Vortex lattice method

About: Vortex lattice method is a research topic. Over the lifetime, 779 publications have been published within this topic receiving 9242 citations.

Longitudinal flight dynamic analysis of an agile UAV

Abstract: Purpose – The purpose of this paper is to describe the longitudinal dynamics of a hover‐capable rigid‐winged unmanned aerial vehicles (UAV) under various equilibrium flight conditions. The effects of the variable‐incidence wing in comparison with the fixed in‐incidence wing on the dynamics of UAV are also discussed.Design/methodology/approach – The aerodynamic modeling of the vehicle covers both pre‐stall and post‐stall regimes using a three‐dimensional vortex lattice method incorporating viscous corrections. The trim states across a velocity spectrum are evaluated using a nonlinear constrained optimization scheme based on sequential quadratic programming. Then linearized dynamic analysis around trim states is carried out in order to compare the characteristics of the conventional platform with the modified platform incorporating variable‐incidence wing.Findings – It is found that with the variable‐incidence wing, the longitudinal equilibrium flights can be achieved with reduced thrust‐to‐weight ratio dem...

Unsteady and Post-Stall Aerodynamic Modeling for Flight Dynamics Simulation

Abstract: 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

Aerodynamic Interference of a Large and a Small Aircraft.

Abstract: The influence of wake from a leading aircraft, Boeing 757, on a following aircraft, Cessna 750 Citation X, is investigated using the unsteady vortex lattice method. The effect of the wakes from both wings and each other is considered and the new position of the wakes from both aircraft calculated. As the midspan of the Cessna wing approaches the Boeing 757 trailing vortex core, the roll moment increases to its maximum value. Strong changes appear in aerodynamic loads of the Cessna as it moves near the Boeing 757’s wake. Rapid change occurs as the Cessna moves in transverse or vertical directions near the Boeing’s wake. The unsteady aerodynamic loads on wings moving along different paths are calculated. The Boeing 757’s wake strongly affects the aerodynamic loads and the wake of the Cessna. Also included in the present results are numerical simulations of wakes and velocity vectors in the Trefftz plane detailing the influence of the wake from the leading aircraft, the Boeing 757.

An analysis method for a ducted propeller with pre-swirl stator blades

Abstract: A method is developed to analyse the flow around a ducted propeller operating in combination with a set of pre-swirl stator vanes. The method uses a boundary element method to solve for the flow around the duct and the hub and a lifting surface vortex lattice method to solve for the flow around the propeller and stators. The three-dimensional flow around the duct/hub, propeller, and stators is computed separately with the interactions between them being accounted for in an iterative manner. The interactions between the duct and the stators and between the duct and the propeller are treated in a non-axisymmetric manner. However, only the circumferentially averaged interactions between the propeller and the stators are considered. Using this method a duct and set of stators are designed to operate efficiently with an existing propeller. This model was then built and tested in the MIT water tunnel at a variety of stator pitch angles. Comparisons are made between the theoretical and experimental values for the forces on the duct, propeller and stators. A very good agreement is achieved in the region of attached flow.

Compact Formulation of Nonlinear Inviscid Aerodynamics for Fixed-Wing Aircraft

Abstract: This paper presents a compact formulation of the Vortex Lattice Method (VLM) which allows fast computation of the nonlinear inviscid aerodynamics of fixed-wing aircraft at low speed and low angle of attack flight. The formulation captures the nonlinearities present in this flight regime, but avoids the costly re-computation that regular VLMs require. Instead ap reprocessing step reduces the calculation of inviscid aerodynamic force and moment to simple quadratic expressions in terms of the flight and control variables. The size of these quadratic expressions is independent of the number of vortices used in the vortex lattice, allowing for fine discretization of lifting surfaces with no penalty in re-computation time. Full six degree of freedom flight simulations that include a VLM aerodynamic model can now be executed very efficiently using this approach. Furthermore, this formulation provides an analytic aerodynamic model allowing direct mathematical manipulation, such as analytic differentiation with respect to flight and control variables, making it useful for optimization problems where analytic gradients could reduce computational costs as well as improve convergence characteristics.