Main Rotor Blade Research Articles

A geoacoustic inversion technique using the low-frequency sound from the main rotor of a Robinson R44 helicopter

A series of underwater acoustic experiments utilizing a Robinson R44 helicopter and an underwater receiver station has been conducted in shallow (16.5 m) water. The receiver station consisted of an 11-element nested hydrophone array with a 12 m aperture configured as a horizontal line (HLA) 0.5 m above the seabed. An in-air microphone was located immediately above the surface. The main rotor blades of the helicopter produce low-frequency harmonics, the fundamental frequency being ~13 Hz. The tail rotor produces a sequence of harmonics approximately six times higher in frequency. The first experiment characterized the underwater sound signature of the helicopter with altitude and range. Using analytical and numerical 3-layer (atmosphere-ocean-sediment) acoustic propagation models a sediment geoacoustic inversion technique has been developed. This technique, requiring only knowledge of the relative location of the sensors and sound source (helicopter), uses the cross-correlation between HLA sensor pairs to produce the estimated time delay of the head wave. The results from the simulations and the latest experiment are presented. [Research supported by ONR, SMART(DOD), NAVAIR, and SIO.]

On the use of strain sensor technologies for strain modal analysis: Case studies in aeronautical applications.

This paper discusses the use of optical fiber Bragg grating (FBG) and piezo strain sensors for structural dynamic measurements. For certain industrial applications, there is an interest to use strain sensors rather than in combination with accelerometers for experimental modal analysis. Classical electrical strain gauges can be used hereto, but other types of strain sensors are an interesting alternative with some very specific advantages. This work gives an overview of two types of dynamic strain sensors, applied to two industrial applications (a helicopter main rotor blade and an F-16 aircraft), FBG sensors and dynamic piezo strain sensors, discussing their use and benefits. Moreover, the concept of strain modal analysis is introduced and it is shown how it can be beneficial to apply strain measurements to experimental modal analysis. Finally, experimental results for the two applications are shown, with an experimental modal analysis carried out on the helicopter main rotor blade using FBG sensors and a similar experiment is done with the aircraft but using piezo strain sensors instead.

Aerodynamic and acoustic analysis of helicopter main rotor blade tips in hover

Purpose The design of main rotor blade tips is of interest to helicopter manufactures since the tip details affect the performance and acoustics of the rotor. The paper aims to discuss this issue. Design/methodology/approach In this paper, computation fluid dynamics is used to simulate the flow around hovering helicopter blades with different tip designs. For each type of blade tip a parametric study on the shape is also conducted for comparison calculations were performed the constant rotor thrust condition. The collective pitch and the cone angles of the blades were determined by at an iterative trimming process. Findings Analysis of the distributed blade loads shows that the tip geometry has a significant influence on aerodynamics and aeroacoustics especially for stations where blade loading is high. Originality/value The aeroacoustic characteristics of the rotors were obtained using Ffowcs Williams-Hawkings equations.

Airborne visualization of helicopter blade tip vortices

Blade tip vortices and their interaction with the helicopter blades play an important role in the generation of noise on rotorcraft. Full-scale vortex visualization is essential for the understanding of these effects and the validation of sub-scale experiments and numerical simulations. In the present work, the reference-free background-oriented schlieren (BOS) method was used to visualize blade tip vortices of a full-scale BO 105 helicopter in hover and under free flight conditions. A ground-based dual-camera BOS system with an artificial background was applied for the visualization of the helicopter’s main and tail rotor tip vortices during take-off. In a second flight test, a similar dual-camera BOS system was used to visualize the main rotor blade tip vortices of the BO 105 during high speed forward, curve, and accelerating forward flight. The camera system was deployed aboard a microlight airplane flying above the helicopter, with vegetation on the ground serving as natural backgrounds. Different natural and artificial backgrounds were analyzed and compared to assess their suitability for the BOS evaluation. The airborne setup enabled the visualization of vortices up to a maximum vortex age of $$\psi _v=630^\circ$$ at distances of up to one rotor diameter behind the rotor plane. Blade-vortex interaction effects and deformations of the vortex system were observed. The visualization results obtained during the inflight measurements show that the reference-free BOS method is highly suitable for the detection of helicopter blade tip vortices during full speed forward and maneuvering flight, therefore removing the restrictions imposed by ground-based measurements.

