajectories and stress intensity factors along the crack path under different loading conditions. Obtained results revealed the features of the crack growth trajectory and stress intensity factors in the presence of residual stresses from shotpeening. These results also demonstrated a significant (2–3 times) increase in the crack propagation life of shot-peened specimens relative to virgin specimens (i.e. without shot-peening), which is in agreement with experimental observations.

Theoretical modeling of crack arrest by inserting interference fit fasteners

The repair technology under consideration involves drilling a number of holes along a crack in a metal part and inserting fasteners (bolts, rivets, or pins) into the holes with a predetermined interference fit. A fracture mechanics-based model is proposed to study the decrease in the crack growth rate after repair. A parametric analysis is performed to discover the effect of geometry and materials on crack retardation. Elastic-plastic contact stress distributions in the specimens during cyclic loading are determined by the finite element method. The results show that a significant enhancement of fatigue life until crack re-initiation can be achieved through an optimal set of parameters: number of fasteners, their material, and interference fit. The model is validated using a comparison of fatigue tests of the specimens.

Edge cracks in stiffened plates

Abstract An analytical method for the determination of stress intensity factors in two-dimensional plane elastostatic problems is extended for the parametric study of edge cracks in stiffened panels. Two problems are studied: the case of a cracked panel reinforced with stringers and the case of two panels, connected by rivets, one of which contains a crack. The loading in each problem is uniform tensile stress applied normal to the crack plane. A parametric investigation of these problems is to discover the influence of geometric factors on the strength of the panels. The results show that the influence depends on the crack tip location relative to the reinforcement. The results for some examples are also compared with finite element calculations.