## 1 Introduction  
Understanding the mechanisms of trim and flight stability in birds is critical to guide the design of bionic micro air vehicles.  
[PERSON] and [PERSON] [1, 2] first investigated the issue of dynamic stability in gliding and flapping flight. Their results show that the wings alone may be sufficient to provide longitudinal static stability in the gliding flight of birds, which make birds resemble tailless aircraft more closely than conventional aircraft with a tailplane. They found that stable gliding flight should satisfy one or more of the following features: (1) fliers can sweep their wings forward in slow flight; (2) wing sections can be twisted down at the tips when swept back (wash-out) and twisted up at the tips when swept forwards (wash-in), which can generate a nose-up moment; and (3) additional lifting surfaces. Inspired by the above theory, seagulls, as the subject of this work, may take the methods of wash-out and forward-sweep to realize their longitudinal balance.  
Quasi-static and blade element approaches were used to analyze the stability provided by a flapping wing. They found that beating the wings faster simply amplified any existing stability or instability, and flapping can even enhance stability compared to gliding at the same airspeed. However, static stability is necessary but insufficient for dynamic stability, because a statically stable system could overcompensate for a disturbance, which could in turn lead to divergent oscillations. A full stability analysis would therefore need to consider the dynamics of the system.  
Short-term dynamic stability and trim of birds have been studied by [PERSON] [3]. Without active control, an interaction between flapping, pitching, and heave can cause resonance. This can be suppressed by active control, either with a fore-aft sweeping motion of the wing or with an up-and-down deflection of the tail.  
[PERSON] [4] developed a two degree of freedom model of the bird, which makes it possible to account for speed and height changes. With this expanded treatment, it can be shown that there is speed stability in the gliding flight of birds, which also holds for flapping flight at speeds below the speed of the minimum power required.  
The aforementioned scholars utilized simple trigonometric functions. They did not employ the true motion laws of bird wings, and they ignored the changes in lift and thrust caused by the folding motion.  
Though there are several studies on the dynamic stability of birds, computational methods for creatures [5; 6; 7], and dynamic stability analysis for flapping flight can give guidance and reference in the study of birds.  
[PERSON] and [PERSON] studied the inertial effects of flapping wings on flight stability through deriving equations from multiple rigid body dynamics [8]. [PERSON] et al. numerically simulated flapping flight characteristics through coupling flexible multibody dynamics with a reduced-order aerodynamic model, using structure solver [9]. For the insects and FWMAVs with relatively high flapping frequency and low wing-to-body mass ratio, the eigenmode analysis technique can be used because it is applicable to a linear time invariant (LTI) system [10; 11]. [PERSON] and [PERSON] studied the influence of the center of gravity location on the longitudinal dynamic stability of hovering in flapping-wing micro air vehicles, using an average method to analyze stability [12].  
The main purpose of this paper is to quantitatively study the trim state and dynamic stability of one particular bird, the seagull, using the real movement, airfoils, and geometry of birds. In order to determine the accurate aerodynamic force, we used computational fluid dynamics which is a relatively precise calculation method. The quantitative results about trim and dynamic stability can enlighten the design of bird-inspired air vehicles.