Their findings could alter our thinking about how birds and insects first learnt to fly. It could also influence future drone designs.
Lund University’s Associate Professor of biology Christoffer Johansson told Reuters: “We are using a technique called particle image velosymmetry where we fill up the wind tunnel with tiny particles that we light up with a laser. We film the particles with multiple high speed cameras which allows us to determine the velocity induced by the animals in the air. We have used that information to calculate the cost of flying for animals and compared when the animals are flying in the free air to when they’re close to the ground.”
The team’s findings suggested that bats save twice as much energy by flying close to the ground as models predicted.
“The wing tips move really close to the ground during their down stroke which we think reduces the cost of generating lift,” explained Johansson.
He added: “Our main finding is that we see a reduction in the power requirements when flying close to the ground of 29 percent. Theoretical predictions are difficult to make but the highest we’ve seen is around 14 percent. So we saw twice the power savings that we expected.”
The ground effect means that a surface acts as an aerodynamic mirror which increases the air pressure under the wings. It is achieved within a single wingspan of the surface.
The study supports one of the competing theories on how birds began to fly – the so-called ‘ground-up theory’. This postulates that flying began on the ground and that by running and jumping with proto-wings various animals were able to run faster and jump higher, eventually evolving into flight.
The research also showed that animals with flapping wings use less energy than steady wings moving near the ground. This could have implications for future generations of unmanned aerial vehicles (UAVs) – or drones.
“If we’re thinking of quadcopters then wings probably won’t have much application but if we consider flapping drones it would make sense to have them fly close to the ground,” said Johansson. “If they’re travelling long distances it could allow drones to save energy and allow them to travel further.”
Various researchers, including a group at Switzerland’s EPFL research institute, are working on developing flapping drones.
Among the best-known such commercial drones is the Avitron Bluetooth Bionic Bird.