## 1 Introduction  
Landing is a crucial stage for the safety and performance of aircraft flight. It is reported that 47% of aircraft accidents happen in the stage of landing. Flying animals in nature, such as birds, have demonstrated much advanced capability of short landing comparing with fixed-wing aircraft. By flapping the wings, birds can make use of the stall delay phenomenon and produce great aerodynamic lift and drag forces at large pitching angles and low flight velocity. By adjusting the flapping motion and pitching angle of the wings and body, the required lift and drag forces can be gained to achieve a safe short landing. As illustrated in Figure 1, the forward flight velocity of birds decreases rapidly during landing although the descending velocity in vertical direction increases. However, the forward velocity of a fixed-wing aircraft must be maintained to prevent stall and produce adequate lift for safe landing. Consequently, a fixed-wing aircraft usually lands with a certain runway distance.  
Inspired by the superior flight performance of birds, research has been conducted on the kinematics of flapping wing motion during landing. [PERSON] et al. [1] observed large pitching angles between 40 and 90 of the wing together with the body of a pigeon during landing. [PERSON] et al. [2,3] recorded the kinematics of motion of the flapping wing and body motion of a pigeon in descending flight. The study results pointed out that the body pitching angle combined with the wing twist plays an important role for a bird to achieve a short landing. The majority of lift and drag forces are produced by the modulation of the pitching angle during landing.  
On the other hand, research efforts have been made on the theoretical analysis and numerical simulation of the aerodynamic performance of the flapping wing. [PERSON] et al. [4] revealed that the leading edge vortex generated by flapping wings led to a significant increase of lift coefficients compared to the steady-state aerodynamics of a fixed wing. [PERSON] et al. [5] found that a flapping wing flight could save aerodynamic power compared to steady flight. [PERSON] et al. [6; 7; 8] employed the CFD method to simulate the unsteady aerodynamics of an insect-like flapping wing and found that increased aerodynamic lift can be obtained at large flapping amplitude and large pitching angle (angle of attack, AOA) beyond the stall AoA. Based on potential flow theory [9; 10] and Joukowski transformation [11; 12], [PERSON] et al. [13] constructed an unsteady aerodynamic model (UAM) to calculate the aerodynamic force produced by the leading edge vortex [14; 15; 16] and trailing edge vortex [17; 18; 19]. [PERSON] et al. [20] further developed the UAM to prevent the adverse effect of vortex penetration using a collision avoidance algorithm and enforced a zero-through-flow boundary condition for both two-dimensional (2D) and three-dimensional (3D) wings.  
The previous research has established a solid base for aerodynamic analysis of flapping wings. However, less attention has been paid so far to the analysis of short landing performance of a bird-like aircraft. In this context, the investigation has been conducted into the kinematic of motion and associated aerodynamics of a flapping wing aircraft for short landing by employing the UAM validated by CFD. Experimental study of a bird-like flapping wing aircraft model was also carried out to measure the wing motion, inertia, and aerodynamic forces. The test data was used to compare and verify the numerical analysis results by UAM. In addition, a parametric study was carried out to evaluate effect of the kinematics of motion in particular the pitching angle and flapping frequency of flapping wings on the aircraft landing performance. The investigation results demonstrate the advantages of flapping wings in short landing performance compared to a fixed-wing aircraft.