It's relatively easy to get something big and heavy to fly. With enough equipment, it's possible to load the object with lots of energy to power the flight, specialized parts to control it, and the computers (or people) needed to direct the flight. But things get challenging as you make things smaller, and it gets harder to squeeze all the requisite parts into an ever-shrinking space. In that, nature has us beat, since something like a fruit fly crams all the energy, control systems, and specialized hardware into an extremely compact form.

We may not be at fruit fly level yet, but researchers are giving the insects some competition. Today's issue of Science reports on miniature flying robots that aren't much bigger than a coin. The power and control are handled externally, but the tiny robots can still perform basic maneuvers, and they have enough lift to spare that they could fly under their own power for a few minutes if the right power storage were developed.

The authors are all from the Wyss Institute for Biologically Inspired Engineering at Harvard, and they clearly find insects inspirational, noting that, despite their simple nervous systems, "flying insects are able to perform sophisticated aerodynamic feats such as deftly avoiding a striking hand." So they set out to build their own.

Simply scaling down mechanics that work for flight on larger objects wouldn't do. Scaling things down just results in too little force, or it creates a situation where surface interactions between the parts inhibit flight, as things like friction begin to dominate. Rather than taking the traditional route to get something tiny aloft—attaching it to some form of rotary engine—they returned to the fly for inspiration, making a pair of flapping wings.

On the fly, the wings work because the angle they take when moving upwards is different from the one they take when flapping down. The authors set that up so it happened passively; as the wings swept in opposite directions, the hardware at the joint where they met the robot's body forced them to rotate.

To get the wings to beat fast enough, the authors created two "muscles" made from a piezoelectric material, which changes shape when a voltage is applied. These flapped the wings at 120 beats a second. Not only is this rate similar to a fly's, but it also created a resonance in the robot's body that amplified the force of each beat. That resonant frequency was so important that the flight control system never changed it, even when it needed to change the force generated by the wing (to fly up or drop lower, for example). Instead, the force was controlled by changing how far the wing traveled with each beat.

That same approach allowed the researchers to rotate their robot while in flight. By having the left or right side do a stronger beat, the robot would turn.

All of this, however, meant that the robot was very dynamic while flying. The control system, which was connected via the same wires that supplied power, had to make constant adjustments to keep it stable. To enable that, the robot had a number of silvery spheres on appendages. These helped with balance and could be tracked by a motion capture system that used them to figure out the robot's position in a three-dimensional space. Overall, the control system's response time was about 12 milliseconds—just a hair slower than a fly's.

Thanks to a carbon fiber body and polymer wings, the whole package weighed only 80 milligrams. Its three centimeter wingspan allowed it to generate upwards of 1.3 milliNewtons of force.

Overall, it's an impressive little device. But we're still a long way off from being able to put the computing power that keeps it stable onto the robot itself. And even if all its spare lift capacity went into batteries of some sort, it would only be enough to allow it to fly for a matter of minutes. For now, millions of years of evolution have the engineers beat.

Science, 2013. DOI: 10.1126/science.1231806 (About DOIs).