Helicopter Rotor Research Articles

Method for eliminating aerodynamic lift vibration of rigid rotor helicopters based on the novel sine-trim model

A novel methodology called sine-trim is proposed from the phasor superposition principle for AC voltages in electrical engineering to eliminate the aerodynamic lift vibration of rigid rotor helicopters. An advancing blade concept rotor aerodynamic analysis model is presented on the basis of free-wake method associated with the inner- and outer-trim modules. The inner-trim module transforms the blade lift distribution curve of a rigid lift-offset rotor into a similar sinusoidal one by adjusting the blade pitch in each azimuth. An acceleration algorithm that could greatly improve the inner-trim calculation efficiency is provided. The outer-trim module selects the amplitude of the lower rotor as the preset trim value, and it is reliable and effective in trimming the overall forces and moments of the helicopter by modifying the lift sinusoid curve parameters of the upper and lower rotors. In the sine-trim, the upper and lower rotors have similar sinusoidal lift distribution parameters, and their phases and amplitudes exhibit a direct inverse relationship. Calculation results show that after being sine-trimmed, the total aerodynamic lift vibration of the rigid rotors could be eliminated regardless of the change in flight and operating conditions, such as flight speed, number of blades, and rotor rotational speed. The optimal blade pitch control distribution, which is a comprehensive superposition of many higher harmonic inputs, is obtained in all azimuths. For the same lift-offset, after sine-trimming, the effective lift-to-drag ratio may have a small degree of loss compared with the conventional advancing blade concept rotor.

Параметры высоковольтных разрядов на лопастях винтов вертолета и создаваемых ими электромагнитных помех

Abstract Parameters of pulsed electric discharges arising between insulated metal parts of helicopter rotor blades, which generate electromagnetic interferences for onboard radio stations operating in the very short wave (VSW) and ultrashort wave (USW) bands have been studied. These discharges appear as a result of the in-flight engine-driven differential charging of helicopters. In-flight and laboratory experiments were used to determine the range of discharge voltages and currents and the amplitude and spectral characteristic of electromagnetic interferences produced by these discharges for onboard antennas.

Improving the Mechanical Behavior of a Helicopter Tail Rotor Blade Through the Use of Polyurethane Foams

The tail rotor blade represents a very important component in the structure of any helicopter, being subjected to many aerodynamic loads. The integrity of this component is essential for maintaining flight safety and improving its characteristics can increase the overall performance of the helicopter. Thus, in some cases, some improvements can be made to the structure, in order to reduce the weight of the component and to improve its mechanical properties and its reliability in flight. The current paper presents a comparative result analysis in the study of the tail rotor blade of the IAR 330 helicopter, where the honeycomb hexagonal core made of aluminum alloy is replaced with a full structural core made of polyurethane foam. The result of the analysis is of practical importance, because it shows the benefits of using polymers in the construction of helicopter rotor blades and similar structures.

Improving the Mechanical Behavior of a Helicopter Tail Rotor Blade Through the Use of Polyurethane Foams

The tail rotor blade represents a very important component in the structure of any helicopter, being subjected to many aerodynamic loads. The integrity of this component is essential for maintaining flight safety and improving its characteristics can increase the overall performance of the helicopter. Thus, in some cases, some improvements can be made to the structure, in order to reduce the weight of the component and to improve its mechanical properties and its reliability in flight. The current paper presents a comparative result analysis in the study of the tail rotor blade of the IAR 330 helicopter, where the honeycomb hexagonal core made of aluminum alloy is replaced with a full structural core made of polyurethane foam. The result of the analysis is of practical importance, because it shows the benefits of using polymers in the construction of helicopter rotor blades and similar structures. Keywords: helicopter tail rotor blade, honeycomb structure, polymers, polyurethane foam, finite element method

Simultaneous pressure and deformation field measurement on helicopter rotor blades using a grid-pattern pressure-sensitive paint system

A grid-pattern pressure-sensitive paint (grid PSP) system is developed and then integrated with a stereo vision system to achieve simultaneous pressure and deformation measurements on rotating surfaces. The grid PSP features a dot pattern, which is prepared by spraying PSP dots on top of a polymer-ceramic binder. A single-shot lifetime method is adopted to obtain the transient pressure field on a rotor blade coated with PSP. Meanwhile, the PSP dots provide feature points for blade surface reconstruction through stereo cameras. A data processing algorithm is developed for calculating the blade motion angles based on the registration targets. This system was applied on a helicopter model test stand to capture a sequence of images on the rotating blade surface. The results from grid PSP were compared with regular PSP to evaluate the performance of pressure measurement. The accuracy of blade motion measurement was also evaluated for pitch angle from 15° to +15°, flap angle from −10° to +10° and lag angle from −10° to +10°, respectively.

