Abstract
Aerial drones have improved significantly over the recent decades with stronger and smaller motors, more powerful propellers, and overall optimization of systems. These improvements have consequently increased top speeds and improved a variety of performance aspects, along with introducing new structural challenges, such as whirl flutter. Whirl flutter is an aeroelastic instability that can be affected by structural or aerodynamic nonlinearities. This instability may affect the prediction of potentially dangerous behaviors. In this work, a nonlinear reduced-order model for a nacelle-rotor system, considering quasi-steady aerodynamics, is implemented. First, a parametric study for the linear system is performed to determine the main aerodynamic and structural characteristics that affect the onset of instability. Multiple polynomial nonlinearities in the two degrees of freedom nacelle-rotor model are tested to simulate possible structural nonlinear effects including symmetric cubic hardening nonlinearities for the pitch and yaw degrees of freedom; purely yaw nonlinearity; purely pitch nonlinearity; and a combination of quadratic, cubic, and fifth-order nonlinearities for both degrees of freedom. Results show that the presence of hardening structural nonlinearities introduces limit cycle oscillations to the system in the post-flutter regime. Moreover, it is demonstrated that the inclusion of quadratic nonlinearity introduces asymmetric oscillations and subcritical behavior, where large and potentially dangerous deformations can be reached before the predicted linear flutter speed.
Highlights
Drones, or unmanned vehicles, are becoming more common for a variety of applications due to their increased reliability, improved aerodynamics, pilot safety concerns with manned aircraft with regards to force limits caused by velocity/maneuverability, and most importantly only being limited by structural and aerodynamic parameters rather than the pilot’s ability to handle these flight conditions before losing consciousness [1].This leads to the importance of investigating the fluid-structure interaction limitations of these unmanned aerial systems, and addressing all physical phenomenon that can arise
A parametric study for the linear system is performed to determine the main aerodynamic and structural characteristics that affect the onset of instability
By understanding the phenomenon known as whirl flutter, designers and manufactures can improved system lifespans and avoid catastrophic failure
Summary
Unmanned vehicles, are becoming more common for a variety of applications due to their increased reliability, improved aerodynamics, pilot safety concerns with manned aircraft with regards to force limits caused by velocity/maneuverability, and most importantly only being limited by structural and aerodynamic parameters rather than the pilot’s ability to handle these flight conditions before losing consciousness [1]. The improvement to motor function and propeller design has made rotor-powered aerial vehicles faster and more maneuverable These systems, with increased cruise/maximum speeds, have brought new challenges, including a structural phenomenon known as whirl flutter. Mair et al [56] presented an improved tilt-rotor model and investigated the stability boundaries and bifurcations when including odd orders of nonlinearity. Both of these analytical studies showed that a correct stability boundary is not found through linear studies, and nonlinear effects must be included. Analytical Model of Rotorcraft Aerodynamics and Dynamics (CAMRAD) II and Rotorcraft Comprehensive Analysis System (RCAS) are both used to investigate whirl flutter and to compare results to one another.
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