Flight dynamics and stability of bioinspired ornithopters

Abstract

The ornithopter, often called as a flapping-wing air vehicle, is an aircraft that flies by moving its wings like a bird. It includes every mechanical realization of the biological flyers, not only birds but also insects and bats. During the process mimicking the biological flyers, some essential features of the system such as controllability and observability need to be retained

however, those characteristics of the biological flyers are difficult to be known due to the intrinsic closed-loops between the neuromuscular controllers and the sensory organs. Toward an autonomous operation of the ornithopters, flight dynamics and stability need to be identified by either way of numerical modeling and flight testing so that a closed-loop feedback controller can be easily designed and implemented to the open-loop ornithopters. Flight dynamics and stability of bioinspired ornithopters were numerically and experimentally studied in this dissertation. Two different types of bioinspired ornithopters in two distinct flight modes were considered: 1) forward flight tailed ornithopter, and 2) hovering insect-like ornithopter. Flight dynamics and stability of tailed ornithopter in forward flight condition have not been rigorously investigated until now as much as those of hovering insects

this study primarily made advances in our understandings on flight dynamics and stability of forward flight tailed ornithopter. The body of tailed ornithopter in forward flight condition oscillates due to the periodically excited forces and moments of wings

the characteristics of these oscillations were studied in both the numerical simulation considering the aeroelasticity of wings and tail and the free flight testing with in-flight whole-body measurement. The stability of the limit-cycle oscillation has been regarded as one of the main interests in forward flight tailed ornithopter

however, both the translational-rotational coupling effect and the absence of tail made hovering insects have “inherently unstable” flight characteristics. Compared to tailed ornithopter, hovering insects have much more control degrees of freedom in wing kinematics for the sustained and controlled flights. When a hovering flapping-wing air vehicle is to be mechanically realized by mimicking real insects, a small number of control parameters need to be carefully selected to make the “inherently unstable” open-loop system controllable and stabilizable. This study newly proposed the stroke plane angle as a control input for stabilizing the intrinsic instability of hovering insect-like ornithopter. A relative rotational motion of the stroke plane with respect to the body turned out to be one of the effective control efforts for the stabilization of model hawkmoth

in the closed-loop response, the stroke plane angle was kept almost the same in cycle-averaged mean sense with respect to the horizontal plane. Inspired from the cycle-averaged constant of the stroke plane angle, “inherently stable” flight characteristic of hovering insect was obtained by instantaneously and continuously keeping the stroke plane angle as a constant with respect to the horizontal plane (i.e. the stroke plane was completely isolated from any changes of body pitch angle). Maintaining the trimmed wing kinematics with the isolated stroke plane successfully made hovering insect-like ornithopter have “inherently stable” flight dynamics like forward flight tailed ornithopter.

the direct time integration of tailed ornithopter flight dynamics model and the wind tunnel testing of tailed ornithopter tethered by a special device were employed to qualitatively see its intrinsic stability. After linearizing the nonlinear flight dynamics model, eigenvalue analysis of the linearized system matrix revealed that flight dynamic stability of tailed ornithopter quantitatively turned out “inherently stable”. Such intrinsic stability caused by the tail which generated a counter moment with respect to the body pitch angle so that the perturbed body pitch angle restored to the original trim value. In the light of the effects of flapping counter forces and torques and the negative stability derivatives in the linearized system matrix of bioinspired ornithopters, just keeping the same trimmed wing kinematics was found to play a positive role to maintain flight stability. In that occasion, most of the dominant stability derivatives in the system matrix of tailed ornithopter had negative values for the intrinsic stability