Stealth drone spins so fast that it disappears

Stealth bombers use exotic materials to lower their radar signature and spy satellites orbit Earth far outside human sight, but a new drone avoids detection in an altogether simpler way: spinning so fast that it blurs into the background.

Emma Alexander and her colleagues at Northwestern University in Illinois created the drone, known as Phantom Twist. It spins at 25 revolutions a second to hide itself in a motion blur like the blades of a fan. But unlike a fan, which has fixed parts that remain clearly visible – the central axle, controls, a stand – every single part of Phantom Twist whirs in circles.

The unusual single-motor design was chosen through a complex, multi-stage and entirely automated process. First, an AI created a list of millions of designs, which was whittled down to around 20,000 that were theoretically able to fly.

Next, a second AI slightly tweaked the placement of each component in every design to minimise visibility from all viewing angles, on average. Finally, a third AI model that mimicked human sight assessed the visibility of each shortlisted design in front of various backgrounds. The researchers simply stepped in at the end to build the best, final design.

That winning design spread the components around so that none visually overlapped when spinning. The drone is small enough to fit in the palm of the hand and weighs just 30 grams.

“The motion blur essentially turns all of the mechanical components into this slight haze,” says Alexander. “And if you’re not paying attention, you might really miss that slight change in the brightness of the environment.”

David Whitaker at Cardiff University, UK, who studies optical perception, says that although the human visual system can detect changes up to 60 times a second in some situations, such as when observing a flashing LED, this rotating drone is moving fast enough that the brain simply merges its parts and the background together – not becoming invisible, but blurring enough that it can be missed if the background closely matches the drone’s colour.

“Imagine a cooling fan, spinning round and round. If it spins fast enough, you lose appreciation of the blades. The visual system can’t cope,” says Whitaker. “When things are moving fast, the visual system merges those moving objects with the background,
so they become relatively invisible.”

Peter Lee at the University of Portsmouth, UK, who wasn’t involved in the research, says that stealthy drone technology has obvious military applications, but that this design – while deploying a nifty optical illusion – has serious limitations.

The trick relies on a very spindly and sparse design, so adding any additional sensors or payloads would make it more visible, he says. And adding weight or scaling the design up in size would also lead to much higher centrifugal forces, potentially making flight impossible or causing the drone to break apart. Added to this, the gyroscopic effect of a spinning drone would make quick changes of direction extremely difficult.

“This style of drone is not manoeuvrable in the way that quadcopters are highly manoeuvrable. So because of the rotation, it won’t be able to bank at steep angles,” says Lee. “It would slow the speed of the rotation, and therefore it would become more visible and it’s probably getting unstable.”

The researchers admit that the drone isn’t highly steerable – it can currently only hold a steady hover – and that it can still be heard easily even if not seen. But they believe that choosing transparent components rather than opaque ones, such as the black carbon-fibre rods that make up the structure, could further reduce visibility. And changes to the AI design process to take into account the visibility of even the smallest components on the device could also bring further gains.

The researchers will present their work at the Robotics: Science and Systems conference in Sydney, Australia on 16 July.