In this paper, the variable-speed coaxial helicopter is modelled by the blade element method, non-uniform inflow method and empirical function method, and the aerodynamic performance analysis method is proposed. To ensure the accuracy of this method, a flight test of the verification aircraft has been performed, and the rotor speed and required power under different forward flight speeds have been obtained. By comparing the experimental results with the calculation results, it is ascertained that the rotor speed trim error of this method is less than 6%, and the required power calculation error is less than 5%, which demonstrates the accuracy of this method. Finally, when the rotor diameter, hovering collective pitch and hovering rotor speed are equal, the required power of variable-speed coaxial helicopter and variable collective-pitch coaxial helicopter is compared through this method. The comparison results show that the variable-speed coaxial helicopter has low required power when it is less than the forward flight speed corresponding to the hovering rotor speed. When the forward speed is higher than that corresponding to the hovering rotor speed, the variable-pitch coaxial helicopter has lower required power.

In this paper, the dynamic balance of a rigid variable speed rotor is tested and analyzed. The vibration acceleration is measured by vibration analyzer instrumentations. The rotor adjustment parameters of counterweight, pitch, and trailing-edge flap are considered. The amplitude and phase of the 1 Ω vibration acceleration are analyzed through an all-phase fast Fourier transform. The experiments are conducted using two rigid rotors with the same geometry. The accelerations of the fuselage in the x, y, and z directions are measured. Through a waterfall diagram of the auto-power spectrum, it is found that the imbalance of counterweight, pitch, and trailing-edge flap causes an obvious increase in 1 Ω and 2 Ω acceleration. The hub counterweight mainly causes the lateral and longitudinal vibration in the disc plane, and the aerodynamic factors such as pitch and trailing-edge flap mainly affect the vertical vibration. In order to achieve dynamic balance for variable speed rotors, the counterweight adjustment should be given the highest priority for the vibration in the disc plane, and the pitch and trailing-edge flap adjustment should be given the highest priority for the vertical vibration. The results obtained from this experiment may be helpful toward reasonable designs of variable speed rotor helicopters.

In this work, the flow field of an autorotating rotor in a water tunnel with various pitches and shaft backward angles was investigated via particle image velocimetry (PIV). The experiments were carried out on a free-rotating two-bladed single rotor. Computational Fluid Dynamics (CFD) based on moving overset grids were developed to study the hydrodynamic characteristics of an underwater autorotating rotor. The simulation results are in good agreement with the test results. The thrust and thrust coefficient of the underwater autorotating rotor were calculated by CFD simulation under different situations. The research demonstrates that rotational speed and thrust have a significant positive correlation with water velocity, pitch, and shaft back angle. In particular, the thrust coefficient scarcely varies with the shaft backward angle. An underwater autorotation rotor with a thin airfoil, negative torque, and a suitable number of blades can increase the thrust and thrust coefficient. The investigation is of significance in enriching the autorotation theory of rotors and helping to develop underwater autorotating rotors.

CFD simulation of the hovering condition of a single-rotor helicopter was carried out by three methods: moving reference frame (MRF), sliding mesh and overset mesh, respectively. The parameters such as velocity field, pressure field and pressure coefficient of the helicopter rotor and its surrounding flow field are obtained, and the simulation results are compared with the experimental data of wind tunnel test. The three methods are compared from the differences between the model and the grid, the accuracy of the calculation results, and the calculation convergence time. The characteristics, application and other meaningful results of each method are obtained.

In this paper, a robust tracking control strategy based on the dynamic feedback linearization method is proposed for the nonlinear and highly coupled dynamic characteristics of coaxial unmanned helicopter. The mathematical model of the coaxial unmanned helicopter is determined by fault analysis. Then the high-order state system is dynamically feedback linearized by extending the state variables, and the dynamic characteristics of the zeros are analyzed according to the expected tracking characteristics of the inner loop. The pole placement of the subsystem realizes robust monitoring of height and position commands by designing robust compensators. On this basis, an outer loop proportional derivative controller is designed for the horizontal positioning subsystem to realize position tracking. Loop tracking simulation ensures the good separation characteristics of feedback linearization method, and trajectory tracking simulation under fault conditions ensures the control ability and durability of the designed controller.

In this paper, a computational fluid dynamics simulation method is developed to study the influence of the rotor overlapping azimuth on the aerodynamic performance of compound coaxial helicopter. The simulation method is verified by comparing the numerical simulation results with the wind tunnel experiment data of the NASA coaxial rotor. Two overlapping azimuths of the upper and lower blades are considered, and the aerodynamic performance of the isolated rotor and the compound coaxial helicopter in hover and forward are analyzed respectively. State 1 means the upper and lower blades overlap at azimuth or , state 2 means the upper and lower blades overlap at azimuth or . It is found that the performance of isolated rotors is not affected by rotor overlapping azimuth in hover, but the total thrust fluctuation amplitude of isolated rotors in state 2 is 76.3% smaller than that in state 1 in forward. In the hovering flight of compound coaxial helicopter, compared with state 1, the fluctuation amplitude of the lift of the wing in state 2 is 42.7% smaller; the lift fluctuation amplitude of the flat tail in state 2 is 52.4% smaller. In the forward flight of compound coaxial helicopter, compared with state 1, the total thrust fluctuation amplitude in state 2 is 83.5% smaller; the fluctuation amplitude of the lift of the wing in state 2 is 61.2% smaller. It can be concluded that the compound coaxial helicopter working in state 2 has better aerodynamic performance than the compound coaxial helicopter working in state 1; changing the rotor overlapping azimuth of the upper and lower rotors has a high engineering application value, which can increase aerodynamic stability and reduce lift fluctuations.

This paper focuses on the wake features of autorotation status mainly including the induced velocity and tip vortex trajectory in order to preliminary explanation the principle of rotation. In this work, the flow features of autorotating rotor were investigated in water tunnel via particle image velocimetry (PIV) technique. The experiments were conducted with a two-blade single rotor which can freely rotate. The stable rotational speed with various pitch and shaft backward angle was recorded and analyzed. The results show that there is a threshold value of stable rotational speed at shaft backward angle and an optimal region at pitch angle. The PIV results showed that the velocity of the internal wake was much lower than the external, indicating a consuming of energy from the flow, which was in accord with momentum theory. The vortices have three prominent parts: tip vortices, wake sheets and root vortices. There is no apparent interference between wake sheets and tip vortices. The vorticity of the tip was observed to increase in near wake due to lesser interference with the sheets. The Beddoes's model was employed to calculate the tip vortex location with respect to the rotor. The experimental results are in good agreement with the calculated results. It is meaningful for the study to enrich the autorotation theory of rotor and develop underwater rotorcraft.

An optimization method of helicopter rotor hovering performance is proposed, which is mainly carried out by means of CFD simulation. This method can be used to obtain the rotation speed of any blade model under the designed hovering tension through several iterations, and then, the tension, torque, power, and power load of the rotor under the rotation speed can be obtained. By comparing different blade pitch, radius, twist, and chord length, the blade parameters at the highest power load can be selected. A series of new rotor models are obtained by changing the model parameters on the basis of C-T rotor. NASA has carried out a lot of experiments on the C-T rotor model and published detailed experimental reports. The transient simulation is carried out by the embedded grid method.