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Numerical study of effect of hangar modification on ship-airwake and helicopter downwash interaction
Get PDFNumerical investigation of the effects of installing a rotating cylinder at the hangar top edge of a frigate on the dynamic interaction between the airwake generated by a Simplified-Frigate-Ship (SFS) during its propulsion and downwash formed by a 3-bladed helicopter rotor trying to perform landing/take-off operations on the flight deck of SFS is undertaken. The SFS hangar is attached with this rotating cylinder to suffice as an active flow device and the flow field so created by this dynamic interaction is analyzed and compared. The modified frigate is modeled using a scale ratio of 1:100. Measurements are taken in terms of rotor thrust coefficient, recirculation length and turbulence intensity at identified locations. STAR CCM+ code that uses FVM (Finite Volume Method) solver to solve the RANS (Reynolds-Averaged-Navier-Stokes) equation along with two-equation ????? turbulence model as a CFD tool are used for carrying out this numerical analysis. Firstly, the airflow analysis is carried out for SFS-2 in isolation in order to establish a baseline understanding of flow followed by airflow analysis for modified SFS-2, the one equipped with a rotating cylinder. Further, simulations for studying the dynamic interface of ship airwake and the rotor downwash are carried out which involve both the helicopter rotor with ROBIN fuselage and SFS-2 in the first case while the helicopter rotor with ROBIN fuselage and SFS-2 modified with rotating cylinder in the second case. These simulations were varied based upon three distinct cylinder diameter to hangar height ratios for each one of the two rotor hovering -planes positioned parallel to eachother. The inference obtained from this study is that the hangar with a rotating cylinder gives better flow field in terms of recirculation length and thrust co-efficient with zero WOD (wind over deck angle)
Time domain identification method with improved robustness for rotorcraft flight dynamics modeling
Get PDFA time domain identification method based on set-membership theory for rotorcraft flight dynamics modeling is developed in this paper. In order to solve the difficulties in applying standard set-membership identification algorithm to identify the flight dynamics model of a rotorcraft due to its inherent complexity, an improved Optimal Bounding Ellipsoid (OBE) method is established. The basic OBE algorithm for multi-input multi-output system is derived firstly, and then an indirect OBE algorithm for state space model is established. The concept of generalized noise is introduced and the noise bound optimization is implemented. Finally, the established identification method is used to identify the flight dynamics models of a helicopter and a quad-rotor aircraft separately. The results show that the method developed in this paper is able to identify the flight dynamics model of a rotorcraft with high accuracy as well as robustness
Matrix pencil method integration into stabilization diagram for poles identification in rotorcraft and powered-lift applications
Get PDFThe development of a safe and reliable VTOL aircraft is largely dependent on the availability of accurate and practical model of the system. Due to the complex dynamic behavior of rotorcraft and related subsystems, system identification based on flight data is essential in producing representative models. Traditional approaches, such as the polynomial based Prony method, are computationally expensive and sensitive to signal noise and disturbances. They also lack any indication of reliability and typically require time-consuming manual post-verification by a specialist. Given the unique complexity of VTOL aircraft, associated poor signal to noise ratio, and presence of tonal disturbances in flight data, current system identification methods are not robust enough. In this paper, a comprehensive two-step method is presented to address the mentioned limitations of current techniques. The proposed method is an output-only identification technique, based on time-domain analysis of the free decay response. Here, a modified Matrix Pencil (MP) method is coupled with the stabilization diagram to identify system poles. The main features of the method are the capability to assess simultaneously multiple time-histories in order to obtain a more comprehensive description of the system and, at the same time, a time saving analysis; the independence of tonal disturbances and noise; the generation of a qualitative measure of confidence on the results. The algorithm was implemented in a software package, and extensive evaluation of the method was conducted using flight data. The results of the evaluation confirmed the capability of the method to detect poles within proximity of tonal disturbances. In addition, indicated an efficiency improvement of 70% in automatic system pole identification in high-noise signals when compared to MP techniques not used in conjunction with stabilization diagram
Main rotor blade tip vortex characterization
