The objective of this investigation is threefold. First, to assess the flight dynamics of an electric vertical take-off and landing (eVTOL) concept aircraft with a propeller-driven rotor. Second, to develop an automatic flight control system (AFCS) for this concept aircraft. Third, to verify the potential safety benefits of the concept aircraft by analyzing the autorotation performance following a total loss of power. The paper begins with an overview of the design of the aircraft and description of the simulation model, including a detailed discussion on the inflow model of the propellers that drive the main rotor. Next, the flight dynamics are assessed at hover and in forward flight. An AFCS based on dynamic inversion is developed to provide stability and desired response characteristics about the roll, pitch, yaw, and heave axes for speeds ranging from hover to 80 kt. Additionally, an RPM governor is implemented to hold the main rotor angular speed constant at its nominal value. Finally, simulations that make use of the AFCS are performed to analyze the autorotation performance following total loss of power.
The use of an active cargo hook for stabilizing external loads during high-speed flight is demonstrated in simulation. A CONEX cargo container with two rear-mounted stabilization fins is used as the subject load. Significant nonlinearities in the dynamics of the external load result
in multiple equilibria and limit cycle oscillations. A full-state feedback linear quadratic controller is developed assuming an isolated load in a wind tunnel model and shown to be successful in stabilizing the originally unstable load at a target airspeed of 100 kt. The design is then completed
to cover the target carriage envelope from hover to high-speed flight. Simulations of a coupled system incorporating a UH-60 Black Hawk helicopter with an actuated cargo hook and the external load show that the controller is successful in providing system stability throughout the target flight
speed envelope.
In this work the use of a Nonlinear Dynamic Inversion flight control system is investigated for use on electric-VTOL aircraft. This included studying the use of an airspeed scheduled switching system that would switch the aircraft’s control architecture from a low speed helicopter control system to a high speed airplane control system. Additionally, a novel thrust control allocation scheme is presented. This new scheme combines the variable collective pitch and variable rotor speed allocation schemes into a unified, complimentary filtering based, control allocation scheme. This new scheme is compared against the original constituent schemes on the basis of time simulations, stability margins and handling qualities. The successful operation of the control architecture switching system, was demonstrated via time domain simulations. It was also found that the combined control allocation scheme did not have better performance than the variable collective pitch scheme. However, the combined scheme did offer some improved performance over the variable rotor speed scheme. Especially when electric motor torque limits are enforced. The combined control allocation scheme was able to perform maneuvers that the variable rotor speed scheme could not.
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