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Tiltrotor whirl-flutter stability investigation using Lyapunov Characteristic Exponents and multibody dynamics
No full textThis work discusses the use of Lyapunov Characteristic Exponents to assess the stability of nonlinear, time-dependent mechanical systems. Specific attention is dedicated to methods capable of estimating the largest exponent without requiring the Jacobian matrix of the problem, which can be applied to time histories resulting from existing multibody solvers. Tiltrotor whirl-flutter stability is analyzed. With respect to the available literature, the proposed method does not require the system to be strictly periodic, no linearization is required about a reference steady solution, and characteristic nonlinear aspects of stationary solutions like limit cycle oscillations are correctly identified and pointed out. A limitation lies in the ability to correctly identify the stability but no information is inferred about the related characteristic frequencies/periods, if any
Wind tunnel test of single-rotor lift-offset due to differential flaps
No full textA single-rotor lift-offset system for a winged compound helicopter is evaluated at JAXA 2m x 2m low-speed wind tunnel in the closed test section using a flyable radio-controlled model rotorcraft. The single-rotor lift-offset is achieved utilizing flaps originally designed for download reduction during hovering flight on a winged compound helicopter. Differential flaps on the left and right wings create a rolling moment on the wing-body to produce a lift-offset state of the rotor while the overall rotorcraft rolling moment is balanced. Two types of rotor blades, which are the optimized rotor blade for high advance-ratio flight and the reference UH-60A rotor blade, were tested to compare the effect of lift-offset on the overall aerodynamic performance. The tests were conducted at advance ratios of 0.3, 0.5, and 0.7. The test results show that the single-rotor lift-offset improves the overall effective lift-to-drag ratio as expected. Furthermore, the adverse yaw effect due to the differential flaps is observed from the test results of isolated wing-body
Design and testing of an active vibration absorber for a helicopter rotor
No full textThis article presents the design and the experimental tests on a prototype of an active vibration absorber for helicopter rotors. The main disturbance forces are those transferred by the blades to the rotor, which are harmonic with a frequency multiple of the rotor angular speed times the number of blades, and the device aims to counteract the two main frequencies of the disturbance. This device is composed of two identical subsystems, one for each frequency, which are put in rotation by a mechanical transmission connected to the rotor hub guaranteeing that the generated force always has the correct frequency for any angular velocity of the rotor. On each subsystem, there are two eccentric masses actuated by electric motors, which are used to modulate the amplitude and the phase of the generated force. The control algorithm of the device is based on the EPLL harmonic tracking to identify the amplitude and the phase of the disturbance, and the positioning algorithm allows to place the masses in the correct position avoiding collisions between them. The device has been built and tested on a dedicated test bench, measuring the generated forces. The results of the experiments are promising since the eccentric masses follow the reference positions, indicating that the device would significantly suppress the vibrations
Comprehensive simulation of a complete tiltrotor with pilot-in-the-loop for whirl-flutter stability analysis
No full textThis work presents the complete aero-servo-elastic model Bell XV-15 tilt-rotor equipped with Advanced Technology Blades (ATB). Multibody and aerodynamic modeling of each subcomponent, using the open-source software MBDyn and DUST, is illustrated and validated considering experimental and numerical results. The design of optimal longitudinal control of the tilt-rotor is presented and validated. Finally, the detailed biomechanical pilot model is coupled with the aeroelastic tiltrotor model. To evaluate the capability of the model to evaluate the aeroelastic stability a is evaluated through a frequency sweep excitation of the model during a time-marching simulation, and the principal airframe modes are identified through the Matlab system identification. This model opens a wide spectrum of different analyses that could be performed, for example, this model will be use to study passengers’ comfort, performances in transient maneuvers such as conversion and pull-up, aeroelastic stability, pilot-induced-oscillation phenomena. Due to the modularity and parameterization of the model, other innovative VTOL configurations could be studied with the purposed approach
L1 Adaptive speed control for a helicopter
Get PDFAdaptive control theory is used to improve robustness of systems against model uncertainties and disturbances. Two of the main drawbacks of adaptive controllers is the lack of criteria to measure the robustness of the controller and the speed of the adaptation loop. In L1 adaptive control theory both problems are solved by the introduction of a low pass filter in the control input. The L1 adaptive control robustness and stability are guaranteed by numerical criteria. L1 adaptive control raised a great interest in the aerospace field due to the presence of nonlinear and uncertain dynamics that affects aircraft flight performances. Such strategy by the way has not been widely used and explored for rotorcrafts. In this article the authors propose an L1 output feedback speed controller for helicopters. The control system architecture is tested by means of a simulation test campaign through the use of a nonlinear helicopter model. Both performances and robustness of the controller are evaluated, even in presence of disturbance
Technology concept of an automated system for integration testing
