Autonomous Soaring Can Boost the Range and Endurance of Small Drones, Researchers Find

Many bird species in the natural world are capable of soaring within thermal updrafts for extended periods without flapping their wings.   Their ability to conserve energy and fly continuously over long distances – sometimes, even while sleeping – has fascinated drone designers looking for a way to extend the flight range of drones that must depend on batteries of limited power and duration.

Most drones on the market can only fly for 20-30 minutes before needing their batteries recharged.  While some in-flight recharging or battery-switching systems are available for larger drones, smaller UAVs generally lack this capacity; this persistent shortfall reduces their flight time and limits their utility for a wide range of commercial and public safety applications.

Drone engineers in China – at the Pre Research Center in Chengdu – are studying ways to design fixed-wing drones that can exploit thermal updrafts in the same way that birds do.  The researchers have created a simulated system that would allow drones to detect updrafts in real time and to adjust their flight modes and trajectory accordingly; they’ve also conducted a limited field test of an actual drone adjusting to the presence of updrafts and reducing its battery-powered propulsion accordingly.  While still in their infancy these early design efforts are showing that properly designed drones could conserve their limited battery power and extend their range and endurance while still airborne.

One of the challenges the designers faced was how to install a sensor system on a small drone to detect the presence of a thermal draft in real time.  Ultrasonic anemometers, porous probes, and wind vane sensors are difficult to install and come at a high cost, particularly for small drones. An alternate updraft perception method is to measure the rate of energy change using GPS, airspeed indicators, and what’s known as a descent polar curve.

While cruder and simpler, the Chinese researchers demonstrated the utility of these non-sensor based methods: the drone tested was able to soar in thermal updrafts for a period of 40 minutes using just 8 minutes of its battery propulsion power.

The researchers did detect a major difference in bird and drone behavior during thermal updrafts.  The drone was able to glide and circle to higher altitudes without propulsion power during major thermal updrafts – just as birds do –  but tended to lose altitude when the updraft was weaker and of less duration.  Birds, by contrast, naturally compensate for this discrepancy by flapping their wings slowly during minor updrafts to maintain their higher altitude.  The drone, as currently constructed, lacked the flexibility to adjust their propulsion power accordingly.

Another issue in need of further consideration is a drone’s perception of horizontal wind speed.  Unexpected and unstable turbulence can weaken the drone’s ability to exploit thermal updrafts;  additional on-board AI-driven data collection and analysis is needed but the increase in computational power also means a heavier payload that a smaller drone may not be able to accommodate.  Advances in miniaturization will be needed to allow for these stronger on-board systems.

This is certainly not the first time drone engineers have turned to birds as a source of inspiration for drone designs.  Engineers elsewhere have designed drones with bird-like feet and claws that can perch on branches and roofs and swoop and dive like birds.  Other bird-like drones are serving as “mobile scarecrows” – either on farms, to protect seeds and crops from aviary poachers, or on piers – to chase away pigeons.  And a New Mexico engineer is currently working on a “taxidermy” drone – a UAV equipped with sensors and cloaked in aviary camouflage — to try to infiltrate bird flocks to record and analyze their movements in real-time.

But this latest research and testing effort in China may have the most far-reaching commercial implications of any such experiment to date.  Allowing drones to conserve power in flight could allow for extended aerial surveillance operations in construction, mining, infrastructure inspection, retail delivery and public safety. It could also allow for a greater reliance on BVLOS operations without remote piloting and monitoring and a reduced dependence on costly ground infrastructure for mobile launching and charging stations.

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