An experimental study on the aeromechanics and wake characteristics of a novel twin-rotor wind turbine in a turbulent boundary layer flow

The aeromechanic performance and wake characteristics of a novel twin-rotor wind turbine (TRWT) design, which has an extra set of smaller, auxiliary rotor blades appended in front of the main rotor, was evaluated experimentally, in comparison with those of a conventional single-rotor wind turbine (SRWT) design. The comparative study was performed in a large-scale wind tunnel with scaled TRWT and SRWT models mounted in the same incoming turbulent boundary layer flow. In addition to quantifying power outputs and the dynamic wind loadings acting on the model turbines, the wake characteristics behind the model turbines were also measured by using a particle image velocimetry system and a Cobra anemometry probe. The measurement results reveal that, while the TRWT design is capable of harnessing more wind energy from the same incoming airflow by reducing the roots losses incurred in the region near the roots of the main rotor blades, it also cause much greater dynamic wind loadings acting on the TRWT model and higher velocity deficits in the near wake behind the TRWT model, in comparison with those of the SRWT case. Due to the existence of the auxiliary rotor, more complex vortex structures were found to be generated in the wake behind the TRWT model, which greatly enhanced the turbulent mixing in the turbine wake, and caused a much faster recovery of the velocity deficits in the turbine far wake. As a result, the TRWT design was also found to enable the same downstream turbine to generate more power when sited in the wake behind the TRWT model than that in the SRWT wake, i.e., by mitigating wake losses in typical wind farm settings.

Calculation of a safety factor of damaged composite main rotor blade according to limit equilibrium theory

Safety factors of the Ansat helicopter main rotor blade are determined in the root section at various flight modes with taking into account the possible damages.

Very High Cycle Fatigue Testing under Spectrum Loading for Endurance Limit Structural Design

A program has recently concluded that generated fatigue test data for the influence of a rotorcraft main rotor blade root bending spectrum (Helix) on the crack nucleation mechanisms in 7075-T651 aluminum. High-frequency tests were performed that generated spectrum fatigue failures out to nearly 109 cycles. Fractographic examination showed a distinct change in crack nucleation from slip initiated to inclusion-initiated cracking as the spectrum peak stress level was increased. Spectrum life predictions were made using three different baseline constant-amplitude S-N curves, one using a traditional rotorcraft original equipment manufacturer fitting methodology, one using the high-cycle fatigue (HCF) portion of a strainlife curve, and one that was fitted to S-N data with test lives out to 3×108 cycles. The spectrum life prediction using the S-N curve that properly modeled material behavior in the very high cycle fatigue regime provided a good correlation to the spectrum fatigue test data. Predictions using the other S-N curves were highly conservative.

Structural characterization of rotor blades through photogrammetry

This paper deals with the use of photogrammetry for the experimental identification of structural and inertial properties of helicopter rotor blades. The identification procedure is based upon theoretical/numerical algorithms for the evaluation of mass and flexural stiffness distributions which are an extension of those proposed in the past by Larsen, whereas the torsional properties (stiffness and shear center position) are determined through the Euler–Bernoulli beam theory. The identification algorithms require the knowledge of the blade displacement field produced by known steady loads. These data are experimentally obtained through photogrammetric detection technique, which allows the identification of 3D coordinates of labeled points (markers) on the structure through the correlation of 2D digital photos. Indeed, the displacement field is simply evaluated by comparing the markers positions on the loaded configuration with those on the reference one. The proposed identification procedure, numerically and experimentally validated in the past by the authors, has been here applied to the structural characterization of two main rotor blades, designed for ultra-light helicopters. Strain gauges measurements have been used to assess the accuracy of the identified properties through natural frequencies comparison as well as to evaluate the blades damping characteristics.

Vibration-based damage detection for a composite helicopter main rotor blade

This work presents experimental results of two damage detection techniques based on modal properties, with the application on a full-size composite helicopter main rotor blade. The damage detection methods used in this study are the coordinate modal assurance criterion (COMAC) and the modal strain energy method, which are respectively based on the comparison of vibration modes and on the comparison of the modal strain energy of a beam. Modal parameters were obtained with experimental modal analysis and damage was introduced artificially on the blade by attaching a small mass to it, changing its global properties in this way. Finally, experimental results for the damage detection technique are shown for both methodologies, and remarks concerning sensitivity and robustness of the methods are discussed.

