Helicopter Rotor Research Articles

Numerical investigation of dynamic stall reduction on helicopter blade section in forward flight by an airfoil deformation method

In the present work, airflow over blade section of helicopter rotor in forward flight is investigated numerically to offer a proposal for reduction or alleviation of dynamic stall on retreating blade. Unsteady 2-D compressible Reynolds-averaged Navier–Stokes equations are solved at flow conditions experienced by retreating blade of UH-60A helicopter rotor. For validation and verification purposes, flows around static and pitching airfoils in constant and variable flow Mach numbers are simulated and compared with other numerical results and experimental data. Having defined two deformation parameters, nine different permanently-deformed airfoils are produced from SSC-A09 airfoil. Flow field around these nine airfoils are simulated in variable flow Mach number with defined pitching angles. Coefficients of $$C_{{\text{l}}}$$ , $$C_{{\text{d}}}$$ and $$C_{{\text{m}}}$$ of these deformed SSC-A09 airfoils are compared with those of original airfoil to demonstrate that airfoil deformation proposed here is able to reduce and alleviate dynamic stall. Quantitatively, $$C_{{{\text{d}},\max }}$$ and absolute value of $$C_{{{\text{m}},\min }}$$ have been reduced up to 49.2% and 25%, respectively. In addition it is worth to mention that in some flow conditions dynamic stall is even eliminated with the proposed airfoil deformation. These are all achieved by permanent airfoil deformation which does not impose design complexities and expenses needed by dynamic airfoil deformation as proposed by others.

Design and control of helicopter main reducer vibration isolation platform with magnetorheological dampers

A magnetorheological vibration isolation platform (MRVIP) with cubic structure Stewart mechanism was designed and an effective semiactive control strategy was proposed based on its inverse kinematics characteristics for the helicopter to effectively reduce the vibration of the helicopter rotor transmitted from the main reducer to the fuselage. Firstly, the structural scheme of the helicopter main reducer MRVIP was proposed. Secondly, the MR damper was manufactured, and the dynamic model was established based on the mechanical test. Thirdly, according to the design requirements of the VIP, the overall structure of the MRVIP was designed. Fourthly, the dynamic simulation model of the MRVIP was established by using 3D drawing software and SimMechanics software. And the vibration characteristics of each direction were analyzed. Finally, a PID-Fuzzy semiactive controller was designed based on the characteristics of the MR damper to improve the performance of the MRVIP. The numerical analysis results showed that compared with the passive MRVIP and the MRVIP based on fuzzy control, the MRVIP with PID-Fuzzy control has the best vibration isolation performance and can effectively reduce the vibration of 6-DOF.

Full-Field Displacement Measurements of Helicopter Rotor Blades Using Stereophotogrammetry

This study presents a stereophotogrammetry approach to achieve full-field displacement measurements of helicopter rotor blades. The method is demonstrated in the wind tunnel test of a 2 m diameter rotor, conducted at the 5.5 m × 4 m Aeroacoustic Wind Tunnel of China Aerodynamics Research and Development Center (CARDC). By arranging the retroreflective targets on the special hat installed directly above the rotor hub, the dynamic motion of the rotor shaft was tracked accurately, and a unified coordinate system was established on the rotor. Therefore, three-dimensional coordinates of instantaneously measured targets attached on the blade could be transformed to the unified rotor coordinate system, thereby providing a basis for consistently calculating the blade displacements at different test conditions. Moreover, location deviations of the blade caused by the vibration of the measuring system or the rotor due to freestream and rotor rotation were also effectively corrected through coordinate transformation. Comparisons of experimental and simulation results for a range of hover and forward flight conditions show good magnitude and trend agreements.

Hardware-in-the-Loop simulation algorithm for helicopter rotor time-varying echo signals

The inherent micro-Doppler (m-D) feature of helicopter rotor echoes has been widely used in signal detection and identification. To meet the high accuracy and real-time performance requirements of the hardware-in-the-loop (HIL) simulation, this paper proposes a real-time simulation algorithm for the echo signals of helicopter rotor blades. The proposed algorithm can not only simulate the echo signals of different helicopter rotors but can also reflect the m-D characteristics of the helicopter rotor echo signals accurately. Compared to other methods, the simulation algorithm exhibits both high accuracy and superior real-time performance. The experimental results prove the feasibility and effectiveness of this method. The results of this study can have various industrial applications, such as helicopter detection and helicopter identification. This method can ensure the reliability of radar echo characteristics research, while effectively reducing the volume and weight of the system; thus, it can effectively reduce costs.

