As anyone with a fear of flying will tell you, flight is a mystery.
The fact that heavy planes full of people can stay aloft is incredible. Even experts admit there is a lot we don't know about flight, including why animals do it so much better than our best aircraft.
A Blackbird jet flying nearly 2,000 mph covers 32 of its own body lengths per second. But a common pigeon covers 75 of its body lengths a second. The roll rate of the aerobatic A-4 Skyhawk plane is about 720 degrees per second. The roll rate of a barn swallow is more than 5,000 degrees per second. Some military aircraft can withstand gravitational forces of 8-10 G (Earth's gravity is equal to 1 G). Many birds routinely experience G-forces greater than 10 G and up to 14 G.
When it comes to flexibility and efficiency in the air, birds, bats and insects easily outperform airplanes. Researchers hope studying animals more closely may reveal some of the secrets behind flight.
Nearly all airborne animals use flapping wings to fly.
When humans first dreamed of flight, many people naturally thought to imitate birds and create flappable wings.
In the 13th century the philosopher and Franciscan friar Roger Bacon proposed the idea of a flapping-wing vehicle, and in the 15th century, Leonardo DaVinci sketched flying machines with flapping wings.
But early attempts to fly like animals fell flat. For a vehicle carrying the weight of a human, the flapping speed required to create the necessary lift and thrust was an insurmountable obstacle. It wasn't until people tried using stable wings, for which the aerodynamics are much less complicated, that they were really able to get off the ground.
The first human flights involved gliders and, famously, a gargantuan steam-powered airship in 1852.
In 1903, the Wright brothers famously made the first controlled, powered and sustained heavier-than-air human flight using a contraption with wings fixed in place. Since their success, most human-carrying aircraft have followed suit with fixed wings.
"Because the nature of flapping flight is so complex, it's difficult to copy," said Sergey Shkarayev, a professor of aerospace and mechanical engineering at the University of Arizona. "People realized it's much easier to deal with fixed wings, as birds do when they soar. But you still need a propeller. Birds do not have this very important invention, like a wheel or propeller. People came up with a combination of a propeller and fixed wings. That’s how the Wright brothers succeeded."
While our 747s serve us pretty well, there are limitations to fixed-wing flight.
"With flapping wings we expect that they will have some qualities fixed wings don’t have, like the ability to do sharp maneuvers like hummingbirds," Shkarayev said.
Flapping wings also allow fliers to hover in one position, fly at very slow speeds and respond more flexibly to changing environmental conditions such as wind turbulence, rain and snow. Such flexibility even enables birds and insects to fly with broken wings.
Shkarayev designs flapping-wing miniature aircraft called micro air vehicles. His fliers have wingspans of 5 to 8 inches and can be remotely controlled.
These machines are highly sought after by the military, and could also have scientific and commercial uses.
"There is a lot of interest in miniaturized technology," said Wei Shyy, chair of the Aerospace Engineering department at the University of Michigan, who also designs small flapping-wing vehicles. "For example, you can send one of these fliers for surveillance in Afghanistan or Iraq. They could send them to fly out to monitor the situation, take videos and see what's really going on in the field. Or you can send these into a nuclear disaster area when you don’t want to send humans."
To better engineer his aircraft, Shyy photographs birds and designs computational models to try to understand their aerodynamics.
When birds and insects fly, their wings change shape constantly to accommodate environmental conditions. But the physics of these changing wings is complicated.
"How much flexibility is really desirable? If you have too much flexibility you can be sloppy, maybe worse than having no flexibility," he said. "That is a big question."
Peter Ifju is a professor of mechanical and aerospace engineering at the University of Florida who collaborates with Wei Shyy.
"I think we've still got a long way to go to figure out some of the bigger questions," he said. "Physically, what are they doing to the air to produce such efficient lift? There are all kinds of flow physics we just don't understand. We can see what they're doing, but we don’t understand how that interacts with the air."
On a basic level, he said, birds are swimming through the air as people do through water. When we swim we push water away from us to create thrust forward and up, while trying to cause as little drag as possible.
"Birds have a similar philosophy," he said.
Animals will always have some advantages over machines, such as the ability to use their nervous systems to sense subtleties about the environment around them and alter their flight accordingly.
"We are not trying to just copy nature," Shyy said. "Some things nature does, we simply cannot make. We are trying to take a fundamental understanding of nature, then apply engineering knowhow."
Natural flight is not just popular for its military applications. A community of enthusiasts has sprung up around ornithopters — another term for flapping-wing vehicles.
"The reason why a lot of people have pursued this idea over the years is not for any practical application, but more to prove that it can be done," said Nathan Chronister, who founded the Ornithopter Zone Web site, a hub for flapping-wing hobbyists. "The idea kind of took hold several hundred years ago before the airplane was invented. It was logical. People watched birds flying and thought maybe we could do that too. It ended up being more technically challenging than the airplane. But there are still those who want to show that we can fly the way birds do, also."
Chronister studies the history of ornithopters and designs his own vehicles ranging in wingspan from 6 inches to 5 feet. He is involved in the Ornithopter Society, which publishes a quarterly newsletter with updates about ornithopter design.
"People who are interested in ornithopters usually are people who enjoy working on technical challenges," he said. "It comes out of an admiration for bird flight, and enjoying the challenge of trying to imitate that."