Get PDFThe paper illustrates a research activity carried out at CIRA with the aim to characterize experimentally and numerically the blade tip vortices of a small scale four-bladed isolated rotor in hover flight and to evaluate their decay process during the convection of the wake downstream. 2C-2D PIV measurements were carried out below the rotor disk down to a distance of one radius. The numerical simulations were aimed at assessing the modelling capabilities and the accuracy of a free-wake Boundary Element Methodology. Several detection criteria were investigated in order to identify a suitable one for the analysis of PIV data. The ?2 vortex was selected as the most robust and reliable criterion and was applied to both experimental and numerical results. The tip vortices were characterised in terms of vorticity, circulation, swirl velocity, core radius and trajectory. The rotor wake mean velocity field and the instantaneous vortex characteristics were investigated. The experimental/numerical comparisons showed a reasonable agreement in the estimation of the mean velocity inside the rotor wake, whereas the BEM simulations predicted and under-estimated effect of the diffusion thus generating a smaller shear layer region with respect to the experiment. The numerical simulations provided a clear picture of the filament vortex trajectory interested by complex interaction starting at about a distance of z/R=-0.5. The time evolution of the tip vortices was investigated in terms of net circulation and swirl velocity. The PIV tip vortex characteristics showed a linear mild decay up to the region interested by vortex pairing and coalescence, where a sudden decrease, characterised by a large data scattering, occurred. The numerical modelling predicted a hyperbolic decay of the swirl velocity down to z/R=-0.4 followed by an almost constant decay. Instead, the calculated net circulation showed a gradual decrease throughout the whole wake development. The comparisons showed discrepancies in the region immediately downstream the rotor disk but significant similarities beyond z/R=-0.5
Aerodynamic analysis of store jettison on a helicopter
Get PDFAerodynamic analysis of the store jettison on a helicopter was performed in this paper. The objective of the present study is to investigate the aerodynamic characteristics of a jettisoned store from a helicopter. Transient, incompressible, three-dimensional and turbulent flow in the store jettison with rotor downwash was analyzed using commercial computational fluid dynamics software. Virtual blade method was adopted to describe the effect of rotor down wash. Overset meshes were used to consider a movement of the jettisoned store. Computational results are validated by dynamically scaled free drop wind tunnel testing. The wind tunnel testing was executed at low-speed tunnel of the German-Dutch wind tunnel (DNW-LST). Calculated results are good agreement with wind tunnel testing results. The trajectories of the store are mainly affected by the weight of jettisoned store
Aeromechanics of a coaxial mars helicopter using high-fidelity CFD/CA
No full textA high-fidelity coupled computational fluid dynamics (CFD) and comprehensive analysis (CA) solver is developed for application on the Mars helicopter. Accurate aeromechanical understanding of a coaxial rotor on Mars is necessary in order to make proper design decisions for future aircraft with longer range and greater payload. The objectives are to understand the performance, structural loads, control loads (pitch link), wake interaction, and blade strike for hingeless and articulated coaxial rotors, so that an informed decision between the two rotor hubs can be made. This will become more important as the vehicle size and payload grows. Lower fidelity tools are not capable of capturing the complex flow phenomena (blade vortex interaction, roll-up and core growth, and 3D unsteady pitching moments at low Re), and therefore this problem requires coupled CFD/CA. Some of the key conclusions specific to Mars are: (1) an articulated rotor in fact benefits from greater rotor separation because pitch angles, not flapping motion dictates separation (2) a hingeless rotor experiences only marginally greater (6?7%) flap bending moments compared to an articulated rotor, (3) the oscillatory pitch link loads on an articulated rotor are nominally 15.5% greater than on a hingeless rotor and (4) the steady pitch link loads of a hingeless rotor are in fact 8 times greater than an articulated rotor. For these reasons, larger future Mars helicopter it appears an articulated rotor might be more desirable over a hingeless rotor, for considerations of structural and control loads
Applying artificial neural networks to the tasks of designing airfoil sections of a helicopter´s main rotor
No full textAn original system for designing aerodynamic profiles using artificial neural networks (ANN) is presented. A panel method was applied for a quick assessment of the quantitative aerodynamic characteristics of the profiles. This approach eliminates the problems of CFD modeling associated with the necessity to use large computational power. The developed methodology and the algorithms based on were applied to the design of aerodynamic profiles. As test NACA23012 base profile has been selected, on the basis of which was generated by a plurality of profiles acceptable to analysis. The analysis carried out with ANN methods revealed a limited area of the most promising candidate profiles for the further optimization of the geometry from the point of view of the chosen criteria. The profile design process is iterative. Already at the first iteration, it can be shown that the resulting profile family in aerodynamic characteristics exceeds the base profile
Numerical analyses of different state of flight of new concept coaxial rotor dedicated to unmanned helicopters
