Conventional multicopter drones are excellent at hovering and VTOL, but they can't cover long distances as efficiently as fixed-wing aircraft. A team of students from the Singapore University of Technology and Design is trying to get the best of both worlds with the Transformable HOvering Rotorcraft (THOR).

According to the design team, the THOR is based on a concept raised in 1913. Called the monocopter, it was "structurally efficient" – the whole frame is used to generate lift – but "strategically inefficient." The lack of counter-rotational systems meant the pilot and cargo would have needed to spin with the rest of the aircraft, making it impractical to actually ride in.

The Singapore University of Technology and Design (SUTD) isn't alone in trying to revive the monocopter. Lockheed Martin and MIT have both created monocopter-style concepts that have their roots in nature and were designed to explore how the idea could improve current UAVs.

The THOR prototype in hover mode

According to the team, the design of THOR improves on the basic monocopter design in a few new ways. The first involved a second wing – and yes, that means it isn't a monocopter anymore. Lockheed Martin's design only used one wing, but the SUTD team have fitted two wings that protrude from the center of the UAV.

STUD took inspiration from the samara seed, a flying tree seed that can be carried long distances by the wind because of its inherently stable, wing-style shape. Although some variations of the seed look like a single wing (or propellor blade), there are other "two-winged" examples as well.

The THOR's opposing wings are mounted at right angles to each other and rotate into alignment when making the transition from helicopter-style hover to fixed-wing-style cruising. The students have also created a passive system to shuffle weight around based on flight mode. When the craft switches from hovering vehicle to fixed-wing aircraft, or vice versa, the centrifugal force involved in the switch is used to move the ballast into a position to keep the aircraft balanced.

A breakdown of the components in the THOR prototype

It's just a prototype at the moment, but the THOR concept could be useful in a huge range of industrial environments. Agriculture, surveillance and payload delivery are three areas mentioned by the SUTD team, who say that most hybrid UAVs are only efficient in one of their flight modes.

There are still a few tweaks that need to be made before the project is ready for public consumption. To make sure it doesn't just drop from the sky if there's a problem, the team is working on an autorotation system that will allow it to glide down safely without power.

The THOR prototype can be seen hovering and transitioning to cruise mode and back again in the video below.

Source: Singapore University of Technology and Design