Ever since the Wright brothers, engineers have been working to develop bigger and better flying machines that maximize lift while minimizing drag. There has always been a need to efficiently carry more people and more cargo. And so the science and engineering of getting large aircraft off the ground is very well understood.

Hui Hu, an Iowa State University associate professor of aerospace engineering, said there hasn’t been a need to understand the airflow, the eddies and the spinning vortices created by flapping wings and so there haven’t been many engineering studies of small-scale flight. But that’s changing.

For a few years, Hu’s been using wind tunnel tests and imaging technologies to learn why dragonflies and bats are such effective fliers. How, for example, do flapping frequency, flight speed and wing angle affect the lift and thrust of a flapping wing? A study based on the dragonfly, for example, found the uneven, sawtooth surface of the insect’s wing performed better than a smooth airfoil in the slow-speed, high-drag conditions of small-scale flight.

Using particle image velocimetry – an imaging technique that uses lasers and cameras to measure and record flows – Hu found the corrugated wing created tiny air cushions that kept oncoming airflow attached to the wing’s surface. That stable airflow helped boost performance in the challenging flight conditions.

And what about building tiny flying machines that use flapping wings? Hu’s using piezoelectrics -- materials that bend when subjected to an electric current -- to create flapping movements. That way flapping depends on feeding current to a material, rather than relying on a motor, gears and other moving parts.

Hu has also used his wind tunnel and imaging tests to study how pairs of flapping wings work together – just like they do on a dragonfly. He learned wings flapping out of sync (one wing up while the second is down) created more thrust. And tandem wings working side by side, rather than top to bottom, maximize thrust and lift.

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