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; 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; 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.
최근 무인 비행체의 다양한 활용 가능성에 주목하고, 건물 내부와 같은 좁은 공간에서도 근접 감시 및 정찰 목적으로 운용이 가능한 초소형 생체모방 날갯짓비행체 개발에 많은 관심이 모아지고 있다. 일반적으로 초소형 비행체는 추가적인 탑재중량이 제한적이기 때문에 무인자율비행에 필요한 센서와 구동기가 최소한으로 구비되어야 한다. 이를 위해서는 날갯짓비행체의 비행 동역학적 특성이 잘 모사될 수 있는 신뢰성 있는 해석 모델이 필요하며, 이를 바탕으로 모델 기반 비행 제어에 필수적인 상태-공간 시스템 모델을 수립하게 된다. 하지만 날갯짓비행체는 다자유도의 날갯짓 운동, 유연한 날개의 구조동역학적 변형, 저 레이놀즈 수 영역의 복잡한 비정상 공기역학적 특성이 상호 연계가 되어, 제자리 비행 혹은 순항 전진 비행에 필요한 공력과 안정성 유지 및 방향 전환에 필요한 제어력 및 제어모멘트를 생성하게 된다. 이러한 날갯짓비행체의 고유한 특성은 일반적인 항공기의 비행동역학 모델링과 달리 높은 난이도의 모델링 기법을 요하며, 아직까지 체계적으로 많은 연구가 이루어지지 않은 실정이다.
본 연구에서는 제자리 비행하는 곤충형 날갯짓 비행체 및 전진 비행하는 꼬리가 있는 날갯짓 비행체의 무인자율비행에 필요한 신뢰성 있는 해석 모델을 제시하기 위해, 해석적 및 실험적 방법을 포괄하여 생체모방 날갯짓비행체의 비행 동역학 및 안정성에 대한 연구를 엄밀히 수행하였다. 비행 중 날개와 꼬리에서 발생되는 공력탄성학적 변형이 비행동역학에 미치는 영향을 파악하기 위해 유체-구조 연계 해석기법과 모션 캡쳐 시스템을 이용한 비행 실험 및 풍동 실험이 병행되었다. 수립된 날갯짓비행체의 비행 동역학 모델은 날갯짓비행체의 자세한 비행 동역학적 특성을 살펴보기에는 활용도가 높지만, 반복되는 날갯짓 운동에 의해 주기성을 가지며, 공기역학 및 비행체의 병진 및 회전 방향의 연성에 비선형성을 가지기 때문에 비행 제어에 활용되기 위해서는 적절한 선형화가 필요하다. 미소 교란 및 주기-평균 이론을 동시에 비선형 비행 동역학 모델에 적용하여, 안정성과 같은 주요한 특성이 성공적으로 반영된 선형 시불변 시스템 모델을 수립하였으며, 이 모델을 바탕으로 시스템 안정성 분석, 센서 및 구동기의 자유도 선별 및 불안정한 시스템의 안정화를 위한 제어기 설계 및 적용이 효과적으로 이루어질 수 있었다.
