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A hybrid Navier-Stokes / viscous vortex particle wake methodology for modeling maneuver loads
Get PDFManeuvering flight and high-speed flight are critical design points in any rotorcraft's operating envelope. These conditions give complex flow phenomena, creating high stresses and vibrations. To accurately predict the flow properties over the relatively flexible rotor blades, coupling between computational fluid dynamics (CFD) and computational structural dynamics (CSD) is required. In this work, GT-Hybrid, a hybrid wake rotorcraft CFD code that is coupled to DYMORE, is used. A vortex particle method has been implemented, in place of the existing lattice wake methodology. Selected UH-60A maneuvering flight conditions; being two diving-turn and pull-up maneuvers, are simulated using the vortex particle method. Results are compared qualitative with those using the traditional wake method and available experimental data; indicating that using the vortex particle method gives similar or better results. Additionally, computational efficiency is improved by using the vortex particle method and time savings exist in every simulation
Model predictive motion cueing for a helicopter hover task on an 8-dof serial robot simulator
Get PDFMotion cueing for helicopter hover is difficult: small simulators require considerable attenuation, rendering motion cues not useful for stabilization, and large simulators are typically not cost effective. Industrial serial robot-based simulators provide large motion capabilities at a moderate cost, but have two distinct disadvantages. First, they are highly dimensional systems with a non-convex motion space, such that efficient use of the entire space is not trivial. Second, they are typically non-stiff structures with a large mass at the end effector, resulting in oscillatory dynamical properties. We recently developed a novel Model Predictive Motion Cueing Algorithm (MPMCA) that resolves both problems effectively for pre-recorded inertial reference signals. The MPMCA requires an accurate prediction of the future course of the reference inertial signals, which is trivial for pre-recorded maneuvers, but not for real-time human-in-the-loop simulations. In this paper, we present a model-based prediction method, which predicts pilot control inputs and the subsequent helicopter inertial signals during a helicopter hover simulation in real-time. The method is tested in a human-in-the-loop experiment and compared with the Classic Washout Algorithm. The results demonstrate that the MPMCA is a promising new approach to motion cueing
An extensive helicopter ground vibration test: from pretest analysis to the study of non-linearities
No full textThe Ground Vibration Test (GVT) is one of the key milestones in the characterization of an aerospace structure, allowing to describe its structural dynamic behavior. Moreover, a helicopter GVT is associated to additional challenges deriving from the rotorcraft architecture, such as a high modal density and non-linear phenomena. In this paper, these challenges are treated by presenting the extensive H145 GVT campaign carried out in June 2017 by AHD and DLR, from its conception to the first analysis of results. Starting from a H145 FE model, the pre-test analysis began with the selection of target modes from the initial numerical modes set based on modal participation and energy considerations. An optimal sensor distribution was also achieved as results of the implementation of sensor placement metrics like the Normal Displacement Method and sensor elimination methods based on MAC analysis. An extensive description of the testing methods and procedure is as well documented, from the use of a dedicated test rig to the excitation of the structure by means of several exciter constellations using different force levels in order to assess non-linear behavior and therefore identify the structural variability. After data acquisition, the efficient post-processing performed using DLR correlation tool allowed the identification of modes family and the creation of a modal model. In the first analysis of results, modal identification has shown the validity of the pre-test analysis by identifying more than 40 modes for the first helicopter configuration and exhibiting an excellent data quality. Comparison between two H/C configurations has given also a first sample of how structural variability can influence the modal layout. Furthermore, focus has been put on the identification and analysis of non-linear phenomena, proving how non-linear behavior can affect significantly the H/C dynamic response and the modal identification. Finally, a comparison between FE and test results for one H/C configuration has been performed, allowing an objective evaluation of the predictive capability of current FE models. On this basis, the path for future works in the field of FE modal updating and structural optimization is clearly defined