Numerical Simulation of the Wake Generated by a Helicopter Rotor in Icing Conditions

Numerical Simulation of the Wake Generated by a Helicopter Rotor in Icing Conditions

Impact Resistance Analysis of a Composite Double-Layer Honeycomb Sandwich Structure

A composite double-layer honeycomb sandwich structure is proposed to improve the impact resistance of fillers in helicopter rotors, 44 groups of those structures are numerically simulated by LS-DYNA finite element analysis software. An approximate expression for fracture energy of the composite structure is derived. The cell diameter, its wall thickness, and material strength are related to the impact resistance of the structure. With a decrease in the cell diameter and an increase in its wall thickness and material strength, the energy absorption rate of the composite structure is gradually enhanced. Among the 44 composite groups, the structure with the cell diameter of D = 3 mm, the wall thickness of 0.08 mm, and 4340 steel possessed the best energy absorption effect, the energy absorption rate is 54.7% more than for the cell diameter of D = 6 mm and 2024 aluminum. The honeycomb structure composed of various cell diameters has little difference, but it exerts a certain effect on the cell diameter.

Application of pressure head theory to determine the aerodynamic characteristics of the helicopter main rotor

A method is proposed for calculating main aerodynamic characteristics of the main rotor of the Mi-26 helicopter: a lift, a reactive moment, a required power of the propulsion system, a speed of the inductive air flow, a throttle response coefficient, a speed of the horizontal flight. This method based on the pressure propeller theory. The parameters of the Mi-26 helicopter and the propeller of the experimental setup calculated according to the pressure theory of the main rotor are experimentally confirmed. The percentage of the discrepancy between the statistical data on the aerodynamic characteristics of the Mi-26 helicopter rotor calculated with the pressure theory does not exceed 1.5%.

Numerical investigation for coupled rotor/ship flowfield using two models based on the momentum source method

The coupled rotor/ship flowfield is investigated using the Navier–Stokes solver when the helicopter rotor hovers in the ship’s airwake. The solid rotor model (SRM) and virtual rotor model (VRM) are employed, considering the different types of rotor simulation by the momentum source method. Qualitative and quantitative rotor/ship coupled flowfield analyses are conducted when the rotor hovers at different locations and with different wind-over-deck conditions. The results show that both models effectively reflect the different vortex interactions and velocity distributions in the rotor/ship coupled flowfield, and the discrepancy of velocity magnitudes between the SRM and VRM is kept within 1 m/s under most conditions. Moreover, a comparison is made of computational costs between the SRM with the moving overset mesh method and the VRM during a helicopter vertical shipboard landing. It is shown that the computational time of the VRM is smoother than the SRM by about 10 times. Considering the computational cost, it is more appropriate to use the VRM to conduct coupled flowfield studies in the helicopter/ship dynamic interface.

Wind Tunnel Test on a Slowed Mach-Scaled Hingeless Rotor with Lift Compounding

A single main rotor helicopter's maximum forward speed is limited due to the compressibility effects on the advancing side and reverse flow and dynamic stall on the retreating side. Compound helicopters can address these issues with a slowed rotor and lift compounding. There is a scarcity of test data on compound helicopters, and the present research focuses on a systematic wind tunnel test on lift compounding. Slowing down the rotor increases the advance ratio and, hence, the reverse flow region, which does not produce much lift. The lift is augmented with a wing on the retreating side. A hingeless rotor hub helps to balance the rolling moment with lift offset. Wind tunnel tests were carried out on this configuration up to advance ratios of 0.7 at two different wing incidence angles. Rotor performance, controls, blade structural loads, and hub vibratory loads were measured and compared with in-house comprehensive analysis, UMARC. A comparison between different wing incidences at constant total lift provided many insights into the lift compounding. It increased the vehicle efficiency and reduced peak-to-peak lag bending moment and in-plane 4/rev hub vibratory loads. The only trade-off was steady rotor hub loads and rolling moment at the wing root carried by the fuselage.

