SerpApi.Org
A small experimental drone rises from its launcher, its batteries and circuit board plainly visible. Then its body begins whipping around as many as 25 times a second. The parts dissolve into pale rings, and the machine becomes less an aircraft than a smudge in the air.
Northwestern University engineers call it Phantom Twist, a single-propeller drone designed to exploit a quirk of vision: eyes gather light over a brief interval, so fast-moving objects blur together. Rather than covering a drone in camouflage, the researchers rearranged its working parts so rotation spreads them thinly across the background.
The prototype is not exactly invisible, nor has the team shown in human trials that observers consistently miss it. But a computer model intended to approximate human perception rated the optimized aircraft about 10 times less noticeable than a conventional quadcopter. The researchers will present the work on Thursday at the Robotics: Science and Systems conference in Sydney.
“Most efforts to hide drones focus on making them look like their surroundings,” said Michael Rubenstein, the Northwestern roboticist who led the work. “Instead, we asked whether we could design the drone itself around the way humans perceive motion. This idea of low visibility through persistent motion is something few people have explored.”
Designing a Flying Blur
A normal quadcopter keeps its body largely still while four rotors spin. Phantom Twist uses one offset motor. Its propeller turns one way while the aircraft rotates in the opposite direction, producing gyroscopic stability. The controller changes thrust at precise points during each revolution to steer.
The architecture was inspired by earlier minimalist aircraft such as Piccolissimo, a one-motor microdrone reported in 2017 that’s only 28 millimeters (1.1 inches) across, and a later Northwestern-led version that used constant rotation for infrared communication and navigation.
The team used AI to generate about 20,000 viable arrangements of two batteries, a circuit board, a motor, a propeller and up to two counterweights. Each had to remain under 40 grams, fit inside a cylinder about 14 centimeters wide and spin between 15 and 25 revolutions per second — fast enough to blur, but slow enough to remain controllable.
The researchers rendered every promising design through a full rotation, viewed it from 10 angles and placed the blur over 100 real-world scenes. An image-comparison system scored how much the drone changed each background. The software refined the 500 least conspicuous candidates, cutting their visibility score by 12 percent on average. The best design scored 0.0104, compared with 0.0207 for a human-arranged version.
“The human eye takes time to accumulate signals, roughly analogous to the exposure time of a camera,” said Emma Alexander, a Northwestern computer-vision researcher, who explained the invisibility effect of a fast-spinning drone. “When an object spins quickly, we perceive it as blurring out and losing distinct features.”
A Quieter Presence, but Not Yet Quiet
The researchers built three prototypes and flew them under laboratory tracking. All hovered for about 10 minutes, with a typical positioning error of about three centimeters. Long-exposure photographs broadly resembled the simulations. The optimized craft appeared as a faint, translucent volume rather than the dark rings produced by the hand-designed drone.
Such drones could prove particularly useful in wildlife research. A 2017 systematic review in PLOS One found that drone size, engine noise and flight pattern influenced animals’ reactions, with birds particularly prone to disturbance. A less visible aircraft might reduce one part of that problem, although Phantom Twist’s propeller still announces itself quite audibly.
The concept joins a broader effort to make robots less conspicuous. Other teams have built transparent soft robots and chameleon-like machines that alter their colors. Phantom Twist instead treats movement itself as the hiding material.
This story originally appeared on ZME Science. Want to get smarter every day? Subscribe to our newsletter and stay ahead with the latest science news.
