Aerodynamicists can only marvel at nature.

Insects, like locusts, need very little energy to fly great distances. Bumblebees can carry their own weight in pollen. And moths can hover over a particular spot - without moving - while collecting nectar.

Scientists say understanding such flight characteristics could help them and engineers build very small flying machines - even smaller than what is possible today.

"Nature has solved the problem," says Richard Bomphrey from the Zoology Department at the University of Oxford.

So a team from the University of Oxford and the German Aerospace Center (DLR) in Göttingen have joined forces to find out more about nature's "best-practice models."

Big locusts only weigh about two grams (0.07 ounces), but they can easily cover 100 kilometers (62 miles) in flight a day. Unlike birds and airplanes, they have two pairs of wafer-thin wings.

"Locusts combine lift and propulsion with a certain wing position," says Andreas Schröder from the DLR Institute of Aerodynamics and Flow Technology.

They lift themselves up by turning their wings with every beat - their movements are like those of a swimmer.

To emulate what happens in nature, the researchers placed locusts and moths in a wind tunnel to measure the airflow behind their wings.

Working with living animals was an entirely new experience for physicist Daniel Schanz, who prepared the experiment in Göttingen.

"Usually, we're measuring metal constructions that can't move and don't want to fly away," says Schanz.

But the locusts were perfect for the experiment.

For one, they have a strong chest area, which allowed the researchers to fix them onto a holder. After the experiment, the holder was removed without harm to the locusts.

Another advantage comes from a natural reflex.

"If they don't feel ground under their feet and find themselves facing headwind, they begin to beat their wings quite regularly because they think they are up in the air flying," says Schanz. So the locusts in the wind tunnel seemed to be flying as if they were out in the open.

Visualizing airflow

An entire flight sequence was reconstructed.

The locusts were blown at speeds of 11 and seven kilometers per hour (7 and 4 miles per hour, respectively). And while this was happening the scientists took pictures of the airflow behind the wings.

By introducing very small droplets to the air, they were able to make the airflow visible.

Eight cameras took 230 images every 23 seconds - from different angles. Two high-powered lasers were used as flash lights.

In the interval of just a couple of microseconds, the scientists took pictures of the particle distribution.

"In the short interval between the two flashes, the flow in the wind tunnel hardly moved at all," says Schröder.

Each camera produced two black and white pictures that look like star constellations. The pictures look almost identically, but there is a subtle difference.

"If you switch back and forth between both pictures, you see that the stars are moving in the smallest way," Schanz explains.

In a next step, a computer analyzed these changes by placing the two images on top of each other.

They found there were areas where particles had clearly shifted.

Using this data, they calculated velocity vectors - the movements of particle groups to determine where turbulence occurs.

Track down the airflow

The researchers then created three-dimensional representations of the airflow behind the locusts using the data from all eight cameras. The resulting tomography showed turbulence and airflow in different colors - rapidly-moving air is depicted in red, slow-moving is blue. And static air is invisible.

"What it shows is the complete structure of the locust's vertebra as it changes with the movement of the wings," Schanz says. It is this that has helped the researchers understand the insects' flight characteristics and efficiency.

But the insect itself can not be seen. Its picture wasn't taken.

"It was technically impossible because the locust would have been directly exposed to the light," Schanz says. This would have damaged the cameras because insects reflect too much of a laser's light. In addition, the locust would have been in the way of at least one camera at all times and would have cast a shadow, making measurements from behind impossible.

The scientists say their results have given them vital insight into the construction of - what will become - their mechanical locust.

Mini insect-like flying machines could be used to monitor pipelines and help detect leaks. They could also provide assistance during disasters. In situations like the 2011 Fukushima nuclear catastrophe in Japan, the little flying devices could have entered reactor buildings and enabled the workers to avoid radiation exposure.

But the scientists say it will be another 20 years before their visionary devices make it onto the market.

Author: Fabian Schmidt /sst
Editor: Zulfikar Abbany