Improvement of solid particle erosion resistance of helicopter rotor blade with hybrid composite shield

In the present study, the solid particle erosion behavior of polyurethane (PU) tape (3M-8663) material, bidirectional carbon fiber reinforced epoxy (CF/EP) and glass fiber reinforced epoxy (GF/EP) composites has been characterized experimentally and numerically. In the experimental part, steady-state erosion rates of aforementioned materials have been evaluated at different impact angles (20°, 30°, 45°, 60°, and 90°) and different impact velocities (70, 110, 150, and 190 m/s), and compared with those of stainless steel (SS304) and pure nickel materials (Ni) which are commonly used for erosion shield of helicopter rotor blades. Tests were conducted at room temperature using silica (SiC) particles (average diameter of 175 µm) as an erodent. Erosion behaviors on the shield surface of the same materials were investigated numerically under different impact velocities (110, 150, 190, and 230 m/s) for a pitch angle of 0°. Numerical erosion simulations were realized with commercial ANSYS-Fluent software using the finite volume method, an erosion model, and a discrete phase method with the Eulerian-Lagrangian approach. The experimental results showed that the GF/EP and the CF/EP composites indicated the maximum erosion rate at the impact angles ranged from 60° to 90°, whereas the PU tape material exhibited the maximum erosion rate at the impact angle of 30°. The results also showed that the PU tape surface clearly provided much better erosion resistance than the GF/EP and CF/EP composite surfaces, and the hybrid erosion shield having the PU/SS304 surface significantly improved the erosion resistance of the helicopter rotor blade up to the impact velocity of 150 m/s. A good agreement was also observed between the experimental and the numerical results.

CFD-CSD method for coupled rotor-fuselage vibration analysis with free wake-panel coupled model

An efficient comprehensive vibration analysis method for a helicopter rotor–fuselage coupling system is presented. This loose computational fluid dynamics (CFD)/computational structural dynamics (CSD) coupling approach with a free wake–panel coupled model is used for system vibration response analysis. The CSD model of the helicopter consists of a fuselage model using a refined three dimensional (3 D) finite element model (FEM) and a rotor model consisting of nonlinear moderate deflection beam elements with 15 degrees of freedom. The unsteady Euler CFD solver is used for the flow field analysis of the entire vehicle. The induced inflow of the quasi-steady aerodynamic force is calculated with the free wake–panel coupled model, which is used to simulate rotor–fuselage aerodynamic interference. Using a full-scale helicopter as an example, the vibration responses of the typical fuselage position in hovering and level flights are analysed. When compared with the literature results and flight test data, the predictions of the proposed method are closer to the test data than those of the traditional method in hovering and low forward ratio flights, and the difference between the two methods is minimal in high forward ratio flight.

Scaling sediment mobilization beneath rotorcraft for Titan and Mars

Using rotorcraft to explore extraterrestrial bodies will allow future missions to explore terrain that is inaccessible to traditional lander/rover missions, and will enable spacecraft to explore much larger distances than previous missions. The upcoming Mars 2020 Rover mission will include the Mars Helicopter (named Ingenuity), a small ( < 2 kg) counter-rotating co-axial vehicle, which, if successful, will be the first rotorcraft to fly on a non-terrestrial body. The Dragonfly mission, recently selected through the NASA New Frontier’s program, is expected to fly a much larger vehicle on Titan ( ∼ 750 kg) arriving in the early 2030s. On Earth it is commonly observed that both sub-scale and full-scale rotorcraft can mobilize sediment, to varying degrees of magnitude, when a vehicle is operating over regolith. In order to determine the feasibility of performing sediment mobilization experiments (e.g. saltation investigations) beneath a rotorcraft through intentional sediment mobilization (due to the wake produced by the rotating rotors) for future missions, the scaling of aerodynamic and aeolian parameters should be accounted for. Furthermore, in an extreme case, this analysis can also be used as a low-fidelity method to begin to understand any potential risk associated with brownout (large-scale sediment mobilization due to the helicopter rotor wake) to the rotorcraft itself. In this work, a scaling analysis is presented that allows a vehicle designer to quickly investigate the magnitude of sediment mobilization beneath a rotorcraft, without requiring detailed CFD simulations. The analysis confirms that the upcoming Mars Helicopter and Dragonfly missions could be used for sediment mobilization experiments.