Innovative Helicopter In-Flight Noise Monitoring Systems Enabled by Rotor-State Measurements

Abstract The present contribution aims at providing a comprehensive illustration of a new approach to rotorcraft noise abatement, especially during terminal procedures, when the vehicle approaches the ground and the acoustic impact is higher. This approach pursues the development of technologies and tools for real-time, in-flight monitoring of the emitted noise. The effect of the acoustic radiation is presented to the pilot in a condensed, practical form on a new cockpit instrumentation, the Pilot Acoustic Indicator (PAI), to be used for performing quieter maneuvers. The PAI is based on the synergetic composition of pre-calculated acoustic data, which are used in a noise estimation algorithm together with the data gathered by an innovative contactless measurement system, capable of acquiring the main rotor blade motion. The paper reports on the current studies in unsteady and quasi-steady aeroacoustic prediction and tip-path-plane angle of attack and thrust coefficient observation. Results on novel methodologies are discussed, together with the main features of the PAI design and development process.

Some aspects of computation methods in strength analysis of flight structures

Primary attention of our investigation during the last ten years has been focused on the development of efficient computation methods and corresponding software for strength analyses of flight structures such as aircraft, helicopter and tactical UAV structures. In these investigations the complete computation methods/procedures are developed and corresponding in-house software for total fatigue life estimations (up to crack initiation and crack growth) of structural components under general load spectrum. In order to ensure efficient computation methods in fatigue life estimations of metal structural components the same low cycle fatigue material properties are used for initial fatigue life estimations and residual fatigue life estimations. The developed computation methods and corresponding software are verified using our own experiments. Most of these methods/procedures and software were applied for the extension of aircraft structure lifetime, both those at home and those in service abroad. An important aspect in designing and extending the life of the aircraft structure represents a precise determination of the load. For this purpose, as a rule, CFD numerical simulations are used. It is especially important to accurately determine the load of the main and tail rotor blades of the helicopter. In these cases, the application of CFD numerical simulation is unavoidable.

Computational analysis of helicopter main rotor blades in ground effect

Computational analysis of helicopter main rotor blades in ground effect

Damage tolerance and classic fatigue life prediction of a helicopter main rotor blade

Fatigue life of the main rotor blade of a helicopter is investigated from two different approaches. The blade is comprised of many different parts such as spar, skin, abrasive strip, doublers, box beam doublers, etc. and it undergoes complex random load spectra. A finite element analysis is carried out to assess the stress distribution in the blade parts. Then, using the Miner’s rule the fatigue life of the blade is calculated following a classic life estimation method under random loading. At the next step, damage tolerance approach is used for life estimation of the blade. Fatigue crack growth properties and threshold stress intensity range of the spar is obtained based on ASTM E647 test method. Then, crack growth analysis is done using Zencrack software. In order to calculate the length of the smallest growing cracks and their fatigue crack growth life, no crack growth and slow crack growth approaches are used. A comprehensive investigation is carried out on the effect of initial crack length and aspect ratio on growth or no growth of cracks. Comparing the obtained fatigue life from two different approaches, some conclusive results are drawn.

Photogrammetric detection technique for rotor blades structural characterization

This paper describes an innovative use of photogrammetric detection techniques to experimentally estimate structural/inertial properties of helicopter rotor blades. The identification algorithms for the evaluation of mass and flexural stiffness distributions are an extension of the ones proposed by Larsen, whereas the procedure for torsional properties determination (stiffness and shear center position) is based on the Euler-Prandtl beam theory. These algorithms rely on measurements performed through photogrammetric detection, which requires the collection of digital photos allowing the identification of 3D coordinates of labeled points (markers) on the structure through the correlation of 2D pictures. The displacements are evaluated by comparing the positions of markers in loaded and reference configuration. Being the applied loads known, the structural characteristics can be directly obtained from the measured displacements. The accuracy of the proposed identification algorithms has been firstly verified by comparison with numerical and experimental data, and then applied to the structural characterization of two main rotor blades, designed for ultra-light helicopter applications.

Piezoelectric Deicing System for Rotorcraft

Deicing using piezoelectric actuators is considered as a potential solution to the development of low-energy ice protection systems for rotorcraft. This type of system activates resonant frequencies of a structure using piezoelectric actuators to generate sufficient stress to break the bond between the ice and the substrate. First, a numerical method was validated to assist the design of such systems. Numerical simulations were performed for the case of a flat plate and validated experimentally. The model was then used to study important design parameters such as actuator positioning and activation strategies, and it was concluded that positioning actuators at antinode locations, and activating them in phase with those antinodes to obtain maximum displacements for a given vibration mode. The findings were then used to apply piezoelectric deicing to structures more representative of a helicopter rotor blade. The method was implemented on a thinned Bell 206 main rotor blade and a Bell 206 tail rotor blade. Deicing performance was demonstrated in an icing wind tunnel. Power input to the actuators was below 19 kW/m2 (12 W/inch2) for all structures.