Development of an automated hydraulic brake control system for testing aircraft turboshaft gas turbine engines

To absorb the power generated by a free turbine, hydraulic brake systems of various designs are used in ground tests of aircraft turboshaft gas turbine engines. Ground tests of aircraft turboshaft gas turbine engines with the use of such hydraulic brakes can result in emergency modes of automated engine control in the area of operation of a free turbine speed regulator. Mismatch between the hydraulic brake loading characteristics and the loading characteristics of the rotor driven by a free turbine of the engine is the main cause of emergency operation of automated control systems.The presented experimental loading characteristics of the hydraulic brake and the helicopter rotor show their significant difference in terms of gain. To eliminate this difference, a possibility of modeling dynamic parameters of rotors by simple automation means was considered. To solve this problem, a linear dynamic model and a block diagram of an automated hydraulic brake control system for ground testing of turboshaft gas turbine engines were elaborated. The law of regulation of the hydraulic brake loading was substantiated. A structurally dynamic diagram of the developed automated control system was presented and calculation formulas for determining the regulator parameters were given. Transient characteristics of the hydraulic brake unit without automation means and with the use of an automated loading control system were calculated. The presented calculation results have shown that the use of automation make it possible to fully emulate characteristics of the helicopter rotors

Helicopter Crash on the Spitsbergen Archipelago: Infrasound and Seismic Signals Decryption

A Convers Avia Airline Mi-8 helicopter crashed near the Russian settlement of Barentsburg on October 26, 2017. The moment of impact was recorded by the Barentsburg seismo-infrasound array 2.5 km from the crash site. The seismo-infrasound array consists of a micro-aperture infrasound group, including three low-frequency microphones and a broadband seismic station located at the same site. A detailed analysis of the obtained records of the seismometer and the low-frequency microphones is presented here. Two types of signals determined by the helicopter fall are detected in seismic channels: a wave caused by the water impact of the helicopter and propagating through the earth, and an air wave that was also generated by the water impact but propagating through the atmosphere. The second type of wave was also recorded by low-frequency microphones from the infrasonic microgroup. The procedure for determining the source coordinate by the complex analysis of acoustic and seismic signals is described. The difference in arrival times of seismic and acoustic waves is used to determine the distance to the source, and the back azimuth is calculated using the differences in the arrival times of the acoustic wave to the spaced microphones of the infrasound microarray. The records of the acoustic signals associated with the operation of the helicopter rotor before the crash and at the moment of water impact are examined. A detailed analysis of the frequency and amplitude composition of the received seismic and infrasound signals has allowed us to not only determine the exact place and time of the helicopter crash, but also to estimate the expected trajectory of its movement before the fall, as well as to recreate some details of the accident.

A New Approach to Modelling and Testing the Fatigue Strength of Helicopter Rotor Blades during Repair Process

Abstract The fatigue test was carried out on an element of a rotor blade removed from the Mi-2 helicopter. The purpose of the test was to check the fatigue strength of the repaired rotor blade. Metal composite rotor blades have a metal spar in the form of a box and the trailing sections in the form of metallic honeycomb sandwich panels. The trailing sections are bonded to the spar. The repair had been carried out at the point where the trailing section became debonded from the spar at the Air Force Institute of Technology in Warsaw using a methodology developed for carrying out repairs of rotor blades’ damage. All types of the Mi family helicopters are equipped with metal composite rotors blades. Depending on MTOW (Maximum Take-Off Weight) and destination of helicopters, blades differ in dimensions, but their design solutions are practically the same. For this reason, the developed repair methodology can be used for all characteristic rotor blades structures for Mi helicopters. The fatigue test was performed at the Łukasiewicz - Institute of Aviation in Warsaw, using a hydraulically driven fatigue machine. The fatigue test was carried out by performing over 1.1 million load cycles. In repair places, upon completion of fatigue testing, no damage was found.

