Flight dynamics modeling and analysis for a Mars helicopter

This study develops a hybrid solver with reversed overset assembly technology (ROAT), a viscous vortex particle method (VVPM), and a CFD program based on the URNS method, in order to study the aerodynamic and acoustic characteristics of coaxial rigid rotors. The aerodynamic load of the “AH-1G” helicopter rotor is first calculated based on the hybrid method and compared with available experimental data. The prediction of the linear noise of the OLS rotor is then performed and the obtained results are compared with available experimental data. These results allow the evaluation of the accuracy of the hybrid method for emulating rotor aerodynamics and acoustics. Afterwards, the hybrid and CFD methods are applied to obtain the aerodynamic and acoustic characteristics of the given coaxial rigid rotor model, while taking into account the trim of the collective pitch. The obtained results demonstrate that the hybrid method has high proficiency in capturing blade–vortex-interaction impulsive loads and high computational efficiency in predicting associated loading noise characteristics. Furthermore, the effect of the hybrid method on the noise characteristics of coaxial rigid rotors under a different advance ratio, blade tip speed, shaft angle, and other conditions, as well as the impact of the upper and lower rotors on the noise contribution of the coaxial rotor are analyzed. Finally, the impacts of the initial phase and the vertical spacing on the sound pressure level are studied.

Due to the distinct atmospheric conditions, there are notable differences in the rotor aerodynamic characteristics between the Mars helicopters and conventional Earth helicopters. Determining the flight dynamics characteristics of the Mars helicopter presents considerable challenges. This study focuses on a coaxial helicopter as the research subject, specifically analyzing the flapping characteristics of rotor blades in the unique Martian atmospheric environment. The unsteady aerodynamic loads and wake effects on the rotor are calculated utilizing the viscous vortex particle method. Furthermore, the aerodynamic interference of the coaxial dual-rotor system is analyzed across a range of forward flight speeds. Subsequently, a novel and efficient trimming method based on a combination of the viscous vortex particle method and dynamic inflow model for Mars helicopters is proposed to enhance the calculation accuracy and computational efficiency. Moreover, a relaxation factor is introduced to modify the induced velocities at various positions on the rotor disk. Finally, a comprehensive flight dynamics analysis is carried out for the Mars coaxial helicopter. And the results indicate that during the forward flight of the Mars helicopter, the rotors generate noticeable aerodynamic disturbances and there is evident coupling among control channels, with Dutch roll mode and velocity-attitude stability exhibiting dynamic instability as flight speed increases, posing challenges for the design of the flight control system.

Aerodynamic characteristics of rigid coaxial rotor by wind tunnel test and numerical calculation

An integrated modelling approach, suitable for flight loads analysis and flight dynamics investigations is presented. These scenarios need fast loop capable simulation models. Typically, they are evaluated many times in the entire flight envelope, for varying mass cases and for changing parameters during the aircraft design process. Further, the simulations models have very different requirements depending on their application. Flight dynamics models must account for large nonlinear rigid Body motions and aerodynamic nonlinearities, manoeuvre loads analysis, additionally need to take flexible deformations into account, and hence require distributed aerodynamics, and for gust loads Analysis unsteady aerodynamic effects are essential. The proposed integrated model is based on nonlinear equations of motion using mean axes constraints. Potential flow based aerodynamics with low computational cost are the method of choice for most aeroelastic applications. Presented in this paper are some important aspects regarding the aerodynamic modelling, which are handled differently compared to other implementations. Instead of the Doublet Lattice Method (DLM), the 3D panel method NEWPAN is used, which is able to capture previously neglected flight mechanical effects such as roll-yaw coupling and induced drag. Since, the Aerodynamic Influence Coefficient (AIC) matrices obtained from NEWPAN are nonlinearly dependent on the flight state, a Reduced Order Model (ROM) based on Proper Orthogonal Decomposition (POD) is setup and subsequently interpolated. The proposed ROM interpolates directly AIC matrices (AIC-ROM) instead of pressures. The advantages of a ROM based on AIC matrices over similar approaches based on pressure coefficients are explained. Similarly to the Doublet Lattice Method, the 3D panel code is able to compute frequency based unsteady Aerodynamic Influence Coefficient (AIC) matrices, which are approximated by a Rational Functions (RFA) to make them amenable for time domain simulations. Unlike other approaches, the physical RFA allows a clear separation of quasi-steady, added mass and unsteady lag terms. Several application examples are presented and their particular needs are addressed. The proposed modelling approach supports flight dynamics by including previously neglected aerodynamic roll yaw coupling and nonlinear angle of attack dependencies, manoeuvre loads, including aeroelastic deformations, and gust loads analyses, including unsteady aerodynamics in one integrated model and is suitable for for investigations regarding active gust and manoeuvre load alleviation, primary flight control design, or performance improvement through lift redistribution.