The exponential decay of underwater acoustic intensity with increasing altitude of low-frequency sound from a Robinson R44 helicopter

A series of underwater acoustic experiments utilizing a Robinson R44 helicopter and an underwater receiver station has been conducted in shallow (16 m) and deep (>100 m) water. The receiver station consisted of an 11-element nested hydrophone array with a 12 m aperture. In the shallow water experiments the array was configured as a horizontal line (HLA) 0.5m above the seabed; whereas in deep water the array was suspended from the surface in a vertical line (VLA). An in-air microphone was located immediately above the surface. In this paper the power spectral density as a function of helicopter altitude will be reported from measurements on a single hydrophone. The main rotor blades of the helicopter produce low-frequency harmonics, the lowest frequency being ~13 Hz. The tail rotor produces a sequence of harmonics approximately six times higher in frequency. Between heights of 30 m to 600 m above the sea surface the underwater intensity was found to decay exponentially with increasing helicopter altitude. Interpretation of the observed low-frequency sound signatures is being facilitated with a numerical three-layer (atmosphere-ocean-sediment) acoustic propagation model in which the source may be moving or stationary. [Research supported by ONR, NAVAIR, and SIO.]

Dynamic Simulation of an Unmanned Hybrid Flying Robot

This paper considers the dynamic simulation of an Unmanned Hybrid Flying Robot (UHFR) with main fuel engine in the middle to carry most of the weight and promises long flights. This configuration will increase the flight time of the unmanned copter for a given payload size as opposed to the traditional quad-copters, where only DC motors are used. A parametric study to investigate the effect of the propellers ratio (main rotor propeller diameter to secondary rotor propeller diameter), the angle of incidence of the main rotor and the twist angle of the main rotor blades on selected performance criteria of the UHFR is presented.

Damage tolerance approach for analyzing a helicopter main rotor blade

In this article, damage tolerance approach is used for life estimation of the main rotor blade of a helicopter under random loading. At first, fatigue crack growth properties and threshold stress intensity range of the spar is obtained based on ASTM E647 test method. Then, crack growth analysis is done using Zencrack software. In order to calculate the length of the smallest growing cracks and their fatigue crack growth life, no crack growth and slow crack growth approaches were used. A comprehensive investigation was carried out on the effect of initial crack length on growth or no growth of cracks.

Contrastive Study of the Radiography on Film and Real-time Imaging

The dissertation introduced the development of the real-time imaging inspection technique, and contrast study of the techniques of the radiography on film and real-time imaging from the principle, technique parameter, system composing, post disposal manner and archive. Taking a verification test of inspected the main rotor blade and composite material insert part with the radiography on film and real-time imaging, the result is that the two kinds of inspection techniques are similar in two kinds of parts. If using the real-time imaging technique can improve the working efficiency, reduce the labor intensity and improve environmental protection. However, there are no evaluation criteria of imaging for the real-time imaging technique now, and still using the criteria films as the acceptance criteria. The radiography on film is the main inspection means, but the real-time imaging technique will be the development direction.

Numerical Simulation and Investigation of Transonic & Symmetrical Airfoil for Helicopter Main Rotor Blade Application

Helicopter rotor aerodynamics is prognosticated to be one of the most perplexing and enigmatic affliction encountered by both researchers and aviators throughout the ages. The bewilderment of the flow field around the main rotor blade ceaselessly remains unrequited in tangible flight environments. Appalling calamities repeatedly befall owing to these unforeseen and equivocal instances. In order to extricate these impediments, one must go back to the brass tacks and apprehend the cause. However, it is every so often exceedingly disconcerting to obtain experimental data due to intricacy, perplexity and substantial price tag. Subsequently, computational simulation is progressively becoming more of a preferred choice in recent times. Bearing this in mind, this study intended to simulate and visualize the air flow configuration of the main rotor blade using symmetrical and transonic airfoils to demarcate their physiognomies and behavior. Results have revealed that the transonic airfoil has a higher lift coefficient (Cl) than the symmetrical airfoil. Contrariwise, if used in an actual rotorcraft, the transonic airfoil can cause stern apprehension in terms of stability and control.