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Evaluation of a real-time simulation environment for helicopter air-to-air refueling investigations
No full textThe ability to perform air-to-air refueling (AAR) can dramatically extend the utility of helicopters, through effec-tively providing unlimited range. For helicopters, AAR is typically performed utilizing the probe-and-drogue aerial refueling method. This is a complex maneuver, where normally both the helicopter and tanker aircraft are operating at the limits of their flight envelopes. In addition, the wake flow from the tanker aircraft can cause a significant disturbance on the refueling helicopter. This paper presents the initial evaluation of an AAR sce-nario constructed within DLR’s flight simulator, the Air Vehicle Simulator (AVES), based on current procedures and pilot interviews. A mission task was defined to assess the scenario in AVES and results are subsequently discussed. For pilots unfamiliar to formation flight or HAAR the results show the difficulty of the flying task itself at the given cueing. Measures for improvement in future investigations are suggested
The vortex ring state of a shrouded rotor
No full textAn experimental investigation into the flow field produced by a shrouded rotor, operating in axial descent is presented. Laser Doppler Anemometry (LDA) was used to determine the induced velocity of the rotors, whilst Particle Image Velocimetry (PIV) was applied to investigate the dynamics of the unsteady flow field produced. Tests were repeated using the same rotor, without the shroud, to assess its influence on the observed phenomena. At low descent velocity ratios, the mean flow field resembled that produced by a shrouded rotor operating in hover. At slightly higher descent velocity ratios, the wake from the shrouded rotor broke down, leading to the formation of a large region of recirculation outboard of the shroud which aperiodically sheds into the free stream. Further increases in descent velocity lead the centre of recirculation to form around the external surface of the shroud. The flow topology appears to be generally similar to the one arising from an isolated rotor operating in descent flight, however the results suggest that the presence of the shroud might slightly delay the onset of the Vortex Ring State (VRS)
Damper model identification using hybrid physical and machine learning based approach
No full textIn this paper the identification of a time domain model of a helicopter main rotor lead-lag damper is discussed. Previous studies have shown that lead-lag dampers have a significant contribution to the overall aircraft dynamics, therefore an accurate damper model is essential to predict complex phenomena such as, instabilities, limit cycles, etc. Due to the inherently nonlinear dynamics and the complex internal architecture of these components, the model identification can be a challenging task. In this paper, a hybrid physical/machine learning based approach, has been used to identify a damper model based on experimental test data. The model, called grey box, consists of a combination of a white box, i.e. a physical model described by differential equations and a black box, i.e. regression numerical model. The white box approximates the core physical behaviour of the damper while the black box improves the overall accuracy by capturing the complex dynamic not included in the white box. The paper shows that, at room temperature, the grey box is able to predict the damper force when either a multi-frequency harmonic or a random input displacement is imposed. The model is validated up to 20 Hz and for the entire damper dynamic stroke
Icing characteristics on NGCTR engine inlet for relevant certification conditions
Get PDFRotorcraft engine air intakes are known to be particularly sensitive to inflight and ground icing and snow conditions. To allow early identification of geometrical features of the inlet that are sensitive to ice accumulation, the design can benefit from early icing analyses through numerical methods. This paper is focused on the icing assessment and optimisation of the engine intake duct of the Next Generation Civil Tilt Rotor (NGCTR) which is being developed by Leonardo Helicopters. Results for catching efficiency and water catch rates are presented for test cases that have been defined with respect to the operational envelope of the NGCTR and the requirements for atmospheric icing as defined in the EASA certification specifications for large aeroplanes, large rotorcraft and engines. It was found that both the water catch rate and the total water catch are lower for the NGCTR in conversion flight compared to the NGCTR in airplane flight conditions. Additionally, ground operations of the NGCTR are not found to be critical for icing in/on the engine inlet due to supercooled droplets
A study on control law and hardware applying to flexible control rotor blades for flight performance improvement
Get PDFFlexible control systems for helicopter blades (e.g. HHC, IBC, Active Twist and Active Flap) have the potential to improve flight performances of rotorcraft. This study selects Active Flap as the flexible control system because it is more feasible than the others and it has been abundantly experienced in author’s organizations. CAMRAD II analyses are conducted to evaluate the effect of Active Flap on hover and forward flight performances. The hover analysis demonstrates that the required rotor power is reduced by 2.5 % with the positive 0/rev (quasistatic) flap deflection, which twists the blade in the nose down direction to reduce the local angle of attack and the local drag. The required power for hover changes non-linearly with respect to the flap deflections because of the independent behaviors of CM and CL variations. In forward flight analysis, the Active Flap is controlled by 2/rev, 3/rev and ideal flap deflections. The 2/rev and ideal controls can reduce the required rotor power for forward flight by 4% resulting from suppressing the negative angle of attack on the advancing blade. Furthermore, ideal control results in lower rolling and pitching moments acting on the hub than other harmonics, thus it can reduce re-trim control which is required when blades are actively controlled and can prevent significant increase of pilot workload. A conceptual design study on the Active Flap system to realize the developed flexible control law is carried out based on the previous research activities which had demonstrated the sufficient achievements. The system consists of two flaps and two drive mechanisms to share aerodynamic loads and prevent force fighting between the drive mechanisms
Active whirl flutter control through dynamic wind tunnel experiments and modelling
