This work investigates the effect of aerodynamic interference in the coupled nonlinear aeroelasticity and flight mechanics of flexible lightweight aircraft at low speeds. For that purpose, a geometrically exact composite-beam formulation is used to model the vehicle flexible-body dynamics by means of an intuitive and easily linearizable representation based on the displacement and Cartesian rotation vectors. The aerodynamics are modeled using the unsteady vortex-lattice method, which captures the instantaneous shape of the lifting surfaces and the free inviscid wake, including large deformations and interference effects. This results in a framework for simulation of high aspect ratio planes that provides a medium-fidelity representation of flexible-aircraft dynamics with a modest computational cost. Previous independent studies on the structural-dynamics and aerodynamics modules are complemented here with the integrated simulation methodology, including vehicle trim, and linear and nonlinear time-domain solutions. A numerical investigation is next presented on a simple wing-fuselage-tail configuration, assessing the interference effects between wing wake and horizontal tail, and the downwash due to the proximity of the wake is shown to play a significant role in the longitudinal dynamics of the vehicle. Finally, a brief discussion of direct wake-tail encounters is included to show the limitations of the approach.

Assessment of Wake-Tail Interference Effects on the Dynamics of Flexible Aircraft

Technological advances in areas such as battery technology, autonomous control, and ride-hailing services, combined with the scale-free nature of electric motors, have sparked significant interest ...

Tilt-Wing eVTOL Takeoff Trajectory Optimization

This paper focuses on full six degree-of-freedom aerodynamic modeling of small unmanned aerial vehicles at high angles of attack and high sideslip in maneuvers performed using large control surfaces at large deflections for aircraft with high thrust-to-weight ratios. Configurations, such as this, include many of the currently available propeller-driven radio-controlled model airplanes that have control surfaces as large as 50% chord, deflections as high as 50 deg, and thrust-to-weight ratios near 2∶1. Airplanes with these capabilities are extremely maneuverable and aerobatic, and modeling their aerodynamic behavior requires new thinking because using traditional stability-derivative methods is not practical with highly nonlinear aerodynamic behavior and coupling in the presence of high prop-wash effects. The method described outlines a component-based approach capable of modeling these extremely maneuverable small unmanned aerial vehicles in a full six degree-of-freedom real-time environment over the full...

Real-Time Flight Simulation of Highly Maneuverable Unmanned Aerial Vehicles

https://cyberleninka.org/article/n/1319427.pdf

Unsteady aerodynamic modeling at high angles of attack using support vector machines

The ability to accurately predict both static and dynamic stability characteristics of air
vehicles using computational fluid dynamics (CFD) methods could revolutionize the air
vehicle design process, especially for military air vehicles. A validated CFD capability would
significantly reduce the number of ground tests required to verify vehicle concepts and, in
general, could eliminate costly vehicle ‘repair’ campaigns required to fix performance
anomalies that were not adequately predicted prior to full-scale vehicle development. This
paper outlines the extended integrated experimental and numerical approach to assess the
of stability and control prediction method capabilities as well as the design and estimation
the control device effectiveness for highly swept low observable UCAV configurations. The
aim of the AVT-201 Task Group is to provide an assessment of the CFD capabilities using
model scale experiments and transferring this knowledge to real scale applications

The NATO STO Task Group AVT-201 on Extended Assessment of Stability and Control Prediction Methods for NATO Air Vehicles

This paper presents selected results from an ongoing effort to develop an aerodynamic database from Reynolds-Averaged Navier-Stokes (RANS) computational analysis of airfoils and wings at stall and post-stall angles of attack. The data obtained from this effort will be used for validation and refinement of a low-order post-stall prediction method developed at NCSU, and to fill existing gaps in high angle of attack data in the literature. Such data could have potential applications in post-stall flight dynamics, helicopter aerodynamics and wind turbine aerodynamics. An overview of the NASA TetrUSS CFD package used for the RANS computational approach is presented. Detailed results for three airfoils are presented to compare their stall and post-stall behavior. The results for finite wings at stall and post-stall conditions focus on the effects of taper-ratio and sweep angle, with particular attention to whether the sectional flows can be approximated using two-dimensional flow over a stalled airfoil. While this approximation seems reasonable for unswept wings even at post-stall conditions, significant spanwise flow on stalled swept wings preclude the use of two-dimensional data to model sectional flows on swept wings. Thus, further effort is needed in low-order aerodynamic modeling of swept wings at stalled conditions.

