Hybrid unmanned aerial vehicles (UAVs) with vertical take-off and landing (VTOL) capability combine efficient forward flight with hovering, making them ideal for missions requiring both high-speed flight and precise maneuvering. Among various hybrid UAVs, tailsitters offer a compact and mechanically efficient solution for applications such as search-and-rescue and environmental monitoring. However, operation across the full flight envelope remains challenging due to highly nonlinear aerodynamics at high angles of attack (AoA)and limited control authority caused by actuator saturation. This work addresses these challenges through a systematic integration of wind tunnel–based aerodynamic analysis, control law development, and flight test validations, culminating in an integrated design and control framework enabling agile, robust, and fully autonomous tailsitter flight.

Firstly, wind tunnel tests were performed on a tailsitter wing under varying propeller thrust, elevon deflection, and airspeed across the full range of angles of attack, resulting in the first publicly available aerodynamic dataset of its kind. The results reveal nonlinear aerodynamic behavior, including during stall, post-stall, and reverse flow. In reversed flow, elevon-induced pitch moments act oppositely to normal flow, though this can be mitigated by increasing throttle. Elevon deflection proves effective at low angles of attack and high airspeeds, but its influence degrades at high angles and low speeds. These findings underscore the need for alternative or supplemental actuation to maintain control authority, especially in vertical or descending flight where traditional surfaces lose effectiveness.

Secondly, in response to the limited pitch control authority observed in conventional elevon-only tailsitters (E-tailsitters), new control strategies are necessitated to achieve full-envelope autonomous flight without actuator saturation. A tailsitter equipped with dual tilt rotors (TR-tailsitter) is introduced, which relies exclusively on thrust vectoring for control moment generation. While thrust vectoring provides ample pitch control authority in hover and vertical flight, it lacks sufficient roll control during forward flight due to wing-propeller interaction. To address this limitation, a TRE-tailsitter is proposed, integrating tilting rotors with conventional elevons. This combined actuation setup provides complementary control, with tilt rotors primarily handling low-speed and vertical flight phases, while elevons dominate during highspeed cruise. To achieve full-envelope autonomous flight, a cascaded Incremental Nonlinear Dynamic Inversion (INDI) controller is implemented, with Weighted Least Squares (WLS) control allocation, which dynamically coordinates actuator allocation between rotor tilt and elevon deflection across different flight regimes, avoiding actuator saturation and ensuring seamless transitions.

Thirdly, to enable fully autonomous field deployment, a pivoting takeoff and landing controller is developed for robust VTOL operation under windy and uneven terrain conditions. By exploiting rotor tilt, the vehicle initiates liftoff from a horizontal ground posture through a controlled pivoting motion around its tail, eliminating the need for landing gear and enabling deployment on uneven terrains. Indoor and outdoor flight tests validate the stability and robustness of the proposed approach in the presence of wind disturbances.

Fourthly, the agility of the tilt-rotor tailsitter UAV is examined through high-speed sharp turn scenarios, where maximizing lift is essential for minimizing turning radius. Wind tunnel data covering various actuator combinations are used to develop empirical models of axial force, lift and pitch moment w.r.t thrust, rotor tilt, elevon deflection, AoA, and airspeed, capturing wing–propeller interaction effects. The derived models and pitching moment trim tests reveal that upward rotor tilt combined with downward elevon deflection enhances lift while maintaining pitch equilibrium. Furthermore, a theoretical minimum turning radius of 8.01𝑚 at 18𝑚/𝑠 coordinated sharp turn is computed, confirming that coordinated actuation enables aggressive maneuvers without compromising pitch stability or speed.

Overall, this dissertation develops a tilt-rotor tailsitter UAV capable of robust, autonomous and agile operation across the full flight envelope.The proposed framework advances the understanding of tailsitter aerodynamics and control, and provides a pathway toward field-deployable UAVs for demanding missions requiring both maneuverability and autonomy.
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Tailsitter aircraft attract considerable interest due to their capabilities of both agile hover and high speed forward flight. However, traditional tailsitters that use aerodynamic control surfaces face the challenge of limited control effectiveness and associated actuator saturation during vertical flight and transitions. Conversely, tailsitters relying solely on tilting rotors have the drawback of insufficient roll control authority in forward flight. This letter proposes a tilt-rotor tailsitter aircraft with both elevons and tilting rotors as a promising solution. By implementing a cascaded weighted least squares (WLS) based incremental nonlinear dynamic inversion (INDI) controller, the drone successfully achieved autonomous waypoint tracking in outdoor experiments at a cruise airspeed of 16 m/s, including transitions between forward flight and hover without actuator saturation. Wind tunnel experiments confirm improved roll control compared to tilt-rotor-only configurations, while comparative outdoor flight tests highlight the vehicle's superior control over elevon-only designs during critical phases such as vertical descent and transitions. Finally, we also show that the tilt-rotors allow for an autonomous takeoff and landing with a unique pivoting capability that demonstrates stability and robustness under wind disturbances. ... Read more

Tailsitter Micro Air Vehicles with two rotors are promising due to their simplicity and efficient forward flight, but actuator saturation due to ineffective pitch control at a high angle of attack flight is a challenge limiting the flight envelope. This paper proposes a novel tilt-rotor tailsitter design which features two tilting rotors as the only means for control moment generation. Incremental Nonlinear Dynamic Inversion (INDI) is applied to the attitude control problem of the tiltrotor tailsitter, whose attitude angle tracking performance is validated by indoor and outdoor flight tests. It is found that actuator saturation is largely avoided by using thrust vectoring which provides sufficient capability of pitch moment generation. However, it is also found that the proposed design with only leading-edge tilting motors excluding any aerodynamic control surfaces has limited roll control effectiveness in forward flight. ... Read more

Tailsitters have complex aerodynamics that make them hard to control throughout the entire flight envelope, especially at very high angle of attack (AoA) and reverse flow conditions. The development of controllers for these vehicles is hampered by the absence of publicly available data on forces and moments experienced in such conditions. In this paper, wind tunnel experiments are performed under different flap deflections and throttle settings at all possible AoA. The dataset is made open access. Our analysis of the data shows for the tested wing, flap deflections greatly affect the lift coefficient and stall occurs at (Formula presented.) AoA as well as (Formula presented.). Wing-propeller interaction is studied by analyzing the propeller induced force in the axis orthogonal to the thrust axis, which is dependent on AoA, airspeed, flap deflections and thrust in a nonlinear and coupled manner. The influence of inverse flow on the wing is also discussed: The data confirm that when the airflow over the wing is reversed, flap deflections will affect the pitch moment in an opposite way compared to the non-reversed case, but this opposite effect can be avoided by increasing the throttle setting. The data show the exact relationship between flap deflections and forces in this condition. Moreover, it is found that the flap control effectiveness for a wing with or without spinning propellers is usually higher around zero degrees AoA than at (Formula presented.) and it is more effective to change the flaps from (Formula presented.) to (Formula presented.) than from (Formula presented.) to the respective (Formula presented.). ... Read more