Development and application of a medium-fidelity analysis code for multicopter aerodynamics and flight dynamics

Abstract

Most of the research on eVTOL platforms comes in two flavors: control design and experimentation, using highly simplified models that ignore much of the physics surrounding multicopters, and high-fidelity, high-cost computational fluid dynamics (CFD) studies that capture extremely minute details. Due to the oversimplifications found in control studies, and the high-cost of CFD, neither tool is useful for preliminary analysis and design.

RMAC is applied to compare a plus- and cross-type quadcopter in terms of their required trim controls, power consumption, open-loop flight dynamics, and control authority. The flight dynamics modes are described in detail, and two unstable phugoid modes are found. The impact of forward flight on these modes is also detailed. RMAC is also applied to a novel, reconfigurable multicopter, to which rotors can be added or removed to suit a given mission. These aircraft are compared primarily in terms of power consumption for a given payload and the related metrics of range and endurance, though considerations of control authority are also included.

The various induced-flow models included in RMAC are analyzed vis-a`-vis their effects on the steady rotor loads in hover and forward flight, as well as the resulting changes in the predicted trim controls and open-loop flight dynamics. Additionally, the effect of blade elasticity on steady and vibratory loads, as well as the impact of the vibratory loads on variable-speed rotors on the aircraft-level vibrations is explored.

A novel expression of the rotor speeds in terms of aircraft-level “modes” is defined in this thesis, termed the multi-rotor coordinate transform. The coordinate transform is defined in closed-form for any regular multicopter, including the quadcopter, hexacopter, and octocopter. This transform is used to define power-optimal “primary” controls and reactionless “extra” controls, the former used to control the aircraft, and the latter generally unused, except to explore the control space. Multi-rotor coordinates are also used to define rotor cant modes, and the effect of these modes on a quadcopter are analyzed.

The Rensselaer Multicopter Analysis Code (RMAC), a medium-fidelity analysis tool, was developed to analyze eVTOL platforms with good accuracy and low-cost. RMAC uses lifting-line aerodynamic analysis to calculate airloads on a rotor of arbitrary planform, coupled with several different rotor-induced inflow models, as well as an elastic blade model. The code has been validated against experimental data obtained through wind tunnel experimentation and system identification of a flying model.

Electric Vertical Take-Off and Landing (eVTOL) are a newly popular platform in modern vertical lift. These small rotorcraft have much simpler mechanically and dynamically than conventional helicopters. Their simplicity has made them extremely useful in a wide variety of applications, including package delivery, aerial photography, and, recently, designs have been proposed for human-carrying vehicles for use as air-based taxi systems.