Designing and FEM simulation of the helicopter rotor and hub

The paper presents a preliminary design of the three-blade rotor. The geometric model was prepared in the Siemens NX 12.0 programme. After the preparation of the model and kinematic analysis, the structure was checked for strength by means of NX Nastran software. In addition, structural solutions of bearing rotors currently used in the construction of helicopters were discussed and a comparative analysis of the developed rotor design with other rotors was carried out in order to define the basic advantages and disadvantages of each solution. The main rotor of a helicopter is considered the most important component of the rotorcraft, which determines its performance. Rotating blades generate the resultant forces and moments in the rotor hub that allow the helicopter to fly. In the case of a single-rotor helicopter design, a tail rotor must be added to obtain directional control. Cyclic and collective control of blade inclination influences the generation of aerodynamic and inertial loads, which depend on the azimuthal position of the blade on the rotor disc. The number of rotor blades and the properties of the blade-hub connections strongly influence the level of load transfer from the blades through the hub to the helicopter fuselage. By incorporating a set of hinges, it is possible to reduce the component loads of the rotor, tilt and rolling moments transferred to the hull. A conventional articulated rotor head with built-in hinges requires frequent maintenance, which increases helicopter operating costs. Furthermore, a complex structure with a large number of components increases the weight of the articulated rotor.

Comparison of micro‐Doppler signatures registered using RBM of helicopters and WSM of vehicles

Jet engine modulation (JEM) are micro-Doppler features obtained from the radar data of aircraft, including jet aircraft and helicopters, and generally viewed as unique phenomenon suitable for identifying aerial targets. The authors demonstrate however, that JEM are not unique, and that the rotating wheel spokes on the wheels of ground vehicles produce a modulation similar to the rotating rotor blades of jet engines and helicopter rotor blades. They also develop a mathematical model to analyse the radar signal scattering mechanism of wheel spokes, and also employ a Ku-band Doppler radar system to collect radar echoes from a wheeled vehicle and a helicopter. Both the theoretical and experimental results verify the existence of rotor blade modulation (RBM) phenomena from helicopters and wheel spoke modulation (WSM) phenomena from vehicles. Both RBM and WSM are similar to JEM features. This fact indicates that JEM is a necessary but insufficient condition for the identification of aerial targets, and additional recognition features inherent in surface surveillance radar are needed to distinguish the JEM spectra arising from aerial targets and ground vehicles.

CFD Simulation Methods for Rotor Hovering Based on N-S Equation

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.

Performance Analysis of Composite Helicopter Blade Using Synergistic Damage Mechanics Approach

The damage behavior of a helicopter rotor blade made of carbon fiber polymer composite has been numerically investigated using a synergistic damage mechanics (SDM) model. Laminates of the quasi-isotropic stacking sequence have been considered in this study. The simulations were performed, corresponding to a hovering condition and for a range of the angles of attack and rotational speeds of the blade. The aerodynamic and centrifugal loads due to different angles of attack and rotational speeds have been computed by using fluid dynamics simulations. A structural analysis was then performed using the aerodynamic loads. Using the SDM model, the matrix crack density and crack multiplication were evaluated to predict the damage initiation site and maximum crack density occurring in the plies of the laminate under the operational conditions. It was observed that maximum crack appeared in the blade at extreme operational conditions; among the different plies, a ply was cracked by the maximum amount. A crack density of around was predicted in the ply at an operational condition with a 500 rpm blade rotation and an 18 deg angle of attack of the blade. The present study demonstrates application of the progressive damage modeling in designing a composite helicopter blade by considering its performance parameters.

