Whether it is the Thales UAS100, the Wingtra Gen2, the SkyEye Tango or many others, the VTOL (Vertical Take-Off and Landing UAV) drones represent a major technological advance in the field of unmanned aircraft. It combines a capability for fast and long distance flight with an unmatched operational flexibility offered by their ability to take-off and vertical landing. Particularly valuable in areas such as defence or maritime, their effectiveness remains to be improved and their performance/cost ratio is only to be improved. In addition, their high speed and quasi-fixed wing make them perfectly suited to the potential of the formation flight presented by the swarm of drones and its promise of energy saving and performance gain. Aerodynamics is thus one of the first factors in the development and optimization of these aircraft.
Endurance and performance of the VTOL drone
A fixed wing base
The specific architecture of the drone, where the cockpit layout no longer exists, often requires in the case of VTOL a special treatment of the tail-section to minimize interactions with the wake of the vertical propellers implanted upstream but also with the main thruster which, is often found at the rear end of a short fuselage.
A multiple propulsion
Whether the energy source is hydrocarbon, chemical, hydrogen, solar or a hybrid of all these, the motorization of a VTOL drone is distinguished by the need to ensure two distinct propulsions: horizontal flight and vertical flight. For the first, the variety of projection capabilities and target speed will lead to choices that will go from simple propeller to jet engine. For the second, the use of two pairs of rotors will be the most common architecture.
A long range with variable speeds and loads
Often intended to ensure long and distant missions, VTOL drones have projection capabilities ranging from 50 to 300 kilometers, and this at cruising speeds ranging from 50km/h up to 350km/h and sometimes even more. Due to the technical and economic necessity of designing multi-mission and multi-sector drones, VTOL are now developed with a great modularity of transport for all types of loads.
Evolution in Swarm of VTOL drones
An ability to flight in formation
Subject to the autopilot management capacity, these VTOL drones have real dispositions for swarm flight. This is due in particular to their relatively high moving speed and fixed wing geometries that will generate interesting wakes characterized by ascending incidences, like upwash, favorable to the following drones. Among the possible configurations, we will find here the V-formation flight regurlarly illustrated in nature by migratory birds.
An optimization of aerodynamic interactions
The upward incidence provides a natural upwash that benefits the lift. Thus, for the same mass, the aerodynamic work required of the structure and the engines is reduced. Moreover, the air flow detachment on the fuselage of the aircraft also have a good chance to be reduced. As a result, the energy efficiency of the drones is greatly improved when they are in an optimized position within the swarm.
A good efficiency by controlling the costs
Note that the swarm requires only a minimum of two drones to exist, and although this configuration is light, it already brings significant gains in energy savings. It should also be noted that these gains are redistributed evenly if a rotation strategy is implemented. Thus, controlling the cost of production and operation of a VTOL drone is not only intrinsic to the aircraft and its development, but can also be done via the swarm flight functionality that will significantly increase the efficiency of the mission.
Applications and growth of VTOL drones
With favourable performances in the fields of defence, logistics, maritime surveillance and other sectors, VTOL drones represent a significant share of the current and future drone market. The world economy is expected to grow by 20-25% until 2030, reaching a value of between $4.5 and $5 billion. Dominated by small aircraft under 25kg which will represent almost half of this market, it remains no less than any optimization of their aero-energy performance, especially through the ability to fly in swarm, will be a competitive advantage of the highest order to win.
