A research team in western China has unveiled a drone capable of travelling through air and underwater.
Although it is not the first "transmedium" drone the world has seen, the Chinese prototype uses a design with improved underwater mobility.
Water is 800 times more dense than air, and stickier. Similar drones developed in Western countries must rotate their blades at a slow speed while underwater or risk snapping. But the Chinese drone used two kinds of blades with one designed to spin 3,600 times per minute in water to generate a powerful thrust.
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A drone can easily lose balance and flip when moving from one medium to the other. During a 90-second test flight, though, the 1.5kg (3.3lb) Chinese drone remained intact after diving into and emerging from the water seven times.
The technology has shown potential in a wide range of applications, according to the researchers, but they said there was room for improvement in its 20-minute flight time and 500-gram (18-ounce) payload capacity.
"The cross-medium UAV [unmanned aerial vehicle] is designed to expand the operating environment and application range of existing aircraft, and can make full use of stealth underwater and high manoeuvrability in the air," said Zhang Shuxin and his colleagues at the school of mechano-electronic engineering in Xidian University in Xian.
Notions of transmedium flight can be traced back to at least the 1930s. Today, some weapons such as submarine-launched intercontinental ballistic missiles (ICBMs) and airdropped torpedoes can travel through different mediums. But they can move in or out of water just once.
Scientists have not yet found a physical model that works perfectly in both the air and water, so the transmedium journey involves many uncertainties. Nonetheless, many countries have invested heavily in research in this sector, hoping to develop a disruptive technology that can change the dynamics of a sea battle.
China, for instance, is developing transmedium drones to be released by a submarine hundreds of metres under the water's surface for airborne surveillance, communication or attack, according to publicly available information.
Chinese military scientists are also developing some high-speed, long-range missiles that can jump in and out of water like a flying fish to evade the defence system of a warship or aircraft carrier.
Although the formal deployment of such weapons has not been openly reported yet, some military experts believe they would have a major impact on existing naval tactics and strategy.
Earlier this year, the US navy confirmed some rare footage of UFOs. Though the public interest weighed heavily on the possibility of alien spacecraft, the US military stressed the unexplained objects could be man-made.
In one video, an oval-shaped object hovered near a US warship for some time, then entered the water with a splash.
The US military said their major concern was not outer space intelligence but new weapons deployed by countries such as Russia and China.
According to some recent studies by Chinese military researchers, UFO sightings in Chinese territories also increased significantly in the past decade, and they believed that most of these objects were man-made.
The US military funded at least one research project more than 10 years ago to develop a saucer-like aircraft that could travel in both air and water, according to media reports.
Yuan Xulong, an associate professor with the cross-media flight dynamics laboratory at Northwestern Polytechnical University in Xian, said there was a chance that the UFO that splashed down in the US footage was a transmedium drone.
Considering its shape and size, which could be more than 10 metres (33 feet) long, the subject would need to have a big power source, such as jet or rocket engines, according to Yuan.
"There are two types of transmedium drones. One is high speed, the other one is slow. Judging by the way it entered water, it might belong to the latter," he said on Tuesday. "Global competition in this field is heating up."
A drone must be smart to swim and fly, according to Zhang's study. The fluid physics where air meets water creates surprising results.
When the drone moved up to the surface and started its aerial blades they found the air turbulence could create holes in the water and disturb the operation of submarine blades still under the surface.
Shock waves produced by repeated, fast entry into the water could also damage motors and sensitive equipment on board. As a result, the researchers needed to develop new flight sequences to reduce the impact.
And a drone that moved from the air to underwater would require additional communication technology on board because radio waves do not travel far in water, according to Zhang and colleagues.
Though transmedium drones today are mostly developed for military use, the researchers expected it to have some civilian applications.
Lifeguards at the beach, for instance, could use the drone to quickly find swimmers in trouble from the air and deliver life-saving equipment to drowning victims.
This article originally appeared in the South China Morning Post (SCMP), the most authoritative voice reporting on China and Asia for more than a century. For more SCMP stories, please explore the SCMP app or visit the SCMP's Facebook and Twitter pages. Copyright © 2021 South China Morning Post Publishers Ltd. All rights reserved.
Copyright (c) 2021. South China Morning Post Publishers Ltd. All rights reserved.