Simulation of active flow control actuator using CFD with application to rotor blade vibration reduction
No full textA computational fluid dynamics (CFD) model is developed to determine the unsteady aerodynamic effects of active flow control implemented by combustion-powered actuation (COMPACT) on a two-dimensional airfoil. Previous work with COMPACT actuators employed pulsed-jet actuation at moderate to high angles of attack to control flow separation and dynamic stall. However, the focus of the present study is to implement actuation in the moderate to low angle of attack range suitable for helicopter rotor vibration control. At low angles of attack, the effect of actuation on the sectional aerodynamic forces of the airfoil diminishes. Therefore, modifications to the COMPACT actuator con1guration are considered to enhance the actuation authority for the relevant operating range. Modifications include placing a ramp on the airfoil upstream of the actuator and relocating the actuator near the airfoil trailing edge. These are considered using both CFD simulations and wind tunnel experiments, which are compared to validate the CFD model. Finally, a surrogate-based reduced-order modeling technique is described to address the high computational cost of the CFD simulations. The reduced-order model is used to accurately reproduce full-order CFD results for the unsteady changes in lift, moment, and drag due pulsed actuation on a static airfoil. Furthermore, the results show that the reduced-order model represents a feasible method for representing the unsteady aerodynamic effects of COMPACT actuation. This will be refined in future work and implemented in a comprehensive aeroelastic code for helicopter rotor vibration reduction
Drivetrain influence on the lead-lag modes of hingeless helicopter rotors
Get PDFStructural couplings between the 2exible main rotor and the 2exible drivetrain of the Bo105 helicopter are investigated by numerical simulation. For this purpose, the rotor hub constraint ? = const. is dropped and a drivetrain model, consisting of discrete inertia elements and intermediate 2exible elements, is connected to the hub. By use of the multibody-software SIMPACK, the coupled rotor-drivetrain system is linearized and the Eigenmodes are compared to those obtained with a constrained rotor hub. The drivetrain has a signi1cant in2uence on the shapes and Eigenfrequencies of the collective lead-lag modes. While the 1rst collective lead-lag Eigenfrequency is raised by the 1nite drivetrain inertia, the second is lowered due to drivetrain 2exibility. To assess the in2uence of modeling inaccuracies on the observed couplings, the study is complemented by a sensitivity analysis. Rotor blade mass axis offset, blade pitch (causing elastic coupling) and blade precone angle have only weak in2uence on the coupled modes. In contrast, variations of drivetrain inertia and stiffness strongly affect the Eigenfrequencies of the coupled rotor-drivetrain modes
Eigenmode distortion as a novel criterion for motion cueing fidelity in rotorcraft flight simulation
Get PDFEigenmode distortion (EMD) is a novel methodology developed to study the degradation of perceived vehicle dynamics as a result of motion cueing algorithms (MCA's) applied in rotorcraft flight simulators. This paper briefly introduces EMD and subsequently describes its application in a pilot-in-the-loop experiment conducted on the SIMONA Research Simulator at Delft University of Technology. The experiment considers a precision hover task performed by two test pilots in three different motion cueing conditions. Each of the evaluated conditions is devised such to best reproduce one of the vehicle modes (pitch/heave subsidences and phugoid) simulated using an independently developed, three degree-of-freedom, longitudinal, nonlinear model of the AH-64 Apache helicopter. The experiment yielded a number of interesting results. For example, the mode participation factors (MPFs) computed using recorded model states showed that the unstable phugoid mode dominates the overall dynamic response in all conditions evaluated. Also, based on the relative distribution of MPF's across the three motion conditions, some indication of a change in pilot control behaviour as a result of motion cues (or lack thereof) was exposed. Finally, subjective pilot ratings suggest that the motion cueing condition optimized for the pitch subsidence mode is preferred, even though this is not the dominant mode in the vehicle's response. The condition corresponding to the heave subsidence mode (i.e., only vertical motion cues) is appreciated least
Towards a European helicopter noise calculation method
