An energy management strategy for a series hybrid electric propulsion system designed for a fixed-wing vertical take-off and landing (VTOL) aircraft is presented in this paper. The proposed method combines an ideal operating line (IOL) and fuzzy logic. Fuzzy logic is used to dynamically and optimally allocate the output power of the generator and the battery pack according to the power requirement of the aircraft and the SOC of the battery pack. The IOL controller is used to optimize the internal combustion engine (ICE) operating point to improve the fuel economy of the system. The detailed aircraft model and energy system model are established. The flight process of a 100 kg scale VTOL aircraft under a typical mission profile is simulated. The simulation results show that running the ICE based on IOL can greatly improve its efficiency The introduction of fuzzy logic to optimize the power allocation of the generator and battery pack improves the overall efficiency of the system. The feasibility and effectiveness of the energy management strategy proposed in this paper are verified, and the design ideas and analysis methods are provided for the energy management of a hybrid electric aircraft.

An aircraft can extract energy from a gradient wind field by dynamic soaring. The paper presents trajectory optimization of an unmanned aerial vehicle for dynamic soaring by numerical analysis and validates the theoretical work through flight test. The collocation approach is used to convert the trajectory optimization problem into parameters optimization. The control and state parameters include lift coefficient, bank angle, positions, flight path angle, heading angle, and airspeed, which are obtained from the parameter optimization software. To validate the results of numerical simulation, the dynamic soaring experiment is also performed and experimental data are analyzed. This research work shows that the unmanned aerial vehicle can gain enough flight energy from the gradient wind field by following an optimal dynamic soaring trajectory. Meanwhile, the variation of flight path angle, heading angle, and airspeed has a significant influence on the energy transform. The solution can provide theoretical guide to unmanned aerial vehicles for extracting maximum energy from gradient wind fields.

With high aerodynamic efficiency and low observable design for flying wing configuration, the control ability decreases and the flying risk increases, especially during the takeoff and landing processes for unmanned aerial vehicle (UAV). To solve the nonlinear characteristic problem of the flying wing configuration at a high angle of attack (AoA) for a low aspect ratio flying wing UAV, a controllable region and stability analysis method is proposed in the paper. Firstly, the aerodynamic characteristics and static stability of a flying wing UAV are analyzed. Secondly, according to the characteristics of aerodynamic data, an efficient longitudinal trim state calculation method under the low‐speed range is proposed. The trim state contour map can be used effectively with the proposed trim state calculation method, which can further analyze the constraint relationship between different trim states and the controllable region of the UAV. Thirdly, the stability of different trim states is analyzed through the phase portrait between AoA and airspeed. With the analysis result, longitudinal attitude and AoA decoupling control laws are designed based on the incremental nonlinear dynamic inversion (INDI) method. The effectiveness and adaptability of the designed control methods are verified through simulation at high AoA.

Dynamic soaring is a special flying technique designed to allow UAVs (unmanned aerial vehicles) to extract energy from wind gradient field and enable UAVs to increase the endurance. In order to figure out the energy-extraction mechanisms in dynamic soaring, a noninertial wind relative reference frame of aircraft is built. In the noninertial frame, there is an inertial force which is created by gradient wind field. When the wind gradient(GW)and the components of airspeed(vzvx)are positive, inertial force(F)makes positive work to the aircraft. In the meantime, an equilibrium position theory of dynamic soaring is proposed. At the equilibrium positions, the increased potential energy is greater than the wasted kinetic energy when the aircraft is flying upwards. The mechanical energy is increased in this way, and the aircraft can store energy for flight. According to the extreme value theory, contour line figures of the maximum function and the component of airspeed(vz)are obtained to find out the aircraft’s lifting balance allowance in dynamic soaring. Moreover, this equilibrium position theory can also help to conduct an aircraft to acquire energy from the environment constantly.

Stratospheric airships are important platforms for near-space enduring cruising capability for several applications. Since energy shortage is one of major limiting factors in stratospheric airship, it is crucial to develop novel methods to reduce the requirement on energy systems. In this article, optimal trajectory of stratospheric airship in gradient wind fields is investigated in order to reduce energy consumption. First, polynomial expressions for wind gradients are assumed, and three-dimensional point mass airship equations of motion are used. Then the variety characters of energy for stratospheric airship are studied, the minimum power output of power system is taken as the optimizing target, and collocation approach is used to convert the trajectory optimization problem into parameters optimization. The characters of stratospheric airship’s cruising in three typical wind field are analyzed, including control and state parameters time histories, power changes for airship system, and so on. This research work shows that the greater the wind gradient, the more favorable to reduce energy consumptions, but the speed of the gradient wind cannot be too high.

Hybrid-electric technology can be expected to improve the performance of fixed wing vertical takeoff and landing (VTOL) aircraft. In this paper, we demonstrated a method of retrofitting a single-energy propulsion system prototype with a hybrid-electric propulsion system. Since the hybrid-electric system has several working modes, the optimal design results have strong coupling with mission performance. Therefore, we propose an analysis method of the mission profile to determine the design point. Finally, the payload-range sensitivity is studied. The results show that the hybrid-electric propulsion system can greatly increase the mission profile of aircraft. The analysis method of the mission profile also provides perspective for the hybrid-electric propulsion system design.