No full textWhirl flutter aeroelastic instability is a well-known limiter of performance in current generation tiltrotors, and as such, its study is of particular interest for future rotorcraft generations. This paper presents two integrated systems aiming to facilitate novel analysis of the whirl flutter phenomenon and its alleviation through active mini-tab control: a bespoke experimental rig, and a complementary numerical model. This is the first experimental and numerical investigation of active whirl flutter control with a promising, and practically realisable flow control technology. The experimental rig is shown to be capable of producing dynamic motion comparable to documented XV-15 flutter, while LQR analysis of the reduced-order model suggests the feasibility of linear control, which offers unique advantages over nonlinear control for practical implementation of the technology. A mutually beneficial relationship is established between physical testing and numerical investigation enabling state of the art data collection and analysis
A novel procedure for the topology optimization of an engine exhaust mixer
Get PDFThe design of the engine primary exhaust of a helicopter must guarantee that the backpressure induced at the turbine outlet is sufficiently low to expel the hot air from the vehicle without compromising engine performances. At the same time, a large enough mass flow of air from the engine bay (secondary flow) is imposed to properly ventilate the bay. To this purpose, the exhaust is composed of two parts: the primary exhaust which conveys the air flow from the engine outlet and the secondary flow which collects the air from the primary exhaust and the air from the engine bay. The exhaust is designed with the aim of minimizing the backpressure at the engine outlet (minimizing therefore the fuel consumption) and of providing a secondary flow to properly ventilate the engine bay. In this paper, the discrete adjoint of the compressible Navier-Stokes equations coupled with the level-set method for the topology optimization is applied to this complex aerodynamic scenario, with the aim of designing a geometry able to minimize the impact on engine fuel consumption while fulfilling all other design requirements. The design case analyzed is the primary exhaust of the tilt-rotor demonstrator developed by Leonardo Helicopter division within the Clean Sky 2 Fast RotorCraft framework. Since the topologic optimization has been only recently introduced in Computational Fluid Dynamics tool, this work duly investigate all the steps required for the optimization. These include the choice of the constraints and the analysis of the error introduced by the Brinkman Penalization Model, which is used to simulate the solid domain introduced in the fluid field including some detail on modelling the walls generated by the optimization process. The impacts of the developed solutions are at the end compared with the primary exhaust developed using standard methodologies. Two novel geometries are produced which fulfill all design requirements. All simulations were performed using Siemens PLM commercial software STAR-CCM+
Flutter assessment of a rotor blade in hover based on indicial aerodynamics considering blade profile, rotor inflow and wake periodicity
Get PDFThe 7AD rotor blade is assessed for flutter stability in hover. For the aeroelastic analyses, the multibody model is tightly-coupled with an unsteady aerodynamic model based on Wagner’s function and related enhancements for the general motion of an airfoil section considering heave and pitch. The mathematical setup of the Wagner function is extended for axial flow to include unsteady contributions related to rotor inflow and wake periodicity. Since the unsteady aerodynamic model is based on indicial functions, a separation of these contributions is possible and allows to study their individual impact on rotor blade flutter. According flutter results are extracted in frequency domain for three test cases which differ in the unsteady aerodynamic model for circulation comprising blade profile, rotor inflow and wake periodicity. As known for articulated rotor blades, also the 7AD blade exhibits a classical bending-torsion coupling. The lowest flutter onset is found for unsteady aerodynamics limited to blade profile, whilst the cases with added rotor inflow and wake periodicity show both the same flutter onset at a 5 % larger rotor speed. Here, the influence of rotor inflow plays the major role, since it increases the torsion damping within the critical flutter coupling. Added wake periodicity neither changes frequency nor damping and hence, does not affect the aeroelastic coupling
Performance of a dual-controlled rotor in level flight
No full textFeasibility of a "level flight" multi-rotor aircraft, a concept for a high-speed multi-rotor aircraft characterized by a horizontal attitude during forward flight, using a separately placed propeller for propulsion is investigated. The performance of a single rotor used in this aircraft at forward flight is evaluated through numerical simulations. Also, a dual-controlled rotor whose control parameters are rotational speed and blade pitch angle was investigated. The numerical results show that the effective lift-to-drag ratio of the horizontal rotor is higher than that of the forward-tilted rotor. Adding control of the blade pitch angle improves the effective lift-to-drag ratio compared to rotors that can only be controlled by rotation speed. The optimal value of the effective drag is determined by the relationship between the power and the drag. The rotational speed changes the rotor power, and the drag reduction depends on the blade pitch angle. In addition, the untwisted blade shows a higher effective lift-to-drag ratio than the twisted blade. The blade twist significantly affects the drag of the rotor
Helicopter noise in urban flight
Get PDFThe present paper deals with a test campaign carried out in order to investigate the interaction between sound waves emitted by an helicopter and a model of an idealized urban contest. Test campaign was performed in Politecnico di Milano anechoic room, in order to analyze data produced by only helicopter noise. Set up consisted in a two blade main rotor helicopter model and a rectangular prism in aluminium as the landing building model. Ground observers perception was recorded by means of a surface microphone and a realistic landing trajectory was approximated as a succession of fixed point measurements. Collected data have been analyzed through acoustic spectra and sound maps. Spectra were used to comprehend physical phenomena, such as reflection, diffraction and shielding, and to analyze the different components of helicopter noise. Sound maps analysis enabled to get a global perspective of involved phenomena and to understand how much people close to building are stressed by an helicopter approaching an elevated urban helipad