Experimental study of secondary vortex structures in a rotor wake

The operation and aerodynamic performance of a helicopter rotor is strongly affected by the structure of its wake, in particular regarding vortex–vortex interactions of hovering rotors. Rotor simulations using modern computational methods have the potential to capture high levels of detail, which recently triggered discussions of secondary vortex braids entangling the primary tip vortices. These structures are highly dependent on the numerical settings and need experimental validation. The current work investigates the wake of a subscale rotor in ground effect by time-resolved and volumetric flow-field measurements using the “Shake-The-Box” technique. Both the Lagrangian tracks of the flow tracers and the derived gradient-based vortex criteria clearly verify the existence of secondary vortices. A post-processing scheme is applied to isolate these vortices in larger data sets. No distinct spatial organization of the structures was observed, but a slightly preferred sense of rotation which agrees to the shear of the wake swirl. The secondary structures were created shortly downstream of the rotor blades, starting at wake ages of approximately $$75^\circ $$.

Aerodynamic/aeroacoustic variable-fidelity optimization of helicopter rotor based on hierarchical Kriging model

Rotor noise is one of the most important reasons for restricting helicopter development; hence, the optimization design of rotor blade considering aeroacoustic and aerodynamic performance at the same time has always been the focus of research attention. For complex rotor design problems with a large number of design variables, the efficiency of the traditional Kriging model needs to be improved. Thus, Hierarchical Kriging (HK) model is employed in this study for rotor optimization design. By using the validated RANS solver and acoustic method based on the FW–Hpds equation, an efficient aerodynamic/aeroacoustic optimization method for high-dimensional problem of rotors in hover based on HK model is developed. By using present HK model and new infill-sampling criteria, the number of design variables is increased from less than 20–53. Results of two analytical function test cases show that the HK model is efficient and accurate in calculation. Subsequently, the helicopter rotor blade is optimally designed for aerodynamic/aeroacoustic performance in hover based on the HK model with high dimensional design variables. The objective function is adopted to improve the rotational noise characteristics by reducing the absolute peak of the acoustic pressure. In addition, the constraints of thrust, hover efficiency, solidity, and airfoils thickness are strictly satisfied. Optimization results show that the Kriging model finds the objective of reducing the noise by 2.87 dB after 248 iterations while the HK model does it only after 164 iterations. The optimization efficiency of the HK model is significantly higher than that of the traditional Kriging model. In the case analyzed, the HK model saves 35% of the time used by the Kriging model.

Система передачи мониторинговой информации с винта вертолёта.

Система передачи мониторинговой информации с винта вертолёта.

Rotor Loads Reduction by Dynamically Extendable Chord

Dynamically extendable blade chord sections show promise for reducing helicopter rotor loads. A rotor model based on an elastic beam concept, and capable of predicting helicopter power, is used. A stiff in-plane four-bladed rotor with a shape similar to the UH-60A rotor is used as a baseline for comparisons. For the control of the 4/rev (per revolution) vertical hub force, it is not beneficial to actuate the extendable chord at hover and low-speed flight. At a high speed of , the extendable chord, with a width of 10% rotor radius and responded to an extension magnitude of 10% chord length, obtained a maximum force reduction of 89.4%. The magnitude of the dynamic chord needs to be optimized according to the flight state. The performance can be enhanced by increasing the extension or width of the dynamic chord. The dynamically extendable chord was not suitable for reducing the 2/rev blade flapwise root bending moment. However, a 3/rev dynamic chord extension showed great potential in reducing the 3/rev flapwise root bending moment and the 4/rev rotor rolling and pitching moments simultaneously. The effectiveness of a 5/rev dynamic chord in reducing the 4/rev rotor rolling or pitching moment degraded significantly as compared with a 3/rev actuation.

Strength analysis of composite rotor helicopter blade, having percussive damage, by limit equilibrium theory

Authors model the limiting state of the helicopter «ANSAT» rotor blade at different flight modes, taking into account the possible damage to the blade in the butt section. The method of variation of elastic constants, based on the limit equilibrium theory and allowing to obtain both upper and lower limits of the limit load, is used to estimate the strength. The stresses in the layers determined in accordance with the deformation law of the associated limit yield surface of these layers, are expressed in terms of the six-dimensional vector of generalized forces. Analysis of the limit stress state is reduced to the analysis of the limit state in the space of the generalized forces. The safety factors of the blade’s butt section depending on the azimuth angle of rotation of the blade are determined for the case when all the generalized forces are proportional to one loading parameter. The possibility of making internal diagnostics of damage areas using x-ray computer tomography is announced. It is proposed to use medical Siemens x-ray tomograph to perform such diagnostics.