Experimental study of the impact of the number of blades on the profile drag of UAV helicopter rotors in hover

Due to the motion of tip and root vortices of helicopter rotor blades and the inevitable blade-vortex interactions in multi-blade rotors, the profile drag of all individual blades is increased, when compared to the absence of any blade-vortex interaction. Different factors like the speed of rotation of the rotor and the number of blades greatly impact the magnitude of the profile drag increase. This effect is additionally worsened with the low- Reynolds operational conditions of small-sized UAV helicopter rotors. In addition, there is an initial upward motion of the tip vortex and thus there are three scenarios for the blade-vortex encounter, depending on whether the vortex emitted by the previous blade is still above, on or below the plane of rotation of the blades. All those uncertainties motivated the authors to carry- out this present experimental study. The results are shown in a convenient form. In hover, the profile drag increase is shown to be greater with increased number of blades and higher rotational speeds as there is less time for the preceding vortex to clear the path of the following blade.

A comparison of helicopter recovery maneuvers on frigates by means of PIV measurements

Helicopter pilots who have to operate on frigates must deal with turbulent and unsteady flows caused by the incidence of the wind on the non-aerodynamic surfaces of the frigate. In addition, the flight deck is usually placed at the stern, right behind the frigate superstructure where recirculation and low-velocity regions appear. For that reason, studying how the aerodynamic effects influence the helicopter rotor during the approach and landing on the frigate could be essential to guarantee safe operations. The aim of this paper is to present PIV velocity maps with the flow patterns generated by the interaction between the frigate wake and the helicopter rotor working during its landing approach usually named as dynamic-interface. Three different landing maneuvers have been analyzed: rear, diagonal and aft-fore. The jet generated by the rotor will be used to obtain induced velocity profiles under the rotor. With these velocity profiles, rotor asymmetries during its working over the frigate are detected. With all data at each point of the maneuvers, a final comparison will be made to select the best procedure for helicopter recovery on frigates under different wind over deck conditions.

Multi-disciplinary optimization of variable rotor speed and active blade twist rotorcraft: Trade-off between noise and emissions

The concepts of variable rotor speed and active blade twist are emerging technologies for the next generation of civil rotorcraft. Previous research has focused on the optimum implementation of these technologies for improved fuel economy and environmental impact. Within this work, an integrated approach is deployed to quantify the concurrent reductions in rotor noise and NOx emissions. A relaxation-based free-wake inflow model, coupled with unsteady blade aerodynamics modeling, resolves the flow-field around the main rotor. Aero-acoustic predictions are performed through an acoustic-analogy-based formulation. Gaseous emissions are then predicted via stirred-reactor modeling, coupled with zero-dimensional engine performance analysis method. This strategy is incorporated into a multi-disciplinary genetic algorithm optimization process based on surrogate modeling. Optimal schedules of combined variable rotor speed and active blade twist controls are derived for a twin-engine light helicopter in descent. The accrued schedules suggest NOx reductions between 6% and 21%, simultaneously with source-noise reductions of the order of 2–8 dB, relative to the non-morphing rotor case. The developed strategy constitutes an enabling methodology for the holistic and multi-disciplinary assessment of morphing helicopter rotor configurations.

Aerodynamic Optimization over Frigate Helicopter Flight Deck by Hangar Shape Modifications

Helicopter operations on frigates imply complex maneuvers for pilots. During the aircraft approach to the frigate, the helicopter rotor is immersed in a changing aerodynamic flow, with high speed and turbulence intensity gradients. This occurs as a consequence of the nonaerodynamic surfaces that compose the frigates and generate detached flow on their decks. Typically, the helicopter operations on frigates are located at the stern. That is, after the superstructure of the ship and just behind the hangar where the helicopter can be hosted. Using a simplified frigate shape model, tests have been carried out at the National Institute of Aerospace Technology’s low-speed wind tunnel by modifying the hangar geometry in a bubble-shaped way to optimize it aerodynamically and improve the flow over the frigate aftdeck. Roof and side walls have been modified by linear , circular , and elliptical geometries. A particle image velocimetry (PIV) technique has been used for obtaining the flow velocity field above and behind the hangars proposed to evaluate the effectiveness of each geometry modification proposed. Finally, a numerical comparison with the PIV results and parameters such as the storage capacity reduction of the hangar due to its geometry change has been carried out.