Get PDFHelicopter noise is strongly dependent on flight conditions, exhibiting in addition a pronounced directivity, complicating noise modelling. In land-use planning, the current best practice stems from fixed-wing aircraft and follows a Noise Power Distance approach that is unsuitable to include these features. The European Commission commissioned the development a novel helicopter noise model to be eventually part of a public "European Environmental Model Suite for Aviation". The model embodies a helicopter noise calculation method based on the current state-of-the-art. A clustering strategy has been used to represent the European helicopter fleet, thus avoiding the need for performing noise measurements on all types of helicopters. The method uses an empirical source model, with noise hemispheres to faithfully describe the noise directivity pattern. Emission characteristics of a helicopter type are described by a set of hemispheres measured for a range of conditions within the flight envelope. Atmospheric propagation effects are accounted for to evaluate the noise hindrance experienced on-ground. The latter is based on established public models for atmospheric propagation, ground reflection and surface impedance
The elevated helipads - Study of wind and rotor wash influence for most common configuration types
No full textProblem of lack of possible places to build new buildings is well known in modern cities. When it concerns helipads, which need large area to be placed and also it have to be carefully checked, how surroundings is influencing on this new construction, the case is even more complicated, because those sites have to fulfil demanding regulations. For hospital helipads it is necessary to have possible quickest way from helicopter to surgery. When area is limited, it is usually necessary to place such construction on a building and such helipad is then called elevated. However no document can provide a strict information, how to place new helipad in its surrounding - only general data is available. Too many factors have to be considered. This is why always a detailed analysis is needed in order to be sure, that flight operations can be done safely. This paper presents the work flow concerning this topic, from regulations to fulfil to results of analysis. Some aspects of different locations and its influence on elevated helipads are discussed. Also details about performing the analysis are presented
Qualification and certification of Special Patrol Insertion & Extraction (SPIE) equipment for military helicopters
Get PDFSpecial Patrol Insertion & Extraction (SPIE) is a type of military operation to rapidly insert personnel in and/or extract personnel from areas where it is not possible to land with a helicopter. The equipment used to perform SPIE operations usually must comply with specifications to ensure adequate performance. Additionally it needs to meet certain airworthiness criteria in order to be certified by the military airworthiness authority. Requirements 27.865 and 29.865 of the civil airworthiness codes provide a useful baseline regarding external loads, which can be adapted to take into account the military operation and environment. This paper discusses the development of the requirements of a qualification and certification programme for a SPIE system to be used on a helicopter. The tests to show compliance with these requirements, both at component level and at system level are discussed as well
AH-64 loss of lubrication study: test of isotropic superfinished AH-64 (Apache) engine nose gearbox without black oxide coating
Get PDFRotorcraft gearboxes are designed to the utmost precision to withstand the tremendous demand and loads required to convert thousands of horsepower and RPM into hundreds of RPM and lift torque. The transmission components and the lubricants utilized incorporate various technologies to enable proper function and longevity. Many of these technologies are "tried and true" over the course of hundreds of thousands of flight hours and decades of real flight experience. As a result, replacement of these legacy technologies requires a tremendous amount of testing and qualification. The US Dept. of Defense conducted a loss of lubrication test of an AH-64 Engine Nose Gearbox (ENGB) with Isotropic Superfinished (ISF) gears instead of the legacy black oxide coated gears. The question addressed in this paper is; will the low Ra value generated by Isotropic Superfinishing lead to scuffing, especially if the Black Oxide Coating is eliminated, and to go deeper, will the gearbox survive the mandatory loss of lubrication test requirements, which is a punishing test designed to simulate a realistic failure scenario within the gearbox. Success for this test is demonstrated by the ability of one gearbox to continue to transmit torque for 60 minutes after loss of lubrication, or of two gearboxes to continue to transmit torque for at least 30 minutes each after loss of lubrication. Upon successful completion of the loss of lubrication test, the gearbox was subjected to a detailed tear-down analysis which indicated no discernable damage occurred to the gear teeth