Dae-Kwan Kim

Wind tunnel tests for a flapping wing model with a changeable camber using macro-fiber composite actuators

In the present study, a biomimetic flexible flapping wing was developed on a real ornithopter scale by using macro-fiber composite (MFC) actuators. With the actuators, the maximum camber of the wing can be linearly changed from −2.6% to +4.4% of the maximum chord length. Aerodynamic tests were carried out in a low-speed wind tunnel to investigate the aerodynamic characteristics, particularly the camber effect, the chordwise flexibility effect and the unsteady effect. Although the chordwise wing flexibility reduces the effective angle of attack, the maximum lift coefficient can be increased by the MFC actuators up to 24.4% in a static condition. Note also that the mean values of the perpendicular force coefficient rise to a value of considerably more than 3 in an unsteady aerodynamic flow region. Additionally, particle image velocimetry (PIV) tests were performed in static and dynamic test conditions to validate the flexibility and unsteady effects. The static PIV results confirm that the effective angle of attack is reduced by the coupling of the chordwise flexibility and the aerodynamic force, resulting in a delay in the stall phenomena. In contrast to the quasi-steady flow condition of a relatively high advance ratio, the unsteady aerodynamic effect due to a leading edge vortex can be found along the wing span in a low advance ratio region. The overall results show that the chordwise wing flexibility can produce a positive effect on flapping aerodynamic characteristics in quasi-steady and unsteady flow regions; thus, wing flexibility should be considered in the design of efficient flapping wings.

Experimental investigation on the aerodynamic characteristics of a bio-mimetic flapping wing with macro-fiber composites

This study describes the development of a bio-mimetic flapping wing and the aerodynamic characteristics of a flexible flapping wing. First, the flapping wing is designed to produce flapping, twisting, and camber motions by using a bio-mimetic design approach. A structural model for a macro-fiber composite (MFC) actuator is established, and structural analysis of a smart flapping wing with the actuator is performed to determine the wing configuration for maximum camber motion. The analysis model is verified with the experimental data of the smart flapping wing. Second, aerodynamic tests are performed for the smart flapping wing in a subsonic wind tunnel, and the aerodynamic forces are measured for various test conditions. Additionally, the effects of camber and chordwise wing flexibility on unsteady and quasi-steady aerodynamic characteristics are discussed. The experimental results demonstrate that the effect of the camber generated by the MFC produces sufficient aerodynamic benefit. It is further found that chordwise wing flexibility is an important parameter in terms of affecting aerodynamic performance, and that lift produced in a quasi-steady flow condition is mostly affected by the forward speed and effective angle of attack.

Longitudinal Flight Dynamics of Bioinspired Ornithopter Considering Fluid-Structure Interaction

This paper addresses the flapping frequency-dependent trim flight characteristics of a bioinspired ornithopter. An integrative ornithopter flight simulator including a modal-based flexible multibody dynamics solver, a semiempirical reduced-order flapping-wing aerodynamic model, and their loosely coupled fluid―structure interaction are used to numerically simulate the ornithopter flight characteristics. The effect of the fluid―structure interaction of the main wing is quantitatively examined by comparing the wing deformations in both spanwise and chordwise directions, with and without aerodynamic loadings, and it shows that the fluid―structure interaction created a particular phase delay between the imposed wing motion and the aeroelastic response of the main wing and tail wing. The trimmed level flight conditions of the ornithopter model are found to satisfy the weak convergence criteria, which signifies that the longitudinal flight state variables of ornithopters need to be bounded and that the mean value of the variables are converged to the finite values. Unlike conventional fixed-wing aerial vehicles, the longitudinal flight state variables, such as forward flight speed, body pitch attitude, and tail-wing angle of attack in trimmed level flight, showed stable limit-cycle oscillatory behaviors with the flapping frequency as the dominant oscillating frequency. The mean body pitch attitude and tail-wing angle, and the root-mean-square value of the body pitch attitude, decreased as the flapping frequency increased. In addition, the mean forward flight speed is found to almost linearly increase with the flapping frequency.

An aeroelastic analysis of a flexible flapping wing using modified strip theory

The present study proposed a coupling method for the fluid-structural interaction analysis of a flexible flapping wing. An efficient numerical aerodynamic model was suggested, which was based on the modified strip theory and further improved to take into account a high relative angle of attack and dynamic stall effects induced by pitching and plunging motions. The aerodynamic model was verified with experimental data of rigid wings. A reduced structural model of a rectangular flapping wing was also established by using flexible multibody dynamics and a modal approach technique, so as to consider large flapping motions and local elastic deformations. Then, the aeroelastic analysis method was developed by coupling these aerodynamic and structural modules. To measure the aerodynamic forces of the rectangular flapping wing, static and dynamic tests were performed in a low speed wind-tunnel for various flapping pitch angles, flapping frequencies and the airspeeds. Finally, the aerodynamic forces predicted by the aeroelastic analysis method showed good agreement with the experimental data of the rectangular flapping wing.

