In the last few years, a new aircraft configuration has been developed: the electric Vertical Take-off/Landing aircraft (eVTOL). To improve aircraft efficiency, structures are lighter and more flexible. Due to the structure's flexible nature and the use of propellers for propulsion, eVTOL are prone to whirl flutter, which consists of a dynamic instability produced by the propeller aerodynamics and structure interaction.
This thesis aims to assess whirl flutter in a multirotor VTOL considering propellers in thrusting conditions. A semi-analytical model was developed and then implemented on MATLAB for two types of configurations: a conventional propeller and two counter-rotating propellers at the tip of a cantilever beam structure.
The propeller aerodynamics in thrust conditions were obtained through blade element momentum theory (BEMT) and extended to counter-rotating propellers. Then, the BEMT flow characteristics were coupled to a quasi-steady aerodynamics perturbation model using strip theory to obtain the forces produced by the propeller whirl motion. Additionally, the beam structure was modeled using a space frame finite element model, in combination with a two-degree of freedom system model for the propeller-pylon structure. Then, the structural system was coupled to the unsteady aerodynamic forces produced by the whirl motion obtaining the aeroelastic model from which the stability analysis was performed.
The model was used to analyze whirl flutter on a multirotor aircraft designed by the company Betronka SPA. It was found that the multirotor does not suffer from whirl flutter for the designed flight conditions, considering the conventional and counter-rotating configurations. Afterward, to study the thrust influence on whirl flutter, flow conditions and propeller angular speeds outside the designed limit were studied. The main findings are: the counter-rotating propeller configuration is more stable compared to the default configuration, increasing the whirl flutter speeds; thrust conditions stabilize the default configuration, increasing the propeller angular speed to encounter whirl flutter by 4%; thrust conditions are negligible for counter-rotating propeller configuration considering constant angular velocity.