Reaching a whale pod, an iceberg or a stretch of open sea to take a measurement is often expensive and, at times, dangerous work. A palm-sized robot developed by engineers at MIT and the Swiss university EPFL hints at a cheaper way to do it. The machine can cruise through the air, plunge into the water to gather data, and then flap its way back out to fly home — all with a single pair of wings.
The device, described in the journal Science, is called a flapping-wing aerial-aquatic vehicle, or FAAV. It weighs only about 250 grams, has a slim carbon-fibre body, a steerable tail and two flexible membrane wings that beat like a bird's. Its wings and tail can be swapped for different sizes. In the air it travels at roughly 6.3 metres per second; underwater it paddles along at close to one metre per second.
The inspiration is the diving bird. Loons, puffins, gulls and petrels — around 100 bird species in all — chase prey beneath the surface and then burst back into flight, mastering two fluids that differ enormously in density. Recreating the hardest part of that trick, the leap from water back into the air, has long eluded engineers.
Testing the robot in a water tank and at a nearby lake, the team led by Raphael Zufferey, who heads the AURA Lab at MIT, found the right combinations of wing size, flapping frequency and tail angle to make the transition smooth. Crucially, the robot manages it without propellers, without folding its wings and without paddling feet. Larger wings improved flight without greatly hurting underwater efficiency, while the angle of exit and the spacing of the tail proved decisive for breaking the surface cleanly.
The researchers, whose co-authors include colleagues at EPFL and Northwest Indian College in Washington state, imagine such robots launched from a boat or a beach. They could fly to a site of interest, dive to take a sample and return with the data at a fraction of the usual cost — then head straight back out for more.