Hybrid VTOL configurations combine fixed-wing efficiency with VTOL capability, enabling operations without launch and recovery infrastructure (catapults, runways, or nets). While adaptable to almost any fixed-wing UAV, the lift system – which is essentially a multicopter, retains the limitations of multicopter drones. This study examines improving the control authority and capabilities of the Hybrid VTOL using thrust vectoring propulsors (TVP).
The study vehicle employs a typical four-lift-plus-one propeller configuration. As is typical It operates in three modes: VTOL, transition, and fixed-wing flight. VTOL mode employs a 4-DOF control system similar to multicopters, using differential thrust and motor torque for coupled attitude and position control.
During transition – the most energy-intensive phase, the system maintains multicopter control while the propulsion propeller generates forward velocity until sufficient wing lift and control surface effectiveness is achieved. The lift propellers then park for fixed-wing flight. Many of these types of vehicles utilize a fairly sophisticated power management systems to optimize energy use during transition between VTOL and fixed wing flight.
Animation: Tilting controls yaw rate, differential thrust maintains attitude. The second slower loop depicts propeller pair tilt directions for clockwise yaw.
The use of differential motor torque for yaw control in multicopter systems presents inherent inefficiencies and operational limitations. The method necessitates non-optimal motor speed operation, resulting in increased power consumption during both yaw maneuvers and position maintenance. Control authority exhibits significant variation with motor speed, leading to degraded effectiveness at low throttle settings and a susceptibility to wind disturbances. Additionally, the inherent latency in motor speed changes – particularly with larger diameter propellers, results in diminished responsiveness with available yaw torque frequently proving insufficient against external aerodynamic disturbances. These limitations collectively constrain the aircraft’s yaw agility due to the non-optimal balance between lift generation and yaw control demands.
Thrust vectoring offers an alternative approach by implementing coordinated tilt control of the lift propellers. This method provides enhanced yaw authority while maintaining optimal motor speeds for efficiency. Implementation requires an additional servo for each propeller and modification of the flight control laws to transfer yaw control from the motor controllers to the tilt mechanism – which is not difficult. The resulting system decouples yaw control from thrust generation, allowing each subsystem to operate in its optimal range.
The conventional approach to lateral control in multicopter and Hybrid VTOL vehicles relies on differential thrust to induce vehicle tilt, creating inherent inefficiencies in both energy management and control. The method uses differential vertical thrust to roll the vehicle and generate lateral force. It results in increased power consumption and compromised stability – particularly with larger vehicles. The coupled nature of attitude and position introduces latency in the dynamic response of the vehicle and limits both performance and wind resistance. These limitations impact operational capabilities and necessitate oversized propulsion systems to compensate for the coupled force requirements.
Thrust vectoring addresses these limitations by tilting only the propellers, rather than the entire vehicle, to generate lateral forces. The difference in the tilting inertia between the two approaches has a profound effect on the latency. This architecture enables direct lateral force generation while maintaining vertical thrust efficiency and level vehicle attitude – or any attitude desired. The decoupled, 5-DOF control scheme provides enhanced dynamic response and improved disturbance rejection without compromising lift efficiency. TVP integration for lateral control uses the same tilt servos added for yaw control and modifies control laws to effectively utilize the additional degree of freedom. The result yields significant improvements in precision during take-offs and landings in challenging environmental conditions.
Animation: Differential thrust maintains any commanded attitude. Tilting controls lateral position and speed. Only the propeller and protection beam tilt, the motors remain fixed
Rendering: Pusher propeller (green) tilting the thrust vector below the vehicle CG raises the nose in pitch at rates greater than control surfaces at any velocity.
The integration of TVP for the propulsion propeller offers significant advantages in transition and the fixed wing flight modes. During transition, it provides direct authoritative compensation for the natural aerodynamic center shift of lift propellers while providing pitch control authority in the low-speed regime where traditional control surfaces are least effective. This capability enables more rapid and stable transitions in the most energy intensive mode of flight.
The added pitch authority compensates for the increased longitudinal inertia of the Hybrid VTOL’s lift booms. The vectoring propulsion propeller offers real-time adaptation to center of gravity changes from fuel burn, offloading payloads, or asymmetric loading conditions, reducing trim drag and improving overall energy efficiency. Furthermore, the enhanced control authority enables improved evasive maneuverability, which is particularly valuable during contested operations.
The implementation of thrust vectoring across multiple control axes in Hybrid VTOL systems offers significant operational advantages over conventional differential thrust methods. By decoupling thrust generation from attitude control, TVP enables more efficient yaw control, direct lateral force generation, and enhanced pitch authority during critical flight phases.
There is also a compelling technical argument that these performance improvements would bring overall system efficiency gains, particularly in vehicles where control authority is a limiting factor in the original design. Those gains include:
Note, Aerofex does not sell Hybrid VTOL aerial vehicles. We develop and sell the propulsors that enable the capabilities shown, for any Group 3 vehicle or larger.
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