The present paper describes the results of an experimental wind tunnel test campaign aimed at investigating the aerodynamic performance and flow physics related to a wing section equipped with two propellers mounted on a boom. The configuration investigated is meant to be representative of a full-scale eVTOL aircraft in cruise flight condition. The use of full-scale components of an eVTOL aircraft made this setup a quite advanced experiment in the recent literature. Pressure measurements and an infrared thermography technique were used during the test campaign, respectively, to evaluate localised effects induced by the propeller blowing on the wing and to provide a quantitative evaluation of the amount of laminar flow on the wing surface with and without the influence of the propeller at different thrust conditions.

Optimisation-based control design techniques are applied to multicopters with variable-RPM rotors. The handling qualities and motor current requirements of a quadcopter, hexacopter and octocopter with equal gross weights (5,360N) and total disk areas (producing a 287N/m $^2$ disk loading) are compared in hover. For axes that rely on the rotor thrust (all except yaw), the increased inertia of the larger rotors on the quadcopter increase the current requirement, relative to vehicles with fewer, smaller rotors. Both the quadcopter and hexacopter have maximum current margin requirements (relative to hover) during a step command in longitudinal velocity. In yaw, rotor inertia is irrelevant, as the reaction torque of the motor is the same whether the rotor is accelerating or overcoming drag. This, combined with the octocopter’s greater inertia as well as the fact that it requires 30% less current to drive its motors in hover, results in the octocopter requiring the greatest current margin, relative to hover conditions. To meet handling qualities requirements, the total weight of the motors of the octocopter and hexacopter is comparable at 13.5% weight fraction, but the quadcopter’s motors are heavier, requiring 16% weight fraction. If the longitudinal and lateral axes were flown in ACAH mode, rather than TRC mode, the total motor weight of all configurations would be nearly identical, requiring about 13.5% weight fraction for motors (compared to 7–9% weight fraction from hover torque requirements).

This study focuses on vibration reduction for quadcopters and octocopters with elastic, two-bladed, fixed-speed, variable-pitch rotors through the use of relative rotor phasing. The study defines phase modes such as a pitch phase mode with relative phasing between the front and aft rotors, a roll phase mode with relative phasing between the left and right rotors, and a differential phase mode with relative phasing between the clockwise and counter-clockwise spinning rotors for both the quadcopter and the octocopter, as well as additional higher harmonic phase modes for the octocopter. Parametric studies on individual phase modes indicate that, for the quadcopter in forward flight, the pitch and roll phase modes can almost entirely eliminate the 2/rev vibratory forces (at the aircraft level), but the 2/rev vibratory moments cannot be minimised at the same time. By simultaneously using multiple phase modes, a Pareto front can be generated and a solution selected based on the relative emphasis on force or moment vibration reduction. For the octocopter, it was observed that individual higher harmonic modes (specifically the 2c or 2s modes) could almost entirely eliminate both the 2/rev vibratory forces and moments, simultaneously. Compared with vibration levels in forward flight that might, on average, be expected if the rotors were randomly phased, a 62% reduction of a composite vibration index can be achieved on a quadcopter, and complete elimination of vibration was achievable on an octocopter, with appropriate rotor phasing.

NASA is conducting investigations in Advanced Air Mobility (AAM) aircraft and operations. AAM missions are characterised by ranges below 300 nm, including rural and urban operations, passenger carrying as well as cargo delivery. Urban Air Mobility (UAM) is a subset of AAM and is the segment that is projected to have the most economic benefit and be the most difficult to develop. The NASA Revolutionary Vertical Lift Technology project is developing UAM VTOL aircraft designs that can be used to focus and guide research activities in support of aircraft development for emerging aviation markets. These NASA concept vehicles encompass relevant UAM features and technologies, including propulsion architectures, highly efficient yet quiet rotors, and aircraft aerodynamic performance and interactions. The configurations adopted are generic, intentionally different in appearance and design detail from prominent industry arrangements. Already these UAM concept aircraft have been used in numerous engineering investigations, including work on meeting safety requirements, achieving good handling qualities, and reducing noise below helicopter certification levels. Focusing on the concept vehicles, observations are made regarding the engineering of Advanced Air Mobility aircraft.

“Urban air vehicles” have been hailed as the next revolution in aviation. Prototypes of various sizes have been flown to demonstrate basic flight (hover and climb), but in most cases there is no demonstration of full flight capability, for example conversion from vertical to level flight (conversion corridor). There are proposals for vehicles in a wide range of scales: from drones specifically designed to deliver goods, to full size vehicles for manned transportation. Most of the concepts proposed include full electric propulsion, multiple (often convertible) rotors (ducted or un-ducted, counter-rotating), and widespread use of composite materials. Start-up companies are seeking funding with high-profile demonstrations in front of the media, but many unresolved technical problems are not been solved. Large aerospace companies have joined the fray. These initiatives are fuelling expectations that achieving the next milestone is within easy reach. This paper aims to fill some gaps in understanding and curb optimism. It takes a holistic view in order to establish a scientific basis for design, manufacturing, operations.

In this paper, we examine some of the physics behind Vertical Take-Off and Landing (VTOL) flying machines, and some of the emerging technologies that are driving the recent upsurge of new VTOL projects. The paper attempts to put these into context by examining some of the projects that have been publicised over the past couple of years, particularly those that transition from hovering into wing-borne flight. Although much progress has been made, there still needs to be significant breakthroughs in technologies, particularly battery technology, before the dream of fast, quiet and environmentally friendly inter-city VTOL aircraft can be realised.