/pdf/a-cfd-database-for-airfoils-and-wings-at-post-stall-angles-2u0mv6ct78.pdf

A CFD Database for Airfoils and Wings at Post-Stall Angles of Attack

This paper summarizes the status of ongoing NASA research supported over the past eight years to advance computational capabilities for modeling civil aircraft loss of control due to airframe damage or wing stall The research is motivated by a desire to exploit the capabilities of computational methods to create augmented flight simulation models that improve pilot training for such loss-of-control scenarios Flight of aircraft with either airframe damage or operating near and beyond the stall boundary encounters additional nonlinear aerodynamic influences on stability and control from dynamic motions that, if not included in flight simulation models, may lead to incorrect pilot responses In the present work, both low- and high-fidelity computational methods are explored for analyzing such nonlinearities The challenge of creating nonlinear reduced-order models from high-fidelity computational data is also addressed At the beginning, few guidelines were available for computing or modeling the dynamic s

Computational Aerodynamic Modeling Tools for Aircraft Loss of Control

SUMMARY
Nonlinear aerodynamics of wings may be evaluated using an iterative decambering approach. In this approach, the effect of flow separation due to stall at any wing section is modeled as an effective reduction in section camber. The approach uses a wing analysis method for potential-flow calculations and viscous airfoil lift curves for the sections as input. The calculation procedure is implemented using a Newton–Raphson iteration to simultaneously satisfy the boundary condition, which comes from potential-flow wing theory, and drive the sectional operating points toward their respective viscous lift curves, as required for convergence. Of particular interest in this research is the calculation of the residuals during the Newton iteration. Unlike a typical implementation of the Newton iteration, the residual calculation is not performed via a straightforward function evaluation, but rather by estimating the target operating points on the input viscous lift curves. Estimation of these target operating points depends on the assumptions made in the cross-coupling of the decambering at the different sections. This paper presents four residual calculation schemes for the decambering approach. The residual calculation schemes are compared against each other to assess computational speed and robustness. Decambering results are also compared with higher-order computational fluid dynamics (CFD) solutions for rectangular and swept wings. Results from the best scheme compare well with the CFD solutions for the rectangular wing, motivating further development of the method. Poor predictions for the swept wings are traced to spanwise propagation of separated flow at stall, highlighting the limitations of the current approach. Copyright © 2014 John Wiley & Sons, Ltd.

Iteration schemes for rapid post-stall aerodynamic prediction of wings using a decambering approach

This paper presents a methodology for flight dynamics simulation inclu ding three-dimensional unsteady and post-stall aerodynamics. This is accomplished by integrating a decambering viscous correction into a linearized unsteady vortex lattice method. Coupling the aerodynamic model with the nonlinear rigid-aircraft equations of motion results in a low-order, medium-fidelity framework for flight dynamics simulation. The numerical studies first evaluate the im portance of unsteady aerodynamic effects on the dominant aircraft modes, illustrating the errors incurred on the prediction of the short period when quasi-steady approximations are employed. Next, the combined impact of unsteady and post-stall aerodynamics is assessed on a maneuvering aircraft. Furthermore, the framework is expected to be a suitable design-oriented tool for flight con trol synthesis and to incorporate aeroelastic modeling. I. Introduction High fidelity aerodynamic models implemented in the form of l ook-up tables [1‐3] are commonplace in certified flight simulators and training devices. Force and moment inf ormation for these look-up tables is commonly developed using some combination of the following techniques: Computational Fluid Dynamics (CFD), experimental testing of models in wind tunnels (static and dynamic), and/or experimental flight testing. Both CFD methods and data from experiments are capable of representing forces and moments in the nominal flight regime, where aerodynamics are expected to be linear, and beyond this range where significan t amounts of flow separation exist (aerodynamic stall). The primary disadvantage of using these high-fidelity repre sentations of forces and moments is the significant effort required to develop the look-up table. Each expected operating condition, defined not only by the aerodynamic inflow angles but also three components of angular rates, derivatives of these rates, and other explanatory variables as desir ed, must be run as a CFD or experimental test case to develop the look up table. It is often not feasible to perform such an extensive study due to limited resources, or because the required level of detail is simply not available at early conceptual design stages. The methodology presented in this research represents a departure from the high-fidelity approach discussed above. A medium fidelity aerodynamics model is considered, based on an unsteady vortex lattice method (UVLM) [4, 5] with a post-stall model based on iterative decambering [6, 7]. The UVLM is a fully unsteady, three-dimensional aerodynamic method, which is very accurate as long as potentialflow conditions are satisfied [ 8] ‐ in practice this requires low speed, attached flow. Howeve r, with increasing angles of attack, the boundary layer on the upper surface of a wing thickens and finally separates. If separation occurs, then potential-flow predictions deviate from the real viscous flo w. The underlying idea of the decambering methodology is that this mismatch due to the boundary-layer displacement thickness and separation can be related to an effective change in the chordwise camber. In other words, the goal is to match the potential-flow solution to the viscous one by introducing a decambering variable, which is effectively a camber correction ‐ it can also be seen as a rotation of the airfoil, modifying the effective angle of incidence to fit vi scous data. This idea has been around for several decades, but most recent progress can be found in Refs. [6, 9‐12]. While the scheme relies on a 2D airfoil data, 3D effects can be incorporated by accounting for the aerodynamic interference among all lifting-surface airfoils. This can be easily done on the UVLM, for instance, when enforcing the boundary conditions. A strip-theory philosophy for engineeringlevel predictions of wing aerodynamics at high angles of attack has been extensively adopted before [13‐17], leading

