Basic Design Limitations for Urban Electric VTOL Aircraft

Urban air mobility has the potential to change the face of aeronautics and transporation, which has created a lot of speculation on what the new technology is capable of. The uncertainty is increased by the fact that urban air mobility is a unchartered field of aeronautics and the proposed designs vary significantly, especially because of new technologies such as distributed electric propulsion which give extreme design freedom. However, there exist a short series of simple physical rules that set limits on what can be done in order to meet stringent constraints of urban eVTOLs, such as noise, range, and payload requirements. This work aims at showing these limits, especially with regards to aircraft footprint and noise, and what technological and physical parameters drive these limits. The method employed uses an original but simple mathematical model to size eVTOLs for a variety of missions based on extensive quantities only, such as power required for flight, battery capacity and empty weight fraction. The assembly of these comparatively simple equations reduces down to solving a quartic equation for sizing an aircraft, which is done rapidly and robustly with a current desktop computer, and thus allows for extensive design space exploration. Using this tools, this work investigates the effects of increasing range, payload, and footprint of eVTOL aircraft for urban transportation needs. It is found that the rotor area to footprint ratio should be greater than 0.2 to avoid large mass penalties due to excessive power requirements. Also, smaller footprint vehicle require relatively larger rotors to be feasible. Main drivers for the take-off noise is found to be footprint, whereas improved battery technology has negligible effect on noise.