Obtaining high surface quality in electrolyte jet machining TB6 titanium alloy via enhanced product transport

Numerous components used in the aerospace industry, such as helicopter rotor hubs and aero-engine casings, require high surface quality because they must endure extreme conditions. Most of those components are made of titanium alloy materials. However, it is difficult to obtain high surface quality for those titanium alloy materials by electrolyte jet machining owing to the high sensitivity of these materials to changes in the flow field. A disordered flow field will result in pitting corrosion and product accumulation, which will degrade the quality of the machined surface. It had been reported that specially designed structures at the tool end could improve the machined surface quality. In this study, several types of tools with different sloping structures at their ends were designed to enhance the machined surface quality in electrochemical jet macro machining. Numerical simulations were applied to investigate the influence of these sloping structures on the machining flow field distribution. The simulation results revealed that an upward-sloping inclined end could improve the electrolyte flow rate, reduce the dead zone, and limit the influence range of the electric field. In contrast, a downward-sloping inclined end caused flow field disorder, which resulted in serious pitting and electrolytic product accumulation. Experiments were also conducted to verify these results, which revealed that the electrolytic product transport capacity was improved and then high surface quality was obtained by the tool with an upward-sloping end face. The groove machined using this tool possessed a smoother surface, sharper edge, and lower surface roughness compared with grooves machined using other tools. In combination with a numerical control system, the proposed tool could be used to fabricate aerospace parts composed of titanium alloys, as the machined surface quality could meet the requirements for industrial production.

Problems of an Aerodynamic Interference between Helicopter Rotor Slipstream and an Elevated Heliport

Abstract An elevated heliport, as it has been defined by FAA (Federal Aviation Administration), is a heliport located on a rooftop or other elevated structure where the TLOF (touchdown and lift-off area) is at least 30 inches (76 cm) above the surrounding surface [1]. One of greatest advantages of such heliports is that they require less free space, which eases its build nearby existing buildings – especially in densely built-up areas. However, design of such heliports is more complicated, than ground level ones, while one must include an aerodynamic impact of the building below the elevated heliport and surrounding buildings. The aerodynamic interference between the helicopter and the buildings may result with decline of flight safety, due to sudden decrease of thrust (when flying above the edge of building) or because of increased turbulence in windy weather, wake behind surrounding buildings causing sudden gusts etc. Moreover, oscillations of pressure caused by helicopter rotor influence on the building structure also must be taken into account due to increased wear of upper part of the building or devices mounted on its roof (for example, elevator drives). These oscillation may also cause vibrations of building’s structure, which is especially important in case of medical heliports – which are a vast majority of elevated heliports (and heliports in general) – because of strict requirements for acceptable vibration level. The article is aimed on summarize aerodynamic issues, which should be taken into account during design of elevated heliport.

A Prototype Test Stand for Testing Insolated Rotor Systems

Abstract The article discusses assumptions for a prototype test stand designed for testing rotors in the conditions of work at their nominal speed, in particular gyrocopter and helicopter rotors. In the work presents an overview of similar solutions of test stand presented in the available literature and solutions proposed by companies, such as Swangate International or Luftfahrttechnik GmbH. Beyond these solutions the article also contains test stand used so far at the Institute of Aviation, these solutions were prepared for testing one type of rotor (for example: only for gyroplane rotors). The article provides the steps of designing a new test stand, which one of the basic assumptions is that it will be a universal test stand, giving the possibility to test different types of rotors. The article discusses two concepts of the stand with electric engine as a drive, gives the advantages and disadvantages of each concept, and indicates the selected solution. In addition to design and applied drive solution of the stand, the measuring part and data acquisition were also discussed. Moreover, the article focus on the advantages of rotor testing using the presented solution, as well as problems encountered during tests, in particular related to construction free vibrations and resonance. Vibration and resonance of the test stand are very important aspect of the entire design process, because it affects the quality and safety of the tests.