The marvels of flight

Now that spring is starting to appear, our neighborhood is awash with the song of birds singing in the trees. Every day while waiting for the bus, I see at least a few geese flying overhead on their way south. And the number of robins digging worms in our yard is actually kind of comical. As I watch all of these birds, I am frequently struck by the fact that birds are remarkable fliers. The ease with which they take off, maneuver in the wind, swoop and swoon like acrobats, somehow manage to avoid running into each other, and then land safely on any number of surfaces (trees, roofs, the ground, cars, electrical wires, or the surface of Lake Washington) is remarkable. How do they do it?

Flying is all about having enough upward thrust to overcome to force of gravity pulling an object to the ground. Birds achieve this thrust using their wings in 2 ways – by manipulating air that is already moving (as in gliding) or by moving air themselves (as in flapping).

Gliding is actually relatively simple. When a bird is already airborne, it can glide in much the same way that a hang glider does. For one, they use thermals, trade winds or updrafts. These are piles of air that are rising upwards. The air moving upwards creates a force on the underside of their wings, pushing them up. For another, the shape of the wings gives them upward thrust. As their wings move against the air, they push the air underneath the wings down. This is a force that must be counteracted (according the Newton’s law of every action having an equal and opposite reaction) by an equal and opposite force up. A force pushing up – and voila! Flight.

Okay, so that’s the easy case. What about flapping? How does that work? Well, when a wing flaps, it pushes air in a strong force downwards. This moves the bird up in the air. A curious question, therefore, is why doesn’t the upward stroke of a wing flapping create a force that pushes the bird down? Actually, bird wings are hinged – when the flap down, they are fully extended. But when they move back up, they are folded up, presenting less surface area to push against the wind. You can think about this like the oar on a canoe. If you paddle with the full surface area of the blade pushing against the water, you get a big push. But if you paddle with the blade parallel to the direction of motion, you don’t get much thrust at all. So just imagine a bird wing being like a canoe paddle that pushes air around, instead of water.

Well, it doesn’t sound too complicated, right? If that’s all it takes for flight, why haven’t humans invented self-flying units with retractable wings that we can use to take off, and why don’t we glide into work every day in our personal hang gliders? Well, a bird can fly not only because of the forces its wings can exert on the air, but because its body is uniquely adapted for flight in ways that a human body is not. For example, birds are extremely light. They have extremely strong, hollow bones – strong enough to get the job done, but light enough not to create too much weight to overcome. Birds have feathers, which are capable of trapping and dispersing more air than hair or skin. Birds have highly efficient respiratory systems, which are extremely good at extracting oxygen from the air. In addition, they have extra air sacs next to the lungs, so the animals never run out of breath. This is really important, as it takes a lot of energy (and thus a lot of oxygen) to maintain flight. And birds eat food that is highly energy dense for their body size, which (again) gives them the extra energy they need to carry out this tremendously difficult task.

The human body is all wrong for us to be able to fly on our own. We’re much too heavy, largely because we’re so dense. We don’t have any sort of extra light, fluffy, airy material like feathers to help us move air the way we want to; our arms and hands are neither strong nor big enough to create enough of a downward thrust to move us skyward. Our lungs are perfectly fine for our usual activity on land, but are nowhere near efficient enough for such an aerobically intensive feat as flying. In fact, we just don’t have enough energy, period.

Of course, we have found ways around our limitations as far as flying goes. We’ve built jumbo jets, ultralights, hang gliders, and powered parachutes. So it is possible to get us off of the ground. But if you were to compare the simplicity of a bird flying with all of the ways that we as humans do it?

Frankly, I don’t think it’s much of a contest which one is more elegant. So you can consider this another entry in my list of “ways in which nature is a much better engineer than man.”

(With apologies to all of the engineers I know out there.)