Optimization framework for design of a hybrid electric vertical takeoff and landing multirotor

Abstract

Since the 1970s, helicopters have been vital in disaster response but face limitations in cost, infrastructure, and
maneuverability. This thesis presents the design and optimization of an emergency response flyer tailored for the
GoAERO competition. Multirotor configurations with hybrid-electric propulsion are investigated to overcome
the limitations of existing fully electric VTOL technologies.

A Multidisciplinary Design Optimization (MDO) framework is applied to hybrid-electric quadrotor, hexarotor,
and octorotor configurations, optimizing the balance between rotor count, mass, and performance. The objec-
tive is to minimize Maximum Takeoff Mass (MTOM) while enhancing dynamic performance and maximizing
the payload-to-system mass ratio. This study addresses a key research gap by integrating early-stage handling
qualities (HQ) evaluation and exploring trade-offs between optimized rotor count configurations. Aerodynamic
forces and energy consumption are estimated using momentum theory, while system dynamics are analyzed
through Newton-Euler equations and eigenvalue assessments.

Results show that the hexarotor configuration achieves the fastest convergence, balancing design complexity and
design space exploring. Configurations maintain a disk loading between 43–63[kg/m2] with hover efficiency
between 5 and 5.8. A higher rotor count improves hover efficiency and disk loading, making performance com-
parable to eHang and Vahana while achieving nearly twice the cruise speed, rivaling helicopters.

At a hybridization factor (HF) of 0.1, MTOM is reduced by 75%, with only a 3% mass variation between con-
figurations. However, at higher HF, mass increases by 20% due to relatively low energy density of batteries
further impacting structural support demands, underscoring the need for a more detailed support structure model.
Stability analysis confirms neutral hover stability, while a real, negative eigenvalue at cruise identifies the surge
subsidence mode, governed by system mass.

Rotor count significantly impacts design flexibility. Hexacopters and octocopters offer better disk loading homo-
geneity, whereas quadrotors face constrained rotor sizing and elevated blade and disk loading, limiting efficiency.
Quadrotors excel in payload-to-mass ratio and simplicity, making them ideal for productivity missions but less
suited for maneuvering-intensive tasks with lower disk loading and control authority, highlighting the importance
of early HQ considerations.

This thesis makes a valuable contribution to the advancement of eVTOL research by addressing early-stage HQ
assessments where on the basis of other literature and configuration optimization, the hexacopter emerges as the
most viable for the GoAERO competition, striking the best balance between mass, handling qualities, and mission
performance.