Effects of Body Aerodynamics and Wing Flexibility on Flight Characteristics of Insect-like Flapping-wing Micro Air Vehicles

Abstract

The dissertation presents the effects of body aerodynamics and wing flexibility on several flight characteristics, including the trim conditions, power requirements and dynamic stability of an insect-like flapping-wing micro air vehicle (FWMAV). The FWMAV is assumed to have the geometry and mass properties of the hawkmoth Manduca sexta. Moreover, wing kinematics and other flight parameters, such as flapping frequency, body and stroke plane angles are also based on measurement data from real insects. A potential-based aerodynamic model, which combines the unsteady panel method (UPM) and the unsteady vortex-lattice method (UVLM), was developed to simulate the unsteady aerodynamics of the insect-like FWMAV. Aerodynamic loads on the streamlined body of the FWMAV are computed by the UPM

while the UVLM is applied to the thin wings. The leading-edge suction analogy model is integrated into the present aerodynamic model to estimate the contribution of leading-edge vortices (LEVs) occurring on insect wings

whereas the vortex-core growth model is used to avoid singularity problems due to wing-wake and body-wake interactions. The motions of the FWMAV in free flight are obtained by a multibody dynamics simulation framework running in the MSC. ADAMS environment. The trim conditions corresponding to flight at various speeds ranging from 0.0 to 5.0 m $s^{-1}$ are found using the trim search algorithm. The equations of small disturbance motion are linearized to obtain the longitudinal and lateral dynamic modal structures of the insect-like FWMAV. To investigate the effect of wing flexibility, the wing structure is modelled with the finite element analysis (FEA) software ANSYS Mechanical APDL by considering all details of the hawkmoth wing. The study results have revealed that the effect of body aerodynamics in low-speed flight is negligible

while at lower flight speeds, there are almost no changes found in the numerical results. On the other hand, the body aerodynamics effect may worsen the longitudinal dynamic stability characteristics of the FWMAV flight and make the FWMAV more unstable to horizontal and vertical gust disturbances. In contrast to the body aerodynamics, wing flexibility offers quite many advantages to the FWMAV in hawkmoths’ favorite flight speeds ranging from 0.0 to 3.0 m $s^{-1}$. Within this speed range, the use of flexible wings enables the FWMAV to enhance its lateral dynamic flight stability

whereas the longitudinal dynamics is almost unaffected. The improvement in lateral dynamic stability is explained by the changes in the derivatives of the aerodynamic roll and yaw moments with respect to the lateral translational velocity. Additionally, using the flexible wings, the amount of required mechanical power could be significantly reduced. These favorable effects are the most apparent in hovering flight, and appear to decrease as the flight speed increases. At 3.0 m $s^{-1}$, the benefits are almost unnoticeable

and at 4.0 m $s^{-1}$, the results show a negative effect on the mechanical power demand. The study also found that this negative effect is related to an upward flow region that appears in high-speed flight and is intensified by wing deformations at the beginning of the downstroke. This phenomenon is unfavorable for the lift generation mechanisms of the wings

and therefore, wing flexibility can lead to a negative effect in high-speed flight. Along with the changes in dynamic stability and power requirements, the trim conditions are also affected greatly by wing flexibility.

however, as the flight speed increases, this effect becomes more obvious. For the trim conditions, the inclusion of the body aerodynamics influences the sweep angle of the wings significantly. In order to offset the large aerodynamic pitching moment due to the body at high speeds, the FWMAV has to reduce the mean sweep angle by moving its wings more backward. Regarding the dynamic stability, the effect of the body aerodynamics on the longitudinal dynamics was found greater than that on the lateral one. It was indicated that the effect on the longitudinal dynamics is primarily owing to the considerable changes in the derivatives of the aerodynamic pitching moments with respect to the horizontal and vertical translational velocities when the body aerodynamics is considered. Likewise, the changes in the derivatives of the roll and yaw moments with respect to the lateral translational velocity account for the effect of the body aerodynamics on the lateral dynamics of the FWMAV. The results also showed that these effects are not only directly due to the body aerodynamic force but also from differences in the trim conditions. In terms of power requirements, the insect-like FWMAV may benefit from the body aerodynamics effect only at 5.0 m $s^{-1}$