Nothing lasts forever: things wear out. However, there is a huge pent up demand for "perpetual" autonomous drones for everything from security to surveying, meteorology and even power generation. Imagine them permanently looking down on secure areas, even houses. Ideally, they should have Vertical Take Off and Landing VTOL so they can operate from a small footprint on ground and in the air and be autonomous in navigation and task so they descend when a storm is coming in or when they need repair or stealth.
Unfortunately, VTOL is very power hungry. In 2016, Latitude Engineering's HG-60B hybrid quadcopter set a 22.5 hour record for endurance using an internal combustion engine multicopter because it cruised as a fixed wing plane. It carried a camera and radio to show it could be useful.
Fixed wing alone goes further. In 2017, a fixed-wing drone from Vanilla Aircraft backed by DARPA achieved a world-duration flight record for drones in its class. Designed for a 30 pound payload to be carried at 15,000 feet for up to 10 days, it flew for nearly 56 uninterrupted hours at an altitude of 6,500 to 7,500 feet after being launched on Nov. 30 from New Mexico State University's Unmanned Air Systems Flight Test Center.
The National Aeronautic Association certified the flight as "achieving the world duration record for combustion-powered UAVs in the 50 kg-500 kg subclass (FAI Class U-1.c Group 1)," according to a DARPA press release. It also captured record for the fourth-longest flight for any unmanned aircraft. A 120-hour flight was planned. The aircraft had to be grounded early on Dec. 2 due to incoming bad weather but still carried a half tank of fuel. Nonetheless, after finding a few niche markets, these improving records with internal combustion power will be lost into the history books alongside records set by steam trains because electric drive is progressing so much further and faster.
There are two approaches to this - solar power and tethered power. Zephyr was a High-Altitude Long-Endurance (HALE) Unmanned Aerial Vehicle (UAV) made of ultra-lightweight carbon-fibre. It flew on solar power from amorphous silicon arrays covering the wings, no thicker than sheets of paper. It was powered by rechargeable lithium-sulfur batteries, for lightest weight, that were recharged during the day using solar power. On 9 July 2010, Zephyr set three new world records including the absolute UAV duration record of 336 hrs 22 minutes (14 days and 22 minutes) and absolute altitude record for a UAV in its category - of 70,740 ft (21,561 m).
ETH Zurich's AtlantikSolar 2 Unmanned Aerial Vehicle achieved continuous flight of 2316km and 81.5 hours (4 days and 3 nights) July 14th-17th 2015 and broke the flight endurance world record for UAVs below 50kg. In addition, the fully solar-powered flight was the longest-ever flight of a low-altitude long-endurance (LALE) Unmanned Aerial Vehicle. AtlantikSolar UAV has completed its next milestone by demonstrating the first-ever fully-autonomous (from launch to landing) solar-powered perpetual flight with significant payload (Color + Thermal Camera) in a 26-hour Search-and-Rescue (SaR) mission.
However, the missions that its team really cares about - Search-and-Rescue missions relying on long-endurance aerial sensing to support authorities e.g. in the European refugee crisis currently unfolding over the Mediterranean Sea - require both of these elements, i.e. ease-of-use through full launch-to-land aircraft autonomy as well as significant payloads to help rescue teams. ETH researchers are convinced that they can be of great help in solving pressing issues such as the European refugee crisis across Mediterranean Sea.
The 26-hour solar-powered Search-and-Rescue flight performed by AtlantikSolar AS-3 from July 19th - 20th 2016 was the first-ever flight worldwide to combine:
Perpetual flight: 26-hours of solar-powered day/night- and thus energetically-perpetual flight
Full aircraft autonomy: No pilot stick moved within 26-hours of flight
SaR payload: The aircraft carried a 10 Watt 300g payload (1 Color camera, 1 Thermal Camera, 1 ODROID onboard computer with WLAN) and performed victim detection from the air during day and night.
Environment-aware: The aircraft performed automatic thermal updraft tracking for increased energetic efficiency and to speed up the battery recharge.
Now things are getting even more dramatic, proving that solar power for aircraft is more capable than commonly realised. Solar Impulse completed its hops around the world on sunshine alone in 2016: it was manned because they wanted to make a point yet it still achieved a longest hop of Japan to Hawaii of 117 hrs 52 mins 4,819 nmi (8,924 km) at 40.9 kn (75.7 km/h) 28,327 ft (8,634 m). Again making a point about manned aircraft, in 2017, PCAero of Germany and partners are preparing to have a solar aircraft take off and rise to 80,000 feet with the photovoltaics even powering the pilot's spacesuit as it does so.
An autonomous version would be easier to achieve. Indeed, the Solar Impulse team's next project, together with sponsors Solvay and ABB, is to develop a large unmanned solar aircraft that can fly continuously at high altitudes to carry out some of the work currently performed by satellites. It will also be able to run applications that cannot currently be performed by satellites, in such fields as communication, measurements and observations for agriculture, infrastructure planning and for other purposes. First prototypes will fly in 2019. Meanwhile, Alphabet Google has abandoned its Titan version but Facebook continues with its Aquila autonomous solar plane at 60,000 feet for delivering the internet to the 4500 million people that lack it. Fixed wing solar aircraft that stay aloft for 5 years are planned and helium-filled dirigibles that can stay aloft for ten years and deploy anywhere in the world in 15 days are being developed by Lockheed Martin and others despite the Northrop Grumman $517 million DOD order for one being cancelled. Yes, there are billions of dollars being invested in upper atmosphere solar drones: High Altitude Long Endurance HALE is the moniker.
The sun shines all the time in the upper atmosphere: not so near the ground, where there are also many needs for perpetual aircraft. That is why Aerovironment with Tether Eye and other developers have surveillance multicopters tethered by a power cable as they fly above their power pod. They can be battery or grid powered from the ground as required. This is not as clunky as it sounds. They will stay up for years and multicopters are far more tolerant of wind than helicopters. Tether Eye already operates in rain, winds of up to 15 kts and it can fly from moving vehicles for "reconnaissance, surveillance and intelligence".
On another tack, the autonomous giant drone of the Makhani project of Alphabet takes off using a megawatt tether and its eight monster fans become wind turbines generating 750kW initially with several megawatts generation in prospect. The TwingTec tethered drone in Switzerland takes off and lands autonomously. It has fixes wings and vertical thrusters do it can act as a kite generating 100kW by a pumping action to a generator on land.
A new report, Electric UAV Drones: Autonomous, Energy Independent 2017-2027 from IDTechEx looks at the whole status and future of forthcoming electric drones from swarming ones to those that dive into the sea and fly out again. Both of these have already been demonstrated. The report gives forecasts, technology roadmaps and project assessments for electric drones. The internal combustion engine is nowhere in sight. Little wonder that the EV overview report by IDTechEx and covering land, water and air is called, Electric Vehicles Change the World 2017-2037. The future is electric.