The performance and inflow characteristics of representative coaxial rotor and tandem configurations have been studied. Two different solvers were used. The first approach is the well-established RotCFD solver that uses a wake capturing approach on a Cartesian coordinate system, with radially distributed rotating body forces representing the forces exerted by the rotor blades on the fluid. The second approach, called GT-Hybrid, uses Navier-Stokes analyses over small regions surrounding the rotor blades, and a free wake model outside the Navier-Stokes domains. There are significant differences in the way the flow was modeled with these two solvers. Nevertheless, the two solvers gave consistent, physically realistic results for representative coaxial and tandem rotor configurations. The inflow velocity field was examined, and the first three terms in the dynamic inflow (uniform component, and the lateral and longitudinal variations) were extracted. Comparisons with a viscous vortex particle method and the dynamic inflow theory was done for the coaxial rotor in forward flight and good agreement was observed. Significant interference of the upper rotor downwash over the lower rotor was captured and quantified for this coaxial rotor. For the tandem rotor configuration, the effects of overlap on the inflow was captured. In addition, the longitudinal variation of the inflow was properly captured with both solvers.

This study investigates the performance and blade airloads for a rigid coaxial rotor of high-speed compound unmanned rotorcrafts. The present compound unmanned rotorcraft uses not only a rigid coaxial rotor, but also wings and propellers for high-speed flights. For the rigid coaxial rotor in this work, CAMRAD II, a rotorcraft comprehensive analysis code, is used to study the performance at a flight speed of up to 250 knots and blade section lift forces at 230 knots. As the flight speed increases, the rotor power decreases; however, the power of propellers increases to overcome the drag force of a rotorcraft in high-speed flight. The effective lift-to-drag ratio of a rotor has the maximum value of about 11.6 which is much higher than the value of the conventional helicopter. The blade section lift forces of the upper and lower rotors at 230 knots show the similar variation trends for one rotor revolution, and the impulses because of the aerodynamic interaction between both rotors are observed. Keywords: 강체 ë™ì¶•ë°˜ì „ 로터, 복합형 무인 íšŒì „ìµê¸°, 성능, ë¸”ë ˆì´ë“œ ê³µë ¥ 하중 Keywords: Rigid Coaxial Rotor, Lift-Offset, Compound Unmanned Rotorcraft, Performance, Blade Airloads

Aircraft flight dynamics trim is the basis for the large civil aircraft flight dynamics characteristics analysis. In this paper, the full nonlinear flight dynamics model of civil aircraft is systematically deduced, an iterative trim method for the six-degree-of-freedom flight dynamics model based on the Downhill Simplex optimization algorithm is designed, and the trim accuracy and computational efficiency of the Downhill Simplex optimization algorithm are analyzed and the variation of trim variables under different initial weights is studied. Results show that the Downhill Simplex optimization algorithm can effectively solve the six-degree-of-freedom full nonlinear aerodynamics trim problem of large civil aircraft under the condition of straight forward and level flight, and the trim deviation can be controlled within the range of ±0.05N/Nm within 500 steps. The flight dynamics iterative trim method based on the Downhill Simplex optimization algorithm designed in this paper can provide reference and technical support for civil aircraft flight simulation, flight dynamics characteristics and flight quality research.