Unsteady and Post-Stall Aerodynamic Modeling for Flight Dynamics Simulation

A low-order prediction method for post-stall aerodynamics of multiple-wing aircraft configurations was developed at NC State during 2003-06. The method is suitable for implementation in vortex lattice formulations. For post-stall conditions, it relies on the modeling of the trailing-edge flow separation due to stall using an appropriate camber reduction, or a “decambering”, at the section’s trailing-edge. The method uses an iterative approach to determine this decambering at all sections of the configuration, while ensuring that the boundary conditions are also satisfied everywhere. Recent work has resulted in speed improvements in the method and the coupling of the aerodynamic prediction with equations of motions for flight dynamics, resulting in the ability to simulate the aircraft flight dynamics at nominal and post-stall flight conditions faster than real-time. Current efforts are focused on validation of the low-order method at post-stall conditions using computational fluid dynamics studies of airfoils, wing, and configurations. Traditionally, simulations of flight dynamics make use of either linearizations and the use of aerodynamic stability derivatives [1] or the use of look-up tables generated for a given aircraft. Developing aerodynamic models in this fashion is configuration specific, and very often the models are only valid for a limited range of operating conditions. This paper presents the current status of an on-going effort at NCSU on augmenting linear aerodynamic prediction methods for use in real-time simulation of post-stall flight dynamics of multiplewing aircraft configurations. The post-stall prediction method is based on a decambering approach [2], developed at NC State in 2006 and made significantly faster [3] in 2008 using lift-distribution superposition principles. Given the instantaneous aerodynamic inflow angles and the angular-velocity components, the post-stall method predicts the aerodynamic forces and moments on the aircraft without the use of any empiricism. Inputs to the aerodynamic prediction method include planform-geometry details and lift, drag, and moment curves for all the airfoil sections including post-stall information. This paper, and previous studies have demonstrated the use of decambering in faster than real-time flight simulation. One aspect of the low-order aerodynamics that has not been explored in previous work is validation. Validation, focusing on the stall and post-stall regimes, is introduced in this paper. Due to the lack of experimental and computational post-stall aerodynamic data in the literature, CFD studies tailored for validation have been initiated in the current work. The current effort uses the NASA TetrUSS CFD package for this purpose. This paper begins with background on the decambering concept, and how it is applied in three dimensional aerodynamics. The recent efforts attempting to validate and improve the method using computational tools are discussed. Finally, results from real-time simulation are presented as well as future work directions for this effort.

Ryan C. Paul

Papers

A CFD Database for Airfoils and Wings at Post-Stall Angles of Attack

Computational Aerodynamic Modeling Tools for Aircraft Loss of Control

Iteration schemes for rapid post-stall aerodynamic prediction of wings using a decambering approach

Unsteady and Post-Stall Aerodynamic Modeling for Flight Dynamics Simulation

Aerodynamic Modeling for Real-Time Flight Dynamics Simulation