UC Berkeley

Electric vertical takeoff and landing (eVTOL) aircraft are employed in several applications such as aerial photography, delivery, search and rescue missions owing to their compactness and ability to hover. However, they inherently have lower endurance and range as compared to their fixed-wing counterparts. Existing approaches to address this issue have explored increasing the mechanical, electrical, or aerodynamic efficiency of the system, replacing the batteries of the drone on ground stations, recharging them in-flight via large wireless chargers or laser power beaming, and several other innovative solutions exist.

Given the demand for long-duration flights and long range for applications such as drone delivery and urban aerial mobility (UAM), this dissertation explores various ways to increase the endurance of eVTOLs by adding modularity and incorporating novel design and talks about the interesting challenges that arise from them.

We begin by calculating how the flight time of an eVTOL is affected by simply adding more battery. We find that there is a fundamental flight time limit for hovering eVTOL that cannot be exceeded by just adding more battery. This motivates the exploration of new designs and approaches to improve their endurance.

First, we explore a simple approach of tethering a series of multirotors to a power source. We evaluate the power requirements to run such a system which helps in design optimization and can be used to guarantee electrical safety.We discover that there exists a critical boundary of thrusts the multirotors can produce that cannot be exceeded due to fundamental electrical limitations. The boundary can be manipulated by changing the voltage of the power supply or the resistance of the cables. We also compare the power consumption for one tethered quadcopter and two tethered quadcopters and show that for large quadcopters far enough from the anchor point, a two-quadcopter system consumes lesser power.

Next, we explore the idea of using the multirotor battery in stages to discard the discharged portion of the battery which results in lower power consumption due to reduced mass. We find that even if we stage the energy source continuously (e.g. gasoline in a combustion engine), there still exists a fundamental flight time limit that cannot be exceeded. We consider two optimal staging problems that aim to maximize the flight time and present analytical or visual solutions which are validated experimentally.

Then, we present the idea of flying batteries -- modular batteries that can fly to a mission quadcopter, dock with it, power and recharge it in-flight, and fly away after discharging, allowing us to repeat the process. This approach lifts the limitation due to energy storage. We present an analysis to evaluate the constraints that need to be satisfied to unlock unlimited endurance for eVTOL aircraft. We then discuss a stochastic model that can be used to predict the probability of success of a long-duration mission when using flying batteries.

Lastly, we present a computer vision-based solution that can be used to get the flying batteries system working in the real world. The solution involves using purely onboard sensing for the mission vehicle via a combination of an inertial measurement unit (IMU) and a camera that can generate pose measurements with respect to the flying battery. Measurements from these sensors can be fused to generate relative state estimates that can be used to dock two vehicles in-flight with a centimeter-level precision.

We strive to experimentally validate the analysis and proposed approaches presented throughout this thesis.