Nature Inspired Aerial Crafts

Transformable HOvering Rotorcraft (THOR) *SG MARK 2018*

The Transformable HOvering Rotorcraft (THOR) is a novel and unique hybrid Unmanned Aerial Vehicle (UAV) that combines the range and speed of a horizontal fixed-wing platform with the hovering and manoeuvrability of a rotor-wing. Unlike conventional hybrid UAVs designs that feature distinct systems to achieve horizontal and vertical flight separately, THOR is highly structurally efficient as all aerodynamic surfaces and propulsion systems are fully utilized in both flight configurations. With its two large wings and compact fuselage, THOR can fly like a delta-wing aeroplane with high speed and long range. But when it needs to be agile and/or hold a position in the air, it flips its wings to mimic the natural autorotation of two-bladed samara seeds, spinning about itself to generate efficient and quiet lift. Moreover, this transition can be performed seamlessly and repeatedly in mid-air while in flight without the need for any additional mechanisms and actuators. Because THOR’s design is inherited from the samara seed, it is also fully capable of autorotation, which would allow the craft to safely descend to the ground in event of propulsion and power failure. This platform has broad, multi-industry implications especially in applications where travelling vast distances and negotiating tight spaces are required simultaneously. Package delivery to remote locations deep within urban jungles, close up inspections of offshore cargo ships and rapid response to disaster relief are just a few examples of potential use cases.

Ongoing research: Dynamic analysis, design optimization, non-linear controls, optimal transition trajectory, multi-modal sensing, robust mechanisms.

Featured in:
  • IEEE Spectrum, “THOR Transformer Drone Hovers and Cruises With No Compromises,” 17 Jul 2017.
  • New Atlas, “THOR hybrid UAV hovers like a helicopter, flies like a plane,” 24 Jul 2017.
  • The Verge, “This transforming drone can fly like an airplane and spin like a seed,” 20 Jul 2017.
  • sUAS News, “THOR Transformable HOvering Rotorcraft from the Singapore University of Technology and Design (SUTD),” 18 Jul 2017.
  • Daily Mail, “The THOR transformer drone inspired by a seed that can hover like a helicopter and fly like a plane,” 25 July 2017.
  • UAS Vision, “THOR Hybrid UAV from Singapore,” 28 Jul 2017.
  • Neowin, “THOR is a hybrid drone that can transform mid-flight from a helicopter to an airplane,” 21 Jul 2017.
  • VR Drone Geek, “THOR Hybrid UAV Accommodates both Helicopter and Plane Flying Techniques,” 26 Jul 2017.
  • AirSoc, “THOR hybrid UAV hovers like a helicopter, flies like a plane,” 27 Jul 2017.
  • DIYDrones, “THOR hybrid UAV hovers like a helicopter, flies like a plane,” 27 Jul 2017.
  • The Drive, “THOR Transformer Drone Can Hover and Cruise Equally Well,” 19 Jul 2017.
  • alt_driver, “Watch this drone transform in mid-air right before your eyes,” 21 Jul 2017.
  • Drones Player, “新加坡科技設計大學發明了 THOR,這是無人機、螺旋槳還是固定翼?,” 31 Jul 2017.
  • Sankei Shimbun, “シンガポール工科大学が新型UAVのプロトタイプを発表”, 1 Aug 2017.
  • Time & Space, “まるでタケコプター! 1枚の羽根で滑空し、空中で停止できる『THOR』がすごい理由,” 2 Oct 2017.
  1. J. E. Low, L. S. T. Win, C. H. Tan, D. S. B. Shaiful, G. S. Soh, S. Foong, “Design and Dynamic Analysis of a Transformable HOvering Rotorcraft (THOR)”, 2017 IEEE International Conference on Robotics and Automation (ICRA), Singapore, 2017, pp. 6389-6396. doi: 10.1109/ICRA.2017.7989755
  2. T. Lembono, J. E. Low, L. S. T. Win, S. Foong, U-X. Tan, “Orientation Filter and Angular Rates Estimation in Monocopter Using Accelerometers and Magnetometer with Extended Kalman Filter”, 2017 IEEE International Conference on Robotics and Automation (ICRA), Singapore, 2017, pp. 4537-4543. doi: 10.1109/ICRA.2017.7989527



Monocopters are a class of rotorcrafts known as free rotors where the entire airframe is continuously rotated to provide lift during flight. This mode of flight, which is employed in nature by the samara of maple trees to cover extended distances through unpowered stable autorotation, can be adapted for controlled powered flight with the same manoeuvrability and degrees of freedom as conventional helicopter. Unlike flapping wing and multi-rotor based designs, the monocopter possesses an inherent gyroscopic stability and mechanically robust as it has very few moving parts and does not have intricate and fragile swashplate and linkage mechanisms in helicopters and flapping wing systems. Most importantly, recent work has shown that a single rotating wing system is fundamentally more efficient and requires half as much power as a flapping wing to achieve a comparable level of flight performance.

Fixed-wing crafts are capable of gliding which allow them to continue to generate lift in absence of thrust. Rotor crafts such as helicopters and multi-rotor crafts are superior to fixed-wing crafts in terms of manoeuvrability and ability to perform hovering. While helicopters can execute autorotation and land safely in event of engine failure, it features an intricate and fragile swashplate and linkage mechanisms. Multi-rotor crafts such as quadrotors are mechanically simpler employing multiple rotors to achieve lift and control authority. However, compared to helicopters and fixed-wing crafts, they have inferior efficiency and mission endurance. Moreover, loss of just one motor is often catastrophic. As demonstrated in nature by birds and insects, flapping wings systems offer advantages in efficiency and manoeuvrability compared to fixed-wing crafts as well as permitting hovering and vertical take-off and landing. However, the complexity of the actuating system and fragility of the wing (which has to be lightweight to allow it to flap) is a disadvantage during transportation and field deployment.

Ongoing research: Autorotation, Geometrical wing analysis, rotational aerodynamics, gyroscopic precession, rapid prototyping, under-actuated design and control.

  1. J. E. Low, Y. H. Pheh and S. Foong, “Analysis of wing twist effects on hover flight dynamics of a single rotor aerial craft,” 2016 IEEE International Conference on Advanced Intelligent Mechatronics (AIM), Banff, AB, 2016, pp. 323-328. doi: 10.1109/AIM.2016.7576787
  2. S. K. H. Win, C. H. Tan, D. S. B. Shaiful, J. E. Low, G. S. Soh and S. Foong, “The Effects of Chordwise Wing Optimization of Single-Winged Samara in Autorotation,” 2017 IEEE International Conference on Advanced Intelligent Mechatronics (AIM), Munich, 2017, pp. 815-820. doi: 10.1109/AIM.2017.8014118