This paper presents numerical calculations, on coaxial rotor systems, that show the variations of rotor thrust share with lift offset, system thrust, and advance ratio. The calculations are based on a coupled analysis between the U. S. Army's Rotorcraft Comprehensive Analysis System (RCAS) and a viscous vortex particle method. The level of agreement between calculations and experimental data is good in hover and mixed in edgewise flight. In hover and zero lift offset–a condition in which the upper rotor typically produces more thrust than the lower rotor–changing system thrust does not significantly alter the rotor thrust share. For advance ratios up to approximately 0.25, independently increasing lift offset increases the lower rotor thrust; this is an aeroelastic phenomenon that is eliminated with rigid blades. Independently increasing advance ratio leads to three distinct regions with different thrust sharing behaviors; these behaviors are governed by changes to the longitudinal skew of the upper rotor wake and its proximity to the lower rotor.

This paper proposes a two-level, data-driven, digital twin concept for the autonomous landing of aircraft, under some assumptions. It features a digital twin instance (DTI) for model predictive control (MPC); and an innovative, real-time, digital twin prototype for fluid-structure interaction and flight dynamics to inform it. The latter digital twin is based on the linearization about a pre-designed glideslope trajectory of a high-fidelity, viscous, nonlinear computational model for flight dynamics; and its projection onto a low-dimensional approximation subspace to achieve real-time performance, while maintaining accuracy. Its main purpose is to predict in realtime, during flight, the state of an aircraft and the aerodynamic forces and moments acting on it. Unlike static lookup tables or regression-based surrogate models based on steady-state wind tunnel data, the aforementioned real-time digital twin prototype allows the DTI for MPC to be informed by a truly dynamic flight model, rather than a less accurate set of steady-state aerodynamic force and moment data points. The paper describes in detail the construction of the proposed two-level digital twin concept and its verification by numerical simulation. It also reports on its preliminary flight validation in autonomous mode for an off-the-shelf unmanned aerial vehicle instrumented at Stanford University. This article is part of the theme issue 'Data-driven prediction in dynamical systems'.

Motivated by the problem of airplane modeling and simulation, this work presents a complete process of building a low cost, high fidelity simulation platform for airplane motion starting from scratch. It includes modeling, simulation and flight-testing for model validation. The six DoF nonlinear simulation model of a small commercial off-theshelf radio-controlled fixed wing airplane is presented. It includes experimental measurements of geometric and inertia model, wind tunnel testing and modeling of the propulsion system, Datcom estimation of the aerodynamic model, and experimental identification of control surfaces actuator model. Whenever possible, some of these models are validated either experimentally or by comparison with available literatures. The proposed procedure to build the airplane model is used to simulate the flight dynamics of the studied airplane. Then, flight-testing of the studied airplane is performed and used to validate the longitudinal motion of the developed flight simulation model. The proposed technique was found to be robust for modeling and simulation of the airplane motion. It also gives very promising results to test the flight dynamics of the airplane and design a reliable flight control system.

Tailless Unmanned Aerial Vehicles (UAVs) offer several advantages over their conventional counterparts, including enhanced maneuverability, higher durability, and better stealth. However, they are challenging in their stability and control due to the absence of stabilizing and control surfaces that typically exist in conventional empennage. A crucial process for analyzing the tailless UAVs’ stability/performance characteristics and determining/validating their flight control parameters is done via the modeling and simulation of flight dynamics. This paper presents a comprehensive and systematic procedure for investigating the flight dynamics of a tailless UAV, including modeling, simulation, analytical verification, and flight testing, while also explaining the interconnections among these elements. It also addresses the common challenge of limited accessibility of UAV essential data through using diverse analytical, empirical, and experimental methods. First, a rapid and effective first-principles modeling approach is introduced to simulate the nonlinear six-degree-of-freedom flight dynamics of a small tailless UAV case study. The modeling process follows a modular framework where well-defined experiments and commercial of-the-shelf software, tools, and sensors are employed to build the necessary sub-models, including geometric, mass–inertia, aerodynamic, propulsion, and actuator models. Then, all sub-models are integrated into a simulation environment to allow the prediction of the UAV dynamic response obtained from the given control inputs. The developed flight dynamic model is subjected to a thorough verification process to ensure its integrity and proper functionality by comparing the simulated trim and natural flight modes with the calculated analytical results. Finally, a set of specific flight tests are performed to validate the developed simulation model and verify relevant performance characteristics for the case-study UAV. The results show that the proposed approach provides a systematic and straightforward method for examining the flight dynamics of small tailless UAVs with reasonable accuracy.