Improved Aerodynamic Model for Efficient Analysis of Flapping-Wing Flight

This work was supported by a Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2007-313- D00122). The second author would like to thank the Brain Korea 21 Project in 2010. The authors would like to thank James D. DeLaurier of the University of Toronto and M. Okamoto of the Akita National College of Technology for their gracious support for this study. The authors also thank anonymous reviewers and the Editor for their valuable comments and suggestions.

Nonlinear Aeroelastic Analysis of a Deployable Missile Control Fin

The nonlinear aeroelastic characteristics of a deployable missile control fin have been investigated. Modes from free vibration analysis and a doublet-point method are used for the computation of supersonic unsteady aerodynamic forces. The minimum-state approximation is used to approximate the aerodynamic forces. The fictitious mass modal approach is applied to reduce the problem size and the computation time in the linear and nonlinear flutter analyses. For the nonlinear flutter analysis, the deployable hinge is represented by an asymmetric bilinear spring and is linearized using the dual-input describing function method. From the nonlinear flutter analysis, three different types of limit-cycle oscillations are observed in the wide range of airspeed over the linear flutter boundary. The aeroelastic characteristics of the missile control fin can become more stable due to the existence of the deployable hinge nonlinearity.

Ornithopter modeling for flight simulation

This paper presents the flight simulation of flapping-wing air vehicles (ornithopters) based on a refined flapping-wing aerodynamic model; the modified strip theory (MST). Compared with conventional types of micro air vehicles (MAVs), flapping MAVs show more complicated flight behaviors due to their complex wing motions and aerodynamics. In this paper, a flight dynamic model of an ornithopter is presented to analyze its stability and controllability. This paper focuses on the stabilization and path-following control of the ornithopter by adjusting the flapping frequency and tail-wingpsilas elevation angle. In spite of its nonlinear and complex behavior, controlling the tail-wing pitch angle can be effective for the stabilization in longitudinal motion of the ornithopter.

Dynamic Model Establishment of a Deployable Missile Control Fin with Nonlinear Hinge

This research was supported by the Agency for Defense Development (ADD) and was partially supported by the Ministry of Science and Technology (National Research Laboratory Program) in Korea. This support is gratefully acknowledged.

Study of flapping actuator modules using IPMC

The Ionic Polymer Metal Composite (IPMC), an electro-active polymer, has many advantages including bending actuation, low weight, low power consumption, and flexibility. These advantages coincide with the requirements of flapping-wing motion. Thus, IPMC can be an adequate smart material for the generation of the flapping-wing motions. In this research, a flapping actuator module operated at the resonant frequency is developed using an IPMC actuator. First, IPMC actuators are fabricated to investigate the mechanical characteristics of IPMC as an actuator. The performances of the IPMC actuators, including the deformation, blocking force and natural frequency, are then obtained according to the input voltage and IPMC dimensions. Second, the empirical performance model and the equivalent stiffness model of the IPMC actuator are established. Third, flapping actuator modules using the first resonance frequency are developed, and their flapping frequency and stroke characteristics are investigated. Fourth, adequate flapping models for a flapping actuator module are selected, and dimensional data such as wing area and wing mass are obtained. Finally, the flapping actuator module is designed and manufactured to adjust the flapping models and its performance is tested. Experimental results demonstrate the potential IPMC has for use as a flapping actuator.

Experimental evaluation of a flapping-wing aerodynamic model for MAV applications

In the preliminary design phase of the bio-inspired flapping-wing MAV (micro air vehicle), it is necessary to predict the aerodynamic forces around the flapping-wing under flapping-wing motion at cruising flight. In this study, the efficient quasi-steady flapping-wing aerodynamic model for MAV application is explained and it is experimentally verified. The flapping-wing motion is decoupled to the plunging and pitching motion, and the plunging-pitching motion generator with load cell assembly is developed. The compensation of inertial forces from the measured lift and thrust is studied to measure the pure aerodynamic loads on the flapping-wing. Advanced ratio is introduced to evaluate the unsteadiness of the flow and to make an application range of flapping-wing aerodynamic model.