Coaxial Rotor Research Articles

Aerodynamic Optimization of the Sizing and Blade Designs of Hovering Corotating Coaxial Rotors

Corotating coaxial rotors are seeing renewed interest in distributed electric propulsion systems and electric vertical take-off and landing (eVTOL) aircraft. The recent literature reports many interesting investigations, using prescribed rotor blades, into the flow phenomena as well as aerodynamic and aeroacoustic benefits of corotating rotors. However, the subject of the design of blade geometries, optimized to a design goal, for corotating rotors is currently lacking in the literature. This paper is written from such a design perspective, by extending a previous generalized approach to the aerodynamic optimization of counterrotating rotors to corotating rotors. The previous requirement for upper and lower counterrotating rotor torques to be equal can now be lifted in the case of corotating rotors, enabling improved versatility in the optimization of corotating blade designs. The optimization is demonstrated on an application example to address the conflicting conditions that index angles (high) for aeroacoustic benefits of reduced noise are at odds with those (low) for aerodynamic efficiency. The approach demonstrated in this paper is to set the index angle for reduced noise and then recover back the aerodynamic efficiency by using the newly developed aerodynamic optimization technique.

Investigation on Aerodynamic Noise Characteristics of Coaxial Rotor in Hover

A numerical method based on Reynolds Averaged Navier–Stokes (RANS) equations and a high-efficiency trim model is developed to simulate the aerodynamics of a coaxial rotor. Farassat 1A equations are used for the prediction of thickness and loading noise. Hover cases of different thrust coefficients with torque balance are conducted. The sound pressure history of different observation points positioned below the rotor disk plane is analyzed. Results indicate that the special noise characteristics of the coaxial rotor are mainly caused by the noise superposition of the twin rotors and the unsteady loads of aerodynamic interaction. A new kind of impulsive loading noise is induced by the blade-meeting interaction. In contract to the single rotor, the loading noise of a coaxial rotor has a much larger sound pressure level in the high-frequency band. The loading noise is obviously enhanced around the blade-meeting azimuths. The maximum noise of the coaxial rotor is located immediately below the rotor disk center, while for the single rotor, it is the minimum location.

Numerical Investigation on Aerodynamic Performance and Interaction of a Lift-Offset Coaxial Rotor in Forward Flight

Numerical Investigation on Aerodynamic Performance and Interaction of a Lift-Offset Coaxial Rotor in Forward Flight

Active vibration reductions of a lift-offset compound helicopter using individual blade pitch control with multiple harmonic inputs

PurposeThe purpose of this study is to use the individual blade pitch control (IBC), reduce actively both the rotor hub vibratory loads and airframe vibration responses for the lift-offset compound helicopter at a high-speed flight condition.Design/methodology/approachThe Sikorsky X2 technology demonstrator (X2TD) is used as the lift-offset compound helicopter. The X2TD lift-offset rotor is modelled and its rotor hub vibratory loads at a flight speed of 250 knots are predicted using a rotorcraft comprehensive analysis code, CAMRAD II, and the airframe structural dynamics is represented with a finite element analysis code, MSC.NASTRAN. When the propulsive trim methodology is applied for rotor trim, the best input condition for IBC using multiple harmonic inputs is searched to reduce the rotor vibration, while the rotor aerodynamic performance (the rotor effective lift-to-drag ratio) is improved or maintained at least. Finally, the reduction in airframe vibration responses is investigated when the best input condition for IBC with multiple harmonics is applied to the lift-offset rotor.FindingsWhen the IBC with the single harmonic input using the 2/rev actuation frequency, amplitude of 2° and control phase angle of 120° (2P/2°/120°) is considered for X2TD rotor, the rotor vibration is reduced by about 26.37% only and the rotor effective lift-to-drag ratio increases slightly by 0.98%. When X2TD rotor uses the IBC with multiple harmonic inputs (2P/2°/45° + 5P/1°/90°), the rotor hub vibratory loads and airframe vibration responses are reduced by 44.69% and from 0.48 to 79.10%, respectively, while rotor effective lift-to-drag ratio is improved by 0.77%, as compared to the baseline without IBC.Originality/valueThis study is the first study to use the 2/rev actuation for IBC to the four-bladed lift-offset coaxial rotor and to investigate to obtain simultaneously the rotor vibration reduction, rotor performance improvement and airframe vibration reduction, using IBC with multiple harmonic inputs.

Optimization of the basin and inlet channel of a gravitational water vortex hydraulic turbine using the response surface methodology

The gravitational water vortex hydraulic turbine is an appropriate turbine to be used under from a small to medium water flow rate and at a low head. This turbine extracts energy from an induced vortex in a basin, using a coaxial rotor with a vertical axis. The gravitational turbine has efficiencies that vary between 17 and 85%. Increasing the circulation was required in order to achieve the highest efficiency. The circulation is a function of some geometric parameters of the turbine, such as the ratios between the basin diameter (D) and the outlet diameter (d), d/D; the basin height (H) and D, H/D; the inlet channel width (w) and D, w/D; the inlet channel height (h) and D, h/D; the inlet channel long (L) and D, L/D; and the wrap-around angle (γ). Response surface methodology was utilized to determine the optimal geometry that results in the highest circulation. The values of the six variables that give the highest circulation of 2.089 m2/s were d/D = 0.167, H/D = 1.840, w/D = 0.2, h/D = 0.599, L/D = 0.500 and γ = 179.976°. In addition, an exponential equation was proposed for calculating the inlet velocity for each treatment. This equation is a function of d/D and the coefficient of discharge, Cd. The results of the exponential function were compared with experimental data from other investigations.