MODELLING THE HELICOPTER ROTOR AERODYNAMICS AT FORWARD FLIGHT WITH FREE WAKE MODEL AND URANS METHOD

The presented work is dedicated to the numerical study of the aerodynamic characteristics of the helicopter rotor. Two approaches to modeling of the rotor are applied: the free wake model developed by the Authors with using steady airfoil characteristics and the Unsteady RANS method based on the Ansys Fluent software. The modes of hovering and horizontal flight in the range of advancing ratio μ = (0-0.45) are considered. The shapes of the rotor wake, the distributions of the normal force coefficient and the fields of inductive velocities for all considered flight modes are calculated. For a particular case with μ = 0.25 there is a comparison with experimental data. The time needed for calculation of the applied methods is estimated. Accuracy of the used methods in the framework of the solved task is analysed with taking into account used models assumptions. It is shown that in the range of μ = (0-0.25) the free wake model provides a fast and reliable calculation of the aerodynamic characteristics of the helicopter rotor. For values of μ &gt; 0.35 it is necessary to take into account the unsteady characteristics of the airfoil.

Feasibility of Rotor Fault Detection from a Fluid Dynamics Perspective

The majority of condition monitoring techniques employed today consider the acquisitioning and analysis of structural responses as a means of profiling machine condition and performing fault detection. Modern research and newer technologies are driving towards non-contact and non-invasive methods for better machine characterisation. Yet current literature lacks investigations into the monitoring and detection of anomalous conditions using fluid dynamic behaviour. If one considers unshrouded rotors which are exposed to a full field of fluid interaction such as helicopter rotors and wind turbines amongst others, such an approach could potentially be beneficial. In this work, time-dependent fluid dynamic data is numerically simulated around a helicopter tail rotor blade using URANS CFD with the Open FOAM software package. Pressures are probed at locations in the field of the rotor and compared to results attained in an experimental investigation where good correlation is seen between the results. A blade is modelled with a seeded fault in the form of a single blade out of plane by 4°. Comparisons are drawn between the blade in its ‘healthy’ and ‘faulty’ configurations. It is observed that the fault can be detected by deviations in the amplitudes of the pressure signals for a single revolution at the probed locations in the field. These deviations manifest as increases in the frequency spectrum at frequencies equivalent to the rotational rate (1 per revolution frequencies). The results described are assessed for their fidelity when the pressure is probed at different locations in the domain of the rotor. Deviations in the pressure profiles over the surface of the blades are also seen for the asymmetric rotor configuration, but may prove too sensitive for practical application.

Grid transformation and dynamic scattering for tail rotor radar cross section analysis

With the promotion and enhancement of stealth technology of helicopter rotor components, the research on the dynamic radar cross section (RCS) of helicopter rotor is becoming more and more important and imminent. In order to facilitate the calculation and analysis of the electromagnetic scattering characteristics during rotor rotation, a dynamic scattering calculation (DSC) method based on quasi-static principle (QSP) and grid coordinate transformation is presented. After analyzing the advantages and disadvantages of QSP, the dynamic principle is used to describe the rotation process of the rotor. Combined with the grid coordinate transformation method, the RCS of the rotor is accurately calculated by physical optics (PO) and physical theory of diffraction (PTD). Then the influence of azimuth, elevator angle and observation distance on rotor dynamic RCS is analyzed. The results show RCS of the tail rotor is indeed dynamic and periodic and its main influencing factors include azimuth and elevation angle. The proposed DSC method is efficient and effective for studying the dynamic RCS of tail rotor.

An Investigation of Rotary Drone HERM Line Spectrum under Manoeuvering Conditions.

Detecting and identifying drones is of great interest due to the proliferation of highly manoeuverable drones with on-board sensors of increasing sensing capabilities. In this paper, we investigate the use of radars for tackling this problem. In particular, we focus on the problem of detecting rotary drones and distinguishing between single-propeller and multi-propeller drones using a micro-Doppler analysis. Two different radars were used, an ultra wideband (UWB) continuous wave (CW) C-band radar and an automotive frequency modulated continuous wave (FMCW) W-band radar, to collect micro-Doppler signatures of the drones. By taking a closer look at HElicopter Rotor Modulation (HERM) lines, the spool and chopping lines are identified for the first time in the context of drones to determine the number of propeller blades. Furthermore, a new multi-frequency analysis method using HERM lines is developed, which allows the detection of propeller rotation rates (spool and chopping frequencies) of single and multi-propeller drones. Therefore, the presented method is a promising technique to aid in the classification of drones.