Passive Longitudinal Stability in Ornithopter Flight

Abstract

T HIS work investigates the effect of aeroelastic interaction between flexible ornithopter wings and the surrounding airflow on overall flight dynamics and stability. Typical ornithopter wings are composed of carbon rod stiffeners with a nylon fabric skin, providing anisotropic flexibility distribution to the wings. High speed camera images of ornithopter flights reveal that the wings undergo passive deformation both in chordand spanwise directions, and this aeroelastic phenomenon is known to heavily affect aerodynamic forces andmoments of the entirewing [1–6]. However, no studies have adequately addressed whether or not flexibility of wings is favorable to flight stability. Generally, for the analysis of flight dynamics and stability of ornithopters, a complex nonlinear flexible multibody configuration of an ornithopter is simplified to a linear rigid-body dynamics model with a quasisteady aerodynamic model. In particular, the passive deformation of a flexible-wing structure is oftentimes not considered or at best assumed to have a prescribed form to guarantee enough lift and thrust to propel the vehicle aloft [7–11]. Among these relevant studies, Dietl et al. [7] focused on the flight stability of an ornithopter using a single rigid-body model with prescribed sinusoidal twist angle distribution profile of the wings and concluded that the system had an unstable limit-cycle trim condition. This paper addresses the issue of the effect of passive deformation in local twist angles of flexible ornithopter wings and how it influences longitudinal flight stability. Two different ornithopter models were constructed based on the ornithopter flight simulation framework used in previous studies [12,13], which can account for flexiblemultibody dynamics andfluid-structure interaction ofwings. Both models were identical except for the wing structure; the reference model has rigid wings with prescribed sinusoidal local twist angle change as in [7], whereas the other hasflexiblewingswith aeroelastically varying local twist angles resulting from fluidstructure interaction. Longitudinal trimmed level flight and transient response to a pitch directional moment disturbance were compared between the two ornithopter models to ascertain the effect of aeroelasticity. II. Modeling and Simulation Methodology