Numerical Study of Coaxial Main Rotor Aerodynamics in Steep Descent

Numerical studies of the aerodynamic characteristics of the coaxial main rotor of the Kamov Ka-32 helicopter in steep descent modes, including the area of the vortex ring state (VRS) modes, have been performed. Used in this paper is an original free vortex wake model of the rotor developed by the authors. The angles of attack of the rotor αR = 30 – 90° and the rate of descent in the range of Vy = 0–30 m/s are considered. The calculations have been carried out under the condition of a fixed time-average thrust of the rotor. The visualization of the rotor wake shapes the flow structures using streamlines, and the flow velocities have been received and analyzed. The VRS boundaries in “Vx–Vy” coordinates have been constructed. The criteria used in this paper are: rotor thrust and torque pulsations, rise of rotor torque and induced velocity relative to the hovering mode. The results of the calculations are compared with the experimental and calculated data of other authors, and a satisfactory match has been obtained. The new results presented in this paper can supplement the existing experience of experimental and numerical research in this field.

A Coupled CFD/Trim Analysis of Coaxial Rotors

A numerical simulation method of computational fluid dynamics (CFD) coupling with a trim analysis for coaxial rotor systems is described in this paper. The trim analysis is implemented using a rotorcraft flow solver, rFlow3D. Six target forces and moments, which are the thrust of the coaxial rotor system, the rolling and pitching moments for each upper and lower rotor, and the torque balance for yaw control, are considered as the trim conditions. The blade pitch angles of both upper and lower rotors are adjusted to satisfy the target trim conditions through the trim analysis by being loosely coupled with the CFD solver. Verification of the trim analysis method and validation of the prediction accuracy of aerodynamic performance are performed based on previous experimental and numerical studies in hover and forward flight using the lift-offset conditions. It is shown that the predicted hover performances of the torque-balanced coaxial rotors are in excellent agreement with the experimental data. It is also verified that the lift-offset conditions in forward flight are simulated using this established trim analysis. Furthermore, reasonable agreements with other computational results are indicated.

Design, development, and flight testing of a tube-launched coaxial-rotor based micro air vehicle

This paper describes the development and flight testing of a compact, re-configurable, hover-capable rotary-wing micro air vehicle that could be tube launched for increasing mission range. The vehicle design features a coaxial rotor with foldable blades, thrust-vectoring mechanism for pitch/roll control and differential rpm for yaw control. The vehicle was stabilized using a cascaded feedback controller implemented on a 1.7-gram custom-designed autopilot. Wind tunnel tests conducted using a single-degree-of-freedom stand demonstrated gust-tolerance up to 5 m/s, which was verified via flight testing. Finally, the 366-gram vehicle was launched vertically from a pneumatic cannon followed by a stable projectile phase, passive rotor unfolding, and transition to a stable hover from arbitrarily large attitude angles demonstrating the robustness of the controller.

Performance and Loads of a Lift Offset Rotor, Part II: Prediction Validations with Measurements

The predictions of an upgraded UMARC comprehensive analysis are compared to experimental lift offset rotor results. The experiments cover a range of collective pitch angles (o) from 2° to 10°, advance ratios (μ) from 0.21 to 0.53, and lift offset from 0% to 20%. The experimental model rotors are from a system of coaxial hingeless rotors, with two blades each, and a first flap frequency of approximately 1.6/rev. The simulation is compared with isolated rotor performance and controls with lift offset, loads, and pitch link forces. Increasing efficiency with increasing lift offset, the impact of lift offset on different loads, and the dependence of pitch link loads on pitch bearing damping are identified in the experiment and correlated with the simulation.

Experimental and numerical investigation of co-axial rotor interaction to thrust

Experimental and numerical investigation of co-axial rotor interaction to thrust

Finite element analysis and optimization of active magnetic bearings for contra-rotating coaxial rotor system

This paper presents the design, analysis and optimization of active magnetic bearings (AMBs) to support the inner rotor in a contra-rotating coaxial rotor system. An FEM based rotordynamic model is developed using 8-dof Timoshenko beam elements assuming both inner and outer rotors to be flexible. The dynamic response of this system due to an unbalanced force is used in one of the constraints of a multi-objective genetic algorithm for controlling the vibration amplitude within 10% of the air gap. A maximum reduction of 46.9% in the amplitude peak is obtained under the present conditions. Hence, this work also offers a tool useful in touch-down bearing design. The PD controller gains are found to govern the stiffness and damping properties of the AMB. On increasing the mass unbalance by a factor of 2.5, number of turns, pole width and maximum current are found to increase by 52.9%, 29.4% and 71.67% respectively.

