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Albatrosses fly fast for huge distances, barely flapping a wing. Can they teach us to build drones that will stay aloft for months or even years?

PHILIP RICHARDSON’S eureka moment came in 1997, on the pitching deck of a ship in the South Atlantic. The vessel was steaming at more than 20 kilometres per hour into a strong headwind, yet a lone albatross swooping elegantly just above the waves was easily keeping pace.

These large birds remain on the wing for years at a time and thrive in blustery environments, especially in the powerful winds of the Southern Ocean. Clearly, these aren’t the conditions to create thermal updrafts – rising pockets of hot air that hawks and vultures ride like elevators. So what gives the albatross its lift?

This isn’t just an academic question. Richardson, an oceanographer at Woods Hole Oceanographic Institute in Massachusetts, realised that if he could unravel the bird’s flying secrets, they could lead to a new generation of uncrewed gliders capable of surveying vast areas of ocean without using a drop of fuel. Such gliders could also work over land, harnessing energy from the wind to extend flight durations from hours to days, months or even years.

Over the past 15 years, Richardson has investigated a variety of possible mechanisms for the albatross’s flight. Initially, he believed the birds were using “wave lift” to get a boost – capitalising on gusts of wind forced upwards by the sloping edges of waves to create updrafts like those found along cliffs or mountain ridges. However, calculations showed the effect isn’t strong enough to account for the bird’s remarkable flight.

Another possibility is that the bird uses the “wing-in-ground” effect: as it flies low over the waves, a pocket of smooth air is trapped between its wings and the water and . Alternatively, the birds could be making use of . By emerging from low behind a wave into faster moving air above, they could utilise sudden changes in wind speed to gain energy.

Altitude and attitude: an albatross in flight (Image: Tom Roberton/Getty)

But the most complex theory involves an idea developed by Lord Rayleigh in 1883 to explain how pelicans fly long distances. The “Rayleigh cycle” is based on the fact that friction slows the winds closer to the waves compared with winds 10 metres up, creating a vertical gradient of wind speeds. As the albatross flies into the wind just above the sea surface, it can use this gradient to gain altitude like a kite, riding the increasingly faster winds as it rises. Then it turns away from the wind and descends, gaining speed by diving. Finally, when the bird reaches its starting altitude it turns back into the wind in its original direction, only moving significantly faster.

Over the years, researchers have refined this “dynamic soaring” idea, suggesting that if the bird rolls slightly as it rises into the stronger winds, it can gain even more energy, thanks to the aerodynamic performance of its wings. This magnifies the air speed effect by a factor of 10 or more, says Colin Pennycuick, who researches bird flight at the University of Bristol. “It is just a matter of putting an efficient wing in the air in the right way.”

Dynamic soaring

To many researchers, including Richardson, this theory seems the most promising to explain an albatross’s abilities. But is it right? To find out, GPS tracking devices were attached to albatrosses by Gottfried Sachs and his colleagues at Munich University of Technology’s Institute of Flight System Dynamics working alongside French researchers. Smaller and more precise than conventional trackers, the devices record the bird’s position to within a few centimetres by taking measurements 10 times every second.

In 2013, the team gathered from tracking 16 wandering albatrosses in the Indian Ocean. Their data reveals a four stage flight pattern: a windward climb, an upper turn, a leeward descent and a lower turn – precisely the pattern you would expect for Rayleigh’s model. This is repeated over and over, with most energy being gained during the upper curve. According to Sachs, other mechanisms like gust soaring, wave lift and the ground effect aren’t significant – it’s all dynamic soaring, he says.

This technique is surprisingly flexible. Richardson has calculated that an albatross can use dynamic soaring to fly in any direction, upwind or down (). When travelling upwind, this generates a zigzag flight – like a tacking sailing ship – at a speed of about two-thirds that of its cruising speed. The model matches closely what Richardson observed in the South Atlantic.

This soaring technique could be incorporated into an ocean-surveillance drone on missions that last for days or weeks, says Richardson – and may even lead to gliders that can outperform an albatross. Current glider designs can withstand accelerations of 100g or more, and Richardson suggests that by copying the albatross they would be able to fly at speeds of 150 kilometres per hour for extended periods. “Gliders could soar much faster than an albatross because of stronger wings and air frames,” he says.

Nor is this limited to ocean-going drones. Anywhere with a strong wind gradient has the potential for dynamic soaring. And drones flying over land can make use of ridge lift provided by hills and mountains, as well as thermal updrafts.

It turns out that some radio-controlled glider enthusiasts already use wind gradients to help their craft go faster. Gathering in the hills of southern California on the lee of ridges, they loop their craft round and round, . The record for this type of flight stands at an astonishing 800 kph, typically driven by winds gusting at around 95 kph. “I was totally blown away by how fast they were flying,” says Richardson.

Autonomous drones are already getting in on the action. In 2008, an autonomous glider built by a team at North Carolina State University in Raleigh came third in the – the first autonomous machine to enter this US-based competition for remote-controlled gliders. It involved a series of challenges, including distance and speed competitions. Called ALOFT, the glider can both spot and use thermal updrafts to gain altitude and the craft has managed flights of more than 100 kilometres on its own.

Could this design be improved by incorporating the amazing abilities of an albatross? Salah Sukkarieh and his team at the University of Sydney in Australia believe so. They have developed sensors and software that can measure wind strength in real time. This allows us to estimate the energy available to a glider, whether it comes from updrafts or wind gradients, says Sukkarieh. And so far it seems to work: the team has added a flight planner and successfully tested the complete system in a glider on short missions.

Add solar panels and batteries and it is possible to imagine long-distance drones that mimic the capability of satellites at a fraction of the cost. They could be used as flying cellphone towers that shift capacity around as required, for everything from music festivals to disaster relief. Perhaps the biggest market is in farming, to record the state of crops, measure growth, detect weeds and pest infestations and allow the application of water, fertiliser and pesticides only where needed.

Even city-based drone delivery services could be wind-powered, says Caleb White and his colleagues at RMIT University in Melbourne, Australia. They studied the feasibility of using lift generated from wind patterns around buildings in urban areas to power small, engineless drones. Their . However, designing craft capable of flying near buildings without crashing will be a huge challenge. “Albatross have really good collision avoidance,” says autonomous-flight expert Jack Langelaan at Penn State University in University Park. “For us it’s a bit more difficult.”

“Lift generated from winds around buildings could power engineless drones”

This article appeared in print under the headline “Easy rider”