Determination of the aerodynamic characteristics of the tiltrotor with the wingtip-mounted coaxial rotors

The article describes the study of rotor-to-wing aerodynamic interaction for the wingtip-mounted coaxial rotors configuration of the tiltrotor aircraft. The influence of the rotor slipstreams on lift-to-drag ratio characteristic was determined. Obtained results were compared with similar characteristics of the equivalent in thrust conventional single rotor slipstreams impact. Using the computational aerodynamics methods (panel-vortex method) the flow around the tiltrotor model with the wingtip-mounted single and coaxial rotors has been simulated. A study of the basic model configuration with conventional single rotors, based on the technical characteristics of the AgustaWestland AW609 tiltrotor, was conducted. Further researches were conducted for a modified model where single rotors were replaced with equivalent in thrust coaxial rotors. The influence of the rotor slipstreams on the aerodynamic characteristics of the model for both directions of rotors rotation in coaxial combination is considered. Also, the dependence of the maximum lift-to-drag characteristic due to the coaxial rotor diameters change has been determined. The results show that the coaxial rotor slipstreams-to-wing aerodynamic interaction effect is the similar to the effect of conventional single rotor, but less intensive. Comparison of the results showed that a tiltrotor equipped with wingtip-mounted single rotors has approximately 20% greater maximum lift-to-drag characteristic than one equipped with coaxial rotors with the same thrust. However, the use of coaxial rotors allows getting higher maximum speed, when conventional single rotors lose the efficiency significantly. Therefore, it is advisable to conduct further research for the possibility of using coaxial rotors for tiltrotor aircrafts. The research results are presented in graphical form. The obtained data provides a basis for further studies of the described problem, and also will be useful for new tiltrotor design works.

Distance and Rotational Speed Analysis of Coaxial Rotors for UTHM C-Drone

The prototype of UTHM C-Drone use a coaxial hexacopter concept for its propulsion system. A coaxial rotor consists of two motor and two propellers mounted above each other and aligned in relation to their axis of rotation. The propellers are based on the T-Motor U15XXL KV29 model used in UTHM C-Drone. The distance between the two propellers is usually relative to the radius of the propeller or can be lesser. The objectives for this study are to investigate the effect of distance between upper and lower propeller in a coaxial rotors system and the effect of rotational speed. This study is important to ensure the C-Drone power efficient and capable to lift 180 kg payload. The CAD model of the propeller and coaxial rotors system were designed based on the specification from T-Motor company by Solidworks software and the flow simulations were conducted using Solidworks Flow Simulation module. The total of six CAD models; one for a single propeller and five for coaxial rotors with five difference of distance cases were constructed. For each model, the total thrust was tested from 50% throttle power up to the 90% throttle power. It was found that the coaxial rotors system can generate more thrust than a single propeller but less than double. It was also found that if the lower propeller rotates faster than the upper propeller, the increment of total thrust is very small. However, if the upper propeller rotates faster than the lower propeller, the total thrust increase significantly. For the case of faster upper propeller, as the higher the throttle applied, the thrust increment ratio will decrease, and the efficiency of the thrust produced will be affected. In addition, for same rotation speed, the thrust generated was lesser when both propellers rotate in a same direction compared to when each propeller rotates in the opposite directions of each other.

Aerodynamic Performance of a Coaxial Hex-Rotor MAV in Hover

Micro aerial vehicles (MAVs) usually suffer from several challenges, not least of which are unsatisfactory hover efficiency and limited fly time. This paper discusses the aerodynamic characteristics of a novel Hex-rotor MAV with a coaxial rotor capable of providing higher thrust in a compact structure. To extend the endurance during hover, flow field analysis and aerodynamic performance optimization are conducted by both experiments and numerical simulations with different rotor spacing ratios (i = 0.56, 0.59, 0.63, 0.67, 0.71, 0.77, 0.83, 0.91). The measured parameters are thrust, power, and hover efficiency during the experiments. Retip ranged from 0.7 × 105 to 1.3 × 105 is also studied by Spalart–Allmaras simulations. The test results show that the MAV has the optimum aerodynamic performance at i = 0.56 with Retip = 0.85 × 105. Compared to the MAV with i = 0.98 for Retip = 0.85 × 105, thrust is increased by 5.18% with a reduced power of 3.8%, and hover efficiency is also improved by 12.14%. The simulated results indicate a weakness in inter-rotor interference with the increased rotor spacing. Additionally, the enlarged pressure difference, reduced turbulence, and weakened vortices are responsible for the aerodynamic improvement. This provides an alternative method for increasing the MAV fly time and offers inspiration for future structural design.