No full textPaper presents a concept analysis of a resilient coaxial main rotor designed in the Warsaw Institute of Aviation which incorporates features of a standard joint rotor with a rigid Active Blade Concept (ABC) rotor. The combination of two extreme cases aims to increase the cruising speed compared to classical joint rotors and cost reduction compared to technologically advanced ABC rotor. In the paper detailed numerical analysis of the rotor has been carried out based on the data obtained during bench and field tests carried out on the rotor blades and the rotor assembly. The calculations were conducted for envelope of deferent flight state: hover, horizontal flight level with different speeds, pull-up maneuver and analysis of high-speed flight (over 300 km/h). Results of calculations among others prove that a properly selected stiffness of the rotor blade attached to a rigid rotor hub allows for reduction of the separation distance between rotors compared to coaxial rotors with articulated blades
Lagrangian particle tracking in deforming sliding mesh for rotorcraft icing applications
Get PDFParticle tracking techniques are presented for simulating the development of clouds containing supercooled water droplets entrained in complex flow systems such as those produced by rotorcraft. The physics of this problem and the associated condition is known as in-flight icing, where extreme ice structures can possibly form, causing severe performance degradation and a reduction in handling qualities. Resolving rotorcraft flow fields frequently entails the use of multi-zone and deforming grid systems to allow independently moving components. The approach advocated here allows particle tracking through mesh with arbitrary motion which includes deformation and particle tracking through non-conformal interfaces which are present in multi-zone problems. These techniques are first described and verified for two-dimensional problems before the results of three-dimensional practical engineering applications are shown
Novel methods for estimating bandwidth and stability margins of pilot-in-loop systems
Get PDFA standard metric to estimate system bandwidth is the pilot cutoff frequency, which is the frequency at which the cumulative power ratio of the pilot’s control response equals 0.5. Implicit with using the pilot cutoff frequency is that the vehicle output being tracked is approximately the integral of the control input (i.e., rate-commanded). However, errors in estimation will occur when the technique is applied to systems whose tracked outputs differ significantly from this assumption. Furthermore, the type of disturbance spectrum impinging on the tracking task has a strong influence. The effect of these factors on pilot cutoff frequency is examined theoretically, and a method for transforming the cumulative control power ratio is developed that enables the transformed ratio to be applied to any tracked state. A method for determining effective time delay, and effective phase and gain margin from the slope of the transformed cumulative power ratio is also developed. Assuming knowledge of the disturbance spectrum and vehicle dynamics, two techniques are offered to estimate system bandwidth and time delay using: 1) A cutoff frequency (dependent on the forcing function) using the transformed stick response cumulative power ratio; 2) Iteration on the crossover frequency and time delay parameters in the closed-loop Crossover Model until a best match is found between the transformed cumulative power ratios of the modeled and observed stick response. The latter approach does not require that the forcing function contain power extending to or beyond crossover. The development demonstrates that bounding the upper frequency of the computed control power is a critical step of the estimation process, as this reduces the effect of uncorrelated high frequency content arising from sources such as the neuromuscular mode and harmonics of pulse-like control on the estimates. A unique bi-directional spatial filter that allows the frequency and slope from cumulative power ratios to be continuously analyzed when using discrete spectra forcing functions (such as sum-of-sines) is developed. The filter also improves estimation when the forcing spectrum is continuous. A new system bandwidth estimation method that uses the vehicle output cumulative power ratio is proposed, which unlike the cumulative stick power approach does not require an assumption about or measurement of the vehicle dynamics. This technique transforms the output by simple differentiation, allowing similar application of the stick power methods (cumulative power ratio cutoff and model matching). Finally, effective time delay and crossover frequency are estimated using the ideal Crossover Model by matching the observed system time response. The novelty introduced here is the that the effective stability margin arising from these two effective parameters closely coincides with the actual system stability margins (phase and gain), irrespective of the differences between the idealized and actual dynamics. This allows the accuracy of any bandwidth estimate to be assessed - establishing the actual bandwidth associated with human-in-loop operation has heretofore proven elusive. The technique lends itself to both manual and automated systems and will be useful for assessing handling qualities. Pilot data from a simulation tracking experiment is used to demonstrate the efficacy of these various estimation techniques