Humans have wing envy.
For thousands of years, we've been dreaming up hare-brained schemes to fly like birds.
The ancient Greeks conjured up Icarus and Daedalus, who made wings from bird feathers,
strings and wax.
Leonardo da Vinci put a little more thought into it, drafting plans for a mechanical winged
contraption known as the ornithopter.
While 20th-century humans are perfectly capable of taking to the skies, there's something
intrinsically unsatisfying about doing so in an aluminum behemoth.
We still want to fly like birds, taking off under our own power and gliding effortlessly
once we're airborne.
After at least two millennia of thoughtful engineering, why are humans still trying to
figure out individual flight?
Why can't we strap on a set of wings and take off?
To understand why, consider the physics of liftoff.
In order to leave the ground, the forces driving a body upward must overcome the downward force
of the body's weight.
You can see this in action during a jump.
When you push your feet down into the ground, Earth provides an equal and opposite reaction,
forcing the body upward.
You can't rely on the ground when trying to achieve sustained flight, since the whole
point is to uncouple yourself from Earth.
So flying creatures and machines have to push air down into the ground, instead of pushing
on the ground itself.
There are two ways to manage the problem of pushing down enough air to achieve liftoff.
You can accelerate a small amount of air downward very quickly, as a hummingbird does.
That takes incredible power, and none of the muscles in the human body muscles can even
come close to accelerating air that quickly.
Alternatively, you can accelerate a large amount of air more slowly, which is the more
efficient technique, especially for heavier fliers.
(That's why a pelican's wings don't flap as fast as that of a sparrow or hummingbird.)
You might think, then, that we could just build really, really large wings and flap
them quite slowly.
But there's a problem here, too.
Bigger wings, and the muscle required to move them, add weight to the equation.
The amount of power required to take off increases by the square of the additional weight.
So a doubling of weight requires a fourfold increase in power output.
Unfortunately, our arm and back muscles just aren't that strong.
Our more powerful quadriceps and gluteal muscles, stretching from our thighs up, however, can
almost do the job.
For a human to take flight on flapping wings, your body would have to be made almost entirely
of muscle.
In other words, humans make terrible hummingbirds.
With flapping and spinning apparently non-starters, a couple of options remain.
Some birds, including albatrosses, take to the air by riding thermal updrafts.
They coast between the rising and falling air currents to manage their trajectory and
execute a landing.
It's a phenomenon similar to hang-gliding.
But the whole process is totally dependent on the right wind conditions, and many scientists
think albatrosses can't fly on a still day, and therefore consider the birds technically
flightless.
We could attempt fixed, airfoil-shaped wings, as we use in airplanes.
When a plane gets going fast enough, the air moving over the top of the wing is at a slightly
lower pressure than the air underneath, creating the phenomenon known as lift.
The problem is that you have to get going far faster than your legs can carry you.
The frustrating physical reality seems to be that, unless we can lose a lot of weight
without losing any strength, humans simply aren't destined to fly any significant distances
under their own power.
But there is a happy ending, of sorts, to this story.
The jet pack, probably the closest thing a human will ever get to flying like a hummingbird,
is right around the corner.
A New Zealand-based company has developed the Martin Jetpack, essentially a helicopter
backpack.
Working with the same basic problem that faces human-powered flight, power requirements increase
at the square of added weight, Martin developed a rotary wing system with the lightest engine
capable of turning the rotors.
What he ended up with was an automobile-style piston engine.
It's lightweight, and it forces far more air through the rotors than it uses to combust
the gasoline that powers the engine.
While it's not exactly soaring like a majestic bird, it'll save you the hard work of flapping
your way up there.