title: Investigation of propeller stall flutter
Creators: name: Higgins, Ross John
Publisher: name: University of Glasgow
Year: 2021
Subjects: subject: Propeller, stall flutter, aeroelasticity, CFD.

description: Propeller flutter can manifest in a variety of ways. This includes classical bending-torsion flutter, stall flutter and whirl flutter. Classical bending-torsion flutter for propeller blades is driven by the coupling, and excitement, of selected modes of motion. Such flutter problems are often a result of structural coupling and in the linear aerodynamic regime. As a result, low-fidelity, fast calculations can be used to determine boundaries and mitigate the effects via changes in the structural design. Whirl flutter is the most complex and involves the coupling of the aircraft wing modes of motion to the gyroscopic and aerodynamic effects of the propeller. This phenomenon can be highly non-linear due to both the structure and flow-field, and any mitigation involves sophisticated modelling efforts with respect to the airframe. Propeller stall flutter is less complex in terms of the structure, however, involves the highly non-linear aerodynamics associated with detached flow. This phenomena, like classical flutter, is driven by the propeller design and conditions, but due to its nature, the stall flutter boundary significantly reduces the overall flutter boundary of a propeller. Hence, the understanding of this limitation must be known to ensure safe operation. The development of modern propeller blades utilising high sweep/taper with thin aerofoil sections can result in a change in the flutter boundary. In addition to this, propellers are coming back into focus due to the development of electrically driven Vertical Take-off/Landing (eVTOL) vehicles and, due to the nature of such a vehicle design, the propellers are being pushed into significantly different operating conditions. This motivation, coupled with the increased computational power available in the modern era, requires the need to reassess what is required to understand the stall flutter boundary associated with a modern, in-service, propeller blade. To this end, a numerical investigation using Computational Fluid Dynamics (CFD) and Computational Structural Dynamics (CSD) was conducted on the Commander propeller blade of Dowty Propellers. This blade was selected from the list of experimentally investigated blades due to the availability of geometry, structural data and applicability in real life applications. A validation procedure was conducted to assess the effects of the computational setup. This included the effects of turbulence, structural modelling and implementation, with a validated process found whilst using Scale-Adaptive-Simulation (SAS) with interpolated structural modes. An attempt was made to extract aerodynamic damping data of the stall flutter phenomenon via the development of a method from the aeroelastic simulations. Such values give an indication of the stability, with links made to typical two-dimensional modelling. The thesis ends on the parametric study of the validated Commander simulation. This was conducted in order to gain greater detail on the effects of key structural and aerodynamic parameters on the blade stall flutter response. The key outcome from this investigation is the need for scale-resolving methods in propeller stall flutter investigations. This study utilises a hybrid RANS/LES model to capture the key detached flow content. This detached flow content results in significant pressure fluctuations, not observed in traditional statistical models, which drive the aeroelastic deformations. In addition, the requirement for a well validated structural model is highlighted including the setup of the structural solver for which an interpolated modal response method is used. It is also found from this investigation that there is a need for a modern experimental test case focusing on propeller stall flutter. The last comprehensive study was conducted in the 1980’s and, with improvements in experimental techniques, greater understanding and data can now be extracted. This new data can be used to validate modern CFD efforts. The novelty of this work lies within the derivation of a method for the extraction of the aerodynamic damping data from three-dimensional simulations. This had previously not be done before and the extracted results correlated with equivalent two-dimensional aerodynamic damping data. Additionally, the development and application of three-dimensional Navier-Stokes based CFD, with a coupled structural model, had not been conducted on propeller stall flutter.