Conventional hybrid VTOL (Vertical Take-Off and Landing) aircraft are always designed by directly mounting multicopter rotors onto a fixed-wing plane. This type of VTOL aircraft may suffer from terrible drag due to the copter rotors. To tackle this problem, this paper describes a novel rotors retractable hybrid VTOL UAV (RR VTOL) that combines the hovering capability of multicopters and the high cruise velocity property of pure fixed-wing aircraft. Computational fluid dynamics (CFD) analysis on rotors folding and unfolding models proves that the retractable rotors can increase the lift-to-drag ratio by approximately two times compared to that of the common hybrid VTOL UAV. Moreover, some aerodynamic performance data of the rotors folded model are also acquired. A preliminary design method for this creative UAV is proposed with new empirical equations derived from many hybrid VTOL UAVs’ data. Emphasis is given to the method for determining the center of gravity position and horizontal tail dimensions as they are the unique aspects of the RR VTOL configuration. Subsequently, the manufacturing process and rotor retraction mechanism of two UAV generations are reviewed and a rapid prototyping method for RR VTOL is summarized. Finally, a successful flight test confirms that the proposed VTOL concept is practical and the drag reduction of rotors retracting function is remarkable.

From the perspective of damage mechanics, the damage parameters were introduced as the characterizing quantity of the decrease in the mechanical properties of powder superalloy material FGH96 under fatigue loading. By deriving a damage evolution equation, a fatigue life prediction model of powder superalloy containing inclusions was constructed based on damage mechanics. The specimens containing elliptical subsurface inclusions and semielliptical surface inclusions were considered. The CONTA172 and TARGE169 elements of finite element software (ANSYS) were used to simulate the interfacial debonding between the inclusions and matrix, and the interface crack initiation life was calculated. Through finite element modeling, the stress field evolution during the interface debonding was traced by simulation. Finally, the effect of the position and shape size of inclusions on interface debonding was explored.

Fatigue failure is a mail form of engineering structures, especially for aircraft structures. Crack initiation and propagation threat to flight safety. So far, there has been a variety of methods in fatigue life prediction. Within them, damage mechanics method is a special engineering calculating method. Based on theory of damage mechanics, a new life prediction model for high circle fatigue of metal is proposed. The equation of damage evolution adopted here is concise and efficient, from which the formula for fatigue life prediction under general fatigue load could be given. Consequently, the damage parameter can be determined through standard fatigue experimental data. This new method simplifies the traditional method of life prediction, and reflects the performance of the material fatigue damage concisely and accurately.