A linear parameter varying (LPV) flight dynamics model (FDM) is proposed to cater for atmospheric disturbance analysis in special flight conditions. A novel FDM which is capable of addressing the influence of turbulent wind is derived under the wind frame. An affine parameter dependent LPV model with wind effects is built based on function substitution method. The optimal solution for the decomposing function of the LPV FDM is obtained by genetic algorithm (GA). The analysis of dynamic response indicates that the genetic-optimized LPV FDM approximates the nonlinear FDM evidently, since it identifies the instantaneous dynamics and flight states varying in a wide range. The simulations of approach and landing against wind shear show that the genetic-optimized LPV FDM captures the instantaneous dynamic response when flying through turbulent wind, indicating that the LPV model can be further applied to turbulent wind special flight analysis and control law design.

A numerical simulation method of computational fluid dynamics (CFD) coupling with a trim analysis for coaxial rotor systems is described in this paper. The trim analysis is implemented using a rotorcraft flow solver, rFlow3D. Six target forces and moments, which are the thrust of the coaxial rotor system, the rolling and pitching moments for each upper and lower rotor, and the torque balance for yaw control, are considered as the trim conditions. The blade pitch angles of both upper and lower rotors are adjusted to satisfy the target trim conditions through the trim analysis by being loosely coupled with the CFD solver. Verification of the trim analysis method and validation of the prediction accuracy of aerodynamic performance are performed based on previous experimental and numerical studies in hover and forward flight using the lift-offset conditions. It is shown that the predicted hover performances of the torque-balanced coaxial rotors are in excellent agreement with the experimental data. It is also verified that the lift-offset conditions in forward flight are simulated using this established trim analysis. Furthermore, reasonable agreements with other computational results are indicated.

The lightweight structures and unconventional configurations being considered for the next generation of aircraft mean that any effort to predict or control the flight dynamics is impacted by the structural dynamics. One of the most severe forms of coupling between aeroelasticity and flight dynamics is an instability called body freedom flutter. The existing tools often assume a relatively weak effect of structural dynamics on the flight dynamics, and are therefore incapable of modeling strong interactions like body freedom flutter. A method of combining different sources of data traditionally used for aeroelasticity and flight dynamics is described by reconciling many of the differences between these models. By building upon past modeling efforts, a level of familiarity in the approach is achieved. Generally the differences from the traditional approaches are subtle but significant. The traditional frequency domain flutter model in a modal coordinate system is converted to a form consistent with a time domain flight dynamics model. The time domain rational function approximation about a non-inertial coordinate system and the unique constraints for the conversion between the inertial and non-inertial coordinate systems are discussed. A consistent transformation of the states of aeroelastic models to flight dynamics models is derived, which enables the integration of data from higher fidelity computational fluid dynamics models or wind-tunnel testing. The present method of integrating multidisciplinary data was used to create models that compare well with X-56A flight-test data, including conditions past the flutter speed.

Based on CFD method and FW-Hpds equations, a numerical method for predicting aeroacoustic characteristics of coaxial rotor is established. In order to accurately simulate the flowfield of coaxial rotor, several techniques including moving-embedded grids, implicit LU-SGS scheme, ROE-MUSCL scheme and Spalart-Allmaras turbulence model are adopted to solve the RANS equations. The aeroacoustic characteristics of coaxial rotor in forward flight are calculated and analyzed by using FW-Hpds equations. The calculated acoustic characteristics of the AH-1/OLS model in forward flight for validation are in good agreements with the experimental data. The acoustic characteristics of coaxial-rotor in forward flight including the sound pressure time history and directivity patterns are investigated in detail. Some meaningful conclusions are obtained from the calculated results. The counter-rotation of upper and lower rotors has significant effects on the sound pressure time history and leads to different phases of peak value at different observation points. The noise radiation intensity of coaxial rotor in forward flight exhibits an approximately symmetrical distribution on both sides of the forward flight direction, which is one of the important differences between the coaxial rotor and the single rotor.