Nature often gives us the best design concepts. Want a speedy swimsuit design? Study some shark skin. Need to cool a large building on the cheap? Take a little inspiration from termite architecture. It's called biomimicry and simply involves trusting millions of years of evolution over a few thousand years of human research and development.
So why don't airplanes have flapping wings, right? That design seems to work just fine for birds, insects and bats. Leonardo da Vinci sketched numerous flapping aircraft and "Dune" author Frank Herbert envisioned a distant future in which humans sailed the winds of a desert planet in flapping ornithopters.* Yet more than a few inventors have tried to flap their way into the sky only to plummet like a stone. What gives?
According to Dr. James Usherwood of the University of London's RVC Structure and Motion Laboratory, this is one area of design where nature didn't quite nail it. Millions of years of evolution can't change the fact that flapping wings are horribly inefficient compared to mechanical propellers -- at least as far as slow flight and hovering are concerned.
In "Flying and Walking: Learning from Nature," (published in this book) Usherwood points out that a slow-flying pigeon requires up to four times the power an equivalent helicopter would need. And hovering is even worse, resulting in pitiful amounts of lift for the amount of energy exerted.
Each pump of the wings requires yet another burst of energy and it all adds up pretty fast. That's why hummingbirds have the highest energy expenditure of any warm-blooded animal -- 10 times that of a human. They're perpetually on the verge of starvation and have to consume colossal amounts of nectar to keep up.
Early aviation pioneers learned the folly of flapping wings (both mechanical and human-powered) the hard way. Yet it seems scientists have recently had to learn this lesson all over again with micro air vehicles (MAVs). These are tiny robotic aircraft that depend on biomechanical flight dynamics. MAV funding typically comes into play due to the obvious surveillance possibilities.
You can probably imagine a future full of tiny, moth-sized government spy drones. Luckily, we won't see such a reality any time soon. The U.S. Army Research Office is really into the concept, but identifies some rather predictable downsides in this report: enormous manufacturing challenges, inefficient energy usage and dependence on less-understood small-scale physics.
Birds, bats and insects have flapping wings. Helicopters and airplanes have propellers. Don't expect that to change any time soon.
* Artist Abraham Katase (who created the fine image above) points out that since Holztman-Effect antigravity devices are all over the Dune universe, the 'thopters may have had a good bit of help ascending into the sky.