Closed Loop Feedback and Control of Coefficient of Lift for High Amplitude Gust Encounters

A large body of work in the field of fluid dynamics has sought to understand the physics of wings moving in rapid motions (or high reduced frequencies) and at high angles of attack. These types of motions are prevalent in small air vehicles but also apply to larger fixed wing aircraft that are encountering gusts. Controlling vehicles through these large gusts is a major goal in the fluid dynamics community. Much work has been done to model these dynamics in a way that controllers can be implemented to mitigate unwanted force and transient effects from these movements and encounters. Currently, researchers develop closed loop controllers using simulation of the unsteady aerodynamics around a wing or an airfoil with computational fluid dynamics techniques. The angle of attack history of these simulations is then used to inform open loop experiments for validation. A large need in this work is the ability to implement these controllers in a laboratory environment where the feedback from the system can be implemented directly to the wing to validate the entire system experimentally. In addition, it is unclear if the models being used to simulate the unsteady aerodynamics for high amplitude lift encounters are accurate or even needed for controlling a wing through these encounters. Many of the unsteady aerodynamic models only work at low amplitudes and low reduced frequencies. An experimental controller that works for these high amplitude and high reduced frequency cases could be used to study the angle of attack history needed to achieve mitigation of the gust effects. This could help improve the existing unsteady models in a way that they could accurately represent the physics in these higher regimes. The current work focuses on a methodology for implementing closed loop feedback and control systems to achieve coefficient of lift control of a AR 4 flat plate undergoing pure pitch in the University of Dayton Water Tunnel (UD-WaT) using two different controllers which are not informed by the aerodynamics but only through sensory input and feedback. This will allow researchers to implement different closed loop control architectures to a physical wing in the lab resulting in a new capability for the fluid dynamics community. This will also allow for the assessment of the need of an aerodynamic controller for accurate lift tracking of a wing. The experimental setup consists of an ATI force sensor attached to a flat plate to sense lift and drag. A linear motor is then used to control the angle of the wing based on a control law programmed on a dSPACE MicroLabBox with the real time control interface. The primary function of the control system developed is to track a given Cl magnitude as a function of time over a range of Cl amplitudes and reduced frequencies. Closed-loop feedback controllers based on PID, full state-feedback, and full state-feedback with a Kalman filter were designed and implemented in the present study. These three control architectures provided the capability to achieve coefficient of lift tracking at various amplitudes well beyond stall and at high reduced frequencies. Each controller performed different angle of attack profiles for the same desired coefficient of lift profile. Analysis of this angle of attack history was done to try and identify trends that can be used for developing a model that capture these dynamics. The Theodorsen unsteady aerodynamic model was used with the angle of attack data to gauge its feasibility for predicting the lift response of the cases run and it was determined that while it does not accurately capture all of the dynamics it might be a good candidate for modification to arrive at an accurate model of high reduced frequency and amplitude effects