The Control Algorithm and Experimentation of Coaxial Rotor Aircraft Trajectory Tracking Based on Backstepping Sliding Mode

In view of the uncertainty of model parameters, the influence of external disturbances and sensor noise on the flight of coaxial rotor aircraft during autonomous flight, a robust backstepping sliding mode control algorithm for the position and attitude feedback control system is studied to solve the trajectory tracking problem of an aircraft in the case of unknown external interference. In this study, a non-linear dynamic model based on a disturbed coaxial rotor aircraft was established for an unknown flight. Then, a non-linear robust backstepping sliding mode controller was designed, which was divided into two sub-controllers: the attitude controller and the position controller of the coaxial rotor aircraft. In the controller, virtual control was introduced to construct the Lyapunov function to ensure the stability of each subsystem. The effectiveness of the proposed controller was verified through numerical simulation. Finally, the effectiveness of the backstepping sliding mode control algorithm was verified by flight experiments.

Active Airframe Vibration Control Simulations of Lift-offset Compound Helicopters in High-Speed Flights

This paper studies the simulations of active airframe vibration controls for the Sikorsky X2 helicopter with a lift-offset coaxial rotor. The 4P hub vibratory loads of the X2TD rotor are obtained from the previous work using a rotorcraft comprehensive analysis code, CAMRAD II. The finite element analysis software, MSC.NASTRAN, is used to model the structural dynamics of the X2TD airframe and to analyze the 4P vibration responses of the airframe. A simulation study using Active Vibration Control System(AVCS) with Fx-LMS algorithm to reduce the airframe vibrations is conducted. The present AVCS is modeled using MATLAB Simulink. When AVCS is applied to the X2TD airframe at 250 knots, the 4P longitudinal and vertical vibration responses at the specified airframe positions, such as the pilot seat, co-pilot seat, engine deck, and prop gearbox, are reduced by 30.65 ~ 94.12 %.

Wind Tunnel Studies on Hover and Forward Flight Performances of a Coaxial Rigid Rotor

The aerodynamic performance of a reduced-scale coaxial rigid rotor system in hover and steady forward flights was experimentally investigated to gain insights into the effect of interference between upper and lower rotors and the influences of the advance ratio, shaft tilt angle and lift offset. The rotor system featured by 2 m-diameter, four-bladed upper and lower hingeless rotors and was installed in a coaxial rotor test rig. Experiments were conducted in the Φ3.2 m wind tunnel at China Aerodynamics Research and Development Center (CARDC). The rotor system was tested in hover states at collective pitches ranging from 0° to 13° and it was also tested in forward flights at advance ratios up to 0.6, with specific focus on the shaft tilt angle and lift offset sweeps. To ensure that the coaxial rotor was operating in a similar manner to that of the real flight, the torque difference was trimmed to zero in hover flight, whilst the constant lift coefficient was maintained in forward flight. An isolated single-rotor configuration test was also conducted with the same pitch angle setting in the coaxial rotor. The hover test results demonstrate that the figure of merit (FM) value of the lower rotor is lower than that of the upper rotor, and both are lower than that of the isolated single rotor. Moreover, the coaxial rotor configuration can contribute to better hover efficiency under the same blade loading coefficient (CT/σ). In forward flight, the effective lift-to-drag (L/De) ratio of the coaxial rigid rotor does not monotonously change as the advance ratio increases. Increases in the required power and drag in the case with a high advance ratio of 0.6 leads to the decreasing L/De ratio of the rotor. Meanwhile, the L/De ratio of the rotor is relatively high when the rotor shaft is tilted backward. The increasing lift offset tends to result in reduced required rotor power and an increase in the rotor drag. When the effect of the reduced rotor power is greater than that of the increased rotor drag, the L/De ratio increases as the lift offset increases. The L/De ratio can benefit significantly from lift offset at a high advance ratio, but it is much less influenced by lift offset at a low advance ratio. The forward performance efficiency of the upper rotor is poorer than that of the lower rotor, which is significantly different from the case in the hover flight.