Get PDFLooking at the trend seen in the aerospace for Verification & Validation (V&V) process, the future of integration testing will move to a more and more automated process thanks to the use of Artificial Intelligence (AI) and robotics. For the integration testing of an avionic system many different scenarios need to be executed and validated by the user running the test. Focusing on the state of the art of the integration testing, there are some issues and inefficiencies while executing the tests. Firstly, the duration of the test sets of safety-critical systems is long and involves very repetitive tasks for the operator. During this time, the system is under test and the laboratory cannot be used for other purposes which decreases the asset’s availability for other stakeholders. Moreover, with distributed offices, the on-site testing could be limitating when the execution of test procedures requires at least one person physically in the laboratory. In order to mitigate these issues ARTO (Automated Robotics for Testing Optimization) has been designed. ARTO is an automated testing system with the capability of executing functional tests that up until now are being performed by test engineers, operators or pilots. It has the objective to automate the repetitive procedures in which human interactions are required, to increase the efficiency, reduce costs and make work remotely from different locations possible. The test sequence is implemented through a dedicated HMI (human-machine interface) and executed by an automated framework, fully able to carry out the tasks needed for each sequence. The system itself consists of four main subsystems: robotics, image & audio processing, framework and user interface. ARTO is designed to be applied in the integration testing field, designed to interact, and operate with the existing testing environment of the Next-Gen Civil Tiltrotor-Technology Demonstrator. It is intended to be installed inside the Simulation and Integration Laboratory cockpit where it can operate on the displays, the keyboards, the knobs and the levers. With the video and audio feedback, it can execute the test procedures that were programmed beforehand and collect test results automatically
Design methodology of urban air mobility for noise mitigation at conceptual design stage
Get PDFThis article proposes a design methodology for UAM vehicles for noise mitigation at a conceptual design stage. A rotor analysis module for accurate noise prediction and a noise prediction module for a conceptual design stage are constructed. The rotor analysis module is developed for accurately predicting aerodynamic force distribution around rotor blades, thus enabling accurate noise prediction. This module consists of rotor analysis using BEMT or BET methods and CAMRAD II, and these methods exchange force distribution data, which is iterated in the module until aerodynamic force distribution by each method converges. The noise prediction module for a conceptual design stage uses compact loading assumption and dual compact loading assumption, therefore loading and thickness noise prediction are completed in a short time. The proposed design methodology is applied to a conceptual design of an eVTOL aircraft with the lift+cruise concept. Through the design optimization process, the necessity of considering noise impact at the conceptual design stage is demonstrated
Immersive maintenance review in customer configuration integrated into the support & services 3D production chain with a user-centered approach: Application to maintenance tooling review
Get PDFValidation of the installation of maintenance tools on aircraft is often difficult, due to the downtime of the aircraft and prototypes that are not representative of the customer's configuration. The objective of the study is to exploit the real-virtual coexistence specific to mixed reality, which is integrated here into the digital production chain of support & services to collect real data on the customer configuration in order to optimize the operational performance of maintenance by freeing itself from the removal of physical parts and the maturity of real aircraft
A simplified model for evaluating eVTOL conceptual designs and with example results for three types of eVTOL aircraft configurations
Get PDFeVTOL Performance Analysis Tool, known as ePERF is a MATLAB-based analysis developed at NTU specifically for evaluating the performance of different types of eVTOL aircraft and is ideal for eVTOL aircraft conceptual and preliminary design phases. The framework is built to have a modular nature allowing for rapid trade studies of different sizing parameters, and it provides a representative performance evaluation of the eVTOL in all phases of flight. ePERF consists of three main modules: a rotary wing module, a drag estimation module for forward flight and an energy module for range calculation. Momentum theory is used for hover calculations. A component level drag build-up method is used for cruise calculations. The energy model considers the input battery parameters and results from the other two modules to compute the energy requirements of the eVTOL based on a given mission profile. The outputs of ePERF include the power and energy requirements for each flight phase, an estimated attainable range, as well as the velocity for best range. In this paper, the ePERF is explained and then it is used to estimate the performance for three different example eVTOL aircraft: (A) a Lift + Cruise type like Beta Technologies - Alia, (B) a tiltrotor type like Joby Aviation - S4, and (C) a fixed Lift + Tiltrotor type like Vertical Aerospace - VX4. The results show that reducing the structural weight of the eVTOL aircraft is one of the key factors in achieving best performance
Algorithm for identification of helicopter open-loop transfer functions and reduced-order modeling
No full textThis paper deals with the development and application of an algorithm aimed at the identification of openloop transfer functions from closed-loop data. Particularly suited for helicopter applications, it allows the identification of the machine transfer functions even in the presence of a controller. The identification algorithm is based on the knowledge of the time histories of arbitrary external inputs and corresponding control actuation and responses. It is successfully applied to the AW-09 helicopter for the identification of the transfer functions relating pilot inputs to vehicle attitudes and kinematics. Then, from their rational approximation the time domain representation of the helicopter dynamics is achieved. The capability of the final state-space helicopter model to simulate vehicle dynamics is assessed by comparing the provided helicopter time responses to arbitrary inputs, with those obtained through the high-fidelity nonlinear solver formerly used to obtain the database for the identification of the transfer function