Video: A bizarre aircraft design dating from the late 19th century could provide the perfect lift for small, agile flying robots (Video courtesy of Hu Yu/Singapore National University)

A small robotic aircraft powered by rotating “paddle-wheel” wings could yet rule the skies, if renewed interest in an antique design bears fruit.

Several international research groups are working on prototype “cyclogyros”, a design first proposed more than 100 years ago.

A cyclogyro flies using “cycloidal propellers” – several wings positioned around the edge of a rotating cylindrical framework, a bit like a paddle-wheel. As each wing rotates, its blades move through the air generating lift and thrust.


And, since each wing rotates through a full circle, altering the angle of the individual blades can pull the aircraft forwards, backwards and down as well up. The manoeuvrability that cycloidal propellers could offer provides benefits over more established flying methods.

Although no cyclogyro has yet flown without being tethered, its proponents say the design could prove more efficient and manoeuvrable than helicopters at small scales. A team of Singapore researchers is leading the race to construct a working cyclogyro with a prototype that hovers on the end of a line.

Quieter ride

Lim Kah Bin and Hu Yu at the National University of Singapore have built a small cyclogyro that hovers and turns on the end of a tether, which delivers power to its electric motors (see video, top right).

The researchers heard about the cyclogyro’s design while working on miniature aircraft with flapping wings. They performed simulations confirming that cycloidal propellers can be more efficient than those used on aeroplanes and helicopters. “They will also be potentially much quieter than screw rotors,” they add.

After studying the performance of different cycloidal designs, the pair modified a toy helicopter, giving it two cycloidal propellers with three blades each, and a small tail rotor for stability.

“On the tether, the aircraft can spin, move directly up and down or fly forward and backward,” says Hu. “This is perhaps the first recorded flight for a cyclogyro,” he adds. “There were some people claiming successful flights, but no video or proof for that.”

Unstable aerobatics

For now, the aircraft still lacks stability. Its tether is needed to restrain it from spinning on the spot or performing loops in the air. But the team hopes to add a tail rotor to make it more stable, and to build the power supply into the craft.

“I’m full of admiration that they got it to fly,” says Daniel Weihs, an aerospace engineer interested in cyclogyros at the Technion Israel Institute of Technology (TIIT), Haifa, “but it does still rely on the tether.” Using four cycloidal propellers instead of two might remove the need for a tail rotor, he suggests.

Weihs and colleagues Gil Iosilevskii and Yuval Levy have also been working on simulated and real-life cyclogyro experiments. “We built models and proved that it can work, but tried to go directly from that to a self-contained flyer carrying its own power,” Weihs told New Scientist. “We tried to leapfrog the tethered stage.”

Reaching for the skies

Other research groups hoping to resurrect the exotic design include one at Seoul National University, Korea. The team there has built a craft that can lift off by a few centimetres attached to several short tethers.

Engineers at the University of Electro-Communications, Tokyo, Japan, have their own designs. A video (mpg format) shows one of their cycloidal propellers with five pairs of stacked wings lifting its own weight. Another clip shows four propellers working together (wmv format).

After a break from cyclogyro development, Weihs now plans further experiments, saying the recent resurgence in interest in the design is encouraging. “It has shown us there are different ways to make them work, and we can try and bring that together,” he says.

Weihs discovered that a cyclogyro was first patented by a German inventor in 1893. The idea was most thoroughly investigated in the early 20th century, with popular magazines suggesting passenger cyclogyros were on the horizon. Unfortunately prototypes struggled. “They broke themselves apart,” says Weihs.

Part of the problem was size. The biggest challenge for cyclogyro builders is making strong but light structures able to handle the strong forces generated inside a cycloidal propeller when it spins, says Weihs. At larger sizes that becomes more difficult.

Small is beautiful

“Cyclogyros are more relevant now because people want to build small, agile UAVs [uncrewed aerial vehicles],” says Weihs. At such sizes they have greater advantages over helicopters, he says.

The parts of a helicopter blade nearest and furthest from the hub are moving too slowly and too fast, respectively, to generate thrust. “With a cyclogyro every bit moves at the same speed, so there is no ‘dead space’,” says Weihs.

Cyclogyros can also be more manoeuvrable, says Weihs. Helicopters must tilt to travel laterally. But cycloidal propellers can generate thrust in any direction so the craft can remain level, or adopt any other position and still fly in any direction.

These advantages are greatest at small sizes. “They are probably not practical above half a metre across,” says Weihs. “You won’t see one carry a passenger.”