Comparative Study of Coaxial Main Rotor Aerodynamics at Forward Flight Based on Free Wake Model and Unsteady ReynoldsAveraged Navier–Stokes Method

This paper is dedicated to the numerical modeling of the aerodynamic characteristics of the full-scale coaxial main rotor. The modes of forward flight in the range of values V = 30-60 m∙s –1 (108-216 km∙h –1) have been under consideration. The simulation has been performed with the usage of two computational fluid dynamics (CFD) approaches: the free wake model (FWM) developed by the authors and the unsteady Reynolds-averaged Navier –Stokes (URANS) equations method based on the Ansys Fluent software. The coefficients of rotor thrust and torque, vortex wake shapes and induced velocity fields have been obtained and analyzed. The FWM and the URANS modeling data match satisfactorily. The FWM demonstrates a significant advantage in computing time and resources costs with sufficient accuracy in resulting rotor’s basic aerodynamic characteristics. That’s why it seems appropriate to use the FWM for preliminary simulations, taking into account before using the high time and resource intensive the URANS method for comprehensive investigation of coaxial rotor aerodynamics at forward flight modes.

동축 로터 해석을 위한 Actuator Model을 포함한 압력 기반의 압축성 유동 해석자 개발

The purpose of present study is to develop a pressure based compressible flow solver with actuator models to estimate the performance of coaxial rotor. In order to aim this purpose, actuator models which used developed for the single rotor performance was modified to analyze the coaxial rotor performance by isolating and correcting the effect of each rotor blade. In addition, the suggested Actuator models were combined with a pressure based compressible solver which was developed previous research. The hovering rotor analysis results calculated by the developed solver were compared with Full-CFD results with a body fitted mesh. The proposed actuator methods can predict the thrust and power of a coaxial rotor as accurately as a full-CFD method. Moreover, the suggested actuator surface model well predicts the interaction effects between near blades than previous manner.

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.

축소 기체 구조 모델을 이용한 고속 비행 복합형 무인 회전익기의 능동 진동 제어 시험

Active vibration control for a 25 %-scale model of a high-speed compound unmanned rotorcraft was experimentally studied. The compound rotorcraft uses wings, propellers, and a lift-offset coaxial rotor system with two blades per rotor. A small-scale airframe structure was manufactured using aluminum and steel and appropriately represents the dynamic similarity of the full-scale model. A ground vibration test (GVT) was used in this experimental work. An active vibration control system (AVCS) using two force generators and three accelerometers was applied for the active vibration control of the small-scale airframe model. Three accelerometers that measure acceleration signals in the vertical direction of the airframe are located at specific airframe locations: the remote cockpit unit, wing root, and wing tip. A 2P rotor hub pitch moment that produces a linear force generator was used to excite the airframe structure. The AVCS using the Fx-LMS algorithm was implemented using MATLAB Simulink, dSPACE ControlDesk, and MicrolabBox. This experimental study shows that the vibration responses of the airframe in the vertical direction at the specific airframe positions are reduced by 47.54 % ~ 84.67 %, and the test results compare reasonably with the simulation results.

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.

Vibration loads analysis of coaxial rigid rotor in rotor interaction

A vibration load computing method for a coaxial rigid rotor was developed and applied to analyzing its vibration load in rotor interference. The structural dynamics model based on the medium-size deformation beam theory and the double-rotor aerodynamics model that contains unsteady free wake were jointly used to develop the coaxial rigid rotor's vibration load computing method. The wind tunnel test data of the XH-59A helicopter was used to validate this computing method. The vibration load of a rotor hub was analyzed in detail. The results show that the 3Ω harmonic wave amplitude of the vertical force Fz of the rotor hub and that of its pitch moment My decrease in high pitch angle, but the 3Ω harmonic wave amplitude of the roll moment Mx of the rotor hub obviously increases. The vibration load of the rotor hub increases with the decrease of the range of the coaxial rigid rotor; the roll moment and pitch moment of the rotor hub are sensitive to phase angle. The results also show that the increase of the 3Ω harmonic wave amplitude of the roll moment Mx is greater than the decrease of that of the roll moment My at the phase angle of 30° and that the level of vibration load of the rotor hub is minimum at the phase angle of 0° in forward flight.

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.

Flight Dynamics Modeling, Trim, Stability, and Controllability of Coaxial Compound Helicopters

Coaxial compound helicopters feature high-speed characteristics that are not possessed by conventional helicopters. The aim of this paper is to develop a flight dynamics model of coaxial compound helicopters and investigate the trim, stability, and controllability. A flight dynamics model including a coaxial rotor model and a propeller model is introduced. Based on the flight dynamics model, the trim of hover and level flights of a coaxial compound helicopter is performed. Considering the influence of flapping motions, stability and control derivatives are assessed under the small disturbance hypothesis by using a numerical differentiation method. Then, stability and response of the coaxial compound helicopter are presented. The results of trim show good agreement with those of the reference, which verifies the flight dynamics model. The speed static stability derivative is stable in whole trim range, while the angle of attack static stability derivative is unstable. The yawing control with collective differential of the coaxial compound helicopter will induce serious roll effect in high-speed flights.