VTOL UAV Dynamic Stability Derivatives: CFD Simulation by Ansys Fluent

Description

Static and Dynamic Stability Derivatives: VTOL UAV CFD Simulation Training

Introduction

The determination of an UAV’s dynamic stability and control derivatives is vital for the development of accurate flight models and control system design. These coefficients, which relate aerodynamic moments to angular motion, are traditionally obtained through costly and time-consuming wind tunnel tests.

In VTOL UAV dynamic stability derivatives project, a numerical approach based on computational fluid dynamics (CFD) combined with the forced oscillation technique is proposed to efficiently extract the derivatives of the dynamic stability of an UAV in the subsonic flight regime.

For more information on stability derivatives, click here.

Vertical Take-Off and Landing (VTOL) UAVs are unmanned aerial vehicles capable of vertical takeoff, hovering, and landing without the need for a runway. They combine the flexibility of helicopters with the aerodynamic efficiency of fixed-wing aircraft. This advanced technology enables them to perform complex flight operations in confined spaces and challenging geographical conditions.

VTOL UAVs are generally divided into two main categories: pure multirotors and hybrid fixed-wing VTOLs (VTOL-FW). Hybrid fixed-wing models use lift rotors for takeoff and then rely on their fixed wings to generate lift during cruise flight. More advanced configurations include tilt-rotor mechanisms with three-dimensional thrust vectoring capability, which significantly improve responsiveness and stability in the roll, pitch, and yaw axes.

One of the most complex aspects of developing these UAVs is the transition phase, during which the aircraft changes from rotor-thrust-based hovering flight to wing-generated aerodynamic cruise flight. To maintain stability during this critical phase, advanced flight control systems (FCC) are required so that they can intelligently switch between multirotor and fixed-wing control algorithms. Accurate calculation of stability derivatives and aerodynamic analysis is essential in these aircraft to prevent sudden altitude loss or stall during the transition.

The geometry of the present model is three-dimensional and has been designed using SpaceClaim software. We do the meshing of the present model with ANSYS Meshing software. The mesh type is Tetrahedral, and the element number is 4,420,514.

The computational domain is specifically structured with three fluid regions to facilitate the motion: a rotating internal fluid region for the blade, an oscillating internal fluid region that completely encloses the drone, and an external fluid region that does not oscillate. This research focuses on applying a specific cosine oscillation to the drone and analyzing the resulting aerodynamic moments to quantify the dynamic stability derivatives.

Methodology

This study used a transient, pressure-based CFD simulation in ANSYS Fluent to analyze the incompressible flow around a UAV to derive dynamic derivatives. The flow physics was modeled using the k-ω SST turbulence model. The key method involved the oscillatory region technique, in which an inner fluid region, containing the UAV, was forced into a simple harmonic oscillation via a UDF, while the outer domain was set with inlet and outlet boundary conditions. This setup, with good time resolution, was designed to capture the aerodynamic response necessary to calculate dynamic stability derivatives.

Results and Conclusion

The most commonly used and important coefficients in the discussion of stability have been extracted after simulation and are given in the table below. Using these coefficients, it is possible to comment on whether a drone is stable or not in terms of static and dynamic. Also, using the extracted data, hysteresis diagrams have been drawn, which can be understood by considering the shape and direction of rotation of these rings, whether the drone is stable or not.